THE NAUTICAL INSTITUTE
TUG USE IN PORT A Practical Guide 2nd edition
by Captain Henk Hensen FNI
N.Cham.
387.166 H526 2.ed. 200';
Autor: Hensen, Henk, Titulo: Tug use in port: a practical guide.
I\!IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII1II 1111 Ex.1
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8
TUG USE IN PORT - 2nd edition by Captain Henk Hensen FNI
1st edi tion publishe d by The Nautical Institute 1997 2nd edition 2003 Publish ed by The Nautical Institute 202 Lambeth Road, London, SEI 7LQ, England Telephon e: +44 (0)20 7928 1351 Fax : +44 (0)20 7401 2817 Publications e-m ail: pubs@nautinslorg Worldwide web site: http:/ /www.nautinslotg This edition Copyright © The Nautical Institute 2003 Sponsored by the Port of Rotterdam Authority Cover picture The Hellespont Metropolis arriving in Rotterdam on her maiden voyage O ctober 2002 with Fairplay tugs in attendance. Courtesy of Port of Rotterdam; Ben Wind Fotografie, the Netherlands
All rights reserved: No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, except for the quotation of brief passages in reviews. Although great care has been taken with the writing and production of this volume, neither The Nautical Institute nor the author can accept any resp on sibility for errors, omissions or their consequences. This book has been prepared to address the subject of tug use in port This should not, however, be taken to mean that this document deals comprehensively with all of the concerns which will need to be addressed or even, where a particular matter is addressed, that this document sets out the only definitive view for all situations. The opinions expressed are those of the author only. Captain Henk Hensen was born in 1935, is a Master Mariner and was a Port of Rotterdam pilot for 23 years. During his years as a pilot he was stationed at the Pilot Office for five years. During that time he started simulator courses for harbour pilots and tug captains and participated in many port studies, including simulator research. He started a database for casualties in the Port of Rotterdam and analysed them with the object of improving safety. Following his retirement he started his own consultancy, Nautical Safety Consultancy, and works as marine consultant on the nautical aspects of port studies, tug advice and simulator training. All photographs and diagramS acknowledged
Typeset by J A Hepworth I Ropers Court, Lavenham, Suffolk, CO 10 9PU, England Printed in England by Modern Co lour Solutions 2 Bullsbridge Ind ustrial Estate, Hayes Road , Southall, Middlesex, UB2 5NB, England
ISBN 1 870077 39 3
CONTENTS Acknowledgem ents
ii
Foreword
iii
Author's Preface
iv
Tug Use in Port - The Ov erview
v
Glossary of Terms
vi
List of figures
ix
Chapter 1
Tug design factors
2
Types of harb our tug
3
Assisting me thods
33
4
Tug capabilities and limitations
43
5
Bollard pull required
68
6
Inte ractio n and tug safety
80
7
Towing equipment
94
8
Training and tug simulation
117
9
Escort tugs
134
10
Tug developments
163
1 :
9
174
References Appendices 1
Port au thorities & towing companies which provided information
178
2
Safety of tugs while towing
180
3
Rules for escort vessels
182
Index
187
TUG USE IN PORT
ACKNOWLEDGMENTS 1st edition The autho r would like to express his appreciation to the Rotterda m Mun icipal Port Manageme nt for their generou s sup port, which made it possible to write this book. Without the expertise and support of many individuals and companies this book could not h ave been completed to the standard which has been achieved. The author is sincerely grateful for their contributions. Although it is hardl y possible to name them all, a small list of the persons and companies that have been so kind in providing information or sharing their insights would include: The Rotterdam towing companies, and in particular Smit H arbour Towage Company; Damen Shipyards, Gorinchem, The Netherlan ds; Mr.Joh. deJo ng MSc, Marine Simulator Centre the Netherlands; Mr. David L. Potter, Marlow Ropes, U K; The Glosten Associates, USA; Captain LarriJ ohn son , Marine Supe rintendent Foss Maritime, Seattle, USA; US Coast Guard; and Thomas Reed Publication s, UK. Furth ermore the author is greatly ind ebted to the following person s:Mr. W. Hoebee MSc, and his staff, and Captain W. Verb aan of the Rotterd am Port Authority, Mr. T.E. Tom asson MSc, of MarineSafety Int ern ational Rotterdam, for their generous and continuou s support. Captain Evgeny Sarmanetov, former St. Petersburg pilot, for his excellent contribution regarding manoeuvring in ice and Captain N. Golovenko, Rotterdam, for the Russian - English translation of this article. Captain Victor ]. Schisler, Long Beach - pilot, U SA and Captain Nigel Allen, Southampton - pilot for their professional contribution on escorting. Those of all the port authorities and towing companies that compl eted the questionn aire and provid ed information regarding tugs and tug assistance in their ports. The respo nse to the questionnaires, which were sent by the Port Authority of Rotterdam to a hundred ports around the world, was much high er than might be expected and the information provided by those ports that completed the questionnaires was invalu abl e. The names of these person s and th e port authoriti es and towing companies are listed in Ap pendix I. Finally, the author is sincerely grateful to Captain Herbert van Donselaar MSc, for sharing his kee n profession al insight du ring the process of writing this book.
2n d edition In 200 2 the bo ok was revised. Again many were helpful and contributed by providing information, sharing their insights and always willing to answer questions. The author is grat eful for the contributions of: Mrs. Heik e Hoppe of IMO, London, United Kingdom; Mr.JoopJansen and Erik Leend ers, Dam en Shipyard, the Netherlands; Mr. Randy S. Longerich, Puget Sound Rope, USA; Mr. Paul P. Smeets, DSM High Performance Fibers, the Neth erlands; Mr. Dav id L Gray, Glosten Associates, USA, Mr. Rob ert Allan, Robert Allan Ltd , USA ; Mr.J on M.Jakobsen, Statoil Mongstad, Norway; Mr. Erling Kvalvik, Norsk Hydro Produksjon a.s, Norway; Mr. Jimmy Brantn er, Marine Towing of Tampa, USA ; Mr. Richard Decker and Mr. John Collins, Seabulk Towing, USA; Mr. Markus van der Laan , IMC Group, the Netherlands; Mr. Dave Foggie, The Maritime an d Coast Guard Agency, UK, while several others could be added. Furthermore, the author is greatly ind ebted to the following persons: Mr.Jaap C. Lems, Director Rotterdam Port Authority and Harbourmaster of the Port of Rotterd am, for his great support; Captain Roger Ward, Tug Master and formerly Marine Man ager with H oward Smith Towage , Melb ourne, Australia,for the valuab le discussions and information exchange on practical aspec ts of harbour towage durin g several years; Captain Gregory Brooks, Tug Master/Instru ctor, USA ; Captain Victor]. Schisler, Long Beach pilot, USA; Capt Arthur Naismith, Voith Training Master; Cap tain Nigel Allan, Southampton pilot, UK; LT Keith Ropella, Chief Vessel Traffic, MSO Valdez, Alaska, USA and Mr. H enrik Hammarberg, Det Norske Veritas, Norway, for their professional contribution; on escorting, escort procedures, and / or regulations. Finally, the Rotterdam Muni cipal Port Managem ent gen erously suppo rted also this revised edition of the book, for which the author would like to express his since re appreciation. Without the help of all those mentioned it would have been impossible to revise the book in the way it has b een don e.
ii THE NAUTICAL INSTITUTE
FOREWORD by Executive Dir ector of th e Port of Rotterdam Mr. P. Struijs
Tug Use in Port, which includes escor t tugs, is a valuable additio n to nautical literature. Twenty years ago few would have b elieved th at it cou ld be possibl e to bu ild in so mu ch powe r and manoeuvrability into th e hull form of tod ay's tugs. With conventional designs it was impossible to achieve this capability, bu t now towage companies which do not em brace thi s n ew techn ology are likely to find the co mpe tition overwhelming. It is against this b ackground, an d I trained as a na val archite ct, th at I welcom e this book. It sets out to demonstr at e the characteristics of th e old and new and in d oing so th e read er can come to appreci ate how to transfer an d adapt towing practices to optimise the use of all tugs in a mixed fleet. Whilst naval arch itects an d m ari ne enginee rs h ave co ncentrated on fuel economy pe r ton mile in deep sea vesse ls they remain unwieldy in confined waters. Similarly the car carrier an d containe r ship, altho ugh generally high er p owered than th e bulk carr iers, h ave special limitations imposed by windage. H appily whils t the deep sea vessel h as b ecome larger and relatively less man oeu vrable tugs have grown in cap ability and so play an essential role in port econom ics. Indeed a port which can not provide effective tug sup po rt b ecom es un viabl e an d it is impo rtant that the towing industry recogni ses this. So Captain H ensen an ex perienced pilot fro m my p ort h as pro vided an essential service in dem onstr ating h ow tugs can b e used to b est effect. T he Port of Rotterd am is pl eased to have pl ayed its p art as a major spo nsor to this publication. This book ex amines towage techniques an d the reader will b e constantly re m inded that shiphandling with tugs is all about competent teamwo rk. On b oard th e ship are the maste r, pilo t and crew, on bo ard the tugs are th e tug masters and crew and they have to work together. To be effective all n eed a good kn owledge of this professional area of activity particular ly as ships are often atten de d b y a mixed variety of tugs. T he foundation of how b est to control operations is laid out in this ve ry p rac tica l guide . The othe r ch apters on tow ro p es, training, bollard pull and esco rt work, all link ed by a common thr ead of safe working me th ods makes this an ideal b ook for study. I b elieve it will favo urably influen ce the way tugs are design ed and used . This is the hallmark by which this book will b e recognised an d ' I have no h esitati on in recommending thi s well illu str at ed text to towage co mpanies , ports, tug m asters, pilots an d sea staff alike. Everybody will ben efit from its practical guidan ce.
TUG USE IN PORT iii
AUTHOR'S PREFACE Wh en ships are assisted by tugs, experi~nce, teamwork , communication and abo ve all insight into the capabilit ies and limitation s of ships and attending tugs are essential for safe and efficient shiphandling. This ap plies to th e tug captain and his crew as well as th e ship master and pilot, particularly nowadays as older conv en tiona l tugs ar e increasingly being replaced by modern types with larger engine powers and increased capabilities. Reputable shipyards build goo d tugs, and designers can predict how well their tugs will perform . However, they do not handle ships themselves and have not experienced the tug assistance required: not in a river, channel or port approach nor in a confined h arbour basin, not during a storm or in strong curre nts nor in the midd le of a foggy night. Not even du ring nice, calm weath er. These are the situations an d conditions in which pilots and tug captains have to hand le ships. So it is essential that they know what can be expected from a tug in any specific circumstance. Only when these professional s are fully aware of the capabilities and limitations of the various types of tugs in gene ral an d of an ind ividual tug, including the effects on an assisted ship, are they able to utilise tugs in the safest and most effective way an d in harmon y with a ship's m anoeuvring devices. Good insight int o the opera tional performance of differ ent types of tugs while assisting vessels is also of major im portan ce for tugb oat companies. It allows the m to determine wha t type of tug will pro vid e optimum service for the port, with respect to the local situation, environme ntal conditions and ships calling at the port. The increasing use of simulation for research and training purposes requires an in-depth knowledge of tug capabilities and limitations, in add ition to the data requir ed for creat ing a tug simulator mod el. Only then can resu lts be achiev ed that are safely applicable to daily pr actice and which form a contribution to safe shiphan dling. Th ere is a trend tow ards ev er more powerful tugs and mor e man oeuvrabl e modern vessels. Th is is leadin g to a reduction in the number oftugs used to assist those ship s, so the role of harbour tugs becomes eve n m ore crucial than before.
There are many reasons, therefore, why a book on tug assistance could be usefuL The aim of this b ook is to improve the practical kno wledge of harbour tugs and their different types, and to give a better insight into the cap abilities and limitations of these tugs while rendering assistance. Not all aspects of shiphandling with tugs are addressed in detail within this book. This work sho uld be see n as a basic guid e to the reader, whilst at the same time en couraging further increase of knowledge. The references m enti oned at the end may prove usefuL ' The b ook is specifically written with th e needs of maritime professionals involved in the day-to-day pr actice and training of shiphandling with tugs in mind, particularly pilots, tug captains and training instructors. It sho uld also b e of valu e to towing companies, shipmasters and mates of seagoing vessels and all other persons or orga nisations involved, one way or ano ther, with tugs and shiphandling. In th e second edition several subjects have been reviewed or extended, based on expe rience and kn owled ge gained during the last five years. Item s that were found to be missing have been included, Ship's fittings for use with tugs have been addressed more specifically, the escort chapter has been extende d, new developments in the tug world have been included, and several refer ences used for th e book have been add ed for tho se who want to read mo re about certain subjects. Th e tug world is a fast changing world , although basic principles for tugs and tug operations do not change that much. It is th e author's earnest hope that this b ook will contribute to improved knowledge of harb our tugs and lead to increasing safety in tug and shiphandling ope rations in ports and port approaches around the world. The author.
iv THE NAUTI CAL INSTITUTE
TUG USE IN PORT THE OVERVIEW The contents of this book are outlined below. A general review is presented first of factors which affect operational requirements for a harbour tug, such as the different tasks for which they are used, the particulars of a port, the environmental conditions and ships calling at the port. •
Various types of harbour tug are discussed in a general way, addressing the diversity of design, propulsion, steering and manoeuvring capabilities. After reviewing assisting methods in use worldwide, tug types are considered in more detail, including the performance of different types of tug resulting from the location of propulsion devices, towing point and lateral centre of pressure. Tug capabilities, limitations and effectiveness with respect to different assisting methods and operating positions relative to a ship are discussed.
•
The number of tugs required to handle a vessel safely is frequently a topic for discussion between pilots and shipmasters. This iroportant subject is discussed taking into account the effects ofwind, current, shallow water and confined waters. The number of tugs and total bollard pull used in several ports around the world is mentioned. Much attention is given to dangerous operational situations for tugs, such as interaction and girting, and to environmental conditions such as fog. Towing equipment is dealt with, particularly in relation to safe and efficient shiphandling. Escorting and escort tugs, being a subject of specific interest nowadays, is dealt with separately. Proper training for a tug captain and crew is essential in order that they handle the tug safely and efficiently. The same applies to the pilot and/or master for shiphandling with tugs. Training is therefore an important subject in the book, including siroulator training and research. All subjects are, as far as possible, related to situations encountered in practice.
PIw,,, S~~Lbi.,
Cmwia
&verse-tractor tug> 'Seaspan Hawk' and 'Seaspan Falcon' (l.o.a. 25·9m, beam 9· lm, bp ahead 39 tons, bp astern 37·5 was) ready w mah fast at thefrrward and port quarters with a bow line
TUG USE IN PORT v
GLOSSARY OF TERMS Assisting methods
The term used to describe the way in which harbour tugs assist seagoing vessels.
Breasted/alongside towing :
A tug securely lashed alongside a ship, usually with a minimum of three lines: head line, spring line and stem line. Also called 'on the hip' or 'hipped up'. A tug made fast so that it can pull as well as push at a ship's side. Depending on the type of tug, its location and the assistance required, it can be secured with one, two or three lines. A tug assisting a ship while towing on a line as is in common use in many European ports.
Push-pull
Towing on a line Box keel
An enclosed keel structure extending from the aft skeg (if fitted) to a point close to the forefoot of a tug. A box keel is sometimes installed on ASD escort tugs to provide a better course stability on astern and additional lift forces, resulting in higher towing forces, when operating as stem tug in the indirect towing mode. In addition, a box keel gives additional strength to the tug's hull and provides a better distribution of dock forces when in dry-dock.
Course stability and directional stability: Course stability is also called dynamic stability, stability of route or dynamic stability of route (see References : H ydrodynamics in Ship Design, Vol. I. H.E. Saunders). It is that property of a ship (which includes tugs) that, when disturbed, damps out extraneous motions set up by the disturbance and to reduce them progressively to zero . Course stability should not be confused with directional stability, which is, strictly speaking, the ability of a ship to follow a certain direction, e.g. by means of an automatic steering system. A ship closely following a selected heading has directional stability but may be course unstable (see below), which resultsin frequent rudder (or thruster) actions to hold the ship on its course . Course stable ship With a constant position of the steering systems (rudders, thrusters, etc.), a ship is defined to be course stable if, after experiencing a brief disturbance, it will resume the original manoeuvre without any use of the means of steering. Course stability on a straight course, with the rudder in the equilibrium position, is mostly only considered. A turn initiated by a brief disturbance of a course stable ship will thus not continue. However, after the disturbance has vanished, the actual course of the ship will generally be altered. A course stable ship needs relatively large rudder angles for course changing. A course stable ship has good yaw checking ability. Course unstable ship A ship is called course unstable, if, after it is disturbed, it will immediately start to turn. Course changing, with relatively high rates of turn, can be achieved with relatively small rudder angles . A course unstable ship therefore generally has poor yaw checking ability. Cross lines/gate lines
Separate lines from either side of the tow to the opposite quarter of the tug or the opposite side of the tug's H-towing bitt.
Dead ship
A ship which cannot use her own propulsion.
Density of air as used
1.28 kg/m'
Density of sea water as used
1025 kg/m'
Escort tugs
Tugs specifically built for escorting at high speeds.
Escorting tug
Any type of tug escorting a ship underway.
F(P}SO
Floating (Production) Storage and Offioading Unit,
Free sailing
A tug sailing independently.
Girting
Risk of capsizing, especially with conventional tugs, due to high athwartships tow line forces. Also known as girding, girthing or tripping.
Gob rope I gog line
A rope or steel wire used on conventional tugs to shift the towing point
vi THE NAUTICAL INSTITUTE
H MPE
High-modul us polyethylene fibre under the trade names 'Spectra' and 'Dyneem a' used for high performance ropes .
H odde
Kinking or twisting of a strand in a rop e whi ch makes it unfit for use.
IMO
International M aritime Organization.
Lbp
Length between perpendi culars.
Loa
Length overall.
LWL
Length at th e waterline .
M BL
Minimum Breakin g Load (of arope},
MG
Initial Metacentric Height.
Messenger
A light rop e attached to the tow line in ord er to heave the tow line on board a ship.
Norman pins
Short iron bars fitted in th e gunwales of the transom to pr event the tow line from slipping over the side gunwales. Som etim es called ' King Pins'.
Nozzle
A tube around the propeller to increase pr op eller performance. The nozzle can be fixed or steerable.
OCIMF
Oil Companies International Marine Forum.
PlANe
Permanent International Association of Navigation Congresses.
Pendant/pennant
A separate part at the final part of a tow line which is mo st liable to wear on board an assisted ship, at Ship's fairleads, etc. The pendant can be of different construction to the tow line.
Propulsion: Azimuth prop ellers CPP FPP VS
3600 steerab!e propellers, which can deliver thrust in any dire ction. Also called: 'Z'pellers', 'Rexp ellers', 'Duckpellers' (azimuth propellers in nozzles). Cont rollable pitch propeller{s}. Fixed pitch propeller(s}. Voith Schn eider propulsion : propulsion system with vertical propeller blad es, also called cydoidal propulsion system.
PRT
Prevention and Response Tug.
Significant wave height
The approximate wave height as seen by an expe rience d observer wh en estima ting the height visually.
Snag re sistance
Resistance of a rope to single yarns b eing pu lled out of the rope when it slides along a surface, such as over a deck or through a fairlead. A snag is a loop of a yarn .
SPM
Single Point Moorings.
Sponson
A strongly flared section in the side of a tug, commencing at or just b elow the waterline, which results in substantial increase in deck area and reserve buoyancy without increasing the b eam at the waterline.
Stemming
A tug coming under the bow of a ship at speed.
Stretcher
That part of a tow line, between the original tow line and pennant, which absorb s the dynamic forces in th e tow line. Also called a spring and often ma de of nylon , polyester or a polyester/polypropylene combination .
Towing point
Point of appli cation of the tow line force. It is the point from where the tow line goes in a straight line towards th e ship.
Towlin e
A flexible hawser used for towing purposes.
Tripping
A tug towing on a line swinging around and coming alon gside a ship's hull due to excessive speed by the ship in relation to a tug's capabilities and towing angl e. The expression 'tripping' is also used for girting. TUG USE IN PORT vii
Tug/ engine power : BHP SH P BP MC R Ton Tug simulation : Int eractive tug
Vector tugs UHMW polyeth ylene (U H MW PE)
Brake Horse Power : power delivered by th e engine. Shaft Horse Power: power delivered to the propeller shaft (approximately 97% of BHP). Bollard Pull, which in this book is expressed in the practical units of tons , equal to 1000 kgf (= 9·80665 kN). Maxim um Continuous Rating (of tug engine). Th e practical unit used in this boo k for force, e.g. for bo llard pull, equal to 1000 kg force, and for 'weight', equal to 1000 kg. A tug simulated on a bridge man oeuvring simul ator, able to interact wi th other bridge manoeuvring simulators, which are simulating othe r tugs and! or the assisted ship . Tugs simulated by just a for ce vector.
Ultra High Molecular Weight polyeth ylene. Material used for dock fend ering and for fenders of tug boats at places whe re a low friction coefficien t is required.
VS-tug
A tug with VS pro pulsion.
viii THE NAUTICAL INSTITUTE
LIST OF FIGURES Figure
Titl e
Page
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Port or Antwerp. Zandvlietsluizen. Tugs should be able to assist ships thr ough th e locks and b ridges Push-pull tugs ope rating in the Port of O saka. Large man oeuvring area near the ber th M.T. Capitol berthing atJetty 4 at Sullom Voe Oil Terminal Tug assisting in open sea close to port entran ce _ _ In colder areas tugs should be able to operate in ice conditions Car carrie r passing Calandbridge in the port of Rotterdam. Th e stem tug is an azimu th tr actor tug Azimuth tractor tugs (53 ton s bo llard pull) of the KOTUG towing company towing an oil rig Conven tional twin screw tugs of 27 tons bollard pull towing on a line H arb our tugs - factors influencing ch oice
1 _ 2 3 3 4 4 5 6 7
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.2 1 2.22 2.23 2.24 2.25 2.26 2.27 2.28 2.29 2.30 2.31 2.32 2.33 2.34 2.35 2.36 2.37 2.38 2.39 2.40
Main typ es of harbour tug 8 Pusher tug Lam Tong .........................................................................................•..................................•........................................• 10 Plan of the na vigation bridge deck and view of the whee lhouse of a modem Hong Kong p usher tug 11 Typi cal fender arrangem ent for a tug pu shing under swell conditions and!or at flaring parts of a vessel 12 Bow fend er made of reinforced truck tyres 12 Tyree used in addition to vertical bow fenderi ng 13 Conventional twin screw tug - type Stan Tug 2909 13 Two gen erally used nozzle types 19A and 37 15 Steering nozzles, one with a moveab le flap the oth er with a fixed fin 16 Construction of a steerab le nozzle with moveable flap 15 Fixed nozzle with a move able flap rudder 15 Schilling rudder 16 Shutter rudder system with a fixed nozzle an d two flanking rud ders 16 'Iowmaster rudder system of tug Hamm 17 Twin screw tug moving sideways to starboard, also called flanking 18 Some assisting methods with conven tional tugs 18 Combi-tug Petronella] , GoedJuJop of Wijsmuller Harbour Towage Amsterdam 19
3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10
Tugs alon gside at approach and push-pull whi le mooring/unmooring Conventional USA tug secured with backing, spring and stern lines Alongside towing (USA) Forward tug secured alongside Alongside towing in Cape Town for a 'dead ship' up to 100 metres in len gth Ru dder or steering tug Conventional tug working stem to stem with a lar ge passenger ship Conventional twin screw tug EsperaTlQl At approach, forward tug alongside and stern tug on a line; push -pull while berthing Towing on a line at the approach and while mooring
~:::~~~:oe::~s=:cc::~~=:.:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::.: :::::::~~
Voith tractor tug 21 Prop eller blad es of a VS tug 21 Prin ciple of Voith propulsion 22 Prop eller control of VS tugs 22 A VS tug sailing ahead and astern 23 Some assisting methods with a tractor rug 23 Azimuth tractor tug Fairplay V•..................................................................................................•.................................................. 24 Integrated Schottel noz zles with open protective frames 24 J oystick for combined control of both thrusters 25 Thruster control unit for combined control of thrust and thrust directio n "" 25 Manoeuvring diagram for reverse-tractor tug 25 Reverse-tractor or pusher tug Lam 'Iimg 27 Thrusters of Cates ' reverse-tractor tugs 27 Assisting methods with a reverse-tractor tug 27 ASD-tug type 3110 28 Free sailing mano euvring capabilities of an ASD-tug and rev erse-tractor tug 29 Some assisting methods with an ASD ·tug 29 Relationship between brake horse power and bollard pull for different propulsion systems 30 Ranges in relationship between brake horsepower and b ollard pull for different tug types 30 Example of thrust vector diagrams 31 An assisting meth.od as used in some USA ports 32 34 35 35 35 35 35 36 36 36 37
TUG USE IN PORT ix
Figure
Title
Page
3.11 3 .12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.2 0 3.21 3 .22 3.23 3.24 3. 25
Ship is passing. narrow bridge and a conven tional tug forward is assisting with two crossed tow lines Towin g on a lin e at the approach nd push-pull while mooring Combination of different assistiog methods Ship app roaches the be rth nearly parallel to th e dock Tug assistance in ice during approach to the b erth an d while m ooring Tug sweeping ice aw.y from between ship an d dock Mooring in ice wh en some 30 me tres free be rth is available in front of th e b ow position Combination of tug and b ow thruster while mooring Good results when approaching the b erth astern an d m ooring star boar d sid e alo ngs ide Tug assistance whe n m ooring in ice with ships and p owerful engines Ship approaching the b erth astern Two tugs stem to stem clearing ice b etween ship and berth whil e othe r tugs keep the ship in position Ship of m edium size departiog Unmooring bow first Channel through th e ice prepared b y ice breakers or strong tugs
37 37 37 39 40 40 40 40 41 41 41 41 42 42 42
4.1 4.2 4.3
Location of the pivot point for a ship at speed Location of the pivot point in a ship with zero speed Forces cre ated on assistiog tug, moving ahead
43 44 45
4.4
Forces created on assisting tug, moving astern
46
4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.2 2 4.23 4.2 4 4 .2 5
Tug working on a gob rope Swivel fairlead on the after end of a tug's deck for th e gob rope The large fairlead is the aft lying towing point on a VS tractor tug Direct and indirect towing methods VS tug operating in the indirect towing mode Heeling forces working on a conventional tug when towing on a line The effect of a r adial hook The effect of a radial hook , Basic differ ence between tug types Comparison between tractor type tugs and conventional tugs when towing on a line with a ship having headway When port helm is applied and the tug pulls to starboard to counteract the port swing Comparison of performance of tug types wh en pushing or pulling Pushing force created by hydrodynamic force on a tug's hull Effect of dynamic forces in the tow line Performance and behaviour of a 40 m etre conventional tug Performance and behaviour of a 30 m etre ASD-tug for pushing Performance graphs for four and six koots speed Performance graphs for eight ko ots spe ed Different tug positions Two conventional tugs assisting a tanker having headway in making a starboard tum VS tug & dbridge of Adsteam Towage, Southampton, UK
47 47 47 48 49 49 50 50 52 53 54 55 56 57 58 58 59 60 62 63 65
5.1 5.2 5.3 5.4 5.5 5.6 5 .7 5 .8 5 .9 5.10 5.12
Bollard pull required to compensate for beam wind Wmd height velocity ratio Bollard pull required in a cross-eurrent Effect of underkeel clearance on current for ce Bollard pull required for beam waves Open b erth constru ction for bulk carriers A tug's propeller wash hitting a ship's hull, reducing towing effectiveness Different towing positions 'C oanda' effect Total bollard pull in tons and average number of tugs for container and general cargo vessels Total bollard pull in tons and average number of tugs for tankers and bulk carriers (based on length overall) (based on deadweight) Total bollard pull in tons and average number of tugs for tankers and bulk carriers
70 70 71 72 73 73 74 75 75 77 77 77
6.1 6.2 6:3 6.4 6.5 6.6 6.7 6.8
Effect of following water when passing through a channel with a deep loaded ship Schematic flow - unsteady flowfield as felt by an observer in a stationary tug seeing a ship approaching Pressure pattern and relative flow field around a bulk carrier Interaction effects on a tug when proceeding along a ship Effect of flow pattern around a ship on tug performance A: Tug is waiting for the approaching ship to come closer to pass the tow line Girtiog and tripping Some specifi c manoeuvres by conventional tugs towing on a line including risk of girting or capsizing
81 82 82 83 85 87 88 89
5.11
x
THE NAUTICAL INSTITUTE
Figure
Title
P age
6.9 6.10
D ue to excessive speed a tug at a ship's side may capsize if the stem line cannot be released Due to low powered tugs and a strong beam wind, a container ship is drifting
90 91
6.11
ADS-tug 'Smit Marn e
93
7.1 7.2
Radial towing hook with rail track Radial towing hook of conventional twin screw tug Saona, Dominican Republic Aft er deck of a conventional tvvin screw tug with a to vvin g winch , i th radial system Additional fairlead/towing point near the stern of combi-tug Hmdrik: P. Goedkoop
94 94
7.3
7.4 7.5
7.6 7.7 7.8 7.9 7.10 7. 11 7.12
_
95
95
Two different gob rope systems Conventional single screw tug Adelaar After deck of ASD-tug Maasbank
95
Standard hook and a disc-hook with spring shock absorbers and different quick release systems Single drum towing winch of azimuth tractor tug Iixelbank Waterfall winch on board RTSpirit The friction drum s of a traction winch Split drum winch of the ASD-tug Melton D ouble winch forward on the reverse tractor tug]ohn Steel wire construction Typical minimum breaking strengths
96 96 97 97 98 98 98 99 102 102
7.16
Fibre rope components and con structions
103
7.17
Table giving comparative weights and minimum breaking loads of eight strand ropes of different fibres Table showing some characteristics of different fibre types
7.13
7.14 7.15
7.18 7.19
105
Tug reaction time and manoeuvring space required depending on towline length
105 108
7.20 7.21
The effect of different tow line lengths Tug ope rating broadside
108 109
7.22 7.23
Static force in a a to, line Two conventional twin screw tugs, Smit Ierland and Smit Denemarkm
109 109
7.24 7.25
VS tug Matchless Reverse tractor tug Charles H Cates 1
II I 112
_
7.26
Quick release hook used on ferries of North Sea Ferries for securing a tow line when a tug is required
112
7.27 7.28 7.29
Automatic hook up system, Aarts Autohook Typical emergency towing arrangement One of the emergency towing systems in three phases of deployment
113 114 115
8.1 8.2 8.3 8.4
Simulator layout with five bridge ma noeuvring simulato rs, a VTS simulator and instruction rooms Desktop computer program Tug.Master, developed by The Glosten Associates, Seattle, USA Bridge layout of a full mission bridge simulator Simulation track plot of a loaded tanker entering a port from the sea
116 121 124 125
8.5
Simula ted ship and assisting tug passing a bridge
126
8.6
Sche matic diagram of an interactive tug operations simulator
127
8.7
Field of view required for interactive tugs
128
8.8
Relationship between direction of view and control handles for an interactive tug with a 225 0 out-of-window view
128
8.9 8.10 8.11
Heeling angle is an important factor in tug limitations. Twin screw tug Smit Siberiii Model and mod el tank test for escort tugs to obtain hydrodynamic data, optimise tug design Model and mod el tank test for escort tugs to evaluate performance
129 132 132
9.1 9.2 9.3 9.4 9.5
M ajor oil spills from tankers and their causes: No. of incidents & volume, World, 1976-89 Typical effect of frequency reducing measures Direction of forces applied by assisting harb our tugs Ph otographs taken during escort trials in Prince William Sound, Alaska, August/ September 1993 Terminology relating to direct and indirect towing methods The reverse-tractor tug LynnMarie Maximum direct bralcingforces azimuth drive Approximation of steering forces of a 36 tons tractor tug Definition sketch offorces on a tug and a ship Importan ce of proper locations of centre of pressure and towing point.. Aquamaster escort tug concept - The Towliner with towing arch Steering forces required based on 15° rudder angle Rudder forces in tons for different load ed tankers, speeds and rudde r angles Tug Lindsey Foss applying steering forces in the indirect mode Plots of a full scale trial with the loaded 125,000 dwt tanker Anofuraou and the pwpose built escort tug Lindsey Foss VS escort tug Bess with m odified tractor tug design Specially designed tanker stern fittings on the former ARea tankers, now Polar tankers
134 135 137 139 140
9.6
9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17
140
141 141 141 142 l43 145 145 146 147 148 149
TUG USE IN PORT xi
Figure
Title
Page
9.18 9.19 9.20 9.21 9.22 9.23 9.24
The Foss Transom Link 151 Ttvc escort tugs of towing company Foss Maritime keeping pace with a ship 153 Large VS escort tug Garth Foss .......................................................................................................................................•........... 154 155 A selection of escost(-ing) tugs at different ports. Situation 2002 156 VS escort tug Ajax 157 Powerful ASD escort tug Hawk Can the escort tug prevent a grounding? 160
10.1
Novel ne w tractor tugdesign ·.v. ith sketch of the original shunters Taiwanese r everse tractor tug No 3 Iao-Yu
10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 10.13 10.14 10.15 10.16 10.17 10.18 10.19
The optimum harbour tug concept ROTO R escort tug concept The Rotor escort tug RT Magic Modified ROTOR tug concept with aft thruster located more aft, behind the aft towing point Typical assist modes with a ROTOR tug SDM New River of Seabulk Towing (USA) Side view of SDM Mark 11 Bow view ofSDM Assistmodes SDMs Characteristics of Design A and Design B of the carrousel tug Combi·tug Mullratug 72 Modified combi-tug Multratug 72 during full scale trials Towing forces based on model tests Carrousel tug outer port design Damen ASD tug 2477 with an open docking skeg, extending as a closed skeg forward Compact tugs. Common assist modes Example of a compact tug - Cape Pasley
xii THE NAUTICAL INSTITUTE
164
164 164 164 165 165 166 166 167 167 167 169 169 169 169 169 170 17l 17l
Photo ; SmitIntmulJionnl
Three difJerrnJ tugtypes wwing ona line. The lugs port silkforward andstarboardside aft are VS tugs of 35 tons bollard puU. Ttu tugstarboard side forward isa twin screw conventional tugof 37·5 tons bollardpuUandtheport tug aft isan ASD-tugof 62 tons bollasdpulL When the tanker hasto berth starboard silk a1mlgside the j etty, the ASD-tugand the VS tugport silk forward can, whm near the bmlt, easily dlange to apushingpositian orpush-puU witlwut rekasing thetowline TUG USE IN PORT xiii
Chapter ONE
TUG DESIGN FAcroRS 1.1
Differences in tug design and assisting methods
METHODS OF ASSISTANCE PROViDED BY 111GS in ports and port approaches around the world differ due to local conditions and specific situations and have often grown from long standing customs and traditions. These differences in assistance methods and practices are often reflected in the requirements for the tugs and hence in the development of a range of tug types.
Over the past few years rapid development has been observed amongst harbour tugs. New types have been designed with high manoeuvrability and considerably increased engine power. Modem steering devices, new towing appliances and new materials for towlines , to name a few, have been fitted. These developments affect methods of tug assistance and the number of tugs used. Following the Exxon Vald~disaster, the requirement to escort tankers in certain port approaches has resulted in the development of specially built escort tugs. As a result of the improved manoeuvring capabilities of modern ships on the one hand and the improved towing performance of modern tugs on the other hand, the number of tugs required for assistance in port areas is decreasing, Due to economic factors shipping companies are facing, captains and pilots are often under pressure to use the minimum number of tugs.
This reduction in the number of assisting tugs per ship places the individual tug in a more crucial role . It requires a high level of operational safety and reliability
from the tug and a high level of suitability for the job to be carried out In order to keep a port's tug services up to date and to ensure safe, smooth shiphandling it is essential to keep abreast of developments in harbour towage and shipping, to have the most suitable tugs available and to have well trained crews for the specific situation in the port. This is all the more important when the investment required for new tugs is so very high. It may be necessary to reconsider the traditional approach. It requires extensive research and knowledge of tugs before an answer to the question "which type of tug or which working method is the best for a certain port" can be given. It requires a profound knowledge of the different tug types, their capabilities and limitations, and a good insight into the local situation.
The capabilities and limitations of different tug types are dealt with in the following paragraphs. The operational requirements that harbour tugs must conform to with respect to ship assistance are mainly determined by the following factors:The kind of port or harbour and approaches, foreseeable future developments and the existing geographical environmental conditions. The type of ships calling at the port. The services required in and around the port and, if relevant, at offshore locations, e.g, SPMs, F(P)SOs or oil rigs.
l'Iw"',/'brl of~"'""'P I G.w. O>olnu FW'" 1.1 Prn1 ofAntwerp. ZondvlUtsluiQrz. Tugs s!wuld beobI. toassist ships tIorough the loda and bridges.
TUG USE IN PORT 1
l'r1rts underdevelopment
In many ports , developments take place such as new berths or harbour basins and new ports are still being designed. At an early stage it is desirable that tugboat companies and pilots should participate in design studies for these new ports, harbour basins, terminals, etc. In this way tugboat companies and pilots can give advice based on tbeir experience of shiphandling with the available harbour tugs. Moreover, tug companies can take account of these new developments when ordering new tugs which are suitable for the new situation. Regular consultation between port authorities, port designers, tugboat companies and pilots will favourably affect the accessibility of ports and harbours . In container ports, especially where space is limited, the requirement for large land space to stack containers may not correspond with the minimum manoeuvring area required for ships and tugs. Specific requirements for tug assistance may be necessary, such as the type of tug , engine power, towing equipment and assisting method. Port approaches Port approaches are under the influence of the open sea and can be wide or narrow, with sandy or rocky banks, winding or straight entrances. Depending on the local situation, tugs may be used in the port approaches and should he capable of working in more open sea conditions with waves and!or swell. Following the Exxon Va/de'1: disaster there is a growing tendency to require an escort for oil and gas tankers in port approaches. Tugs used for escorting must comply with very specific requirements.
PADto: .4.1II1uK-
Figure 1.4 Tug arsisting in open sea dos« toPOTt entraTla
1.2.2 Environmental conditions
Geographical environmental conditions are very important from a tugboat company's point of view. The majority of older ports are situated in river estuaries and are The particularly subject to the influence of tides or usual course taken by vessels through a harbour or coastal waters seasonal effects. Fairways and rivers are constan tly subject to changes. Differences in water depth, bridge passages and lock entries may require the adoption of time windows. The accessihility of these ports, therefore, can be rather complicated. Tugs have to handle ships ,---- - - - - - -- - - - - - - - - -- - - - - - -- -.., safely and efficiently, Especially in these ports, therefore, the requirements to which a tug must conform may change continuously from the entrance or approach up to the berth and the final mooring. In some ports this problem is solved by using different types of tugs for the various parts of the route .
As already mentioned, ports close to the sea may be influenced by waves and swell, leading to additional requirements for tugs. The same applies to tugs that have to operate at offshore locations or in ports in colder areas where ice may be encountered. Limited water depths in port areas where harbour tugs have to operate will give rise to special requirements with regard to a tug's maximum draft.
Fwn' 1.3 M.T 'Capitol' berthing atJetty 4 at Sullom ", e Oil Tmninal TUG USE IN PORT 3
Phot.o: KOTUG I FotostUlh:oRijnmond Robert Nagtllurlu
Figure 1.7 Azimuth tractor tugs (53 tonnes hollardpull) of the KaruG towingcomparry towingan oil rig. Dependingon theport, harhour tugs should also be able tohandle offihare equipmen~ barges, fWaling cranes and soon.
These activities also demand a specific type and size of tug, as well as specific manoeuvrability, equipment and towing methods, as is the case with tugs that have to operate, for example, at SPMs, F(P)SOs or at oil rigs.
port. They have built up their exp eri enc e with these tugs and with the tug's crews. They know the advantages and the shortcomings of their tugs and are thu s able to anticipate. Changing over to a new system or to a new PLATAFORMAS+][ type of tug may be associated with difficulties, will take time and should be weighed carefully. Training and 12..5 Assisti ng me th od In use instru ction will be needed, especially wh en the type of tug and the way it operates is totally different from the The method of assistance used by tugs will depend on: existing system . A well planned changeover to the new system will be ne cessary. All this should be taken into Port,jetty, terminal layout andloroffshore installation. Types of ship . account when considering the introduction of a new Environmental conditions. tug typ e or assisting method. Navigational complexi ty of river, channels and port approach. 1.2.7 Safety requireme nts Whe ther bridges and locks have to be passed. Often on tradition. Tug assistance always includes risks for the tug and her crew. These risks can be minimised by good training and by a well designed and equipped tug. Th e typ e of The type of tug used is largely dep endent on the assisting method. Tugs have to meet, as far as possible, tug also influences the level of safety. Depending on the type of port, the environmental conditions, the ships the requirements related to the assisting method. The assisted, the assisting methods and the port regulations, assisting method may also depend on the availability of mooring boats . When no mooring boats are available a the safety requirements may differ by port. On the other ship has to be brought up very close to the berth or hand tug owners should require, regardless of the port even alongside to be able to pass the mooring lines . In situation, the highest level of safety, which could dictate these circumstances tugs should be able to push at the a certain type of tug and tug equipment. ship's side. 1.2.6 Available experience
1.2.8 Summary
Pilots and tug captains are accustom ed to the assisting methods used in the port and to the types of tugs in the
No port is the same. Many factors influen ce the choice of typ e of tug. such as local customs , environmental TUG USE IN PORT 5 CADE A PAGINA 6:
1.5
no t only wha t forces have to be considered but how,
Conclusion
when and und er what conditions and circumstanc es, such as ship's speed, co nfine me nt, environm ental
It is clear that no port is the same with respect to tug requirements. Port layouts differ, as do the types of ship frequentiog the port, the environmental conditions, local tradi tions and consequently the types of tug and the assisting methods.
conditions and underkeel clearance. This is the way more and more modern ports and/or tug companies work nowadays. Th e outco me may b e a tractor type with azimuth propellers or Voith-propulsion or even a conve ntiona l type of tug. Escorting of tankers will set additional requirem ents.
Wh en a new tug is nee de d a simple answer to the que stion "which typ e of tu g and/ or which towing method is most suitable for the port" cannot easily be given. Too many factor s play a role. It takes reliabl e research, weighing all the advantages and disadvantage s against each othe r, in order to establish the requirements for the most suitable tug for the port. Most important is
On the other hand tug own ers want to ope rate as few different types of tug as po ssible and prefer that th e available typ es ar e put into action as frequently as possible. H arb our tugs should, therefore, be as versatile as possible.
Factors influencing harbour tug choice Other
Passage! Berth
Environmental Conditions
Types of Ship
Sea/Approach
Swell
River
w aves
Services
Safety ofThgs
Tug type
Tug type
Budget
single screw
experienc e
Portl
price
Assistin g Methods
Existing Th",
General
Offshore
Towing
Conventional
cargo
installation s
on a line
ships
AvaUable Experience
required
Barges Channel
Wind
Container vessels
Push-pull Floating
Water depth
Current
Locks/Bridges
lee
Car carriers
Dockyards
j etties in open sea
Fog
Ro-rc
Escorting
cranes
Alongside
Jetties in protected
A ssisting
State
twin
methods expe rience
regulations
towin g
Escorting
Tractor tugs VS
ships
Tractor tugs
Tanke rs! VLCCs
Azimuth
water
Tug
Conventional screw
Financial Aspects
O perating costs
Classification regulations Environmental conditions
ASD tugs
Gas Harbour basins
tankers
Terminals
Bulk carriers
River berths Ferries Mooring buoys Mooring boats
Reverse tractor tugs SDM s (Ship Docking Modul es)
Passenger ships
Figur« 1.9 Harhour tugs - faCUITS influencingchoice
TUG USE IN PORT 7
TYFEOFTUG
I Propulsion forward
Tractor tugs
'--
L--
Voith Schneider
Azimuth propellers
J
J
I Propulsion aft
J
Conventional type
'--
ASD-type * (Multi tug)
f---
Reverse-tractor typ e
f---
Combi type
*
f---
• Tugs that can operate as a conventional tug and as a reverse (tra ctor-) tug Note : The ROTOR tug discussed in par. 10.1.1 is in fact a tractur tug with a dynamic skeg, being a third thruster. The SDM (Ship Docking Modu le) discussed in the same paragraph do es not belong to any of the categories mentioned ab ove.
Figure 2.1 Main types ofharbour tug
8 THE NAUTICAL INSTITUTE
Chapter TWO
TYPES OF HARBOUR TUG 2.1
Classification of harbour tug typ es
T UG TYPE S ARE NMIED AITER THEIR MAIN CHARACTERISTICS,
i.e. the type of propulsion, propulsion manufacturer, location of propulsion or steering system. Names include conv enti onal tugs, Voith-Schneider tugs, Z-peller tugs, Kort nozzle tugs and tractor tugs, amongst others. Th ere is no uniform naming system in use and this can be
confusing. For example, when talking about a Z-peller tug, what is meant? Is this a tug with azimuth propellers forward or with azimuth pro pellers aft? The difference near stern of a ship d o es not see m so great, but considering tug performance while rendering assistance, it is. Afte r all, that is what
towing hook instead of a towing winch aft. Because an AS D-tug can operate as a'reverse-tr actor tu g, it is often m entioned together with reverse-tractor tugs.
Although the term ASD-tug is frequ ently used, it is not such a good name, b ecause rev er se-tractor tugs
also have azimuth propulsion un der the stern . Multitug is a b etter name.
Modified older tugs with a360° steerable bow thruster (combi-tugs) and equipped ....,th an additional towing point at the after end of the tug. These tugs can operate as a n ormal conven tional tug or like a tractor
tug when using their aftermo st towing point.
tugs are used for - to render assistance. As will be seen later, it is better to classify tugs according to their location of propulsion and towing point It makes things easier to understand.
So the following types of tug can be see n, all belonging to one or both of the above groups :
Naming tugs this way there are only two ma in classifications , which can be groupe d as follows:
Conventional tugs. Trac tor tug s with azimuth propellers or Voith prop ulsion. ASD-tugs.
a) Tugs with their propulsion aft and towing point ncar midships. These are basically conventional types of tug . This category includes all normal conventional types such as single scr ew and twin scr ew tugs.
Reverse-tracto r tugs.
Combi-tugs. The tab le in figure 2.1 gives an overview of the classification of harbour tugs. The re are, of course, m any d iffe r ences in
b) Tugs with their towing point aft an d propulsion forward of midships. Thes e are tractor tugs. In this catego ry are: Tracto r tugs with Voith propulsion . Tracto r tugs with azimu th propellers. T here are intermediate types of tug that can be classified either as conventional or tractor tugs,
depending on the way they operate. These are:
co nstru ction , hull de sign, propulsion and rudder configuration and so on with in each tug type. The different types of tug are therefore discussed in more detail star ting with some gene ral aspects regar ding tug performance and safety of operations.
2.2
Important general requirements for good tug performance
For good harbour tug safety and performance, the following factors are important:
Reverse-tractor or pusher tugs - tugs with azimuth propellers aft and towing point forward, built to
2.2.1 Thg performance and safety
operate mainly over th e tug's bow, as can be see n
Response time
for exam ple in J ap an , Hong Kong and Taiwan. Tractor tugs normally work with their towing point - the tug's st ern - towards the ship and their propellers - near the tug's bow - away from the ship. Reverse-tracto r tugs operate in -th e sam e way regarding the towing point and th e pro pellers, consequently the tug itself lies in the reverse direction. Azimuth Stem Drive (ASD) tugs. These are multipurpose tugs with azimuth pr opellers aft which are built to ope rate over the tug's bow as a reverse-tractor tug as well as over the tug's stern like a conve ntional tug. Most ASD -tugs have a towing winch forward and one on the after deck while some have simply a
Harb our tugs should have a short respo nse time and their manoeuvrability should b e such that the tug can react in a minimum of time. It is therefore im por tant
that measures are taken to increase the manoeuvrability of harbour tugs and shorten their response time . Not only is a short response time required when assisting a vessel, but also for making fast. Due to ever d ecr easin g n umb er s in a ship's crew, th e time ta ke n to make tugs fast is increasing. Thus the requirement for tugs regarding fast an d easy h an dli ng o f towing equi pment becomes an e lement of in crea si n g importance in orde r to im prove their response time.
TUG USE IN PORT 9
Effectiveness and safety of operations It is not only manoeuvrability, but also bollard pull and underwater shape that make a tug effective and th er efore suitable .Ior th e j ob . For exampl e, lar ge containe r vessels with containe rs stacked six high on deck need powe rful tugs in case of strong wind s. Wh en a ship is underway at spe ed, loss of tug's effectiveness due to th e ship's speed and/o r towing direction should be as small as possible. The effectivene ss and safety of a tug is also related to factors such as the tug's stability and suitability of towing equi pment. R equired manoeuvring space Th e manoeuvring space requ ired by assisting tugs sho uld, depending on the situation, be as small as po ssibl e . Thi s ca n be achieved b y good tu g manoeuvrability, limited tug dimensions and proper towing equipme nt.
Other practical aspects of importance for good tug performance and safety of operations are as follows: 2.2.2 Wheelhouse construction and layout Visibility A tug's wheelhouse should be placed and constructed such that, at his manoeuvring station, the tug captain has a good view of the tug's fore and aft ends and tug sides . He must also have a good view of:
Th e towline and towing equipment. The working deck. Contact areas between tug and ship. The assisted ship. Other assisting tug boats. The direction of ope ration. This requires a field of view at the manoeuvring station(s) as un ob stru cted as po ssible, with an angle of view as close as possible to 360 °.
TIu HQrIg KongSalvagt & 10wage Co . Ltd.
Figure 2.2 Pusher tug 'Lam Tong' (l.o.a 26·7m, beam 805m, bp 4JT) with a cockpit uhedhmue. S'" has vertical andhorizontal heauy dutyfentkring with water lubrication at thebow plur vertical sum andhminmtal side fender systems
10 THE NAUTICAL INSTITUTE
In addition to the all round view, well de signed whee lhou ses also have small windows that face upwards, which is important wh en making fast to vessels with a high forecastle, stern or freeboard. On some modern tugs very small wh eelh ouses are constructed wi th large window s and a nearly 360 0 view . Manoeuvring stations When making fast to a vessel and while assisting, a tug captain should be able, in one glance from his man o euvri ng statio n, to see th e mo st essentia l information available from outside, without jumping from side to side in his whee lho use and without getting painful legs, neck or back. T he ess en tial outside information co mes from :
a) The towline(s) - their dir ection an d tension. b) The assisted ship : such as relative heading an d speed, distance off and the way the assisted ship reacts to th e applied tug forc es. When pu shing, essential information also comes from the co ntact area between tug and ship. c) The combined ship/ tug dir ection of movement with regard to channel or fairway boundaries, other traffic and nearby berths and banks. Depending on the typ e of tug and th e assisting m ethod in use, this essential information may come from
totally different or eve n opp osit e directions. Th e directions may change during one and the same trip and are dependent, in any case, on the assisting method . In a reverse-tractor tug, which is assisting from over the
tug's bow , nearly all the essential outside inform ation com es from forward and should be available in one outside look from the man oeuvring station. T his can b e a chieve d with one forward facing statio n . If the manoeuvring station is well planne d, the tug captain may h ave an unob struct ed view in th e working direction , even from a seated po sition, of the winch ,
working deck, bow and side fend ers and th e assisted ship. For all other typ es of tug and/or other assisting methods th e visibility re quirements m ay be totally different. For instan ce, a tractor tug used for push-pull op erations works over th e stern. The n an aft facing manoeuvring panel is need ed. When the same tug is free sailing a forward facin g man oeuvring p an el is required , Depending on the wh eelhouse construction, a central manoeuvring panel for this type of tug could be useful, capable of being oper ated in both dir ections, forward and aft. On other tugs more manoeu vring panel s may be required, of course, depending on th e wheelhouse size and constru ction . Som e harbour tugs even have three manoeuvring panels facing forward and one facing aft. Care should be taken in order that reliable change-over between manoeuvring panels is possible without the risk of failures or mistakes.
Co n tr ols at the man oeu vring panels should be arrange d such that they can be ope rated in a logical way in relation to the tug's direction of movement. Pushing a lever down an d away in the direction the tug cap tain is fa cing should result in an incr e ase of
towline length ca n th en alway s be adjusted wh en requir ed without calling a man to the towing winch . The numb er of crew members on mo dern harbour tugs is very limited nowadays.
movem ent in that direction. Turning a whe el or moving
Communicatio n
a joystick to the left should turn the tug in that direction , rega rdless ot"wheth er the direction of movem ent is ahead or as te rn. Any illogical way of control or com plexity in control easily lead s to human control
Good co-operation between the pilot and tug captain is a basic requireme nt for safe and efficient shiphandling with tugs. Such co-operation is only possible with good procedures and efficiently wo rking communication
failures, particularly when under ten sion .
syste ms. Radio com munication systems on boar d tugs
sho uld the refor e be reli able. A double VHF set is It is clear that the wheelhouse layout and the numb er, location an d orientation of manoeuvring pan els depend largely on the typ e of tug and the usual assisting method and should be carefully con sidered , also taking into ac coun t the op tim u m vi ew n eeded fr om th e manoeu vring station wh en co ming alongside a ship or b erth. Mod ern tugs so me tim e s h av e one ce nt ral mano euv ri ng panel in an opti mal design ed small wheelhouse, like a kind of co ckpit.
At th e man oeuvrin g stations the captain should also have a goo d view of his instruments, including the radar. Communication and quick release systems, which will be discussed later on, should be within hand reach at all manoeuvring pan els. Towing winch control from the whe elho use is also recommend ed for harb our tugs. The
recommend ed.
2.2.3 'lUg superstructure and underwater d esign Tugs regularly have to work near a ship's bow or stern, whe re the flar e and overhang are often fairly pron ounced . It is necessary, ther efore, that the tug's supe rstructure is located well inboard of the deck edge , so that risk of tug damage can be avoide d as mu ch as possible when working near the ship's bow or stem or when the vessel or tug is rolling when alon gside a ship. Underwater design of the tug should be such that the propulsion units will not hit the ship's hull wh en th e tug is rolling alongside. In this regard harbour tugs have to assist all kind of vessels, including submarines
in some ports. Tug pro pellers may hit the submarine hull whe n a tug is required to come alongs ide for assistance or for bringing the pilot on board. In that case Single screw harbour tugs were usually b est. 2.2.4 Fendering Tugs should be equippe d with go od fendering. Appropriate fendering protects both the assisted ship and tug fro m dam age and decreases the tendency to slide along the ship's hull when the tug is pushing at an angle to th e ship' s hu ll. Fenders are construc ted of rubb er or synth etic rub ber products. Bey o nd th e m echanical requirements ofload versu s deflection and ene rgy absorption, which is giv en in curves, attachment
methods and structural limits, con sideration should also be given to the material used in the fender. The material used sh ould have good resistan ce to pollut ed water, ozone, UV radiation an d high and low temperatures. The following factors are of importance in the choice of a tug's bow and/or stem fendering:
Photo:Author
Figure 2.3 Plan of tlu navigationbrit!t,e deck and viewofthe wheelhouse of a modern HongKongpusher tug. The captain is handlingthepropellercontrols and the mate thetowing wind!
The way the tug is assisting vessels, for instance towing on a line or push-pull, and whether the tug will pu sh by the stern andlor by the bow. The size and engine power of the tug wh ich are important factors for the horizontal load an d kinetic energy transmitted durin g contact and pushing. Size of contact area.
The type and size of vessels to be handled e.g. ships with large bow flare and/or overhangin g stem. Tugs TUG USE IN PORT 11
pushing near the bow or stem of these ships may need extra fend ering on top of the bow to pr event damage to tug or ship. Th e environmental con ditions such as waves and swell. Th ese conditions will give rise to addi tion al forces in the fend ering, for which it mu st be able to compensate . The tug's bow and stem construction. Tug fendering varies enormo usly. One freque ntly u sed fen der sys tem is the extru de d profil e typ e. Extruded fend ers are produced in differ ent lengths and in a wid e variety of profile s and sizes. They can have a hollow D-shap e profil e, can b e rectangular, cylindrical or solid, can b e pr ecurved to fit the tug bow or stern, be chamfered or drilled. Extruded fenders are very flexibl e fr om the p oint of vi ew of d esign . Extrusion is a manufacturing method whe re by un cur ed rubber is forc ed through a die to produce th e required profil e and then the lengths offormed rubber are vulcanised. Moulded m odular or block fend er systems offer many of the advantages of extrude d fenders and, in addition , allow for sec ure attachment and ease of repair
since with this type individual blocks can be repl aced. Weldable fenders with steel backings are yet anothe r fend er type, used when very sec ure attachment is
required. A tug's bow andlor stern can b e equipped with horizontal fendering, for instance extruded fenders of cylindrical profile, or with verti cal block fendering. A combination of these typ es is often used . Block fenders can easily be replaced when damaged, and for fend ers on bow and stern which are inte nsively used, basic verti cal blo ck fendering is very suitable. Other typ es of fendering include tho se made of reinforced tru ck or aircraft tyres which are cut to a
Photo: Schuyler Rubber Co. lnc., USA
Figure 2.5 Bowfender made ofreinforced truck tyres
specific size and compressed onto steel supporting rods. This fender typ e, made in the USA, is suitable for b ow fend ers, stern fenders and side fend ers. There is one specific type which has a large absorption ability an d is very soft, thus ha ving a lar ge contact ar ea an d 'sticking ability' when under load. Tugs may also be fitted with foam-filled or pneum atic fenders, especially wh en working in exposed ar eas. Sometimes 'non-m arking' fend ers are required, for instan ce wh en ships with white or grey hulls have to be handled , such as cruise or navy vessels . In that case
manilla rop e fend ers, in addition to the standa rd tug fendering, may still be used or the tugs may b e equipped with grey rubber fendering. Bow fend ers should have a large contact area an d radius to reduce th e pressure on the ship's hull . The same applies to the stern fend ers of tractor-tugs since these tugs ar e pu shing with the ir stern . Tyr es are often used in additi on to bow and stern fenders to protect the fenders and enlarge the contact area and ar e often used along tug sides since they can easily be replaced when damaged. The following is an indication of some p ermissible hull pressures, whi ch vary by ship' s typ e an d size: General cargo ships of 20,000 dwt and less
400-700 kN lm'
Oil tankers of m ore than 60,000 dwt VLCCs RoyalBakkerRuM", TheNetherlands
Figure 2.4 1jpicalfi nder arrangement f or a tugpushing underswellconditionsand/or atflaringparts of a vessel, consistingof vertically instal/ed moulded blocks and horizontal hollowcylinders of .. - extruded rubber
12 THE NAUTICAL INSTITUTE
Container ships : 3rd gener ation 4th gen eration 5th and 6th gen erati on (Superpos t Panamax)
< 350 kN/m' 150-200 kNlm'
<300 kNlm' < 250 kNlm' <200 kNlm'
G as carriers (LN G/LP G) and Bulk ca rrie rs
< 200 kN /m'
Fender mat er ial should h ave a large coefficient of friction in order to keep the bow or stern in position when the tug is pushing un der an angle to the ship 's hull. Sliding along the ship's hull , tug berth or alongside othe r tugs, and rolling and pitching along the ship's side du e to waves will easily damage tug fend ers. To avoid early damage of the fend ering, as for instance the side fend erin g, or wh ere no grip is required , fenders can be used with a low friction coefficient or can have a top layer of UHMW polyethylen e, which has an extremely
Specific types of fenders can be pro vided with water lubricati on to reduce the friction between tug and ship and so prevent damage and wear, especially when pu shing again st a slab-sided ship in swell con ditions. This type of fendering can, for instance, be found on tugs in the port of Hong Kong (see ph oto of the reversetract or tug Lam Tong - figure 2.2). Th e height of a tug's fen dering above water level is a factor to be considered. When pushing under an angle at a ship's side while the ship has headway or sternway, the hydrodynamic forces on the tug create a list. It is evident that the higher the bow fender above the water the larger the resultin g heeling mom ent will be. Harb our tugs assisting submarines may also h av e
un derwater fendering to avoid contact damage to th e submarine's hull. In addition, an ASD or reverse-tractor tug's hull may be expanded with fender ed steel sponso ns on the quart ers to ensure that th e nozzles of the tug's azimuth propellers never come in contact with the
submarine be ing assisted, the so-called 'propulsion uni t protective sponsons'.
Photo:Author
Furthe r relevant inform ation for harb our tug design in general and for ASD-tug design in particular can b e found in 'Designers' Che cklist No 1. Azimuth Stern Dri ve Tugs (ASD)' (see References).
Figure 2.6 Trw usedin addition to t mtical bowfendmng
low friction coefficient. The coefficien t of friction of rubber to stee l is approximately 0·8. The friction force F = c x P, whe re P is the impact force of the tug and c the friction coefficient. UHMW polyeth ylen e has a friction coefficient of 0·15.
Suitable and reliable towing equi pm ent is also important for good harb our tug performance and safe working. This is dealt with in Chapter 7.
o
.,
Damm Shipyards. 17u NttJurlmuis
Frgure 2.7 Consentionaltwin screw tug - type Stan Tng 2909. L».a. 29-6m, beam 9-3m, bp depending on installed engint power: 3D-60 tons
TUG USE IN PORT 13
2.3
Conventional types of tug
2.3.1 General The largest number of tugs still belong to this typ e. They can be seen all over the world and are still built in large numbers. Conventional tugs are used for push-pull assis tance, alongside towing and in part icular, in European ports, for towing on a line. The re is a large va riety of conve ntional tugs. The most simple one is a single screw tug with a single plate rudder. Mainly due to the location of the towing point, the tugs have limit ations regarding performance and safety. When towing on a line the main risk is of girting. A towing winch with a quick release m echanism lowers this risk. The same app lies to a quick release towing hook , if it work s under the extrem e condition of girting, which is not always the case . The astern power of conventional tugs is gen erally low. Wh en making fast near the bow of a ship , interaction for ces b etween the ship and the tug should be allowed for, which can better be don e with tugs th at can produce goo d side thrust, such as tractor-tugs. Girting and interaction are de alt with in Chapters 4 and 6. The towing point of the se tugs gene rally lies abo ut 0·45 x LWL from aft, although shorter distances may b e found . Th e aft tow ing point on American conventional tug s lies furth er aft, whic h allows the opportunity to extend th e deckhouse further aft. A more aft placed towing point limits the tug's effectiveness when towing on a lin e at spee d but this way of towing is not normal pr actice in the U SA. In USA ports whe re tugs are used for towing on a lin e, conventional tugs can b e found with a more
forward lying towing point. Exp erien ce is an important factor in h andling conventional tu gs safely while assisting ships under .speed and with a well qualified captain these tugs can be v ery effective while rendering assistanc e. To increase
the tug' s effectiveness and/or manoeuvring capabilities ther e are several possibilities, as explained below. 2.3.2 Propulsion and rudders Propulsion and propeller control Nearly all tugs are equipped with diesel engines th ough an occasi on al old harbour tug with a steam engin e may still be found somewhere outside a maritime mu seum. Diesel engines on harbour tugs are high or medium speed engines. The high engine revolutions have to be brought down by redu ction gearing to the required propeller revolutions. To reverse the propeller thrust, different syste ms are in use.
.
The direct-rev ersing system is the oldest and can still be found on som e conventional tugs. The engine ha s to 14 THE NAUTICAL INSTITUTE
be started on ahead and on astern . On some tugs engines can be controlled from the whee lho use, while on othe rs it still has to be done by an engineer. The number of manoeuvres is limited by the volume of starting air . available . The response time from ah ead to astern and back differs by tug and by the dir ect-reversing handling system fitted. Diesel-electric propulsion systems can still be found in some harbour tugs. The diesel engin e(s) drives electric generators \v·hich in turn drive electric m otors . T hese
electric motors drive the propeller. This system is easily contro llable fro m the wheelhouse . It h as th e large advantage that it can deliver any propeller shaft sp eed ahea d and astern without delay. The system is exp ensive, though . It has high initial costs and high er maintenance expenses compared to oth er systems . Most commo n now ad ays on harb our tugs ar e high an d medium speed diesel engines with reduction gears and pneumatic-hydraulic couplings. (See References for ' Operational benefi ts of hi gh-speed electronic diesel engines'). Other type s of couplings can be used. On tugs with fixed propellers the propeller thrust is reversed by m eans of a reve rse-reduction gear, while on tugs with controllable pitch propellers (cpp) thrus t is re versed by changing the prop eller pitch . Torqu e pro blems may arise wh en a fixe d pitch pro peller is reve rse d at high tug speeds. These problem s can be re duce d or overcome by prop er design (= the correc t comb ination of engi ne, prop eller and gear) and tuning ofthe who le propulsion system. Shaft brakes are used, dep ending on engine and prop eller typ e. Engine revolutions and propeller pitch are re motely co nt r olle d fr om the whee lhouse . Manoe uvri ng, espec ially with a cpp, is very smooth. When th e cpp control system is equipped with a combinator control, prop eller re volutions are regulated in accordance with propeller pitch . The pitch of a cp p js regulated by a hydraulic system. Cpp control systems, including remote control systems, th e hyd rauli c system an d emergency stop require regular checkup s an d goo d maintenance . Failure in the hydrauli c or remote con trol system can cause serious dam age to tug, ships or be rths. Modem cpp systems have reliable backup systems. Propeller effidency and manoeuvrability The propellers of conve ntio nal tugs can be fitted in op en fram es or fitted in no zzles. Go ing full astern, an ope n fixed pitch prop eller will - in general - deve lop about 60% of its maximum ah ead thrust. An open cpp going astern de velops some 40 to 45% of maximum ahe ad thrust. The lesser efficiency astern of a cpp has to do with the specific design and working of a cPl' . Prop ellers are designed for maximum efficie ncy going ah ead . A fixed pitch propeller will turn, whe n astern thrust is requ ired, with the same pitch in the reverse direction as on ahead. Th e propeller blad es of a standard cpp have a sm aller width near the hub an d the refore,
when the blades are set for ahead, a larger forward pitch angle than near the tip of the propeller. When the blades are turned for astern thrust, the lower part of the blades will consequently have a smaller pitch than the top of the blades , which results in less efficiency going astern compared to a fixed pitch propeller. Nozzles increase thrust and conseq uently bollard pull significantly. Mr. Ludwig Kort, an aerodynamicist, designed the first nozzles as far back as 1927. The first one was introdu ced into service in 1932 and was originally designed to protect canal banks from pro peller wash. The effect of a nozzle is most pronoun ced with high propeller loads at low speeds. Harbour tugs have to perform in that way. Nozzles increase thrust by 1525% in towing and pushing conditions.
".
"
propellers and the same type of nozzles. With a specific propeller design a much higher value can be reached for astern performance of controllable pitch propellers, but then ahead efficien cy will be lower. Note:
The 19A nozzle, and several variations more or less on this design , are used for azimuth thrusters, either with fixed or with controllable pitch propellers, b ecause astern thrust is achieved by turning the nozzle. A nozzle seen on several tugs with azim uth propulsion is the Nautican nozzle, which is the same as the Lips HR (= high efficiency) nozzle. Ahead efficiency of this nozzle is higher than of nozzle type 19A and 37, approximately 8% in bollard pull conditions, while astern performance of the Nautican nozzle is better than of nozzle type 19A, but not better than of nozzle type 37. As said, astern performance is not relevant for tugs with azimuth thrusters.
Figure 2.8 Twogenerally usednoz:;:k typtJ 79A and 37
+-j+-+- ~
o. p
.-
Figure 2.9 Steering noales, one with a moteable flap 1M other with aftxedfin
Willi &&n. Iogrn;"";;",. G
Figure 2.10 Construction ofa suemble nozzk with moteableflap
Various typ es of nozzles (figure 2.8) have been developed while research is still going on. Nozzle type 19A is very common because of its cost-effective design and is typical for ahead thrust requirements. Nozzle type 37, a 'backing nozzle', has been deve loped to give better efficiency going astern, which results in only a little less efficiency going ahead. The same applies to the Hannan Ring Nozzle, which is a normal type 19A nozzle with slots cut in at the after end giving good astern thrus t abou t 70% of the ahead value with-fixed pitch propellers and special blades and 60-65% with ordinary blades. Nozzle type 37 is a type of nozzle often used for conventional harbour tugs. Conventional tugs with controllable pitch propellers .n nozzles (nozzle type 37) achieve, whe n pulling astern, tbout 45% of maximum ahead ballard pull, while this gure is about 65% for tugs equipped with fixed pitch
Nozzles increase the efficien cy of the propeller but decrease steering capabilities. The fItting of a nozzle is equivalent to increasing the lateral area of skegs. Special rudder systems are therefore often used. Nozzles can be also be steerable. Their manoeuvring performance is superior to norm al rud der arrangements. Rudder angles of no more than 25° - 30° are used due to the greater side thrust. A tug's manoeuvrability when going astern with a nozzle ru dder system is very good. When going astern the tug will swing to port or starboard depending on the direction of the steering nozzle. A vertical fin or a movable flap may be fitted at the end of the steering nozzle. (see figures 2.g and 2.10). Some twin screw tug s hav e two indepe ndently contr olled steerable no zzles, so increasing the tug's manoeuvrahility furth er. TUG USE IN PORT 15
Conventional tugs can be single screw, twin screw and even triple screw, e.g, the USA harbour tug Scott T. Slatten. Manoeuvrability of twin and triple screw tugs will, in general, be better than of single screw tugs. In general tugs are equipped with balanced, semibalanced or spade rudders. By far most tugs have balanced rudders. Single plate rudders are also stillused. With the spade, balanced or semi-balanced rudder the leading edge of the rudder extends forward of the rudder shaft. This , together with the shape of the rudder, results in higher propeller efficiency and a lower steering couple, so a smaller steering gear can be used. Spade rudders are hanging free, are not attached to a heel, and are consequently more stoutly constructed than a balanced rudder. Single plate rudders decrease propeller efficiency, need a higher ste er in g couple and consequently a larger steering gear. The manoeuvrability of conventional tugs can be increased by the use of specific rudder types or rudder systems. Several different rudder systems are in use, often in combination with nozzles, such as:
Movableflap-rndthTs There are several types of movable flap rudders, such as Becker, Barke, U1stein,]astram and Promac Stuwa. At the end of the rudder blade is a movable flap, controlled by linkage, comprising about 2030% of the total rudder area. Maximum helm angle differs by type and is about 40-50 °. Each type of flap rudder has its own specific characteristics. The flap angle is a function of the helm angle and with a Becker rudder, for instance, it will be about three times the l'/w1D..H",,
nome
Schilling rudders Schilling rudders can also be found on tugs e.g. the 16 THE NAUTICAL INSTITUTE
new tug Sayya/at Abu Dhabi. Schilling Monovecrudders have no movable parts. Horizontal slip streamguideplates are fitted at the top and bottom of the rudder. The rudder itselfhas a high liftblade profile with a wedgeprofile, socalled 'fishtail', at the end of the rudder blade.The rudder develops 30-40% more lift compared to a conventional rudder and maximum lift is obtained at a rudder angle of approximately 40°. The rudder can be usedup to 70° angle and at this angle the propeller slipstream is thus deflected 90 ° and works more like a side thruster. When moving astern the rudder is more effective than normal rudders. With a Schilling Monovec rudder, turning on the spot FlgUTe 2.72 is almost possible while Schilling rudder speed is dropping very fast. Two Schilling rudders, called SchillingVecTwin, can be used behind a propeller and make the vessel very manoeuvrable. Each rudder has a separate steering gear. The rudders can be turned by joystick a maximum of 105° outboard and 40° inboard. A maximum side thrust of 70% of ahead thrust can be achieved. Depending on the two rudder angles, it allows the degree of thrust from a conventionally mounted propeller to be controlled and the thrus t direction vectored through 360°. Thu s the need to reverse the shaft direction or propeller pitch is eliminated.
Flanking rudders Flanking rudders are installed in front of the tug's propeller and both single screw and twin screw tugs may be so fitted. Flanking rudders are often installed in conjunction with other rudder systems, such as a single rudder behind the propeller or a Towmaster rudder system and are especially used in conjunction with fixed nozzles. In general there are two flanking rudders situated before the propeller nozzle. The flanking rudders are operated by separate controls and enhance steering performance when moving astern or when towing astern on a towline from the tug's bow. Figure 2.73 Slwtur rudder When going ahead th ey lJsfmlwith afixed Male are kept amidships. and twojlmJeingrudders 1bwmaste1' system
The Towmaster rudder system is a shutter rudder type used in conjunction with fixed nozzles. It consists of several rudders mounted behind and sometimes also ahead (flanking rudders) of each nozzle. Behind the nozzle are normally three and ahead of the nozzle two
rudders. Rudder angles are possible up to 60°. The Towmaster system provides good thrust and steering characteristics ahead and astern at the expense of increased complexity. Astern thrust can be more than 70% of ahead thru st. Even recently built tugs are still equipped with this system , such as tugs of the Kuwait O il Company, the tug AI-Ha wtah of the Saudi Arab ian Oil Co., tug Pegasus of the Mobil Refinery, Port Stanvac, Australia and the tug Neeltje P and her sister tugs of Terminales Maracaibo , Venezuela. The Michigan Vane Wh eel used on some tugs in the USA is a comparable system, with several high aspect ratio rudders, e.g. three, be hind a fixed nozzle; the same applies to the Nautican High Aspect Ratio Triple Rud der system.
2.3.3 Manoe uvri ng conve nti on al tu gs S ingle screw tugs T hree aspects are im portan t in manoeuvring a norma l single screw conventional tug:
The aft location of the rudder and propul sion. The transverse effect of the propeller when turning for astern. The low astern po\\'er.
When ahead thrust is appli ed with port or starboard helm, the tug's stern moves in a direction opposite to
the intended direction of turn due to the aft location of the propeller an d rudder. This con trasts with tracto r tugs where the steering forces are applied in the direction of turn . Thi s is a subje ct further dealt with in Chapter 6 when discussing interaction effects between tug and ship. Turning on the spot, or nearly on the spot, is only possible with the previously mentioned high lift rudders. No sideways movement of a single screw tug is pos sible,
not eve n with h igh lift rudders, though sideways m ovement is p o ssibl e with high lift rudde rs in co njunction w ith a bow thruster.
Plwt4: DammShJ.jJyo.uu, VIt Nttlterlands
Figure 2.14 Towmaster rudder sysUm of tug 'Hamm'.L.o.a.38m, beam 11m, Bp 70 tons ahead and50 tans astern
Other systems Besides the rudder systems me ntione d above, many other systems exist, such as different types of fishtail ru dders and the pr eviously mentioned triple screw tug Scott T.A llenwi th her thr ee ru dders, of which the cen tre rudder ca n b e ope rated in depende ntly from th e outboard rudders. Bow thruster Conv entional harbour tugs are sometimes equipped with a tunnel bow thruster. The effectiveness of a tunnel thruster is not high whe n the tug has speed ahead. With only two knots speed the effective ne ss of th e bow thruster may alread y be reduced by 50%. Seagoing harbour tugs operating in po rt areas as well as at sea for offshore work often have a bow thruste r, which enables them to keep position better near oil platforms.
C o nv ention al tu gs may b e equipp ed wit h a (re trac table) 360° steerable bo w thru ster. Th ese bow thrusters are much m ore effecti ve and can op erate in
any dir ection . Tugs with this kind of bow thruster are the previously menti oned co mbi-tugs.
Th e tr an sver se effect or ' paddle whee l effect' is caused by the pro peller wash hitting the stem at right angles when the propeller is turning for astern. Nearly all single screw tugs have a right handed propeller, which means a clockwise turn ing propeller going ahead in case of a fixed pitch propeller and an anti-clockwise turning propeller in case of a controllable pitch propeller. When the prop eller is set for astern, propeller wash hits the tug's stern on the starboard side and the stem moves to port - consequ entl y the bow turn s to starboard. The more sternway the tug h as the more effective the rudder is and it may even be possible to bring the tug onto a straight course by applying rudder. The paddle whee l effect togethe r wi th the low astern power results in poor performance going astern in single screw tugs.
When moving astern a tug's stern can be controlled when the tug is equi pped with a steering nozzle or with Towmas ter or flanking ru dders . Steering nozzles or flanking rudders can be set for the directio n the stern has to move. Twin screw tugs Twin (or triple) sc re w tugs are much more manoeuvrable than single screw tugs. They can turn on the spo t without m akin g headway and can easily man oeuvr e straight astern. Turning can be done by reversing one propeller and setting the other for ahead while applying helm in the intended direction.
Prop ellers of twin screw tugs, wheth er controllable or fixed pitch, are often inward turning except on tugs designed to ope rate in ice conditions. The advantage of in-turning prop ellers is higher propeller efficiency. A disadvantage with fixed pitch propellers is the larger TUG USE IN PORT 17
turning diameter, because the starboard propeller is left handed and the port.one is right handed. Wh en using the propellers as a couple, the transverse effect of the screws opposes the turn .
t I
have to push with the stern . The manoeuvre itself is already difficult unless the tug is equipped with a b ow thruster or if it is a twin screw tug. However, whe n pushing with the stern the tug' s propellers are so close to the ship's hull that the interrupted water flow towards the prop ellers will result in low prop eller efficiency. In addition, the stern fend ering of con ventional tugs are norm ally not designed for pushing with the stern. In such a situation it is better to release the tug from the bow or stern in order to be able to pu sh at the ship's side. For tug op eratio ns at th e ship's side a n ormal conventional tug can push but it is not the most efficient one for pulling on a tug's bow line, due to the limi ted astern power. Specific rudder configurations, suc h as the Towmast er system for example, will increase astern thru st. Normal single screw conventional tugs can neither pull at right angles because of the transver se effect of the propeller, no r can a single screw tug pull at
Figure 2.15 Twin screw tugmoving sideways tostarboard,
right angles with a cross current or strong cross winds.
also calledflanking, by sating 1M port engineon ahead andstarboard engine onas/ern while applyingport htlm. In tIu case ofin-turning fixedpit
The same kind of pro blem arises when the assisted ship is moving ahead or aste rn while the tugs are pulling. It will then be imp ossible to stay pu lling at right angles. Additional measur es should then be taken, such as a line from the stern of the tug to the ship to keep the tug in the best pulling dir ection. A bow thruster does not . improve the situation as the conve ntional tug operates while pu lling with th e tug's bow headed towards th e ship's hull. Steering nozzles, Towm aster an d nanking ru dders mak e it easier to keep the tug at righ t angles when pulling. Twin screw conve ntional tugs can make use of their propellers to keep the tug at right angl es, alt hough this will be at the expense of los s of
With inward turning fixed pitch propellers a tug can move sideways (see figure 2.15), so-called 'flanking '. When the tug has to move sideways to starboard, one wou ld thin k of setting the starboard propeller to ahead and the port propeller to astern . Th is works only when th e tug is equipped with a bow thruster. H owever, witho ut a bow thruster this pro pe ller setting does not move the whole tug sideways, bu t only the stern to starboard. By setting the propellers in the op posite way, with the starboard propeller astern, the port propeller ahead and the rudder to port, the tug w:ill move sideways to starboard witho ut gathering headway, depending on trim, wind and current influence . The transverse effect of the inner propeller will enhance the side thrust.
effectiveness.
2.3.4 Conventional tugs in shiphandling Conventional tugs are used for all methods of tug assistance but are not equally suitable for all methods. When assisting a vessel under speed a conventional tug is effective when towing on a line but as a stern tug, owing to the location of the towing poin t, it has sever e limitations. When the ship has more than approximately thr ee knots headway the after tug can only assist at one side ofthe ship and cannot shift to the other side nor is it able to control the speed of the assisted ship. The towing point being near midships implies a risk of girting.
Figure 2.16 Some assisting methods with conventional tugs
Wh en towing on a line, co nventional tugs are no t
suitable to changi ng over, while th e towlin e is still fastene d, to pushing at the ship's side. Thi s might be desirable, for instance, on arrival at a berth. For a quick change-over from pulling to pushing and vice versa while the towline is still fastened the conventiona l tug would 18 THE NAUTICAL INSTITUTE
The capabilities and limitation s of conventional tugs in relation to oth er tug types are discussed in C hapter 4. Som e assisting m ethod s with conventional tugs are show n in figure 2.16.
with a special rudder and/ or propell er arrangement which increases propeller efficien cy. - - .../'
,; --------
-----
,- I
-------
-- -
- -
-.:: ~
-- -,-- I_ '~
•
•
..
.« (--- -1- --Figure 2.17 Camhi-tug 'Petronella], Goedkoop' ofWijsmulhr Harbour Towage Amsterdam. Lio.a. 28·5m, beam 6·9m. Main engine 900 bhp. Om epp infixed nowe andtwin rudders. Retractable 361J' steeroble bowthruster of 420bhp, typeAquamaster UL 316/2600. Bollardpull ofmain engine 15t. Bollardpull main engim + bow thruster2Ot. Maximum speed ahead 11·9 knots, astern 102 knots when usingboth main engineand bow thruster: The tug is equipped with a specialfairleadat the stern and a towing winch. Line '1' shows the tow line in its (normal' posit£on and '2' the tow line passing through thefairlead .
Side stepping
By installing a conventional single screw tug with a 360 0 steerable b ow thru ster, also called azimu th bow thruster, these disadvantages can be ov ercome . Tugs equippe d with such a bo w thruster are the so-called combi-tugs. The first combi-tugs app ear ed in the early 1960s. A tug equipped with this type of bow thruster can, with the aid of the ma in propulsion and the bow thruster, turn on the spo t, sail straight astern at a fair speed and mov e side ways as well (see figure 2.(8). Setting this type of bow thruster in the same direction as the propulsion also gives additional bollard pull ahead and astern and increases maximum spee d. In most cases this type of bow thru ster is equipped with a nozzle and can be of retr actabl e or fixed typ e. An azimuth bow thruster with a no zzle pr op eller below th e keel, in contrast to a tu nnel bow thruster, achieve s high efficiency in any dir ection even when th e tug is moving quickly. This provides an additional increase in the tug's man oeuvrability, As an example, an azimuth bow thrus ter of 400 hp can increase the top speed of a tug of 27 metr es length and engine power of 1500 bhp by half a knot. With just the bow thruster working a speed of about five knots can be achieved. The towing force of the tug is increased by five tonnes if the main propulsion and the bow thruster work in the same directio n. This is all in addition to better manoeuvrability.
2.4.1 D esigning and mano euvring combi-tugs
For older tugs this is a satisfactory and inexp ensive way of improving manoeuvrability and bollard pull. As exam ples of converted tug s, at Am sterdam, The Netherlands, some older tugs have been converted to combi-tugs and at San Pedro, California, USA , the tug San Pedro (now Pacific Combl) has been converted into a combi-tug with a similar conversion to the tug Point Gilbert and Flying Phantom of Cory Towag e (no w Wijsmuller Marine) in the UK. The San Pedro has b een equipped with a 600 bhp bow thruster, which has increased the tug's bollard pu ll by 400/0, from 25 to 35 tons and has improved the manoeuvring capabilities. Moran Towing Company, USA, revitalised its fleet of single screw tugs by installing retractable azim uth bow thrusters and a larg e fairlead aft. New tugs are also equipped with azimuth bow thrusters, all of them of the retractable type.
As discussed above, the manoeuvrability of single Screw conventional tugs can be improved by the use of high lift rudders. How ever, th e disadvantage of many single screw tugs without steerable nozzles, Towmaster system and/or flanking ru dders, is that moving straight astern is hardly possible and no single screw tug can move sideways unless fitted with a tunnel bow thruster in combination with high lift rudders. The astern power of single screw tugs is also low, unless the tug is equipped
If the azimuth bow thru ster is not in use it causes extra resistanc e. This is one of the reaso ns for making the b ow thruster retractable . In shallow waters a retractable type is necessary_ Care is required in using the azimuth bow thruster when underkeel clearan ce is small and it should be retracted in good time . A good working alarm system whe n the water depth is not sufficient for safe working of the bow thruster is strongly recommended.
Turningon the spot
Going astern
Figure 2.18 Free sailing manoeuvres witha ccmbi-tug
2.4
Combi-Tugs
TUG USE IN PORT 19
2.4.2 Combi-tugs in shiphandling Combi-tugs can tow on a line forward as well as aft. As a forward tug th e co m b i-tug operates like a conventional tug, but has the advantage of increased maximum speed , manoeuvrability and ballard pu ll. Also, the risk of girting is reduced and response time is less du e to the higher manoeuvrability. As a s te rn tug combi -tugs can" ope ra te as a conve ntional tug at low spee ds and can easily work over the tug's stern at higher spee ds b ecause of the azimuth bow thruster. However, since conventional tugs have their towin g point approximat ely 0·45 x LWL from aft, working over the tug's stern nee ds an additional towing poin t near the stern to prevent girting, especially when the assisted ship has a higher spee d. On conve ntional tugs the towin g point can be moved aft by a gob rope, and on some tugs by a gob rope from a gob rope winch . The gob ro pe is then led from the winch through an eyelet or swivel fairlead at the tug's stern. At the free end of the wire is a large shackle which can be pu t around the towline. By heaving on the gob rope winch the towing point can th us be brought as far as possible aft.
This system is furth er explained in paragraph 7.2. A gob rope arrange me nt normal ly needs two persons on deck. With the redu ced numbers in tug's crews a handier
and safer sys te m wa s develo ped b y the form er Goedkoop H arb o ur Towage Company of The Netherland s (n ow Wij smuller H arb o u r To wag e Am sterdam). A strong fairlead has been attached to the deck close to the tug' s stern. This fairlead can b e opened at one side so that the towline can easily be put in or taken out. \ Vith this additional towing point at the tug's after end the combi-tug can operate similarly to a tractortug, that is with the stern towards the assisted ship . To show the capa bilities of a combi-tug consider an arriving sh ip. The comb i-tug makes fast aft an d approa ches stern first to the stern of the ship to pass the towline (see figure 2.19 position 1). The ship to b e assisted may still have rathe r a high speed, e.g. about seven to eight kn ots. As soon as the towline has been secured and the aft towing point is in use by means of a gob rop e or fairlead, the combi-tug can control the vessel's speed (position 5) or assist in steering (positions 2 and 3). To reduce ship's speed, the tug's propulsion and the bow thruster will be set in the same direction to increase the tug's pulling force. Assisting steering is achieved by th e tug shee ring out to port or starboard with the main propulsion going aste rn and the bow thruster working sideways. In position s 2 and 3 th e incoming water flow creates lift forces on the tug an d conseque ntly a force in the towline. When th e ship's spee d reduces, the effect of the tug in position 2 and 3 will b ecom e less due to the reduce d lift forces. The gob rope is then released or the towline take n out of the fairlead . The origin al towing point is then in use again and the tug can ope rate again as a normal conventional tug (position 4). In circumstances where there are strong cross winds
and/or curre nts, and much effort is requi red from the tug to compe nsate for those forces, the tug is m ore effective whe n it proceeds with the assisted ship as a normal conventional tug (position 4) and thu s can use its full ahe ad power. When required, th e bow thruster can b e used to incr ease bollard pull. The lift forces on the tug caused by the water flow increase the force in the towlin e.
A combl·tug as slamtug
If so required thetug can , eve n when the assisted ship has forward speed, shift to a position beh ind the shi p's stern by using th e gob ro pe or fairl ead, b ow thruster an d main pro pulsion (position 4 --7 5). This can be don e faster compared to a normal conventional tug. Co nversely, moving from a position abaft the stern to a po sition m ovin g with the assisted ship is, because
of the bow thrus ter, possible at a somewhat higher speed than with a normal conve ntional tug.
Combl·tug pushing
Figure2.19 Some assisting methods witha camhi-tug
20 THE NAUTICAL INSTITUTE
It has b een mad e clear that th e advantages of a combi-tug are greatest wh en the tug op erates as a stern tug on a line. For that rea son this type of tug often assists during quite long p assages as a stern tug for speed an d steering control. The cornbi-tug can also be used at the ship's side, such as for push-pull ope rations.
When operating at the ship's side, a combi-tug has many of the disad van tages of a normal conventional tug . The co mbi-tug can either push with the bow or with the stern . When push ing with the bow while the ship has some speed , the how thruster can be help ful to keep the tug's bo w in po sition and prevent slidin g along the ship's h ull. T he bow thruster will also give an additional transverse pushing force [see figure 2.19). When pushing with the stern, the effectiveness of the tug is reduced due to the restricted wate r flow towards the propeller an d it is mo re difficult to bring or ho ld the tug at right angles to the ship's hull when th e ship has some spe ed, due to the low power of the bow th ruster. In particular, when working over the tug' s bow, pulling effectiveness at speed is low.
2.5
Tractor-tugs 'with cycloidal propellers
2.5.1 Design Tractor tugs have their propulsion under the forebody. Those wit h a vertical blade system, o r cycloidal propulsion system, are the so-called VeithSchneider or Voith tugs (VS tugs). The first vertical axis propeller, simil ar to the Voith Schn eider propulsion system , was developed in the early 1920s by Professor Kirsten of the Aeronautical Engineering Faculty at the
Figurt 2.20 VOith traaor tug Note: A second towingpoint is only fitt ed in a small number of VS tugs, and is discussedfurther in Chapters 4 and 9 1 Voith·Schneidn praptlhr
2 Proptlur guard
platt
Figure 2.21
Propeller blades ofa VStug Phow:
J M. VO>lh GmbH. Grml4l1J
University of Washington in Seattle , USA. Tugs with Voith Schneider propulsion system appeared in th e 1920s and 1930s. In the 1950s Wolfgang Bear of the Voith Company designed a shiphandling tug with the cycloidal propulsion under the tug's forebody and the towing gear on the aft deck. Many limitations of conventional tugs were overcome by the introduction of this totally new concept, which was called a Voith Water Tractor. The cycloi dal propulsion system is, in fact, a kind of controllab le pitch propeller (see the side-view of Voith tractor tug, figure 2.20). The engine works at constant rpm and magnitude of thrust and the thru st direction is regulated from the wheelhouse. Different engine rpm settin gs can be selected. Full engine rpm is required when full towing or push ing power is req uired or a t high free sailing sp eed s. In other situations lower rpm settin gs can be used. The VS propulsion system for tugs consists of two units with vertical propeller blades whose pitch and thrust direction can be regulated uniformly through 360· without delay. The protection plate (2) protects the propeller blades and works like a nozzle, thus increasing propeller efficiency. During docking,the tug stands on these protection plates an d on the skeg (4).
3 J'oith turbo coupling
The large skeg is typical for tractor tugs an d in particular for VS tractor tugs. It gives course stability and bri ngs- the centre of hydrodynamic pressu re further aft, which is advantageous to both safety and towing performance when towing on a line, especially towing perform ance whe n operating as an after tug at higher speeds.
4Sug
5 Ferukr 6 10wing unndi 7 Towing staple 8 Second towing position
The towin g winch (6) is located aft of midships. It may also be just a towing hook. The towing po int, a large fairlead or towing staple (7), through which the towing line
9 WheelJwwe
j. M. Thith. GmbH, Gn many
TUG USE IN PORT 21
_.
passes, lies far aft and usually exactly above the middle of the skeg. The hull is relati vely wide and flat to provide sufficient space for the two propulsion un its. VS tugs have heavy duty fendering (5), especially at the stem, because when pushing, the tugs push with the stem. Most modem tugs have small wheelhouses with optimal visibility. The same applies to modem VS-tugs, like the one shown in figure 2.20. The small and optimum wheelhouse (9) often has one central manoeuvring panel for propeller control.
..-u ar-.s or
pItCh ...... ~
..
..
"
Th e principle of a cycloidal VS propeller is sh own in figure 2.22. Links leading to the steering centre N are fitted to th e vertical propeller blades. The steering centre N can be moved out of the centre a by two hydraulic cylinders. O ne hydraulic cylinder wor ks in the longitudinal dir ection an d the othe r one in the transverse direction. The propeller blades create a thrustin a direction depending on the location of the steering centre N . In sketch I there is no thrust; the propellers are 'idling' . In sketch 2 the steering centre is moved by on e hydraulic cylinder to port. Th is offset location of th e steering centre N results in forward thrust. In sketch 3 the steering po int N is moved by the two h yd raulic cy lind ers to port and forward, which gi ve s th rus t in the indica ted dir ection S. So, the thrust can b e regulated for an y directi on by moving N. The nominal direction of thrust is perpendicular to the lin e O -N and the magnitude of thrust is proportional to the distance O-N. In tugs, there are always two VS propeller units, which are installed side by side.
J M. Vilith GmbH,
GmruJ7I]
Figure 2.22 Principk Voith propulsion
of
Th e maximum draft, including the propulsion un its, of a VS tug is relatively larger than that of conventional tugs, due to the w eight of the propulsion units, the propell er location and dimensions. The location of the propulsion units is approximatel y 0·25 x LWL from 0·30
22 THE NAUTICAL INSTITUTE
..
~
-.s lhWnl- 11)0'"
~
"-...s. l/YUIt • 0
• ••
..
""
pllch..-. f\AI'tl. . e ("11m) ....h.... appro",. 5 por'l (Il a/board)
enu.. aheld
.••
..
>0 "·" "" 10
"It"""'"" ",, '
(lttllfTl!
apptox. 80 %
lI1ln........ " 1U'1.pplOX. 50 %
..
.. pI1cto . . . Uw-cl (IsIem)
.. ..
........ 11 porl (NrboIrd) 1hr'......ad (litem) •• nr-fIII . . . . . 1011 ...
.
,o"'. ,,', .'' r. , ... .. ' ,,'''' ,,, ,,,
" j. M. VoilA GmhH, GmMny
Figure 2.23 Propea.r control ofVS tug'
forward. The towing point lies 0· ' - 0·2 x LWL from aft, although this may differ by tug depending on operational requiremen ts. 2.5.2
Propeller co ntrol
Th e direction and magnitude of prop eller thrust is remotely controlled from the whee lhouse. Th e remote control may be mechanically operated bya push-pull rod gear. This is a ver y reliable system for tugs and best when the distance between wheelhouse and propeller is short. With long distances between the bridge/ wheelhouse and propeller and whe n several manoeuvring stands are installed oth er remote control systems are recommended . Hydraulic, pn eumatic, electric and even computerised rem ote controls, even with jo ystick control, are alternative solutions. How propeller thrust is regulated can be seen in figure 2.23. Transverse thrus t is contro lled by the wheel and longitudinal thrust is controlled by pitch levers. So thrust setting is a combination of transverse and longitudinal thrust. Transverse direction has pri ority. When full transverse thrust is used (wheel hard to port or starboard) no longitudinal thrust will be available, even when the pitch levers are set in pitch position. It can be seen that the full 100% thrust cannot be applied in any direction. The two units of a VS tug can be co n tr oll ed ind ependendy or together for longitudinal thrust but onl y controlled togeth er for transverse thrust.
2.5.3 Manoeuvring
2.5.4 VS tugs in shi phandling
VS tractor-tugs are highly manoeuvrable, can turn on th e spot, deliver a high amount of thru st in any dir ection and sail straight astern at high speed. Astern thrust is nearly equal to ah ead thrust. Many of the disadvantages of conventional - especially single screw
VS tugs are used for towing on a line and for operations like push-pull (see figure 2.25). For towing and pushing operation s the maximum longitudinal pitch Intensidade is limited (to approximately pitch 8 for towing/pulling and pitch 9 for pushing) to avoid overloading the engine . In push -pull o pe r atio n s the disadvan tages of
- tugs, such as low astern power, no or low side thrust and in so me situations transv erse effect of the prop eller,
do not apply to VS tugs. Because it is possible to apply side thrust tractor tugs are also safer when making fast near the ship's bow and interaction forces can be better
compensated. Sailing ah ead as well as astern is easily achieved by use of the wheel, as shown in figure 2.24. Turning on the spot can be done by setting the wheel hard to port
conve ntional tugs of having low astern power and/or
not being able to pull at right angles to the ship do not ,apply to VS tugs. As already mentioned, VS tugs hav e nearly equal power astern and ahead and can apply thru st in any direction. While towing on a line a VS tug forward or aft can change to pushing without releasing the towline, which is very handy while approaching the berth (see figure 2.25, situation 3). The forward tug can change to a
====1~.
,
j. M. JIOith GmbH, Gmno.ny
Figure 2.24 A VS tug sailiug ahead and astern
or starboard. A VS tug can be moved sideways e.g. to p ort. The po rt pitch lever is set for ahead and the starboard for astern, while turning the whee l to port. The turning moment of the propellers is eliminated by the action of the wheel and the tug moves sideways . Propeller effectiveness is less on astern therefore ahead pitch should be set somewhat lower than astern pitch. VS tug propulsion produces little wash, which is invaluable when skimming oil and, for example, when working with full thrust close to deep loaded lighters as
Extremely usefull
can be the case in narrow harbour basins.
The full bow of tractor-tugs and the flat and wide hull bottoms which are nece ssary to create sufficient room for the propulsion units adversel y affect their sea keeping behaviour. According to the experience of some VS tug captains, so do the bottom plates of the VS propulsion units in roug h sea conditions. A nu mbe r of VS -tugs, p arti cular ly those used for escorting, are designed such that they better meet the demands of operating 'skeg-first'. This, however, does not alter the basic principles of the tractor tug.
Figure 2.25 SOTM assistiugmethods witha traaor tug
pushing position at a ship's speed up to approximately two knots. A towing winch is always useful with this kind of op eration in order to con trol the length of the towline and to enhance safety. VS tugs can also make fast directly to a ship 's side as push-pull tugs (see figure 2.25, situation 4) approaching the ship either stern or bow first. Ship's speed should then not be more than about five knots. Although VS tugs are not the most effective type of tug as.a forward tug towing on a line for a ship under spee d, due to performance restrictions imposed by the location of th e towing point, they are very suitable as after tug for course and spee d control. Course control can then be carried out with ships having headway and, contrary to what is possible with conve ntional tugs, to starboard as well as to port.
TUG USE IN PORT 23
Course control is carried out at higher speeds by the indirect me thod (see figure 2.25, situation 2), making use of the hydrodyn amic forces on the tug's hull, or at lower speeds by the direct method (see figur e 2.25, situation I). Forces in the indirect method can be far highe r than the tug' s ba llard pull . When braking forces are required, pitch levers should be adjusted to ship's speed to avoid overloading the engine and a minimum of whee l should be used. Th e different manoeuvres that can b e carried out with a VS tug are show n in the ma noeuvring manual ofJ.M. Voith GmbH.
2.6
Tractor tugs with azimuth propellers
2.6.1 Design Tracto r tugs with azimuth propellers have two 3600 steerable thrusters under the forebody. Th ere are several m a nufactur er s of az imuth th ruste rs, in clu ding Aquamaster, Schottel, KaMe Wa, Niigata , Kawasaki, V lstein an d Bru nvoll. So me of the Eu ropea n manufacturers mentioned h ave merged. Differ ent names are used for azimuth thrusters, such as Z-pellers, Rexpellers and Duckpellers, amongst others. Although th e thru ster systems are gene ra lly sim ilar, each manufacturing comp any has its own specific design .
Schout/· wtlft, Gnmany
Figure 2.27 Integrated Sdiouel ;'o<.des with open protectioe frames, decreasinga tug's maximum draft by approXimately 0-5m without affecting the tug's performame
clutch, which enables the pro peller spe e d to be controlle d in a stepless manner from ze ro up to maximum. This more or less eliminates the need for contro llable pitch propellers and is much less expensive. Azimuth propellers are fitted in nozzles to increase propeller efficiency, (for nozzle types, see par. 2.3.2) In the event of grounding, propeller protection is provided either by protection or docking plate s. Docking plates are fitted undern eath or in fron t of the prop eller an d give on ly limited prot ectio n for the pro pellers. Protection plates serve also when do cking.
The first azimuth propellers were intro duced into service in the \960s. The first tug fitted with azimuth prop ellers was the German harbour tug]anus (1967). Azimuth propellers can be fixed pitch , e.g. mostly with Niigata, or controllable pitch . Fixed pitch propeller revolutions can b e regu lated by a speed modulating
The basic design of th e tug itself does no t differ much from VS tr actor tu gs. The displacem ent of a VS tug is m ore than that of a comparable azimuth tractor tug of the same engine power, du e to the hi gh er weight of the VS pro pulsion systems and to the requireme nts for more stiffening due to the wide r hull openings for the VS units. .An azimuth t racto r tu g of the s am e dimensio ns and engine powe r will therefor e have less hull draft. Towing point location is generally similar to that in VS tugs. The skeg is sometimes smaller and the location of the towing point is often less strictly related to the location of the skeg as with VS tractor tugs. Th e towing point lies approx imately 0·1 x LWL from aft and the propellers are fitted at 0·30 - 0·35 x LWL from forward. A smaller distance is Schotte~ 1M NetMrlands found, 0·25 x LWL for instance, on some Figure 2.26 Azimuth tractortug 'Fairplay V'. L.o.a. 26·7m,beam 8·8m, bp2!!t. Infront Italian tractor-tugs at Genoa, Italy. Thrusters of the thrusters is the doclring plate 24 THE NAUTICAL INSTITUTE
Photo: Author
Figure 2.28 J oy,tUkfor combined control ofboth thrusters. The direction oflug's motemmt is indicated around thej oy'tidc. Speed control is carried out by separate loxn
Plwto: StorJ:·KwaJlJ, T1u Nethtrl.zruls
Figure 2.29 Thruster control unitfor combined control of thrust and thrust direction. The unitsare asailableforfixed pitrh and conlrollablepitrh propellers
o Clutchoff _
• and! Of! Oi..CIlonof Aa..... b.~ irId
Niigata Engineering Co. UrI.Japan
Figure 2.:jO Manocuvringdiagram for reuerse-tracior tug. When the tug has a Uni-leter type manoouaingpansl; theUni-lever is used in combination with the dual speed control handles. When the tug has the standard typc manoeuvring pane~ manoeuvring is t/Q1Jl by thesteering whee~ the dualahead-astern handles anddualspeed control handles. A comparable system, :4quaduo' ofAquamasterlKilMeJ#z is installed in ASD-Iug'of Adsteam Towage, UK
TUG USE IN PORT 25
placed further forward increase a tug's effec tiveness while assisting. The thrusters deliver practically the same amount of thrust in any direction, though astern thrust might be about 5% less. \'!hen the thrusters int era ct, as wh en producing side thrust, total thrust efficiency will be less. Thrusters should then be set at a small angle to each other. 2.6.2 Propell er control T hrusters can be controlled by a single dev ice for each thruster separately in respect of the amount of thrust (propeller pitch for cpp or prop eller revolutions for fpp) and thrust direction or controlled togeth er by a joystick. Altern atively, by a control system consisting of two steering levers (ahea d-astern handles), a steering wh eel to give angle adju stment to both thrusters and two speed control lever s. For the latter two method s sec the manoeu vring diagram (figure 2.30) of Niigata for joystick, steering wheel and control handle po sitions and the resulting tug movements for a tug with azimuth thrusters at the stern.
2.6.4 Azimuth tractor tugs in ship handling The assisting capabilities of azimuth tractor tugs are comparable to those ofVS tractor tugs. They ar e suitable eithe r for operati ng at the ship' s side or for towing on a line (see figure 2.25). Azimuth tracto r tugs fitted with a smaller skeg and!or a towing point not located at the correct position are less effective as a stern tug compared to the VS tractor tugs, whe n ope rating in the indirect towing me tho d at highe r speeds. On the other hand, because of the ir 100"ver underwater resistance - mainly due to th e relatively sha llowe r draft - and the ability to pr ovide ne arly 100% thrust in any directio n, azimuth tractor tugs will be mor e effective at spee d when direct towing as a stern tug and as a forward tug when towing on a line, again dep ending on a proper location ofthe towing point. The influen ce of the location of the towing point on the p erforman ce of tr actor tugs is furt her discussed in Chapter 4.
2.7
Reverse-tractor tugs
When combined thruster control is by joystick (also called a Uni -lever, Combi-lever, master pilot, or similar names), th e thru sters are automatically set for the most suitabl e direction in order to manoeuvre the tug as indicated at the joystick control. Some azimuth thruster types have joystick control for the direction of tug's movement while the amount of thrust has to be regulated separately. Others have combined control of thrust force and direction.
2.7.1
Design
Tugs with combined joystick control can also control each thruster separately, but on some tugs this may be too complicated due to the number of handles to b e operated. Combined joystick control of both units is
were to tow on a line at speed like a co nve ntional tug.
limit ed to pre-programmed tug manoeuvres, so separate
control of the thru sters has some advantages owing to the large number of possibilities, especiall y wh en ship handling man oeuvr es arc com plicated. It should th en be possible that thru st and direction for each thruster can be regulated in a simple and logical way. Azimuth thrusters with controllable pitch propellers hav e the advantage that pitch can quickly be reversed for astern thrust However, wh en full power astern is
required thru sters should be turned for astern. 2.6.3 Manoeuvring The m anoeuvring characteristics of azimuth tractor-
tugs are more or less comparable to those ofVS tractortugs. They are also safe working tugs and highly manoeuvrable, canturn on the spot, move sideways and have nearly the same ballard pull ah ead as astern. Because of the relatively shallower draft, sometimes another skeg de sign and almost 100% thrust in any direction, the manoeuvring characteristics of thes e tugs
may be somewhat different compared to VS tugs. 26 THE NAUTICAL INSTITUTE
Reverse-tractor tugs, also called pu sher tugs, are tugs with two azimuth propellers under the stern . They are more or less specifi cally de signed for th e assisting method used , for instance, in a large number of ''\Test Pacific ports - assisting over the tug's bow. These tugs have a large towing winch forward and only sm aller towing equipm ent aft e.g. a towin g hook. The towing point aft often lies too far aft to be effective if these tugs Sometimes the towing point lies nearl y above the thru sters aft. Azimuth propeller systems in use are J ap an ese or Europ ean made and can b e fi tte d with fixe d o r controllable pitch propellers in nozzles. In th e case of fixed pitch propellers, revolution s can be regu late d by a spee d modulating clutch , which controls the pro peller spe ed in a stepless manner from zero. Be cause the
thrusters are fitted under the stem the maxim um dra ft of rev erse-tractor tugs is less than that of comparable real tractor tugs. Hull draft is less th an the hull dr aft of a similar VS tractor tug, for reasons alrea dy explaine d when discussing azimuth tractor tugs.
The propulsion units are located approxi mately 0·[ x LWL from aft. The pushing point and forward towing point is at the forward part of the bow. Wheelhouse construction is completely adju sted to th e assisting method. The manoeuvring station is designed in such a way that the tug captain ha s an un obstructed view of the forepart of th e tug, the towline and th e assisted ship while seated behind the manoeuvring pan el and the assorted instrumentation and control handles around him .
tractor tugs do the same but are then heading in the reverse direc tion. Tha t's why th ese tugs are call ed reve rse-tractor tugs.
o
0
.............
What has been mentioned about azimuth tractor tugs with respect to manoeuvring also appli es to a large extent to reverse-tractor tugs. They can be used for towing on a line or for assisting at th e ship's side as shown in figure 2.33. They can easily change, whe n towing over the tug's bow, to a pushing position at the
r
.~~ '.
PROFILE
UPPER DECK PLAN
1M Hong KongSalvagt & Towage Co. Ltd
Figure 2.31 Reverse-tractor orpushertug 'Lam Tong', l.o.a. 26· 1m, beam 8·5m, hp 431
Figure 2.33 Assisting methods with a reverse-tractor tug
ship's side or for pu sh-pull while b erthing. A towing winch is useful to enable the towing line always to be a suitable length or to pick up any slack in the line. When operating at the ships side these tugs are very effective at speed. Alth ough this type of tug is also used for towing on a line , as a forward tug it will not be effective in steering ships having headway. The tug has to move astern an d Mas o ASD tem essa capacidade! its towing point lies at the forwardmost end of the tug, giving a similar decrease in steering efficiency wh en speed increases as with a tractor tug.
C. H Caus & Sf11/.1 Limi.ttd, Qll'u.4a
Frgure 2.32 Thrusters ofCates' reome-traaor tugs
2.7.2 Propeller control, manoeuvring capabilities and shiphandling Prop eller control with rever se tract or tugs is the same as with azimuth tractor tugs. Because of the two azim uth thru sters and the forward lying towing point reversetractor tugs ar e highly manoeuvrable and safe working tugs. They can tum on the 'spot and mov e sideways, (see fig. 2.35) The astern power of these tugs is generally ab out 10% less than ahead power, du e to the shape of the after hull. The name reverse-tractor tug implies that the tugs operate similarly to tractor tugs but in the opposite way. Tractor tugs always operate with the towing point towards the assi sted ship and th e pr opulsion units away from the assisted ship. Reverse-
As a stern tug, reverse-tractor tugs are very suitable for steering and speed control for ships at speed, whether making use of the indirect or direct method. In th e indirect m ethod rev erse-tr actor tugs are in ge n er al somewhat less effective in steering compared to a similar VS tug in the same situatio n, but in the direct method rev er se-tractor tug s might be so me mor e effective because of the lesser draft The effectiven ess of tugs is dealt with in more detail in Cha pter 4.
2.8 Azimuth Stern Drive (ASD) tugs 2.8.1 Design Conventional tugs hav e certain advantages and so do reverse-tractor tugs. ASD-tugs are nearly the same as reverse-tractor tugs but are de sigrled in such a way that they can op erate like a reverse-tractor tug as well as a conventional tug, thu s com bining the advantages TUG USE IN PORT 27
-,9.
Shipyard Damen; 'I1le Netherltmds
Figure 2.34 ASD-tug type 3110. L.o.a. 30·7m, beam 70·6m, bpdepending oninstalled enginepower 37-57 tons (ahead) Note:: Underusuer body design ofthis ASD-tug type has been optimised during recent years, which includes a large skeg extendingfrom approximately 0.3 x water length from aft till theftrefoot, with the deepest part aft.
of both types. ASD-tugs have a towing winch forward and a towing winch or towing hook aft. The aft towing point is at a suitable location for towing on a line, viz. 0·35 - 0·4 x LWL from the stern. Like reverse-tractor tugs, they have two azimuth propellers fitted under the stern at roughly the same location, about 0·1 x LWL from the stern.
increasing the tug's manoeuvrability, its position keeping abilities, maximum ballard pull ahead and astern and maximum achievable sideways thrust.
The azimuth thrusters of ASD-tugs are made by the
Propeller control is the same as with azimuth tractor tugs. The manoeuvring capabilities of free sailing ASDtugs and reverse-tractor tugs are shown in figure 2.35. These tugs can deliver thrust in any direction, though maximum stem thrust is some 5 to 10% lessthan on ahead.
same manufacturers as the azimuth thrusters of tractor
tugs. In addition, Holland Roer Propeller (H RP) can be mentioned. Their maximum draft is less than that of comparable tractor tugs, as mentioned when discussing reverse-tractor tugs. They may be equipped with a tunnel bow thruster, especially when used for offshore operations. Tunnel bow thrusters are not very effective when a tug has speed ahead, but are very useful for position keeping. Interest in this typ e of tug is still growing because of their manoeuvrability and multipurpose capabilities. The latest development is installing an azimuth bow thruster. This has been the case with the 4000 hp ASD-tug Z-Two of towing company Tugz International LLC (USA). A retractable azimuth bow thruster of approximately 1000 hp was installed, so 28 THE NAUTICAL INSTITUTE
2.8.2 Propeller control, manoeuvring capabilities and ship handling .
Conventional tugs are effective as forward tugs towing on a line, while reverse-tractor tugs are effective aft and are also very suitable for push-pull operations. ASD -tugs are very effective and suitable for all kinds of shiphandling, owing to their ability to assist like both a reverse-tractor tug and a conventional tug. When towing forward on a line like a conventional tug (see figure 2.36, 1) the ASD-tug is very effective, although the risk of girting exists. The risk is minimised when the tug is equipped with a reliable quick release system .
As a stem tug on a line an ASD -tug works over the bow (situation I and 2). This is effective for speed contro l and course control to both sides. Effectiveness when assisting in indir ect mode (situation 2) is generally somewha t less wh en compared to VS tractor tugs, but ASD-tu gs may be somewhat more effective wh en direct pullin g (situation I), because of their relatively shallower draft.
~ ]2]) -~ {~
~ ~
c:
If an ASD-tug is equipped with an azimuth bow thruste r as mentioned in par. 2.8.1,then the manoeuvres discussed can be exec ute d faster and more effective .
IID)-
~
W W :tID! »ill lID)
Like reverse -tractor tugs, ASD -tugs can also easily change from to win g on a lin e to pu sh-pull without releasing or changing the towline position (situation 3). The forward ASD-tug should th en assist like a reversetr actor tug (situation 2). A bow thruster is, as for a reverse-tractor tug, useful for bringing and keeping the tug's bow in position at the ship's side. For this kind of ope ration a tow ing winch is ve ry useful in order to control the length of the towlin e and to pick up the slack wh en necessary. ASD-tugs are also very suitable for assisting at the ship's side, because of their high reversing power and their 3600 steerable thru sters.
Figure 2.35 FTU sailing manoeuvring capabilities
of an ASD-Iug and reuerse-tractor tug
~
t
2.9
Tug performance
With r espect to tug performance it is goo d to understand some basic principles. The first item deals with performance at speed, which is discussed in detail in Chapter 4, and the second mainly with ballard pull conditions. I) Wh en the tug's propeller wash is more or less with the direction of the water flow, the propeller is said to be operating in positive flow cond itions. This is, for instance, when a bow tug is pulling a ship having headway. Wh en the tug' s propeller wash is more or less against the direction of the water flow, it is said to be operating in negative flow conditions. This is, for instance, when a stem tug is braking a ship's speed . Although greater thrust is produced when operating in a negative flow, torque loadings on the propeller an d engine increase considerably, particularly with increasing speed of the water flow. As the negative flow may also result in an unstable flow trough the propeller, it may produce fluctuating loads and vibrations.
/
1 .
\
~I • • ~~ C§2· ·: . ~
'lJ . lJ <1Q.! .~ 4
Figure 2.36 Some assisting methods withanASD-tug
2) The line pull is essentially dependent on the square of the propeller revolutions , and the engine power is dependen t on the cube of the revolutions.This means that if propeller revo lutions are doubled, the force will increase with a factor of four , while the requir ed engine power increases by a factor of eight. This re la tions hip not on ly applies to ballard pull conditions, but approximately to most tug operations in port. The efficiency of an open propeller - as already mentioned - can be increased by fitting a nozzl e. Tugs with the same BHP may have a different bollard pull depending on whethe r the propellers are fitted in a n ozzle or no t. Also, th e typ e of pro pe ller fitted is im por tant. To de termine the towing force of a tug, bo llard pull tests are carried out at different engine ratings, particularly at the manufacturer's recommended TUG USE IN PORT 29
continuous rating (MCR). Tests can also be carr ied out at engine overload conditio ns , for instance with a maximum rating that can be m aintained for a mi nimum
of one hour , and also with just on e propeller working. Bollard pull tests are carried out with engines ahe ad and increasingly, especially for azimuth tugs, on astern . Bollard pull tests should be carried out with sufficient underkeel clearance, no current an d waves and not too
much wind. The tug should pull straight ahead, or straight astern when astern b allard pull is measured. The towline should b e of sufficient length to avoid the tug's propeller wash having an y influence on the towline force. Bollard pull is m easured by a device insert ed in the towline. It can be a measuring devic e based on an ordinary spring system , the 'clock', or an electronic device.
Classification societies issue regulations for ballard pull tests. For instan ce, according to the rul es of Det Norske Veritas (DNV) , the towline length should not be less than 300 metr es, the water depth n ot less than 20 metres within a radius of 100 metres around the tug, wind sp eed not more than 5m/ sec and current not more than one knot. An in strum ent giving a continuous read-
out and a recording instrum ent repre senting ballard pull graphically as a functi on of time should, according to DNV, be connected to the load cell. The figure certified as th e tug's continuous bollard pull will then be the towing force re corde d as being maintained without any tend ency to decline for a duration of not less than ten minutes. Although conditions mention ed in the DNV regulations do me et the requirements, unfortunately several other regulations for ballard pull testing do not sufficiently take into account the conditions required for accurate and reliable b allard pull testing of modern powerful tugs. In the report called 'Bollard Pull' (see References) a new ballard pull trial code is proposed that ensures obje ctive results and repeatability as well as comparability for trials p erformed at different locations andlor with different tugs. Figure 2.37 gives an indication of the ratio BHP Bollard Pull for different propeller configur ations. The value s shown in the tabl e are more or less the maximum valu es. Because the relati on between bollard pull and engine power dep end s on several factors, such as hull form , nozzle typ e and propeller load, the valu es may vary as shown in figure 2.38. The relationship between engine power and bollard pull vari es considerably with the extent of engine power and in such a way that a conven tional tug with 700 BHP and a fixed propeller can attain two tons/IOO BHP, whilst for conventional tugs with about 6000 BHP with nozzles, towing force may even be less than 1.3 tons / 100 BHP.
30 THE NAUTICAL INSTITUTE
Voith propeller
] ·] 5
]·55
Open fixed pitch propeller
]·3
] ·8
Az imuth prope llers in nozzles (ahead)
] ·35
] ·8
] ·5
2·0
Fixed/controllab le pitch propellers in nozzles [conve ntional tugs)
Figure2.37 Relationshipbetween brake horse powerand bollardpull fOr differentpropulsionsystems (see'text} Brake HorsePower (BHP) is measured at theflywheel ShaftHorsePower (SHP) is measured at thepropellershaft SHP~± O·97 x BHP
Propeller performanc e is also shown in so-called thrust vector diagrams. Several kinds of these diagrams exist, all of them giving different information. Thrus t ve ctor di agrams give inform ati on on pr op uls io n performance with ze ro speed in different directions, which is also importan t inform ation to-assess the tug's assisting p erforman ce. An example of thru st vector diagrams with an indic ation of thrust forces is given in
VS tugs
1·0 - 1·15
]· 35 - 1·55
ASD tugs
1·]5 - 1·35
] ·55- ]·8
Con v entional twin screw tugs with fixed/controllable pitch propellors in nozzles
1·25 - 1·5
] ·7 - 2·0
Figure 2.38 !IJlnges in relationship between brake horsepower andbollardpullfOr different tug types
figure 2.39 . It gives propulsion performan ce at zero speed for equal installed po wer. Side thrust an d the influence of int eraction of propellers on side thrust are clearly shown in th e diagram. In this thrust vector diagram the ahead values given are also more or less maximum valu es. Th e astern thrust of ASD-tugs may vary between 90% and 95% of ahead thrust. In the diagram the astern thrust of conve ntional tug s with controllable pitch pr opellers is given . The astern thrust of conventional tugs .with fix ed pitch pr opellers is higher and around 65% of maximum ahead thrust, but it dep ends strongly on th e n ozzle typ e, propeller/rudder desigu and configuration. For examp le, the Towmaster system may improve ahead thrust to even more than 1·50 tons BP/IOO BHP, whil e a very good
-r
1l0 'llo o f ." .,d
6" iol ~ I""~
+
,, ,
"._-
-l
•..·f---- ·
ASD e TRACTOR AZIMUTH
,
I
.....-_ .~ . _ . -T.- ..r. .. .; .. . .
, i ~
- ' - -l ·~ · · I·
-.
,
, .. +-:
Astern e
---·-..t: -··r- .. - . -_'-- .- -(j"t . • _.: -.. _," , : ,.,_~, ,'1- •.
\.
"-
t ~
I
r
'-'. "
".
-~
-I
;
"
.--
_.~.,-
t-
Convenci onal with nozzle
,
.~-"
Athwarthship
1
•
L
-'
Figure 2.39 Example of Thrust v"ctor Diagrams Legend a) Tractor tug: Voith h) Itaaor tug: azimuth: prapelkr in naw es c) Stem drive tug: admuthpropeller in naaJ
astern thrust of more than 70"10 of maximum ah ead thru st 'an be achieved. Note: Particularly for the mor e sideways thrust it is fficult to say how accur ate the thrust vector diagrams e. Simulated or calculated performance diagram s ould therefor e, as far as possible, be validated in full Ie trials,
Official full scale trials by Clyde Consultants UK with a VS tug have shown that thrust in the mo re athwartships direction may be much less than indicated in the thrust vector diagram. The athwar tships thru st me asure d was less than 40% of the ahead thrust while deve loping over 80% of the shaft horsepower. O n the other hand the athwartships thrust of tractor tugs with azimuth thrusters can be high er tban indicated when thrusters ar e set at a small angle to each othe r.
TUG USE IN PORT 31
Ewrgrtm Marine Corp. (Taiwan) Ltd. tJrUi plwto:J Plug, Ltkko ITES
Figure 2.40 An assisting method asused in some USA ports. TIu container shipis assisted in the portofLosAngeles by twoconventional tugs. The stern tug operates lih a rudder tug. The smaller photograph shows howthestern tug 'Pointe Vi'enle' ('Q1lven!ional tug, twin screw, length 32m, bollardpull ahead 46·5 tons, astern 28-5 tons) pushes the ship~ stern towards theberth. The assisting method issimilar to tha: used in a "'rge number ofmst Pacifi< ports, such astkose in]apan, Taiwan andHong Kong. However, in those ports reverse-tractor tugs are used andoperate in the push-pull mode while mooring
32 THE NAUTICAL INSTITUTE
Chapter THREE
ASSISTING METHODS 3.1
Introduction
IN THE FIRST CHAPTER DIFFERENT TYPES OF PORT we re
discussed. In these ports tugs may ren der one of the Prestar following services: Tug assistance during a transit to or from a berth, including assistance during mooring and unm ooring operations .
Tu g ass is ta n ce m ainly du ring m oorin g an d unmooring operati ons onl y.
width adversely due to the larger drift angle. Steering ability is less at lower speeds, and is adversely influenced by wind and current. O n the othe r hand, spee ds up to six knots become rather high for effective tug assistance. When port configuration is such that tugs are mainly used for m oorin g and unmo oring ope rations, then tug assistance may comprise :
Th e approach phase toward s turning basin or berth. To what extent tug assistance is considered to be necessary dep end s on parti cular s of: The ship - including type, size, dr aft, length/width ratio, lo ading co n ditio n, windage and Air resistance manoeuvrability. The berth and nearb y manoeuvring area - including typ e and size of berth, alignm ent, b erthing space, manoeuvring space near the berth, size of turning circle, water depth,.influence of current and wind ,
and availability of mooring boats. The transit rou te - such as width, length and depth, the bend s in that route, maximum allowable spee d, traffic to be expected and whether moored ships have to b e passed, plus th e influen ce of current, wind , waves , shallow water and banks. The important difference between tug assistan ce during mooring/unmooring ope rations and during a tran sit lies in the difference in ship's speed, which is a maj or fa ctor of importance for selec tin g the mo st appropriate type of tug and meth od of tug assistance . The vari ous methods of tug assistance employed in ports aro und the world are reviewed in this chapter, including the typ es of tugs used . In the next chapter the large influ en ce of ship's speed on the performance of the differ ent tug types in relation to the assisting methods is considered. Tug assistanc e as may be required in ports is first addressed in more detail to obtain a better insight int o what tugs should be capable of doing. Tug assistance during a transit may comprise: Passage through a rive r or chann el. En try manoeuvres into a harbour or turn ing basin from river, channel or sea.
Passage thro ugh narrow harbour basins. Passing narrow bridges or locks.
Turning in a turning b asin. Moorin g and unmooring ope rations.
Contrary to transit speeds, ship's speed during these manoeuvres is normally very low or zero. Although tugs should be capable of controlling a ship's heading and speed and compensating for the influence of wind and current whil e approaching the turning circle or berth, the influence of ship's spee d on the performance of different tug typ es is less predominant. Tugs assisting during transits, taking into account the assisting meth od applied, should be cap able of: Giving steering assistance and controlling shiP's speed
Steering assistance whil e the ship has headway may be necessary in narrow pa ssages, when passing bridges or negotiating sharp and!or narrow bends in the fairway, river or channel, or when en tering harbour or turning
basins under the varying influence of current and wind conditions. Controlling ship 's heading and speed may be required when approaching the harb our or turning basin or whe n entering a lock. Compensatingfor wind and current during transit while a ship has speed
While transiting a channel, riv er or harbour basin a ship under the influence of wind and!or current may exp erience drift. This can be co mpensate d for by steering a drift angle or by a higher speed. A higher speed is normally not possible in confined p ort areas and due to the limited width in narr ow passages only small drift angles are acceptable. Tug assistance is then nec essary.
Higher the speed more directional stability it has
Tugs assisting ships during a transit normally also Over the larger part of a transit route the speed of a vessel is mostly within the range of about thr ee to six kn ots and sometimes even higher. Atthese relatively low ship's speeds the influence of wind , curre nt and waves is more pronounced , affecting the required path
assist during mooring/unmooring operations an d the
final approach and departure manoeuvres as tugs used for mooring/unmooring operations onl y. All th ese tugs sho uld, with the assistan ce meth od appli ed, be capable of effectively: . TUG USE IN PORT 33
Controlling transverse spe ed towards a berth while compensating for wind and current during mooring/ unmooring operations
impact on the assisting meth ods used in Europe as well as in the USA, which should be kept in mind whe n reading this paragraph.
Duri ng moo ring operations a ship's longitudin al ground speed is prac tically zero and, wh en there is no current, the ship has hardly any speed through the water. The same applies when a ship has to be turned in a turning basin. Mainly crosswise pu shing and!or pulling force s have to be applied by th e tugs.
In some ports combina tions of methods are used, depending on th e local situation. For specific situa tions or circum stances, assisting methods are applied other than those in normal use. So it is possible that in po rts wh ere tugs normally wor k alongside, th ey w ill occasionally assist while towing on a line, for exam ple when n arrow bridges have to be passed or wh en ship s have to enter a dry dock. Changing the assisting method can b.eco me necessary at seas ide terminals, where tug
Th e tug assistance requi red as outlined above has b een somewha t simplified. In any particular case the complete tug assistance procedure may consist more of a combina tion of the separate aspects that hav e b een describ ed . En vir onm en tal co nditions h ave a lar ge influence . For instance, when tugs are used mainly for mo oring/unmooring, the influence of currents can b e such that although ship's ground speed is low, say two kn ots, th e speed through the water can be rath er high. With a bow curre nt of two kn ots, the speed through th e water is already four kn ots. Situation s th en become comparable to tug assistance durin g a tran sit with the higher ship's speeds and the associated requirements for the assisting tugs. Additional services such as mooring boats also affect th e extent and method of tug assistance. When no mooring b oats ar e available the tugs must be statione d and operated in such a way that the ship cao b e pushed up to a b erth. It cao be concluded that the port configuration, th e influ en ce of the en vironmen tal co n ditions and port se rvices have a prominent bearing on the requirements
for tugs aod the method of tug assistaoce, while ship'S speed is an essential factor.
3.2
assistance is affecte d by wav es. If in calm weather it is
norm al practice to assist alongside a vessel, it may be considered safer to tow on a line whe n weather and sea conditions de teriorate in order to avoid parting
towlines ao d losing contro l of the vessel. According to resear ch carried ou t in 1996 into assisting me tho ds in use in po rts around the world, the two method s are gene rally app lied in the following ways, assum ing two tugs assist a vessel:
Tugs alongside during approach to the berth andpushing or push-pull while mooring This method is normall y used in th e majority of ports in the U SA, Caoada, Australia, Malaysia, South Afri ca and also at large oil terminals in Norway. While th e method used in these ports is similar, the type of tug differs. The way tugs are secure d using this method dep ends mainly on th e type of tug. When using tugs with om nidirectional propulsion they are mad e fast at the forward and aft shoulder, generally with one b ow line fro m the tug in case of ASD/reverse-tractor tugs an d with a lin e from the tug's stern when tr actor tugs are used (see figur e 3.1) .
Assisting methods
3.2.1 Assisting methods in use
In the U SA tugs m ay b e secured alongside a ship by on e, two or three lin es, depending on th e type of tug, Mas rebocador amarrado não é o método europeu? Não! O Metodo europeu opera puxando e o americano no costado
The different ways ships are handled by tugs in various areas and ports ar ound the world can indeed mainly b e traced back to large differ en ces in local circumstances. Methods of assistance that different tug typ es are used for have alrea dy been mention ed bri efly whil e discussing th e various types. Assessment of assisting meth ods in use all over the world shows only two m arkedly differ ent meth ods: Tugs towing on a line. Tugs ope rating at a ship's side. In Europeao p orts towing on a line .is mainly used , whil e in the USA and West Pacific ports tugs usually operate "a t a ship's side , altho ugh in differ ent ways dep ending on th e type of tug used . Parti cularly in Europe and in the USA there is a tendency towards the use of mo re flexibl e types of tug . Thi s tend en cy has an 34 THE NAUTICAL INSTITUTE
•
4
r::
Figure 3.1 Tugs alongside at approach andpush-pull while mooring/unmooring
ON THE HIP -bacaças Para o rebocador não sair de posicao
Figure 3.3 Alongside towing (USA) Figure 3.2 Conoentional USA tug secured with backing, spring and stern lines. In situation 2 the ship mczesastem: Ifshipmoves ahead the stem line will leadforward. Depending onthe assistance required and local situation, ont, two or three lines may he required
the local situation an d the assistance re quired. Conventional tugs normally operate with two or three lines made fast, though in some cases only one line is deemed sufficient (see figure 3.2). The forward line is a tug's backing line to be made fast to the ship. Th e spring may come fro m the forward winch through a tug's most forward bow cho ck or fairlead . O n other tugs bo th lines may come from a winch . The third line, the stern line, is nee ded when a tug has to work at right angles to a ship to pr event the tug from falling alongside when the ship has forward or astern mov ement thr ough the water, or to compensate for the tran sverse effect of a tug's prop eller when going astern . This line may come from a winch or be fastened on a bitt. It also compensates for the influence of the ship's propeller wash when the ship's prope ller is going astern. A forward as well as an aft tug may be secured in this way. O wing to their bette r man oeuvrability, twin screw tugs or tugs with steerable nozzles normally operate with fewer lines whe n assisting at a ship's side. Usually one or two lin es will then be sufficient. In the US A other m ethods are also used by tugs ope rating at the ship's side. When breaste d or alongside towing, also called 'on the hip ' or 'hippe d up', tugs forward andlor aft are lashed up solidly alongside a vessel (see figure 3.3). This alongside towing is also ope rate d in many other ports in the world, but mainly when han dling barges. When a tug is lashed up , tug an d sh ip wo rk lik e a twin screw shi p with two independent rudders. Wh en lashed up forward to a ship with the tug's bow facing aft, the tug's engine and rudder combined act like a kind of steerable bow thruste r (see figure 3.4). A ship can .the n turn on the spot or move sideways. Alongside towing is also used in USA ports to handle a 'dead ship', and occasionally ap plied in a similar way in some othe r p orts - for instance in the port of Cape Town ships up to 100 metres in length are sometimes handled as a 'dead ship ' by a VS tug lashed up alon gsid e (see figur e 3.5).
In US A ports me tho ds are also used that differ from those discussed above. For example, in certain situations tugs may work stem to stem with a vessel. A ship moving
Figure 3.4 Forward tugsecured akmgside. Asshown the ship can Starboard tum on the spot and when the tugapplitS hardportrudder and engine ahead; the ship mootS crosswise. Ship'S aIlend power to be equal to tug's aheadpower
Figure 3.5 Alongsid< towing in Cape Town for a 'dead ship' up to 700 metres in kngth
astern can be steered by a tug pushing at the ship's bow. Pushing at the port side of the b ow will give the ship a swing to starboard, pushing at th e starboard side of the bow will give the ship a swing to port. In some ports in the USA and in the Panam a Canal a stern tug is used as shown in figure 3.6. A rudder tug can control a ship's spee d and a conventional tug can steer a ship in th e require d direction by giving forward thrust and applying starboard or port rudder. Other typ es of tug such as VS tugs also use this m ethod. A similar me tho d is some time s used on Dutch inland wate rs.
Tug's boWto port...nIp wtR 90 to starboard .
Tug'l bow to S'tafbQ8Jd. ship Win00 to port.
Tu g's engil1l!l astern, ship's speed will decre ase . Tug may be fastened with ona or two linea.
Figure 3.6 Rudder orsteering tug
TUG USE IN PORT 35
r'f;>i
,
,
'.\' 'v' I
9H
Figure 3.9 At approach, forward tuga/Qngside andstem tug on a line; push-pull while berthing
tugs have to assist while towing on a line, for example when assisting ships to ente r dry docks or floating docks.
Photo: Moran Towing, USA
Fz"gure 3.7 Conventz"onal tugworking stem tostem with a largepassengershz"p
Forward tug alongside and aft tug on a line during approach towards a berth and push-pull while mooring Thi s method, which does not differ much from that mentione d abov e, is mainly found in the ports of] apan, Taiwan and Hong Kong (see figure 3.9). The after tug is mad e fast by a tug's b ow lin e amidships or at th e starboard or port quarter aft and follows the ship. The forward tug is mad e fast at the forward sho ulder, also with a b ow line. The after tug is used for steering and speed control. During b erth ing manoeuvres the tugs change over to the push-pull meth od. Tugs in these ports are all of similar design, spec ifically constructe d for this typ e of operation . T hey are rever se-tr actor tugs or sometimes ASD -tugs, with 36 0' steerable thrusters under the stern and made fast with a line from the tug's forward winch . For certain specific manoeuvres these
Photo: A. Jv. &mpt
Figure 3.8 Conventional twinscrew tug 'Espera1l
36 THE NAUTICAL INSTITUTE
Apart from th e co unt ries mentioned above this metho d is applied in some other ports around the world either with reverse-tractor tugs or with tractor tugs. Furthermore, conventional tugs are sometimes used for this me thod, as is th e case in some US A ports whereby th e stern tug ope rates like a rudder tug . While b erthing this tug stays close be hind the ship's stern and pushes it towards the berth on the tug's bow line.
Tugs towing on a line during transit towards a berth and while mooring This is th e assisting m eth od used spe cifically in Europe, most often when conventional tugs are assisting
vessels, but othe r typ es of. tugs are also used for this me thod. The method is also applied in many other ports of the wo rl d, es pecially in p orts working with conve ntional tugs (see figur e 3.10). In many of these ports, ships are assisted by tugs during transit towards the berth , e.g. on th e river, from the river into the harbour an d thr ough harbour basins up to a berth . The advant age of this meth od of assistance is that it can be used in n arrow wa te rs. This m ethod is also used, therefore, when passing narrow bridges or entering locks an d dr y-docks. In suc h situations the forward tug sometimes has two towlines, so-called cross lin es or gate lines or both lin es may come from a doub le winch at the tug's b owas can be the case on some reverse-tractor tugs. The tug can then react very qui ckly an d only a little manoe uvring space is req uired (see figure 3.11). The typ e of tugs used were originally conventional tugs with a small engine an d a streamlined un derwater body. These were very effective when a ship had some spee d, by making use of th e tu g' s m ass and the hydrodynamic forces on the tug's hull. The increasing size of ships requ ired the introduction ofmore powerful tugs. Mo dern conve ntional tugs are more manoeuvrable an d have mo re engine power and gene rally a smaller
PROVA
the better the capabilities are applied to shiphandling. The method is, for instance, practised in the Europoort area of Rotterdam an d at the port of Coteborg, where mainly tractor~ reverse-tractor or ASD·tugs are used .
Al a Vert10.... speeod a
corwenllonllllugcan move10 e.,_ poslllon1 & 2 for$pHd
eontrcl and Sl-w.g or10 J:0:llaon 3 Ill!'st_rIo;.
Q ·- '
3 , · ·
·
"-
,,
,
2
"----EJ
\
, J
'/
W'
Figure 3.72 Towing on a line at the approiUh andpwh-pullwhik mooring Figure3.10 Towingon a lineat tk approiUh and whik mooring
Combinations ofthe above systems
In many ports various tug typ es are operated and to assist larger ships more than two tugs are often requir ed. Moreov er, port entry or berthing manoeuvres can be so complicated that not just one assisting method is used but a combinati on. As an example of a combined meth od the assisting method applied in an Australian port for large bulk carriers entering the harbour is shown in figure 3.13. PROVA Canal do panama
Figure 3.11 Ship ispassing a narrow bridgeand a omxentional tug forward is assisting with two crossed tow lines. The tug can react quickly and only little manoeuvring space is required
len gth/width ratio. These tugs are still effective when a ship has speed. Du e to the limitation in capabilities of conventional tugs, new tug typ es have been introduced such as tugs with azimuth propulsion. Also, VS tugs have ' for m any years been used for towing on a line. When more than two tugs are used during berthing the forward and aft tug will usually stay on the towing line to control approach speed towards a berth while the other tugs .push at the ship's side.
Figure 3.73 Combination ofdifferent assisting methods. Raxrsetractor tugs orASD-tugs awngside and ona lineaft.A conventional tugforward. A good configurationJor steering and, inparticular, when only a short stoppingdistance is available. Nearer the berth: one ofthe tugs alongside has toshift totheother side topwh
3.2.2 Relationship between type of tug and assisting method . As can be seen, th ere is a relatio nship between type
Tugs tawing on a line during approach towards a berth and push-pull while mooring
Thi s assisting meth od is becoming common practice in ports where towing on a line is carried out with highly manoeuvrable tugs such as tractor, rever se-tractor or ASD-tugs (see figur e 3.12).The more familiar pilots and tug captains becom e with the capabilities of these tugs,
of tug an d assisting meth od used. An essentia l factor is whether a tug sho uld be suitable to operate at a ship's side, tow on a line, or bo th . For the attentive reader it will also be clear that th e most suitable tugs are not always available or used. In the ports ofJapan, Taiwan and Hong Kong there is one assisting method an d mainl y one type of tug.The TUG USE IN PORT 37
reverse-tractor tug with its omnidirectional propulsion at the stern is well suited to operate the assisting method in use - on a line at a ship's stern and alongside at the forward shoulder. ASD-tugs are, however, also used for this inethod. It is anticipated that for these ports the reverse-tractor tug is the type that will usually be ordered in the future. There is often a steady development towards a particular tug type. For instance, twenty years ago there were still several VS tugs in the Port of Yokohama. This type has now almost totally been replaced by the reverse-tractor type. In Europe towing on a line is general practice,
Originallyjust with conventional tugs but now for many years with VS tractor tugs too. Due to the limitations of conventional tugs, various tug types with omnidirectional propulsion are increasingly being used, resulting in a change to more flexible assisting methods. This is the case in many other ports where originally mainly conventional tugs were used. In the USA tugs operate at a ship's side most of the time, and for many years the conventional tug was practically the only type to be found. The limited manoeuvrability and low astern power of these tugs is partly compensated for by the use of extra towlines, installation of high engine power, specific propeller/ rudder configurations and/or specific assisting methods. In many ports of the USA and Canada there is a tendency towards the use of more flexible tug types tractor tugs as well as reverse-tractor or ASD~tugs ..As in many ports elsewhere, conventional tugs will
nevertheless continue to be built in the future. In Australian, New Zealand and South African ports tugs mainly operate at a ship's side. The majority of the tug fleet already consists of those with omnidirectional propulsion and new buildings will mainly comprise this type. .
The increasing variation in tug types offers an opportunity to select the .most suitable tug for a port, taking into account port particulars, existing assisting methods and future developments in port and shipping. (see also section 4.3.4: Towing on a line compared with operating at a ship's side).
3.3
Tug assistance in ice
to do so. But all an icebreaker and tugs can do before a ship's arrival is to break the ice. They cannot completely remove ice from a berth, so certain procedures have to be followed for berthing and unberthing. Depending on a ship's size, strength and engine power, berth location and ice conditions, ships may berth or unberth with or without tug assistance. How tugs can be used during berthing and unberthing in ice is considered in this section. Further information about types of ice and pilotage in ice can be found in books mentioned in the references.
Mooring in icy conditions is usually time consuming. Each port has its own method of assistance in ice conditions. The methods discussed here are based on experience in one of the larger Baltic ports, where shipping is impeded by ice for several months each year. Methods in other ice ports may not differ greatly. 3.3.2 'IYPes of ship for manoeuvring in ice As mentioned before, ships may berth or unberth in ice with or without tug assistance. It depends on the size of ships, strength and engine power, berth location and ice conditions. Regardless of a ship's size, strength and engine power, not all vessels can pass independently through ice owing to their construction and/or loading condition. A vessel operating in ice should be so ballasted and trimmed that the propeller and rudder are completely submerged. If this cannot be done and the propeller blades are exposed above the water or are just under the surface, the risk of damage due to propellers striking the ice is greatly increased. Such vessels and other vessels which may damage their propellers or rudders when they have sternway and/ or when a ship's engine is working astern and light draft vessels with bronze propellers which cannot be ballasted or trimmed sufficiently require tug assistance.
With respect to berthing procedures ships can be divided into two main groups: Ships that can work with their engines on Dead Slow on a spring line, without the danger of parting: e.g. small vessels and ships with controllable pitch propellers. Ships with large engines, high starting power and high propeller thrust at minimum propeller revolutions, not able to work at Dead Slow without parting the spring line, even when a double line is used.
3.3.1 Introduction
3.3.3 Preparation before berthing or unberthing
During winter months, shipping traffic to and from several ports in the world is impeded by ice. Ports are kept open as long as possible by icebreakers so that ships can be berthed. When ice is not too thick, ships themselves may be able to break it. In other cases an icebreaker, if available, or tugs otherwise, are required
Before mooring, a berth should be prepared by an icebreaker or by tugs when ice is too thick for the ship itself. Ice should be broken near the berth and an approach route towards the berth should be made. Prior to departure ice should be broken around a ship and a departure route should be made.
38 THE NAUTICAL INSTITUTE
3.3.4 Tugs and tug as sistance The way ships are handl ed by tugs in ice conditions depends largely on the type of tug. Tugs need to h e adapted to work in ice conditions. Th ose with light draft and prope llers fitted in no zzles have ve ry limited
VS tugs. Tugs with pr op ellers in no zzles. In addition, full scale trials were carried out in 1984 in Finland with two ice-going tugs, one fitted with an ope n pr opeller and the other with a steerable nozzle, to
capabilities, b ecause whe n they are moving astern the
investigate thei r perform ance in ice conditions. D uring
nozzles im mediately fill with ice. Even with tug engines on ahead ice can fill the nozzles. Wh en this happens the tug should imm ediately be stopped and the nozzles cleared by repeatedly reve rsing propeller thrust. That is why this type of tug, and other tugs having prohl em s in ice, should not tow on a line. Th e assisted vessel might not react fast enough and/ or not be abl e to sto p im me diately to avoid dang er of collision or worse.
a twenty hour test the nozzle of the latter tug was blocked twelve tim es and the tug had to be stopped each time.
For these tugs in part icul ar, but also in gene ral, towing on a lin e in ice co nditions is not wi thout risk, as
explaine d later.Towing on a line is only acceptable wh en a ship is moving at a very controlled low speed on a straight course or when taking easy bend s in a channel or river and during berthing or unb erthing ope rations. Assistance in ice conditions during arrival and departur e is then carried out mainl y by pu shing and includes br eaking th e ice and swee ping away th e ice from between ship and berth. Without the help of tugs it is almost impossi ble, in mo st cases, to remove ice from be twee n a ship an d berth. Whil e pr ep aring a berth location, tugs often work very close to the do ckside. Some objects may stick out or overhang, so tug side s should be clear of overhanging fend ers, etc. Tugs sho uld, of cours e, always be very careful when working between a ship and the dockside. With resp ect to tug towi ng wires or ropes, the y sho uld retain their strength in low temperatures but should nev er be allowed int o icy water because it will then be very hard to handle them . The most reliable tugs in ice conditions are nonmal ice strengthened conve ntional tugs with open propellers. Twin screw tugs ar e pr eferable because of their b etter mano euvring properties.
Propellers and rudders may have ice pro tection and nozzles may b e fitted with protection bars or ice knives fore and aft of the nozzle. Alth ough nozzle construc tion itself may be adapted to ice conditions, in particular shallow draft tugs with nozzles are very limited in their performance when operating in ice, due to the fact that nozzles ar e often blo cked with ice. This does not mean however th at thi s ty pe of tug is worthless in th ose conditions . They can create an effective surface stream for moving ice in situations as explained later. Deep draft tugs are more reliable during towing operations. Based on expe rience gain ed in some of the larger ice ports, the following tug types are not very suitable for servic e in ice co nditions:
Having said that, some tugs with azimuth prop ellers in nozzles that have to ope rate in ice conditions have
bee n built recently e.g. for Finni sh and Danish owner s. Performance in ice of tugs with azimuth thru sters in nozzles can be improved by a proper design such as adequate clearance between the hull and the thr usters and by short reaction times for pitch change s or for turn ing the thrusters ad equ ately to get the ice out as quickly as possible when they are blocked. 3.4.4 Berthing in ice A berth should be approache d at a small ang le. As soon as the forward spring is secured the engine should be set to Dead Slow Ahead. Prop eller rev olutions or pr op eller pitch sho uld b e incr eased gradually, jus t avoiding breaki ng the spri ng. It is best to double the spring an d the rudder should be used to swing th e stern of a vessel in and out and away from the docksid e. Th e water flow caused by the propeller will force ice out from between the ship and the dockside and wash it away astern of the ship. The engine should be kept run ning until the propeller wash has swept away all loose ice. The ship can then be berth ed. In this way, provided it is weak ice, it can b e rem oved co mpletely from between the ship and berth . In the case of den se and thick ice the assistance of tugs is requi red . In some cases berth location could be such that a berth can be approached parallel to the dock (see figure 3.14)_In this case ice may be pu shed away by the bow. If there is unbroken ice on the starb oard side it will push the ship towards the b erth and pr ev ent h er swinging out. Care should be taken to avoid any ice getting between ship and dock. It may be necessary to
Figure 3.14 Shipapproad!es the herth nearly parallel to tk dock. Ice ispushedaway by tk how. TMship ;, pressed towards tlu herth by unlrroken iceonthe starboardside TU G USE IN PORT 39
move the ship forward and astern a few times to move the ice out or to press the ice togethe r betwee n ship and dock. Th is can only be done in the case of young and weak ice.
Sometimes, app roaching parallel to the dock may not be possible due to the prese nce oflarge pans of ice or dense, thick ice directly in the ship's track. Othe r methods should then be adop ted such as the use of tugs. Several procedures for the use of tugs in ice during an approach toward s a berth while berthin g or unbe rthing are n ow considered.
In general, while approaching a berth in ice, the bow of the vessel should be kept as close as possible to the berth with the assistance of a tug pushing at the bow (see figure 3.15A , B). Th e ice b etween the bow and the dock will tend to push the bow aside. After the forward spring has been secured the tug can break the ice outside the ship and the n wash th e ice away from between the ship and the dock (see figure 3.15 C, D). The ship itself
Figure 3.76 Tug sweeping ice awayfrom between ship and dock
. -
---- .,..
.
can swing its stern in and out by rudder action and use
of th e engine, as explained.
Figure 3. 77 Mooring in icewhen some 30m free oerth is
availahlt infron t of the bowposition
Figure 3.75 Tug assistance in ice duringapproach to the berth andwhile mooring
Sweeping ice away from round the bow area can also be done effectively by a tug just ahea d of the ship (see figure 3.16). With its stern directed towards the ship's bo w, the tug can sweep ice away by putting its engines ahead . In this case the ship should not pass any head lines, which would prevent the tug working in this way. Since ice at the bow is usually squeeze d between bow and dock, getting it out is very difficult, Good results can be achieved when there are 20-30 metres of free berth ahead of a ship's planned position. The ship should approach its b erth ahead of th e planned position (position I of figure 3.17). Breaking ice at the out er side of th e ship and sweeping ice away from b etween the ship and dock are then carried out. The ship can then be brought alongside and mov ed astern while th e tug is con stantly pushing the bow toward s the dock. 40 THE NAUTICAL INSTITUTE
Figure 3.t8 Combination oftugandbow thruster whilt mooring
A bow thru ster can also be very effective in sweeping ice away (see figure 3.18). A ship sho uld approach the berth at an angle. After the forward springs and head lines are ashore, the stern is take n as far as possible out by rudder and ship's engine. The bow thruster should then be set to take the bow off in order to create a water flow between ship and dock. Th e bow sho uld be held to the dockside by the ship's ropes and by the pushing tug. Th e water flow of the bow thruster will swee p ice away from between the ship and dock. Anoth er method by which good results are obtaine d is moving the ship astern towards the berth to moor
with its starboard side alongside (see figure 3.19). After approaching the berth at a smal l angle and securing the back spring, the engine should be set for astern. Th e pro peller stream is normally very strong and will move the ice be tween the ship and dock quick ly in the direction of the bow. The bow should be swung in and out by tug or b ow thru ster. Thi s method is used and suitable for large r vessels, as prop eller thru st astern is lower than on ah ead and consequ ently the tension in the sp ring line(s) will be less.
Figure 3.27 Ship approaching the berth altern. OMaft tug secured. Occasional bursts ahead onthe engine blow away the ice
With large ships, good results in removing ice from between ship and berth are someti mes obtained with two tugs working stem to stem . These two tugs, moving togethe r forward an d astern between the ship and berth, sweep ice away. The safety of these tugs is ensured by an additional three tugs keeping the ship in position as sho wn in figure 3.22. Obviously, a large nu mb er of tugs Figure 3.19 Good results when approaching the berth altern and mooringstarboard side alongside
These be rthing pr ocedures wh er eby a ship uses engine and spring lines is not suitable for ships with large engines and high starting power and!or high power on Dead Slow. All operations in ice with these ships are norm ally carried out by tugs. After approaching the berth at a small angle, a spring line and head line are made fast forward (see figure 3.20). O ne stern tug on a line is used to take the stern from the berth and a second tug is used for pushing th e stern towards the berth. Thi s tug will also clear the ice. Propeller wash is not used. Berthing will, in gene ral, take a lon g time.
is required in this case.
Figure 3.22 Two tugs stem to sum dearing ice betwem shipand berthwhit. ather tugs keep theshipin position
3.4.5 . Unberthing in ice Before unb erthin g, tugs shoul d br eak ice around the ship and in areas of about 20·40 metres distan ce from the bow and stern .
Figure 3.20 1Ug assistance when mooring in ice with ships and powerful engines
In some cases, wh en po ssible.ft is better to appr oach the berth astern with a stern tug towing on a line (see figure 3.21). By giving short 'kicks ahea d' on the ship's engine to stop the vessel, ice will be pushed away from the dock in the dir ection of ship's move me nt
Some vessels can be taken off the berth by the stern with the assistance of a stem tug towing on a line (see figur e 3.23). At the bow the ice between bow an d dock will pr event the ship from coming too close to the berth. In addition, the stem tug "ill drift the ice between the ship an d dock, which again prevents the ship from coming too close to the dock whe n moving astern. Som etim es it may be necessary to unberth the ship bow first (see figure 3.24). A second tug may th en be need ed to br eak ice ne ar the stern and to prevent the stern from coming too close to the berth . Someti mes even the assistance of a third tug may be req uir ed to crush ice at the outer side of the ship. TUG USE IN PORT 41
Figure 3.23 Ship ofmedium sb.e departing. Before departure tugs have broken ice around he.'in areas some 20-4Om from bow andstem
Figure 3.25 Channelthrough the ice prepared by ice breakers or strong tugs. A ship moving astern through the ice is safis t. M en the stern tngisstopped in orby ice theship can immediately be stoppedby propeller
departur e. Tugs handling the ship can assist the ship in swinging and break ice whe n necessary. 3.4.6 Safety of tugs in ice Tugs are at great risk whe n towing on a lin e through a channel in ice. As previously mentioned, when a tug
Figure 3.24 Unmooring howfirst. A stem tug is required when ice
near the stemneeds to he hroken andwhen the stern may touch the berth when thsbow ispulled off Sometimes a third tugis required to break ice alongside the vessel
When a depa rting ship has to be swung around after bein g unberthed this should be carried out in a pr epared area or channel in the ice. This area or channe l sho uld b e prepar ed by large tugs or icebreak ers pri or to
42 THE NAUTICAL INSTITUTE
has to stop du e to nozzle blockage with ice, the ship should also be stopped immedi ately. The tug may also en te r dens e ice and consequen tly lose speed very quickly. The assisted ship , therefore, sho uld always use engines with utmost care. Even the n the safety of the tug is still at risk. It is for these reasons that the safest meth od of towing on a lin e is m ovin g a ship astern (see figure 3.25). The engine should at all tim es b e ready to go ahead. When necessary, th e ship can b e stopped imme diately. Further practical and useful information regar din g navigating and manoeuvring in ice can be found in 'Marin e Towing in Ice-cover ed Waters' by Peter E. Dunderdale and in 'Ice Seam anship ' by George Q Parn ell (see References).
Chapter FOUR
TUG CAPABILITIES AND LIMITATIONS 4.1
Introduction
Now THAT VARIOUS ASSISTING ~l ETHO DS and types of tug hav e been introduced 10 the read er the more practical subject - effective shiphandhng with tugs - is addressed.
lies far forward. As soon as a ship gath ers speed the pivot poin t moves aft. O nce a ship is in a stea dy turn with rudder hard over the pivot point settles in a position approximately one third of the ship's length from the bow (see figure 4.IA).
When a ship is stopped in the water, meaning she has no spee d through the water, the effect of, let us say, a 30 tons bp tug is th e same irr espectiv e of type , assuming that the tug operates in the most effective way. Differences in tug performance mainly become apparent whe n a ship has spee d throu gh the wate r. Th e emphasis in this cha pter, therefore, is on tug performance while assisting ships und er way. Wh en considering effective shiphandling with tugs there are, apart from the essential issue of bollard pull, two very imp ortant aspects to be considered: Correct tug positioning. The right type of tug. Differ ent tug opera ting positions are consid ered in relation to their effect on a ship. The performance of different tug types are discussed, taking into account both the vari ous assisting me thods and the different tug positions relative to the ship. With respect to type of tug, specific aspects of various tug types are necessarily discussed in a fairly gen eral way, since ther e are so m any variations in de sign within each type. Reviewing them all individually goes far beyond the scope of this book.
4.2
Basic p r inciples an d d efinitions
For a good understanding of tug performance and shiphandling with tugs so me basic principl es and definitions are first considered. Th ese include the pivot point, towing poin t, pu shing point and lateral centr e of pressure, direct and indirect towing and tug stability. 4.2.1 Pivot p oint The pivot point is an imaginary floating point, situated somewhe re in the verti cal plane through stem and stern, around which a vessel turns wlren forced into a directional change. The form of the subme rged body, rud der size an d type, trim , und erkeel clearance and direction of movem ent all affect the position of the pivot point of a vessel. The exact location of the pivot point is ther efore not stationary but variable. For effective tug assistance the location of the pivot point of the vessel to be assisted is very important. It affects the choice of operating positions for the assisting tugs. When a ship is dead in the water and forward thrust is applied with port or starboard rudder, the pivot point
B
Figure 4.1 Location of 1M pivotpointfor a ship at speed SituationA: Ship turning with starboard rudder. The pivotpoint lies between how and midships Situation B: A tug ir pushingforward. Althaughthepivot point lies further aft, the effidforwa rd is la» because of theopposing hydrodynamuforces also centredforward. When stasboard rudder ir also applied thepivotpoint movesfUrlkr forward Situation C: A tug ir pushing aft. The Iaural resistanceforward contributes to 1M swing. Thepivotpoint liesfarforward,partieu"'rly when starboard rudder ir also applied
For a good understanding, figure 4.1 requir es a little expl anation . In this figure three ships ar e shown with differ ent forces working on the ships. A force applied to a ship, for instan ce a tug force or a rudder force, gives a transverse force and a turning moment, resulting in a lateral velocity and a rate of turn. The arr ow V is the direction ship's centre of gravity (G) may move as a result of the lateral velocity caused by the rudder force or tug force, an d th e forward velocity of the ship. The lat eral movement of th e ship is op pos ed b y th e hydrodynamic forces centred forward on the ship having headway, which also creates a turning moment. Thi s turning mom ent opposes (situation B) or assists (situa tion C) the turning moments created by th e tugs. The location of the pivot point (PPj results from the motio n of th e sh ip cansed by the vari ou s forces mentioned working on the ship. TUG USE IN PORT 43
Tem a ver com a estabilidade direcional Com grande boca
Beam y full bodied ships have a sma ller turning Estreito diameter and a furth er aft pivot point than slen der ships. When a ship is down by the he ad turn ing diameter is also less and the pivot point lies further aft than wh en on an even keel.
Turning diameter is independent of ship's spee d as long as engine pr op eller revolutions or propeller pitch match a ship's spee d but is depend ent on rudder angle applied. When in sha llow water, such as in mo st port areas, turning diameter increases considerably, due to the larger hydrodyna mic forces oppos ing the turn . A ship moving astern has its pivot point som ewhere between stern and midships when turning, e.g. by use of a bow thruster. The exac t position of the pivot point, therefore, is different for each individual ship and ship condition.
The pivot point also changes position wh en, in addition to rudder for ce, othe r forces such as bow thru ster or push/pull forces from an extern al origin, such as tugs, are appli ed . When, in order to assist a ship unde r speed and in a turn, a tug starts pushing at the bow in the direction of the turn, the pivot point moves aft. This is because the ship tends to turn around a point which lies further aft than when only rudder force is app lied . Although the lever arm of tug force would be rather long the effect is not very pronounced, so there is another aspect to be con sidered. As explained earlier, a tug pushing for ward tri es to move the bow to starbo ard, say. This creates an opposing hydrodynamic force, also centred forw ard (see figure 4.1B). The hydrodynamic moment counteracts the turning mom ent exercised by the tug. The effect of the pushing tug is very small. This is also one of the reasons why the effect of a bow thruster is small on a ship making slow to
[r-
; - . - - - ~ "?1
1::O~ :-·· · - ·b --- -
: <,
A
moderate speed ahead. In addition, the tug's underwater resistance counte racts the turn.
It should however be noticed that the effect of the forward tug differs with ship's hull form, dr aft and trim . For conve ntional ship form s, on eve n keel in de ep or sha llow water, the opposing hydrodynamic force is ind eed centre d forward, as m entio ned in 'Performa nc e
and effective ness of omni-direc tional stern drive tugs' (see References). When, for instance, taking a tank er in b all ast an d trim m ed by th e stern, the opp osing hydrodynamic force is centre d much more aft, resulting in a mu ch larger effect of the pu shin g tug forward. When a tug starts pu shing a ship underway at a po sition aft, the piv ot point shifts forward. The pushing force has a long lever arm and the lateral resistance forward then contributes to the swing (see figur e 4.1C). It is evident that the furth er forward and/ or aft of the pivot point that tug forces are exe rted on a ship, the longer the lever arm and hence the more effective the assistance will be. A ship dead in the water (see figure 4.2A) with one tug pu shing (or pulling) forward and one with th e same bollard pull, pu shing (or pulling) aft, pivots around its midship s when on even keel. For th e same size of vessel and same conditi ons, rate of turn depends on the tug's
bollard pull and on the lever arms between tugs. The longer the lever arm the larger the turning effect of the tugs. Wh en a tug pushes at th e bow or stern of a ship that is stopped in the water, the ship turns aro und a point located approximately a shi p's width from the stern or bow respectiv ely (see figure 4.2B). Other forces of externalorigin that affect the po sition of the pivot point ar e wind and current. In port areas, wind and current may var y in speed and direction depending on locati on . Relative wind and curr en t directi ons may also vary during a transit to or from a
berth due to changes in a ship's h eading. For instan ce, wh en en tering a h arbour basin fro m a riv er the current
gradually decreases but als o changes in rel ative direction . As a result, the influence of wind and current on a ship fluctuate . Dep ending on the angl e of attack and point of appli cation, wind and cur rent may decrease or 'in crease the rate of turn, moving the pivot point furth er forward or aft, or may have onl y a sideways effect. 4.2.2 Towing point, pushing point and lateral centre of pressure. Direct towing and indirect towing. Skegs
Figure 4.2 Location of the pivotpointin aship with$0 speed Situation A: Tugsofequal power pushinglpullingforward andaft. The pivotpointlies amidships. The tugs towing on a line have a longer lever andso a larger effict SituationB: Forward tugpushing; the pivot point lies far aft. When an after tug is pushing, thepivotpoint lies farforward
44 THE NAUTICAL INSTITUTE
The relative po sition s of the centres of thr ee different resultant forces ar e mainly resp on sible for a tug 's performance. These are centre of thrust, the tow or pu shing point and the lateral centre of pressure of the in co m ing water flow. In particul ar, th e mutual relationships between towing or pu shing point, centre
of thrust and centre of pressure affect not onl y the effectiveness but also the safety of a tug.
later are m erel y an in dication and are base d on
The towing point .For tugs towing on a line, th e towing hook or towing "linch is not necessarily the tov..i ng point. Th e towing point is that point from where the line goes in a straight line from the tug toward s the ship. For tugs pu shing at a ship's side the contact point or pu shin g point is of im por tance . Befo re discu ssing the cap abilities and limitations of different tug types the towing and pushing point in relation to the location of propulsion and centre
Wh en w ater flow towa rds a tug comes from abeam, caused either by crosswise movement of a tug through the water or by a current at right angles, the centre of pressure generally lies behind midships in a positi on about 0·3 to 0·4 x LW1. from aft. For conventional tugs it is prob ably more often in the vicinity of 0·3 x LW1. from aft and for tractor tugs closer to 0·4 x 1.W L from aft. Reverse-tractor tugs and ASD·tugs may have a more forward lyi ng centre of pr essure, dep ending on the hull design.
of pressure are co nsidere d.
The lateral centre ofpressure The lateral centre of pressure is a non stationary point. Its location depends on the underwater hull form including appe ndages such as rudder and pro pellers, on the trim of the tug and the angl e of attack of the incoming water flow. The influ enc e of rudder and propellers on the location of the centre of pressure seems to b e rath er high .
Tractor tugs and especially VS tugs hav e a large skeg aft, resulting in an aft lying location of the centre of pressure.
Incoming water flow exerts a force on the tug. The point of application of this force is the lateral centre of pressure . The dir ection and magnitud e of the for ce dep ends on the underwater lateral plane and shape, the angle of attack, the und er keel clearance and on the speed squared. Speed, therefore, is a dominant factor. The exact location of the lateral centre of pr essure and the magnitude and dir ecti on of the resultant force created by the incoming water flow for different angles of attack and speeds can best b e determin ed in a towing tank. The locations of the cen tre of pr essure mention ed
observations and information e.g. from Voith.
When a tug turn s with its bow into the direction of wate r flow, the centre of pressure moves forw ard, The
smaller the angle between incoming water flow and tug's head ing the more forward the centre of pressure lies. For conventi onal an d tractor tugs the centre of pressur e does not generally move forward of amidships (0.5 x 1.W1.). Reverse-tract or tugs an d ASD-tugs m ay expe rience a position of centre of pressure forward of
midships with a forward incoming water flow. When a tug is turnIng with the stem into the water flow the centre of pressure moves aft an d with an acute angle of incom ing water flow will lie far aft. Figure 4.3 shows a tug moving ahead, towing on a line, assisting a ship und er speed . The resultant for ce created by incoming water flow is force F, assume d to be centred approximately near ami dships . Force F can be r esolved into lift force 1. and d rag force D, comparable with the lift and dr ag forces on rudders or aeroplane wings. Lift force 1. gives an additi on al force on the towlin e and drag force D has to be overcome by the tug' s thrust. Towing point T lies a little behind th e ce n tre of pressure. T h e for ce in th e to wlin e in combination with force L creates a co unte r-clockwise turning m oment.
L
F "" Resultant hydrodynunic force, on tugbulland appendages L = Lift Icece D = Drag force C = Lateral centre of pressure T - Towing point Ps = Location of propulsion at stem Pt ... Locencn of propulsion fot ltact ,n tuV
Figure 4.3 Forces createdonassistingtug, moving ahead
Consider two locations of propulsion position Ps for stern driven tugs, a conventional tug for exam ple, and position Pt for tractor tugs. The smaller the distance b etween T an d C the smaller is the turnin g moment. Thus less steering power, b y eit he r rudd er d efl ecti on o r om n idirec tio n al pr op ellers, is ne ede d t o counteract that turning mom ent. Conseque ntly, more engine pow er is avai lable for towing. If propulsion is located aft at Ps, starboard rudde r is need ed, giving a little m ore drag but also an additi onal force in the towline. If propulsion is located forward (Pt) then sideways steering power is ne eded, but in the opposite dir ection. This consequently decreases the towline force. With increasing speed, force F increases
and consequ ently lift force 1.. The higher the sp eed th e m or e stee r in g effo r t is ne eded. Th erefore, the high er the speed the larger the TUG USE IN PORT 45
tug would increase its effectiveness as a forward tug, However, this would have consequenc es for its effectiveness as stern tug wh en operating .in th e indirect mode whereby use is mad e of the hydrodynamic forces on the tug 's hull. Ther efore a compromise has often to b e found for the location of the towin g p oint an d also for the underwater profile of a tug.
, F
Figure 4.4 Forces createdon assisting tug; moving astern
difference in towline forces between a conventional and tractor tug. As a forward tug the tractor tug is more effective if it is pos sible to operate stern first. Heel inclinação
Towline forces also create list. Considering the direction of steering forces it is evident that with the propulsion located in position Ps the sideways steering forces increase the tug 's list, while with propulsion located in Pt steering forces counteract the list caused by the towline force . When an ASD -tug is operating like a conventional tug its high steering forces result in larger heeling forces. This is also due to the fact that the centre of pressure of this tug type lies generally somewhat further forward, resulting in a larger turning moment to overcome. The larger heeling moment is more or less compensated for by 'the large beam of this tug type .
In figur e 4.4 th e tu g is m oving astern through the water. The centre of pressur e lies much furth er aft e. g. at l ocation C for con venti onal tugs as well as for tractor tugs. Tractor tugs are considered first. The towing point T is very dangerous, not only b ecause of the large heeling moment caused by the hydrodynamic force on the tug's hull, but also because large crosswise steering forc es (at Pt) have to be exerted by the tug in ord er to compensate for the turning moment created by th e incoming water flow, giving additional forc es in the towline and additional heeling forces. At higher speeds and /or too large angl es of attack of incoming water flow the resulting he eling forces may cause capsizing of the tug. The large vertical distance between the propulsion units and towing point also contributes to the high he eling moment. Therefore although towline forc es are high for tractor tugs it is much safer to loc ate the towing point aft at a small distance abaft C, the centre of pressure for smaller angles of atta ck. (In VS tractor tugs the towing point lies gen erally just above the middle of , the skeg.) The tug then comes in line with the towline when its engine s are stopped and very little steering power is needed to keep the tug in the most effective position when the indirect towing method is applied.
point on tractor tugs is located further aft for safety reasons and for better performance as stern tug . This is explained later. The consequence of the further aft
Neither do conventional tugs operate as shown in figure 4.4 becaus e with higher speeds it is almost impossible to steer the tug safely and is th erefore very dangerous. If the angle of attack increases, the increase in towline forces might cause the tug to capsize. At very low speeds conventional tugs often operate broadside, for instance as a forward tug steering a ship whi ch is
towing point on a tractor tug is an even less effective
moving astern or as a stem tug steering a ship moving
tug as forward tug. More sideways steering power is needed to counteract the larger anticlockwise turning
ahead. Especially on single screw tugs, this can only be done with a gob rope or by passing th e towline through a fairlead situated aft, as is the case on som e cornbitugs. The gob rope system is dealt with in more detail in Chapter 7. Using a gob rope the towing point can be shifted to a position somewh ere between the after end of the tug and the towing bitt or winch. By shifting the towing point from TI to T2 (see figure 4.5), the tug can stay broadside on and steer the ship by moving ahead or astern using the tug' s engine. By shifting the towing point to a po sition at the stern of the tug, the tug can be pulled astern by a vessel without the danger of capsizing. The tug can then use its engin e to control the ship' s speed. Twin screw tugs often use the propellers instead of a gob rope to keep the tug in the position as indicated in figure 4.5.
Although the towline position discussed here is the most effective for both conventional and tractor tugs when operating as a forward tug on a line, the towing
moment, resulting in a further decrease in towline force.
By giving more engine power in order to achieve the same towline force as a conventional tug would exert, the tug comes more in lin e with the towline, resulting in higher turning moment and drag force to be overcome. At higher speeds drag force may become so large that a tug is unable to react SUfficiently to the force . and swings around. The consequence is that when working forward a conventional tug is more effective when towing on a line than a tractor tug. The better the omnidirectional thrust performance of a tractor tug the more effective it will be. Reducing the underwater resistanc e of a tractor 46 THE NAUTICAL INSTITUTE
I
I
seen in some performance diagrams in section 4.3.2. As soon as a ship starts turning she gets a drift angle and speed of ship's stem, being at the outside of the
/
tum, increases initially, so tug's speed has to increase,
pi;
resulting in even higher steering forces. The indirect towing method is further dealt with in Chapter 9 - Escorting.
-"y*T2=----j T1 I
From this brief explanation of direct and indirect towing it is apparent that the locations of the centre of pressure and towing poin t are very critical. A more forward lying towing point in a tractor tug results in higher towline forces, but the safety of operations and
I
FrguTt 4.5 Tug WIlTking onagob rope Ship has a "try low speed ahead: Tug can steer the bygoing ahead orastern. on the engine. Corwentional twin SCTtW tugs don't always need a gob rope; they can mate a (l)Uple by the prop,llm to stay broadside
.,,,,1
Direct and indirect towing method The direct and indirect towing methods are explained in figure 4.8 (overleaf). P is the location of the propulsion, C of the centre of pressure and T is the towing point.
The direct towing method is carried out by an after tug on a line at low ship speeds . The tug pulls in the required direction, either to give steering assistance and!
or to control the ship's speed. Tractor tugs assist with their stem directed towards the sterJof the assisted ship and ASD/reverse-tractor types of tug assist with their bow towards the stem. Whether tractor tugs or ASDI reverse-tractor tugs are more effective in steering control
depends on the relation between the distance P'T and CoT, the tug's engine power and thrust performance in the pulling direction, but also on the tug's underwater plane . The smaller the distance CT in relation to PT the better the tug's performance in the direct towing mode.
Plwro: SJutland Islo.nd.r Caunci1
FiguTt 4.6 Swivelfairlead on the afier endofa tug's d,,* for the gob rope
The indirect towing method is applied by an after tug at speeds higher than five to six knots. With the indirect towing method, the tug makes use of the hydrodynamic forces created by incoming water flow on the tug's skeg and/or underwater body. The aft lying towing point of the tractor tug, and consequently the small distance between towing point (T) and centre of pressure (C), implies that only a little crosswise steering power of a tug is needed to keep the tug in the most effective position to exert the highest steering forces to the assisted ship . The ASD-tug/reverse-tractor tug has a larger distance between the towing point (T) and centre of pressure (C). Consequently, more crosswise power is needed to keep the tug in the most effective position, thus decreasing towline force . Photo:AutWr
In the indirect towing mode tugs can give high initial steering forces to a ship underway at speed, as can be
Figur« 4.7 The 11Jrg' fairlead is the oft lying towing point on a VS tractor tug
TUG USE IN PORT 47
bulbous bow, can be found on a number of ASD tugs, which also brings the centre of pressure more forward .
1" '"iI'
Ii IIi r I I
Pushingpoint
H~HH
Wh en pushing at a ship's side, the larger the distance between the pr opul sion unit(s) (P) and the pushing point (Pu) in relation to the distance between the centre of pr essure (C) and the pushing point (Pu), the better the tug can work at right angles (see figure 4.17).
-- ::= =_. - -=:::::
®
®
S1c£gs and their ejfed The tug's unde rwater fonn should be such that the tug can perform in the best possible way. Skegs can contribute to a tug's perfo nnance and tugs are often designed with some sort of skeg.
I I
I
I I ,
I I I •
: I l Ii
II II II iI
........,;;:.-
"' ''J. J.lI I II
\)'
J., ,I
_ '.--- --
! ' . ~ --
--
,/
~
I I
~
.
.
"
~ ,
~
I ,' ...,
IT- - . --.:...1_ J o
(j)
~
® Figure 4.8 Direct and indirect towing methods Top: Direct TowingMetJwd - A: Tractor tug B: ASDIReverse-tractor tug Position 1.' Steering and retarding Position 2: Retarding Bottom: Indirect TowingMetlwd - A: Tractor tug B: ASDIRevtrse·traclor tug Position 7: Steering and retarding Position 2: Retarding
as a result performance decreases. A more forward lying centre of pressure in AS D/reverse-tractor tugs doe s not affect tug,safety but increases the tug's perform an ce as a stern tug. To minimise steering effort in keeping a VS tug in line with an escorted vesse l when no assistanc e is required, a second to wing point is installed at the after end of some VS tugs, which pins the tug under the towline an d reduces the steering effort required. Wh en steering assistance is,:required then the original towing point more forward ' is used again , which should be po ssible with out releasing th.e towline. In ASD-tugs, specific designs are used to brin g the centre of pressure more forward e.g. in the USA ASD tug Kinsman Hawk. This tug is designed with a deep forefoot which results in a more forward po sition of the centre of pressure and the stern is cut away significan tly to provide a clean flow to the azimuth propellers an d to push th e tug's ce ntre of pressure forward as well. Forward skegs at the bow, or in combination with a 48 THE NAUTICAL INSTITUTE
A pure harbour tug should in general be most effective at ship speeds below six to seven knots, when the assisted ship is slowing down and has to stop its mai n engine, losing its controllability to a lar ge extent and during turning, berthing and unberthing ope rations when hardly any use can be ma de of the ship's own manoeuvring devices, except for bow and stern thru sters (see paragraph 5.1). Such a har bour tug should be able to apply the high est possible tow in g forces in all the required directions and with a short response time. High pushing forces may be needed with the tug operating at right angles to the ship still having speed. A low underwater resistance is therefore needed. On the other hand, a tug may have to operate at higher spee ds, and escorting of ships may be one of the tug tasks. Then a well de signed under water body, which may include a skeg, plays an important role in generating high towing forces in the indi rect mode by making use of the hydrodynamic forces working on the tug's hull. As can be seen a skeg m ay be effective for one task, bu t ineffective for other tasks. With regard to skegs it sho uld therefore be well cons idered what is expected from a tug. Ther e is a large variety of skegs. Mainly the following skegs can be found on tugs, of which some have already been m entioned whe n discussing tug types : a) The skeg on tracto r tugs . This type of skeg provides better course stability when free-sailing ahead (with skeg aft). It generates additional towing forces when operating as stern tug in the indirect towing mode because it increas es the tug's lateral underwater area and brings the centre of pr essure more aft, closer to the towing point. The skeg may have a specific form to generate the highest possible lift forces. b) An aft skeg on tugs not be ing tractor tugs: A vertical fin attache d to the tug's und erwater hu ll in the centreline of the after sectio n at some distance before
Tugs with azimuth propellers may heel over appreciably if thrust is suddenly applied athwartships. These tugs tend to be powerful with respect to their size and the deeply immersed point of application of thrust, implying a long heeling lever, results in a large heeling moment. Whether the indirect or direct towing mode is applied this heeling moment counteracts the heeling moment created by towline force. When conventional tugs tow on Photo:]M. Voith GmbH, Gmruzny a line the heeling moment caused by transverse Figure 4.9 VStugoperating in the indirect towing mode steering thrust enlarges the heeling moment created by towline force, as explained when the propellers, to give the tug a better course stability discussing lateral centre of pressure. The same when free-sailing ahead. happens when ASD-tugs operate like conventional tugs c} A flat vertical skeg, or box keel, in the centreline of while towing on a line. In figure 4.10 heeling forces due several ASD-tugs and reverse-tractor tugs, which to towline force, lateral resistance and steering force are extends for some distance before the propellers to shown for a conventional tug. the forefoot. It provides better course stability when All these aspects should be taken into account when free-sailing ahead and often, depending on skeg fonn, particularly astern. It generates additional towing tug stability requirements are considered. Means of forces when operating as stern tug in the indirect increasing stability and reducing the heeling effects of towing mode and when ASD-tugs operate as external forces on a tug include the following: conventional tugs at a ship having speed. d) Skeg at the bow of an ASD or reverse-tractor tug. High GM and good dynamic stability Such a skeg improves the 'course stability when freeGood static and dynamic stability is required because of the high dynamic forces a tug experiences. A tug sailing astern (not ahead) and increases a tug's perfonnance when operating as stern tug in the needs considerable residual dynamic stability when, due to a sudden force, she heels over considerably. The tug's indirect mode and to some extent as bow tug when beam has a large influence on its GM (IuitialMetacentric operating bow-to-bow at a ship having headway. Height). Making a tug beamier results in a larger GM and righting moment, assuming all other factors Combinations of the skegs mentioned can be found influencing its stability are unchanged. The length/width as well, for instance of skeg types c and d. ratio of harbour tugs is decreasing and many modern tugs have a length/width ratio of between approximately When reading the following paragraphs and the 2·8:1 and 3:1. Several harbour tugs with even lower capabilities of the various tug types in the different length/width ratios have also been built, such as the USA situations it is good to consider at the same time the tractor tug Sroward (l.o.a, 30m, bp 53 tons) with a length/ possible skegs and their effects. width ratio of 2·5: 1 or the Canadian reverse-tractor tug Tiger Sun [l.o.a. 21·7m, beam 1O·7m, bp 70 tons) with a 4.2.3 Stability Operational stability, one of the basic design requirements, is of great importance for harbour tugs due to the nature of their work. Conventional tugs, when towing on a line as a forward or after tug, can experience very large athwartships towline forces. The same applies to ASD·tugs when towing on a line as a conventional tug. High towline forces can also occur when conventional tugs are operating in the way shown in figure 4.5. Tractor tugs and ASD/reverse-tractor tugs also experience high athwartships towline forces when indirect towing. At high speeds these forces can be far in excess of a tug's bollard pull. Towline forces can increase even further due to dynamic forces caused, amongst other things, by irregular engine performance and/or tug control, tug movements due to waves, and when towlines are used with too little stretch, such as steel wires.
Tow5neforoe
51
ngloroe
"it"""-""'_lateral -reslstance
Figure 4.70 Heelingftrc
TUG USE IN PORT 49
Projeções laterais
Tugs are some times designed with sponso ns, which create larger righting mom ents at smaller heeling angles.
Stability curve for tug
£
Reducing the transverse resistance ofthe hull Making the lateral area smaller allows a tug to be pulled more easily through the water instea d of rolling
.
:, a. :
d
c
G_
---_.
Z ponto ??
0, 6
l.:)
~llg.W.Yer cen.!.:_
_ _
0,4 0,2
a -1"-----.-------''--r---,..L----,---'-'-..
Figurt 4.72 Tlu
gen erated and the lower centre of pressure, result in
b
0,8
N
For a good performance these tugs need a high lateral resistanc e in orde r to be able to generate high towline forces. A skeg may be added to increase lateral area (which also lowers the centre of pressure) and lateral resistan ce. The higher towline for ces that can b e
'"--"""""'lI~;....-; - - - - -
E L
o
inc reases its capability of workiog at right angles to a ship's side with a ship underway and reduces its heeling moment. For tugs making use of the un derwater body, like conventional tugs towing on a line and tugs using the ind irect towing method, this is contradictory to their required perform ance.
B
1,0
~
over. Low transve rse resistance of a tug's hull also
larger heeling angles and consequently in higher stability requirem ents. A radial hook , as shown in figure 4.11, reduces the heeling angl e considerably.
1,2 - ----- - - - - ---- "B'-- -------------
..?
tug's safety and consequ ently safety of ope rations is also shown in figure 4.12. The situation is for a specific conventional tug and a radial hook with a radius equal to half tug's width. A certain athwartships towline forc e is applied to the towing point near the centre of the tug. The towline force is su ch th at it almo st results in capsizing the tug, bec ause the maximum stability lever is only a little more than heeling lever 'B'. No safety margin is left. With a constant towlin e for ce, he eling angle is approximately 31°. In case of a radi al hook, the same towline force is appli ed initially at the same height ab ove the lateral centre of pressure. Th e heelin g lever 'A' resultingfrom thisforce de creases fast with increasing heeling angle, and in this specific case maximum heel ang le caus e d b y a con stant towlin e for c e is approxim ately 18°, with a.large safety margin left. The system itself is furth er discussed in paragraph 7.2.
Fig. 4.1/ Tlu
Reducing the height ofthe pushing point The vertical distance between the pushin g point and lateral centre of pr essure should be as small as possible io order to redu ce the heeling moment created by lateral resistan ce whe n a tug is pushing at a large angle to a ship's side.
Reducing the height ofthe towing point The height of the towing point abov e the lateral centre of pressuse should be as small as possible in order to reduce the heeling mom ent created by towline forces. If a tug is equipped with a towing winch the lead of the towlin e may be such that either it goes straight from the winch towards the ship or it passes first through a towing bitt or fairlead. In either case the height of the fixed points from where the towline leaves the tug should be as low as possibl e above the lateral centre of resistance . Using a towing arm or radial hook (see figure 4.11) or similar gear, a tug heels until the heeling moment is counteracted by the larger induced righting moment. A radial hook is a substantial improvement for tug safety and performance.
A towline with good shock absorption characteristics This is required to reduce sudden heeling momen ts caused by high peak forces in the towline: Towin g winch es can be equipped with load reducing systems, althou gh these are not suitable for narrow port areas, when such a system would slacken the towline at high load s, for instance, when the tug is close to a dock wall.
Questão de prova
50 THE NAUTICAL INSTITUTE
Tug freeboard being such that the deck edge is not immersed at too small a heeling angle According to th e form er Briti sh Department of Transport, Merchant Shipping Notice No. M.1531 ofJune 1993, thi s angle should not be less than 10° (see Appendix 2). Openings in superstructures, deckhouses and exposed machinery casings situated on the weather
deck, which provide access to spaces below deck, should be fitted with watertight doors . Such doors should be kept .closed during towingoperations. Air pipes, vents, exhausts should be designed to be as high up as possible and/or should be fitted with an automatic means of closure. The Intern ational Maritime Organization (IMO) has establishe d recommendations regar ding static stability curve requirem ents. These recommendations apply to ships in international trade over 24 metre s in length. In addition , recommendations on weather criteria have
been established for ships of 24 m etres in length and over. These apply to reserve stability with respect to wind, wind gu sts and waves. No specific recomm endations for the stability of tugs, which take into account towline force s) are given.
The IM O stability criteria and all related aspec ts are speci fied in the 2002 IMO publication "Code on Intact Stability for All Types of Ships Covered by I M O Instruments". The publi cation consists of the text of Resoluti on A .749 (18) as amen ded by resolution MSC. 75(69). National autho rities or classification societies often have their own specific regu lations or guidelines. For example, the stab ility requirements of the Un ited States Coast Guard (USCG) for towing vessels are much the same as the static stability curve requirements of the IM O. In addition, USCG requ ires tha t tugs shall either meet the static towline pull criteria or the dyn amic towline pull cr iteria. The static towline pull criteria include a required minimum G M by which no deckedge immersion will occur due to the heeling effect of de flected propeller thrus t at full h elm, taking into account the tow hook he ight above the centre of the propeller shaft. The dynamic towline pull criteria require a ce rta in residual righti ng ene rgy at the point of equilib rium of the righting and heeling arm curves. The hee ling arm curve sho uld be calcu lated on a give n formula which takes into account the deflected propeller thrust and height of towing po int.
T he American Bu r eau of Shipping gives recomme nda tions for residual dynam ic stability base d on a towline pu ll at 90° of 50% of b ollard pull for twin screw tugs with no rmal propellers an d 70% of b ollard pu ll for tugs with azimuth or cycloidal pro p ellers. Heeling arm should be taken from the top of the towin g bitt to the centre of buoyancy or for an approximation to h alf the mean draft. Othe r semi-static methods are used, allowing for a constant athwartships towline force acting on the hull , causing it to be dragge d bodily thro ugh th e water. The r equi rements of the previo usly m enti on ed British Shipping Notice are such that the minimum GM of a , tug should b e sufficient to limit the heel to an angle of deck imm ersion when being towed transversely thro ugh the water at a spee d of four knots. This results in the
following simple relationship . The GM in the worst anticipated condition should not be less than: 0·076K i.C. Where: K = 1·524 + 0·081. - 0-45 r L Length of vessel between perpendiculars r
=
(in metres) Length nf radial arm of towing hook (metres) Freeboard (metres)
=
Block coe fficient
f
CB
The effect of a rad ial towing h ook is included in this formu la. The same kind of requ irem ent can be seen in Norway where a five knot transv erse spee d with a tow of 65% of the bollard pull should be possible witho ut deck immersion.
Unfortunately, in a tug's working environment large dynamic forces far in excess of static and semi-static
val ues may be develop ed and these are almo st imp ossible to estimate accurately. When designing tugs, therefore, stability and in partic ular reserv e stability should be considered very carefully, taking into accou nt all relevant factors including type of tug, required assisting methods, propulsion system and working conditions . It is clear that good stability not only improves a tug's safety but to a large extent a tug's capabilities and performance. With respect to escort tugs, stability requirements are further discu ssed in paragraph 9.5.1.
4.3
Capabilities a n d limitations
The capabilities and limitations of different tug types are now considered, based on the two principal methods of tug assistance: Tugs towing on a line. Tugs operating at a ship's side . Fur thermore, the performance of different tug types and th e effect of tug assistance on a ship's behaviour is highlighted. Rudder tugs, mo re or less comparable to tugs operating at a ship's side but able to assist in steering to port as well as to starboard, are mainly dealt with in Chapter 9, while discu ssing escorting with no rmal harbour tugs. 4.3.1 Capabilities and limitations of tug types Good cooperation between pilot and tug captai n is indispensable for smooth, safe shiphandling with tugs. Safety applies both to the ship concerned and to the tug and her crew. Goo d cooperation is based on a good understanding of the capabilities and limitation s of the attended ship and, in pa rticular, of the assisting tugs.
TUG USE IN PORT 51
conve ntional tugs often tum their tug at the beginning of the manoeuvre round the towing point on a tight towlin e. It speeds up the manoeuvr e but is not ne c es sary and n o t adv o cated , because it results in a short
pull in the wrong direction which may ad ve rsely affec t the man oeu vr e, especially for light ships .
t Figure 4.73 Basic dijfimu;e betweentugtypes The main difference hetween types oftugwithrespect toperftrmance when towing on a line: Conventionol types oftug.,.lowingpointlocaud forward of propulsion. Iiaaor types of lug- rowing pointIocaud aft ofpropulsion
Tugs towing on a line The capabilities and limitations of tugs towing on a line are closely related to the location of the towing point and the propulsion units, as explained in section 4.2.2. That's why, in Chapter 2, tugs were classified according to these locations. Of course, a tug's manoeuvrability and stab ility are also factors of major importance whe n considering capabilities and limitations, but that applies to any situation and to any typ e of tug.
t
A tractor tug (see figure 4.14A) is less ef fec tive in giving steering assistance or crea ting sideways forces
o n a sh ip h aving spee d tha n a conventional tug. As explai ned in section 4.2.2, a tractor tug lies more in lin e with the t owlin e a n d consequ ently a relatively high er sideways resistan ce has to be overcome at the expense of effective towline pull. A conve ntional tug (see figure 4.l4B) can tum the tug arou nd th e towing point, has a lower resistan ce to over com e owing to the smaller angle of attack of the incoming water flow and can make better use of the hydrodynamic forces, all of which contribute to a more effective towline pull .
In figure 4.13 a conventional tug is shown with its propulsion aft and towing po int near midships. It could also be an ASD -tug wh en towing on a line and using the after winch or towing hook in the way conventional tugs do . The other tug shown in figure 4.13is a VS tractor tug. It may also be a tractor tug with azimuth propellers
Th e effectiveness of a conventional tug increases, depending on the angle (b), and of a tractor tug decreases with increasing ship 's spee d. The higher a shi p's speed the larger the difference in effectiveness between tractor and conve ntional tugs. The low er th e un de rwate r r esi stan ce of a tr a ct or tug an d th e b e tt er th e omnidirectional thrust performance th e h igh er th e effectiv en ess . With r espect: to thi s, it h as b een experienced that for th e same ship's spee d an azimuth tractor tug can op erate at a larg er towing angl e (a) than a VS trac tor tug and consequently can apply high er sideways and steering forces on a ship, owing to a beller thrust performance in directions other than ah ead or
or eve n a reverse-tractor tug. As can be see n, the location
aste rn.
of the propulsion an d the towing point in a tractor tug are opposite to those in a con venti onal tug. Th e co nsequences of this are discussed now.
or deli ver crosswise forces to an assisted ship to starboard as well as to port. H ow ever, the re is a
With a tra ctor tug care should be taken that, with increas ing speed, an gle (a) is not ge tting to o lar ge otherwise the tug cannot ov ercome sideways resistance any mor e and will swing around on the towlin e secured at th e aft towing point and will come alongs ide th e vessel. If the towlin e is on a quick 'release towing hook or on the winch, the line can be released by the quick release mechanism. It can be concluded that a trac tor tug forward is very limited by a ship's speed.
difference in respo nse times b etween the performance of tractor and convention al tugs. When required, a tractor tug can move easily and quickly from on e side to the other e.g. from starbo ard bow to port bow to deliver steering assistance or to ke ep the bow up into the current or wind. This is du e to its ability to deliv er side thrust from th e forward located propulsion un its. A conventional tug takes a little longer. In addition, to manoeuvr e a tug from one side to the oth er, captains of
For a convention al tug angl e (b) can be very lar ge without an y problem . A conventional tug can create large force s in the towline, even with a large towing angle (b), by in creasing angle (c). With increasing ship's Devida speed du e att enti on should be given to a tug's heading. Wh en angl e (c) between a tug's head ing and incoming water flow becomes too large the tug might not be able to come back in lin e with the assisted ship an d, as a
Forward tugstowing on a line Forward tugs towing on a line are dealt with first (see figure 4.l4A, B). Irrespective of the type of tug, a forward tug towing on a lin e can give steering assistance
52 THE NAUTICAL INSTITUTE
the tum as sho wn in figure 4.15. The effect is gr eatest at low sh ip 's spee d with n ot too heavy ships. A similar meth od - ru dd er hard over towards th e berth, engine on dead slow ahead and the for ward tug pulling off the b erth - can be applied when unberthing with just one tug. C are shou ld be ta ke n n ot to overtake th e tug . When forward tugs towing on a line give stee ring assistance, this generally results in a force vector tending to increase ship 's speed.
t
1
There is another important aspect to be aware of wh en tugs operate on a line - they often have a tendency to keep towlines tight when no assistance is required. This also has an unwanted speed in creasing effect on th e assisted vessel and should be avoided as much as possible. Pilots th erefore often order tugs to keep the towline slack when no assistance
t
is required . 1c -Indirect method for high speed above 7kn
When reverse-tractor tugs, and ASDtugs operating as reverse-tractor tugs , assist c o as a forward tug on a lin e they operate in a Figure 4.74 Comparison between tractor type tugs andconventional tugs when towing sim ilar way to a tractor tug but with the tug's on a line with aship having headway b ow direc ted towards th e ship's bow. These A: Tractor type oftug madefast asforward tug B: Conventional tug (or ASD-tug) tugs have a comparable performance to asjorwaDrd;,:g , , t ra c t o r tugs and the d iffer ence in C: Traclor type of tug asafter tug : "011oenl,,,,= type oftug asafUr tug effectiveness depends on the sam e factors consequence, athwartships towline forces may get too as mentio ned earlier when discussing the dir ect towing me thod. See also paragraph 6.3.12, section operating high. TIlls may also be the case with an ASD-tug when operating like a conventional tug. The high athwartships bow-to-bow. towline forces might overturn th e tug if the towline Stern tugs towingon a line cannot b e released in time. This is called girting, which For tugs operating as a stem tug on a lin e the situation also h ap p ens when a ship's speed is too high in relation is totally different. It depends entirely on the type of to the tug's speed or po sition. tug and ship's speed whether steering assistance can be given to bo th sides. From the po int of view of assistance It often happe ns that quic k release hooks canno t be it is also very impor tan t whe ther a stern tug can control opened in case of eme rge ncy, especially when towlin e a ship's speed. Whether th is is possible or no t dep en ds forces are ver y high an d th e towlin e, if fastene d dir ectly also on th e type of tug and ship's spee d . to th e towing h ook, h as a lar ge ve rtic al angle with the plan e of the tug deck. Towing winc hes with quick release In figure 4.14C a tractor tug is show n again. At lowe r syste ms are safe r. Nevertheless, ship's speed sho uld speeds a trac tor tug can give steering assistance by th e always be carefully contro lle d wh en tugs are towing on direct towin g m eth od (see position I a, Ib). Giving a line forward an d, as far as possible, the pilo t sho uld steering assistance in po sition Ib will not increase the close ly observe the behaviour of the tugs. ship's speed. On the contrary, in this po sition braking force s are also applied. A speed in creasing force vector When a ship's speed is very Iowa conventional tug is applied in position 10. In position la a tractor tug is can give very effective steering as sistance when less effective than the conventional tug of figure 4.14D operating as shown in positio n Ib (see also the photo (p osition I). This situation is comparable to ' that of of the tug Smit Siberie - figure 8.9). A tug's resistance forward tugs towi ng on a line, as previously discussed . creates high steering forces wi thout increas ing ship's If required, a tractor tug can easily change from posit ions speed. The tug itself uses most of its engine p ower to I to posi tion 2 for speed co ntrol or to a position to give stay free fro m a ship's hull and this results in additional towline force . steering assistan ce to port. Even at higher speeds (e.g. seven kn ots) a tug canBecause safelyof swing around from position la to position 2 owing to the aft location of th e towing With a good conventional tug forward on a line, po int. In some ports position l a, instead of position 2, sideways forces on a ship can be exe rted by applying is also used as a standby position. ru d de r whilst at th e same tim e the tug is counteracting Para velocidade acima de 3 kn: pode emborcar a memos que use a gob rope quetransferirá o towing point To aft
TUG USE IN PORT 53
kno ts, co nventio nal tugs can move from positio n 1 to a
po sition bro adside astern the ship as sh own in figure 4.5 (see also figure 7.5). The tug th en lyin g bro adside on can give stee ring assistance to both sides. Twin screw
tugs often don't need a gob rope to operate in a similar way, owing to their high er manoeuvrability. It is clear tha t at speeds abo ve about three kno ts, only steering assistance can b e given and only to one side. At very low spee ds steeri ng assistance can b e given to both side s an d a ship's spe ed can b e controlled . A conv enti onal tug is ve ry restricted in its m ovements as a stern tug owing to the location of th e towin g point. BECAUSE OF
<
At higher speeds th e indirect towing method is normally used for steering control' (see po sition Ic). Steering assistance at higher speeds can be given to po rt as well as to starboard. At the sam e time the tug is able to control the ship's speed. ASD-tugs and reverse-tractor type tugs perform in a similar way , but with the tug's bow now directed to the ship's stern. An ASD /reverse-tractor tug will generally be somewhat less effective than a tractor tug when using the indirect towing m ethod for steering assistance. The factors influencing performance and effectiveness of these tugs in comparison to tractor tugs have already been mentioned when discussing the indi rect and direct towing methods. A conventional tug can only give steering assistance to one sid e; in figure 4.l4D this is only to starboard. When giving steering assistance a conventional tug delivers longitudinal forces which may increase a ship's sp eed. Moving to a position to starboard of the ship's stern, for instance, to give steering assistance to port or to compensate for wind or current forces at that side is
impossible at speeds higher than one to two knots . At speed s over about three knots, it is dangerous to manoeuvre from position I to position 2 in order to control the ship's speed. A tug may come broadside on with too high towline forces and may capsize unless the towline is released in time by the quick release mechanism . When a tug is equipped with a gob rope winch, by which the towing point can be transferred to a position at the after end of the tug, the tug can swing around from position I to position 2 at somewhat higher speed . At very low spe eds, of not more than about thr ee 54 THE NAUTICAL INSTITUTE
When a conventional tug is work ing close to or behind a ship's stern, a ship sh ould be very careful in using its propeller or the tug migh t be overturned by propeller wash. A tractor tug and ASD/reverse-tractor tug, on the other hand, will in gen eral not be hinder ed by ship's propeller wash due to the location of the towing point near the tug's stern or bow. If working on a short towlin e, however, excessive vibration of the azimuth propellers may be experienced, du e to th e turbulence from the ship's propeller. Lengthening th e towline will redu ce this effect. A tractor tug, approaching a sh ip stern wa r ds , experiences the influence of a ship's prope ller washon the skeg. Careful steering is then required to keep the tug on a straight course. This is also the case when the tug is secured and has to stay straight behind the vessel, as mentioned while discussing direct and indirect towing methods. From the above it is clear that prior to secur ing tugs forward or aft the position of the different tug types in general and of conventional stern tug s in particular should be well considered, taking into account the forces of wind and current to be compensated, b ends to be taken, etc.
Each type of tug has several version s with varying capabilities, which should be regarded as well when positioning tugs. A twin screw conventional tug, for instance , will gen erally perform better than a single screw tug. The same applies to a conventional tug equipped with a rad ial towing arm. This will increase a tug's capabilities and safety compared to the same tug without such 'an arrangement. In addition to what has been discussed already, therefore, performance and safety of a conventional tug depend lar gely on good manoeuvrability and appropriate towing equipm ent. Also, combi-tugs with their azimuth bow thruster have better cap abilities than ordinary convention al tug s, esp ecially when a combi-tug's towing point can be shifted to an alternative pos ition far aft. The cap abilitie s of these tugs wer e explained in paragraph 2.4 . Tugs operating at a ship's side Tugs operating at a ship 's side while the ship has some speed are shown in figure 4.16. Three types are
shown - a tractor tug (which can be a VS tug or one with azimuth propulsion), an ASD/ reve rse-tractor tug and a conventional tug. Pushing mode
Wh ether one type of tug is more efficient in pushing th an an other dep end s on how well a tug can push effectively with out incteasing ship's speed. Th e bet ter a tug can work at right angles to the hull of a vesse l und erway, the more effective it is. It dep end s largely on th e ratio a:b (see figure 4.l6A ): the relati on ship between the lever of propulsion {P- PU}and the lever of hydrodynamic forces {C-PU}. The better a tug can overco me the turning mo me nt resulting fro m hydr odynamic force by the mo ment created by sideways thrust of the propulsion, th e better a tug can work at right angles to the ship and th e more power is available for pushin g. In addition, the vertical location of the centre of pressure, stability and freeboard are important factors. Tug fendering should prevent a tug sliding along a ship's hull, otherwise one or MO towlines are required.
Owing to its aft lying centre of p res sure a conventiona l tug may find it difficult to come to or remain at right ang les whe n a ship has spee d through
the water. Conventional tugs generally have a large underwater plane and an important consideration for effective pushing is steering performanc e, which is less
tha n that of tugs with omnidirection al pr opulsion systems. Dep ending on th e situat ion convention al tugs use stern lines to stay at right angles to a ship's hull when the ship gathe rs speed, as shown in figure 3.2. However, excessive spee d imp airs safety as the line may part or result in capsizing the tug. Devices increasing the steering performance of co nve ntional tugs, such as
high lift rudders and Towm aster systems, increase their pushing capab ilities. The ASD /reverse-tractor tug with its highly efficient steering propellers and the far aft lying propulsion in combination with a generally more forward lying centre of pressure is very effective at pu shing. Tug company C.H . C ates & Sons of Van couver claim s that th eir reverse-tractor tugs can deliver a 90° side push at ship speeds up to eight knots instead of the usual four kn ots for conventional tugs. Three to four kn ots is gene rally the maximum sp eed for effective pu shing b y conventional tugs, altho ugh it depends on the ir engine power and prop eller/ ru dder configuration. Tractor tugs are also mu ch mo re effec tive than conventio nal tugs due to their omnidirectional propu lsion.
if«- -b- - >;i a
:
: I
I
:
I
I
I
)oj
I !
T
A
B
Fig. 4.16 Comparison ofperftrmance oftugtypes when pushing or pulling Comparison of different tugtypes when pushingor pulling at a ship"side. The ship has headway. Locations ofthe lateral centre ofpressure are approximaud. Apartfrom the underwater resistance tugperformance depends on: a) maximum heel; b)propulrion performance - omnidirectional propulsion systems are very suitahl. owing to1M possibility ofapplyingforces in any required direction; c) ratio a:b (a = distance between propulsion andpushing ortowing point, b = distance between lateral centre ofpressure andpushing or tounng point. The kzrger the leoer a in relation tolsoerb 1M less side thrust required tok epposition and 1M more thrust avaikzblefor effective pushingorpuUing TUG USE IN PORT 55
Whether tractor tugs are ma re or less effective than ASD reverse-tractor tugs dep ends on the ratio a:b as sho wn in figur e 4.16A, th e tug's under water body, its engine power and thru st perform ance in the requir ed direction. There is ano the r aspect which determines a tug' s capability for operating at th e shi p's side, viz. th e maxi mum heeling angle. In this respect the height of the pu shing point is important. The heeling mo me nt caused hy hydrodynami c forces incr eases by the spee d squared. This is counteracted by sideways steering forces and by a tug's stability. The higher the pu shing point th e larger the heelin g mom ent and the less it can be compensated for.
I I I
I
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I I
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I Incoming waterflow
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I I
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i
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, I I I
I I
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Conventional tugs, du e to their lar ge underwater plane, ex pe rience heelin g mom ents which are more difficult to comp en sate for by their lower steering forces. Wide beam tractor tugs and ASD/reverse-tractor tugs with their effective and - for tractor tugs - deep set ste erin g pow er, are in a much bett er po sition to compe nsate for ' heeling moments. As said, they are capable of remaining at right angles to a ship's hull at much higher spee ds than conventional tugs. At high ship speed s, tugs can push at a smaller angle. Lift forc es also create pushing forces, which can be rather high (see figure 4.17) . This effect can be seen in graphical format in figure 4.20.
Fig. 4.17 Pushingforce created by hydrodJ1lflmicforce on a tug's hull A tuglaeping position at an angle with the ship 'shull may also exert rather highpushingforces caused by the waterflow, depending on the ship's speed andthe tug's undenoater hullform
Pulli ng mode
ratio a:b, a tug's underwater size and profile, its engi ne
Tugs operating at a ship 's side need goo d astern power, which sho uld be about the same as their ah ead po wer. Tug s wi th om nidirectional propulsi on are ther efore very suitable for pu sh-pull work. In figure 4.16Bi th e same thr ee typ es of tug are pulling, secur ed with one lin e. The ship is und erway thr ough th e water. The situation doe s not differ very much from situations
power and thrust performance in the pulling dir ection.
when stern tugs are towin g on a line in the direct mode,
as discussed earlier. Only for conventional tugs is the situation rather different. The longer tugs can pull effectively with increasing ship speed the better. It is obvious that the conv entional tug will swing around. The tug ne eds a stern line leading forward to be able to pull at right angles. For the situation shown, the paddle-wheel effect of the tug's propeller also add s to the swinging motion . Tugs with twin screws, steering nozzles, a Towmaster system or flanking rudders perform b ett er. The m aximum shi p 's speed with con ventional tugs pulling, eve n using a stern line, can
only be low. Tractor and ASD/reverse-tractor tugs perform much bett er, because while pulling they can apply forces in the direction of ship's movement. That is a big advantage of omnidirectional propulsion systems engaged in push-pull operations. Whether one of these types is more effective than another dep ends on the same factors mentioned when discussing the direct towing meth od, nam ely the 56 THE NAUTICAL INSTITUTE
Hydr
An imp ortant aspec t to take into account is loss of pulling efficien cy due to a tug's propeller wash hitting a ship' s hull. This for ce can be as lar ge as its ball ard pull, some time s even larger. The effect is far less if the distan ce between tug prop eller an d ship's hull is increased. Tractor tugs therefore pu sh and pull with their stern so as to keep their propellers as far away as po ssible from a ship's hull. In addition, tractor tugs with azimuth propellers, when pulling, can set their propeller thrusters at an angle, thus diverting the propeller wash. The sam e applies to ASD /reverse·tractor tugs. Hi gh er pulling effectivenes s can also be achieved using a lon ger towline.
This can only be don e when onl y pulling is requi red, not pulling and pushing, otherwise it length en s response time. The effect of propeller wash is furth er discussed in Cha pter 6. When changing from pulling to pushing tug captains should be aware of the dynamic forces in a towlin e. Particularly with a steep towline angle and in wave conditions these forces may draw the tug quickl y in the direction of the ship when its engine is sudde nly stopped. When stern thrust is also applied a tug may hit a ship' s hull with force (see figure 4.18). See also th e note at the end of paragraph 6.3.2 regarding damage to ships caused by tugs.
From the foregoing it is also clear that ASD-tugs, reverse-tractor tugs and tractor tugs operating at a ship's side have better performance when braking assistance
The ability to provide stopping assistance is nil for forward tugs towing on a line and limited to very low speeds for stern tugs towing on a line. Ship's engines should be handled with care when conv entio nal tugs
is required than normal conventional tugs. This is due to omnidirectional propulsion, which provides almost
tug, tug po sitions should be carefully pl an ned in
the same bollard pull astern as ahead.
advance.
Summary
The pu shing effectiveness of conventio na l tugs decreases quickly with increasing ship's speed; pulling is only possible at zero or low speeds, dependin g on whether a stern line is used . Ship's speed should be carefully controlled so as to take account of the limited capabilities of a conventional tug operating at a ship's side.
Stopping assistance
Many differences in performance, capabilities and limitations of different tug types have been reviewed. For the reader's convenience a brief summary follows
of the most important aspects . It is assumed that all tugs discussed are suitable for their tasks and have the required stability, sufficient freeboard, proper towing equipment and manoeuvrability.
are clo se to the stern. Due to the se limitations as a stem
Tractor and reverse- tractor tugs Tractor and reverse-tractor tugs towing on a lin e as
Conventional tugs
forward tug are able to render assistance to both sides.
Conventional tugs can be very effective when towing on a line a ship having speed through the water. They can assist in steering and in compensating wind and
As forward tugs only steering assistance can be given,
current forces, but often also deliver an unwanted force which increases a ship's speed.
fu forward tug on a line a conventional tug can assist in steering to both sides 'but as stern tug it has its limitations . At higher speeds, steering assistance can only be given to one side. Only at very low speeds is steering control to both sides and control of ship'Sspeed possible. As both a forward and a stern tug, capsizing (girting) is possible as a result of the position of the towing point in combination with induced strong transverse forces.
To minimise risk of girting a completely reliable quick release system should be used. A radial towing hook or equivalent system also decreases the risk of capsizing.
and these tugs may also deliver an unwanted force which increases a ship's speed. As forward tug these tugs are not as effective as conventional tugs for a ship underway at speed. Fig 4.14 As stern tug, reverse-tractor and tractor tugs perform
very well. They can provide steering assistance to both sides and control a ship's speed even at rather high speeds, although a reverse-tractor tug is generally somewhat less effective than (V S) tractor tugs in providing steering assistance at higher speeds (indire ct mode) . Risk of capsizing hardly exists during normal port operations and when operating as stern tug, they are hardly affected by a ship's propeller movements. Tractor and reverse-tractor tugs operating at the side of a ship at speed through the water are effective in pushing and pulling and in applying braking forces. It should be noted that tractor tugs have a relatively large maximum draft, which can be a disadvantage in shallow waters. ASD-tugs
ASD-tugs are multi-functional and can be effective as a forward tug on a line when operating as conventional tug. As forward tug, ASD -tugs can also operate as a reverse-tractor tug . As stern tug on a line ASD-tugs generally operate as a reverse-tractor tug with the same high performance. When pushing and pulling at the side of a ship at speed, ASD-tugs are very effective, also in applying braking forces. 4.3.2 Effectiveness of tug types Figure 4.18 Effictofdynamic fOrces in the towline Pulling with a short and steep towline creates high fOrces in the towline, which are very much enlarged hy waves andswelL As soon as tugengines are stopped, the tugwill immediately be pulled backwards towards the ship by force F caused by stored energy in the elastic towline. So, when thetugcaptain isasked tostop pulling heshould be aware ofthis effiet andwhen ordered tochange overfrom puUing to pushing, astern thrust should be applied very carefUlly
Model testing and full scale trials have been used to determine tug capabilities. Most tests focus on the abilities of one specific tug or tug type. Voith has done, and still does, considerahle work regarding VS tractor tugs . Aquamaster has carried out several studies regarding tugs with azimuth propellers. Recent studies and full scale trials that have been undertaken mainly assess specific tugs and tug type escorting capabilities. TUG USE IN PORT 57
Most of these studies and trials, therefore, only involve some specific aspects of ship assisting manoeuvres required during daily tug handling. Of course, several variations in the design of a specific type of tug exist. Simulation programs provide the possibility of gaining insight into a more extensive range of a tug's ability using, for example, a full mission bridge simulator. When these programs are carried out in close cooperation with pilots and tug captains and are, as far as possible, verified in full scale sea trials, the results give quite high reliability. Simulations are mainly carried out for one specific tug or for a very limited number of tugs of which all details of rudder, propulsion , stability, maximum list, hydrodynamic coefficients and so on are known .
increase very quickly at speeds above four knots. These longitudinal forces increase ship speed. When no bow line is used the longitudinal forces but also the transverse forces exerted at speeds higher than five knots are less , so tug perfonnance is less. In waves of approximately six feet high, tug performance drops quickly at speeds higher than three knots . According to the same study, the effectiveness of conventional tugs with inferior rudder performance decreases quickly at ship speeds of about four knots. PlIshlng Forces (with bow 111\8)
-
50
4s
Desktop computer simulation programs exist, based on a steady situation - equilibrium of forces - by which the performance of different tugs and tug types can be determined. With these simulation programs capabilities produced by different variations of tug design can be predicted.
Real capabilities and in particular limitations are, of course, experienced during daily shiphandling only, but the results of simulation programs can verify some of what is explained in this book.
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As indicated in the graph, the pushing angle becomes smaller as soon as the ship gathers speed. The transverse pushing forces exerted by this tug decrease with ship's speed higher than five knots, but longitudinal forces 58 THE NAUTICAL INSTITUTE
50
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30 20
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Figure 4.79 Performance and behaviour ofa 40m conventional tug 60
Tug Force P tlonnesl
Drill Angle and Propeller Angle .
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The conventional tug has twin screws and three rudders, length overall of 40m, beam lim, 5750 BHP (open propellers), SOt bollard pull and draft 17ft. The maximum pushing forces of this tug were determined at various ship's speeds taking into account, amongst other things, maximum heel at deck edge immersion. The graph shows maximum transverse pushing forces and the longitudinal forces exerted at the same time. It also shows the tug's pushing angle. The tug is pushing with a bow line.
•
Pushing Angle
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80
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longitudinal Force
'
Performance ofa conventional and an ASD-tug when pushing at a ship underway at speed The graphs in figures 4.19 and 4.20 are based on simulation studies and provide an insight into the capabilities of a conventional tug and an ASD-tug when pushing.
70
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Performance diagrams
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ANGULO TUG- SHIP
.--
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Pushlllll Angle
-
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35
5
Simulation programs don't normally take into account all factors influencing tug performance, such as ship-tug interaction, flow field around a ship, influence of water depth and confinement on the flow field, and the influence of ship's wake on a tug's braking performance, which are discussed in Chapters 6 and 8. There may, therefore, be some inaccuracy in simulation results, depending on the situation.
~'
B
4 • Speed V {knots)
v
p
'" Pushlnll.....g1. b; DfftAnllla c: Propeller A"II1ft
Figure 4.20 Poformance andbehauiour ofa 30mASD-tugfirpushing
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Figure 4.21 Petfbrmana graphs forfour andsixknots speed
TUG USE IN PORT 59
In practice a speed of five or even four kno ts is a rather high limit for conventional tugs to exert transverse forces effectively. The study results ma y be affected because not all factors influenci ng tug perfo rmance could be taken into account. Naturally, differences in pe rformance exist be twee n va rious ty pes o f conventional tugs. In general, however, the upper limit at which effective sideways pu shing for ces can be exerted is found to be about three kn ots. This is also proven by full scale trials in the USA in 1982 with a 1700 HP twin screw tug with no zzles, two steering rudders, four flanking rudders and without the use of auxiliary towlines. Th e length of the tug was 30m. In ad dition, effective pu lling forces were po ssible at maxi mum speeds ofless than one knot.
As can b e seen in the graph this tug performs very well. The tug exerts only transverse forces and n o speed increasing longitudinal forces. Th e higher the spee d the larger the hydrodynamic forces on the tug's hull and the larger the lift forces create d by the hull. At about eight and a half knots, 80% of the transverse pushing force is developed by the lift force.
The main conclusion is that at ship speeds higher than around four knots, and for less manoeuvrable tugs three knots, the performance of conventional tugs is very poor. At these higher speeds transverse pushing forces are minimal, but longitudinal forces increase very quickly, thus increasing ship's speed, which is not desirable.
no ship's speed in cre asin g lon gitudinal force s are
Next the perfo rmance of an ASD-tug when pushing is considered. Particulars of th e tug are: 31m length ove rall, beam 10·7m, 3600 BHP, SOt bo lla rd pull , maximum allowable heel 6'. • 00
.I.
Tug stability, freeboard and height of the pushing point have a large influence on maximum achievable pushing forces. Limiting factors are maximum engine revolutions, engin e torque and excessive heel. The two graphs show a large differen ce in pushing effectiveness between ASD and convent ional tug s. An ASD-tug is still effective at a much highe r spe ed while exerted on the ship.
Performance ofan ASD and VS tug while towing on a line These diagrams (figures 4.21 and 4.22) have been produced by the TUGS IM simulation progr am of Damen Shipyards, The Netherlands. Tug performance in the diagrams is limited by a tug's max imum list deck edge imme rsion, and maximum engine load is acco unte d for.
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FiguTt 4.22 Ptiformanet grapbsfor tiglu knots speed
60 THE NAUTICAL INSTIT UT E
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The TUGSIM program operator actually steers the tug, controlling the rudder and/or thrust angles and thrust magnitude. This prevents theoretical solutions being calculated in which a steady situation exists, but situations in which the tug could never be manoeuvred. The graphs show an ASD and a VS tug towing on a line at different speeds - four , six and eight knots. Particulars of the tugs are given at the top of the graphs. The m ain obj ective of the graphs is to show the maximum steering and braking forces which can be achieved. The ship is at the centre a of the graph, sailing in the direction indicated by the arrow. The ASD·tug is operating as forward tug up to a towing angle of 90 0 as conventional tug and as stem tug the ASD-tug operates as a reverse-tractor tug. As stem tugs the ASD and VS tugs operate in the direct or indirect mode, whichever is the most effective. The following general characteristics can be seen in the graphs. The performance of the VS tug in giving steering assistance as forward tug towing on a line decreases quickly with increasing speed, while up to a speed of about six knots the performance of the ASD· tug in giving steering assistance is decreasing much less at small towing angles and is even increasing at large towing angles. At eight knots the ASD·tug can still produce high steering forces in contrast to the VS tug. Normal conventional tugs often perform in a similar way, bu t are generally limited more by the tug's stability. At four knots the tugs operate as stern tugs in the direct mode and are both effective. At six knots the ASD·tug performs better in the direct mode, while the VS tug starts to perform better in the indirect mode in applying steering forces. The braking performance of both tugs in the direct mode is high . At eight knots and in the indirect mode high steering forces can be applied by .b oth tugs . The VS tug is somewhat more effective, although it is less powerful than the ASD·tug. Highest braking forces are achievable at speeds of eight knots by both tugs operating in the direct mode and towing at a small angle (lowest part of the curve). Both tugs perform about the same when taking into account the difference in bollard pull. Thus the following generalities can be observed. As a forward tug towing on a line the ASD-tug performs better. As a stem tug on a line and at higher speeds the VS tug performs rather better in giving steering assistance and the ASD·tug and VS tug perform about the same in applying braking forces . Another aspect is clearly shown in the graphs: the speed increasing force vector of forward tugs towing on a line. For example,
take the ASD·tug of the six knots graph while towing on a line as forward tug. The tug as shown in the indicated position develops a steering force to starboard of 35 tons, but at the same time a force in the direction of ship's movement of about 15tons. This force increases the ship's speed, which is in most cases not welcome. On the other hand, all the stern tugs as shown in the graphs when applying steering forces also develop braking forces. This normally has a large positive effect. It keeps the ship's speed low and , in addition, enables the ship to apply additional engine power for steering, without increasing the ship's speed. All this is in line with the capabilities and limitations of tugs as discussed.
Speedcontrol - brakingassistance Tractor, ASD and reverse-tractor tugs perform very . well as stern tugs for steering assistance and speed control. This has resulted in competition between the designers of cycloidal propellers (VS) and azimuth propellers (Aquamaster) about which type of tug, VS or ASD, performs best as stern tug at higher speeds . This is mainly due to the discussions with respect to escorting, dealt with in Chapter 9. However, one aspect is briefly discussed here, the braking performance of tugs equipped with azimuth thrusters, because it is important for daily assistance in ports. In this respect some new terms have been introduced by Aquamaster. It should be noted that when stopping assistance is required by a VS tractor tug or ASD/reverse·tractor tug, for instance at speeds of more than five knots, the braking force that can be applied is higher when the tug is pulling at a small angle with the ship's centre line rather than pulling straight astern, as can be seen in the TUGSIM performance graphs.
When braking assistance is required at high speeds by a conventional tug operating over the bow as stem tug, it may not be possible to reverse fixed pitch propellers due to the high propeller load which has to be overcome by the engine, although the effect of it can be reduced by proper design and tuning of the engine. For the same reason, at a crash stop VS pitch levers should be set in accordance with the ship's speed and azimuth thrusters have to be rotated to astern but can be set, with independently controlled thrusters, at an angle with the tug's centre line to avoid stalling. In the case of azimuth thrusters with control1able pitch propellers, astern pitch should be applied in accordance with ship's speed when a ship having a rather high speed has to be stopped. Because of the low performance of controllable pitch propellers going astern, turning the thrusters like thrusters with fixed pitch propellers is more effective in applying braking assistance. In the direct assisting method, Aquamaster claims that at speeds of up to eight knots braking forces can reach values up to one and a half times the bol1ard pull TUG USE IN PORT 61
astern with azimuth thrusters (of ASD/reverse·tractor as well as tractor tugs) rotated 180°, the thrusters thus working in line with the tug's centre line in negative flow. At spee ds higher than eight knots braking forces dro p off dramatically, regardless of the powe r app lied. Engine load then also increases rapid ly to an overload condition. This braking meth od is called the Rever se Arr est Mode by Aquamaster. A second way of applying braking force in the direct assisting method is the so-called Tran sverse Arrest Mode. Lar ge arresti ng forces can be created by pointing the thru sters outward at an angle of approximate ly 90'. These forces result from momentum drag and are ge ne rated when the prope llers acce lerate the athwar ts hips compone nt of the was h . The forces inc rease with speed an d excee d the astern bollar d pull at speeds higher than eight knots without overloading the engine . So, be low eight kn ots the Reverse Arrest Mode can be used (thru sters rotated 180' in line with the tug's centre lin e) and at speeds higher than eight knots the Transverse Arr est Mode (thrusters at an angle of 90' with tug' s centre line) can be applied. See figur es 9.5 and 9.7 for the different terms used and the achievable braking forces. Although eight knots is a high speed for tug assistance in port areas, it is goo d to know how thrusters can be use d to deliver hig h re tard ing forces. This way of applying br akin g forces can be utilised by all types of
The location of th e pivot point is taken into account. Forward tug no . I, towing on a line , is capable of exerting quite high crosswise steering forces on a ship . The effect can be limited becau se of the transver se forces near a ship's b ow to be overcome, as explaine d whe n discussing th e pivot point. It is clear that for a par ticular
ship th ese tran sverse forces ar e proportional to the draft and underk eel clear anc e. Also, the more th e tug is pulling in lin e with a ship's h eading th e mo re the tug will increase a ship's speed. Position of tug no . 2 is not so good for the steering assistance required. The tug has to over come th e same transverse forces as tug no . 1, but the lever of cro sswi se steering forces exerted by the tug is much sh orter and the tug's underwater resistance opposes the turn. Also ,
when a tug is unable to push at right angles to a ship's hull it will increase a ship 's speed. Rega rding tug no . 2 it should be kept in mind that this tug might even have an opposite effect. Simulation stu dies carried ou t by, amongst others, Dr. Paul Brandne r an d descri be d in his the sis 'Performance and effectiveness of omni-directional stern drive tugs ' (see
Refere nces) show that a tug pushing at the bow of a loaded tank er on a steady course, with an initial speed of four knots, the engine on Dead Slow Ahead and rudder amidships, has a tendency to turn against th e
1
steerable thrusters, but is most efficie nt whe n using
propellers in nozzles.
4.3.3 Effective tug position Positioning tugs depends on several factors. Firstly, ship's' p articulars such as type, size , draft, windage,
man oeuvrab ility have to be cons idered. Seco nd ly,
2
Cp
fact or s such as the in flu en ce of e nvironmental
3
cond itions, particulars of the passage or fairway toward s the be rth, available stop ping distance, size of turn ing circle, berth location, an d so on have to be taken into
account. Togeth er these factors determine wha t should be expe c te d fro m tugs - steering ass is tance, co mpensating ex terna l forces of win d and current,
assistance in stopping th e ship or a combination thereof. Of that Ship's be rth ing side is also an important factor to be tak en into account whe n pos itioning tugs. And, of course , it is very important to know the numb er, type and bollard pull of available tugs-, In figure 4.23 differ ent tug position s are given. A ship has h eadway and h as to make a turn to starb oard. Tugs have to assist. Whether a particular type of tug is
6
5
more or less effective in one or more of the positions
shown has been discussed already and is summarised in paragraph 4.6. Atten tion now turns to the effect on a ship whe n tugs are ope rating in one of these positions .
62 THE NAUTICAL INSTITUT E
Figure 4.23 Different tugpositions
pushing direction of the tug . The tests were carried out with a depth/dr aft ratio of 1.2. This effect has also been experience d durin g full scale trials. During thes e trials a loaded tanker was on a steady course at five kn ots sp eed, the ru dde r amids hips , and the engine was stopped. A conventional tug started to push on the port shoulder.After an initial tum to starboard the ship started to turn to port , while speed increased. It does in no way say tbat for other ship types or oth er loading con ditions, th e same effect might b e expe rienced. The opposing transverse force at th e b ow differs by ship type, draft, trim and und er keel clearance (see above for tug no. 1). In the report mentioned above, test results of other loading conditions are given. If the same tug is pushi ng at the sho ulder of the tanker when in deep wate r, in hal last condition and trimm ed by the stern , the tug do es turn th e tank er in th e requir ed directi on and the effect do es not differ much from a tug pushing at the quar ter (tug n o. 4). Apart from what hasjust been mentioned, the positions of tugs no. 1 and 2 are not always inadequate . It depends on the situation and circumstances, because the tugs are in a good position to compensate for drift forces caused by wind and/or current from starboard. If required, tug no. 1can easily compensate for the wind and current forces from port as well. This flexibility in operation is an advantage of the forward tug towing on a line.
Tug no. 6 is in a similarly effective position to tug no. 5, but has the disadvan tage that this tug increases ship's speed. The same would be the case with a rudder tug (not shown in figur e 4.23). The difference in effectiveness between a forwar d pu shing and aft pushing tug can also be seen whe n a ship gathers spee d. For instanc e, assume that tug no. 3 and no. 4 are of same typ e and bo llard pull and both pu shing at right angles. At zero speed the ship, on even keel, moves crosswise. For reasons exp lained, as soon
as ship's speed increases, the effect of tug no. 3 is smaller than that of tug no. 4 and the ship starts turning to starboard. The same applies to tugs of similar capabilities when towing on a line forwar d and aft. O CP se desloca para frente
For swinging, e.g. whe n the ship is stopped in the turning circle , tugs no. 1 and 5 or 6 are in the b est position du e to the long lever of exer ted tug for ces. The most effective tug positions have now been reviewed. Which position s should be used during passage towards a be rth and while mo oring/unmooring depends on what is required from the tugs and this depends on the ship, local situation, circumstances and ship's berthing side. If steering assistance to starboard is required during passage towards a berth then tugs no . 3, 4, 5 and 6 are in a good position. Tug no . 5 can even give steering
Tug no. 3 can assist th e starboard turn by going astern. In doing so, an additional starboard turning couple is created by the tug's and ship's engines working in opposite directions. By going astern the tug is slowing down ship's speed, and thu s increasing the effect of the ship 's engi ne on th e rudder. The tug 's und erwater resistance contributes to the starboar d swing. If tug no. 2 had a bow line, both tugs 2 and 3 are in a good posi tion to take off ship's head way, if required. Tug no. 4 is in an effective position to assist the starbo ard turn by pu shin g, b ecause of the 10J;lg lever and forward centred lateral resistance, which contribute to the swing. The tug's und erwater resistance gives additional turning effect to starboard. When tug no . 4 cannot work at right angles, ship' s speed increases, but as a result of the high er rate of tum caused by the pu shing tug and consequently the high er drift angle, ship's speed is hardly affecte d. If the tug has a bowline secure d, it could also assist in the starboard swing by going astern, in the same way as tug no. 3. In that case the whole tug has to b e pull ed crosswise through the water by the ship's stern an d hence opposes the turn . Tug no . 5 is in a very effective position. The lon gest poss ible lever for steering for ces and the transverse forces centred forward contribute to the swing. Also, the tug does not increase ship 's spee d. On the contrary, the tug also provides re tarding forces while app lying steeri ng assistance.
Pho/.(; :Author
Figure 4.24 Two amoetuional tugs assisting a tanker ha.ing headway in mal
TUG USE IN PORT 63
assistance to both sides. The same would be the case with a rudder tug. If crosswise drift forces from port have to be compe nsa ted for in a narrow fairway , tugs no. 3 and 4 are in a good positio n and also tugs no. I (whe n this tug shifts to port), 5 and 6. In case stopping assistance is required tugs no . 2 and 3 (with bow line s) and 5 will assist effectively. If tug assistance is required d ur ing mooring/ unmooring operations then several combinations are
possible, also dep ending on tug type . For mooring of large ships even four tugs may be used. Oft en tugs nos. 3 and 4 are used for pushing and I?..and 5/6 for controlling the approach speed towards the be rth. If tug no.! is a tractor tug, reverse-tractor tug or ASDtug ope rating as a re vers e-tractor' tug, then tug no .l together with tug no.5 can easily push as well as control the ship's approach speed towards the berth during mooring
Wh ether the required tug forces can be delivered effective ly, depends on a correct assessment of the required bollard pull and the right choice of the type of tugs with respect to the tugs' positions and assisting methods.
4.3.4 To wing on a lin e compared with operating at a ship's side In paragra ph 3.2, different assisting met hods were discussed. Which assisting meth od is most appr opriate for a parti cular port dep ends on the local situation and In spite of that circum stan ces. Nevertheless, it is good to have an idea about the adv antages and disadvantages of the two basic meth od s. In paragraph 3.2 the small manoeuvring lane within which tugs towing on a line are able to operate an d the limitations of tugs ope rating at a ship's side du e to waves were mention ed. Taking into account the ca pabilities and limitati on s of tugs, th e foll owin g addition al comme nts are given . Different types of tug can be used for towing on a line, some more effective th an othe rs. In a fairway passage towards a b erth tugs are normally position ed so that the influe nce of wind an d/or curre nt can be co m pe nsated as mu ch as possible and changes in heading can be made in a safe, efficient way. A ship can also be rth eithe r side.using this system . Towin g on a line, therefore, has the adv ant age that tugs are normally positioned at the safe side of the ship and are flexibl e regarding be rthing side. Even in the worst case, when wind and/or curre nt are getting too strong, tugs on a line can assist up to the last moment, minimising th e risk of severe damage. Wh en omnid irectional propulsio n tugs are used for towing on a line they are able to change over to the push-pull me thod during berthing witho ut the need to 64 THE NAUTICAL INSTITUTE
release the towline. This shortens berthing time, because no time is wasted in retrieving towlines or repositioning
tugs. In add ition, a ship can be kept und er better control because towline s stay fastened wh ile tug s eithe r pu sh or pull. Tugs at a ship's side are positioned acco rding to berthing side, to the forces of wind or current to be compensated for and/or the changes of heading to be made dur ing transit toward s a b erth. Wh en pos itioned to compensate for wind and/or curren t forces this may be the wrong position for berthing. Tugs the n have to b e shifted before mooring takes place - common practice in some po rts. H owever, this mean s that a ship
has no or little assistance during shifting of the tugs an d may start drifting. When position ed to compensate for wind and/ or current forces, risk is involved for both tugs and ship when these forces are underestimated and a ship star ts drifting. Wh en it becomes too da ngerous for the tugs they may try to get .out from between the ship and the leeward or downstream fairway or cha nn el banks, leaving the ship without any assistan ce. 4.4
Operational limits
Harbour tugs can operate in all conditions of cur rent and wind . However, during fog the situation is differ ent. Fog in confined port areas makes tug assistance very risky. In good visibility a tug captain assesses his po sition and spee d in relatio n to the speed and heading of the attended ship, and also in relation to the surrounding area, such as buoys, beacon s, rive r banks and quays . Compared to ship movements, tug movements are
much faster, making it difficult to manoeuvre from the tug's radar. In addition, tugs often operate close to a ship's side, resulting in a distorted or partly blank radar picture. Furtherm or e, during fog a tug captain m ay lose a good view of his towlin e. Altog eth er this makes tug assistance during fog much m ore difficult than when Visibility is good. For this reason restrictions on tug assistance under poor visib ility conditions exist in a number of ports. Several ports lie close to ope n sea and jetties may be situated in open waters. Consequently, tug assistance may also be required in open sea. For harbour tugs, passing towlines in wave conditions can b e difficult. H arbour tugs ope rating at a ship's side have short and often rather steep towlines. Wh en tugs operate on a wave exposed ship's side, dyn amic forces in the towline may reach high va lues and lines are liabl e to part in deteriorating wave conditions. So, very stro ng and sometimes double fibre lines of high stretch properties are often used. However, if circumstances permit, tugs can also
change over to towing on a line, allowing them to handle a ship more safely since if towlines are longer tbey can Questão de prova
better absorb dyn ami c forces. When tugs are equipped with lowing winches line can the n be paid out as deemed nece ssary and be sho rtene d when conditions improve or when entering port. On tbe other hand, in wave con ditions harbour tugs can, instead of towing on a lin e, operate more effective ly and at higher wave heights at the ship's le eside,if circumstances and ship manoeuvres allow. It all depend s on the local situation. In wave conditions the risk of girting for conventional tugs towing on a line is high er than in calm water and To achieve or to execute passing towlines can be carried ou t more safely with mo re highly m anoeuvrable tugs. Tractor tugs will in general, the refore, operate m ore safely and can provide assistance in somewha t larger wave heights. It has been reported that th e m ovem ents of VS tractor tugs may b e m ore violent in w ave conditions. Anyway, waves limit th e op er ating effectiveness of harbour tugs when towing on a lin e as well as operating at a ship's side when exposed to waves . Perfor man ce decrease s wit h increasing wave height An indication of the up per limits for ope rations by harb our tugs is: Maximum sign ificant wave height: Conve ntional tug types :1·5 - 1·8 m Tractor types of tugs (incl. reverse-tractor tugs), ASD tugs : 2·0 m
Visibility: In several por ts a visibility of 0·5 mile is found to be the limit.
4.5
Design consequences
Wh at h as b een d iscu ssed with resp ect to the performance of differ ent types of tug has resulted in an alternative design for some new VS trac tor tugs. Th e reason why is clear. A tractor tug is ve ry effective as a stern tug on a line . It ope rates with the stern dir ected towards th e ship and the tug captain facing aft. Thi s is also the dir ection of the assisted ship's movement. Wh en ope rating at a ship's side a tug captain is also usually facing aft an d the same applies during mooring and unmooring operations. So wh at can b e see n now ad ays is a totally new concept VS tractor tug, as for instance in the Norw egian/ Swedi sh Bess an d Boss- the wh eelh ouse is turne d 180°. The stern is high er to give better pro tection against incoming waves (see figure ·9.16). These tugs will be conside red when discussing escorting. Similar changes to VS tug design can be found, amongst othe rs, in the VS tu g R edhridge of Adsteam Tow age, UK, where the Escapamento tug funn els are placed forward of th e wheelhouse, giving an op tim um view aft for the captain . Th e stern in this d esign is also raised . In ad d ition, alternative towing poin ts can be used , as m entioned in section 4.2.2.
Photo:Boh DoJ1tJUSQTt., Soutllampton, UK
Figure 4.25 VS tug 'Redbridge' ofAdsteam Towage, Southampton, UK. (/.0.0. 33m, beam 112 m, hp 43 tons). A newdesign, meeting several operational requirements, it has an optimum v£ew of the after ded: from the wheelhouse, unobstructedby fimnels and a much higher sheer at thestern to keep the aft duk: clear of water when running astern at speed. particularly in wave conditions andwhen esC()fting
4.6
Conclusions regarding tug types
Assuming normal po rt operations with maximum ship speeds of six to seven knots, it can be concluded with some reservations - that the suita bility of different tug typ es can b roadl y be ranked as follows:
As forward tug towin g on a lin e: ASD- tugs Combi-tugs Conventional tugs Tractor tugs/ Reve rse-tractor tugs As stern tug towing on a line: Tractor tugs/ASD-tugs/Reverse- tra ctor tugs Combi-tugs Conven tional tugs When operating at a ship's side: ASD-tugs / Reverse-tractor tugslTractor tugs Co mb i-tugs Conven tional tug s Th e above ranking is, of co urse, a general one. Differen ces in design of a particular type can change the ranking, especially tug types with more or less similar characteristics such as tractor, ASD and reverse-tractor tugs. Conve ntional tugs will never reach the high m an o euvra bility of om n id irectional tugs. Bu t conventional tugs also have many differences in design and man oeuvring devi ces, making one much more ma noe u vrab le tha n a no the r. Wen chosen d e ck equipment can improv e a tug's performance. For instance, installation of a radial h ook in a conventional
TUG USE IN PORT 65
tug can make tha t tug supe rior to a similar conventional tug witho ut such an arrangem ent. The same app lies to AS D-tugs, when they operate as a conventional tug. It should be borne in mind , too, that the abov e bro ad ranking refers to a tug's effectiveness. Wh en safety of ope rations is the major requirem ent, then tractor and reverse-tractor tugs are recommend ed . Although fire figh ting is no t discu ssed in thi s book it is also an important factor to be considered with respect to a tug's manoeuvrability. Finally, the maximum dr aft of a tug, e.g. of tractor tugs, can make them un accept able for certain po rts regardl ess of their high manoeuvrab ility.
4.7
Some other practical aspects
There are other aspects which are important for safe and efficient shiphandling by tugs. Cooperation As stated in section 4.3.1 pilots, ship masters an d t ug captain s should know each oth ers capabilities and limitations regarding ship and tug manoeuvres. This kn o wl ed g e is the basis of good coo peratio n an d und er st anding b et w e en th em . O n ly th en will m an oeuvr es go smoothly and a ship be hand led safely and efficiently. Wh en ma no euvring, the pilot should keep , as far as is possible, a close eye on the assisting tugs. He will the n see how the tugs are performing, can take action when they don't act as expected, or when a tug's safety is at risk. Communications between pilots and tug captains For good cooperation between pilots and tug captains a goo d communication system is indispensable. Portable radio-communication sets have b een used for years by pilots. When of a good mak e these sets are very hand y and work satisfactorily. Radio sets should be tested prior to a pilot boarding a ship and it is best tha t every pilot has his ow n set.
Tug orders sho uld be given clearly and be open to : onl y one interpretation . Tugs should be addresse d by • nam e or by opera ting po sition. Tug capta ins should co nfirm and repeat the orders given, stating the ir tug's
nam e or po sition. Any possibility of misun derstanding should be avoided. M any ports prefer to use a standard syste m in English, but it will take yea rs before such a system could be intro duce d worldwide (see also paragraph 9.5.1 Communi cations an d Inform ation). In near ly all ports the language between pilots and tug captains is a kind of slang and is therefore not always comprehensible to the master of a ship. Althou gh pilots and tug captains understand each other well enough, it is a strange situat ion b ecause th e ship master is still responsible. It would therefore be bet ter if tug orders were given in English, according to an int ern ationally 66 THE NAUTICAL INSTITUTE
ag reed stan dard vocabulary. Using und er stand able English is fine in English speaking countries, but it does cause pr obl em s in m an y n on-Engli sh speaki ng countries. In parti cular, tug captains often speak only the local language. An international standard vocabulary is, for that reason, hardly feasible. In addition, a standard vocabulary canno t cover non-stan dard situations. In critica l situations pilots and tug captains should be able imm ed iately to understand wha t is wan ted. A change in co m m unication proce d u res m igh t re su lt in misunderstandings. This sho uld be avoided. Nevertheless, tug captains should always be inform ed by pilots about the intended ship and tug man oeuvr es. Furthermore, the use of a basic system for tug orders in a port is necessary, even though only a local system, but sho uld be standard for all local pilots an d tug captains .
Tug use H arbour tugs handling a ship should have a reserve of power, be able to react fast and to han dle a ship in such a way that a minimum of space is.required for the ship and assisting tugs. The slower tugs react, the longer the towlines and the sma ller the tug power, th e mo re m anoeuvring space is required for a ship and assisting tugs. H owever, man oeu vrin g space is usuall y very limited in port areas .
Tug size and power shou ld be relative to ship size. Large and powerful tugs should normally not handle small ships. Tug actions in that case could induce too large moveme nts of the att ended ship, resulting in inefficient shiphandling and in a worst case dam age to th e ship . In additi on, bollard pull of the separate tugs h andling a ship should not mutu ally differ too mu ch, Tug configuration should be plann ed we ll in ad van ce, ta king in t o accoun t avail ab le tu gs, the capabilities and limitations of different tugs, manoeuvres to be carried out, the influen ce of wind, curren t, and so on . A nice example of tug configuration can b e seen in the ph oto of the bulk carrier at page xiii. Three different tug types are used in an appro priate configuration . The ship has to round a starboard ben d. O ne conventional tug is assisting the ship, position ed starboard forward wher e it can be effective in app lying steering assistance. In add ition, this tug can tow with a larger towin g angle than a VS tug. One VS tug is therefor e pos itione d at the port bow. At the port quarter aft is a powerful ASDtug, in the be st position for ass isting in a turn to
starbo ard. The ASD-tug can assist in the turn in the dire ct mo de without increasing ship's speed and the tug can, if required, control the ship's speed. T he seco nd VS tug with less boll ard pull than the ASD -tug'is therefore positioned on the starboard quarter aft. Repositioning of tugs may sometimes be considered necessary durin g a trip, bu t should be av oided as far as possib le, p art icul arl y if shifti ng the tu gs in vol ves
releasing and refastening towlines. This tak es time, especially with the limite d number of crew .members on board now adays. During the time of shifting a tug, the ship has less or no tug assistance and in the worst case towlin es may foul ship's or tug's prop eller. Entangle or jam
Speed Ship' s speed sho uld b e car efully con tro lle d in relation to th e limitations of the tugs involved. Thi s gene rally means th at speed should be low, taking into acco unt the effect of current and wind . In any case , the lower a ship's speed th e m or e effectively tugs can operate. Also, other factors playa role with regard to ship's speed, factors which can affect th e tug assistance required and tug safety, such as interaction and shallow water effects, which 'are discu ssed in Chapter 6,
Decreasingeffectivenessoftugs when a ship gathersspeed The difference in pulling effectivene ss tha t arises between a forward tug an d stem tug when a ship gathers speed has been mentioned earli er. In addition, an effect to keep in mind is th e decreasing effectiveness of tugs in gen eral wh en a ship, initially stopped in the water, gath ers speed . T his has som etimes resulted in waiting time for ships. Example: A container ship has to d ep art from a
harbour b asin with strong onsho re winds. Tugs are
ordered . Total bollard pull available seems sufficient to pull the ship off the berth. So far, no problem. However, as soo n as the ship's engines are started and she starts moving, the tugs towing on a line take position to b e able to keep pace with th e ship, so their effectiven ess decreases. The ship may drift alongside th e berth again and addi tional or stro nge r tugs have to be ordered, which tak es tim e. When mo or ed po rt sid e to and d ep arting astern out of the harbour basin with an onsho re wind the effect is worse due to the transve rse effect of the ship's propeller.
Ship pulled orpushed around by a bow tug gathersspeed. A tug pulling at right angles to the bow of a ship stopped in the water will give th e ship a lateral velocity and a rate of tum, causing the ship to pivot around a poi nt somewhere near the stem (see for instan ce figur e 4.2B ). As a consequence the ship' s lateral centre of gravity follows a curved path. A body following a circular path experiences a 'centrifugal force' , and such
a force also acts on the ship's centre of gravit y moving along the curved path. A 'centrifugal force' is always directed outward and perp endicular to the curv ed path. This 'force' originally acts almo st in line with the ship, thus causes the ship to gather headway. The fluid forces also contribute to thi s effect.
TUG USE IN PORT 67
Chapter FIVE
BOLLARD PULL REQUIRED 5.1
Introducti on
TUG CONFIGURATION, THE NUMBER OF TUGS and total ballard pull used are norm ally based on a pil ot's exp erien ce and may vary depending on port conditions and circum stan ces. In general this system work s well. However, with increasing ship size it is more difficult to determine what exactly is needed to handle a ship safely. Exp erienc e alon e in such a case is too narrow a basis and ma y not cov er all situations and conditi ons which might be exp ected . Information on wind, current and wave forces may be essential. This could be particularly the case whe n large co ntainer ship s, car carriers, deep
dr aught tankers or bulk carriers have to be handled in unfavourable environmental co nd itions and in confin ed port areas.
Anoth er con sideration is that, because of economic
pressure, shipping companies often try to min imise tug assistance costs. Thi s can easily lead to a dispute between the pilot, master or shipping agency about the minimum number of tugs to be used. Ships equippe d with bow thrusters and/or stern thrusters often use one or two tugs less. Side thrusters, however, have limitations to their maximum p ower and effectivene ss , which decreases very rapidly wh en a ship gathers headway. The tug assistance required is, therefore, often subject to discussion about acceptable limits of safety. Pilot and master, if well prepared, can avo id these discussions and are in a better position to take the right decision. Dep ending on the local situation, tug assistance on arriv al or departure gen erally comp rises three phases:
able to compe nsate for wind and current forces. Tugs have to assist fully. For ships influ enced
by wind, current and waves this
last phase, when a ship is stopped in the water, is most imp ortant for assessmen t of ba llard pu ll requir ed. It is this phase which will mainl y be consider ed, th er efore. In considering ballard pull required , the av ailability of side thrusters is sometim es taken into account,
be cause a side thru ster may replace part of the ballard pull required. Wheth er this is the case dep ends on the ship, th e local situation, the circu mstances and por t regulations .
5.2
Factors influencing total ballard pull required
The following main factors influence tug assistan ce:
Portparticulars, including: Restrictio ns in the fairway, port entrance, pas sage to a berth, turning circle, manoeuvring space at a berth
or harbour basin, available stopping distan ce, locks, br idges, moored ve ssel s, water d epths, spee d restrictions, and so on .
Berth construction, including: Type of ber th: open, e.g. j etty, or solid. Theship, including: Type, size, draft and und erke el clearan ce, trim,
Windage, and factors such as engine power ahead!
The phase whereby a shiphas reasonable speed The ship can still use her engines and rudder to compe nsate for drift forces caused by wind, current . and!or waves, by steering a drift angle. Depending on the situation, tugs may assist.
The intermediatephase W"hen a ship has to reduce speed, entering a dock, harbour basin, turning circle or approaching a berth. The ship also has to be stopped within a certain distance. When re ducing speed, a ship's steering perfor mance also decreases. The pr opeller has to be stopped, the influence of wind and current increases and tug assistan ce is ne eded more frequently and to a larger extent. The phase involvingthefinal part ofthe arrival manoeuvre. The ship is practically dead in the water, such as in the turni ng circle and!or whe n berthing. The ship is very restricted in manoeuvring performance and not 68 THE NAUTICAL INSTITUTE
astern, prop eller type, manoeuvring performance,
and availability of side thru sters and specific rudders.
Environmental conditions, including: Wind, current, waves, visibility, ice .
Method oftugassistance, including: Towing on a line, ope rating at a ship's side o r a combination of methods. The port is more or less a constant factor. Parti cular s of port layout, such as fairway, port entrance, passage to the berth, turn ing circle and berth location , determine a basic number, type and total tug ballard pull for a particular class of ship. This is based on local experience and sometimes, for more difficult situations, on simulator resear ch. An indication of ba llard pull required for tankers, bulk carriers and con tainer ve ssels is given b elow. Berth construction has to do with the tran sverse approach speed towar ds a berth, which is also dealt with in this chapter.
In addition to tug assistance requirements following from port layout and berth con struction, the varying factors influencing the required total bollard pu ll for a particular ship are: Wind . Current.
. Waves. These factor s have to be con sidered in relatio n to ship d etails such as size, draft, und erk eel clearance , etc. The man oeuvring p erformance of a ship may influenc e requir ed. tug assistanc e in a po sitive or negative '''lay. The towmg method should also be taken into account. Reduced visibility is also considered a factor of im~ortance regarding tug assistance. This is tru e, but it mai n ly concerns sp ecific safety procedures for tug assistance during fog. Reduced visibility, therefore, is not discu ssed further in this chapter. Tug assistance in ice conditions was dealt with in Chapter 3. The total force acting on a ship cou ld, in theory, be compensated for by tugs wh en b ollard pu ll equa ls the total forces of wind, current and waves. However, there are some important factors to be taken into accou nt: Tugs must h ave sufficien t reserve power to push or pull a ship up against wind and curre nt or to stop a dr ifting ship quickly eno ugh. Tugs are not always pulling or pushing at right angles to a ship . For instance, du ring arrival or departure manoeuvres, a ship may have so me forward or astern
...?
movement. Tugs try to k eep pace with a ship, and thus use engine power in th e directio n of ship 's movemen t at th e ex pe nse of pu ll or push forces. The same happens in situations where there is a current and a ship has relative speed through the water. Bollard p ull actuall y available m ay, due to wear an d fouling, no longer be a full 100% compared to the original bo llard pull tests . Forward and after tugs often cannot pull or pu sh at full power simultaneously, even when the required bo llard pull forward an d aft is carefully considered , taking into account possib le yaw moments caused by wind and/or current or trim . A ship may start to swing. At one en d of the ship the tug then has to reduce power in or der to stop the swing. The propeller wash of tugs towing on a line may hit a ship 's hull an d decrease pulling effectiveness. This can be infl uen ced to a certain ex ten t by correct towline length and towing angle, as exp lained later.
So , whe n calculating th e for ces of wind , cu rrent and waves on a ship, a spec ified safety factor sho uld be taken into ~ccount for bollard pull required. In the graphs showing b ollard pull required to keep a ship up against a b eam wind , cross current and beam waves, a safety f~ctor ?f 20% is included. For tugs pulling at a ship's SIde th is safety factor is n ot sufficien t due to the large loss of pulling efficiency, which is separately considered.
5.2.1 Wind forces The forces on a ship ca use d by wind can b e calculated by the formulae : Late ra l force: 0·5 C ywP V ' AL Newton FYw = Longitudinal for ce: Fx0·5 C xwP V' ~ Newton Yaw moment: M xyw = 0·5 C xywP V' Cyw Cxw C XYw
p V
Ac L.p Newtonmetres
Lateral wind force coefficient. Longitudinal wind force coefficient. 'Vinci yaw moment coefficie nt. Density of air in kg/m' . \Vind ve locity in m / sec .
A,.
Longitudinal (broadside) wind area in m' .
~
Transverse (head-on) wind area in m 2 . Length between perp endiculars in m. .
Lsp
The lateral force, longitudinal force an d yaw moment coefficien ts depend on a ship's form, draft and trim, superstructure such as bridge, dec khouses, masts and ramp, and angle of attack of the wind. It sho uld also b e noted that deck cargo, as on co ntainer vessels, shou ld b e included in calculati ng wind areas. The coefficients C yw' Cxwan d C xywdiffer by ship and can be determined by means of m od el tests in win d tunnels. For several ship types the win d coefficients are know~ for all ang les of attack and certain loading
conditions. For tan kers they can be found in 'Pred iction of Wind an d Current loads on VLeCs'. Lateral forces are largest and most important for calculating ballard pull required. C yw varies between approximately 0·8 and 1·0 for beam winds, dep ending on ship's type and loading condition, but lies mo stly between 0.9 an d 1.0. With value 1·0 for C yw' 1·28 kg/m! for density of air and calculating the outcome in kilograms instead of Newtons, the formula for beam wind forces can be simplified to: FYw= 0·065V'AL kgf. To allow a safety margin of 20%, 25% should be added to th e previous formula, resulting in the following handy formula for estimating ballard pull required for beam win ds: FW = 0·08 V' A L kgf The graph in figure 5 .1 is based on this formula, whereby I mlsec = 2 knots' The calculated required ballard pull in the w ind grap h of figure 5. 1 is approxima tely 5% higher wh en, for wind speed in knots, a more accurate equivale nce in m/sec is taken. The safety factor of 20% included is in some cases eve n higher, b ecause for a lateral win d for ce coefficie nt th e value 1·0 is allowe d, whic h is sometimes only 0.8 or 0.85, although difficult to assess in d aily practice . T he
TUG USE IN PORT 69
:I 0
.e 60
~
i'l
11
10 50
1000
~
"-e
7000 8000 9000
1000 2000
Figure 5.1 Bollardpull requiredto compensate
3000
9 -e ~
Longitudinal (broadside)Wind Area (Square Meters) 2 3000 4000 5000 6000
for beam winds
40
8
Note 1: 17m is equal to lOOOkgforce (= 9·8 leN) .
~
til
.S ~
-
Nou 2: For ltzrge gas carriers see note in text
20
5 4 3 2
=0,8x1000x(25**2)
10
original data from UK National Ports Council 1977
0 50
100
150
200
250
300
. 350
400
450
500
Required Bollard Pull in Tons graph is only valid for tugs towing on a line or pulling at a ship's side on a rather long towline .
Note: For loaded tankers the outcome is too high, because the lateral wind coefficient of fully loaded tankers is approximately O· 7. For fully loaded tankers, however, it is generally more the mass that counts . Care should be taken when calculating the required ballard pull for large liquefied gas carriers. The lateral wind coefficient for these ships varies between 1·05 for gas carri ers with pri smatic tanks and 1·2 for gas carriers with spherical tanks (see References for 'Prediction of Wind Loads on Large Liquefied Gas Carriers'). Therefore, for ga s carriers with prismatic tanks 5% and for gas carriers with spherical tanks, 20% should be added to the outcome calculated by the formula or indicated by the graph in figure 5.1.
Wind velocity also varies by height, as shown in the graph in figure 5.2. The graph is based on the following formula:
vw
Vw
v
(1O/h)'/1
v.
wind velocityat 10metres height (mls). the wind velocityat elevation h (m/s).
h
elevation above ground/water surface (metres).
w
For calculating wind force in the equations, its velocity at 10 metres height should be used. For wind velocities obtained at a different elevation, adjustments to the equivalent 10 metre velocity can be made with this formula. On the oth er hand, wind indications given . Haigh t IH) .bov. S.. Left' ln M. t r..
40
For winds not coming from abeam the total ballard pull required can roughly be derived from the ballard pull required for beam winds. It can then be seen that when the angle of attack of the wind is between abeam and up to approximately 30 degrees each side of abeam, the ballard pull required is nearly the same as for beam Winds. In general, yaw moment is maximum for quartering winds but depends, amongst other things , on type of ship, loaded condition, trim and deck cargo . Wind does not blow constantly with the same force wind velocity fluctuates continuously. Therefore not just mean wind velocity should be accounted for, but wind that may be experienced in gusts and squalls. A wind meter, properly installed with a recording device at a pilot station, gives the best information . If considered necessary gust factors, e.g. from PIANC, can be applied tentatively ...?? to find the relationship between mean wind speed and associated maximum speeds for shorter periods.
35
/
30
/
25
.I
20
,s
/
'0
s
o
0 .5
.s-> 0 .6 Ralio Wind
0 .7 V.lo~Uy
V
0 .6
/
V 0 .9
1.0
Figure 5.2 Windheighl velocity ratio
70 THE NAUTICAL INSTITUTE
1.1
al IH) to V,lo olty _t 10 M Haight
1.2
Un derwater Late ral Area (Square Metu .) 1.0 0.8
~ 0
!
1000
2000
,
3000
0.6
-
, , --1- - - - -- , 01
0
~
0
••
SO
... 0
.S 100
..'3 •" "• ."•. '3
-e 150
•"'
'"
,, ,
0.4
0.2
200
250 300
5000
,, - - 1-
~
•" U
4000
, ,
_ _ __ L
~
,
,,
,,
___ _ __ L
,
~
_
-- -1------_ ,.J.
,, ,
___ _ _ _ L
,
,
, ,,,
_ _ _ '- _ _ J.
,
_ _L
_
, ,
- r
r
,
---- --~ - -- - --T - ---
,,
,, ,
,
_
---- - - T------T------~---
1 I I
I I 1
p V
LBP T
_
-- r
I I I
I I I
-- - - - -~- -----T- - --- - r---- - -T---
I
1
I
1
I
r
I
I
I
I
I
':i'
a
1.5
~ ~
~
,; 12 •
~ 1.1
I
FIgure 5.3 Bollard pull requiredina o oss-current Note: Ton is 'qual to l000kgforct (= 9·8 kN)
Original datafrom UK National Ports Council 1977 by a win d m ete r on top of a shi p's m ast give sa fe ap proximations for evalua tion of th e lateral wind force an d bollard pull required . A ship drifts under the influence of wind whe n the wind forces acting on her are not compensated for by tugs. A factor influencing drift velocity is underkeel clearance. A drifting ship has a relative spee d through the water, as with cu rre nt. The drift spee d of a ship decreases with underkeel clearance, because the forces create d by the oppos ing water increase when underkeel clear ance gets sm aller. This is co nside red later whe n . discussing current forces. Of course, a sm aller drift spee d do es not imply that less bollard pull is needed. A drifting vessel has to be stoppe d and pulled back through th e water. Stopping a ship from drifting and pulling back also needs more po wer in shallow water than in deep water. The amount of wa ter moving with a ship whe n drifting, th e .added ma ss, also in crease s wit h decrea sing underkeel . clearance, requiring additional bollard pull to stop an d pull b ack a drifting vess el in shallow water. 5.2.2 Current forces The current force s acting on a ship can be calculated in th e sam e wa y as wind fo rces . For th e sa ke of co m p le te n ess, th e formulae u sed in O CIMF publication s are given:
=
T Newton •
L.i T Newtonmetres
Lateral current force coefficient. Longitudinal current force coefficient. Current yaw moment coefficien t. D ensity of wa ter kg/ m" Curren t vel ocity in m/ sec. Length betwee n perpendiculars in m . Draft.
.,g
3.0
--'------- .: , , .,. - - - - - ,,
r... T Newt on
C", = CXYe =
_
I
Lon gitudinal force: Fxe = 0·5 CX, P V'
Cy , =
_
4 " __ _ _ _ _ 1
_
T .....
Yaw moment: Mxy, = 0·5 e xy, P V'
2
_
_ _
Lateral force : Fy , 0 . 5 C y, P '" y-
The current coe fficients, CYe' C Xt and C XYe] differ by a ship's un derwater shape, draft, trim and angle of attack, and are also affected by underk eel clearance which has a very strong e ffec t on the coe fficie n ts . T hese are determined by using ship mod els in test tank studies .
l.oS
For th e bollard pull required the maximum tran sv ers e forc es exerte d by a cross w ise current are important. Th e transverse force is calculated
using the formula: Fy, = 0·5 C y , P V'
r...T.
The lateral force coefficient for cross currents in d eep water is around 0 ·6 . This is, amongst o thers, th e
OCIMF-coefficient for loaded tankers. When C y , equals 0·6, density of salt water is 1025 kg/m", adding 25% for loss of tug' s effectiveness and giving th e outco me in kilograms instead of Newtons, th e following simplified formula for calculating the approximate bollard pull required for cross currents in deep water can b e used:
Current velocity is take n in metres /second, th e outcome in kilograms . This formula is only valid for deep water, i.e, more than six times ship 's draft Since und erkeel clearance in port areas can be sm all, the current forc es in the se conditions are at least as important as they are in deep water. With underkeel clearance decreased to 1·5 x ship's draft , bollard pull required increases conside rably to ap proximately:
r, =
110 V·
r... T
kgf
With an under keel clearance of 20% of ship's draft, the bollard pull requi red is rou ghly: Fe = 150 V' LB, T kgf When underkeel clear ance is further redu ced to 10%, the bollard pull required is nearly five tim es as high as in deep water, approximately:
TUG USE IN PORT 71
Fc = 185 V' L BP T kgf 25% has in all cases been included for safety reasons. Th e graph in figure 5.3 gives an indic ation of ballard pull required for cross curre nts and is based on th e afor ementioned formulae and OCIM F coefficients for loaded tankers. The outcome includ es a 20% safety mar gin. The graph is only valid for tugs towing on a lin e or pulling at a ship's side on a not too short towline . Th e effect of reduce d underk eel clearance on current force is also clearly show n in figure 5.4. Starting with a current force of 10tons, the same current velocity causes a strongly inc reasing force on the same ship whe n underkeel clearance decreases. . With a smal l underkeel clearance, curre nt forces decrease quickl y when the angle of atta ck of the cur rent b ecome s less th an 90 ° to a ship' s centr e line . Lon gitudinal for ces th en incr ease. The effect of the curren t forces on a ship may then even be in the opposite dir ection to that expec ted, in particular when with a sm all underkeel clearan ce the current is coming in at about 20-30° on the bow. When, e.g. after unmooring, turning with th e assistance of tugs a deep load ed bulk carrier with a small underkeel clearance in a river with ?..? current, the ship may gath er headway and move against the cur re nt directi on coming in from th e port or starboard bow. Pilots have experienced such effects and whil e turning h ave constantly to apply astern power to check the ship's headway. Not only do curre nt forces incre ase considerably with d ecr easing underkeel clearance. Small underkeel clearan ce also results in a larger turning diam eter, a decrease in rudd er effe ctiveness and an inc rease in * sto pping distan ce. To com pen sate for the se effects, the assis ta n ce of tu gs might b e w elcome for sa fe shiphandling. Un derkeel clear ance also conside rably affects th e du ration of swinging round a ship . The transverse forces to be overcome fore and aft of midships
in crease with de cr e asin g underke el clearance .
Consequ ently, the duration of swinging round increases, unless more ballard pull is used. 5.2.3 Wave forces D ep ending on environmental conditio ns in an d aro und a port, wave forces may also be a factor to be considered when establishing the ball ard pull required. H arbour tugs can only operate effectively up to a certai n maximum wave height (see Chapter 4), so only short beam seas are considered. It is difficult to calculate wave forces exactly. It is assumed that a ship's dr aft is lar ge eno ugh to reflect the waves completely. Because of th e relatively short wave peri od it is furthe r assumed that waves do not cause any ship motion. In pr actical terms it means we are considering conditions such as those found in windy but she ltered areas. The waves are sho rt and steep and the wave length is sma ll relativ e to th e length of the ship. We are not considering ope n areas, wher e ocean waves or swell might impinge up on th e ship and cause it to heave, roll and pitch. The for ces per metre of ship' s length du e to these short period waves then amounts to appro ximately:
Fwave = 0·5 P g r' 'e, Newton Because a ship's hull is not flat over its wh ole length and draft, the total force on a ship caused by sh ort period wave s is roughly: Fw . . . = 0·35 P g L p L ~a
o E
(H,l.
H,
Significant wave height from trough to crest, as indicated by an experienced observer whe n estimating visually.
A 25% safety margin is again adde d, an d converting to kilograms instead of Newton an d wave amplitude in
0.5 x Draft
Figure 5.4 Effietof underkeelclearanu on current force
72 THE NAUTICAL INSTITUTE
Newton
Density of seawater in kg/m 3 • Length of waterline; assume length betwe en perpendi culars. Wave amplitude, equal to 0·5 x wave height
111'fli! ~
1;.'
40 Ton .·2 .·7.·? ·z. ;.z ·,·z·,·z.:·z·, 0.2
x Draft
5.2.4 The effect of sh ip's mass and berth
Kil og ram Force p er M length b etwe en PP
500 480 460 440 420 4 00 380 I 360 -I 340 320 300 260 260 240 - 220 --- 200 180 16 0 140 >-12 0 10 0 80 60 40 20 I----
I
co nstructio n
I
-7
I I
I
--l
----
---
As menti on ed ea rlier, tugs should have sufficient reserve power to stop a driftin g ship. A comp arable situation exists during berthing. An arriving ship is stopped parallel to a berth or jetty and is the n pu shed , .??alongside. Wind, curre nt an d even pulled or heaved waves may also pu sh a ship towards a berth . Due to these forces a ship gains transverse speed which should be slowed down by tugs to 'd ead in the water ' or to a safe berthing speed at the mom ent a ship touches the fenders . So, tugs have to oppose the forces of wind,
-++1-
I I
----
-r-r-r--
I
I
---
il
----,-1
f----- - 1f--f--f j = + = 1----"==:1-~_ - -----1--->-- ---
---
-~~~=1~- 7
- '----
fc"::~ -~ lI-
0.2
current and waves, and in addition have to reduce the
tran sverse appro ach speed of a ship towards a berth, which requires additional bollard pu ll. Of course, the wind may blow offshore and tugs may need full power to push or pull a shi p alongside. But even when there is no wind, curren t or waves, bollard pull is needed to
1/
/4
./
Irr----+ I I
o
o
--
0 .4
0 .6
0 .8
1
control a ship's transverse speed. 1.2
1.4
1.6
1.8
2
Sign i fi cant Wave Heig ht (M )
REOUIR ED BOLLARD PULL f OR BEAM WAVES (Olll y valid fo r .h ort per iod wavu!
Figure 5.5 Bolla rd pull requiredfor beam waves
significant wave h eight, th e simplified formula for roughly calculating the bollard pull required to hold a: ship up against short period b eam waves reads:
Fwave
= 112 LH s' kgf
O n the basis of this formula the bolla rd pull requir ed is represented in the graph in figure 5.5. An example: A sbip h as a length between perpendiculars of 200m, an d estimated wave height is 1m. The force of the beam waves on the ship is then (see formula):
The larger a ship's displacement the more bollard pull is needed to stop sideways movement, Not only the displacement but also the water mass moving with a ship influences bollard pull requ ired. This is called (added' or 'hydrodynamic' mass. Virtual mass is the sum
of displacem ent and adde d ma ss. The exact am ount of add ed mass is difficult to determine. The added mass in c rease s w ith decreas ing underk e e l cl e ar an c e . Furthermore, it depends on a ship'S underwater shape and is very lar ge with a sideways motion. It then normally varies be twee n 25% to 100% of a ship's displa ceme nt. Many for mulae u sed for calculating virtual mass of a b erthing ship, especially for fend er design, indicate values ranging from 1·3 up to more than 2·0 times the disp lacement.
112 x 200 x 1 x 1 = 22400 kgf = 22 tons
Photo:PorIof Glatlsto1U, AlLIualia
Figure 5.6 Open berth constructionfor bullc carriers
TUG USE IN PORT 73
Berth construc tion also affects approa ch spee d. Solid berth s reduce a ship's approach speed because a water cu shion builds up between ship and berth. Open berths or j etties do not reduce approach speed as the water can flow away in any direction.
For fend er calculations it is gene rally recommend ed to app ly for a wate r depth of 1·5 times ship' s draft as virtual mass 1·5 times the displacement an d for a water depth of 1·1 times ship's draft as virtual mass 1·8 times the displacement. As virtual mass 1·8 times displacem ent is taken and be rth co nstruc tio n is th en accounted for. A ro ugh ind ication can thus be made of tug forces required to stop sideways movem ent: For op en berths:
0·09 D x V', - - - tons
S 0·07 D x V,' For solid ber ths: - - -- - S Por que menor ? V, D S
=
tons
Initial speed in m etres/ sec Displacem ent Stopping distance in m etres
This formula is based on zero final spee d and the calculated force is in ton s. Final safe approa ch speeds for VLC Cs are gene rally a maximum of 6-8 em/sec. In th e following thr ee examples an initial speed of 0·5 knots (0·25 rn/sec) is assume d and tugs start pulling whe n a ship is 30m away from a berth. Transverse speed sho uld be zero when a ship touches a j etty or berth.
A 250,000 dun ballasted lanker Length overall 340m, beam 38m, draft 9m (29·5ft) and displacem ent 124,000 tons. She has to be berthed alongside an open j etty. According to the. formul a the total tug po wer require d to stop the ship in 30 metres is approxima tely 23 tons. A 250,000 dun loaded tanker Draft 20·4m (67ft) an d displacemen t nearly 300,000 tons - much mor e power, almost 60 tons, is needed. A container ship Length overall 294m, beam 32·2m an d draft 12·2m (40ft), displ acem ent 80,000 ton s. Sh e has to b e berth ed at a solid berth. Based on the assumptions above the b oll ard pull re quired to sto p her in approximately a shi p's width i~ 12 tons. Th ese calculations give an indicatio n of th e forces required. In lin e with expe rienc e they show that large di spl acement ships requite large stopping forc es. Furtherm ore, berth constructio n is a factor influ encing
approac h speed.
74 THE NAUTICAL INSTITUTE
Lo ad ed tanker s an d bulk carrie rs wit h large displacement nee d the largest tug power for controlling transverse speed. T hese ships are less affected by wind. When ther e is any current, ber th construc tion should be such that the cur rent runs in line with the berth or jetty, though unfortunate ly this is not always so. In any case, tugs should have sufficient re serve power to compensate for any current and /o r wind effect. In general, when handling heavy ships , tugs use a substantial part of thei r power to control tr an sverse approach spee d towards a be rth. As dra ft decreases the b all ard pull required for con tro lling tran sverse spee d becomes less, as indicated in the examples for a load ed and ballasted tanke r. Lateral wind area increases and consequently available b ollar d pull can be used to keep the ship up into wind, current and/or wav es, if necessary. Newer tugs are ahl e to operate for a limite d time at l lOOfo MCR. This means that for a sho rt peri od these tugs can deliver additio na l bollard pull, an advantage in critical situations.
For ships affected by wind, current and!or waves a safety margin is included in the graphs, also for the purpose of controJling transverse speed. For load ed vessels, tanke rs and bulk carriers, ballard pull required for controJling tr an sverse speed is included in th e formula in section 5.3.1. 5.2.5 Thg wash effects
In certain pull ing situations, a tug's propeller wash impinges on a ship's side, bow or stern, reducing pulling
effectiveness. The smaller a ship's underkeel clearan ce
Photo:Author
Figure 5.7 A tug', propelkr wash hilling a ship~ hull, reducing lowingeffectiveness
the larger the negativ e effect of propeller wash hitting th e hull. Increasing propeJler rev olution s or thrust worsens' the situation bec ause counter e ffec t also inc reases , caused by a larger, more co nc entrate d prop eller wash. Proper towline length and towing angle
"
"
4a
ta
crea tes a low pressure resulting in a force F. This has to do with the Bernoulli effect, which is explained in the next chapte r. The result is tha t the pulling force T is opposed by the reaction force R and the only force left is force F, giving the ship a forward and starboard instead of port turning movement.
Figure 5.8 DifJerrnl lowingposilions
Compared to positio ns If and l a of figur e 5.8, positions 2f and 2a may show'less loss of effectiveness. Regardin g loss in effectiveness due to propeller wash towing positions and towing dir ections 3f and 3a are considered the most effective. Tugs operating at a ship's side , in positions 4f and 4a, have a large loss of effectiveness whe n pulling. When operating in the pushpull mode to wline lengt hs are short and pulling effectiveness can even be less than 50%, depending on how close the tug's pro pellers are to a ship's hull. Tug propellers shou ld be as far as possible away from a ship's hull. Conventional tugs, towing on a line, have their propell ers closer to a shi p's hull compared with tractor, reverse-tractor and ASD-tugs. Th e latte r two types, when towing or pu lling over the bow, have their prop ellers furthest away from the ship's hull. Thi s is of spe cial import ance for tugs operating at a ship 's side or in narrow harbour basins where they often have to work
F
Force due to Ooande Effect
on short towlin es du e to limited manoeuvring space. VS tugs have less pronounced propeller wash compared with conventional tugs and tugs with azimuth thrusters, in p articular tho se wit h prop ell er s in n o zzles . Consequently, the negative effect of VS propeller wash hitting a ship's side is less. Tugs with azimuth thrusters can set their thrusters at a small angle, at least with
Figure 5.9 'Coa7lJ1Jl '
reduce thi s adver se effect. The less the underkeel clearance and the more power needed, the longer a towline shou ld be. In figor e 5.8, several towin g po sitions are given for a ship stopped in the wate r. In positions If (f=forward) and l a (a=aft) there is a fair po ssibility that the pulling tugs experience loss of pulli ng effectiven ess due to propeller wash hilling the bow and stern almost at right angles . A ship's hull form, shape of bow and stem and whether she has a large bulbous bow, influen ce loss of effectiveness. For th e pulling tugs, e.g. tug If, it might even be poss ible that the tug's wash effect causes a turning m om ent on the ship in an opposite direction to that expected fro m the orientation of the tug. Such an effect is shown in figore '5.9. The ship is loaded, has a bluff bow a n d a sm all u n d erk eel cl ear an ce . A conventional tug is pulling at right angles to the ship' s hull on a short towline . The consequ en ce is an alm ost total loss of towing effectiveness by the reaction force R of the propeller wash hittin g the ship's hull. In add ition, the bulk of the accelerated water flow goes aroun d the bow of the ship and remains allached around the curved surface. This is called the 'Coanda Effect'. The flow
independently controlled thrusters, thus deflecting the wash. So, loss of pulling effectiveness of forward and aft tugs towing on a lin e can be minimised by appropriate towline length, towlin e angle and/or thruster selling. A towing winch is very useful for adjusting towline length in accor dance with circumstanc es. For tugs op erating
at a ship's side, wh en pulling, th e larger the distance between propellers and ship' s hull the be ller. For tugs operatin g at a ship's side and holding her up into the wind, current or waves on short towlines, the required pull in the graphs in figores 5.1, 5.3 and 5.5 should be increase d by, say, at least 20%, resulting in a total safety margin of 50%.
5.3
Bollard pull required
5.3.1 Bollard pull re quire d b ased on environmental con ditions an d displacement In the following assessme nts of req uired bollard pu ll it is assumed that equal tug powe r is required forward and aft, which is not always the case. Yaw mome nts can be caused by wind and dep end on the wind force, angle TUG USE IN PORT 75
The onshore wind (figure 5.1) 117 tons The crosswise current (figure 5.3) 42 tons The waves 281 m x 30 kg (figure 5.5) = 8 tons Total ballard pull required = 167 tons
of attack and on the ship's profile above the water, which varies with draft, trim and deck cargo . In addition, current may cause a yaw moment, depending on the
current velocity, angle of attack and ship's underwater profile which varies with draft and trim. Although with beam win ds or currents a ship may ex pe rience a,yaw moment, they are generally largest with quart ering winds and currents. Yaw momen ts cause d by currents ev en increase with decreasing underkeel clear ance. Yaw
To compe nsate for wind, current and waves, four
tugs with at least 40 tons ballard pull are needed. In the total b allard pull required for wind, current and waves a safety factor of at least 20% = 33 tons is included . This reserv e p ow er is also, am on gst other things,
moments caused by wind and! or current may result in a higher ballard pull requirement forward or aft.
sufficient to control approach speed towards the berth. Without an y cu rre n t or wav es, fo ur tugs of approximately 30 ton s ballard pull would be n eed ed
Th ere is another aspect to be tak en into account. When for example pulling a ship offthe berth, the lateral
wind forces.
Of,
wh en available , two of 60 ton s, to compe nsate for
underwater resistance becomes effective. On a ship 1\I105t container ship s, car carriers, ro -ro ships an d so on are equipped with bow thrusters or b ow and ste rn
having a large stern trim , the centre of pressure of the lateral resistance lies aft of mid ships. When forward and aft the same amount of ballard pull is used, the after tug(s) have to use more power than the forward tug(s) to pull the ship parallel off the berth. A ship down by the head may require more ballard pull forward than aft.
thrusters. 100 HP of a bow thruster is about 1·1 tons force, 100 kWabout 1·5 tons force, for a ship 'dead in
It is because these turning moments vary so much, only the required total ballard pull is considered. How much ballard pull or how many tugs forward and aft are required should be carefully considered each time,
container vessel is equipped with a bo w thru ster of 2500 HP (1840 kW) then 28 tons less ballard pull is required forward . If just the influence of wind is to be compensated for, this would lead to a reduction in th e number of tugs of 30 tons from four to three, i.e . one forward and two aft.
based on an assessment of the actual situation and circumstances.
Therefore, experience is an indispensable factor. As mentioned earli er, master and pilot are in a better position to assess requirements for tug assistance and
unwanted effects to be avoided if the y have a good understanding of the forces and other factors influencing a ship and of tug performance.
Shipsafficted by current, wind and/or waves The graphs in figure s 5 .1, 5.3 and 5.5 give an indication of th e ballard pull required by ships affected by wind, current and! or waves. As an example for using the graphs: Container ship: length overall 294m , length between perpendiculars 281m , beam 32m , draught 12·5m, water d epth 13·8m. Top of containers to waterline approximately 22m . Onshore wind at right angles to the berth. Wind speed 30 knots (7 Bft). The location of the container berth is not too good, with a cross current
of 0·5 knots. Short period waves of 0·5m height are also coming from a direction perpendicular to the berth. Ratio draft/water depth 13·8: -12·5 Area above water, approx. 294 x 22 Underwater area, approx_281 x 12·5 Displacement
= [·1
= ±6500 m' =±3500 m' = 75,000 t
When towing on a line or pulling at a ship's side on not too short a towline, the following compensatory total ballard pull is required : 76 THE NAUTICAL INSTITUTE
the water'. The effectiven ess of stern thru sters is generally somewhat less. If th e ab ove m entioned
It is clear that whethe r a bow thruster can repl ace a tug depends on the forces to be compen sated for and the ballard pull of the available tugs. It also dep ends on the local situation, circumstances and p ort regulation s as to whether side thrusters can repl ace on e or m ore
tugs. For certain situations, for example whe n passing narrow bridges wher e tug assistance is required, it is
preferable to have a forward tug on a line. Regardless of the fact that a ship is equipped with a bo;'" thruster, its effectiven ess decr eases very quickly as a ship gathers forward speed. At a speed of two knots through the water, effectiveness is usually reduced by 50% compared to zero speed. At four knots the effectiveness of a bow thruster is reduced almo st to nothing. At such speeds a bow thruster cannot replace a forward tug. It should also b e noted that the effect of a bow thruster on a ship becomes less with d ecreasing underkeel clearance, due to the higher force s n eeded to turn a ship, to move a ship sideways or to stop a sideways movement and to compensate for th e influence
of currents. Therefore a ship equipped with a bow thruster, which normally uses no tugs, may require tug assistance in shallow water conditions .
When tugs operate in push-pull mode and have to hold a ship on short towlines up into wind, current and! or waves, the required pull in the graphs in figures 5.1, 5.3 and 5.5 should b e increased by at least 20%. In th e case of the container ship with a bow thruster and an
Number of Tugs TotalBollard Pull
4
200
3
150 AverageNumber of Tugs Avcrage Bollard Pull
2
o
I
I
I
300 rn. Length o.a.
Figure 5.10 lbtal bollardpull in Ionsand average number of tugsfor containerandgeneral cargo vessels asused in a number ofports around the world.Dependingon the port andlocal circumstances less tugs mo.y be usedwhenships are equipped with sidethrusters TotalBollard Pull
Number of Tugs
5
250
4
- 200
3
150
2 -
o
120
150
200
250
300
350 m. Length o.a.
Figure 5.11 Total bollardpull in tons and averagenumber of tugs for tankers and bulle carriers as usedin a number ofports around the world (based on length overall) Number of Tugs Total Bollard Pull
250
4
200
3
150
...;
2 50
20.000
100,ODO
200,000
300,000 Deadweight Tonnage
Figure 5.72 Ibtalballard puUin tons andaverage number oftugs1Mtankers andbulk carriers as used in a number ofports around the world (based on deadweight)
TUG USE IN PORT 77
onsho re wind of 30 kn ots, a total ballard pull would then be n eeded of about 140 (117 tons + 20%) - 28 (bow thruster) = 112 tons: roughly a 40 ton tug at the forward shoulder and two of 35 tons at the after shoulder.
ship s, co ntainer vessels, tanke rs and bulk carriers in a
number of ports is shown. With regard to these graphs, on departure and for ships partly loaded or in ballast, fewer tugs or less ballard pull than indicated is sometimes used.
Ships with large displacements Lo ad e d tanke rs and b ulk carri ers have lar ge displacements. For the se typ e of ships the following formula can be used, based on the displacement of the ship s: ) displacement Required ballard pull = - -- - x 60 ) +40 (tons) 100,000 ) Note: Some times tugs have to assist in station keeping at offshor e install ations, such as SPMs and F(P)SO s. Although required ba llard pull as discussed generally also applies to these tugs, the reader is further referred to the information included in the OCIMF publication 'Recomme ndat ions for ships' fittings for use with tugs' (see References). 5.3.2 Number and total ballard pull of tugs as used in a number of ports There is no uniform system in use in ports around
the world giving a relationship between size of ship and numb er and power of tugs required. Calculations are m ostly based on len gth overall, but deadweight, displace ment or gross tonn age are also used as factors.
The same applies to ships equipped with bow and/ or stern thrusters. Sometimes, if equipped with a bow thruster, though not in every p ort, one tug less is used than indicated in the graph and when equipped with both bow and stern thrusters two tugs less than indicated may sometimes be allowe d. Furthermore, p orts or terminals may have a limited number of tugs available to assist ships varying in type and size. The graphs in figures 5.10, 5.11, and 5.12 give the min imum, maximum and average total bollard pull used in a number of ports, including the average number of tugs. For ballard pull used the upp er line of th e graph is assumed as the requirement for more difficult situations and the lower line for normal and easier situations. The average bollard pull used shown in figures 5.11 and 5.12 for bu lk carrie rs and tankers is more or less compara ble with the ou tcome of the p r eviously mention ed formula b ased on displacement for ships of deadweight up to about 230,0 00 ton s. 5.3.3 Summary For ships affected by wind, such as container vessels, ro-r o vessels, car carriers, gas carriers, tankers and bulk
Decisions on the number of tugs to be used in ports and the ba llard pu ll required are mainly b ased on experience. For the majority of ships and situations a mo re or less standard number and/or ballard pull is used. Large sh ips an d mor e specific situations or circu mstances are generally assessed separately by the pil ot and/o r port au tho rities to det er mine th e tug assistance required and if necessary an d possible this is done in consultation with the master. Simulation studies are som etimes need ed to assess requirements for spe cific
situation s or specific ships . In most ports shipmasters or pilots are free to order the numb er of tugs and /or ballard pull they consider necessary to han dle a ship safely. In some ports it is compulsory to use a fixed numb er and power of tugs, dep end ing on the type, size and draft of the ship, environmental conditions and location of the berth . It .may also depend on whether a ship is to berth port or starboard side to. This ob ligation, th ou gh ge ne ra lly a mi nimum req uirement, exists in a numbe r o f Far East and Australian ports and at some large oil terminals. For ships equipped with side thrusters, a reducti on in the number of tugs or required power is sometimes allowed. In figures 5.10,5.11 and 5.12 tug use for general cargo 78 THE NAUTICAL INSTITUTE
carriers in ballast, the ballard pu ll required can b e approximated using the wind graph for cross winds. The influence of current and waves can be accounted for using the current and wave graphs. When assessing ballard pull req uired, the assisting mode - whether on a line or operating at a ship's side should be taken into account. For tugs operating at a ship's side on sho rt towlines the results of the wind, curren t and wave graphs sho uld be increased, roughly estimated, by 20% whe n pulling. For ships with large di splacem ents, balla rd pull required can be approxima ted using the formula based on displacement. Th e graphs showing ballard pull used in a number of ports give an indicatio n of the ballard pull require d for more difficult and more no rmal situations.
Ships with side thrusters, partly loaded or departing may use less ballard pull than indicated. Ho wever, this depends on the local situa tion, circu mstances and port regulations. Co ntrol of tr an sver se spee d to wards a b erth is include d in the graphs and formulas. For a rou gh check the formula as shown in section 5.2.4 can be used.
5.3.4 Influence of tariffs on availability and number of tugs used
Due to the high costs of tugs and .their crews the inefficiency of tug fleet use may result in the availability of tugs coming under pressure, which is the case in a
Shipping comp anies have to pay for the use of tugs, though in some ports tug tariffs are included in port dues . Tug tariffs are usually based on the size of ship and number of tugs or total ballard pull used. In many ports ships are charge d extra for tug assistance durin g adve rse weat her conditions such as strong wi nds, ice or
fog. Th e same applie s to tug services duri ng night h ours, at weekend s an d whe n tug assistance takes longer than a specific basic time period. Tug tariffs often affect the number or ballard pull of tugs used. That is why some attention is pa id to this subject, bearing in min d that circumstances and tariffs differ by por t. Ship arrivals and departures have an irregular pattern and may be influenced, amongst other thi ngs, by the working hours of dock labour and tidal restrictions. This means that ships may arrive dur ing peak hour s, for examp le during hours of slack or high tide. The number of tng s in a po rt is to some extent determined by shipping traffic during these peak hour s. A numb er of tugs rendering assistance during bu sy hours will be un employed outside thos e hours, so peaks in shipping traffic affect efficient employment of a tug fleet in a negative way.
To run a fleet more efficiently and to redu ce costs a tug com pany could consider redu cing the number of tugs. However, this automatically affects the availability of tugs during peak hours, caus ing waiting tim e or resulting in fewer tugs being used for a parti cular ship movem ent, thus affecting safety. A category of ship with an irre gular pattern of tug use is tho se with side thrusters, twin screws and high lift rudders, such as large container vessels, cruise vessels, ferries, car carriers and fo-ro vessels having a large
windage. These ships often don't use tugs, or only a minimum number, except when the wind is increasing.
This can happen after weeks of calm weather. These ships affect the availability of tugs, especially du ring adverse we ath er co nditions, re sulting in a furth er
number of ports. However, the availability of a sufficient number and ballard pull of tugs, especially during peak hours, is an esse ntial factor in good service to the
shipping industry and to running a port efficiently. But co nsidering the po sit ion o f towing co m p ani es , . availability alone does no t pay. Neither does inc rease d tug power, unl ess tug tariffs are also base d on total ballard pull used. Dep ending on shipping traffic in a port, a mor e efficient tug fleet - witho ut affecting the availability of tug assistan ce - can be achieved by the use of less units but of higher power. Less tugs can thus be used per ship, for example, for large tan kers or bulk carriers. These types of ship normally use a standard number of tugs of certain ballard pull. Beyond peak hou rs less tugs will the n b e unused. In ports whe re pr oblem s eme rge regarding th e availability of tugs and tug power a review of the existing tug fleet may be necessary including a review of tug tariffs. Thi s may result in less units of high er p ower, as mentioned above.
Regular meetings between p ort authorities, towing . companies, shipping compani es an d pilot organis ations is nec essary , in ord er to ke ep port se rvices at an acceptable level without raising tug tariffs too much. It might be 'wort h cons idering wheth er a b asic tug tariff
could be included in a port tariff to ens ure minimum availability of tugs. As indicated, in certain p orts tug tariffs may influence the availability of tugs and conseque ntly a pilot's work . Pilots should be permitted to assess the minimum tug requirement to handle a ship safely. On the othe r hand, it is quite reasonable that the cost of tug assistance is a factor taken int o account by a shipping company when ordering tugs, although economy should never have priority above safety. The cost of tugs is frequ ently the background of discussions between masters and pilots, when the number required is discussed, except for ports where the use of tugs is compulsory or strictly regulated .
decrease in tug fleet efficiency. In addition, ships are getting larger and consequently tug power has inc rease d con siderably in recent ye ars. Port dim en sion s h ave ofte n no t exp an de d proportionately to the increase in ship sizes so large ships in restricted manoeuvring ar eas plac e a heavi er demand on towing assistanc e. More powerful tugs also me an higher costs for towing companies.
A good contrac t be tween shipping companies and tug owners, stating the numb er and ballard pull of tugs to be used, and covering circumstances when additional tug power might be neede d, e.g. adverse weathe r conditi ons , is stro ng ly recommended. When tug assistance is necessary it can then be expected that the required numb er and b all ard pull of tugs will b e available with out additi onal cost.
TUG USE IN PORT 79
Chapter SIX
INTERACTION AND TUG SAFETY 6.1
Introduction
PREVIOUS CHAPTERSDEALT \nTH THE CHARACTERISTICS and effectiveness of various types of harbour tug. Anot her very important aspect, sometimes mentioned in those chapters, is the risks harbour tugs m ay encounter wh en ren derin g assistance. It is an essential p oint wh en
engage d in shipha nd ling ope ra tions. Essential, because it is not only the safety of a tug and her crew that could be at risk but also the safety of a vessel. When rend ering assistance tug captai ns and pilots shou ld be fully aware of the risks invol ved. Since a number of unsafe situations can be traced back to interaction effects, attention is
first paid to this subject and also the influence of shallow water on several interaction effects and the tug assistance requi red,
6.2
Interaction and shallow water effects
6.2.1 Interaction effects infl uencing tug p erformance
Tug hull - shiphull interaction The influence of this effect on tug performan ce is particularly marked when a tug operates at a ship's side . This kind of interaction is also influen ced by shallow and narrow waters and in parti cular by ship's spee d, affecting tug safety as well. Ship propeller/ship hull - lug interaction Th ese interactions affectperform ance when operating as stern tug in the propeller slipstream or ship's wake. The effect of ship's wake increases in shallow and narrow wa ters.
The points above show that there are several kinds of interaction affecting tug performance. Tug hull - ship hull interaction affects tug safety as well. This effect and ship propeller/ship hull - tug interactions are dealt with in this chapter. The others have been discussed in previous chapters. Sma ll und erkeel clearance affects some of the interaction effects, as indicated. It is worth considering som e o ther effects of shallow wa ter.
6.2.2 Shallow wate r effects with respect to tug There ar e different kinds of in ter action . Some influence tug performan ce, others affect tug safety, some both . Interactions influ encing tug performan ce are:
Tugpropeller - tughull interaction For examp le, the astern thrust of a reverse-tractor tug/ ASD-tug is 5-10% less than ahead thrust, as a result of pro peller wash hitting the afterbody of the tug and so reducing bollard pull when astern thrust is applied. Interaction oftugpropellers Thi s is espec ially the case with azimuth thrusters and VS propellers. Depending on thrust direction, the two propellers of tractor and ASD /reverse-tractoi tugs interact to a certain ext ent and affect a tug's
performance. Tug - ship interaction due to tugfendering Fender characteristics such as energy abso rp tion capabilities and friction coefficients influence the interaction of forces between tug and ship and also tug performance. Tug - towline interaction Tug reactio ns such as tug list and consequently tug performance are in fluenced by towline characteristics, especially by its dynamic load abso rption capabilities. Tug propeller - ship hull intera~tion The reduction in pulling performance d ue to tug propeller wash hitting a ship's hull has bee n dealt with in a pr eviou s chapter. In the case of small und erkeel clearance this effect is more pron ounced . Pu shing tu gs are al so affec te d by thi s typ e of interaction when prop ellers are close to a ship's hull, due to interrupted water flow toward s the pro pellers. 80 THE NAUTICAL INSTITUTE
assistance
Some effects of shallow wate r have already been dealt with whe n discussing the bo llard pull required in relation to current forces and a ship's displacement. The
relationship between decreasing underkeel clearance and increasing bollard pull required to hold a ship up into a current or to stop a sideways moving ship h as been mention ed previously. There are othe r sh allow water effects necessitating tug assistance and requiring the full atte ntion of pilots and tug captains. There are situations where these effects occur and tug assistan ce is then very welcome: Shallow wa ter, meaning sm all under ke el clearan ce , has the following effects am ongst othe rs:
Increase of banksuction and bow cushion effects A ship proceedin g to one side of a river or cha nne l and close to a bank experiences suction forces towar ds the bank. These forces are no t uniformly distributed over a ship'S length. Their resu ltant acts somewhere abaft of midships. The overall effect is a bodily attraction towards the bank - bank suction an d a yawing effect away - bow cushion. A ship can proceed in a stable situation parallel to the bank by applying rudd er towards the bank. But as soon as this stable situation is disturb ed e.g. by an irregu lar profile of the bank, even a submerged bank, or by careless steering, a ship may sheer away from the
bank. If this happens it is difficult to control the ship and she may even sheer to the othe r side of the river or cha nneL
T he sm aller the underkeel clearance the more pronounced bank suction and bow cushion effects are. They can be kept under control by keeping away from banks as far as possib le and by adjusting ship's speed. Bank suction and bow cushion effects increa se proportionately with ship's speed, viz. by the square of the speed. At a speed of four knots attraction towards a bank and yawing moment away from it are four times as high as at two knots. Also, the lower a ship 's speed the more reserve power is available to
Following warerfrom channel turns and
moves ship aside
Bay
give a ' kick ahead' with rudder hard over to co unteract a sheer. In shallow waters and whe n close
to banks tugs should be on the alert to counteract an unexpected sh eer. Bank suction and bow cushio n effects have all to do with a Mr. Bern oulli, which is exp lained in the next section.
Decrease of rudder effict Possible increment of transverse effict of thepropeller Increase of turning circle radius The turning circle radius in shallow water is mu ch lar ger than in deep water. The initial rate of turn is much sma ller. Manoeuvring a bend in really shallow water is therefore more difficult than in deep water. Tug assista nce may be. required to take a bend properly. The lower,a ship's spee d, the m ore reserve
~ Wa G following snip up channel
po wer is available to control movement and the more
effectively tugs can operate .
Increase ofstopping distance due to larger virtualmass In shallow water a ship drags a large amou nt of water along with her, increasing to as much as 40% of her displacement when keel clearance reduces to 20'/0 of the draft. When unde rkeel clearance is small, more astern power and consequently more tug power are need ed to stop a ship than in deep water. When passing thro ugh a channe l with little underkeel clearance the large amount of water following a ship occasionally leads to ano ther int eresting effect. When a ship comes to an abrupt stop in a basin at the end of a ch ann el the following mass of water need s time to slow down and ove rtakes the shi p. It may push her ah ead , may increase the rate of turn , or push her sideways when she is tu rni ng. O ne has explained that this effect is cause d by the so-called 'adde d mass'. However, it is more likely to be the water flow in the channel following the ship and filling the gap behind which causes the delayed effect when a ship comes to an abrupt stop. This effect has been expe rienced , for examp le, with large tanke rs in a Caribbean port (s ee figure 6.1). The extent of the effect is direc tly related to ship's speed. It is clear that low ship 's speed is very impo rtant whe n pro ceeding in sh allow waters. Shallow wat er effects also enlarge int eraction effects between ships . Fro m the point of view of safety duri ng tug ope rations one sho uld always be aware of the possib le occurrence of these effects.
Figurt 6.1 Effict offollowing waltr whrn p=ing throngh a dumnel with a dttp loaded ship andcoming to a stop at the end ofthe dumnsl
6.2.3 Interaction effects influencing tug safety
Flow pattern around a ship The interactions which most endanger tug safety are those happening whe n ships are sailing or man oeuvring close to one othe r. It is the water flow arou nd a ship which produces int eraction effects. Wh ether a ship is moving through the water or water is moving along a ship does not make any differenc e; water spee d relative to the ship is the same. For furth er exp lanation see figure 6.2, where the ac tua l flow pattern th at cou ld be experienced, for inslance, by a tug stoppe d in the water is shown. In figure 6.3 the water flow relative to the ship's speed is given. Boca
If a ship underway through the water had no beam (nautical) or draft, the water round the ship would have constant spee d relative to the ship - the relative speed of the water would be the same as the ship's speed. Naturally, however, a ship does have beam and draft, so water is pushed sideways an d downward s by the ship an d still has to pass along the ship from bow to stern in the same time but on a path that is longer than the length of the ship. Hence most of the time during its passage along th e ship th e r el ative velocity of th e water flow is in creased.
TUG USE IN PORT 81
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• FIgUre 6.3 Pressure pattern and relativeflowfieldaround a bulkcarrier
This is where Mr. Daniel Bern oulli comes into the picture . An 18th century Swiss philosopher, h e established the relationship between water speed and water pressure. He showe d that an increase in water speed results in a decrease in water pressure and vi ce
versa, wh ereby the change in pressure is pro portional to the square of the speed change . So, when water speed is doubled pressur e redu ces to a quarter. Follo wing Bernoulli's theory the re are redu ced pressure areas round a ship where the relative velocity of water flow is increased. If stream lines wer e parallel aroun d a ship the reduction in pr essure would be uniform . Well ahead of a ship the stream lines are equally space d, but at a certain point they ar e wedged apa rt and as they go ro und the body of the ship they are compressed. At the stern the stream lines tend to spread again in an effort to fill the gap astern of the ship . Wh en stream lines diverge the speed of the water reduce s and, according to Bern oulli, pressure increases. 'When stream line s converge, water speed increases and wa ter pressure reduces. This boils down to conservation
wave making and wave br akin g (at the bow) resistance. The wave length found in such a wave p att ern is a function of the speed of the ship. Pressure field s caused by the Bernoulli effect are the main cause of the wave pattern arou nd a ship at low speeds. It means that at the bow ther e is a high pres sure area, a bow wave, followed by a low pressure field around the midsection while at the stern there is again a high pr essure area, altbo ugh lower than that at the bow.
Du e to viscou s resistance or skin friction water is drag ged along with a ship, a little at the bow bu t mor e and more toward s the stern. It forms a fairly dead layer of water, called th e b oundary layer , in cr easing in th ickn ess fro m bow to ste rn. Abaft the stern the boundary layer form s the frictional wake. This boundary layer and wake astern of a ship result in a less marked spreading of stream lines, resulting in a smaller high pr essure field near the stern tha n at the bow. Particularly in the case of wide b odied ships, wate r speeds up round the forward but less round th e aft shoulde rs, causing a local wave trough .
of energy in fluid flow. At low spee ds a ship's wave making re sistance is minimal. Th e wav e p attern
gen erated by a ship tra velling at higher speeds causes 82 THE NAUTICAL INSTITUTE
In shallow wat er th e flow und erneath a ship is restricted and mor e water has to pass along the sbip
sides than in deep water. Co nsequently along th e ship sid es the water has a higher speed and the reduction in pressure is larger, while high pressure near bow and stem increase, assum ing the same ship's speed as in deep and open waters. When in sh allow and in narrow waters,
Wh en th e tug approaches the stern from a position behind tug no . I, it experiences an increase of speed due to the relatively low water speed. Th e tug may be pushed sid eways to starboard as well by the incoming waterflow (see also figure 6.2).
the water flow between a ship and the ban ks is much mor e co nfine d, causing an even high er water speed and
a much larger redu ction in pressur e along a ship side and a furthe r increased pressure near bow and stern, with the highest pr essure near the bow. Thi s also explains bank suction and bow cushion effect A ship proceed ing on one side of a channel has a more confine d water flow at the side nearest the bank, causing highe r water speed an d lower pressure at that side. The ship is forced towards th e low pressure side. Due to the boundary laye r, also formed along the ban k, the spa ce between bank and ship narr ows towards the ship's stem , causing the re sultant force to act som ewhat
aba ft of midships, giving th e ship a yaw m oment away from th e bank. In addition, th e high pressure near th e bow close to the bank increases and forms a pressure cushio n, causing the bow cushion effect. The effect of a steep bank is bigge r than that of a sloping bank, because with a sloping bank some sideways inflow of water is possible causing a sm aller reduction in pressure.
The most relevant pr essure fields arou nd a ship have now been explained. T he imp ortant role that the ship's sp eed plays is clear. Besid es th e im portanc e of an appro priately low speed, it is also importa nt to keep in mind that inte raction effects will in crease when underkeel clea rance is sm all and when close to banks. In ter action effects between ships or between a ship and a tug are generated in th e same way as between a ship and a bank. It is again the distance off and the relative spee d of th e water between the ship and the tug which causes the degre e of interaction . Tug - ship interaction with respect to tug safety In figure 6.4, a tug is slowly overtaking a bulk carrie r an d tr avellin g p a st th e sh ip . T he mos t r el ev ant int eraction effects on the tug are now considered. Th e approximate stream lin es aro un d th e ship ar e shown.
Wh en coming nearly abeam of the stern (position I) the tug is sucked towards the ship because the speed of water increases between tug and ship's hull causing a low pressure field an d con sequently a sucti on force towards th e ship. Since the tug's forepart is closer to the ship than the stem the tug expe riences a starboard turning m om ent. A lift force caused by a cross flow on the tug also pushes the tug towards the ship. As it pro ceeds th e tug's bow reac hes the trough near the aft sho ulder of the ship, causing an increase d turning effect to starboard and the tug needs more power in order to m aintain spee d due to the higher water speed encountered. Wh en abe am of th e aft shoulder the tug is sucked mor e towards the ship, du e to the local wave tro ugh. In addition, there may still be some lift force expe rience d du e to cro ss flow. As soon as th e tug mov es furthe r forward and parall el with the ship's hull it exp eriences a sudden outward turni ng mom ent, caused b y th e tug's bow cushion . In addition, the tug's stem is near the wave
trou gh at the aft shoulder (position 2) wh ere th e water speed between the tug's stern an d ship's hull is high. As a consequence the stern is sucked towards the ship. The tug is also sucke d bodily towards th e ship. Near th e ship's midship section th e tug is still sucked towards the ship with an ou tward tu rni ng m om ent (position 3), all caused by effects identical to bank suction an d bow cushion effects. Near the bow the situation ch anges quickly. Wh en th e tu g reaches th e forward sho ulder, du e to th e higher wate r spee d and the local wave trough the tug needs more pow er to proceed at th e sarne sp eed. When pas sing the forward shoulde r suction forces increase rapidly due to increased local flow velocities. As soon as the after end of the tug reaches the wave trou gh the outwar d turning moment increases
again (position 4).
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Figure 6.4 Itueraaion effects on a tug whenproce,ding along a ship
TUG USE IN PORT 83
When movin g a littl e further forward (be twee n po sitions 4 and 5) the outward turning moment suddenly changes into an inward turn ing moment. This is due to the cross flow near the bow of the ship acting on the tug's rudder or skeg as a steering force. Du e to the lift force caused by the cross flow the tug drifts sideways away from the ship.
Between positions 4 and 5 tug pow er can be red uced to keep the same spee d since th e relativ e wate r speed reduces. Tugs not aware of the chan ge in tu rn ing moment and maintaining their power setting run with increas ing spee d to starboard and possibly d ramatic consequences . Attention should also be given to the fact that the cross flow acting on the underwater body of the tug causes a decrease in effective stability.
Manoeuvres to pass safely past a ship, including the positions wh ere towlines are passed, are now considered
for two main types of tug. Conventiona l tugs with propulsion and stee ring aft and ASD /reverse-tractor tugs with stee rab le propulsion aft are all considered conve ntional tugs. Tractor tugs with steerable propulsion forward are the other main type. The steerable bow thru ster of combi-tugs tends to give a similar effect to the pro pulsion of tractor tugs,but the power ofthe bow thruster is low compared to the propulsion of tractor tugs. Tugs approaching the stern to pass or pick up a towlin e should be well aware of the increased speed and possibly sideways movem ent to avoid a collision with the ship's stem . A conventional tug when in position I should apply port rudder to coun te rac t th e turning moment. ??? However,
port rudder also creates a sideways force in
the same direction as the suction forces. Th erefore whe n
near this position conventional tugs shou ld keep well aw ay from th e shi p . Tractor tugs can direct th eir propulsion away from the ship, thus counteracting the starboard turn and the suction force, whichis safer. Position I is also a position where towlines are passed. Conventional tugs should be particularly careful because of the turning moments and suction forces in this position.
Between position I and 2 the situation changes. A conventional tug sho uld, within a sho rt space of time, change fro m port to star board rudder. In doing so, the sideway s steering force created now points away from the ship. Tractor tugs have to set their propulsion in the direction of the ship's hull to counteract the turning
Positions 4 and 5 are also positions wh ere towlines are passed. A conve ntiona l tug can keep a steadier position, because the application of rudder to counte ract turning moment also involves counteraction of the suctio n and lift forces. A tractor tug when cou nteracting turning mo me nts sets th e pr opulsion in the same direction as the suction and lift forces and at the po sition s whe re suction forces occur the tug may come too close to the ship's bow. For a tractor tug it is more difficult to keep a steady position close to the ship's bow to pass a towline. Nevertheless, a tractor tug is safer because when coming too close to the ship's hull the stee ring for ces with a tractor tug are dir ected away from th e ship.
From position 4 a tug gene rally stee rs somewhat inwards to come closer to the bow to pick up or pass the towlin e. It is evident that this should b e done with utm ost care, due to the changing influences on the tug near the bow. The inter action effects described h ere only give an indication of the influences on a tug. The effects differ by ship type and loading condition. For instanc e, the diversion of stream lines ahe ad of a ship is less with a fine form ed ship, resulting in lower high pressure near the bow and consequently a smaller bow wave. The cha nge in turning moment expe rie nced on a tug near the ship's bow occurs further aft at slender sh ips . These ships also have less pronounced sho ulders, so effects in these regions are less pron ounced . There is also a shorter, flat area around the mi dsection , so changes in interaction effects qui ckly follow each othe r whe n passing along a slender ship , e.g. a containe r vesse l.
moment but at the same time a side ways force is
At position 3 and 4 the rudder of conventional tugs is still to star board counterac ting the suc tion forc e. Tractor tugs have to keep their propul sion to starboar d to compe nsate for the bow-out turning mo me nt, and still in the same direction as the suction forces. Especially near po sition 4, suction force s and turning moments to star boar d may be marked.
A tug's un derwater body and appendages ha ve th eir influences as well, especially on th e turni ng moments. Although interaction effects differ by ship and tug, these do exist and on e sho uld be aware of them. The smal ler the dist ance between tug and ship the lar ger int eraction effects are . Shallow wate r and narrow waters have an increasing effect on inter action between tug and ship. Most imp ortant to keep in mind is that th e influ ence of all interaction effects increase s sharply with speed an d are mo st dang erous near a ship's bow.
A littl e furth er on, between positions 4 and 5, a conventional tug should abruptly change from starboard to port rudder. If not aware of the turning moment the tug might swing to starboard and end up und er the bow of the ship. A tractor tug should change the propulsion from starboard to port to avoid coming und er the ship's bow.
Ship speeds can be rath er high whe n tugs are coming alongside or making fast. Speeds up to five kno ts are quite normal for tugs taking or passing a towline near a ship's bow or stern. Higher speeds are not uncommon , even up to nin e or 10 kn ots. The int er action effects ar e th en large, especially for tugs taking a lin e at the b ow.
introduced in the direction of the suction forc e, which is not safe.
84 THE NAUTICAL INSTITUTE
With such high speeds highly manoeuvrable tugs with a high, free sailing speed are required and, of course ,
very experienced tug captai ns. 6.2.4 Thg- ship in teraction with respe ct to tug p erformance The flo w pattern a ro u n d a ship affects tu g performance when ope rating close to a ship' s hull,
altho ugh it is difficult to say to what extent due to the int eraction between flow patterns generated by both ship and tug . To make it even more difficult, with chang es in tug po sition the situatio n may change rap idly.
tug no'. 3 the shorte r th e towlin e and the closer to the ship' s hull, the larger the int eracti on effects are. The towing effectiveness of tug no . 4 decreases with a short towline due to the redu cing effect of propeller wash imp inging on the ship' s hu ll. The effect is larger in tugs with propulsion aft. It is advisable for tugs towing on a line, like tugs nos. 3 and 4, to use a some what longer towline length and operate at a farth er distanc e from the ship's hull, which is also safer. This reduc es interaction effects and
the negative effect of the tug's propeller wash impinging on the ship's hull.
It has been explained tha t the relative speed of water along a ship's hull between bow and stem increases in speed compared to a free stream. With wide body ships the wate r speed near the forward and aft shoulders might be even mor e than at the ship's midsection. A ship stearning at, say, three knots through the water may have a speed of four knots relative to the water flow along the ship an d relative ship's speed at the shoulders may be high er still. A tug pushing at a ship's side i~ affected by this increase d water speed and tug perform ance is adve rsely affected, particularly when ope rating nea r the shoulders (see figure 6.5 po sitions I and 2). As alrea dy
In position 5 a tractor tug, which could also be an ASD/reverse·tracto r tug, is operating in a ship's wake as well as in the prop eller slipstream. The wake and propeller slipstream have oppo site directions. It depends totally on the assistance required whether or how wake and /or propeller slipstream influen ce tug performance.
explained, shallow and narrow waters increase water
Th e wake is a com bine d influe nce of potential wake and friction al wake . In figure 6.2 the frictional wake behind the ship's stern and the incoming water flow
flow spee d along the ship sides, furthe r decr easing a tug's effectiveness.
For instance, when retarding force s are required, a ship's
propeller is normally stopped or astern thrust applied. Compared to a free stream situation the wake causes a decrease in the tug's underwater resistance and propeller braking performance, assuming the same amo unt of engine power is used, resulting in a smaller towline force.
near the stern, which cau ses the potential wake, are Fo r tugs towing on a line the situation is more
complicated. Firstly, tugs are operating in areas where they are un der the influence of different inte raction effects as mentione d in sectio n 6.2.3. Secondly, tugs whe n in positions 3 and 4 an d renderin g assistance -
frequently change position an d heading. Thirdly, interac tion effects differ by ship's hull form, loading co nditio n an d speed. So it is h ard to say whe the r int er action effects affect the performance of a tug or tug type whe n towing on a line in positions such as 3 and 4. Apart from speed, an important aspect is towline len gth an d the distance to a ship's hull. With respec t to
shown. As rela tive water speed in th e ship's wake dec reases in shallow and narrow waters, the negative
effect of the wake on a tug' s braking performance increases. The effect of the propeller slipstream is opposite. It can be conclude d, as int eraction effects differ by ship, that so does the influence on tug performance when tugs are ope rating close to a ship and in the wake or propeller slipstream. It is difficult to assess what the influence is on tug pe rformance . The most marked influe nce is experienced by tugs pushing at a ship's side and tugs applying braking forces in a ship's wake .
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TUG USE IN PORT 85
6.3
Tug safety
6.3.1 Introdu cti on The exp lanat ion of variou s interactio n effects on a tug when close to a ship un derway at speed has already showed some of the risks involved for the tug. Th ere are, howev er, various other situations which involve risk for an assisting tug. Not all of the following situations are related to the same kind of interaction as discussed earlier. Interaction
betw een ship's propeller and tug is considere d alon g with several other situations related to tug safety. Some have already be en addressed while discussing the capabilities and limit ation s of various tug typ es, but are also mentioned here for the sake of completeness. Most situations are well known to experienced pilots and tug captains. Still, it is worth paying attention to the risks in which harbour tugs are often involved, because many serious accidents have been reported. The more one knows about these risks, the better one can anticipate and take the right meas ures . Besides, pilots often hav e only a limited view from the bridge on the assisting tugs. They are not always aware of the critical situations a tug may find itself in. The following ri sk y situations are just a few examples; it is impossible to cover all situations. Wh at is mentioned here may be representative for similar situations encountered by pilots and tug captains. Several of the situations to be discussed are related to the method of tu gs towing on a line . T his is understandable, because with this method of assistan ce tugs often op erate close to the bow or stem of a ship underway at spe ed, locations wh ere interaction forces can have large and altern ating effects. On the othe r hand, in ports wh ere tugs normally operate at a ship's side, it is also possible that in specific situations these tugs tow on a line as, for instance, in confined areas, in dry docks or when passing bridges. It goes without saying that readers could probably name other critical situations from their own ex perience.
Critical situations a tug may be involved in can simply be divided as follows: While passing a towline. While the towlin e is secured. Next, atte ntion is first paid to the m anoeuvre of a tug coming alongs ide a ship at speed. This is a practical example of interaction. 6.3.2 Coming a longside and dep arting fro m a sh ip's side Whe n considering tug-ship interaction it is safest, when coming alongside a ship underway at spe ed, to approach near th e midsection wh ere a more uniform 86 THE NAUTICAL INSTITUTE
flow pattern exists. At positions furthe r forwar d or aft the int eraction effects are larger and less pr edi ctable. Dep arting from a ship's side ca n some times b e problematic, as the following example shows, In some ports the pilot bo ards a ship from a harb our tug that is to assist a ship. The ship has headway and th e tug is coming alongside near the pilot ladd er. After b oarding the pilot it can be difficult to manoeuvre the tug free from the ship' s hull. This can happ en with twin screw tugs having an und erwater body which is rath er flat at the sides. Trying to get free from a ship's hull by moving to a far forward or far aft position along the hull does not help. Thi s can be explaine d by the earlier discussion on flow patt ern s around a ship. Tug captains note from experience that when they apply astern thrust with the inner propell er, complet ely again st th e expected manoeuvr ing procedure, the tug comes free from the ship's hull . The explanation is that the wat er speed between ship and tug hull decrea ses and con sequ ently pressure rises. The increased water pressure between the two hulls, in combination with bow cushion effect, force the tug to come off. A nice example of Bernoulli's law!An other solution is to decrease ship's speed, because the high er a ship's spee d the larger the suction for ces. Tugs with azimuth propellers controlled in the way sh own in figur e 2.30 have the thrusters pointed somewhat outwards wh en proc eedin g at low speeds. Wh en coming alongside a ship having a low speed the wash of the inward prop eller causes an increase in wa ter
speed between tug and ship and the tug may b e sucke d violently towards the ship . This becomes more pr obl ematic for tug s with fixed pitch azimuth prop ell ers not equippe d with speed modulating clut ches. Such tugs have a relativ ely high minimum propeller speed, causing mu ch prop eller wash at minimum tug speeds. This has resulted in much contact damage while landing alongside a stationary or moving vessel and during berthing and unberthing, which, however, can be avoided by proper tug handling. The sam e may happen when the clutch-on/clutch-off system of the separate azimuth propellers of tug s with a single lever contro l are not in complete balance.
Note: Operating close to a ship and coming alongside a stationary or moving vessel should always be don e with care and in a controlled way. Approaching a ship with an inappropriate speed has resulted in den ts in ship hull s and damage to tugs and even oil spills have occurred o n several occasions caused by mooring ass ist tugs penetrating bunker spaces_ 6.3.3 Passing a towline near th e b ow The mo st risky situations for a tug wh en ope rating close to a ship's bow have already b een discussed while co nside ring interaction effects . Some oth er situations
such circumstances is to go full astern. Some damage might then occur to th e tug, but the situation is not disastro us. A tractor typ e of tug is safer in such a situation, b ecause the steering forces are directed away
from the ship .
o
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A
Figure 6.6 A: Tug is wailingjiJr the approadlingship tocom, do", 10 pass the tow line. TMr, is riskofan untxpteted sheer 10 partdut to the ship~ how pressure wavt. B: Conomtionol tugpreparing to take the tow line at ship's bow. Due to interaction effects and inadequate reactions from the tug captain, the tug .comes under the ship's how
are now highlighted [see figure 6.6A and 6.2). A tug has to make fast at the bow of an approaching ship and is steaming at some distance ahead. Tug speed is less than the speed of the ship to be attended and the tug is waiting till the ship gets close enough to pass a towline. However, due to the changes in the stream pattern caused by the overtaking ship the tug may experience a turning moment. When the tug captain is aware of this effect in time he can, irrespective of the type of tug, take measures to co unte ract the turning m om ent.
A large turning moment can be experienced, particularly when attending loaded ships with a fullshaped bow and still having reasonable speed. With this type of ship th e bow wave may also have another specific effect ontugs awaiting the approaching ship. It has b een experienced by tug captains that when attending VLCCs or large ore-carriers having a speed of about four to five knots and a small underkeel clearance, the bow pr essure wave may b e such tha t the tug is pushe d forward and the tug capta in may even be forced to reverse thrust in order to come closer to the ship'S bow. Another example of interaction is shown in figure 6.6B. A conventional tug approaches a ship under speed to take a towline at the bow. At a particular moment the tug captain considers his tug too close to the ship's hull and tries to clear the ship 's side using engines full ahea d while steering to port. Due to this action the tug is pushed against the ship by the steering forces and moves steadily forward along the ship's b ow, unsuccessfully trying to get free. Finally the tug comes broadside under the bow and is run down. The only satisfactory manoeuvre in
Taking or passing a towline at the bow ofla rge loaded wide bod ied ships is not so dangerous. Wh en abeam of the fore part of the bow the tug is pushed aside by the earlier menti oned cross flow. Tug captains leam from experience that wh en near the for e part of the b ow and steering a little inwards towards th e bow, the tug do es not get closer. However, when the tug is moving further forward it experiences the earli er mentioned turning moment towards the ship . This effect will probably be ' largest with a small underkeel clearance. Without going into furth er detail, it can be concluded from the foregoing that op erating a tug near the bow of a ship under speed involves risks. These vary depending on the typ e and loading condition of the ship and increase with a high er ship's speed . As alr ea dy mentioned, ship's speed can be rather high when tugs are making fast. Therefore when approaching the bow of a ship to pass or take a towline careful attention and quick reaction is needed from a tug captain in order to avoid dangerous situations developi ng . Skilful tug captains know the inte raction effects and related risks near the bow by experience. Therefore, not just good tug manoeuvr ability but e x p e rie n c e too is an
indispensable factor. It is not the tug captain alone who masters the situation near the bow or is solely responsible for the extent of risk into which his tug gets involved. As already stated, an important factor is ship's speed which is unde r the control of the pilot or master. An experienced ship's crew standing by forward in good time and keeping sufficient heaving lines of the proper length and strength ready available is important This can help to avoid a tug captain being forced to come too close to a ship's bow. Sometimes quite thick lines of insufficient length are lowered from the forecastle, forcing a tug captain to come very close to a ship's bow and involving increased risk.
On the other han d, whe n a tug is pushed away from a ship and a too short messenger line is used by the tug itself, this lin e may break during transfer of the towline from the win ch. The towline then drops into the wate r and may foul a tug's propeller which brings about another dangerous situation. When a tug has to make fast on a ship's line , the line should be hung at a suitable height above the water, ready to be paid out as soon as the tug has got hold of the line . 6.3.4 Passing a towline at the stern Whe n making fast, after tugs are often very close astern of a ship - sometimes just abeam of the after end of the stern in order to pick up or pass a towline. The TUG USE IN PORT 87
interaction forces at the se lo cations are not so large or
dangerous. However, wh en approaching a ship having headway from astern, the tug captain should be aware that when coming close to the ship's stern , the tug is pushed towards th e stern , as has been explained earlier. One should always be aware of the ship's propeller. Wh en a tug is makin g fast at the stern a ship's prop eller should alwa ys be sto ppe d in case of a fixed pit ch propeller. A con trollabl e pitch propeller should be set for minimum pitch. A prop eller turning ahead disturb s the water and m akes it more difficult for a tug to keep a steady p osition be hind the stern. This effect is also expe rienced by tugs making fast near the aft sho ulder. An unsteady tug position affects smooth hand ling of a towline and in the worst case an unsecured towline may drop in the water and foul a tug' s or ship'Spropeller. A critical situation also arises when a tug is passing or taking a towline close behind a ship's stern , or is preparing to do so, and suddenly the ship applies astern thrust hy giving astern on the engine or by reversing the 'pitch. Particularly wh en large ships with powerful engines suddenly apply astern thrust a deep wave trough is crea ted close be hind th e ship's stern, sucking a tug toward s the ship. A tug may touch a ship's stern causing dam age to the ship or tug. This kind of acciden t has happened occasionally. Even with smaller ships this effect is noticeabl e. Wh en, for one reason or ano ther, a ship's propeller has to be used for astern thrust, a tug captain should b e informed by the pilot to allow him to manoeuvre his tug out of the dangerous area.
The conclusion is that when tugs are making fast at or near the stern , a ship's propeller should be stopped and in case of a controllable pitch propeller be set for mimimum pitch. When for some reason or another the propeller has to be used , the tug captain sho uld be informed. Now so me critical situations are discussed whe n towlines are secured. Some situations relate to speci fic manoe uv res as used in some large ports.
6.3.5 Overtaking a bow tug on a line Girting - Tripping In figure 6.7A a tug with propulsion aft is assisting a ship in making a tum to starboard. Ship' s spe ed may become too high for the tug (position I), for instan ce because the tug is pulling too mu ch to starb oard or because the pilot has increased engine power to improve rudder effect in order to make the jurn properly. In the given situation it is very likely that the tug will come abeam of the ship's bow (position 2) an d eve n in a position furthe r aft with the towline coming un de r high tension (position 3). It is almost imp ossibl e for the tug captai n to man oeuvre his tug back in lin e with the ship and the tug is liable to capsize. This may not only b e caused by the stro ng athwartships forces in the towline, but while trying to bring the tug back in line with the SS THE NAUTICAL INSTITUTE
2
3
( (
•.•..•.
\
C
Figure 6.7 Girting andtripping Two examples ofgirting (A & B, both with a conventional tug): A - due to excessiveship 1 speed with respect to tug limitations B - due to misunderstanding Exampk C slwws tripping with a tractor lug
ship, the tug captain appli es high steering forces, adding to the heeling forces. With a reliabl y working qui ck release system the tug captain can release the towlin e, so avo iding cap sizing. On the other hand, if the pilot recognises the dangerous situation arising in tim e he may be able to reduce ship's speed. In doing so the towline force reduces, creating the possibility for the tug captain to come back in line with the ship. It is obvious that the more mano euvrable tugs are, e.g twin screw tugs, the less likely they are to get involved in similarly dangerou s situations. In addition, proper stab ility, freeb oard an d deck equip ment contribute marked ly to safe operations and enlarge the capabilities of a tug. Doors and other openings on deck should be closed during towing operations.
The above situatio n is less dangerous for a tractor tug because of the aft lying towing point, A tractor tug swings around on the towline and comes alongside an attended ship un less the towline is released in time so-called 'tripping' (see figure 6.7C). Similar situations can arise with a tractor tug when the towing angle - the angle be tween ship's heading and direction of the towline - is getting too large with respect to the forward spee d of a ship. The tug is unable to come back in line with the ship and swings around.
Although the above mentioned situations do occur, the following comparable situatio ns are also possible. The danger of 'girting' or 'tripping' does not only exist wh en a ship round s a bend. Even when a ship is proceeding on a straight course girting can OCCUI. In that case excessive spee d of the ship is the main cause.
When a ship increases speed to a level which is rather high for a forward tug towing on a line, the tug captain prob ably does no t keep position right ahea d of a ship's bo w, because that is too dangero us. The tug steers out
towards a position more aside in order to keep well clear of the ship's bow. It is understan dable that if ship's speed fur the r inc reases, a com pa rab le girting or tripping situation will arise for the tug as indicated before. Alth ough pilots should be aware of the implications oftoo high a ship's spee d for th e safety of assisting tugs, it is again an indica tion of the importance of good co mmunications between tug captains and pilots. Th e
pilot may not have a goo d view of what is happening at the bow and the tug captain should therefore inform the pilot in good time if he considers a speed increase too high. An oth er exam ple of how the danger of girting can arise is shown in figur e 6.7B. A ship is making a turn to port, say, to enter a harbour basin . Because the tug captain h as not been informe d that the ship has to enter head first in to the basin he starts pullin g to starboard to contro l ship'S heading, assuming the ship is veering off cours e. If the pilot is not aware of thi s, the same dangerous situation for the tug as described above devel ops, in particular when the pil ot obs erves a decrease in rate of tum due to the tug captain's action
and in cr eases eng ine power while applying a large rudder ang le. This is ju st an example, to show how imp ortant it is for tug captains to be well inform ed about a pilo t's intentions. O n the oth er hand , of co urse, the
tug captain could have asked the pilot what his intentions were,
A furth er exa mp le. A tug has taken position right ahead of a ship, waiting for the ship'S crew to release the towlin e. With the small number of crew members on board ships nowadays , this may take some time. In the meantim e the ship is already increasing spee d. In the case of beamy full-bodied ships it may happ en that the tug, with the towlin e still not yet released, gets pu she d forward by the bow wave of the ship an d thus reaches a speed which can' be higher than the free running speed of the tug. When the tug moves sideways towards a position abeam the bow, due to the danger of the increasing ship's spee d, the forward pushing effect of the bow wave diminishes. The tug may not be able to keep pac e with the ship while stillwaiting for the towline to be released. A dangerous girting or tripping situation may arise. This example shows again the importance of appropriate speed and good communications.
Figure 6.8 Some speafic manoeuvres by conomtumal tugs towingon a lineincludingriskojgirtingor cap,i;jng when a ship" speed is too high with respect to tuglimitations
6.3.6 Forward tug st eering broadside In seve ral ports, ships e nter h arb our basins stern first D eparture is then easier an d in case of emergency most
ships are able to leave without tug assistance. Ent ering a harbour basin stern first can be don e with e.g. two tugs of which th e forward tug is a conventional tug operating broad sid e as shown in figur e 6.8A. The forward tug, acting as a drogue, steers the ship effectively by going astern or ahea d on the engin e and so applying steering forces to port or starboar d. Th e tug usually uses a gob rope, although with twin screw tugs this is not always the case. This metho d of tug op eration has already be en des crib ed in Chapter 4. For small ships often only one forward tug is used ope rating in th e same way. The ship ma intains sternway using its engine. A dangerous situation arises when a tug's capab ilities and limitations are not sufficiently taken into account. Wh en a ship's astern speed is becoming too high, tug heel caused by high athwartships towline for ces may increase until the tug capsizes. This may not only be caused by the large transverse resistance of the tug as it is pulled bodily through the water, but also by the water acting on the tug speeded up by the wash of the ship's pro peller. Tug stability, freebo ard and deck equipment determine the limits of safe ope ration. TUG USE IN PORT 89
Care should be taken when using the engine ahead. A ship sho uld take care not to gath er headway, otherwi se
sh e will collide with th e tug due to the small distanc e between bow and tug. 6.3.7 Stern tug steering broadside See figure 6.8 B. This situation is simila r to th e pr evious one. Th e ship is now moving ahea d an d th e after co nve ntional tug is the steering tug ope rating in
the same way as the forw ard tug discussed earlier. Th e main difference between the two situations lies in the
close presence of the ship's propeller. Wh en operating in this way the ship generally has a very low forward speed. However, it is essential that the ship' s propeller is handled with the utmost care. A very dangerous situation arises if the engine is suddenly set, say, to half ah ead. The water flow on the tug together with the wild propeller wash may cause the tug to list severely and in the mo st serious case the tug may capsize. This has happened more than once.
for instance, when assisting a dep arting ship. A ship has just left her berth an d has been turned aroun d in the turning basin by assisting tugs. Th e after tug is on the port quarter. The ship still has to pass through a channel and it may be necessary to have th e after tug stand by on the star board quarter to compe nsate for wind or current forces . It m ay also b e necessary to compensate for the transve rse effect of the ship's pr op eller when she uses engine astern to wait somewhe re in the chann el. The tu g h as to man oeuvr e from p ort to starb oard quarter, close underneath the stern . Because of the risk of girting this manoeuvre sho uld be carried out while the ship is nearl y stopped in the water. This kind of manoeuvr e also involve s great risk due to the ship's prop eller. A pilot not aware of the tug man oeuvre could
go ahead on the engine or apply ahead pitch while the tug is near position 2. The conventional tug comes in to danger. This kind of tug man oeuvr e, w he never considered nec essary, should always be carried out with the utmost care .
6.3.8 Stern tug manoeuvring from a stand by position on starboard or port quarter
towards a position astern the ship See figure 6.8C. During a certain phase of manoeuvring it may be nec essary for a ship with headway to have the port or starboard position tug (Position 1) move astern of the ship (positions 3 or 4) to assist in steering or for speed control. This might be necessary when a ship has to wait in a river, swing or
b e stopp ed . This manoeuvre is dangerous to conventional tugs when carried out at too high a ship's speed. This is at speeds of more than about three knots, and depends on tug manoeuvrability, stability and freeboard . In situations 2 and 3 risk of girting exists due to the high athwartships towline for ces that may occur. If a tug capsizes it has been observed that the tug is pulled underwater stern first. The manoeuvre just described is no problem for tractor or reverse-tractor tugs, even with a fairly high ship 's speed. Conventional tugs with a gob rope system , whereby the towing point can be transferred towards a far aft position, can also swing around at a higher speed. The gob rope system should be strong enough and fully reliable otherwise such a manoeuvre becomes really
Figure 6.9 Due to excessive speeda tug at a ship's side may capsize ifthestern line cannot bereleased
6.3.10 Tug operating at ship's side Conventional tugs operating at right angles to a ship's side may use quarter lines or stern line s as shown in figure 6.9 to stay in po sition'when the ship moves ahead.
When the tug is secured in that or a similar way, excessive speed should be avoided to pr event p ossible parting of the towlin e or capsizing the tug. 6.3.11 Fog
dangerous for the tug. A conventional tug manoeuvring from a position astern of the ship (e.g. po sition 3) to a position on the starboard or port quarter can only_do this at minimum ship's speed, otherwise risk of girting may arise . 6.3.9 Stern tug manoeuvring from starboard to port quarter or vice versa See figure 6.8D . Sometimes it is necessary for a conventional after tug to move from a position on the port to starboard quarter or vice versa. This may happen, 90 THE NAUTICAL INSTITUTE
All the situations mentioned above can cause a critical situation for tugs. However, during dense fog these situ ations may involve even more risk . A s
mentioned in paragraph 4.4, during fog it is very difficult for tug captains towing on a lin e to ori entate them selves with respect to an attended ship and surrounding are a, in spit e of the availability of a radar. Furthermore, the pilot loses his view of the tugs. It is absolutely ne cessary, therefore, that ship' s speed is kept very low during fog and that tug captain s are kept well informed about intended manoeuvres. Communications between pilot and tug captains should be optimal.
It should be noted that although the use of towing bit ts may be necessary for ce rtain specific m anoeuvres, the ir use is not recommended for tugs towing on a line
during fog conditions. In case of emergency it may be almos t impossible and certainly dangerous to release the towline under tension rap idly. Th e same applies to quick release hooks, unless they are one hundr ed per cent reliable. A good towing winch with a quick release system which can be operat ed from the wheelhouse as well as at the winch is safest in th ese conditions. O n the othe r hand, tug captains some times prefer to have a ship's line on the towing bitt or towing hook during fog co nditions. The sh ip's lin e can th en be released by the tug crew as soon as the tug captain thinks the situ ation is becoming critical. If he has to wait for the ship's crew to release his towline in a developin g critical situation, it could well be too late. 6.3.12 Some other practical aspects Bulbour bows Alth ough there is a mark on a ship's bow indicating tha t she has a bulbous bow, tug cap tains canno t see the bulbous bow when it is underwat er. Even when only partly submerge d the ex act positi on is difficult to determine. This is a prob lem for forward tugs when taking position to pass or take a towline or when they are assisting us ing a very short towline. It is most ' dang erous wh en the stem of the tug touches the bulb ous bow, and the ship has a rather high forward speed. The tug may be severely dam aged and lives may be lost. Tug captains h ave to be p arti cul arl y ca reful whe n ope rating close to a bulbous bow, especi ally during fog and darkn ess.
pin s should be raised on board the tug to prevent the towline slipping along the sides. This avoids the towlin e fouling the ship or tug propeller. In the case of a fixed pitch propeller the ship's propeller shou ld be stopped when the ship's stern tow lines are released.
Underestimating wind and current fo rces Underestimating wind and current forces can create risky situations for a ship and have resulted in accidents. Tugs operating at a ship's side can also be endangered (see figure 6.10). Tugs can be jammed b etween ship and shore when they don't get out in time. The situation is particu larly d an ger ous when tugs are secured by towlines. Th e bollard pull of tugs to compensate for wind and curre nt forces should be more than sufficient to avoid such situations.
s
Sudden changes in a ship heading and speed While passing or taking a towline, tugs are very close to a ship's hull and a tug captain's atte ntion is fully focused on keep ing in po sition and on line handling. It sho uld b e understood that during th ese operations sud den changes in ship's heading or spee d without warning can create critical situations for a tug.
Engin e starts of the large high powered container ships may seriously affect the controllability of a tug ope rating behind the ship's stern. It is necessary, therefore, as alrea dy m enti on ed
earlier, to inform assisting tugs about intende d ship's engine/ propeller and course changing manoeuvres. This applies too for tugs operating at a ship's side. In that way tugs can anticipate expected man oeuvres.
Releasing towlines If a crew on board ship is not able to release a tug's towline when requir ed, problems may arise if a ship is alr eady increasing speed. This is particularly the case with heavy steel wire towlines on powerful tugs. The towline has to be slacked off by a tug in order to make it possible for a ship's crew to release it. The slack towline is then dragged through the water. Wh en ship's spee d increases the resistance of the towline also increases, creating more tensi on in the lin e. Rele asing it then becom es almost impossible. Ch ain stoppers, when used, may br eak. It is a difficult situa tion which can onl y be avo i ded b y pr oper co n tro l of sh ip 's spee d, an expe rienced ship's cre w, sufficient crew members on station and good coope ration between ship and tug crew. (
'"
The aforementione d situation can get very critical for the tug when th e ship is furth er increasing her speed and as a result overtakes the tug. Th e risks stemming from these situati ons have been discussed already. Finally, a tug's towlines sho uld not be dr opped into the wate r but preferabl y be lowered onto the tug's deck guide d by the tug's crew. When applicable, Norman
Fzgure 6.10 DUl to lot» powered tugs anda strong beam wind, a contain" ship is drifting and thetugs are getting jammed betuum theship and thegeneral cargo b"th
TUG USE IN PORT 91
S hip design consequences Du e to the use of ten sion winch es on board ships the number of bo llard s at their foreca stle and stern may be reduced. The location of the remaining bollards is not always optimal for towlines. This may affect proper securing of towlin es and lengthen the time for securing tugs, especially when more than one tug is used forward and aft.
transit spe eds and intended mano euvr es. If the ship has spe cial mano euvring devices or limitations regarding mano euvring, tug securing, mo oring and anchoring equipment, the ship captain should inform the pilot. With respect to the information exchange the reader is al so r eferr ed to th e earlie r m ention ed OC IMF publicatio n 'Recommendations for ship 's fittings for use with tugs'.
Th ere are specific ship type s, such as submarines
Operating bow-to -bow Th e r elati vely low effective ness of tractor tug s, reverse-tractor tugs and ASD -tugs (whe n operating as reverse-tractor) as bow tug towing on a line with a ship having headway, including the reasons why and the risks involved , have been discussed in par. 4.3.1.
and aircraft carriers, whe re it can be probl em atic to pass
or secure towlines, du e to the underwater form of the hull or overhanging structures. With m odern merchant ships such as fa-r o vessels it can som etimes be awkward to secure a towlin e in such a position that a tug can
operate effectively. Ship designers should take into account that, eve n when ships are very mano euvrable,
there will always be situations during a ship's life when the assistance of one or more tugs is nece ssary. On the other hand tugs should meet, as far as po ssible, the requirements of ships calling at the port regarding safe and effective towline handling. Apart from the nee d to have sufficien t and properly located bo llards and fairlea ds available for securing the number of tugs that may be needed , it is furthermore n ecessar y that bollar ds and fairl ead s are in good cond ition an d suitable for the towlines to be used, and strong enough to withstan d the forces that can be applied by the modern powe rfu l tu gs. Not meeting this requirement has resulted in failures . Regarding this important subject, recommendations are give n in the OCIMF pub licatio n 'Recommendations for ships' fittings for use with tugs' (see Referen ces); recommendations that app ly to tankers, but which are also relevant for other ship types, particularly large gas carriers, bulk carri ers and container ships. Additional recommendations are included for escort tugs and tugs engaged in station keeping at offshore installations, such as SPMs and F(P)SO s. Information exchange pilot- ship master - tug cap tain Informing the ship captain about tug manoeuvres by the use of tug ord ers in understandable English has been addressed in paragraph 4.7, while at othe r locations in this book, and particularly in this chapter, several arguments are given why tug captains should be properly informed by the pilot about the int ended ship manoeuvres. A proper information exchange between pilot, ship captain and tug mas ter is needed for a safe an d smooth han dling of the ship by the attending tugs.
In formation for the ship captain to be provided by the pilot may include the number, type and bollard pull of tugs to be used (including, if necessary, the rea son why the specific numb er and/or total bollard pull of tugs has bee n advised], the re ndezvo us position and time of th e tug(s); whe re at the ship and how tug(s) to be fastened; when tug(s) to be released and how to be done; 92 THE NAUTICAL INSTITUTE
For reverse-tractor tug s and ASD-tugs th is way of op erating is generally called 'b ow-to-bow' . When ope rating in this way with a ship hav ing head way th~ tugs are sailing astern . Dire ctional stability of these tug typ es when sailing astern is gen er ally rather low , particularly at higher speed s. Pulling straight astern at a relative high spee d might not immediately present a problem, but as soon as the tug deviates from the straight line, for in stance, to give steeri ng assistanc e to the ship, position keeping becomes difficult. It may easily result
in a loss of contro l. The reader is with resp ect to this referred to the report 'Performance and effectiveness of omni-directional stern drive tugs' (see References). A high underwater resistan ce, e.g. a large skeg, worsens the situation, while a bow skeg may improve the situation to some extent. The re striction in
movement of the bow by the tow rope increases the difficulties in maintaining a safe position and direction relativ e to the ship under tow. Furthermore, if the tug is working on a short towline, tug-ship int eraction effects may playa role, destabilising the tug's position (see par. 6.2.4), while time left to react is minimal. A co mparable situation has led to accidents, amongst othe rs in a severe collision between ship and tug. See the investigati on report of the collisi on between R iver Yarra and tug WJ Trotter (see References). This incid ent resulted in a speed restriction fo r bow-to-bow o per atio ns . A maximum speed of five knots was introduced.
6.4
Summary and conclusions
Several in teractions exist, some influ encing tug performance, others tug safety or even both. Interaction effects influencing tug safety are the kind of int er actions which occur be tween ships whe n close to each other. T he se inter action effects are m ore pro nounced in shallow and narr ow waters and when a tug is in the rela tively close Vicinity of a ship and increase sh arply with increasing ship's spee d.
Because tugs have to ope rate close to ships which are often un derway at speed these effects should always
as possible, keep an eye on assisting tugs.Tug captains should inform pilots about developing or suspected risky situations and contact the pilot whe never in doubt. Ship masters sho uld inform the pilot about the man oeuvring capabilities of the ship, and relevant aspects of ship's mooring, anchoring and t ov..·line securing equipment. Th e pilot should inform the ship master about the tug assistance and mano euvres to
be executed.
source: Foto & Video Produlcties . an der KWt~ theNetherlands
Figure 6.11ASD-tug'Smit Marne- ol Smit HarhourTowage, Rotterdam, assisting a omtainervessel having headwaywhile operating ' boui-to-bmo", TheSmitMam«' (l.o.a. 30.6m, beam 70.6m, draught 5m, bollordpull ahead 67.2 tons, astern 56.4 tons} is huilthy Damen Shipyards, theNetherlands.
be taken into account Considering the interaction effects and all other risky situations that hav e been discussed, it is clear that the following aspects are essential for safe shiphandling with tugs: Experience in recognising risky situation s and
knowing how to deal with them. Good kn owledge of the limitations of tugs. Appropriate ship's speed taking account of interaction effects and tug limitations. Careful use of ship's propellers when tugs are op erating close to the stern or when passing or releasing towlines at the stern. Tug captains should be informed in good time about the intended use of ship's propellers. Optimal communication, information exchange and cooperation between pilots, ship masters and tug captains. Pilots should inform tug captains well in advance of intended manoeuvres and should, as far
Tugs should be fully appropriate for the assistance required and should comply with the following minimum requirements: sufficient bollard pull , high manoeuvrability and free running speed, good stability and sufficient freeb oard , suitable towing equipment with a properly working quick release system and an optimal horizontal and vertical angle of view from the wheelhouse. Tugs operating at a ship's side should be sufficiently powerful and secured so that the risk of becoming jammed between ship and shore due to wind, current and/ or wave forces can be avo ided. Sufficient ship crew memb ers should be available and well prepared on station to secure tugs with minimum delay. Ship 's crews should not drop a tug's towline into the water but lower it gently onto the tug's deck. Ships should be designed so that sufficient towlines can safely and pr operly be secured for effective tug assistance . Finally, openings in superstru ctures, deckhouses and expo sed machinery casings situated on the weath er deck of tugs, providing access to spaces below deck, should be fitted with watertight do ors, These openings shou ld be kept closed during towing op erations, so enhancing tug safety. If these openings are not closed, water can easily flow into a tug when she is forced into a list. Earlier in this bo ok the imp ortan ce of good towing equipment in relation to tug perform an ce h as been mentioned a number of tim es, In this chapter the importance of proper towing equipment and quick release mechanism in relation to tug safety is emphasised. In the next chapter attention is paid to the deck equipme nt of harbour tugs.
TUG USE IN PORT 93
Chapter SEVEN
TOWING EQUIPMENT 7.1
Introduction
IN PREVIOUS CHAPTERS, ON SEVERAL OCCASIONS, th e importance of the location of the towing point in relation to safe ty and performance of a tug was
mentioned . The strong relationship between safety and performance was emphasised because the more safely a tug can operate under different circumstances the smaller its limitations are. For instance, the higher the
towing point of a conventional tug the larger the list when towing on a line. This is due to the athwartships forces and increases the risk of girting. Consequently a high towing point limits towing performance which is particularly the case for conventional tugs but for other tug types as well. There are ways to enhance tug performance and safety, such as: Making the towing point transferable or having more than one fixed towing point, which affects tug performance as well as safety.
Radial system Moving the towing point along a semi -circular track, in particular using th e rad ial towing h ook, ha s already been addressed in section 4.2.3. An exam ple is shown in figure 7.1. A radial system can also be used with a towing winch. In that case the lead of the towline goes from the winch via a fixed fairlead and th en a second fairlead which moves along a circular rail on the tug's deck. The principle is similar to the radial towing hook (see figure 7.3). Radial systems cause smaller heeling angles so higher athwartships towline for ces can be applied, resulting in an increase in tug performance. It is currently only used by conventional tugs , though the int ention is to use a similar system in other tugs types as well, see for instance 'The Towliner '-concept discussed in par. 9.5.1 and the Carrousel tug discussed in par. 10.1.2 Additionalfixed towing points Moving the towing point along the centre line of the tug can be achieved, firstly by making use of more than
Fitting a quick release system, which is an emergency safety system. Another important aspect mentioned in previous
chapters is the need to be able to vary the length ofthe towline when assisting a ship. For example, when dynamic forces due to waves are high, to reduce the counteracting effect of a tug's propeller wash on a ship's hull, or when tug manoeuvring space is limited. In this chapter ways of varying the towing point and towline length by means of deck equipment is discussed. The importance of good deck arrangement in relation to performance and safety is dealt with , as well as whether and how a towline can be released in case of
Figure 7. 7 Radial towinghook with railtrack
emergency - a quick release system. Of course, attention
is also paid to the towline itself, being the crucial connection between tug and ship .
7.2
Additional towing points and gob ropes
The possibility of varying towing point location particularly enhances the performance and safety of conventional tugs. Transferable towing point systems can be distinguished by facilities which can move the towing point: Along a semi-circular track. Along the centre line of the tug in a longitudinal direction.
94 THE NAUTICAL INSTITUTE
Photo: Author
Figure 7.2 Radial towing hook ofconventional twin screw tug 'Saona~ Dominican Republic (l.o.a 26m, beam 7·8m, bollardpull3Ot).
Photo: Author
PlwUJ: Alltlwr
Figure 7.3 Afterdeckof a conventionaltwin screw tugwith a towing winch with radialsystem
Figure 7. 4 Additional[airk adJtowingpoint near the stern of comhitug 'HeadriJc P. Goedlwop: Amsterdam, Holland. The[airkadcan he opened topvl thetowline in or take il out. Particulars ufthetug are given in Chapter 2
one fixed towing point. This provision is found on board some combi-tugs, as mentioned when discussingthis type of tug . The addition al towing point enhances the capabilities of the tug as stern tug to a large extent and enables it to perform almost as a tractor tug. On the photograph the additional towingpoint can clearly be seen. Some VS tractor tugs designed for escorting have an additional towing point far aft to minimise the steering effort required to keep the tug in line with the escorted ship. When required to deliver steering forces the original towing point has to be used.
Gob rope systems A second method, used only on conventional tugs, is the use of a gob rope to vary towing point location in a longitudinal dir ection. This can be done in two ways. Firstly, by using a certain length of wire, one end secured to a side bollard, the other passing through a fairlead or small H-shaped bollard situated on the centreline of the work deck. This end of the wire holds a large shackle which can be attached around the towline as shown in figure 7.5A. The large shackle is free to slide along the
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3 Brakirlg
@ F lf,UTt
7.5 Two different goh rope systems
TUG USE IN PORT 95
towline. When the towline moves into a more abeam position the gob rope tightens and relocates the towing pointbetween the original fixed towing point and the position of the fairlead or H-bollard. The shackle of the gob rope should always be large enough to allow the towline to slip through it if the towline breaks or has to be released in an emergency. As well as wire gob ropes, fibre lines are sometimes used and different gob rope arrangements can be found. By using a gob rope a conventional tug, at low ship's speeds, can operate in the way shown in figure 7.5B for steering control to starboard or port by going ahead or astern on the engine.
An improved arrangement is to have a separate gob rope winch (see figure 7.5C), controlled from the wheelhouse if possible, with the gob rope wire led through a central swivel fairlead at the utmost end of the stern. A large shackle is attached to the wire, which again can slide along the towline (see also figure 7.6 -
tr. __
PIw",.. SmitHarhour 7lwago.1ID_ Holkmd
FtgUre 7. 7 Afterdeck ofASD-tug'Maasbank' (1.o.a. 37·4m, beam 11m, bollard pull 62 tons) showing the rowing pins
pull has been measured. The gob rope arrangement must be able to withstand these high forces. The gob rope is used by a conventional tug when operating as stern tug on a line and the ship is moving ahead. When a ship is moving astern and the tug acts as forward tug a conventional tug, when required, can operate in the same way as described. Several ASD-tugs are equipped with hydraulically operated towing pins (see figure 7.7), which have more or less the same function as the gob rope system, viz. shifting the towing point to aft. However, these towing pins are principally used when towing at sea.
7.3
Towing bitts, hooks and winches
7.3.1 Method of towing and varying towline lengths
Photo: A'Uthor
Figure 7.6 Cotuentional single screm tug'Adelallr' (l.o.a. 26·6m, beam 8·1m, bo/lardpull 30 tons) o/fOrmer rowing company].. Kooren Towing, Rotterdam, Holland, using agob TOP' to steer a ship entering a barbour basin stern first
the tug Adelaar). By varying the length of the gob rope the towing point can be shifted, even to the after end of the tug. The system enhances tug capability and is, for instance, compulsory in German ports. At low ship's speeds, conventional tugs fitted with this arrangement can operate in the way shown in figure 7.5Dfor steering control (position 2) or speed .control (position 3) simply by shifting the towing point location. By heaving on the gob rope and bringing the towing point to the tug's after end, the tug can swing around from position I towards position 2 or 3 at somewhat higher ship's speed than without this arrangement. This is comparable to a tractor tug having its towing point near the stern as well. It should be borne in mind that high peak forces can occur in the gob rope; 70% or more of the ballard
96 THE NAUTICAL INSTITUTE
It is not always easy, or may even be impossible, to vary a towline's length when this would be the best option. This largely depends on the way the towline is secured on board the tug and this in turn depends on the tug's deck equipment, the assistance required and whether ship lines or tug towlines are used. In tugs not provided with a towing winch, towing bitts, bollards and/or quick release hooks are used. Even when equipped with a towing winch, towing bitts and bollards are used when more than one line has to be fastened aboard.
When ship lines are used they are often secured to the towing hook. In this case the crew of the assisted ship have to vary the line's length if so required and this may take some time with the shortage of manpower nowadays. Even though not equipped with a towing winch, tugs often use their own towlines. For tugs towing on a line, these are usually affixed length with an eye spliced at each end. One eye is secured to the ship's ballard and
the other to the tug's towing hook. The length of such lines cannot be varied. These tugs often have two (or mo re) towlines of different fixed lengths. The length of a towlin e secured to a tug's bollard or towing bilt can be changed, although it takes time and manpower to do so an d can only be done when th e line is slack. In an eme rgency it is almost impossible to relea se a towline secured to a towing bitt, wh ereas when
secured to a quick release towing hook, release should not b e problematic. So, although different methods of towline usage and securing exist, they hardly allow towline length to be varied efficiently unless a towing winch is used. It should be noted carefu lly that operational safety is involved if to wlines canno t be slac ke ned or rel eased in an eme rgency.
absorb, as much as po ssible, the impact energy of the hook itself. With disc towing hooks the hook is a rou nd plate with a hook shaped opening for the towline. As soon as a line under tension is released, the stored energy causes an enormous acceleration of this disc bu t avoids
the large impact on hook and deck construction. Towing hooks can be equipped with spring shock absor bers to reduce high dynami c peak forces in the towline. Load monitoring systems for towing h ooks also exist.
Towing hooks are normally equipped with a quick release system, ope rated locally and by remote control from the wheelhouse. Systems vary from a simple one, manually op erated by a steel wire up to electricpneumatic or electric-hydraulic remote control system s. 7.3.3 Towing winches
7.3.2 Towing hooks Di fferent types of hook in addition to the radial towing hook are on the market and in use. There are two basic systems - the normal standard towing hook and the disc to\~ng hoo k (see fIgure 7.8). The disc towing
Installing a towing winch allows adju stments to be made to the length of the towlin e at any time to fIt the circ umstances . O n m odern tugs the winch can be
contro lled from the wheelhouse. The advantage is that normall y no additional manpower is needed for adjusting towline length . A towing winch allows faster and easier handling of the towline, especially whe n heavy ones are used.
At the winch control panels one should have a good view of the towi ng winch because, for instanc e, the lin e
can be trapped between lower layers preventing it from being paid out freely. If not noticed the towline is, instead of being paid out, automatically heave d in again with all its conse que nces. When the towing w inc h is controlled locally at the winch the tug captain should have a good visual contact with the person handling the winch .
MampaEJ, Dordru.nt, HolIo.nd
Figute 7.8 Standard !wok anda disc-hookwitA springshock absorbers anddiffirmt quick ukase sysl
hook has been developed to absorb the energy stored in a towline un de r ten sion whe n being released. Particularly whe n using fibre towlines with large stretc h a lot of ene rgy can build up in the line. Wh en towlines un der tension are released th ey have a large imp act on the hook and deck construction whe n normal standard hooks are used . Som e of these have rubber buffers to
Photo:Author
Figure 7.9 Singkdrum towing winch ofazimuth tractor tug Texelhank' (I.o.a.27·9m, beam 9·7m, holl.ard puU 45 tons) ofSmit Harhour Towage Company, Rotterdam
TUG USE IN PORT 97
TJpes oftowing winch Different typ es of towing winch exist. The most common is the single or double drum winch. In case of a double dru m, one dru m is gene rally used for harbour towage and the othe r for a towline used at sea.
An other type is the friction ortraction winch. Basically it consists of a towage section and a se parate storage section. Th e to'.. ·age section consists of two grooved
drums lying parallel to each othe r and driv en in uni son . The towlin e pa sses ar ound both drums ab out five or six times and is then led to a separate storage drum ,
Waterfall uiinches can b e found on sea going tugs an d on some harbour tugs tha t are also are used at offshore locations e.g oil rigs They have two or three drums. Ea ch drum is located a bit higher and furth er back than the other, like a waterfall. They are mainly used on anchor handling tugs. A waterfall winch with two drums can be used as follows: the top drum ho lds the main tow wire , while the lowe r drum ho lds a working wire for ancho r handling. On some harbour tugs the top drum holds the sea towing lin e, while on th e lower drums the towline s for harbour work are stored.
which can be situate d below deck. The storage drum is driven in such a way that it keep s the towlin e under slight tension . This is usually about 2% of the nominal pulling capacity of th e win ch. This type of winch, originall y used on sea going tugs, offshore work boats and even on ships such as LPG carriers, is occas ionally used on harbour tugs though in very limited numbers. The winch is very suitable for fibre rop es, espe cially on recovery, since the line is alway s under slight ten sion and easily spooled onto the storage drum. When the towline is under high loa d it is not pulled down onto the storage drum, beco ming trapped and crushed . The disadvantage is the large deck space required, whi ch is limited on harbour tugs and the lack of flexibility in choice and type of higher strength fibr e towlin es that are available tod ay, since the profile of the grooves on the winch drums are designed to suit a specific diameter of rope. The use of an incompatible diam eter of rope compared to the winch groove profile results in rop e deformation, increased wear and shorter rope life.
Photo:Author
Figure Z70 Wat'rfall winch on board 'RTSpirit' (see paragraph 70.7.7) of towing company KOTUG, Rotterdam, The Netherlands. 17ze winch has one upper drum and two lower drums.
Phow: Damm ShipytlTtfs. Holltmd
Flf,Ure Z12 Splitdrum wind! oftJuASD-tug \14elton' (I. o.a. 32·7m, beam 12m, bollardpull ahead 60 tons, astern 56·6 tons) ofAdsteam Towage, UK, built by Damen Shipyard, Holland. Accordingto the yardthe application ofthissplit drum winch is different. Each part of the winch carries a separate tow line. Consequently, each drum part hOJ afairlead at the forward H-hitt. The[airleads are ofstainless suel, reducing thewear on modern fibre towlines
Flf,Ure Z77 17ze friction drums of a tractionwinch
98 THE NAUTICAL INSTITUTE
A type of winch very suitable for fibre towlines is the split drum winch. With undivided drums the outer layers of the towline tighten excessively wh en high bollard pull is app lied. This causes crushing and damage to the layers of the rope nearer the centre of the drum. The split drum win ch has a single drum comprising a tension section and a lirie storage section. On the tension section the amount of rope for normal use is available.
towline tension and rope payout with a display in the whee lhouse . Most harbour tugs don 't have such devices at all. Automatic spooling gears , which spool the towline properly around the towing drum, can be found on harbour towing winches. However, because of the short tov...lines used in harbour towage in relation to the long towlines at sea, many harbour towing winches are not equipped with a spoo ling gear. Wh en so equipped they are not always use d, particularly when a ste el wire towline with stretcher and pendant is used. The spooling gear is the n of little use for the short part of the main towline.
Photo: Bildsennce, GotJimhorg, Swtdm
Figure 7.13 Double winch forward on the tmerse tractor tugJ olm' (/.0.0. 32·9... beam 1(}2m, hollardpull ahead 53 tons, astern 51 tons) of rowing company Buksir og Berging, Norway
When more rope is needed, the split pr ovided in the separation disc ena bles any length of rop e to be taken from the storage sectio n. It has the advan tage that the fibre line has only a few layers on the tension drum, so wear is less. In practice, however , even with this type of winch, it some times happens that a synthe tic line b ecomes trapp ed b etween a slacke r lower layer, pr eventing the line from bei ng furthe r freely deployed witho ut manual assistance. A disadvantage of the split drum is that it is somewhat more difficult to ope rate. H owever with good training and some exp erience tha t should not cause a problem . Som e h arbour tugs, e.g. several Jap an ese built reverse-tractor tugs, are equip ped with a doublewinch at th e b ow. Two bow lines can be used independently at the sam e time . Towing winches used on h ar bou r tu gs ca n b e equipped with a self rendering or tension device, which is a towline load reducing system . The rendering facility is in case the shock loa d on the towing gear should · exceed preset parameters. T he winch auto matically heaves whe n line tension is below a ce rtain val ue. Towline length can be preselected. The facility can be adjusted by mean s of a ten sion control, allowing the winch to render more easily when working in difficult conditions such as waves or swell. Such systems are not suitable for oper ation in narrow p ort areas . Harbour tugs can simply b e equipped with a device measuring
Towing winches are driven thro ugh reduction gears by a mot or wh ich m ay be powered by hydraulic pressure or electricity. Most harbour tug towing winches h ave hydraulic drives. Electric controls are sens itive to moisture and corrosion and the relatively sim ple AC pole changing motor is inferior to hydr aulic drives in pull/ speed characteristics. The reasons are that speed is variable in steps only, the high starting torque and on/off control only. The AC/ DC (\'lard-Leonard) drive has excellent drive characteristics, but is expe nsive and more sensitive to moisture, corrosion and over heating than any other type. However, the re are many variations in elect ric and hydr aulic drive typ es that may significantly influence performance, reliability and costs. One example is the frequ en cy-controlled towing winch. Co mpared to a hydr auli cally drive n winch, a freq uency-controlled win ch has a num be r of advantages, such as a smooth and stepless control, easier installation, space savings (no hydraulic power package required) and immedi ate readiness for use. Th e control system can be arranged un der deck, so avoi ding corrosion and moisture. Costs are slightly higher than those of a hydraulic winch .
Towing winch characteristics For ship assistance by harb our tugs, the followin g aspects of a towing winch are impo rtant: Brake holding powe r Maximum pulling capacity Rated pull or pull/ speed char acteristics Slack line spee d
Brake holding capocity is the holding capacity of the brake and usually refers to the first layer. The more layers, the lower the br aking capacity. Braking cap acity of towing winches for harbour towing is two to three times the ballard pull of the tug, altho ugh it is often dependen t on a towing company's policy. Lower values can be found such as a br ake holding capacity equal to b allard pull. There is an important relationship betwee n brake holding capac ity and the minimum breaking strength TUG USE IN PORT 99
of a towline . Th e latter is higher th an the ballard pull of the tug, whi ch is co ns id ered later. With low br ak e holding capac ity an d a high safety factor in the towlin e, th e b rake slips before th e towline b reaks, pr eventing to o hi gh lo ads in the towli n e an d conse que ntly length enin g the towline's life. However, a relatively low brake holding capacity may limit a tug's perfo rmance because in ce rtain situa tions, such as stee p towline angl es, towline forces can be mu ch higher than ballard pull and in order to avoid brake slippage tug power has to be redu ced . The op posite can also be the case. With a high b rake holding capac ity, e.g. three tim es th e ball ard pull, th e towline m ay br eak before the brake slips, unless quick rel ease is used in time. O n th e other hand a tug 's perfor man ce is less limited, which can be of impor tance
or curre nt are high. For example: A large co ntaine r ship has to be berthed during high on shore winds. The tugs towing on a line are pulling at full po wer. Because the av ailable mano eu vri ng space in th e h arbour basin narro ws, th e tugs h ave to sh o rte n towline s. T h e m axim um pull of the towing winches is less th an the ballard pull, therefore the tugs re duce power to be able to sh orten the towline. Because the hauling speed of the towing winches is also low while pu lling, power is furthe r reduced to be able to shor ten line as quickly as possible an d it takes rath er a long time before th e line is at the cor rect length . All th e tim e th e ship is dr iftin g, which easily results in an accide nt. Therefore th e higher the pull and related spee d of towing win ch es th e bett er. Note: Towlin e length should normally b e adj usted in goo d tim e, particularly in tugs with winch types th at can n ot be put in gear whil e th e towlin e is under load.
during ship assistance in adverse weathe r and/ or current
conditions. Therefore when buildin g a new tug th e brake h old ing capacity of the towing winch is an important fact or, to be carefully co nsidered in relation to the minimum breakin g strength of the towline and required performance of th e tug .
It should be n oted that with a steadily increasing towline force th e brake m ay slip at the brake h olding power of the win ch. However, in case of shock loads th e brake will m ostly not slip at that stage du e to th e inerti a of th e braking system and the towline m ay break. Wh en a load reducing system is op erational th e brake syste m is disengaged an d the winch dri ve engaged. Mod ern towing win ches may have an adjustable brake holding power, with automatic release on a pre set lin e ten sion, while winch brakes op en in case of ' dead ship'.
Other im po rta nt aspec ts are the maximum pulling capacity and pull/speed characteristics of a towin g winch. The maximum pulling capaci ty of a winch is th e stall heaving capacity or stalling load . That is the m aximum line pull th e winch exe rts at first layer when control is in heave and the line is kept stationary. The maximum achiev able pull dep ends on the driv e typ e and control. A 15 tons towin g winch means a maximum pulling cap acity of 15 ton s. As drum spe ed increases th e pulling capac ity redu ces. Pull/speeds characteristics or rated pull of a win ch give th e pull at a nominal or rated speed.
Slack linespeed is also import ant, because when letting go tugs the faster th e tow line can be retrieved the bett er, an d th e risk of fouling the sh ip or tug propeller is less. In por ts wh ere tugs are towing on a line during b erthin g operations they often h ave to change to pu shing at a sh ip's side. The faster the towline can be retrieved th e soone r tugs are availab le to pu sh . Achievable slack lin e spee ds are different for differen t typ es of dri ves an d thi s must also b e co n sid e re d w h e n spe cif y ing th e performance an d drive type of n ew eq uip me nt. Towin g win ches are pro vided with quick release system s which can be ope rated at th e winch and from the wheelh ouse . As explaine d in a previous chapter about conventional tugs, risk of girting ex ists. W'h en these tugs are equippe d with a towing winch with a qu ick release system the risk is minimi sed , b ecause wh en danger of girting arises th e towline can be slacked or sli p p ed en ti rely by m e an s o f the qu ick r el e ase m echanism .
7.4
Q uick release systems
By the nature of th eir work h arbour tugs can easily ge t involved in risky situa tions. From th e poi nt of view of safety of tug and crew , qu ick release systems are of the utm ost imp ortance. That is why specific atte ntion is give n here in parti cular to qui ck release towing ho oks.
Pull / spe ed s ch arac teristics are given, for instance , in the
following way: 10 ton s x 10m/ min, which mean s that the winch is cap abl e of pulling 10 ton s at a hauling spee d of 10 m etres per minute. . So me h arb our towing compani es establi sh a m aximum pull for th eir tug s which is half th e ballard pull, though a number of harbour tugs can b e found . having a maximum pulling capacity equa l to th eir ballard pull. Max imum p u Ili ng capac ity a n d pull/ sp eed char acteristics are parti cularly imp or tan t whe n towing in narrow port areas and when influen ces of wind and! 100 THE NAUTICAL INSTITUTE
In case of em ergency, towing lines unde r hi gh tension can not or can only with difficulty be released from a towing bitt. If release is absolute ly necessary this can b e very dangerous for the cre w. An axe could be u sed, bu t th is on ly w orks with light towlines. An altern ative em ergency m eth od sometimes used in th e US A is a qui ck release stra p. It is a short line with an eye at on e end. The eye is put on the towing bitt and the free en d pas sed through the eye of th e towline an d also secure d to th e towin g bitt. In an em ergency th is line is cast off an d the towlin e is released with little danger for the tug' s crew. But it is clear th at, in ge ne ral , towing on a towing bitt has con sequences for safety.
A s m ention ed earlier, towing winches an d towing hooks are normally equipped with quick release systems. Experience teaches that in many cases it is nearly impossible to open th e quick release hook in very critical
with a qui ck releas e system are safest. Th e same applies to quick release towi ng h ooks, provided those hooks are fully reliable.
situations, often with dram atic co nse que nces. Wh en a
7.5
Towlines
7.5.1
Towline requirements
tug is listing cause d by ve ry h igh ten sion in the towline, as is nearly always the case in critical situations, it is
often impossible to open the hook. One cannot rely on such a syste m. Quick release ho oks sh ould the refore be tested un der the m ost seve re circumstances that m ay b e experienced du ring critical situations. They sho uld also b e well maintain ed . In vestigations are recommend ed into whethe r towing hooks can b e construc ted in such a way th at with steep towing angl es th e ultimate lead of the towline towards the towing h ook is kept parallel to the deck pl an e, as shown in th e photograph of th e tug with a towi ng winch with radi al syste m (figur e 7.3). There are m od ern typ es of quick rel ease ho ok s. O ne of these is th e hydraulically locked towing hook (by Brusselle Marine Industries in Belgium). This looks mor e or less like a n ormal stan dard towing hook bu t th e hook itself is kept in p osition by a hydraulic cylin der. As soo n as the quick rel ease system is operated, hydrauli c pressure falls an d the ho ok opens. A similar but furth er improv ed towing h o ok fro m th e sa me co m pany, developed in close co-operation with th e Belgian tug comp any D RS, is the hydraulically locked towing bitt, The towing hook is a sm all bitt which can tumble an d is al so k ept in p o siti on b y a h ydraulic cy li n d e r. Construction is such that in normal op eratin g conditions the tow line cannot slip off th e small bitt, When the quick release sys te m is operated th e h ydraulic cylinder tumbles the b itt an d the towline slips off. Anoth er quick release system for towing hooks is an automatic release system. Such a system is use d on older . Russian tugs in St, Petersburg. It is an ingenious mechanical system which basically works as follows. At a certain preset maximum heeli ng angle an iron ball, locked when the tug has no or only a small list, comes free and falls down. Due to the weight of the iron ball the wire connected to the . quick release is tighten ed and the hook ope ns. Mod em electronic systems which automatically release the towline at a preset angle also exist. Irrespective of the system used for towing h ooks only one thing is important. That is that th e system must be fully reli abl e and function trouble-free under normal and severe circumstances. As for -wheelhouse lay-out, quick release controls should b e situate d so that they are always within th e cap tain 's hand reach. When releasing or cutting th e towlin e, the line should run freely overboard and n ot b ecome jammed somewhere on deck. The con clusion is that the m ethod of towing, whether by towing bitt, ho ok or win ch is also import ant for the safety of th e tug and its crew and th at tow ing win ches
A towline must fulfil certain basic function s. Firstly to func tion as the load carrying link b etween tug and ship an d seco ndly to cope with dynamic load s resulting fro m relative m otion between tug and ship .T his leads to the followin g b asic requirements for towlines for harbour tugs : Strength. A towline sho uld be of sufficient strength to cope with th e forces th at can b e experienced du ring shiph an dling ope ratio ns. Stretch . Dynamic loads should be well compe nsate d for by a towline in orde r to avoid excessive lo ads in the lin e
an d attac hmen t points. Weight/diam eter. A towline should b e manageabl e on b oard a tug as well as on bo ard a ship. When no towing winch is used a towlin e sho uld be flexibl e eno ugh for easy handling. Life. Wh en in use a towlin e should suffer a minimum of .wear, distortion and loss of strength, providing as long a life as p ossibl e, All th ese aspects are conside red while discussing different typ es ofrope . 7.5.2 Steel wire ropes and synthetic fibre ropes Although sh ip lines are used in a number of p orts (see sectio n 7.5.6) many harbour tugs use their own towlines. They can be of various types: steel wire, synthetic fibre, or partly steel and fibre. There ar e many different types of fibr e lin es, consisting of on e typ e of fibr e, or a com binatio n o f fib res and vario us con structions. It is only p ossible, th er efore, to giv e general information on towlin e composition and con stru ction. The b est information regarding a specific ty pe of wi re or rop e can b e ob ta ine d fr om th e manufacture r. D evelopment in conventi onal as well as modern syn the tic fibre s is conti nuous, much research is being carried out, and this will result in furthe r improved p erformance of man-made fibre ropes. Somewh ere during the design stage of a tug, it sho uld be decided whe the r fibre or steel towlines are to b e use d, because the typ e of towline u sed influences such item s as winch drum size and th e type and size of fairleads.
Steel wireropes A steel wire rope consists of a number of strands woun d around a central cor e of fibre or wire. Each stran d
TUG USE IN PORT 101
~dW'ITl
Ordinary lay: A method of making a wire rope whe re the lay of wires in a strand is opposite to the lay of strands in the rope. It is usual to describe wire rop e in terms of strand s,
number of wires and type of core e.g. 6 x 36 IWRC. The first numbe r is the number of strands, the second number gives the number of wires in each strand and the letters IWRC (Independent Wir e Rope Cor e) give the type of core . Ropes with more wires have greater flexibility and fatigu e resistan ce bu t resistance to abras ion is less. Fibre
cores allow easier handling and are ideal for use with smaller wire sizes and wher e wire is to be han dled manually.
•
6x36WS+IWRC
Figure Z14 SteelWiTt construction
in turn consists of a number of wires wo und to form a
strand. Wire rop es are constructed in various ways. The following definitions and illustrations (see figure 7.14) are helpful in id entifying different wire types:
Lay: The twisting of stran ds to form a rope, or wires to form a stran d, durin g manufacture. Right hand or left hand lay: Th e an gle or direction of strands relative to the centre of a rop e. When looking along the line of the rop e and the direction of the strands is anti-clockwise it is called left hand lay. If the dire ction is clockwise it is called right hand lay. Cross lay and equal lay: Term s describing the lay of wires used to make up strands. In a cross lay strand all wires have a different lay length. High stress concentration at the cross-over points leads to early internal failure. Equal lay wire ropes tend to last longer, mainly du e to less intern al wear. Th ey also withstand cyclic loading better and are stronge r. Th ere are a number of co nstructions
availabl e for equal lay strands: Seale, Warrington , Filler or a comb ination, all depending on the numb er, different dimensions and combinatio n of wires in the different layers of a strand. Th e most suitable is the Warrington/Seale construction.
Lang's lay: A method of making a rope where the lay of wires in a strand is the same as the lay of strands in th e rop e. It has better wearing properties than ordinary lay but tends to untwi st so has only limited use. 102 THE NAUTICAL INSTITUTE
Wh ere steel wire ropes are used on towing winches it is advantageous to use a steel wire core. ' Vire s constructe d with a stee l wire co re offer greater resistance to the crushing forces experienced on winches, are 7%
to 8% stronger and stretch slightly less than a fibre core wire of the sam e diameter.
Wir e ropes can be supplied in different gra des of steel, usually 180 kgf/mm' (1770 N / mm ' ) or 200 kgf/ mm' (1960 N/mm' ). The latter has a higher minimum breaking stren gth and gene rally better performance. In the U SA othe r indicatio ns used for ten sile strength include Improved Plow Steel (I PS) which h as about the same tensile strength as 180kgf/ mm' (1770 N/mm' ) steel wire an d Extra Im proved Plow Steel (XIPS) whic h has a higher tensile strength. Figure 7.15 shows some typi cal minimum breaking strength s of 6 x 36 WS (Warrington/ Seale) IWRC wire rop es.
24 26 28 32 36 40
241 283 328 428 542 669
37 44 50 66 83 103
41 48 56 73
92 114
Figure ZIS 1jpi£a1 minimum breaking strength.r Maintenance ofsteel wires Steel wir es sh ould be pro p erl y maintain ed and regularly inspected. Visual inspection is vital, p articularly around eyes and th ose shackled to stretche rs, as the shackle tends to increase wear on the wire at this point.
Inspection should focus on such aspects as: broken wires in stran ds, corrosion, rope deform ation (kinks, flattened areas, misplaced outer wires, etc.).
•
Three-strand rope components
8·strand rop e
One pll1it pil th
3's tran d rop e
12·strand br aid
Parallel strand
6-strand rop e
Double br aid Source: Fihre &pe 'RchnicalManual
Figure Z76 Fibre rope componentsand constructions
Syntheticfibre ropes Du e to the increased bollard pull of tug s, th e diameter and weight of steel wire towlines has increased. Consequently they are increasingly difficult to handle , not only by a tug's crew but also by the low number of crew memb ers available aboard ships to fasten or release towlines. Escort tugs put an additional demand on towlin e performance not only becau se of their large bollard pull but also due to the high towlin e forces
rop e vertical the direction of th e strands corresponds to the diagonal line in the letter S or Z.
Three strand ropes: The thr ee stran d rope, or hawser-laid rope, is the mo st common of twisted ropes. They have a tend en cy to 'kink' or 'heckle' which significantly reduces strength. Specific strand constructions ca n redu ce the tendency to kink. The rope has good abrasion resistance.
experienced when operating in indirect towing mo de.
Because of their strength, stretch, and weight, there is a growing preference for fibre towlines. The differ en t type s and cons truc tio ns of fibre towlines all have the ir own specific cha rac teristics . Dep ending on the rope type and its application, rope making consists of spinning the fibres into initial yams, initial yarns are further twisted into final yarns. Final yar ns ar e then twisted to form stra nds or plaits. Strands or plaits are formed int o ropes. To pr event the rope unJaying, the strands are laid up in the opposite direction to the yarns. As an exam ple, th e com pone nts an d the way of co nstruction of a three-strand rope ar e sho wn in figure 7.16. Some definition s will b e given and some of th e m ost co mmon rope typ es will be reviewed . Ther e are several differ ent constr uc tion method s, also of rope types di scu ssed below, d epending on the manufacturer and fibre type. Ne ve rtheless th e ove rview gives an impression of rope typ es and rope characteristics .
Left hand and right hand lay: The same as with wire ropes. Left hand lay 'is also called S·lay and right hand lay Z-Iay. Wh en holding the
Six strand ropes: Six strand rop es with co re are twiste d rop es similar
to conventional wire ropes. It is not as prone to hockling as a three stran d rope .
Eight strandropes: Eight strand plaited ropes, also called square braid, are made up of four pairs of two strands. The pairs of strands are alternately left hand lay and right hand lay. The bal ance between left and right hand stran ds makes them virtu ally unkinkable and very flexible. The rope has a square profile, and it is mo re dur abl e th an twisted rop es. It has a high ene rgy absorption capability an d essentially the same strength as a thr ee strand rope of the same dim ensions.
Twelve strand ropes: Twelve stran d br aid s consists of twelve twist ed stra nds that have be en braid ed into a singl e braid construction. A single braid construction leaves a void in the centre. The hollow is instrumental in the easy splice procedure. Holl ow braids are non-rotating and ar e a very efficient way to utili se fibr e. Fibres used include nylon, polyester, polypropylene, composites of polyester and polypropylene, and HMPE fibr es. Also other construction methods of twelve strand rop es exist. TUG USE IN PORT 103
A new type of rope of HMPEfibres is the 12 x 12 strand rope, which consists of twelve individual 12-strand ropes that have been braided together to form the final rope . With this type of rop e individual strands can easily be repaired by using traditional splicing methods. Twelve stra nd rop es of HMPE fibre s are, amongst oth ers, frequently used for towlin es, including escort tug
Polypropylene
towlines.
Polypropylene has about the same ela sticity as polyester but is significantly weaker than eithe r polyester or nylon. Polypropylene is the lightest of the man-made fibres and floats in water. It has a low me lting point and tends to fuse under high friction. Prolonged expo sure to the sun's ultr aviolet rays can cau se polypropylene fibr es to disintegrate.
Double braid or braid-on-braid:
Combinations ofmaterials
Doubl e br aided ropes are constru cted from an inner br aid ed core rop e and an outer braided cover rope . It is re ally two rop es in one . The engineering of doubl e braided rop es includes th e use of different fibres in the
Seve ral manufactur ers make rope s comprised of mixtures of polyester and polypropylene fibres. The ir strength lie s gen erally so m ewhere b etwe en corresponding ropes made only of p ol yest er o r polypropylene. Depending on how fibres ar e arranged in the yarn s, abrasion resistance and cy cl ic lo ad performance can be almost as good as for pure polyester. The combination of po lyester and polypropylene gives the ropes optimum resistance to internal fusion damage. Po lypropyle ne always fuses fi rs t, stab ili sing the temperature of the whole rope an d its melting point, conseque ntly protecting the polyester yarn component from any fusion damage.
core and cover to control properties such as elongation,
specific gravity (ability to float), abrasion resistance and coefficient of friction. In a 'standard' doub le braid design the braided cover rope and core rope supplement each other in strength and share the load almo st equally, which can be achieved when the fibre s have a fair amount of elongatio n. H igh perform ance fibres (e.g, Sp ectra, Dyneema, Kevlar) have a very low stretch , consequently it is very difficult to get both cover and core to share the load if the entire rope was made of such fibres. Wh en, for instanc e, Dyneema or Spectra fibr es are used in double br aid, the cover is m erely a prot ective jacket, often m ad e of polyester, and does not cont ribute to the strength of the rop e.
If selecting a ro pe for a certain application, it will be clear from the foregoing that consultation with qualified ma nufacturers and /or engineering consultants is needed in order to be able to make the mo st efficient and costeffective choice.
Description ofdifferent fibres for ropes Firstly we will look at conventional fibres - polyester, nylon and polypropylene and some combinations of these.
Polyester Polyester is the heaviest of the conventional fibres an d does not float. It is also th e most durable. It has high strength, both wet and dry and an exceptional • abrasion resistan ce. It does not lose strength rapidly due to cyclic loading. Polyester has a low extension under load . The low friction coefficient allows it to slide relatively easily around bitts. Its relatively high melting poi nt reduces the chances of fusion .
Nylon Nylon is the nam e for the polyamide fibr es. Nylon does not float. Dry ny lon is sligh tly stro nge r than polyes ter rope and is the strongest of the man-mad e fibres, exce pt for Aramid, Dyneema and Spectra. Wet strength is about 80-85% of dry stre ngth . Wet nylon loses strength much faster under cyclic loading than polyester. Thus a heavily used nylon rope becomes weaker than a heavily used polyester rope of the same size. Nylon has high stretch and is more elastic than the other two fibres.
O ther m ixtures can be found, such as combinations
of nylon, polyester and polypro pylene or a melt mixture of polyes ter and polypropylene. All have th eir own specific charac te ris tics , n ot on ly because of th e combina tion of materi als, but also as a re sult of th e differen t construction meth ods used . In very cold areas the performance of ropes made of synthetic fibre changes in different way s. A few examples : Strength of polyester ropes increases by about 20"10 at an extreme temperature of minus 35-40·C, although icing causes a larger int ernal abrasion, consequently reducing the breaking strength . Nylon loses up to 100f0 strength at th ese cold temperatures, with an additional strength loss due internal abrasion caused by icing.
If working in very cold areas one should be aware of the changes in towline performance, which also may apply to the tow lines made of the modern fib res mentioned hereafter. Now the newer synthetic materials for ropes are co nsider ed, viz. Aramid {with trade names such as Kevlar (Du Pont) an d Twaron (Akzo Nobe l) an d the HM PE (H igh Modulu s PolyEthylen e) an d UHMPE (Ultra Hi gh Modulus PolyEth ylen e) fibres wh ich are available for use in high-perform ance ropes un der the tr ad e n am es Sp ectr a (Allie d-Sig nal) and D yn eem a (D SM). The nam e HMPE will b e used from now on for bo th HMPE and UHMP E fibr es.
Aramid and HMPE (Dyneema, Spectra) Aram id and HMPE fibres have a large breaking
104 THE NAUTICAL INSTITUTE
strength and very low stretch. Th e 'Fibre Rope Technical Information and Application Manual' (see References) shows the following differences in properties between Aramid and HMPE fibre properties when compared to other fibres, such as nylon, polyester, polypropylene, polyethylene and old er fibres :
Polyester
1·44 0·98 1·14 1·38
Polyester/Polypropylene Polypropylene
0·91
Aramid
HMPE Nylon
Ropes made of Aramid do not float and ropes made of H M PE do float. Weight for weight HMPE is the strongest fibre. The surface an d internal abrasion resistance of HMPE ropes is excellent and of Aramid ropes fair respectively good. Friction coefficient of HMPE fibres is very low. HMPE fibres have a melting point of 1500 C and Aramid of 425 0 C. Aramid has a fair resistance and HMPE an excellent
..
425 150 215·250 • 250 250/165 165
1% 10/0
20% 12% 9% 8%
Notes:
•
Extension shown is at 50% breaking load of a worked eight strand rope 215 0 C for Polyamide 6, 2500 for Polyamide 6·6
.... Densitydepends on thecombinationof materials, generally
about 1·1 g/cm'. Figure 718 Tabl, showing some dunaaeristia of differmt fibre types
resistant to ultraviolet sun rays.
H M PE has bette r shock load absorption abilities than Aramid. Aramid has a 5% lower strength and H M PE the same strength when wet.
overlay fmish '. Testing has shown that these marine over lay finishes add strength and abrasion resistance to nylon, polyester, and aramid yarns and rope under wet and wet/dry conditions of use.
The tables in figur e 7. 17 and 7.18 give an indication
Ropes th at will be exposed to severe enviro nmental an d mech anical stresses can b e pro tected by the ' application of exte rnal coatings . The most commo n ma terial is polyure thane, although other materials are also used . Coatings can be app lied to protect ropes tha t will be exposed to seve re weather, cycling abrasion, marine grow th build up, or long exposure in water. Coatings can furthermore be used to improve abrasio n
of some characteristics of fibres and performance of
different rope types. The ex tension at 50% breakin g strength m en tioned in-the table and as given by one rop e manu facturer are for worked ropes, as the stretch
of new rop es is higher. The stretch of nylon in wet conditions is also higher.
resistance, snag resistance, to provide protection against
When reading the tables it should be kept in mind
ultravio let degradation or for colouring coding.
that rope characteristics such as stretch, minimum
breaking load, etc . also depend on the construction method of the rope, as indicated previously.
Handling and maintenance offibre ropes, including tow lines Conside r first the danger of 'snap-back' of fibre lines. Snap-back is common to all lines. Even long wire lin es under tension can stretch enough to snap back with cons iderable energy. Synthetic lines are much m or e elastic, except for Aramid and Dyn eema/Spectra lin es, in creasing th e danger of snap-back, striking anyth ing in th eir path with trem endous force. Synthetic lin es no rmally br eak su d denly and wit hou t warn ing.
Finishes and coatings In creased kn owledge of yarn -to-yarn friction and abrasion within rop es under operating conditions has led to th e d evelopment of special overlay finish es that can be applied to yarns during th e fib re producing or rop e manufactur ing pro cess. The term mos t comm only used for water- res ist ing overlay finishes is 'marin e
40 44 48 52 56 64 72 80 88
71
86 103 121 141 184 232 287 344
11 1 133 159 186 214 276 345 424 514
99 120 142 166 193 252 319 394 477
30 36 42 49 56 72 90 110 131
121 147 175 205 238 311 393 485 587
35 41 48 55 65 83 107 130 159
72 88 104 122 142 185 234 290 351
21 25 29 33 38 49 62 76 91
98 118 141 165 191 250 316 391 473
42 50 59 69 80 103 130 158 190
Figure 717 Table giving compatati... weight. andminimum breaking loadsof8-strand ropes of differentfibres (1) refers In Dyneemfl!Stte/itt Extra; (2) refers In Eurof/
When ever possihle one sh ou ld keep away from synthetic lines under tension and when approaching the se line s it should be done with care. Twisted rop es can be harm ed by kinking, which may form into hockles if not properly removed. 'When a kink form s, the load mu st be re move d an d the kink gentl y worke d out. Rop es must be kept clear of chemicals, chemical vapours or other harmful substances. They sho uld not be stored near paint or where th ey may be expose d to paint or thinner vap our s. The susceptibility of the rope depends on its chemical structure and fibre . Nylon is, for instan ce, attack ed by acids and ble aching agent s. Polyester is attacked by some alkalis.
Damage 10 towlines The ex perience of several towing comp anies is that
mo st damage to fibre towlines is th e result of problem s on the ships b eing towed, such as corroded and deeply groo ved fairle ad s, sharp edges between fairl ead and bollard s and square stems of ships. It should, h owever, be n oted th at the cau se of gro ove d fairleads and bollards does not in all cases lies on board the ship. Many ships have fibre mooring lines. Groo ves in the fairlead s, bollards, etc. may b e caused in ports where tugs are using stee l wire tow lines, or fibre
rope towline s with steel wire pennants, which th en cause pr oblem s for tug s in oth er ports usin g fibr e ro pe towlines.
7.5.3 Composition of towlines Excessive heat can damage synthetic lines, especially p olyprop ylene . Poly ethylen e and Aramid are vuln erable to ultraviolet rays. Care should b e taken wh en dragging synthetic lines alon g th e deck. Avoid sharp edges, rough surface s or surfaces with a small bending diam eter. When dirt, grit or ru st particles are allowed to cling to or pen etrate into synthetic rop es, internal abrasion will result. The rop e should be clean ed before storing. . To distribute wear equally along all parts of the towline, ends should be rev ersed periodically. A further reason is that braided ropes, whi ch are torque-free,
develop twists when constantly used on a winch by the direction of turn of the winch, or by rolling on th e winch drum due to uneven layers. A braided rope can also get twisted through rep eated handling on a capstan. Twists make rope handling more difficult and reduce rope strength when not removed. If a twist develops, it should be removed by rotating the rop e in the opposite direction when it is relaxed. Fairleads, warping drums, roller heads, etc. sh ould be in good condition and damage to fibre line s by rust and grooves in fairleads should be avoid ed (see figure 7.12 for photograph of ASD -tug Melton with stainless steel fairlead s).
The composition of towlines used for harbour towage can be as follows: A single steel wire. A steel wire towlin e, stretcher and steel wire pendant. A fibre rope towlin e and steel wire pendant. A fibre rope towline with or without a fibr e rope pendant. Alth ough steel wire has little stretch , only steel wire towlines are used. Dynamic load s in the towline can be compensated by towing h ooks fitted with springs or by towing winch es v'lith tension control. Wire rop es used as towlines on towing winches are
gen erally 6 x 36 IWRC, tensile strength 180 kgf/mm', wires in strands equal lay Warrington/Seale, strands ordinary lay. On very powerful harbour tug s towlin es of tensile strength 200 kgf/mm' can be found. Usually a steel wire towline is right hand lay, though wh en a towing winch is used with a spo oling device it dep ends on the heaving and spooling direction of the winch wheth er right hand lay or left hand lay is required. When wire towlines are not stored on a win ch the same type of wire towline can be used, h owever with a fibre core . "When a steel wire towline is used in com bination
It is recommended to use a pennant particularly for fibre towlines to minimise damage at the ship's end of the main towline. A co w hitch connection between a
fibre pennant and a fibre towlin e, as often used, reduces strength of the total towing connection by approximately 15%. Splices in a rope decrease minimum breaking strength by at least 10%. Towline, stretcher and pennant (if used), must be inspected at regular intervals and these inspections should include, as far as possible, inspection of inner strands, eye s and splice s. Finally, although all aspects m entioned above for proper rope handling and maintenance are important, of at least equal importance is proper tug handling to minimise as far as possible shock loads in the towline. 106 THE NAUTICAL INSTITUTE
with a stretcher and pendant, the steel wire pendant will generally be of th e same.constru ction as th e towlin e but usually of rather smaller diameter or of used towlin e of the same diameter. In case of ex treme towline forces
the pendant will br eak first and only this part has to b e replaced. Nylon as w ell as poly ester or poly e ster/ polypropylene is used for stretchers, for instanc e in eight str and braided construction. The stretch er is often doubled as grommet. The length of stretcher is usually about 10 metres. Although nylon has large stretch, it degrades in strength and abrasion resistance when wet and is subject
to torsional damage whe n used in conjunction with a steel wire towline (see also pa ragraph 7.5.2 'Finishes and coatings' ). Th er ef or e po lye st er an d po lye ster/ polypropylen e are often preferred for stretchers. It is recommend ed that stre tche rs have a larger breaking stre ng th than the steel wir e tow li n e OC IMF recommend s nylon tails have at least 37% higher b reaking stre ngth th an the ro pe. This is b ecause expe rience shows that cyclic loading degrades synthetic lines, particularly nylon, mo re quickly than wire und er similar load conditions. The stretcher should therefore have a 25% higher dry br eakin g strength than the wire. As nylon has a lower br eaking strength when wet an additional 10% should be added, giving a total 37% allowance for redu ction in strength . Th e same at least will apply to nylon stretchers in relation to minimum breaking strength of the towline.
When fibre towlines are used the type of towline depends amo ngst other thin gs on th e loads and in particular the dynamic loads that can be expected and whe ther a towing winch is used or not. As type of tug, ope rating methods, conditions and circumstances differ by port, different type of fibr e towlines are used, such as towlin es made of polypropylene, nylon, poly ester or poly ester/polypropylene. Different constructions such as doubl e braid, 12 strand, eight strand, six strand and th ree strand can also be found . Th ree strand rop es are not optimal rop es for towing winch es. A pendant may be connected to fibre towlines to pr otect the main towlin e from abrasion. Steel wire as well as fibre rop e (including HMPE fibre ro pe) is used for pendan ts. Nylon towlines are used pa rticularly in wave and swell conditions becau se of the ir high stretc h . One to wing company wo rk ing primarily under these conditions pr efers thr ee stra nd loose laid nylon, after having tried out othe r rope types and constructions, because of the stretch and ease of handling. The line is belayed onto bollard s/bitts on b oard the tug. Modern fibres such as Dyneema and Spectra are inc reasingly used for towlines for escort tugs as well as for harb our tugs. The lines can be 12 strand, eight strand or oth er construc tions, dep ending on the manufacturer and user's n eed s. A tail of th e sa me fibre typ e, sometimes with cov er, is often connecte d to the main towline to pr event it from early wear, while pennants made of e.g. nylon or polyester are used as well. This is because of the low stre tch of ropes ma de of Dyneema or Spectra, which has consequences for dynamic load ab sorption in the towline and which easily results in high peak loads. This may be the case if no use can be made of a load redu cing syste m on the towing winch (which is mo stly the case on harbour tugs), but particularly wh en short towlin es are used . The nylon or polyester pennants add some stretch to the towline.
polyester main towline, which results in more stre tch in the towing connection. The be st way found to connect the grommet to the towline is to p ass the rope of which the grommet is made tluough the main towline's eye and then spliced to a grommet. Th e on-board end of the grommet may be prot ected against shaving by a se iz ing. It is imp ortan t to use compatible ropes, othe rwise the p enn ant may cut thro ugh the main towline. Th e large advantage of the system is that the grommet can be rotated ov er time to spread 'wear and tear'. Experience with these modern fibre towlines is still building up , expe rience that can be used for further improvement. As already mentioned in the beginning of the form er paragraph, there is a large variety in rope types, rope composites and construction methods, and consequently in rope characteristics and applications . Therefore, when selecting a rope for a towli ne of a tug, a ca reful consultation with rope manufacturers and/or suppliers is nee ded regarding the most suitable rope type an d recommended use, taking into account tug's capabilities, working meth ods an d conditions. Th e reader is further referred to paragraph 9.5 where specific information can be found on escort tug towlines,
which is also of relevan ce for no rmal harbour tugs. 7.5.4 Basic towline length The towline length for tugs towing on a lin e is now co nsidered. H owever , it will be sh own th at some conclusions are also applicable to other tug ope rating me thods. Wh en towing on a lin e a tug captain determines the length of the towlin e on the basis of his insight and expe rience. This concerns tugs with towing winches and tugs using ship lines as towline. On tugs without a towing winch and using their own towlines the available length is usually limited to a preset towline length, as mentione d earlier. The towline length used while towing on a lin e dep end s on factors such as type an d length of tug, size and deck height ofthe ship to be assisted, environmental conditions and available manoeuvring space for the tug. Ship's speed is also important. These factors may result in longer towline lengths in one port than in anoth er an d may also differ dep ending on the tug captain's ex pe rie nce. Towline length al so in flue nces ship manoeuvr es, as will be explaine d
Towline length in relation to ship's path width To show how t owline l ength affec ts ship ' s mano euvre s, a for ward tug towing on a lin e is
A system used in e.g. several Australian ports is a Dyn ee ma gro m m et co n n e cte d to a d ouble-braid
co nside re d . Fro m figure 7.19 it is clear that whe n requir ed to chang e from pulling direction I to pulling TUG USE IN PORT 107
direction 2 tug A needs more time in comparison to tug B owing to the longer distance to be covered. Tug B, with the shortest towline, can react
much faster when required, for instance to stop a sudden sheer of the assisted ship. So, with a short towline faster tug reactions are possible than with a long towline. This applies to tugs towing on a line as well as for tugs operating in the push-pull mode at the ship 's side. When the length of the towline is doubled the reaction time will also approximately double.
L
,
I
\I"
II I
"II
p
The manoeuvring space required
by a ship is smaller when tugs react quickly. A ship passing through a harbour basin with the assistance of tugs, for example, needs a manoeuvring lane of a certain width. This path width is smaller when tugs work on short towlines, because the ship does not have much time to sheer or drift. As soon as it happens and the pilot or tug captains notice, tugs can react very.quickly. The total required manoeuvring lane width for the combination of ship and tugs is also narrower, because tugs towing on short lines require less space . So, it works to double effect. Working on a short towline therefore has three important advantages: Figure Z20 The sffea ofdifferent lowline lengths 1
/
11
Faster reaction time of tugs. Reduced ship's path width. Less manoeuvring space required for the
combination of ship and assisting tugs. These aspects are of particular importance when manoeuvring space is limited as is the case in most port areas. It all sounds very logical. However, the experience of some ship masters is that in a number of ports long' towlines are used too often. It then takes too long before a tug can exert towing forces in the required direction. In the meantime the ship is drifting or swinging in the wrong direction. Some comments should be made. The advantages of short towlines include quick reaction times of tugs and minimum required manoeuvring space. However,
Figure Z79 Tug reaction time andmanoeuvriug space required depending ontowline length .
108 THE NAUTICAL INSTITUTE
it will to some extent reduce a tug's effectiveness due to the counteracting effect of the tug propeller wash on the ship's hull. Tugs should therefore have sufficient bollard pull to compensate for part of the loss in
effectiveness resulting from the relatively short towlin es. In add ition, the higher the bo llard pull the faster tugs can restore a ship's position or heading, for instance
when the ship starts drifting or veers off cour se. So th e available bollard pull also influences a ship's path width . When manoeuvring spac e for a ship is very limited tug reaction time should be very high such as when assisting in dockyards and when passing narrow bridges. Two short towlines should be considered in this case for the forward tug as shown in figure 3.11.A tug secure d tha t way can react much mor e quickly,
Figure 7.27 Tug operatingbroadside while ship is moving astern Sta tic force in towline TIP (.
•
T_W•• lo . c lt , p\l mn.g ro.e it
'tl.gl
The effectiveness ofa tug on a short steep towlin e Irrespective of assisting method, the vertical towline angle can be quite large when shor t towlines are used. There has been a lot of discussion about whethe r, apart from the interaction effects of a tug's propeller wash, tug efficiency is otherwise affected when the towline is shortene d.
5 .5
For an ex planation that no lo ss in effectiveness occurs 5 4.5 4
I
3.5
/
3
2 .5 2
1. 5
,
o
»> ro
W
30
40
/ 50
/ 60
/
70
eo
Verical towline ange
Figure Z22 Staticforce in a a towline
when the towline is shortened, see figure 7.20. Both tugs are exactly the same and both are pulling ahea d with equal full power P. Thi s gives a force T in the towline. This towline force has a vertical compone nt, which lifts the tug a little out of the water, but is compe nsated for by the tug's inc reased apparent weight L. Force L together with the towline force T gives a resultan t force R, equal to the pulling force P of the tug in a stale of equilibrium. The towline force T = T' on the ship, can be resolved in a vertical force L' and in a horizontal force pl. The forces pi, which are the tugs' pulling effects on the ship, are equal to the towing forces P of the tugs. So it can be con cluded that shortening the towline does not affect a tug's effectiv eness .
Howev er, there is an important aspect to be taken
into account and that is friction force L' . The figure shows that when using a short towline this friction force is ve ry large, resulting in hi gh temperatures and considerable wear so imperilling the towline's life. Wh ere tugs have to work with such short and steep towlines strong pen dan ts are recommended , if they can be used, because they can easily be rep lace d whe n damaged.
Photo: F. v. La mam
Figure Z23 Two conventional twinscrew tugs1 'Smit Ier land' and 'Smil Detumarkm' (Lo.a. 284m, beam 8·5m, bollardpull 28 tons) operating broadside at 1M stem oj a tanker enlering a basin at 1M port ofRotterdam
Tug safety in relation to towline length Although using a short towline has advantages, one should carefully consider the towline length of a forward tug assisting a ship under speed. When using a short towline the distance between forward tug and ship's bow is very small. Conseque ntly, the time available for a tug captain to react is very limited and when ship's speed is high the reserve engine power of a tug to reac t quickly is small. Th at is why he has constantly and closely to observe ship's course and spee d changes. On the othe r hand pilots have to be careful with rudder an d engine man oeuvres and have to keep a tu g captain well informed abo ut inte nded manoeuvres, becau se the TUG USE IN PORT 109
>
safety of tug an d crew is involved . For thi s reason forward tug capta ins don't like to tow on a short towline in dense fog or wh en an attended ship has ra ther high spee d. Moreover, with increasing speed other effects such as interaction effects might com e int o play. When tugs are operating broadside as shown in figure 7.21 , the steeper the towline the larger the righting force L. A short towline in this case has a po sitive effect on tug safety. 7.5.5 Strength of towline and safety factors As stated in the introduction to this chapter, the towline is the cru cial connectio n b etween tug an d ship. It sho uld be a reliable connection, not limiting a tug's perfo rm an ce. Th e length of towlines as well as different typ es having been discussed , attention now turns to the required strength of towlines. S tatic f orces in short and long towlin es A tug captain towing on a line may be force d in certain situations or circum stances to use a ve ry short
and steep towline, shorter and steeper than he would normally use. This may happen in situations such as when dry -docking pra ctically empty ships with large freeb oards, wh en assisting high freeboard ships in narrow b asins and whe n entering locks or passi ng narrow bridges. Such situations are quite common to harbour tugs and towline strength should be capable of coping with them . For forces in the towline look at figure 7.20 again . With an equal towing force P for the tugs the force T in the line of the tug with the steep towline is conside rably high er than in the line of the tug with the longer towline. How static forces increase compared to vertical towlin e angle can be seen in the graph of figure 7.22. Up to a vertical towline angle of 40° the influence is not so large. Howev er , wh en th e vertical towlin e an gle further incr eases the force in the towline increases very rapidly. At a vertical towline angle of 60° the forc e is already twice the exerted towing force of the tug. A vertical towlin e angle of 45-,50° for tugs secure d at a ship's side is not too large but whe n towing on a lin e it is a large angle, although it does happ en . In this case th e static force in the towline is already 1·5 times as high as the towing force of the tug. The to wlin e force furth er incr eases by the tug's underwater resistan ce wh en the tug is also drawn in the dire ction opposite to its pulling dir ection. In the case in figure 7.20 the tugs would then be pull ed backwards. There is not always a direct relati onship between towline forc e and the towing force exerted by the tug. In situations where the tug is steering broad side to a ship which has stem way (see figure 7.21), the force in the towlin e is caused on ly by th e tug's un derwater resistance. Tugs operating in the indirect towing method, particularly at high speeds as is the case with escort tugs, 11 0 THE NAUTICAL INSTITUTE
expe rience very high towline loads mainly due to high lift forces gene rated by the tug's underwater body and skeg, if fitt ed . H owev er, the main fac to rs for the max imum static forces in the tow line during normal harbour operations are the tug's bollard pull and the towline angle . Dynam icfor ces in a short and long towline In addition to static forces, dynamic forces can also occur in a towline and can reach high values. Th ey are generated, for instance, by sudden acceleration s of the tug, wrong tug manoeuvres, waves, swell and so on,
creating shock loads in the towline. Horizontal tug accelerations can be kept un der control to some degree by careful manoeuvring. However, this is not th e case with vertical accelerations du e to waves and swell. It is obvious that these vertical acce lerations, which can even
be created by the wash of passing ships, hav e a large effect on forces in a towline, especially short and steep towlin es. The longer a tow line and th e high er the elasticity, the better dynamic forces can be absorbed and the lower the peak values of towline loads are . That is why much attenti on has to be paid to the strength an d elasticity of a towline, especially when tugs have to work in wave and /or swell conditions with sho rt towlin es. It can be concluded that boll ard pull and vertical towline angle are not the only causes offo rces cre ated in a towline, but that dynamic forces also play a ve ry impo rtant role. A tug's mass is an imp ortant factor in dynamic for ces and th ese occur irr espective of th e meth od of tug assistance.
Assuming again a vertical towing angle of 45-50°, towline force certainly reaches higher values than the previo usly mention ed 1·5 times bollard pull, due to the dynami c forces gener ated. How large these dynamic forces are dep en ds, amongst othe r things, o n length, type and/o r compos ition of the towline . But towline forces in excess of twice the bollard pull of the tug are not un common, par ticularly whe n towlines with little stretch, such as stee l wire, are used. It is clear that whe n
brake holding power is less than this value the bra ke of the towing winch may slip sometimes. This is, of course, only wh en th e minimum break ing strength of the towline is sufficient to cope with the high dynamic forces. Safety factors regarding towline strength Th e question now is what the towline strength should b e in relation to th e bollard pull of a tug . This is considered starting with a steel wire towline. Two aspects ar e imp ortant wh en using steel wire towlines . Steel h as some elasticity. This me ans that under load a steel wir e elongates and when the load is removed it retu rns to its original length. This is only tru e up to th e so-called 'elastic limit', approximately two thirds of the minimum breaking load of the wir e. When load exceed s this limit it results in permanent elongation of the wire.
The so-called 'enduranc e limit', approximately half the minimum breakin g load , is also of great influence on the life of a steel wire. Tests have shO\\TI that when a steel wire cable has several times en dured a load higher than the 'endu rance lim it' its life is very short and it br eaks without ever being exp osed to a load up to the ' elastic lim it' . It is clear th at shock loads play an important role.
par. 9.5.1 in the escort tug chapter. Although only an approximatio n, the safety factor of at least 4 ti mes the b ol lar d pu ll corre sp onds reasonably well with those applied by a number ofl arge harbour tug companies , viz. 3·5 to four times the bollard pull . A factor of six times the bollard pull can b e found, and also muc h smaller safety factors, twice the bollurd pull for instance. Such a low safety facto r affects a towline's life.
Taking into account the towline force of two times the bollard pull of a tug, the min imum breaking strength of as teel wire towline should then b e at least four times the bo llard pull of the tug, in order to stay within the 'elastic limit' and 'endurance limit'.
Peak values in towline loads due to dyn amic forces are lower in 'convention al' fibre lin es than in steel wire
ropes. These fibr e lines have b etter dynamic load abso rbing characteristics. According to OCIM F, due to the lower reco mmended allowable loads the safety factor for these synthetic (mooring) lines should be 1020% higher tha n for steel wire ropes, depending on the typ e of fibre rop e. Because of the lower peak loads occurring in 'conventional' fibre lines in combination with a higher safety factor, in practice approximately th e sam e safety factor is assume d applicable to steel and fibr e towlines. For the time b eing, this will also include towlines of harbour tugs mad e of the mo re mod ern H MPE fibres. More information regarding this rope type may become available in the near futur e, for instan ce by O CIMF pu blication (see References) or othe rwise . See, however, also the relevant sections of
Note: It has alr eady been indicated that th e bollard pull of a tug is n ot the only important factor for the minimum breaking strength of a towline. But for harbour tugs it can be con sidered the most important because othe r factor s such as mass or un derwater plane of a tug generally have a close relation ship with tug size and conseque ntly with the installed engine power and the bollard pull of a tug.
For escort tugs the h igh towline forces that can b e gener ated in the indirect mode are mu ch high er th an the boll ard pull and th erefore a more appropriate criterion for the required minimum breaki ng strength of the towline. 7.5.6 Ship's mooring lines as towli n es Using ship's moo ring line s as towlines is n o t recommend ed . Strength and compos ition may not be in accordance with tug towing force, particularly of more powerful tugs. Taki ng into ac count th e r ecomm endations of Classification Socie ties for moor ing lines,
the min imum breaking strength of these lines should be roughly 50 tons for a bu lk carrie r of 50,000 dwt and 70 tons for a bulk carrier of 200,000 dwt, Assumi ng a bo llard pull of 30 ton s for attend ing tugs, then th e minimum br eaking strength of the towlines should be about 4 x 30 = 120 tons. A bulk carrier's moo ring lines do not meet this breakin g strength at all, not eve n with a safety factor for tug towlines of 2-2·5. Ship's line s used for tugs ar e also frequently used for mo oring and ar e subject to inte nsive wear. The qual ity of these lines may also be affected by sun, oil, chemicals and so on . Consequently they usually have a mu ch lower br eakin g strengt h and often low reliability.
7.6
Photo:Author
Figure Z24 VS tug 'Matchless' (l.o. a. 27m, beam 9·7m, bollardpuU 34 tom) oJ Port oJChennai, India, matkJast withtwoship's lines. Ttu eyes oJthe lines are ledthrough the tug'sjilirluuiandsecured on the towing bitt
Towline handling
As tug power in creases, espe cially whe n steel towlin es are used, the towlines b ecome mor e difficult to handle. Fibre towlines, particularly those ma de of th e newest fibres, have a much lower weight but ar e only used on a limited though increasing number of tugs. A gradual change in the use of towlines can b e expected. On board ship s the numb er of crew members is still gradually decreasing. This is eviden tly without sufficient TUG USE IN PORT 111
appreciation of the wor kload and manpower requirements associated with arrival/departure activities
such as towline and mooring line handling. For the few remaining crew me mbe rs it is a difficult job to secure and release towlines withi n an acceptable time. The
the person standing on the line. Wh en letting go do not simply throw the lin e ofT the bits and let it run out; always slack it back to the fairlead using a messenger line and lower it as far as possible in a controlled way onto the tug's deck.
reduction in crew size is an ince ntive for deve lo pment
of alternative systems for attend ing towlines of harbour tugs. There are ships where boatmen are engag ed who board the vessel togeth er with the pilot and assist the crew in attending a tug's towlines and when m oorin g.
7.6.1 Safe handling of towlines aboard shi ps Most of the follow ing rules for safe handling of towlin es aboard ships are listed in the OCIMF booklet 'Effective Mooring': A sufficient number of heaving lines of proper length and stre ngth shou ld be ready at mooring stations in good time for hauling tug towlines aboard. The condition of a tug's towlines is unknown, and crew at mooring stations are not normally aware of
when a tug is actually towing or what load is applied to the line. It is therefore important to stay well clear of the towline at all times. When a tug is bein g secured or let go, the person in cha rge of the mooring should monitor the operation closely to ensure tha t no loads come onto the line before it is properly secured, or whilst being cast off. Never let a tug go until instructed to do so from the bri dge; do not respond to directions from a tug's crew. If the towline is provided with an eye, heave this past the bitts so that there is sufficient slack line to work with, stopper off the line, then put the eye on the bitts. Do not try to manhandle a line on to a bitt if the re is insufficient slack line. If the line has no eye and is to be turned up on the bitts then it should always be stoppered off before handling. Do not try to hold a line in position by standing on it just because it is slack - if the tug moves away so will
Photo: Author
FiguTt 726 Qyick release /wok used onferriesof NorthSea Ferries jOr securing a low line when a tug is required
7.6.2 Some methods for passing, taking and!or securing towlines Cranesfor towline handling Tugs can b e equipped with a crane fitted with a hydraulic clamp to deliver a towlin e to a ship to be assiste d. Such cranes for towlin e handling ca n, for instance, be found on board SOfie of the reverse-tractor tugs of the Canadian tug com pany C.H Cates & Sons
Photo: C. H. Ctms & Sons LimiUd, VanlOUlitf, Canada
Figure 725 Rsoetse tractor tug 'Charles H Cates l ' (/.o.a. 22-5m, beam 8·5m, bollardpull ahead 38 Ions, astern 32 Ions) with line haadling crane
112 THE NAUTICAL INSTITUTE
Ltd . The heavier the towline the more adva ntageous such a system can be. However, the increased use of ligh ter , h igh -str en gt h towli nes make these cra nes virtual ly redundant. Quick release hooks on boardf erries Ferr ies do not usually use tugs, though in adverse weather conditions it may so metimes be necessary. For
easy fastening of a towline and to be able to release it in
aboard the ship to be assisted. Th e crane is controlled from the whee lhouse . The system can be used , for instance, at terminal s where the same ships call regulasly, because in order to use the system the ships must be fitte d with a numb er of thes e h ook-up points at conve nient locations for the tug assistance required. The deck mou nted and/or hull mounted hook-up points must also be placed in such a way that the ship can be handl ed in a loaded as well as in a ballasted condition.
a m inim um of time and with only one pe rson, some
ferr ies have a quick release hook fitted on the fore and after deck for towline conuection. see figure 7.26.
active. Co nnection and disconnection to passiv e points
Automatic hook up system A system tha t has been prop osed is an automated ho ok up system, the 'Aasts Autohook' of Aasts Autohook B.V., Amsterdam. No crew is requir ed on the deck of the ship or tug to secure or release towlines. Securing
is by me ans of the manipulator. For disconn ecting fro m the active points there are two po ssibilities, either by manipulator or from the ship by remote contro l from the wheelho use or locall y, activating a hydraulic cylinder which lifts th e connector out of the hook-up point.
Th ere ase two types of hoo k-up points : passive and
or rel easing the towlin e can be achi eved in a minimum
of tim e and at a rather high ship's speed . At the end of the tug's towline a simple ball is fastened, the connector. The connector is placed by a specially designed tug's crane , the manipulator, in a hook-up point
The system can be fitted to any type of tug but certain tugs have been designed specifically for this system. The Triple A design concerns harb our and terminal tugs, whether stern driven or tractor type . The Triple E typ e is also equipped for escorting, emergency towing and emergency response dutie s, such as firefighting and oil spill control. Co n tr olled pl acing of the co n ne cto r by th e manipulator in the hook-up points of a ship having a rather high spee d could be difficult at night, in reduced visibility or in wave and swell conditions, pa rticularly near the shoulders. At the stern of the ship it should b e easier. Probl ems ma y arise wh en a line breaks, thou gh weas will be less becau se the towlin es do not pass thro ugh ship's fairl eads.
I 1
J! ,
Wi
' /
I
• ri • g- = , "..
.J
!
Figure 727 Automatichook upsystem, Aarts Autohook One of theproposed tug designs, TripI< E, with the manipulator, the propoud hook up points on a tanker, the connector and a passive hook upp oint for detk: mounting
Emergency towing equipment Emergency towing equipment has less to do with harbour towag e, but is mentioned here becau se of its importance in sh ip handling in an eme rge n cy_ Emergen cy towing equipme nt is mo re of a safety requirem ent for open sea, to facilitate towing the tank er out of danger in order to pr event the risk of pollution in case of emergency such as loss of propulsion and! or manoeuvrability, altho ugh it may also be suitable for co n necting th e towline of an esco rt tug (see pas agraph 9.5.1)
Emergen cy towing asrangem ents ase requ ired by regulation II-I!3-4 of the 1974 SaLAS Conve ntion, of which a new text was adopted by resolution MSC.99(73) at MSC 73 on 5 December 2000. The ame nd ed regul ation entere d int o for ce on I July 2002. The following is requ ired by regulation II- 1!3-4: I Emergency towing arrangements shall be fitted at both ends on board every tanker of not less than 20,000 tonnes deadweight. 2. For tan kers constructe d on or after I July 2002 : _I The arrangements shall, at all times, be capab le TUG USE IN PORT 113
of rapid deployment in the absence of main power on the ship to be towed and easy connection to the towing ship. At least one of the em ergency towing arrange ments shall be pre -rigged ready for rapid deployment; and 2 Eme rgency towing arrangements at both ends shall be of adequate strength taking into account th e size and deadweight of the ship, and the expected forces during bad weathe r conditions. Th e design and construction and prototype testing of eme rge ncy towin g arra nge me nts shall b e approved by the Administrati on , based on th e Guidelines develop ed by the Organ ization.' 3. For tankers construc ted before I July 2002, the design and constructio n of eme rgency towing arrange me nts sh all b e approved b y th e Administ r ation, b ased on the Gu id elin es develop ed by the Org anization.' *
Referto the Guidelineson emergency towing arrangements
for tankers adopted by the Maritime Safety Committeeby resolution MSC.35(63). IMO has a do pted am endments to r esolution MSC.35(63) at session MS C 75 on 22 May 2002 by resolution MSC.132(75) to bring the contents in line with the new requirem ents of regulation 11-1/3-4 of the 1974 SOLAS Convent ion . T h e 'Guid elin es for Emergen cy To wing Arrangements for Tankers' apply to tank ers, including oil tank ers, gas carriers and chemical tankers. According to th ese guidelines the major components of towing arrangements sho uld include (see figure 7.28):
Non prePick-up gear
optional
Towin g pennant
optional
Fairlead Strongpoin t
Roller pedestal
yes yes yes
requirem ents
yes yes yes yes depending
yes yes yes
on design
Chafinggear
Forward
Aft
Strength
ilihiJ;l
2UhiIl
requirem ents
yes
depending
yes
on design
At least one of the emergency towing arrangemen ts should be pre -rigged and capable of being d epl oyed in a controlled mann er in harbour conditions in not more
th an 15 minutes. The pick-up gear for the pre-regged towing penn ant should at least be designed for m anual o pe ra tion by on e person , allo wing fo r n o power available and the po tentially adverse enviro nme ntal conditions that may prevail du rin g em ergency towing operations. The non pre-rigged emergency towing arran geme nt should b e capable of b eing dep loyed in h arb our co nditions in not more than one h our. The forw ard
emerge ncy towing arr angeme nt sho uld at least be designed with a means of securing a towline to the chafing gear using a suitably positioned ped estal roller to facilitate connec tion of the towing penn ant. Pre-rigged
ow
sse
Towing pennant "" Strongpoinls '-----Pick-up gear
<,
-
- - --
Fairleads
<, Towingconnection
""'- Marker buoy
Figure 7.28 1j'PicalWlergmcy towingarrangement
114 THE NAUTICAL INSTITUTE
Strength
Pre~
~
J
1
Chafing gear
emergency towing arrangements at hath ends of the ship may he accepted. More detailed requirements are given by the 1M0 and hy Classification Societi es regarding strength of towing components, length of towing pennant (IMO: at least twice the highest seagoing ballast freeb oard at the fairlead plu s 50 metres), locations of strongpoints and fairlead s, size of fairleads, type and length of chafing chains (if used) and so on. Large numbers of tankers are already fitted with this emergency towing equipment. Different systems exist and other systems are planned. The main components, in line with IMO requirements, are: A strong point to which the towing conne ction on board the tanker is secured. A ship's fairlead. The strong point can be integrally designed with the fairlead. A ship's towing connection, which can be a chafing chain to which a towing pennant is connected. The pennant can be made of Dyne ema or Spectra fibre which floats. In . addition, a nylon shock absorber may he used. Instead of a chafing chain and fibre towing pennant a steel wire towing pennant may be t,
used, stored on a winch drum.
A pick up gear consisting of: .. • A messenger line (t o be) connected to the pennant and
made of synthetic rope, often of the floating type, or a combination of synthetic rope and steel wire. A pick up line, connected to the messenger, with one or two light buoys. Or just a floating messenger line with mark er bu oy. Pick up gear and towing pen nant are optional for the n on pr e -rigged e me rgency towing arrangement. Deploym ent of eme rgency towing systems depends on th eir design . Most sys te ms h av e to be depl oyed manually by launching the pick up gear or, locally or remote controlled on board, by an air rifle which shoots a pick up line away from the tanker. The salvage tug takes the messenger on board by the pick up line and deploys the emergency towing p ennant and chafing cha in, if used, by heaving on the messenger line. The ship's em ergency towing pennant is then connected to the tug's towline . One of the available systems can, as an optio n,
also be rem ote controlled when th e crew has already left th e ship . A remotely controlled air rifle shoots a line to be picked up by a salvage tug. Other systems also exist for which no crew is need ed for deployment. So, th ere is a large variety o f emergency towing arrangements
that have be en develop ed, many of which are reviewed in 'A guide for the em ergency towing arrangements'
(see References).
Figure Z2!} One oftheemergency towing systems in three phases of deployment - S mit SajiFast - withSmit hro.dcet, chafing chain (of such a length that tlu end/ink reaches ahout 4 metres outside thsfairlead), falTkad andtowingpennon! (about 100metres Dynterrill/Deenaf/
TUG USE IN PORT 115
MarintSaftty lntttna.tional ROlterdam (sina Dmmbrr 2000: Maritime SimulationROlurdam)
Figure 8.1 Simulator lay-out withfivebridge manoeuvring simulators, a VTS simulator andinstruction rooms. 1M bridgemanoeuvring simulators can operat« int
116 THE NAUTICAL INSTITUTE
Chapter EIGHT
TRAINING AND TUG SIMULATION 8.1
Reasons for training
TRAINING IS PART OF THE LEARNING PROCE SS for a pilot or tug captai n. It is a continuous proc ess which do es not stop the moment pilots or tug captains ar e app ointed . Learning continues during their wh ole career. Training can include pr actical 'o n-job' training and a more theoretical phase. In several ports tug captain training is still only carried out 'on the job' and the same is true for most ports with re spect to pilot training on the subject of tugs an d tug use. Traini ng 'o n th e j ob', gath eri ng ex pe rie nce in pr actice, is essential to becom ing a skilled pilot or tug ca ptain. H owev er, if only using 'on-job trainin g', a sys tem of 'trial and error', there are risks involve d because of the 'errors'. It is time co nsuming and can
ther efore be ex pensive, also because of the po ssible 'errors'. Besides, tug captains or pil ots only p ass on to a trainee the exp eri en ce they have built up themselves. Thi s includes an y shortcomings and accumulated bad habits and may not, therefore, result in the most efficient and safest use of tugs. This may particularly be the case when 'on-j ob training' is carr ied out by just one person . Providing pilots an d tug ca p ta ins wi th b oth theoretical and practical background knowledge of the capabilities an d limitations of tugs, an d of wha t can be e xpected in prac tice when tugs render assistan ce, gives
a better understanding of tugs and their performance and results in more efficient and safer ways of building up pr actical experien ce during 'training on the job'. This also applies to simulator training, which should not be
type. Wh en, as is the case in some ports, tug captains shift between different tug types, the need for prope r b asic traini ng incre ases.
In additi on to basic training, focused on the local situation , there can be several othe r reasons for traini ng, such as: Specifi c situations, co nditions or bottle-necks in a port
requiring special attention . Port developments, for exampl e a new harbour basin or berth. Specific large or deep draught ships expected to call at a port. A new type of tug to be introduced into a port, It will b e clear that training is not limited to new pilots or tug captains. In particular th e four traini ng purpos es m entioned ab ove are for experie nced tug captains and pilots as well.
Depending on port requirem ents, tug captains and crews are often trained in fire fighting and pollutio n control. Som e knowledge of these subjects would be welcome even when not required by a port, since tugs have to handl e all kind s of ship , some with dangerou s cargoes. Em ergen cy tug assistance may be required and the more kn owledge about the risks involved the better. However, this cha p ter only deals with trai ning in sh iphandling with tugs and th e use of sim ulators , particularly matters to b e considered when using full mission simulators as a training tool for tug operations.
8.2
Different training objectives
see n as a substitute for 'training on the job', but as a
substantial imp rovem ent. The importance of proper training has grown since the appearance oftugs with different propulsion systems such as azimuth thrusters or Voith Schneider propulsion. Tugs are a costly investment and should therefore be used in the most efficient way. Not only that , but port developments don't always keep pace with increased ship size or draft and a minimum of tugs is often used due to economic pr essure. All this results in dimini shing safety and op erational margins and a more essential role for th e remaining tugs, This role can also be enhanced by th e increased power of tugs, resulting in the use of fewer tugs per ship. Pilots and tug captains should therefore po ssess the ability to use or handle a tug to its fullest capabilities, 'which can be achieved by proper training. Exp eri ence can be gained m ore quickly and the high est level achieved when a tug captain handles just one tug or tug
As m enti on ed, there can b e different reasons for
training in tugs and tug use apart from norm al trainin g on the job. These different objectives are considere d though it depends entirely on the local situation of a port which of the following cour ses is required, althou gh basic training is always very useful. 8.2.1 Bas ic the oretic al-p racti cal training Theoretical-practical trainin g cannot be carried out without the knowledge of experienced pilots and tug captains. They sho uld have th e ability to pass on their knowl edge a n d ex pe r ie nce in a cl e ar and understandable way. The reason the term theoreticalpractical training is used is becau se training sho uld not b e purely theoretical but sh oul d have a str ong relations hip to daily practice. Basic theoretical-practical training gives tug captains and pilots an insight int o the most relevant aspects of TUG USE IN PORT 117
ship ha nd ling with tug s. It takes into account th e capabilities and limitations of tug type s used, type of ships calling at the port, specific characteristics of the port and environmental conditions, with the objective of achieving efficient and safe tug use. A basic training is intend ed for both trainee pilots and tug cap tains, but can also b e usefu l for ex pe rienced pil ots and tug captains, when they have not had an earlier opportunity to attend such training. In a large nu mber of ports theoreti cal-pr actical
Wh en a port has only one type of tug the same, but type-related, kn owledg e is required by pilo ts. It is no t only necessary to have knowl edge of the differ ent tug types in use in the port in gen eral, but also of each tug in particular. This is ne cessary because with in a certain
typ e the design of variou s units may show mar ked diffe re nces no t o nly in ap pea ra nce but a lso in perform an ce and capab ilities. In additi on to the training subj ects mentioned above, a pilot should be trained to b e able to:
co urses in tug ass istance are given . Trainin g
arrangements and target groups, including the tools used, differ between ports. Withou t going into too much detail, the most important aspects of basic trai ning are considered next. For basic training in shiphandling with tugs the following main subjects are important: For pilot training: Ship handling. Knowledge of the capabilities and limitation s of tugs while ren dering assistance.
For tug captain training: H andli ng a free sailing tug. Knowledge of the capab ilities and limitations of ships and of tugs while rendering assistance. It is ass umed that pilots h av e already ga ine d experience in and know ledge of shiphandling and tug · captains of at least handling a free sailing tug. Other aspects have specifically to do with shiphandling with tugs and are discussed below in more detail. What kno wledge oftugs and tug use is required bya pilot?
The following knowledge is required to gain insight into the performance of tugs: Knowledge of what tug types are available in the port. Understandin g various tug types and their pro pulsion and steering syste m functions.
Th e bollard pull of tugs, ahead as well as astern. Know ledge of how different tug types operate when rend ering assistance, including the use of towlines and towing equ ipment. Knowledge of the capabilities and limitations of tug typ es when rendering assistance and how tugs can be used in the most advantageous way. This applies to situations when the ship is stopped in the water as well as when making headway or going astern. Un derstanding the interaction effectsbetween tug and ship an d insight into how interaction may affect tug performance and safety and how these influences can be limited. Apart from in teraction effects, kn owledge of the relationship between: a) Ship's engine and rudder manoeuvres and spee d. b) Tug performance and safety. 118 THE NAUTICAL INSTITUTE
Establish the required bo llard pull for ships, taking into account factor s such as ship particul ars, underkee l cleara nce, env ironmental conditio ns, particulars of the passage to the berth and berth location. Determine the m ost effective positi ons of th e available tugs and tug type s, taking account of whe n, where and how tug assistance is required during passage towards the berth, at the berth and whe n departing. The knowledge gained above contributes to effective and safe tug use . What is useful for a tug captain to know about ships?
For optimum shiphandling a pilot should have a good insight into what a tug can do, including its limitations. For the same reasons it is useful to provide tug captains with knowledge about the manoeuvring capabilities of ships they assist. Th e following are recommended: Basic knowledge of manoeuvring characteri stics of ships , especially m edium and low speed manoeuvring, including the influence of wind, cur ren t, shallow water and banks o n a sh ip 's behaviour. Basic kn owledge of the working of different ship propulsion and rudder type s and their effect on tug assistance.
Performance of bow an d stern thruste rs. Relationship between a tug's position and ship 's response to the forces exerted by a tug. Basic understanding of the interaction effects between tug and ship and insight into how int eracti on can affect tug performance and safety and how th ese influences can be limited .
Apart from the interaction effects, knowl edge of th e relationship between: a) Ship's engine and rudder manoeuvres and speed . b) Tug performance and safety. This knowledge gives a tug captain a basic gen eral insight into a ship 's mano euv ring behaviou r and capabilities. Taking into acco unt different shi ps an d th e situations and circumstances in a po rt, the knowledge gained may contribute to imp rove d anticipation of a ship's be haviour and a pilot's intentions.
A tug captain should also acquire knowledge of the following: The capabilities and limitations of tug types while rendering assistance and in particular of the tug he has und er command, which should also include the capabilities, limitation s and efficient use of the propulsion and steering con trol systems of the tug, also in case of a single lever control system, and how to respond to propulsion and steering control system failures. H ow to make use of the capabilit ies of his tug in the safest and most advantageous way when passing or rel e asing towline s, when com ing alo ngside or
departin g from a ship's side and whe n rend ering assistance, taking into account all the risks involved
related to tug or tug type. Proper towline han dling and appropriate towline lengths . The most effective po sitions for various tug types, takin g into account when, where and how tug assistance is required such as for compensating influ ences of wind or current, and with respect to
part iculars of the passage towards a berth and berth location. Safety regulations and measures, for instance the need to m aintain watertight closed condition of spaces below when a tug is rendering assistance .
in paragraph 4.Z Optimum information exchange between pilot and tug captain and between the tug captain and his crew reg arding tug placement, destination , intended manoeuvres, prop eller use, towline use, etc. How can basic training be given? Th e knowl ed ge of exp erienced pilots an d tug captains is a requirement for successful basic training,
which can be given as follows: By a clas sical co urse" m aking use of o verh ead transparencies, slides andlor vide os. By a classical course and the use of simulations . Simulators can be used to give participants insight into various aspe cts of ship handling with tugs. For some training objectives , deskt op sim ulatio n programs are appropriate, or in some cases remote co n trolled tug models, whe the r o r not in combination with ma nned ship models, otherwise full mission bridge simulators can be used. For junior pilots part of the tra ining should be undertaken on board tugs, while trainee tug captains should accompany pilots on board ships for a time .
Several of the training subjects for pilots and tug captains are similar. Combined training is therefore very effective, particularly when part of the training is given on a full mission simulator. However, the contents of
As with pilots, the knowledge gaine d contributes to safe an d efficient tug use. Some towing companies have goo d training manuals, which include several of the aspects mentioned above . It sho uld be noted that theoretical-practical training gives a basic insight, but the requir ed experience can only be acqui red 'on the job'.
Additional tra ining aspects \Vith th e exceptio n of th e b asic m an oeuvr ing characteristics of ships, all the imp ortant training aspects have been discussed in the foregoing cha pters of this bo ok.
Training for pilots and tug captains has been dealt with separately up till now, but as they sho uld work as a team , training should include more time together. A very im portant objective of training should be the creatio n of good unde rstanding and cooperation between pilots and tug captains. Not only between pilots and tug captains but also amongst tug captains, because they h ave to coordinate manoeuvres in such a way that the most effective tug forces are delivered to a ship. When, for example, two tugs are assisting a ship and one makes a mistake, the effect of the othe r tug may also be spo iled. To achieve goo d coo pe ra tion it is essential to include the following elements in all training co urses:
Effective com municatio n b etween pilot an d tug captain; attention to this aspect has already been paid
basic training may differ between ports because of the differences in level and background of pilots and tug captains. The background of pilots may also be such tha t they have already gained considerabl e experience in tug assistance , especially in po rts where pilots ar e recruited from local tug captains. Whether completely or pa rtly combined training should be given for pilots and tug captains, therefore, should be considered locally. Rega rdless of basic tr aini ng, regular meetings between pilots an d tug captains , common practice in a
lar ge numb er of ports, are very useful to disc uss pr oblems encountered daily and suggestions of ways of solving them. 8.2.2 Training for sp ecific situations and conditions This kin d of training is sometime s required for problematic areas in the port or po rt approaches or for difficultenvironmental conditions such as strong currents or fog. Restrictions in force for certain port areas, harbour basins or be rths with respect to tidal currents or wind are sometimes conside red too stringent, especially from an eco nomic point of view, and relaxed regulations are
issued. For pilots an d tug captains the situatio n then becomes more difficult du e to th e greater influence of wind and!or curre nt and training will familiarise them with the new and more severe conditions and smaller margins. In m ost cases such training followsa feasibility study, often carrie d out on a ship manoeuvring simulator TUG USE IN PORT 119
in close co-op eration with pilots.
It also tak es time to become fully fam iliar with tug capabilities and limitations when rendering assistanc e.
Training is aimed at a specific situatio n, and so
attention is focused on a specific location in the port, the particulars of that location, the environmental conditions, certain class and type of ships and the tug assistance required . Training is then given in the right ship and tug man oeuvring procedures invol ving th e required boll ard pull and tu g pl acement with the objective of being able to handle ships safely in the given situation. Bollard pull and tug placement ma y be varied during the cours e, trying to establish the optimum method of tug assistance. 8.2.3 Tr aining for a planned new port, harbour basin or berth In mo st cases the training for this kind of situation is based on the findings of a feasibili ty study of that particular port o r port area. It includ es the range of
Accordi ng to a spokesman of a port that bought azimuth tra ctor tugs, it took approximate ly one month under the supervision of a capable instructor to convert an exp erienced tug captain to b e fully co m petent in omnidirectional propulsion. A new type of tug perform s differently. When the previous tug h as, for instance , b een a conventional tug
and the new one is a tractor tug, the capabilities of the new tug are much greater and limitations fewer. This influen ces tug assistance as pr eviously provided. The method changes, particularly if the tug is used to its full advantage. As a consequence, a new type of tu g influen ces manoeuvring pro cedures on board Ships. A new type of tug therefore also influen ces a pilot's job.
th e type and size of ships and tug assistance. For a planned new por t the type of tugs may still be un known. This type of training does not differ much from the one
Training for a new tug type should ther efore not onl y be training in tug handling. Pilots should be involved togethe r with the tug captains. Training sho uld comprise the total procedure of shiph and ling based on the new type of tug an d its capab ilities and limitations, taking into account port characteristics, ships calling at the port
above, but is aim ed at a totally new situation.
and env ironmental con ditions.
Such training provides the po ssibility at an early stage to famili ari se pil ots an d tug captains with th e n ew situation. Again, the right ship and tug manoeuvring pro cedur es, the required tug ballard pull and optimum tug placement are subjects to be exercised .
Tr aining m ay also foll ow a sim ulato r study, to determine the effect of a new tug type on access ibility of the port. This type of training also app lies to escort tu g ope rations, dealt with in the nex t chapter.
environmental co nditions, the planne d wate r depths,
8.2.4 Training for specific ships coming to a port Trainin g for specific ships is mostly training for ships w-jth such size, windage or draft that they are marginal
regarding port dimensions, water depth s and /o r env ironmental conditions . Training may follow a previous study which dete rmined the maximum environmental conditions and required bollard pull. The aim of training is to familiar ise pilots an d tug captai ns with handling the specific ships in the port, arriving and / or depar ting proced ures und er maximum allowable conditions, whereby ship and tug manoeuvr es are practised with the required ballard pull and correct tug placement. '
8.2.5 Training for a n ew type of tug to be used in a port
8.2.6 How the specific training co urses can b e given Combin ed training Apart from training for a new tug typ e, the training situa tions mentio ned a lway s conc e rn optimum
shiphan dling with tugs and mo stly under more severe conditions and!or with small margins. In practice pilots and tug captains have to work as a team and both have to become familiar with specific situations, co nditions
and ships. It is best, therefore, that b oth pilots and tug captains who have to work in the ar ea concern ed or have to handle specific ships participate in such training cou rses . They will learn from each other through di scussions during the course, which contribute to training objectives.
A new type of tug has consequences for tug captains as well as pilots. Tug captains should be trained to handle the new type of tug, in particular when the prop ulsion system differs fro m the one with which the captains are familiar. Voith , th e manufacturer of th e cy clo idal
T he same ap p lies to tr ain ing in the sp ecific ship handling capabilities of a new tug type. A new tug type concerns both pilots and tug captains. Such training may include pilots becoming familiar with the new big itself, which can be achieve d by training 'on th e job' on b oard. Whether combine d training of pilots and tug captains can be arranged dep ends on the local situation,
propulsion system , employs an instructor. However, this
as mentioned earlier.
is not always the case with azimuth propulsion sup pliers. It tak es time before a tug captain gets used to a new propulsion system . After sufficient expe rience is gaine d whe n free sailing, the tug captain can start assisting ships.
The use ofship manoeuvring simulators Training for shiphand ling of a new tug can be carried
120 THE NAUTICAL INSTITUTE
out cl assically, making use of overhead transp arencies,
AzllTlUlhPor1: _U Alimuth S'tbd:-I7 Srw:I.O pOft RPM: 1.1I00 S'tbdRPM:IU W... tte lght: 0.0 fe et
_C't .,,, PilOt l-er. ~ Stbd lnoer.-2.1 STW:I.O
Port RPU: tOl S'tbdWM:tlO w.....Height: 0.. feet
Figure 8.2 Desktop annpuler program Tug.Mast", droeloped by T7u Glosten Associates, Seattle, USA. The program thatcalculates equilihrium solutionsfor a stem tug towing on a line, e.g. anescort tug, can he customind fir aparticular tugandhe used asa performanceprediction program andasa training tooL A number ofASD and VS-tugs can be simulated. T7u tugs can be controlled by keyboard andmouse. Speed, wave height andtowingpoint (VS tugs) can hevaried. Detailed inJvrmalion onfOrm, moments, freeboard, heel andtowline augle are displayed. CTUGSIMisa similar program forconventional tugs.
TUG USE IN PORT 121
slides an d videos, showing the performance of the new tug. A desktop computer training program , such as th e one shown in figur e 8.2, if customised for the specific tug, is a goo d tr aining tool. The same may apply to remote-controll ed tug m od els if the correct tug model is availab le. It all dep ends on what kiud of training is needed and the availab le po ssibilities for training. In most cases , however, a ship manoeuvring simu lator is most suitable, providing the simu lator is appropriate for the new type of tug and the metho d of tug assistance. Co mbined trainin g of pilots and tug cap tains in a ship m anoeuvring simulato r teaches them how to use the tug in the m ost advantageous way for shiphandling in the specific are a of th e p ort, taking into acco un t all relevant aspects, such as for instance ships, wind, current and waves Alth ough rather expensive, a ship m anoeuvr ing simulator is a ve ry effective and flexibl e training tool for such a comb ine d tr aining of pilots and tug captains an d therefore the most suitable also for th e othe r training objectives , viz. training for specific situations and conditions in the p ort, tr aining for a planned ne w p ort, h arbour b asin or be rth, an d tr aining for specific sh ips coming to the p ort. It is used for those purposes in a growing num be r of ports.
8.3
Calculating and simulating tug performance with desktop computers
8.3.1 Thg performance calculation programs The real p erforman ce of tugs and different tug typ es is no t always well kn own, which is rather p eculiar. Tugs are built to r ender ass istance and, alt ho ug h ve ry important, the only thing generally kn own is the b ollard pull of th e tug - the for ces that can b e delivered wh en pulling in one of two directions, ahe ad or astern, at full power in a stationary situation. Tugs have to render
figu re 8.2), USA ; Damen Sh ipyards and M arin e Simulation Rotterdam , T he Netherlan ds; Au stralian Maritim e Co llege an d M aritim e Sim ulation Centre the Netherla n ds. T hese simulation p ro grams produce a graphic representation of a tug's pe rfor ma nce at different speeds and towing or push ing angles. Indirect towi ng methods can be included an d some programs account for waves as we ll. These tug p erformance calculation programs are generally based on a force -equ ilibrium-simulation , a static state, taking into acco unt such characteri stics as tug hull, skeg, ru dd ers, propulsion devices, towing point! pu shing point location s, stab ility an d tug maximum list, maximum engine lo ad and assisting m eth od s. Not accoun ted for are th e difficult to d etermin e interaction effec ts such as tug hull/ ship hull interaction, tug propeller / ship hull interaction an d the influence of water dep th and confinement on these factors. In teraction b etween tug prope llers an d b etw een tug hull and tug propeller(s) may also n ot b e fu lly accounted for. Neve rthe less, th e pro gram s give a good basic insight into p erform anc e of one or more tug types in different ope rating m od es. In the des ign stage of a tug these pro gram s allow a review to be made of a wide range of optio ns such as tug hull an d skeg p aram eters and towing point p ositions and allow rap id elimination of u nsuitabl e co nfigurations to b e carried out. In addition to p erform an ce calculations of various tu g typ es, some of these p rogr ams allow the most effective tu g positio ns and tug con figurations to be tested . Tug p erformance can be rep resen ted in so-called p olar di agr ams, showing the m aximum t owin g or pu shing forces at different speeds and towing angles and/or th e m ost relevant pushing angles . In p aragr aph 4.3, a numb er of these p erforman ce di agr am s were shown whe n discussin g tug cap abilities an d limi tations.
assistance, as far as possibl e, in all towing directio ns and
n ot ju st wh en a ship is stopped but also at differ ent spee ds. In sight into wh at a tug's p erforman ce really is at different spe eds and towin g angl es is therefore require d. In d ail y p ra cti ce a pilot an d tu g captain w ill ex pe rience a tug' s p erformance by the response of the ship to th e tug's efforts. But that does not say wh at for ces th e tug actually delivers when op eratin g at th e ship's side or towing on a lin e. T he m or e tug typ es th at com e onto th e market th e m or e sho uld be known ab out th e differences in p erform an ce. It is imp ortant for pilots an d tug captains to kn ow what tugs an d different tug types can do, but also for a tug fleet owner, especially whe n ordering a new tug . A ch oice has then to be m ad e b etween differen t tug types. Tug p erforman ce calculation progr am s h ave b een develope d by a number of companies an d simulation institutes. To name a few, The Glosten Associates (see
122 THE NAUTICAL INSTITUTE
T h ese p ro grams h av e b ecome m ore importan t b ecau se of th e development of purpose built types such as escort tugs. In p arti cul ar, with high esco rting speeds, dynami c for ces can reach high valu es and are the refore very im portan t. Some progr ams also take acco unt of the dynamic be haviour of tu gs and the influe nce on towline forces an d h eeling m oments, wh ile towlin e characteristics are also included in th e program . 8.3.2 Fast-time m anoeuvring simulation programs A number of fast-ti me m an oeuvring sim ulation programs exist in wh ich tug assistance m ay play a ro le in one way or ano ther. Such programs can, for instance, be used to investigate wh eth er a certain ship , following a p la nned route , can ente r a p ort unde r give n
environmental conditions. A tug controller mode can be selected, wh ich merely calculates the tug forces required. As th e tugs them selves are not simulated but only the forces available, limitations of tugs or differences in tug types are not taken into account at that stage . These programs are mainly used for initial port design or, for instance, to approximate the limits of environmental conditions in order to reduce the number of runs to be executed when further research is carried out on a full mission bridge manoeuvring simulator. Simulator tim e and cost s on a bridge manoeuvring simulator can thus be reduced. More soph isticated fa st-tim e m ano euvring simulation programs generate tug forces ba sed on available data of tug type performance , including differences in assisting methods, tug types, speed and environmental conditions. These programs are mainly used for the evaluation and design of tank er escort configurations. Together with th e ship simu latio n program and tug control program, which is responsible for the assistance strategy, the total ship -escort tugs system is simulated after an engine failure, rudder jam or collision course. The results are evaluated with respect to track and/or course control capabilities of the shiptugs system after these ev ents occurred . Advanced versions of these programs also take into account the times needed for tugs to arriv e and/or become effective. 8.3.3 Real-time simulation on d esktop simulators Some real -time simulation programs on desktop simulators, also called part task simulators, provide an opportunity to control a ship by engine and rudder, while several tug typ es can be chosen to assist. Tugs can assist in different modes, e .g. at a ship's side or towing on a line . These programs can be used for different research purposes such as port lay-out, required tug assistance or ballard pull, maximum wind and current conditions. Pilots can make use of these simulation programs for certain basic training objectives or to get an insight into how to deal with a new or problematic port area or the handling of a new typ e of ship . They can try out alternative strategie s or tug configurations, extreme wind and/or current conditions. Simulations can often be replayed in real time and fast time . The programs give a good idea of the different possibilities in a given situation and are much cheaper than using a ship bridge manoeuvring simulator. For most training objectives, manoeuvring on realtime desktop simulators differs greatly from the real world. Manoeuvring is done on the information from a display, which provides a so-called bird's eye view. Reality on board is different, particularly when manoeuvring in confined waters . In such situations a pilot reacts to information mainly obtained from an outside view. This provides a pilot with actual and instant
information regarding ship 's pos ition, speed, distance off, heading and influence of current and wind. In addition, a pilot on board a ship not only has a totally different but a much m ore limited view than when manoeuvring using a display. His perception is different, and consequently he may react di fferently for ship mano euvres and tug assistance required. Furthermore, cooperation with tug captains, an important factor when man oeuvring in confined waters , is hardly possible. Bridge manoeuvring simulators, which are dealt wi th in paragraph 8.5 and following, have an outside view and reflec t reality on board ships in a better way, while cooperation with tug captains is possible. In this chapter most attention is therefore paid to this research and trairting to ol, and in particular to the simulation of tugs.
8.4
Simulation by remote-controlled tug models
Particularly in the USA th ere is an increasing use of simulation by remote-contr olled models for training and for performance stud ies of different tug typ es or various tug d esigns . T his has become feasible thro ugh the construction of very realistic operational m odels by the model builder Ron Burchett in Canada. Existing ASDtugs and VS-tugs, for instance, are built in a scale of I:24 with their specific propulsion systems, with realistic controls, correct stability, working winches and fenders . Models of conventional tugs, ship s and barges have been built as well. The largest problem with scale models is the accelerated time factor. Ship or tug models behave exactly like real ships, only much faster, viz. if the model scale is 1:25, five times faster (square root of the scale). A tug model approaching a ship at a certain speed will do so at five times that speed in reality. The same applies to wind and current speeds. A wind speed of e.g. 10 knots working on the mo del is in reality a wind speed of 50 knots. The control systems of th e model tugs mentioned have an adjustable built-in time delay for propeller and steering control. This does not alter the fact, however, that all speeds observed are five times as high as in reality for a model scale of 1:25. Also the feeling differs from reality, the tug captain is not on board his tug, but is operating a model at some distance, which might affect realism of tug handling. When taking into account these effects, tug models can be a tool for tug performance studies and for trairting, such as with regard to tug manoeuvring and ship assist capabilities and tug limitations. With respect to this, using radio controlled tug models in combination with manned ship models (Port Ash, Australia) is an even better training tool. A disadvantage of trairting with models, in addition to what have been mentioned, is th e limited number and typ e of models and the inflexibility in conditions and circumstances
TUG USE IN PORT 123
8.5
Thg simulation using bridge manoeuvring simulators
Bridge manoeuvring simulators, also calJed fulJ mission bridge simulators, are equipped as a ship's bridge with all the usual instruments - control handles , wheel , radar, communication facilities, chart table, and so on. The outside world is projected on a screen, normally based on computer generated image (CGI) technique s. The angle of outside view can be up to 360° and on several simulators it is possible to switch between a view ahead from the centre of the wheelhouse to a view from the starboard or port wing. Simulators with a smaller angle of outside view, say 225°, can usually also switch towards a stem view. Ship models are often represented in three degrees of freedom: surge, sway and yaw. Roll can also be simulated whether by visual presentation or by a hydraulic system. Some simulators equipped with a hydraulic system can , in addition to roll, also simulate pitch and heave, thus representing six degrees of freedom. Simulator institutes may have up to three or four full mission simulators, which can interact, as explained in section 8.5.2. Not all these simulators are usuall y equipped in the same way. The main bridge simulator may have a 360° view and a hydraulic system, while
other simulators may not have a hydranlic system an, just a 225° out-of-window view or even less. Research projects on full mission bridge simulato r: are mainly conducted for areas of a port with limited space and where frequent manoeuvring takes place. This is the area of the local experts - pilots and tug captains . Simulator institutes have accumulated a significant amount of nautical knowledge but cannot have in-house all the nautical experience of the local pilots and tug captains . The experience of th ese ex p er ts is indispensable for accurate simulation. In these areas margins ~e ofte?, so s~aIl that one canoot afford any maccuracies. This practical exp erience is necessary in order to assess whether the simulation is correct, the simulated tugs operate as they do in reality, the sunulated manoeuvres can be carr ied out in reality and so on . Pilots and tug captains must assess the simulation from their point of view to get a realistic simulation and to obtain results which are achievable in practice. Th e same applies to tr.aining courses , whether basic training is given or followmg on from a former research project A number of training o bj ecti ves have been mentioned. They mainly concern the accessibility of an existing or newly developed port or port area. The accessibility or entrance criteria for a port or port area are determined by the type and size of vessels in relation to the port dimensions, environm ental conditions, the number and type of tugs available and on pilots' and
PIv>,., MarimSafttJ _
Figure 8.3 Bridge layout ofafiJi mission bridgesimulator: Ttu field of Ww is 36(f
124 THE NAUTICAL INSTITUTE
.... IIDtJmmn
tug captains ' experience . As larger vessels try to make use of an existing p ort infrastructure, the accessibility
of a port can only be guaranteed by improvem ent in manoeuvring procedures, increased experience of the
pilot and tug captain s and improvements in th e type , ba llard pull and/or number of tugs used to assist a vessel. The opposite is also possible . By increased exp erience and improvem ent o f m ano euvring proc edures or of tug assistance, it is po ssible that larger vessels can enter a port or a certain port area und er give n envi ronmental co nditions.
Th e effects of improvement can be established by operational research carried out on a full mi ssion bridge simulator. In addition, im proveme nts associated with
expe rience can be achieved by training pilots and tug captains on full mission bridge simulators. Full mission simu lators may also be used for research and training in es cor ting as will be mentioned when discussing interactive tug sim ulation.
The whole simulation process on full mission bridge simulators is not discussed in detail , but attention is given to some essential aspects of tug simulation in general and of interactive tug sim ulation in particular in order
to achieve tug simulation which reflects the pr actical real world situation as .much as possible. 8.5.1 Requirements for corre ct tug simulations Regardless of how tug and tug assistance are actually simulated, to en sure adequate research and training,
correct simulation of the following factors is essential: The [orce (magnitude and direction) that the tugs can exert on an assisted vessel und er different conditions,
situations an d spee ds. The space assis ting tugs nee d to oper ate u nder different con ditions, situations and speeds . For tugs towing on a line the required space depends on the tug dimensions and towline length used . This space is in addition to the space required by the vessel. The response time of the tugs. T hese factors must b e accurately simulated and should b e considered and!or validate d carefully before a resear ch or training project starts, depending on the specific tugs and tug assistance simulated. It is, to a large extent, these facto rs that determine, for a given ve ssel and environmental conditions, the minimum required
manoeuvring space for ship and tugs. In other words the minimum required horizontal dimensions for a port, harbour basin or fairway. Tug captains and pilots with significant experience in a port know what they might expect dur ing assistance to a vessel. They pos ition a tug where it is needed before the nee d occurs. Both tug captains and pilots ant icipate expected situations. This anticipation, based on experience, is also a factor of major importance and
Ftgure 8.4 Simulation trade plot oJ a waded tanker rn1tring a port from thesea. 'lUg positions with towingandpushingdirections are shown. Tlu study war tarried out by MarineSaftty Intmtational Rnttmlam
must be taken into account in evaluating operations with
tugs. How tug simulation can best be achieved depends on how tug assistance is simulated. Developments in and various methods of tug simulation are reviewed,
including their limitations. 8.5.2 Development in tug simulation towards interactive tugs Tug simulation was introdu ced into shiphandling sim ulat ors many years ago. The pro ced ures have changed from sim ple vector tug models to mo r e sophisticate d models ove r th e years as the use of tugs has become an essential part of ship h an dli n g simulations. S imple vector tug models In a vector tug model, the tug is simulated by a force vector, indicating magn itude and direct ion of applied tug force. With the m ost simp le one, the influe nce of ship's speed is disregarded . It is clear that this system has many shortcomings: no simulation of correct speed and towing direction depe nde nt tug fo rces, no simulation of the space required by the tug, incorrect
TUG USE IN PORT 125
reaction time, and no considerations of the limitations
of the l)1gs, etc. All th ese factors will, in one way or an oth er, affect the simulation results.
tug space may not be fully take n int o account. Tug captains often participate to impr ove the simulation, based on their experience .
Simple vector tug models combined with tug captain experience Th e same simple vecto r tug model is used . However, to comp ensate for the shortcom ings, tug captains are asked to assist durin g the simulation . Th ey should have experience in the specific part of the simulated port and practical experience in handling the type of tugs being simulated. The tug captains intro duce practical aspects into the simulation. Based on their expe rience they advise th e simulator ope rator on which tug forces can b e applied realistically, on tug limitations, pr op er towline length with regard to mano eu vring space , reaction time s and other practi cal aspects. They can also anticipate the situation expected. This represen ts a significant improvement in the use of simple vector tug model s.
Several simulator institutes hav e develop ed a m ore sophisticate d form of vector tugs, whi ch m anoeuvre auto matically into stan d by, connec t or assist mode, activated by simple com mands on the computer screen of the ope rator.
Advanced vector tug models Applied tug forces ar e ship spee d and towing angle dep endent: a step forward. Different tug typ es can be selected. For towing and pushing forces to be appli ed , use can be mad e of data obtained by tug performance
tug captain's reaction may come too late.
calculation programs, which may even include a tug's
limitations cause d by waves. Other force calculating programs are used as well. There are still shortcomings in response times and tug limitati ons and the requ ired
Tugs simulated on a monitor (bird's eye view) and operated by tug captains The tug captains have their own control handles for cou rse and spee d control of the simulate d tug, so that the y directly manoeuvre the tug. They might even have a tug wh eelhouse with the required instruments and control handles. The tugs can make fast the tow line, pull and let go the lin e if ne cessary. Thi s system comes closer to reality. The problem is tha t th e tugs on a monitor are so small tha t slight changes in tug position and in heading can hardl y be observed in time and the
Interactive tug simulation Modern computer techniques make inter a cti on between full mission simulators possible (see figure 8.6). Som e or all tugs and the assisted ship can b e run on . separate bridge simulators, all interacting. The limitation on the number of interactive simulators is more a
question of the cost of the facilities than of technology.
Pho(Q: Hamhurg MaritimeReutJrcJt.
Figure 8.5 Simulated ship andassisting tugpassinga bridge. TJu tugis eonnolled asa vector tug by an operator underthe supervision of a tug captain 126 THE NAUTICAL INSTITUTE
8.5.3
Important aspects for interactive tug simulation
Visual presentation and orientation ofcontrol handles visual plesenlelic:n
vlsualpr&5&lllation
For the simulation of interactive tugs on e
has to consider the following with respect to visual pr esentation, especially for tugs towing on a line :
Figure 8.6 Sdumatic ditlgram ofan Interactive Tug Operatiom Simulator Tug cap tains have their own tug, with wheelhouse, bridge instrum ents, propulsion and/or rudder controls, co mmunicatio ns and an out-of-window view. Different tug typ es ca n be simulate d, all with th eir specific characteristics. With a well tuned simulation system the exerted tug forces, tug force directions and required m an o eu vring space co me clo se to reality. Th e sho rtcomings of vector tugs can thus be ove rcome. Reaction times are as in reality, because the tug captain is running his own tug. Besides, tug captains bring their own experience with the m. They are able to see each othe r, wh en not obstructed by the vessel, on the out-ofwindow view for further enhancement of the operations. A furthe r advan tage of interactive tug simulation is th at tug captain s now actively participa te in study and training proj ects, giving greater satisfaction, which will contribute to the study and training results. Wh en simulating a ship assisted by a number of tugs, one or mo re tugs may be simulated on an interactive tug simulator , while othe r tugs may still be simulated by vector tugs. This can be for several reasons such as the costs of the required number of bridge simulators b eing considered too high or manning pro blems of the interactive tugs. Simulation of in te rac tive tugs is mu ch mor e complicated and handling th e tugs should be as close as possible to real world operations . This requires attention to a number of important aspects related to th e practica l handling of simulator tugs, which is discussed below.
A pilot refers the assisted ship's position relative to the surrounding area, such as banks, buoys, mo ored vessels and othe r conspicuous points. A tug captain refers tug's position and speed, heading, distances off to the sur rounding area, and the assisted ship's position . For the pilot the view of the surrounding area is important. For a tug captain the surrounding area is important, but of equal imp ortance is the view of his towline . The view of his towline gives the tug captain the main information about the tug's position in re la tion to the assisted shi p and its performance . The out-of-wind ow view, including the view of the tug's fore or after deck, shou ld always be in accordance with the way propulsion and rudder or thruster controls are operated. There should be no misunderstanding which . directio n the tug will move when control handl es are used The foregoing has consequenc es for the horizontal angle of the out-of-window view.The required minimum angle of view depends on the type of tug and the method of tug assistance , as will be shown by some examples in figure 8.7. A 225° field of out-of-window view will b e assumed , which is common for many bridge simulators. The captain of tug no. I in figure 8.7 must look forwar d in the directio n of movement as well as aft at his towing line an d at the assisted ship . A 225 ° field of view is no t sufficient for him ; he needs almo st a 360 ' field of view. He also needs it for making fast, as can be seen when following the tug along positions a, b and c. The tug captain will lose the ship from sight when between position b and c. He cannot position the tug to secu re the towline. The captain of tug no. 2, a conventional or an ASD / reverse-trac tor tug, can ope rate with a view of 225°_ A tractor tug, no. 3, normally operates with the stem towards the ship . A field of view of 225° in the directi on of the tug's stem will be sufficient. An out-of-window view of 225° is also eno ugh for tug nos . 4 and 5. H owever, whe n the ship also has to move astern, then the direction of ship movement and of the towline are opposite. The tugs will then need almo st a 360° out-ofwindow view. In narrow spaces and during berthing TUG USE IN PORT 127
Because a tu g capt ain frequen tly changes hi s direction of view and sometimes for a rather long time,
particularly in th e case of tugs towing on a line , the contro l handles are often positioned in a wrong direction which is very confusing. Cont rol ha nd les not op erating according to the tug as projected on the scre en easily caus e mistake s wh ich spoil the manoeuvre . This is another reason why a 36 0 0 out-o f-w indow view is often required for correct simulatio n of tugs towing on a line .
Furth erm ore, when cha nging the tug simulator from one tug typ e to ano ther, for instance from a stern dr ive tug to a tractor tug, control handles should be cha nge d accord ingly because a stern drive tug mo stly assists ov er the bow and a tractor tug over the stern.
It can be concluded that an alm ost 360 0 out-ofw indow view is essential for the tug captain in many
situations. The lack of a sufficient out-of-window view is sometimes co mpensated for by using an ex tra monitor
with a bird's eye view, giving add itional inform ation to the tug captain abo ut the tug's position with regard to the surrounding area or the assisted ship's position . The
monitor has some disadvantages as has already been mentioned.
Simulato r institutes having a full mission bridge simulato r with a 360 out-of-window view, could 0
cons ider using a 225 0 field of view for ce rtain training
and research projects for the simulation of the assisted ship and a 360 0 field of view for one simulate d tug.
Figure 8.7 Field ofview requiredfir interactive tugs The captain oftug 7 needs an almost 36(J' out-ofwindow view. An out-of window view of225' for the captains of the pushing tugs 2 and 3 issufficient. Thesame is the casefor tugs 4 and 5 when theship moves in the indicateddirection. Men the ship would move backwards these tugsalso need an almost 36(J' field ofview
From the forego ing it can b e concluded th at when using interactive tugs the following is generally required regarding the angle of out-of-window view :
SCREEN
ope rations a 360 field of view is often nee ded for all assisting tugs. 0
Th ere is ano ther aspect which shou ld be taken into accou nt with a 225 0 out-of-window view, and that is the handling of the prop eller, rudder or thruster con trols. As mentione d at the beginning of this paragraph , on several simulators it is possible to switch between a view
ahead to a view to starboard, to port or to astern.
Ij:
In figur e 8.8 the simulated tug's wheelhouse is shown
c.n,~,.tbn
with th e proj e ction sc reen. When, for in stan ce , a
WHEELHOUSE
conve ntional tug is simulated an d the tug's bow is seen on the screen th e tug moves forward whe n forward propeller thrust is applied. Wh en the tug captain wants to have a stern view, the afterdeck is projected on the scree n, the propeller contro l handl es maintain their positions an d whe n they are moved into the direction of the projected stern, ahead thru st is still app lied and the tug moves forward instead of wh at should occur, viz. astern.
128 THE NAUTICAL INSTITUTE
Figure 8.8 Relationship between direction ofview andcontrol handlesfir a.n interactive tug with a 22SO out·ofwindow view. When the tug's b07:V is projected on the screen control hamdles are correctly positioned. When the stem isprojected control handles are 18rY wrong. 17urefore, control hamdles should he in accordance with the tug's projection on the screen
An angle of view of approximately 225 0 is often sufficient for tugs operating at the ship's side and for tractor and ASD /reverse-tractor tugs when towing on a line and the directions of towline and ship's mov em ent are roughly the same. In narrow spaces
and for berthing man oeuvres, however, a 3600 field of view is mo stly needed. An out-of-windo w angle of view of almost 360 0 is required for conventional tugs towing on a line and for other tug types when towing on a line and the
into account, simulation of tug performance in wave co nditions becom es very realistic. This is an imp ortan t factor for research and training in these conditions, for
exa mple for escort tugs. Such simulation projects may includ e studies regarding th e requ ired tug type for escorting, necessary bo llard pull, achievable towline forces and the related training for tug captains and pilots.
directions of tow line and ship's m ovem ent are opposite.
Otherpractical aspects Apart from the requir ements for correct modelling of the tug with regard to hydrod ynamic aspects, stability,
Ifusing a 225° angle of view for the tugs, an additional
free bo ard, manoeuvring char ac teri stics, location of
moni tor can be used to compensate to a certain extent
towing point or pushing point, propeller thrust and
for the lack of sufficient out-ofwindow view.
H an dling of controls shou ld always be in accordance with the way the tug is projected on the screen.
The captain of an interactive tug
shou ld be able to make a go od assessm ent of the tug' s speed and position, the sh ip's speed an d position, the towline direction and the tow line force, ·In additio n to the requirem ents for the angle of out-ofwindow view, the following aspects are impo rtant for visual presentation
of any interactive tug simulation : The assisted sh ip's side, bow and
stern should no t be of uniform colour , but accentuated (textured ) to give a sense of relative motion
to the tug captain. Tug movem ents are much faster
Photo: Author
an d mo re frequent than Figure 8,9 Httling angle is an important factor in tuglimitations. movements of the assisted ship . TWJ'n screw tug 'Smit Siberil ' The update frequency of the out(I.o.a. 28·6m, beam 8·7m, ballardpuU 35 tons) of-window view should b e eng ine lo ad, and so on, in addition to th e earli er sufficiently high to give a smooth picture . mentioned requirements, there are a number of other The towlines sho uld be made visible in the out-ofimportant practical aspects which should also be taken window view and towline forces should be clearly into account, nam ely: displayed in the tug's wheelhouse since on mo st simulators no visual information can be obtained by Heeling angle the tug captain as to whe ther the towline is slack or A tug captai n reacts to visual information and also to under tension. the tug's ang le of hee l. The heeling angle is an important factor regarding tug limitations. Heeling Tug p erformance in wave conditions angle du e to towline forces as well as steering forces Although with three degrees of freed om , tug should therefore be simulated as well as possible. movements due to waves are not represented , the effects Engine noise may be taken into account with respect to the forces on A tug captain also reacts to engine noise. For instance, tug and ship. Limitations of the tug due to movements a tug captain reduces power to avoid overloading and/or the high dynamic towline loads caused by waves m ay not b e t aken into accoun t on all interactive the engine. This can occur when a tug has to brake a sim ula to rs . Wh en a b ridge simulator is used for shi p's speed. Engine noise should therefore b e interactive tug simulation which also represents roll, or simulated for interactive tugs. a bridge simulator with a hydraulic system representing Control handles heave, roll an d pitch, an d the se limitations are taken The handling of propeller, rudder or thruster controls TUG USE IN PORT 129
of the simulated tug should be similar to the real tug. Towlinelfi nder characteristics Towline and! or fender characteristics should be well simulate d. The chara cteristics and capabilities of the
Th erefore, when using full mission bri dge simulators for research or training it depends on a number of factors which kind of simulation is most suitable. In many cases it may result in the use of one or more vector tugs.
tug's towing equipment, such as a towing win ch,
should also b e taken into account. Wind indicatOT For simulations where wind plays an important role, the tug captain should have a continuous good insight int o the relative direction of the wind working on the assisted ship, eithe r by an app rop ria te wind indicato r or by means of a mo nitor showing clearly the relative wind direction for the ship. When all the above aspects are taken into account, the interactive tug is the most suitable tool for research and training projects . However, full mission bridge sim ulators are not yet abl e fully to reflect the real world. Further study and improvements are necessary on such subjects as mentioned in paragraph 8.7. Practical input will still b e necessary during the coming years. 8.5.4 Method oftug simulation to be used Although tug simulation has some limitations and furthe r improvements are .required, in se ve ral training
and research projects it has been proven to be a very suitable tool. Regarding the kind of tug simulation to be used the objectives of the training or research project have to be considered. Account has to be taken of the capabilities and particularly the limitations of different tug simulation pos sibilities. For certain situations this could result in a decision to use vector tugs instead of interactive tugs, while for other research and training projects only interactive tugs will meet the requirements. A full mission bridge simulator is a rather expensive tool for re search and for training . Wh en, in addition to the sim ula ted ship, bridge simulators ar e used for interacti v e tugs, two, three or even more simulators may
run at the same time, raising the costs con siderably. So costs may be a limitin g factor in the use of interactive tugs. The number of simulators might be such that not all assisting tugs can be simulated by interactive tugs. In addition to the interactive tugs, vector tugs will then be used. Furthermore, each interactive tug needs at least one but usually two captains, and they may not all be available. Some tugs then have to be simulated by vector tugs. There is another aspect which.applies to a number of ports . Tug captains are not always in a position to enable th em to participate in research or training proj ects. In these cases the only solution mght then be to use vector tugs instead of interactive tugs. Tug captain s from oth er ports could be used for certain projects, but they don't have the local experience required for corre ct simula tion. 130 THE NAUTICAL INSTITUTE
Wh en the angle of out-of-window view is a lim itation in using an interactive tug and the sim ulator institute
has a br id ge simulator with a 360 0 an gle of view, consideration could be given to using this simulator for a tug while the assisted ship could be simulated on a bri dge simulator with a 225 angle of view. 0
Wb en the costs of bridge m an oeuvring simulators ar e found t o be to o high , fo r ce r tai n proj e cts mano eu vring simulatio n pro gr am s on d esktop computers can be used.
8.6
Simulator training
. 8.6.1 Enhanced training possibilities In this chapter, vario us training objectives have been discussed, including the require ments for pro per tug simulation. A full mission simulato r with vector tugs and the input of expe rienced tug captains is a suitable tool for training in shiphandling with tugs. Togeth er with the pilots, tug captains can learn, for instan ce, strategies and procedures for entering a port and for be rth ing an d unb erthing, the influen ce of wind an d curren t, the tug effort required and ship's response to it As m en tion ed, anoth er aspect of participa ting in training with pilots, and also in research projects, is the po sitive effect on
pilot/tug captain co-operation. In discussing different mano euvr es, the y learn from each other. It is clear that tug captain training has been improved by the use of interactive tugs. They op en up much better training p ossibiliti es an d tug ca pta ins ca n, in co-
op eration with pilots, actively parti cipate by operating their own tug. Alth ough there are some limitations, interacti ve tug sim ula tio n op en s up th e follo wing possibilities for active trai ning of tug cap tains in: Improved strategies and proc edures for entering or leaving a port, man oeuvring in port ar eas, berthing/ unberth ing with vessels calling at the port or specific vessels expected to call at the port. A new port or new port area. A new type of tug, such as an ASD or reverse-tractor tug. They can be train ed in handling of th e thruster controls and in learning new capabilities of the tug with regard to ship handling. Escorting. New tug captains can be trained in: Subjects suc h as communicati ons, op e rational
pro cedures, co-ope rating with pilots and other tug captains, basic manoeuvres and avo iding
dangerous situations.
8.6.2 Steps to be taken for a simulator training set up To start simulator training for p ilots and/or tug captains with interactive tugs th e followi ng is an indication of the steps to be taken as far as is relevant in clo se co -operation with a simulator institute :
former the various scenario s to be simulated, require d tug assistance and related communication procedures should be carefully specified. Of great importance in training is the capability of the instructor. An ins tructor sho uld have extensive practical exp erience regarding the training subjects, a
sound theoretical backgrou nd kno wledge and be able An accurate defin ition of training needs and training
objec tives.
to pass on his exp erience in a professional way to trainees .
A definition of training requirements.
Subj ects to be considered include: the situations an d
The steps indi cated above are, as far as is relevant,
circumstances for which traini ng is require d, number
also applicable to simulator training where tug assistance
an d experie nce of tu g cap tai ns, numb er and .expe rience of pilots, co-ope ration requir ed between pilots and tug captains, type and numbe r of tugs, type
is simulated in ano ther way than with interactive tugs. Also, for research projects, whe n tug assistance is an
of ship, environmental co nditions, communication s
and ope rational procedures. An assessment of whethe r the simulato r institute can m eet the training requirements with regard to: simulator facilities such as the type and number of in teractive sim ulators , suitabi li ty of th e in teractive tug simulator for the type of tug and method of tug assistance;
ship and tug models; simulation of particular s of the given area, such as the outside view, aids to naviagti on, wind ,
current, waves, water depth s, shipping traffic and moored ships; co mmunica tion faciliti es in cluding , if relevant, VTS co mmunication; instructors
A validation phase . After reachi ng agreement with a simulator institute and pr eparing the simulator the following should be tested, amongst other items, during the validation phase: tug models: free sailing, and whilst interacting with th e ass is te d ship wh ile t owing/pu sh ing manoeu vres are carried out at different angl e s and
speeds; it should be tested wh ether manoeuvres can be carried out in a realistic and an ·acceptable manner; appli ed towing and pushing forces; ship models: mano euvring perform ance and the effect of applied tug forces on the ship ; environmental conditions, including water depths, and the ir influences on ship an d tug; wh eelhouse layout, including tug engine, rudder and/or thruster controls, display of towline forces, bri dge instrum ents; simulated wind and engine sound; out-of-window view, including the view of the assisted ship and tugs as proj ected on the screen and the view of the towline; communication facilities and procedures.
A definition of train ing programs. Training program s sh o ul d pref er abl y includ e simulation parts as well as theoretical parts. For the
essential part of the study, similar steps should be taken.
8.7
Areas of tug simulation that need further attention
Simul ated tugs should perform realistically with regard to type capab ilities, achievable towing forces, response times and limitations. Tug simulat ion should be such that no tug maooeuvres can be execute d that are not possibl e or are too dangerous in real world
operations. Th e shortcomi ngs of vector tugs can partly be compe nsate d for by the participation of tug captains. For inte rac tive tugs, which repr esent m uch m o re
sophisticated simulation, it has bee n indicated what is essential from a practical point of view regarding realistic simulation. Howev er, for further improvement of tug simulation in general, and inte ractive tugs in particular,
the following areas need atte ntion. Some have been mentioned previously when discussing interactions and tug safety. It depends on the simu lated situation to what extent the following aspects are important The rea de r is also referred to the b ook 'Ship Bridge Simulators. A pr oject handb ook ' , wh ich ad dresses ship and tu g simulations in detail (see References).
Tug model tests Model testing is the only feasible method available to obtain correc t hydrodyn ami c data for a tug hull moving through the water. It can also give a goo d insight into tug perform ance. These data are important for simulator models, particularly for escort tugs, which ope rate at high spee ds. Some of the following can be included in the model tests. Effect ofangle of heel and trim onforces on a tug's hull and apperubIges Usually the hull force data are obtained with the tug (model) constraine d in the horizontal plane, i.e. no effect of heel angle and trim is pr esent. In some dedicated tug simulator m odels, however, th e effect of heel is taken into account throu gh extensive model testing, or in an
appro ximate manner. The effect of heel angle will be prominent in more ex treme situations and co nditio ns.
TUG USE IN PORT 131
Photo: Marin, 17uNethrrlo.nds
Photo: Marin, 17uNethrrlo.nds
Figures 8.70 and 8. 11 Model and model tank testJor escort tugs to obtain hydrodynamic data, optimise tug design and evaluate performance. Study by Wifsmulkr Engineering / Marin Influence ofwaves on tugperformarue In several ports harb our tugs have to operate outside breakwaters in the open sea, as do escort tugs. Waves limit tug performance d ue to the high dynam ic load s generated in th e towlin e. Under such conditions tug captains often redu ce power to avoid parting the towline. It is also possible to more or less contro l towline forces by a load reducing system. Tug motions and dynamic for ces in the t owlin e d ue t o waves affec t tug per fo rmance, wh ich also d ep ends on t owli n e characteristics such as stiffness and on towline length.
cond itions the negative effect of propeller wash can be rather high. The very specific 'Coanda effect' (see section 5 .2.5) canot be reproduced by any of th e present simulator mod els
To wing and pushingforces TOWing and pushing forces should be as realistic for vector tugs as for interactive tugs. Full scale tria ls can be used to verify simulated tug forces. The b etter towing and pushing forces can b e simulated for vector tugs, the better use can be made of these vec tor tugs for simulation of tug assistance.
Influence of flow around ship and of waterdepth and confi nement Water flow around a ship influences the perform ance of tugs pu shing at a ship's side of a ship having way on, and tugs ope ra ting in a ship's wake. These effects are not inclu de d in simulator mod els. Th e effect of water depth on the hyd rodyn am ics of a ship is included, but not the effect shallow water has on the flow aroun d a ship and subsequently on the performance of a tug in that flow field. Furth ermore, the highly com plicating factor of confineme nt is not included in any model. Neithe r is the effect include d of the following water flow in a channel as mention ed in section 6.2.2. Finally, the influ ence of the ship's propell er slipstrea m and wash on a tug operating near an d b ehind a ship's stem should be considered. Infl uence ofa tug's propeller wash on a ship's hull Some models incorp orate an approxima te method to allow for this. H owever, it is only valid for conditions wh er e th e prop eller wash hits the hull directly. Not included are the effects of towline length and shallow water. Th e n egative effec t of tug propell er was h impinging on a ship 's hull can b e minimised b y len gthenin g the towlin e. However, in narrow harb our basins this is rar ely possible and under more extr em e 132 THE NAUTICAL INSTITUTE
Thruster - tug hull interaction and thruster - thruster interaction With regard to thruster-hull interacti o n, some simulator m od els include an approximation for this, others ignore it entirely. As far as is known, thrusterthruster inter actions are generally not included for tugs with nozzle propulsion. Out-of the window viewfor interactive tugs In general, as most close quarter manoeuvres are carried ou t predomin antly u sin g visual cues, the imp ortan ce o f an all roun d v iew ca nnot be overemphasised . Dep ending on h ow detail ed the proj ection on the scree n is, distan ces off are often difficult to assess. A pr op er assessment of distan ces is essential for close quarter manoeuvring.
VISualization of towline behaviour A tug captain reacts to a large extent to his towline, such as dire ction and tension . O n mo dem simu lators the towlin e is made visible in the out-of-window view. Towlin e forces are displayed on a monitor. In real life, wh en towing or pulling forces are requir ed while the towline is slack, a tug captain will gen tly m ano euvre his tug till the lin e is tight an d only the n increase power.
Although complicate d, if it were possible to visualise in the out-of-window simulato r view whether the towline is slack or tight, it would be ano the r step forward.
8.8
Conclusion
co-operation with local nautical experts is essential . By utilising the combined expe rtise of hydrodynamicists, p il ot s, tug ca p tains an d scien tis ts , soph ist icated s imu la ti o ns s tu dies an d tra ini ng, incl u ding tug operations , can be pe rformed.
Finally , va rious tr aini ng m eth od s have b een disc usse d. See ing the acci den ts that happ en, the
In world ports there exis t an almost un limited number of different tugs. Fur ther, each port has its own method of pro viding tug assistance . Th is requires a very high degree of flexibility to simulate all form s of tug assistance in a realistic way. 'Iovving on a line increases the deman ds on the ma th em atical mo delling of tug behaviour, due to the combination of hydrodynamic
emphasized enough. Trainin g 'of tug captains and also of pilots shou ld focus not only on tug assistance and the capabilities and limitations of tugs, but also on the risks involved when tugs operate in the close vicinity of ships
behaviour, tug limit ations, lin e and '.. . inch characteristics.
and when rend ering assistance, while learning from the
For a corre ct and realistic appli cation of tug simulations
accidents tha t happ ened .
impo rtance of a proper profession al training can not be
TUG USE IN PORT 133
Chapter NINE
ESCORT TUGS 9.1
The background to escorting
port-by-port basis an d first escorting of tank ers starte d on th e Solent in 1991.
EsCORTING BYTUGS IS KOTHI NG l'EW. This chapter
should therefore not be seen ap art from the foregoing. In the past, as well as today, this kin d of service has been available in many ports arou nd the world, particularly
In Norway tanker escort became mandatory in the Grenland area in 1979 after the accident with the gas tank er Humboldt in the na rro w approach channe l to Por sgrunn on Norway's eas t coas t in March 1979. Escorting with a sp ecial-p urpose built tracto r tu g tethered to the ship started in 1981.
where ports are situ ated along rivers and canals or
behind locks. Wb en large ships started to enter thes e ports the y were, initially, often escorted bya number of tug s from the riv er entrance, or from a locati on wh ere
Following the sinking of the bulk carrier Mercantile Marcia in 1989, with a maj or spill of heavy bunker oil, esc o rting of tanker s in exc ess of 30,000 dwt was introduced at Mongstad and Stur e on Norway's west coast. In Norway escorting of tankers is n ow mandated by the gove rn ment for port approaches of all major ta nke r ports . In Swed en escorting of tank er s was introduced, am ongst othe rs, in the Port of Gothenburg in 1990. In Finland escorting of tankers starte d in the early nin eties. The tragic accident of the tanker Aegean Sea off the Spanish coast in 1992 led to escor ting of tankers in the port appro ach to La Corufi a (Spain).
th e river becomes more confined, up to th e berth. Escorting is often practised in situations involving large tows like offshore ri gs or ships with limited manoeuvrability due to engine or rudder trouble. It is also practised in adv erse weather conditions or when a river or canal configuration or a spe cific situation is such
that tug assistance may be required during the passage for certain categori es of ships . In ge neral , how ever, these escort services are limited to port areas and adjacent
rivers and canals, while the type of escorting discussed in this chapter concerns main ly escorting of tankers in
port approaches.
A US Coast Guard study in 1990 rep ort ed th at 20% of oil entering the marine environment is caused by tanker accidents. There is a variety of othe r entry sources such as tanker ope rational losses and municipal and industrial wastes. In general, a small nu mb er of tank er accid ents is responsible for mo st of the spillage , while the majority of tanker accidents result in negligibl e oil pollu tion .
Specific attention to the escorting of tank ers started in th e USA aroun d 1975. Towing co m p any Fo ss Maritime, USA, began escorting tank ers whe n it was mandated to do so by the State of Washington that year. The purpose of the legislation was to minimise the likelihood of oil spills in Puget Sound on the west coast ofthe USA. Tankers over 40,000 dwt had to be escorted by tugs. Tug escort of laden tankers has also been a featur e of tanker operations in Prince William Sound, Alaska, since 1977. On 24th March 1989 the Exxon Valdenan aground during her outgoing passage of the Valdez Arm, Alaska, resulti ng in a hu ge oil spill. As a consequence, renewed atte ntion was paid to escorting loaded tank ers in the United States. Shortly after the Exxon Valdez disaster the Oil Polluti on Act b ecame law (1990) in the United State s. It had tak en some 15 years to formulate this act " which, amongst other things, emp owered the US Coast •• Guard to set up new regulations for tanker escort, sc specifically in th e waters of Prince William Sound and Pug et Sound.
Th e largest oil spill in the world was that of the Atlantic Empress (West In dies, 1979) with 270,000 tons of oil lost, followed by the ART Summer (off Ango la, 1991) with 260,000 tons, the Castillo DeBellver(South Africa, 1983) with 250,000 tons and the Amoco Cad,z (Lands End, Great
Escorting of tankers, oil tankers and some times also ga s tankers, is app lied in m any o th er countries,
particularly in Europe, where escorting of tankers is practised in several ports with larg e oil and/or ga s terminal s. Mo st of such European cou ntri es will be m en tioned below, and for so me countries or ports also
the cause that led to the introduction of escorting. In the UK the decision for escorting tankers is taken on a
20
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•
C Olllt i O" t At ......l n Ot
Figure 9.1 Majoroil spills from tankers and theircauses: No. of incidents & t olume, World, 1976-89
134 THE NAUTICAL INSTITUTE
Britain , 1978) with 230,000 tons. Some other spills are th ose of the Exxon Valdez (Alaska, 1989) 40,000 tons, the Aegean Sea (Spain, 1992) 70,000 tons, the Braer{Shetiands, Gr eat Britain, 1993) 85,000 tons and the Sea E mpress (Milford Haven, Grea t Britain, 1996) with 65,000 tons of oil spilled.
ships and pilots. Shipping traffic - number, size, draft, speed and
Tanke r groundings and collisions seem to accou nt for about 30% each of oil spillage volume due to tanker accidents (see figure 9.1). Many factors contribute to these accidents, including techni cal failures, reduced visibility and hum an failures . In 1993 the UK P&I Club published its third Annual Analy sis of Major Claim s, cove ring the period 1987-1992. According to this, 50% of all pollution claim s were due to human factors.
Statistics avail ab le on pa st accidents invol ving
Wheth er these accidents could be prevented by the use of escort tugs dep ends on several factors, such as the real causes of the accidents or the location. One can ask the following questions: Did these accidents take place in a port area or port approach, being locations where escort tugs would normally ope rate? Wh at wer e the technical failures leading to the accidents and what was the cause of these failures? Were human failures involved, eithe r on board the tanker or ashor e? Would escort tugs have been able to make up for these human failures? Were the environmental conditions such that escort tugs could have pr ovided any reasonable assistance? .
9.2
Studies on escort requirements
The answers to the previously mentioned questions sho uld be part of a thorough study into wh ether escorting by tugs is suitable for a particular port or port approach. Such a study should include:
cargo .
Arrival an d departure policy for ships of differ en t types, dimensions andlor draft regarding vertical tide , currents, wav es, wind and lo r visibility. Pilotage. transiting ships and the causes of accidents. The enviro nme ntal impact of an incident The available tugs and tug assistance. Size, typ e, loading condi tions and manoeuvring particulars and und erkeel clearance of ships which are con sidered to ne ed escorting. This review may result in an extension or adaptation of certain procedure s or port se rvices regarding arrival/
departure policy, aids to navigation, th e vessel traffic system, tug assistance or pilotage. When the situation is such that no fur the r improvemen ts of th e existing situation are possible or the improvem ents made are insufficient to reduce the risk ofgroundings and spillage, a risk asses sm ent stu dy can b e carrie d o ut. This determines the probability and severity of an incident and consequently the areas of concern. Measures such as the provision of escort tugs can then be con sidered in order to enhance safety of passage. The severity of an incident with respect to oil spillage will, for instance, be less in port approaches or ar eas with san dy banks. Figure 9.2 gives the results of a Norwegian study showing the effect of som e measures. Th e results shown in the graph are not generally appli cable, but are only valid for the area studied. To find out whether escorting tugs are able to reduce risks during a passage, a numb er of accident scenarios
A review of th e present situation and a risk assessment.
Based on the findings of the risk assessment, a study focused on whether escorting could reduce the risks during a passage. Defining esc ort tug requir ements, es co r tin g
for the areas of concern should be developed. These should take into account factors such as the navigational re straints, th e ships concern ed, spee d, underkeel clearance , environmental con ditions including wind,
pro cedures and training requirements.
A review of the pr esent situation should consider the following aspects in relation to each other, but not limited to: Particulars of a port and its port approach, such as: Restrictions, bends, distances, water depths, vertical tides. Environmental conditions at all parts of the
" "
"
passage, i.e: currents, winds, visibility, waves,
swell, ice, day/night . Islands, piers, hottom structure and channel sides - rocky or sandy, flat or steep. Available anchorages. Traffic separation schemes. Aids to navigation. Vessel traffic services and informati on exchange with
NO I'Il s ~ .. ~ , IolI Ull fi S
'flt ~ I' ICI
El c c, l rUG
PIiOl I
Elc orl !IIG
Figure 9.2 1Jpiealeffiet offrequency reducingmeasures Groundingundnpower anddrifting - calculatedincident reduaion with escatt
TUG USE IN PORT 135
cu rrent, w av es and swe ll and, if ne cessary, oth er
shipping traffic. Scena rios sh ould b e develop ed for engine and rudder failures and possibly scena rios for a ship und er pow er, steering various dangerous courses whi ch , if no measures we re taken, would result in grounding and/or collision. Response times, i.e. the time b etwe en the moment a failure happ ens and th e moment the tugs are effective, sho uld also be included in the failure scenarios and be based on realistic assumptions, because this time is very critical to effectively limit the advance and tran sfer of a tanker after a failur e.
When the number of available tugs is found to be sufficient to provide the additional service of escorting, studies should give answers to the question as to whether these tugs are capable of pr eventin g a grounding or collision in case of failur e on board a tank er or whe n steering a dangerous course. Different assisting methods for these tugs can be assessed, with tugs secure d or not, The study results might include: A recommendation for a parti cular tug configuration of available tugs, and Definitions of acceptable environmental conditions and safe ship speeds, or Recommendations and requirements for the design of a totally new type of tug. The simulation technique mentioned in paragraph 8.3.2 is very suitable for investigating a large number of different scenario s, tug typ es and tug configuratio ns. Where the study outcome results in the design of a purpose built esc ort tug , p erforman ce calculatio n program s can be used at an early stage to pr edict the performance of different tug types and various design altern atives. Model tests may b e required to optimise tug de sign, evaluate tug performance and investigate safety limits for escort op erations. From the foregoing it can be concluded that the requirem ents for a purpose built escort tug ma y differ by port, such as with regard to tug size, type and ca pabilities, as ports differ b y approach , lay-out, conditions, circumstances, ship's type and size . For the earlier m entioned spe cific port related accident an d failur e scena rios, it sho uld be studied wh eth er th e purpose built escort tug(s) is capable of preven ting a collision or grounding. Such a study may also lead to port related escort regulations or operational pro cedures regarding safe escort speeds, wh eth er the escort tug should be teth ered to the ship (active escorting) or not (passive escorting), maximum allowabl e enviro nme ntal conditions, etc., if nec essary depending on the zone to be passed . In a d d itio n to the p ort sp ecific esc o r t tug requirements based on research, there might be general 136 THE NAUTICAL INSTITUTE
legal requiremen ts, e.g. na tional regulation s, to be met with respect to the capabilities an escort tug should have in controlling a disabled vessel. In cooperation with expe rienced pilots, ship masters and tug ca ptains, escort tug suitability an d related procedur es can finally be tested on full missio n bridge simulators . They can simulate the escort tug (s), ships to b e escorted, area s o f co ncern , an d en vir onm ental conditions, provided the simulator meets the demanding requirem ents for this kind of simulation.
Note: Model tests and simulation techniques are very useful tools to get insight into the capa bilities of escort tugs of various designs and into the effect such escort tugs have on an esco rted ship's behaviour in various conditions and circumstances . Limitations of simulation, an d of mo del tests, may lead to an ov erestimation of an escort
tug's pe rformance, which may then includ e a risk for the escort tug, its crew, as we ll as for the escorte d vessel.
To wha t exte nt study results differ from reality can on ly be verified during full scale trials under com parable conditions. This applies to norm al and certainl y to wave and swell conditions, as escort tugs often do ope rate in expose d areas. Full scale tests are carried out to ve rify th e cap ability of a n ew escort tug in delivering th e required steering and braking forces, for instance, for an escort tug class notation of DNV. DUring such trials sea conditions ar e usually rathe r fair. In wave and swell co nditions, how ever, high peak forces can occur in the towline if an escort tug has to apply maximum stee ring forces in case of a failur e on board the esco rte d ship. Simulatio n of dyn ami c forces in th e tow lin e, for instance, that result from out-of-phase mot ion responses
of ship an d tug to the waves are extremely difficult to simulate. The same applies to a realis tic sim ulation of th e ch arac teristics and d yn amic p erforman ce of a towline and towing winch. The escorted ship d oes affect the wave pattern, which again affects the escort tug' s capabilities (see also Referen ces for 'Creating th e Virtual Tug' ). So, a tug's escorting p erforman ce in wave conditions, and particularly safe tug manoeuvres and limits of safe tug oper ation , also takin g in to acco unt va rio us dire ctions o f in coming wave s, c an no t be determined accurately. This also includes the effect an esco rt tug ma y have on the esco rted sh ip in such conditions . Summarising, verification by full scale trials of model tests and simulator research is need ed for both aspects : escort tug capabilities as well as the effect an escort tug may have on a disabl ed ship in normal and in wave conditio ns . Further res ea rch starting with full-scale measurements may be required to get b etter insight int o the whol e interaction process between tug and shi p during em ergen cy escorting in sea conditions. With respect to the latter, for comparabl e reasons
Maritime Research In stitu te Nethe rlands (MARIN) proposes a j oint industry project which comprises: 'full scale measurements of towline, winch and tug behaviour under well-define d tug assist operations; modelling of dynamic towline loads, tug motions and stability; assessment of operationa l safety, as well as design and operation practice' (MARI N Report, April 2002).
ship's speed in case of engine failure in order to avoid
grounding. A lot of effort is required by tugs to restore ship's headi ng or rate of tum when, due to engine or rudder failure, a large loaded tanker with head way takes a sheer, particularly if underkeel clearan ce is small. Escort tugs should also be capable of controlling, within reasonab le margin s, ship's po sition wh en speed
9.3
Escorting objectives and methods
The objectives of escorting are :
has dropped, me aning that tugs should be capable of pushing as well as towing, which requires good fendering and the correct static bollard pull. Differ ent methods of escorting are in use, viz .:
To reduce the risk of pollution in port areas and port approaches due to groundi ngs or collisions caused by.techni cal or hu man failures on board a tanke r. To apply steering and braking forces to a disabled vessel by escort ing tugs and to keep it afloat, or limit th e impact of co llision o r groun ding if th ey unfortunately happ en . ' 'lhether steering, braking or both forces are required depen ds complete ly on the situation. Wh en failures occur it is stee ring forces in particular that are mostly
required to keep a ship out of a dangerous area. It might eve n b e necessary in certain situations not to redu ce
Escorting by a number of norm al harbour tugs. Escorting by specifically designe d escort tug(s). Escort in g tugs accompany a shi p eit her with towline{s) secured or free sailing at close quarters, ready to make fast and render assistance if a failure occurs. ;
Escorting by more or less normal harbour tugs is generally carried out only in port areas, over a relativ ely short distan ce and at low speeds. Escorting with specifically designed escort tugs is carried out in port approaches, over longer distances and at higher speeds.
9.4 Escorting by normal harbour tugs 9.4.1 Tug use In so me ports around the world only one harbour tug, which can be of any type, is used for regular escorting of tankers. In other ports the numbe r of tugs is based on size of ship and available suitable tugs. Dep ending on the situation tugs are secured or not. The escorting distan ce is gene rally only a few miles, though ships are some times escorted over A
longer distan ces throug h rivers an d channels. Usual speeds are about five to six knots, but when the tugs are unsecured or for longer escor t distances spee ds up to nin e kno ts are not uncomm on.
B
Figure 9.3 Direction offorces applied by assisting harbour tugs Escorting harbour tugs assisting a tanker in different modes. Tanker hadan enginefailure and veers tostarboard. Tugs are braking 1M sheer. Directionsoflongitudinal andtransverse forces applied by the tugs are shown
Escorting by normal harbour tugs can be carr ied out with tugs ope rating at a ship's side, which may include a rudder tug, or by tugs towing on a line or a co mb ination of these methods. The me tho d used depends largely on local . p ractice and available type of tugs. Wbeth er tugs are secured or not depends mainly on the restrictions of th e fairway an d envi ron me ntal co ndit io ns . T h e following should be taken into account:
It takes time to secure tugs, even whe n sufficient ship's crew are
available and where nee ded . There is no forewarning of the type TUG USE IN PORT 137
of failure neither whe n nor where. In event of failure there is no prediction of h ow the ship will behave. She may go straight on , vee r to starboard or veer to port. Securing tugs can take several minutes. This has cons equences for tug respons e time, the time b etv..-een
the moment failure happens and the moment tugs are effective. Several very costly minutes may be lost. On the other hand, for tugs operating at a ship' s side, securing or not m ay have consequenc es for the number
of tugs required . Wh en tugs are secure d at one side and the ship veers du e to a failure, th ey might not be at the correct side to cope with the shee r. This implies that tugs are n eeded on b oth sides if secure d. Wh en not secure d, available tugs can be directed by the pilot to the required position. Forward tugs towing on a lin e are more flexibl e in applying towing forces to port as well as to starboard. The same applies to after tugs towing on a line whe n equipped with omnidirecti onal propulsion. Based on th e restri ction s of th e fairway with r espect to ship dimen sion s and draft and taking into acco unt the available number and type of tugs, it should be carefully considere d whether the tugs will b e secure d or not. Tug positions sho uld be included in these consider ation s. Current and wind also playa part in the decision . Different esco rting tug positions are now considered. In figure 9.3A and B tugs are show n and the direction s of the appli ed for ces.
In th e example (figu re 9.3A), a load ed ta nke r underway at spee d has an engine failure. The ship veers to starboard, which cannot be stoppe d by the ship's rudder. As explain ed in section 4.3.3, tug no. 1 is not in a po sition to counter act the shee r effectively, but the position of tug no. 2 is much more effective. The same applies for the rudder tug no. 3. Effectiveness of the rudder tug in applying stee ring forces do es not differ mu ch from a tug at ship's side with lines secure d, except for wav e conditions. In that case the effectiveness of a tug ope rating at a ship's side declines fast. The most significant differen ce with tug no. I is not only that tb e rudder tug is in an effective position, but is able to app ly stee ring forces to starbo ar d as well as to po rt. Regarding tug no . I it should be kep t in mind that tbis tug might even have an opposite effect. Thi s b as been further explained in paragraph 4.3.3, Effective tug p ositions. If the ship veers to port instead of starboard, tugs nos. I and 2 ar e ine ffective in braking the shee r. If tugs are secured at a ship's side in order to anticipate failure they are need ed on both sides or at least a rudder tug should be used, pro vided the tug is sufficien tly powerful. When the tugs at a ship's side have a bo wline they can apply br aking forces as well as stee ring forces. 138 THE NAUTICAL INSTITUTE
Tugs at the ship'S side applying br aking for ces also create a turning moment. Th is is anothe r reason why
tugs are needed at both sides. A rudder tug can apply braking forces without creati ng a high turning moment. When tugs are not secure d at a ship 's side but stand by at a close distance, they can take po sition dep ending on the situation that arises due to a failure. At spe eds higher than th r ee to four kno ts conventional tugs lose their effectiveness in applying steering forces, while applied pushing forces increase. Pushing forces have a tendency to increase ship's speed, which should generally be avo ided. Wav es further decrease a tugs' effectiveness. Tugs with omnidirectional propulsion are more effect ive, including at high er speeds , in applying steering forces without increasing ship's spee d. There is another aspect to be taken into account, which could b e impor tant, for instance, in situations
involving partly loaded tanke rs and strong beam winds. Alth ough tug no. 2 and 3 are trying to stop the sheer, they will push the ship, togethe r with the wind, into the dir ection of th e dan ger ous area, while tug no . 1 is pushing in a safer direction . W he n tugs are no rmally towing on a line (figu re 9.3B), it sho uld also be conside red whether they should be secured or no t. When securing near the bow, ship's spee d should not be more than about six to seven knots. Wh en towing on a lin e with a stern tug having omnidirect ional pr opul sion or a combi-tug with an aft lying towing p oint, braking forces and steering forces to port as well as starboard can b e app lied . As with tugs operating at a ship'S side, a forward tug towing on a line increases ship'Sspee d when applying steering forces. The effectiveness of a forward tug in opposing sheer is low compared to a stem tug, as exp lained in section 4.3.3, altho ugh the tug pulls the ship away from the dangerous area. Using the escorting method with tugs towing on a lin e, ships can be contro lled at somewhat higher spee ds than with conventional tugs operating at a ship's side. When suitable conventional tugs are use d forward and tugs with omnidirectional propulsion aft, escorting speed can be aroun d four to five knots. The limitation on escorting speed depend s main ly on the capabilities of the forward tug, but also on the size, draft and underkeel clearance of the escorted ship and, of course, th e res tr ic tio ns of th e fai rway. When a conventional tug is used aft instead of an omn idirectional tug, ship's speed sho uld be l ow - say a maximum of thr ee to four kn ots - to permit contro l of the vessel in case of a failur e. Conventional tugs aft can only apply br akin g forces and steering forces to both sides at a very low ship's speed, while a conventi onal tug forward cannot app ly any braki ng forces. Escorting by conventio nal h arb our tugs is still possible in a numb er of comp ulsory escort areas in the
USA, alth ough escort tugs with om nid ir ecti onal propulsion are increasingly u sed and the ir escort perform ance is being further inve stigated. A sum ma ry is given in par agraph 9.6 of escort regulation s in force in the USA and Europe. Wh en escorted by normal ha rbour tugs, tanker spee ds can not be high. Thi s is reflected in the regulation s, which state that escorted tankers should not exceed a spee d beyond which the escorting tugs can reasonably be exp ected to bring th e ta nker safely un der control within the navigational limits of the fairway. Summary Escorting using norm al harbour tugs is comparable with tug assistance in ports as are escorting speeds. The number, type and bollard pull of harb our tugs used for escorting should be carefully considered taking into account the restrictions of the fairway, ship size, draft and freeboard, underkeel clearance and environmental conditio ns. It should also be ca refully considered whe ther escort ing tugs should be made fast to a vessel or not, Wh en tugs have to make fast at a ship's side, it may influence the numbe r of tugs required.
The speed of the escorted tanker with a maximum of about five kn ots should allow tugs to influen ce tan ker movement effectively in the event of a casualty. Rudder tugs and tugs positioned at port or starboard quarter are at the most effective locations to oppose a sh eer. Rudd er tugs are most flexible because of their capability of applying steering forces to both sides. These tugs all apply pushing for ces at the same time which may in cr ease a ship's speed. The effe ct is less when omnidirectional tugs are used, which are also more effective at higher speeds. When tugs at a ship's side have a bow line these tugs can, like a rudder tug, also apply braking forces. . A forward tug towing on a line is more flexibl e in applying steering forces both to port and starbo ard. The same app lies for a stern tug towing on a line, provide d the tug has omnidirection al propulsion or is of th e cornbi-tug type . These types of tug can, as a stem tug, apply braking forces as well, which is not possib le for a forward tug towing on a line. If a shee r is towards a dangerous area, the applied steering forces of the after tugs are directed towards the dan gerous area and the steering forces of the forward tugs away from it, PhDtos: MARIN, 17u Nt!htTlandJ
Figure 9.4 Photographstaken during escort trialsin Prince William Sound, Alaska, August/September 7993. Three tugpositions are shown: 7) pushing at tlu bow, 2) nearthestem and 3) a tug operatingas rudder tug'. They are twinscrew tugs with three rudders. Tugs 7 and 3 have a botlardpull of 68 tons andtug 2 of50 tons. The tankers are theloaded 'SIR Benicia: 770,000 dwt andthe :4rco Independence: 262,000dun, 80% loaded
9.4.2 Escort training and planning Also when norm al harbour tugs, con ventional tugs for instance, are used for escorting, training and escort planning are important suhjec ts, although depending on the local situation . These subjects are amongst others discussed in the next par agraph. TUG USE IN PORT 139
Before discussing important aspec ts of escor t tug p erform an c e , a numb er of somewhat difficult terms are first explaine d. Figure 9.5 shows wh at are generally called the direct and indirect towing methods. In add ition, term s are given as used hy propulsion m an ufacturer Aqu am aster (figu re 9.5 B). The indirect arrest m ode is recommended for initiating a turn, while the co mbination arrest mode is recommended for opposing a turn at lo w and at h ig her spee ds . T he ac h ievable braking force s in the reverse an d transverse arrest modes
have been discussed in section 4.3 .2 and ar e shown in the graph in figure 9.7. According to Aquam aster th ese for ces see m to correlate ve ry well with full scale trials.
r' d' OiRECT TOWING METHoO'l
Figure 9.5 Terminology relating to direct and indirect towing methods A: Tractor tug. B: ASD/reverse-tractor tug Position 1: Stmingandretarding. Position 2: Retarding
9.5
Escorting by purpose built tugs
In dir ect mod e, achievable steering for ces decrease whe n spee d in cr eases. At speeds ab ove norm al harbour speeds of about five to six kn ots and, amo ngs t othe r things, dep ending on the bollard pull of the tug, higher steering forces can be achieved in indirect mode (see figur e 9.8). For spee ds between thr ee and seven knots a method used by escort tugs in a growing nu mber of USA ports for applying steering forces, is the 'powered indirect
9.5.1 Type of tugs, performance and operational requirements 1jpes, tenn inology andfactors affecting performance T he nam e escort tugs is basically used for tugs spec ifically designed to escort ships over long distances an d at relatively high speeds. Escort tugs are all of the omnidir ectional type, wh ether ASD/ reverse-tractor or tractor. Most escort tractor tugs hav e VS pr opulsion.
The tugs are secur ed (tethered) to a ship'S stem or un secured (untethered), but ready to pro vide imme diate , assistance in case of emergency. 'When secured to the stern of the escorted vessel, escort tugs are abl e to apply high stee ring and/or br aking forces if requir ed in case a failur e happen s on b oard the assisted ship. Steering forces at high spe eds are gen erated in the so-called indirect method .
140 THE NAUTICAL INSTITUTE
Courtesy: Captain Gregary Brooh. USA
Figure 9.6 The reverse-tractortug 'Lynn Marie', which has aforward skeg, applying steeringfotces by using the 'Powered Indirect Manoeuvre'. (Forparticulars of the 'LynnMarie see figure 9.21)
To ~ •
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60 Reverse Arrest
50 Har bour assistance _ _Escortin g
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Figure 9. 7 Maximum direct hrakingforces azimuth drize
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Figure 9.8 Approximatirm ofstmi ngfinces ofa 36 trms tractor tug
anker through water}
Incoming relative flow as seen by observer on tug
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t: Steering force Braking force - -- - -
!! Drift an Ie
Side thrust
,
/ Total force on hull and skeg
Figure 9.9 Definition ,ketch offorees ona tug anda ,hip SkeM ofa traaor lug assisting a tanker in the indirect mode. Thepropeller thrust keeps thetranszerseform andlongitudinalforw, resultingfrom the hydrodynamit:fore< onthe huUand,keg andfrom the towlirufore<, in balance
TUG USE IN PORT 141
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manoeuvre'. The tug then drives itself out further than position A lar B I (see figure 9.5 for In dir ect Towing Method) and de pending on the speed may reach a pos ition at which the towline is at a 90 degrees angle to the ship's centreline. Then full power is given, with the tug at perhaps up to 70 degrees angle to the incoming water flow. High
.. •
•
ste ering forces can be generated ,
higher than in the direct towing meth od. In the five to seven knots spee d ran ge line pulls of 75 - 125% of the tug's ballard pull have been measured. See with respect to these forces, the forces shown in figure 9.8 for the same speed range. Capabilities of the escor t tug, of course, play an
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furtherm ore important to note that with this method steering forces can be delivered mu ch faster than with the direct towing
HI Heeling arm without to....-ing arch H2 Heeling arm with towing arch
Figure 9. 11 Aquamaster escort tug canupt - The Towlina with towing arch
method , wher eby the whole tug' s body has to be pulled thr ough th e water against the incoming water flow from position 2 to position I (figure 9.5 'Direc t Towing Method'). It has been experie nced that this can take a long time, particularly in this five to seven knots speed range . The same method is sometimes used in other ports around the world by VS tugs during normal
approximate 90'. For the highest steering forces, angle (a) differs by tug type and is gen er ally larger for ASDI reverse-tractor·tugs, which can clearly be seen in th e TUGSIM perform an ce graphs of eight knots speed in section 4.3.2.
harbour assistance.
conce rns the perform ance of azimuth stem drive tugs
It is further worth mentioning a specific way of escorting by using two escort tugs as a tandem, both tether ed, which is utilised in the Port of Lon g Beach, California. It is called team towing or tand em escort towing , for which mod ern VS tugs or ASD /reversetractor tugs can be used. With this met ho d relatively smal l escort tugs can be used to hand le heavy ships. Sp ecific tug procedures have been develop ed for this method. Escort spee ds while utilising the team towing system are relatively low, gene rally approxi ma tely six knots, wi th a po ssibl e upper limit of eight kn ots, dep ending on tug design, crew training, and the sea conditions to be faced during the escort, Alth ough escort tugs should also perform well at lower speeds, the indirect mode is furth er discussed because escort speeds can be up to 10 or even 12knots. In figur e 9.9 a tug is operating in indirect mode and th e fo rces act ing on th e tu g an d ship are sho wn. Co nce rni ng tug performance, the magnitud e of the transv er se forc es an d in parti cul ar the points of application of th ese forc es ar e m ost import ant. For generating the high est steering for ces, an gle (a) is pred ominant, wh ile keeping th e tow line ang le at
Discussion amongst propulsion design ers mostly compared to tractor tugs, pa rticularly tracto r tugs with Voith propulsion. Two escorting tugs are shown in figure 9.10 of which one is an ASD -tug and the othe r a V S tug. In this figure the most impo rtant aspects of tug pe rforma nce in the indirect mod e are shown. The centres of pressure are approximated for an angle of inflow (angle a) of 90'. Firstly, the larger lever x is compared to lever y, the less sideways thru st is ne ed ed to balance hydrod ynamic forces at the centre of pressure (C O P) and the high er the towline forces will be. Secondly, th e lar ger th e vertical distance between towing point T and centre of pressure C O P, lever a, th e larger any list will b e. However, the lar ger th e vertical distance b etween propulsion poin t P and towing point T, lever b, the more list is reduced by the sideways thrust of the propulsion. As escort tugs should be designed such that any required sideways thrust to balan ce the hydrodynamic forces at CO P is small, the height of the towing point above th e centre of pressure becomes particularly important with respect to heeling moments.
When comparing the ASD-tug to the VS tug as shown and assuming the same stability, then it can be seen that TUG USE IN PORT 143
with equal towline forces the ASD-tug will have a larger list. Thi s is be cause of the higher vertical distance between towing point T and centre of pressure COP and the smaller vertical distance between towing point T and prop ulsion point P to oppose heeling moment. The relation x :y as show n in figure 9.10 is about the same for both tugs. However, it should be borne in mind that the centre of press ure moves in the direction of the towing poin t when the angle of inflow, the drift angle, becomes smaller. The horizontal and verti cal locations of the cen tre of pressure at different angles of inflow can only be determi ned by mod el tests and will depend on th e hull form and appendages, such as the skeg and propulsion units. It can be con cluded that the longitudinal and vertical locations of the centre of pre ssure and towing point are very important. For a VS tug the positions of the towing point and centre of pressure are mo re or less determined
by the skeg. Good perfo rmance from an ASD / reversetractor tug can be achieved by a not too high and slightly more aft lying towing point than shown in figure 9.10, and by a hull form such that the centre of pr essure lies as far forward as possible. This has, for instan ce, been achieved in the Aquamaster escort tug concep t Towliner (see figure 9.11). Thi s is an ASD-tug with a bulb and b ox keel. A towing arch is suggested for the lead of th e towline , b eing a similar system to that discussed in section 4.2.3. Other ASD-tugs may have a bulb and forward skeg, which also results in a more forward lying ce ntre of pressure.
Apart from the aspects already mentioned, form and lateral are a of th e tug's underwater body are imp ortant factors for genera ting the highest possible towline forces in the indirect mod e. For tha t reaso n specific high lift skegs are develop ed for VS escort tugs. Man y ASD escort tugs are equipped with a long skeg underneath the hull or with a box keel as with the Towliner conce pt, while tug' s underwater form is often su bj ect of continuous research.
Tug's stability should b e well considered if an ASDtug or reverse-tract or tug is to be equipped with a skeg und erneath the hull, because it does increase the towline forces, an d consequently th e he elin g for ces. It is, furthermore, good to note that when a tug's lateral area reduces, performan ce in applyiog steering and braking forces reduces. Thi s will be the case when a tug's bunkers are nearly empty. On the other hand, a minimum ballaSt and fuel onboar d may imp rove a tug's performance with respect to some important aspects: The hazard of early deck immersion reduces and th e tug becomes mo re responsive.
Th e performance of VS tractor tu gs is often compared to that of ASD -tugs, though comparison is difficult because the tugs differ in many respects. In gen eral, tractor tugs seem to b e able to exe rt somewhat 144 THE NAUTICAL INSTITUTE
higher steer ing forces in ind irect mo de than present ASD escort tu gs, wh ile the ASD -tugs can apply so me what higher b ra king forc es, th ou gh this m ay chan ge by speed. Steerin g forces are very important for escorting at high er speeds, though it depends on the local situation as to wha t is chiefly n eed ed . Fo rtn er US towing company Hvide Marine op ted for a tractor tug with azimuth propulsion, the Broward, becau se of the high br aking forces that can be achie ved . So far attention has been paid to th ose aspects important for a goo d performa nce, such as location of ce ntre of pressur e, h eight of to wing poin t, tug' s und erwater form and lateral area. Thi s should, of course, be seen in comb ination with an optimum stability, which is add ressed later.
Note: The design asp ects discusse d in this sectio n are specifically aimed to im prove a tug's performan ce in the indirect towing mode. Som e of the se featu res have a negative effect for the direct towing mode, such as e.g. the large skeg undern eath an ASD -tug. Such a skeg increases a tug's und erwater lateral resistance , making
it, for instan ce, even more difficult to apply steering forces in a fast and effective way in the direct towin g mode. Ship s' spe eds during tug assistance and th e most important tug op er ating m odes sho uld th erefore b e taken into account with respect to th e design aspects, particularly tho se relating to tug's underwater body. Braking and steeringforces Escort tugs have to deliver steering and/ or braking force s in case of e mergen cy . Steeri ng force s are considered to be particularly import ant. That is tru e as long as there is sufficient roo m ahead and bends to b e navigated are not too sharp. In that case a ship can be steered and kept free from dangerous areas. However, " it depends on a number of factors whe th er the steering assistance of an escort tug will be sufficien t to keep a ship in sa fe waters. For exam ple , enviro nm enta l conditions may have such an influen ce that a ship starts drifting into a dangerous dir ection as soon as speed decreases due to an engine failure, regardless of th e steering assistan ce pr ovided .
Wh en an engine or steering failure h appens while th e m an o eu vring area o r di stan ce ah e ad is v ery
"restricted, braking power is required. Th e m ost effective means to take way off, provi ded the re is sufficient ro om, is to initi ate a turn. This has the effect of slowing down the tank er and redu cing head reach. After an engine and/o r rudder failu re h as been recognised and before assistance -is given by the escort tug, the ship may already have built up a rate of turn. For large loaded tankers it is hard to sto p such a turn and bring the tank er back onto a safe course. In most cases, if circum stances allow, it is better to assist the
40,000 dwt bulk carrier
40
70,000 dwt bulk carrier
60
150,000 dwt tanker
88
300,000 dwt VLCC
116
30,000 m) gas carrier
32
60,000 m ! gas carrier
43
on board the esco rte d vesse l the rudder becomes blocked at a certain rudder angle. Whether it should then be possible to counteract rudder forces depends again on the local situation . Rudder forces on a ship with the rudder blocked at a certain rud der angle are reduced when the propeller is stopped, and in case of a controllable pitch propell er, when pitch is set for zero . It enlarges the po ssibilities for an escort tug to steer the tan ker. Det Norske Veritas assumes the rudder lift forces witho ut propeller turning to be 0.53 times the forces with the pro peller turning. If circumstan ces allow the ship could also be stopped by using its engine and with the assistance of the escort tug.
Figure 9.12 Steenngforas required based on 15" rudder anglt
escorted tank er to turn, for instan ce, 180 degrees or 360 degrees, particularly at higher escort speeds. It should, howev er, be noticed that assisting tank er turns at
relatively high spee ds imposes high loads on the tug (and tanker) and may be un safe, as tug speed will incre ase appreciably above tanke r speed when on the ' outside' of th e turn. While turn ing , ship's speed decr eases quickly; conse que ntly after a sho rt period delivering stee ring forces in the indirect mod e, the tug has to switch over to the direct mod e (combination arrest mod e, see fig. 9 .5) to stay effective.
Figure 9.13 gives an indi cation of the rudder forces, (i.e lift forces) of thr ee large tank ers at different speeds , an d rudder angles with the propeller turning while ' matching ship's speed. The rudder forces are based on a study carried out for the Norwegian Sture Crude Oil Term inal. It giv es an indi cation of the required stee ring
forces to steer a tanker at different spee ds in case of a rudder failure and of the rudder forces to be overcome, if nec essary, in case the rudder is block ed at a certain angle .
It depends entirely on th e situation during a failure what kind of assistance is required. But, as indicated,
escort tugs should be able to apply high steering forces. These should meet a ship 's rudder forc e with the propeller turning whil e matching ship's speed. The Norwegian Hesnes Neptun Group has worked out the steering forces required for safe handling of a number of differ ent sizes and types of ships, as shown in figure 9.12.
It is important to keep in mind that the required steering and stopping forces increase when underkeel clearance decreases, as discussed in Ch apter 6. It should also be noted that after an engine or rudder failure, beamy full-bodied ships have the tendency to develop the fastest rates of turn.
For navigating a not too sharp bend at 10 knots speed, which means for many tankers a telegraph setting of half speed, or full maneuvering speed, a rudder angle of 15 degrees, on which the values are based in figure 9.12, can normally b e regarded more or less as a maxirnun . The related rudder forcesli.e lift forces) at this spee d give an indication of the requ ired steering forces in case of a rudder failure.
Therefore, insight into ship's behaviour is important
with resp ect to escort requirements. The bo ok 'Ship Bridge Simulators. A proje ct handbook' (seeReferen ces) includes relevant information on ship man oeuvring
particulars. \'/hat maximum steeri ng and braking forces a local escort tug should b e able to apply should be based on a study of failure scenarios representativ e of the ships and areas con cerned , including the local situation and
In other cases higher steering forces may be required, which can be the case when due to a techni cal failure
i t .l " ~~~t~~\tfii:B:':~' :'f,ik",f,~~~~%:}~~~~~t~~'1~,;~~~itfr*,:;§~3 ~·~;'\'~t'~ dt' ~·""l,,'$?·'"';~I\·~~·~~~{YJ;!£a:t)~;:~~~~~t~1~t,;~~~Jti.t _'~f;·i"'.'t1i'~};·;:';'h' i\.", .t.;x~.:~,...,r,;<:;"t'''':*;~~-./;~:;4-:.-,:>,.,,* : ~~ u:-er ~s~.z ~ anu ~u}l er a.p.g ~}, ~l" }. ~ =Tht;t~:j1f J;'>h'¢;j"&\~~{~~{~"'t;@"'V''\~, ",.", %,o;;,~:;.?: 100.000 dwt
~
200,000 dwt
300,000 dwt
;.", pee "_', l~~-)?:{;:~~1;;;,:.,:.
10'
15'
25'
35'
10'
15'
25'
35'
10'
15'
25'
35'
6 knots
25
30-
45
30
30
50
60
50
40
55
80
60
8 knots
35
55
75
60
55
85
115
90
70
100
140
105
10 knots
60
85
120
90
90
130
185
145
110
155
220
165
12 knots
85
120
175
135
130
190
260
205
160
230
320
245
Figure 9.13 Rudderforces in tonsfor differentloaded tankers, speeds and rudderangles. Rudder f orces art largen at approximauly 25" rudder anglt. Roundfigures are used
TUG USE IN PORT 145
Seve ral full scale trials have been carrie d out, in cluding one in 1991 nea r the Isle of Wight, U K. A normal stern drive tug of 53 tons ballard pull escorting a 130,000 dwt tan ker showed that it cou ld steer th e tank er over a range of 5·9 to 8·8 knots using the in direct m eth od and below 5·9 knots using the dir ect meth od. At a speed of 10 knots the tanker could be stopped in 15 minutes over a distance of one an d a quarter m iles, in
almos t a straight line. The gra phs in sectio n 4.3.2 show achievable steering forces Figure 9.14 lUg Zindsey Foss' applying steeringforces in the indirect mode at a speed of eight knots for a norm al ASD an d VS tug . These circumstances, as menti on ed in paragraph 9.2. Practical forces approximately equal the ball ard pull, while the tests should be carr ied out to validate the results as far maximu m achievable brakin g forces are already much as possible. The failure scenarios, taking int o account higher than the ball ard pull. When speed increases active as well as passive escor ting, may for instance furth er the steering forces increase considerably. include: It should be noted th at amo ngst other things the Steering a tanker on a straight course and through n egative effect of the ship's wake on the achievable b ends in the fairway after a rudder failure andlor braking forces is not included in the graphs. engine failur e or stee ring as well as stopping the tanke r after such failures. Several othe r full scale trials have been carried ou t, Steerin g an dlo r sto pping a tanker with rudd er of which results depend on tug type, ship's size an d draft, escor t spee d, failure scenario and experience. Results jammed at a certain rudder angle, or the same but of one will be m ention ed below. It conce rns a full scale including an engine failure. trial with the fully loaded 125,000 dwt tanker Arcafuneau Differ ent escort spe eds. in April 1997. The large VS escort tug Lindsey Foss was tethe red to the stern of th e tanker (see figure 9. 14). It can be expecte d that for the given failure scenarios, Parti culars ofthe escor t tug ar e given in figure 9.2 1.The a tether ed escort tug can react faster and consequently tanker had a spee d of eight knots. The wind was on the needs to apply relatively lower steering forces than whe n port qu art er with a spee d of 25 knots, wh ile sea passive escorting is utilised. In that case there is a mu ch conditions we re nominal. larger tim e delay before an escor t tug can be effective. As mentioned alr eady, in the meantim e the tank er may WIth the ship on a steady course, the rudder was have built up a rate of turn, or have travelled in the put hard-a-starb oard . Thirty secon ds later the failure wrong direction and, to stop such a turn with a load ed was 'recognized' and the engine stopped. After another tanker and bring it back onto a safe course, the escort thirty seconds , thus a total time delay of one minute, tug sho uld be abl e to gen erate very high steering forces, the tug was ordered to stop th e turn by applying steering particularl y in shallow waters. The requir ed forces may forces in the indirect mode. At the time the ship was eve n b e t oo hi gh for any tug whe n the fairwa y back on the original course it was more than 500m off dimensions are ve ry restricted . So, not only a tanker's track duri ng two similar tests! The results of one such dim ension s and displacem ent are imp ortant factors, but test are pr esented in figure 9.15. The results show the also the local situation and conditions such as spee d, imp ortan ce of a tethe re d escort tug and of a proper underkeel clearance, environmental conditions, fairway recognition time , while tug the master's experience plays co nstraints, whe ther active or passive escorting is a crucial role as well. Alth ough the one minute time applied and the type of failures that may happ en . Th e delay can be considered as rather large (an alert and outcome of th e failure sce narios stu dy sho uld b e well trained bridge team will recognise a failure and weighed in a sensible and practical way regarding tug take action much earlier), the results illustrate that eve n requirem ents, escort method an d escort speed. For the with a large purpose built escort tug and a not too large same tanker size, requirem ents for maximum achievable tanker, off track distances can be large and may inc rease stee ring and stopping forces of an escort tug may, cons iderably at higher spee ds. The results also show ther efore, differ between ports . why it is so imp ortant to have full scale tests . Photo: FossMaritime, U.S.A
146 THE NAUTICAL INSTITUTE
_IWI
.- -
,,- ... .....,....,"
Free sailing speed of an escort tug dep ends am ongst
'.
others on the maxi mum escort speed as determined for
a port or port approach. For a number of rea sons escort tugs should have a reasonab le over-speed compared to I.... the maximum escort speed, due to the factthat the escort tug should be able to overtake the escorted ship withi n 'M' a limited time span. It sho uld be able to ove rcome th e ship's propeller slipstre am wh en approaching the ship's n .. stern to pass or connect the towline and the escort tug r"\' . should have sufficient reserve power to handl e safely any interaction effects that might arise b etween tug and lOU - 'on m. ».. ,<0" • ship, which can be very strong at high speeds, Finally, 'M' in adverse sea condition s a tug's maximum spee d may Courtesy: Floss Maritime, USA decrease faster than a large ship' s speed. Figure 9.75 PUiIs ofafUU scale trialwith the loaded 125,()()() dwttanker 'Arco]untau' andthspurpose huill trcort tug ~indsry Foss' {distances in Stab ility /i et). The tanker while haoing a speed ofeight knots hadasimulatedrudder Stability has been addressed in section 4.2.3 and is failure with the rudder blotked at hard-a-starboard extreme ly important for escort tugs, Towline forces can • The maximum braking and steering forces that can reach very high values, up to one and a half to two times : be achieved by a specific escort tug depends on the the bollard pull at 10knots escort speed in indirect mode, escort speed and also on sea conditions, Performance while escort speeds may even be higher. Waves and tug of tugs decrease in wave conditions, as will be the case manoeuvres can further increase towline forces, another with escort tugs. In wave conditions at high escort speed s reason for keeping the towing point as low as possible it might not be po ssible to apply the maximum steering to reduce heeling moment. A m ethod applied to reduce and brak ing forces andlor the captain could, for reasons heel angle is the construction of hull side sponsons, which provide a substantial increase in reserve buoyancy of safety for his tug and crew, decide to app ly lower steering forces, or the ship's speed could be slowed down and result in large r righting mom ents. in order to enable the escort tug to apply the steering andlor braking forces required in case of a failure. Good static and dynamic stability, taking into account changing trim during escorting. is necessary to operate safely at high escort speeds, It should also be remembered Maximum wave height for the large st purpose built that towline length and characteristics influence a tug's escort tugs seems to be aro und four metres. Not much hee ling angle , Forces reac h higher values when low data based on practical experience is available. With stretch towlines are used , often the case with escort tugs. this wave height steering and braking assistance can still Th e longer these towlines are the better the dynamic be applied, provided the tug has a towing winch with a forces can be absorbed. High stretch towlines, however, load red ucing system. may cause larger movements of the tug. Furthermore, certain tug assist manoeuvres at high speeds and in wave condition s may become risky, as A minim um metacentric height of three metres is m ay b e the case wit h tr an siti ng from one assist ge nerally recommended. It is advisable to avoid manoeuvre to an other, for instance from braking to excessive values of metacentric height. In adverse sea con ditions these might lead to acceleration forces th at the indirect steering mod e, if not carried out in acorrect could be prejudicial to the tug an d its equipment. It way. makes , furthermore, life an d work on board almost impossible and so affects safety of operations. Good insight should therefore be obtained into to what extent wave conditions affect an escort tug's performance and what safe worki ng limits are at various Classification society Det Norske Veritas (DNV) escort speeds and wave conditio ns. With respect to this, gives dynamic stability requirements for escort tugs in th eir rules for escort vesse ls, which are include d in the the tug master's experience is again a crucial factor. Appendix. R equired maximum speed free sailing The heel angle at which maximum steering and Maximum escort speed usually lies between 10 and braking forces are determined should be well considered. 12 knots . However, safe escort speeds depend on factors It has to do with ope rational safety. If deck immersion is such as tug design and capability, weather, sea and swell regarde d as the limit, then only a small safety margin is co n ditions, configuratio n of channels, underkeel left, A golden rule used by an experienced escor t tug clearance, the nature of the bottom and traffic. train ing master is: 'D o not immerse the deck line. As escort speed can be up to 12 kn ots, the maxi mum A maximum heel angle based on righting energy free sailing speed of escort tugs should be higher. The criteria, as is the case with the D NV escort tug rules, maximum free sailing speeds of present escort tugs is includes a certain margin of safety for the dynamics about 14 knots, but varies between 12·5 and 15-16 knots.
•
~
....
TUG USE IN PORT 147
in o peratio ns. Further research is need ed in orde r to
co me to ge ne rally acce pte d , safe an d workable stab ility requiremen ts and crite ria for differ ent types of escor t tugs. Design developments ofescorl tugs An escort tug must, obviously, be seaworthy and able to perform escort duties by utilising her best capabilities. ASD-tugs, free sailing or escorting in teth ered mod e run bow first. This is the normal, fastest and, for the deck crew, the safest operating direction particularly in high wave conditions, at high speeds and when performing in the indi rect towing mode. At high free running speeds tractor tugs norm ally run bow first. H owever, whe n escorting in teth ered mode they run stem first, with the lower afterdeck in the sailing direction. Maximum spee d when running stern first is lower. At higher speeds and in wave conditions water comes over the after deck easily. The design of a number of VS escort tugs has changed, therefore, as can be seen, e.g, with the Bess and Boss. At the skeg end, the shee r and after bulwarks are mad e high er and the hull form is more pointed . In addition, the wheelhouse is turn ed 180°, thus providing the captain with an excellent view in the operating direction. The
of lift. Voith claims an 18% increase in steering force compared to a conve ntional skeg. Additional towing pointfor escort tractor tugs . When astern of a vesse l und erway with a towline fastened, a tractor tug may sheer from one side to the oth er, caused by the incoming water flow on the skeg and the location of the towing point, centred ab ove the middle of the skeg. To bring the tug to a more stable position , a number ofVS escort tugs ar e equipped with a second towing point at the after en d, which could also be useful for azimuth tractor tugs (see also page 152 Operating reliability an d fail safe). Wh en running in line and aste rn of a tan ker the towline then pas ses through a fairlead, a kind of hook or towing pins at the after end of the tug, similar to the towing pins shown in the photograph of th e a fte r dec k of tug Maasbank (figure 7.7)_ Wh en a failure happ ens aboard a tank er and the tug has to pro vide steering assistance, it should be able to take the towlin e out of this far aft lying towing point, otherwi se achievable stee ring forces are lower. This is indeed po ssible on a number of tugs, where th e hook or towing pins can be ope ne d hydrauli cally in order to use the origin al towing point above th e middle of the skeg again .
Towing pins hav e b een dev elo p ed specifically designed for escorting. These pins make it poss ible to release the towing line wh en und er tension , eve n with the towline angled upw ard s, from the most aft lying towing point. In additi on to the use of the secondary towing point
--
.
0 0
00 00 0
for reasons mentioned above, tests with radio-controlled
--
} . M. Voieh Gm1JH
Figure 9.76 VS escort tug'Bess' with tTUJdijied traaar tugdesign (I.o.a. 36·2m, beam 72·2m, draughJ 5·2m, boUard pull 57 Ions)
same change in design is more or less the case with a number of other VS escort tugs. There are continuous developments in the design of escort tugs based on ex pe rience , research and new insights, all concentrating on improvement of the escort tug capabilities. Design devel opments focus on aspects such as optimum skeg and hull form, optimum location of towing point(s), and in particul arfor ASD-tugs the height of the towing point. Specifically for VS escort tugs, design attention for a good perform ance when sailing skeg-first can b e added. Dev elopments on skegs and skeg form concern both VS escort tugs and ASD escort tugs. Modern VS-tugs have high lift ske gs, also called hydrofoil-shaped skegs. One of the latest skeg deve lopments for VS escor t tugs is the Voith Turbo Fin (VT F). This VTF has a rotating tube at the end of the skeg, which causes a considerable increase 148 THE NAUTICAL INSTITUTE
models showed that in extre me conditio ns th e use of th e sec ondary towing p oint adds to th e sa fety of ope ratio ns . In waves th e aft deck is mo re easily submerged when the main towing point is used . At high spee ds and in rou gh conditions, the use of the secondary towing point makes it more difficult for the tug cap tain to get int o tro uble. It also seems to ma ke th e tug's motion s less seve re. Deck equipment, towlines and lowline handling Towing equipme nt of harb our tugs has been dealt with in Chapter 7. Am ongst others, towing winches, towlines and towline handling have been discussed. The foll owin g applies more in parti cular to escort tugs, alth ough mu ch of the items discussed b elow are of importance for harbour tugs as well, particularly whe n involved in escorting and/or using towlines mad e of HMPE fibre s.
s
R equirementsfor towli nes and tug deck equipment The minimum breaking strength of a "towline of an escort tug should be at least two and a half to three times the maximum achievable braking and steering force, which gives some allowance for e.g. p eak loads when taking into account pr esent OCIMF safety factors of 2·0 - 2.2 for synthe tic lines.
Escort tug rul es of cla ssificati on society DNV requires the towing line to have a br eaking strength of
•
at least 2·2 times 'the m aximum mean towing pull' as measured during active esc o rt tests, which is spe cified
in the rules. Th e rules require the towing winch to have a load reducing system. All towing equipment should have high operating reliability an d be designed for the highest towline loads that can be expec ted. Towing winches on escort tugs should have high brak e holding power, a fast lin e deplo yment and retrieval capability and a high pull, in particular if the tug is equipped 'With a towline ten sion contr ol.
Because of the high towlin e loads it is recommended that the towing winch has a load reducing system to avoid ex cessive load s in the towline, which particularly may occur in wave conditions.
Th e high pull of the towing winch enables the towline to be paid out and recovered when the line is und er high tension, whil e rapid lin e handling is essential to allow imm ediate p ositioning of th e escort tu g, particularly whe n in an emergency, an untethered escort
tug has to make fast to a ship . Ship 'S deck equipment requirements De ck equipm ent construction on board the escorted vessel should be suitable for high towlin e loads and for the typ e of towline used. This is a very important aspect, b ecause there have been several complaints regarding the lack of suitable strong points and fairleads on board ships to be escort ed, deck provisions not properly sized and located and not strong enough to withstand the high peak loads generated in the towlin e of the escorting tug. A reduction in escort sp eed may be warranted if the ship 's fittings are not strong enough to withstand the towline forces that would be imposed on them.
In the y ear 2002 OCIMF published "Recommendations for Ships' Fittings for use with Tugs . with Particular Refer enc e to Escorting and Other High Load Operations". This do cum ent provides a proper guidance to the tanker industry regarding the provisions of ships fittings for use with tugs. The guidance includes, amongst oth ers, the safe working load and dimensions of fittings (including certification). Certain ports may use the OCIMF recommendations as a requirement for ships to be escorted in their port. Polar tankers, for example, have specially designed stern fittings to accept the high towline loads of large and powerful escort tugs, such as the Lindsay Foss. The stro ng point and fairlead of the emergen cy towin g arrangements required by SOLAS could be used for securing the escort tug' s towline, pro vided they are suitable for this dual purpose and provided also that su ch use does not in any way compromise the deployment and use of the emergency towing
Photos: Foss Man'time. U.S.A
Figure 9.77 Speciallydesignedtanker stem fittings on theformer ARGO tankers, now Polar tankers
arrangements for their SOLAS purpose (see the above mentioned O CIMF publication). Towlineperformance and use Many towing com p an ies u se to wlines m ad e of
HMPE Spectra or Dyn eema fibres, with pennants of the same mat erial and sometimes with nylon stre tchers, though steel wire towlines with a nylon stre tche r, polyester or polyester/polypropylene towlines are also used for escorting. Towline lengths used for escorting ar e generally 100 to 150 metre s, thou gh smaller towline lengths, e.g. 60 to 80 metr es, are also used at the tug ca ptain's discretion.
Mod ern HMPE fibre lin es are light and easy to handle, important factors not only because of redu ced cr ew numb ers on board sh ips, but b ecause an eme rgency respon se may be need ed wh en n o power is available on the deck of the ship and the crew may have to lift the towline ab oard manually. Another feature of this type of towlin e is that it floats and do es not easily foul propellers. Spectra or Dyneema fibre towlines have a very high br eaking strength but th eir stretch is very low, which should b e taken into account. To minimi se abrasion , towlines and/or towline penn ant s should be protected TUG USE IN PORT 149
against chafing. Fairleads, for example, should be free from rust, sharp edge s and grooves. On escort tugs, therefore, more and more stainless steel fairleads are used.
There is ongoing resear ch in the field of HMPE fibres and ropes in ord er to increase performance .
Exp erience gained on bo ard tugs is mo st important to understand the factors that play a role in the functioning and lifetim e of towlines, experience that can be used for further impro vem ents. As already said, many escort
tugs use towlines made of Dyneema or Spectra fibres (at the moment mostly Dyneema is used). Wear is largest at that part of the towline that is taken on board the ship. A pennant increases the main towline's lifetime .
(An active working ten sion control of the towing winch may cause additional wear on the tug's part of the main towline, from the winch through the tug's fairlead.] The pennant, often made of the sam e mat erial as the main towline and of the same and sometimes larger breaking strength, is either cow-hitched or spliced eye-througheye to the main towlin e. This is the case when a single p ennant is used. Other systems are also used, for instance a grommet as pennant, which makes it possible to distribute the wear over the whole pennant. ( see also paragraph 7.5.2 and 7.5.3).
gauge whe n a rope should be removed from service (see References for the report 'Residual Str ength Testing of Dyneema, Fibre Tuglines'). For that visual inspections and br eak tests were performed on towlin es used on the escort tugs of Crowl ey Marine Services. The towlines were 12-strand ropes mad e from Dyn eem a SK 75 fibre. Pen nants on b oard the Crowley tugs are gen er ally used for one year and the main towlines for two years. After one year the main towlin es are reversed , 'end -for-end'.
The average number of jobs carried out with the main towline was around 1200 an d with the pennants around six hundred . What can be learn ed from this study, based on tests with the one year used pennants and the two year used main towlines, can be found in th e conclusions of the study : The ends of the towline , up to 65m from the end, had on averag e a strength retention of 61% of the original strength, which me ans a loss in stre ngth of almost 40%. Strength retenti on of the mid section of th e towline was high er (81%), while the pennants had on average a strength retention of 63%. Abrasion, compression and line twi sts resulte d in the
Towlines made of HMPE fibres are high performance ropes , perfect ropes for the high towline forces that can be generated by escort tugs. Two aspects require attention, viz. th e low stretch and the strength reduction of the towline during a certain period of use or after a certain number of jobs. The latter applies, of course, to other types of towlines as well.
If short towlines of low stretch are used, it easily results in high peak forces due to the low dynamic load absorption of the towline. This effect is less for escort tugs operating in port approaches. Th ese tugs normally
total strength reduction of up to 40%. Abrasion and cutting damag e accounted for a strength loss of 5-10% Compression from the drum accounted for a strength loss of 12%. Line twists of one to one and a half turns p er foot equated to a 15-20% strength reduction. Shock loading seem s to hav e no effect on th e residual strength of the towline, if due diligen ce is exe rcised in tug handling.
with a load reducing system. However when such a system is not available, and when operating with short
Important findings ar e that shoc k loading seem s to have no effect on the residual stre ngth of th e HMPE towline, provided the tug is handled in a co ntro lled manner. The' study results show how imp ortant goo d
towlines, as is the case in harbour areas, the low stretch
towline care is. Abrasion, cutting damage and lin e twists
. may present a problem. A pennant with more stretch, • e.g. of nylon or poly ester, or a stretcher, can then be used, although this reduces ease of towline handling. In all cases much depends on the tug captain's ability in controlling his tug in such a way that peak loads in the towline are as much as po ssible reduced.
should as far as possible be avoided and a twist in th e lin e, if possible, be removed before storing th e lin e on the Winch.
use rather lon g towlines and often have towing win ches
Strength reduction in a towline will take place over a certain period of tim e and/or after a number of jobs . It is important to know the level of strength reduction and the factors that playa role, taKing into account the fact that much depends how towlines are treated on board the tugs. Sam son Rope Technologies and DSM High Performance Fib ers (pro ducer of Dyneema HMPE fibre) carried out a stud y with the objective to develop retirement criteria to be used by a towing company to 150 THE NAUTICAL INSTITUTE
The importance of a proper safety factor is also shown by this study, seeing the rather large average reduction in towline strength of alm ost 40%. Tests of residual strength of other HMPE towlin es and towin g companies show values of strength reduction of 50% (an d higher). DNV, for instance, requires the towing lin e to have a breaking strength of at least 2·2 time s "the maximum mean towing pull" as measured durin g
active escort tests. Assuming a strength redu ction of 50%, such a safety factor can be gradually degraded to 1·1, which means there is hardly any safety margin left. One should be well aware of this fact - a fact that doe s not just apply to towlin es made of HMPE fibre s, but to towlines made of other synthetic fibr es as well.
With respect to this there is another aspect to be take n into account, and tha t is what is regar de d as the breaking strength of a towline. In the United States it is the pr actice to use spliced ro pes to develo p br eaking st re ngths . Br eak in g stre ng ths are r eported approx imately 15% higher when no spliced samples are used . The possibly lower strength of the connection between the pennant and main towline, depending on th e type of connect ion, is also a factor to be taken into account. In fact, the minimum bre aking strength of the total towline should be tak en into account for th e required breaking strength of a tug's towline. Based on wh at has been discussed , the following aspects are important for the condition of towlines on board escort tugs, particularly those mad e of HMPE fibres, and for safety of towing: Logbook. It is recommended to maintain a log of towline usage, noting factors such as the number ofj obs performed, the results of visual inspections and facts that may have influenced the towline's service life.
A program of residual rope strength testing. A program aimed to determine th e effects that influence the service life of a rop e and to develop retirement criteria for when a towline and pennant
should be removed from service. A realistic minimum breaking strength of the towline. The minimum breaking str ength of the towline should clearly be defined, stating whether the strength refer s only to the rope of which the towline is made, or also including the weaker parts such as splices and, if relevant, the pennant-towline connection. An appro priate towline's safety factor. The requi red towline's safety factor to be based on a realistic minimum breaking stre ngth of the towline, taking into account the reduction in rope strength during th e lifetime of the rope. Proper rope car e, which includ es proper tug handling.
ships, making fast is generally don e at a rather high ship's speed. Wh en making fast, the escort tug generally comes close to a ship's stem to pass the towline. This is easier for stern drive than for tractor tugs, since th e latt er
experience the effect of the ship's prop eller wash on their skeg . Sea and swell conditions make it more diffi cult to p ass a towline sa fely or even m ak e it imp ossible. Wh en a towline slips into the water, caused by tug movem ents due to sea and swe ll or unp rofessional line handling on b oard the tug or attend ed ship, it may foul the tug's propellers, making the tug useless. When conditions are such ·t hat it b ecom es difficult to pass a ·towline, a line-throwing gun can be used to pass a heaving line and messenger line, which is possible, for instance, at the approa ch towards the Sture and Mongstad terminals at Norway's west coast an d at San Francisco, U SA. Letting go an escort tug whilst underway can b e carried out at fairly high speeds but it should always be done safely. Wh en the towline is to be rele ased by a ship's crew, the y should he ordered to stand-by and await the tug, whi ch steams up through the ship's wash until almost tou ching th e stem. Wh en the tug is in position and signals the ship's crew to let go, the towline should be lowered gently so that it does not fall into the water and foul the tug's pr opeller. When appro aching a berth and the escort towlin e is be ing released, the ship's crew should be instructed to handle the towline in a similar way. Different sys tems ar e u sed to reduce th e time required for secur ing. Foss Maritime, for example, has developed a special towline connection link for that purpose (see fig 9.18). On b oard their tr actor tugs escorting and assisting tankers in Puget Sound is a special towline connection, the FossTransom Link. This enables a free-sailing escort tug to establish a towline connection in a minimum of time and with a minimum of
manpower when a tanker loses power or steerage. No
Proper rope care has been discussed in paragraph
7.5.2. Finally, furth er study is needed to get a better insight int o the long term performance of towline s made of synthetic fibre s in general , and specifically of escort towlines made of HMPE fibre s, how performance of th ese lin es can b e improved and/or strength loss can be minimized, which also should include minimum safety factors to be applied, based on the specific towline characteristics, towlin e use and strength decrease.
Connecting and disconnecting towlines As escort tugs may keep pace with a ship while not secured, they must be able to secure quickly and efficiently, in order to be able to deliver the required steering and braking forces within the shortest po ssible tiroe. In such situations and for tethered escort of arriving
Phcto: Foss MarUimt, U.S.A
Figure 9.18 The Foss Transom Link TUG USE IN PORT 151
crew mem bers of the tanker are required to sec ure the escort tug and only one m an is required on the tug deck. In addition, this link allows the escort tug to 'm ake-up' to th e escorted vessel on dem and wh ile avoi ding the hazar ds associated with tethered escorts. The link must be seen in connection with a readi ly available towline pennant on board th e tanke r being escorted. Thi s sh ould b e of high tenacity fibre , such as Spectra, hanging over the stern to which a messenge r line is connected. Th e link is a large hook mad e of ex tra stro ng, lightweight, titaniu m alloy. A towline penn an t is spliced onto th e eye of th e ho ok. Th e pennant and the winc h mounted towlin e are connected in such a way that th ey are readily separated. The link is m ounted in a cradle at the tug's transom. When assistanc e is requir ed , the tug man oeuvres ste rn first close to the tanker' s sterri. By pi cking up the me ssenger line the ship's towin g pennant is taken on board th e tug by just one ma n. T he pennant's eye is insert ed into th e jaw of the h ook , which is equippe d with a sp ring load ed gate to hold th e pennant in pl ace. The tug th en moves away from th e tanker and the link connec ting the sh ip's pennant and tug' s towline is pulled from th e transom-mounted cradle as th e tug pays out th e Spectr a towline. Tank ers calling at Prince William Soun d, Alaska, sho uld also have a towing pennant mad e fast at th e stem, ready for use. This em ergency hawser should be a nine inch Spectr a line (or equivalen t) at least 300 feet lon g. Att ached to this Spect ra towing pennan t, a suitable floating m essenger line sh ould be connected and be re ad y so th at it can be depl oyed rapidly to th e escorting tug. This arr angem ent should allow the tug to approach the tanker' s stem , take up the messenger, pull the towing pennant free and secure withou t requirin g any assistanc e from the tanker crew. Simil ar systems may ex ist in other esco rt areas, though wh ether such systems are utilised or requir ed depends on how escorting is carri ed out.
Operating reliability andfail safe Escor t tugs oft en operate as a Single u nit over relatively large distances, so operating reliabil ity must be high. If for some reason or othe r a tug expe riences a loss of p ropulsion whil e giving steering assistance, its towing point sho uld be such that hydrodyn am ic forces will turn th e tug safely towards a safe position. Thi s was addressed in Chapter 4 when discussing the towing point of tractor tugs. The same applies to escort tugs of th e ASD/reverse-tract or type. In addition, model tests have sh own th at for azimuth tractor tugs op erating in the transverse arr est mode at high spe eds it is safer to use a far aft towin g po in t to avoid cap sizing when one of th e propulsion units fails and no immediate action is tak en by th e tug captain. When on e propulsion unit fails it should , with a well designed tug, still be possible to give steering assistance. 152 THE NAUTICAL INSTITUTE
Wheth er this will be p ossible in an eme rge ncy, starting from a position be hin d a ship's stern , sh ould b e tested .
Communication and information exchange Good radio co mmunicatio ns between pilots an d tug captains are always necessary an d a goo d inform ation exchange.Wh en p roceeding at speeds up to 10 to 12 knots with an escort tug secure d aft, and often n ot in the p ilot's fi eld of vision , a good and r eli a b le communication system is indispen sabl e. Ships may b e escorted ove r lar ge distances an d may take several h ours. Th is sho uld not affect a tug captain's alertness bu t m ay do so. Regular radi o con tact and information exchange between pilot an d tug captai n is require d, therefore, regar dless of the fact that the information that comes available to the tug captain via his instruments is increasing. Good com m u n ica tio n b etween pilo ts a nd tug captains is of particular imp ortance during failures, for instan ce, when a disabled tanker should be navigated round a bend in th e fairw ay. The escort tug should the n steer th e tan ker through th e b end, continuo usly taking into acc ount the required rate of turn, which includes in creasing, decr easing or stopping the applie d steering forces at the correct m om ent, checking the rate of tu rn by applying steering forces at the other ship 's side , and steering the tanker on a new steady co urse. Thi s requires continuous information exchange between pil ot an d tug captain. Good communication between pilots an d tug m asters does also include clear tug commands, not op en for any misinterpretation . Altho ugh a difficult item , uniformity in basic escort tug co mmands betw een escort ports of one country is need ed , an d p referab ly between escort ports of differ en t coun tries . In th e Vessel Escort & Response Plan for Prince William Sound some standar d tug commands are mentioned . With respect to thi s mu ch promotion work has b een ca rr ie d out b y Cap tain Schisler, pilot/instructor Lon g Beach, U SA, with hi s report "Pro posed Standardized Tug Commands as th ey apply to Assist and Escort Tugs" (see Refer en ces). Essential information regarding tug securing mus t be exchanged between pilot, sh ip captain and tug master pr ior to th e start of th e escort voyage as m enti on ed in th e section ' Escort planning' of th is paragraph. If th e ship h as certain limitation s regarding manoeuvrability, tug securing, mo oring and anchoring equipment , th e ship captain should inform the pil ot, and th e relevant autho rities.
Active and passive escorting..Versatility ofescort tugs Escorting may take place unt eth ered or teth ered. The first is also called passive escorting and the second active escorting. Whether escort tugs are engaged in passive or active escorting depends on factors such as the constriction of the fairway in relation to a ship's dimensions and draft, environm ental conditions and the time needed for securing
- the same factors as whe n normal harbour tugs are used for escorting . A decision on tethered or untether ed escorting should be well judged.
In all these cases it is much safer to fasten the tugs as soon as possible, in orde r to be ready immediately whe n assistance is req uir ed,
In restric ted channels and fairways, onl y a tethered escort provides th e possibility of avoiding a grounding or collision . When active escorting an d just following the ship in line, the escort tug should not interfere with pilot man oeuvr es.
When escorting in passive m ode, tugs sh ou ld keep pace with a ship at clo se distance, positioned abeam, slightly forw ard or aft of th e esco rted tan ke r. A good position can b e ab out four p oints on th e bow and ap proximately two cab les off. In this position tugs provide an ad dit io nal lookout, for sm all craft for instance. When required, the tug can be secured m ore quickly to th e escorted tanker than if it had to overt ake from a position astern. However, the best tug po sition during passive escorting is be st ar ranged locall y.
Some port ap proaches are subdivided into areas for pas sive and for act ive esco rting. Alth ough escort tugs are built for th e sea con ditio ns prevail ing in the esco rt area, the choice between active or p assive esco rting also depend s on the swe ll and sea condi tions. These may b e such that it is h ardly possible to pass a towline safely or to p ro vide any useful ass ista nce in th e case of an em ergen cy. Visib ility can also be a limiting factor for safe esco rting. Some ports and terminals give a visibility of I mil e as the lower limit for escorting but it also dep ends on sh ip size and constriction s in th e fairway. Som e oil p orts wh er e escorting is applied have the same tankers callin g at the port regul arly, well equipped for escorting and famili ar with the escort procedures. Other oil ports and terminals may b e visited by all kinds of tankers, tanker s with a captain and crew with no ex pe rie nce in escorting an d with out an y sp e ci al equipment for fast and reliable towlin e sec uring . In addition, the escort tugs may not have the light m od ern fibre tow lin es. Extra manpower on b oard th e esco rte d vessel is then required for towlin e handling, which often pr esents a problem due to the reduced manning.
Pro vided that an escort tug can be made fast imm ediately at a ship's stern if required in the ev ent of a failure on board an escorted ve ssel, the passive mo de enhances the op por tunity to provide ot her u seful assistance , such as pushing at the fore or aft shoulde r or picking up or p assing a towlin e at the bow. This may be requ ired to keep a shi p free from a dangerous area when it loses spe ed after an engi ne failure and starts drifting due to currents or wind. Escort tugs should b e d esigned and equipped, the refor e, in such a way that they ca n safely and efficiently provid e assistance in different ways, whi ch also places great demands on fendering and on static ballard pull. ASD·tugs h av e th e advantage that th ey can also effective ly tow on a line at rather higher spee ds when using their after to wing winch .
Photo: TrITt Oftnws. U.s.A
Figure 9.79 Twoescort tugs of towingcompany Foss Maritime keeping pace with a ship TUG USE IN PORT 153
A tethered tug is limited in its operations. It is not without reason, therefore, that the USA federal rules on escorting tankers in Puget Sound and Prince William Sound require at least two tugs so as to improve the possibility of rendering useful assistance in case of an emergency This even when purpose built escort tugs are used. The same kind of rule can be found in the port of Sullom Voe, UK. If an escort tug is used as the primary tug, the second tug could be a normal conventional harbour tug. Escort tugs, except for the very large ones, are also used for berthing/unberthing operations. When used for berthinglunberthing they sometimes have a specific towline for escorting (Spectra/Dyneema) and another for berthing operations. Escort planning Escorting should be well planned in consultation with the pilot and tug captain(s) and, if possible, with the ship's master. Escort plans should include the following: Dimensions, draft and manoeuvring particulars of the tanker. Destination, transit route, passage times, planned escort speeds, emergency anchorages.
Shipping traffic and hazards. Environmental conditions likely to be encountered. Size, type and bollard pull of escort(ing) tug(s) and method of escorting; when there is no tethered escort, the required position of tugs relative to the vessel. The maximum towline forces the escort tug is
capable to generate at the escort speeds. The SWL (safe working load) of the fairlead, bollard and/or strong point on board the ship to be used for escorting. The escort tug rendezvous position. Communication equipment and channels. Requirements regarding towing equipment and towline handling. The ship's master should be informed in good time about the escort plan. In compulsory escort areas of the USA a pre-escort conference is mandatory, covering subjects as mentioned above. A standard pre-escort checklist, adjusted to a specific situation, is an effective tool for that purpose. Escort tug standardisation After years of research, development and practical experience some standardisation of indicating escort tug performance would be useful. An optional class notation for the independent rating of escort tugs was launched inJanuary 1996by the ClassificationSociety, Det Norske Veritas (DNV). The class notation expresses tug performance in terms of the maximum continuous steering force the tug is capable of providing to a vessel proceeding ata given forward speed. The DNV class notation and requirements, which apply to hull design, 154 THE NAUTICAL INSTITUTE
Photo:}. M. Voith, GmbH
Figure 9.20 Large VS escort tug 'Garth Foss' (l.o.a. 47·2m, beam 14·Om, draught 6· 1m, bollardpuU80 tons)
towing winch, towline strength, fail safe and full scale testing, are mentioned in Appendix 3. At present the American Bureau of Shipping (ABS) is the only other classification society having a specific class notation for escort tugs. In the USA The Glosten Associates, Inc. cooperated with a number of industry representatives in developing an American Society for Testing and Materials(ASTM) standard for escort tugs, the Standard Guide for Escort Vessel Evaluation and Selection (first edition published November 1998; see References). The purpose of this standard is to facilitate a common understanding and approach to the evaluation and selection of escort tug(s) to match a ship's manoeuvring and stopping requirements within the navigational constraints of a particular fairway. Much of the proposed standard is a description of acceptable methods for computer simulation which can be used for escort evaluation and tug selection. The useful guide describes in detail the whole process how to come up with a ship or waterway specific escort plan. All factors of influence are addressed, fairway specific as well as ship and tug related aspects. Presented are methodologies to determine escort tug capability for a certain fairway and/or ship(s), by full scale trials and computer simulations, as well as methodologies for escort tug selection. 9.5.2 Escort tugs in use The table in figure 9.21 gives a selection of tugs in use for escorting. Although it does not include alI such tugs and all escort areas, it gives a good idea of the types of tugs, dimensions and power, and the escort areas where they are used. The types of tugs are ASD/reverse-tractor tugs as well as VS tugs and the bollard pull ranges between approximately 40 and 140 tons.
Name
Type
Escort area (1)
BHP and BP (2)
Dimensions (3)
Owner
Nanuq, Tan'erliq
VS
Prince William Sound, Alaska
10,192 / 95 t
46·6 x 14.6 x 6·6
Crowley Marine Services, USA
1999
10,192 / 136 t 5,400 /55 t
42·7 x 12.8 x 4·9 36·6 x 12.6 x 5.2
Crowley Marine Services, USA
Crowley Marine Services, USA
2000 1997
LOOP (Louisiana Offshore Oil Por t), USA
7,300 / ± 75 t
47·3 x 15.7 x 5·2
Edison Ch oucst Offshore Inc.,
1992
Strait ofJuan de Fuca / PugetSound
8,000 /8 0 t 5,400 / 55 E
Al ert, Aware, Protec tor class tugs
Atte ntive (PRTs)
ASD VS
Loop Responder
VS
Lindsey Foss; Garth Foss Protector class tug(s)
Respon se
VS VS VS
Year built
U SA Foss Maritime Com pany, USA Crow ley M arine Services, USA Crowley Marine Services, US A
1993, 1994 1997
7,200 /68 t
47·2 x 14·0 x 6·1 36·6 x 12·6 x 5·2 39·5 x 14·0 33·5 x 12·2 x 5·0
Seabulk Towing, U SA
1995, 1996 1995
2002
Hawk, Eagle It
ASD
Tampa, Florid a, U SA
6,700/ 77 t
Broward
Z-drive tractor
Port Everglades, USA
4,300 / 53 t
30·5 x 12·2 x 5·8
Seabulk Towing, USA
Harbor class tugs Marshall Foss
VS ASD ASD
Los Angeles / Long Beach, USA
4,800 /4 9 t
Crowley Marine Services, USA
1998
6,250 / 76-68 t 4,400 / 59 t
32·2 x 11·0 x 4·6 29·9 x 12·2 x 5·1 32·9 x 11·2 x 4.9
Foss Maritime, USA Harley Marine Serv ice s, USA
2002 2001
Re v. tractor
San Francisco, USA
6,400 /76- 71 t 4,400 / 61 t 4,000 / 50 t
29·9 x 12·2 x 5·0 32·9 x 11·2 x 4·6 32·6 x 11·6 x 4·4
AmNav, USA
2001
Baydelta Maritim e, USA Foss Maritime , USA
1998 1982
4,00 4 / 50 t
30·8 x 11·1 x 4·8 30·8 x 11·1 x 4-8
Atlantic Towing, St.J ohn, Canada
Millennium Dawn
Lynn Marie Delta Linda Andrew Foss Atlantic Will ow
Atlantic Larch Atlantic Oak
ASD VS Rev. tractor Rev. tractor
Port Hawk esbury , No va Scotia, Canad a
ASD
4,004 / 50 t 5,000 / 62-60 t
30·8 x 11·1 x 4·8
1997
2000 2002
Plac entia Hope , Pla centia Pride
VS
Placentia Bay, Newfoundland
5,600 / 55-50 t
38·0 x 13·0 x 6·0
Newfoundland Transhipment, Canada
1997
Ajax
VS
Sture, Norway
10,400 /93 t
41·6 x 15·9 x 6·8
0stesj0 Rederi AlS, Norway
2000
Bess
Statoil Mongstad, Norway
5,400 /57 t 5,168/ 52 t 6,800 / 65 t
36·3 x 12·3 x 5·2 35·2 x 12·4 x 5·3 38·9 x 13·7 x 5·5
Bukser og Bergtng, No rway
Bo x er
VS VS VS
1994/95 1997 1998
Cramond, Dalmeny Hopetoun
ASD ASD
BP HOWld Point Terminal, Grangernouth, Scotland, U K
4,800 / 60-55 t 9,600 / 125- 110 t
34·4 x 10·5 x 4·6 43·5 x 13·5 x 6·0
BP Exploration, UK
1994 1996
Silex, Thr ax
ASD
Fawley Esse, U K
5,000 / 60-57 t
35· 1 x 10·8 x 5·0
Solent Towage Ltd. , U K
1994
Redbridge Lyndhurst
VS VS
BP Terminal , South ampton, U K
4,100 /43 t 4,000 /43 t
33·0 x 11·2 x 4·8 30·0 x 11 ·0 x 5·3
Ad stearn Towage, UK
1995 1996
C!
Tystie, Dunter
VS
Sullom Voe, Sh etlands, U K
5,400 / 55-51 t
37·6 x 13·4 x 5·7
She tland Towage Ltd., UK
1996
C
Anglegarth, Millgart
ASD
Milford Haven , UK
5,100/66-60 t
32·7 x 11·8 x 5·1
Wijsmuller Marine, U K
1996 /97
m
Sertosa Veintisicte
VS
La Co ruii a, Spain
3,800 /4 2 t
29·5 x 11·0 x 5·0
Sertosa, Spain
1993
Ukko, Ahti
ASD
Refin eri es, so uth coast Finland
6,700 / ± 70 t
33·5 x 12·8
Fortum O il & C as O y, Finland
2002
Bob
o
en
Z o ~
"'0
~
01 01
Figure 9.21 A selection. oJescorti-ing) tugs at different ports. Situation 2002 (7) Not all escort ports are mentionedl whileinsome a/theports mentioned more escort(~ing) tugs operate than indicated (2) Whentwofigures are given for the bollard pul~ thesecondfigure gives ballardpull on astern (3) Length, beam and draught in metres
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OUTBOARD PROFILE
CrJurUJy: 0sttnrj. A/S. Norwrry
Figure 9.22 VS escort tug :4jax'. Lo.a. 41·6m, beam 15'9m, opeTational draft 6·8m,engine power 10,400 hhp, hollardpuU93 tons, steeringforce at 10knots ISS tons, hrakingfora at 10knots 180 rons,free sailing speed IS knots, towingwindt 200 tons puU, 300 tons brake
It is not easy to give a sharp definition of an escort tug, seeing all th e different tugs u sed fo r escor t ope rations. Basically an escor t tug is a tug specifically built for escorting of ships, in particula r tankers, at relatively high speeds. However, harb our tugs, often with enh anced escort capabilities, are also often used for escorting, in ad ditio n to th eir normal harb ou r operation duties. Both types are therefore shown in the list, and it is for that reason that the term escort(-ing) tugs is us ed for th e li st. It dep ends tot ally on th e re quirements of a port wha t kind of tug is used for escorting .
9.5.3 Training and pilotage Escorting has been introduced to reduce the risk of pollution ar ising from failures on h oard a tanker. Expensive escort tugs are dep loyed as a safeguard, sometimes over large distances. The full advantages of escort tugs can only be achieved DY proper training of all pe rsons directly involved . With escorti ng spee ds up to 12 kn ots the huma n element becomes extreme ly im p ortan t. Thi s means that training sho ul d be an essentia l p art of learning an d building up escort experience. Train ing shou ld naturally includ e practical on the job training for tug captains an d crew, but also some theoretical traini ng. This should include training 156 THE NAUTICAL INSTITUTE
of tug cap tains and crew, pilots and possib ly also ship's masters, in :
Escort procedures and communications.
Escort speeds. Ship 's possible behaviour after a failur e. Capabilities and limitations of escort tugs, vario us assist manoeuvres, including the most effective way
of app lying steering an d braking forces in case of an engine or rudder failure on board the escorted vessel. Towing equipment and towline handling. Suc h traini ng can be give n based o n videos of escorting. Desktop com puter sim ulation of emergency situations (see section 8.3.3) can be a significant tool for escort education and eme rgency preparedness. A large number of failur e scenarios can b e simulated under various circumstances and environmental conditions and the effect of tug interventions can be compared. The utmost importance of a quick response to a failure ca n also b e shown . For getting insigh t into the capabilities of a certain escort tug, a program can be used as shown in figur e 8.2. For esco rt tr aining of pilots, tu g cap tains and ship mas ters, use can also be made of a full mission simulator, pro vided such a simulator can b e m ade
suitable for training with escort tugs. With a simulated escort tug and assisted shi p, procedures and failure scenarios for the most critical lo cati ons under different environme ntal conditions can be exercised. Different assist manoeuvres can be trained for, as well as changing from one assist m ode to another, for instance, from
bra king m ode to the indirect stee ring mode, or steering a ship havin g a rud der failu re through a bend in a controlled mann er. Pilots are one of the essential links in escorting. For p orts acco mmodating ot her shipping, or with a large number of pil ots, it is rec ommended th at a lim ited number of pilo ts ar e selected and used for escort jobs, a so-calle d choice pilot system. Training can th en be intensified and the system in creases the experience of th ese pilots in a quicker way. In very sen sitive areas or for large tankers a second pilot might even be required.
The othe r essential human link is the tug captain and his crew. In case of a failure much dep ends on how fast a tug capta in can react and bring his tug in the correc t po sition to apply the steering and!or br aking forces requ ired . A high level of expe rience in handling his tug is of utmost importance, particularly regarding all th e possible tug mal)oeuvres that might be required to control a disabled vessel in the most effective way. The high er the escort speed and the more adverse the co nditio ns, the more important be come s a tug captain's ex perie nce.
It may never happen th at a pilot and escor t tug cap tain h as to come into action due to a failure on board
an escorted vessel. Nevertheless expe rience should be maintained at a high level. Regular training and
,
1
.~
-
J.,
Aquamesu r-Rasma Ltd.
Figure 9.23 PowerfUl ASD escort lug 'Hawk' (1.0.0. 33-5m, beam 12·2m, ballardpull 75 Ions)
The im portance of a well-designed purpose built escort tug in combination with a high level of experience has furtherm ore b een proven by risk assessment analyses carried out by DNV for several oil terminals. Th ese studies show tha t a purpose built escor t tug with ap pr o p r ia te m anning r edu ce s th e ris k p icture significantly, while an escort tug not properly equippe d or man ned incr eases the risk dramatically.
Note: When normal harbour tugs are used for escorting, instead of specific escort tugs, the same typ e
of training can be utilised . In addition to the pr eviously menti oned training subjects, the most appropria te tug placem ent can be exercised, if nee ded, an d the effect of sh ip's speed on tug efforts afte r a failure ca n b e demonstrated. 9.5.4 Summary of escort tug re quirements
instruction is a n ece ssity, the refo re, making use of
exp eriences alread y gained. The best way such training and instruction can be performed is: A regu lar refresher co urse on a full mission simulator
for pilots and tug captains, togeth er with, if possible, ship masters. Real life exerci ses with a tanker and escort tug. This could, for instanc e, be done with an in coming tanker if time and circumstances allow, and the ship maste r agrees. How important training is can best be illustrated by the following conclu sions made by classification society DNV. Since 1990 DNV has attend ed several full scale te sts an d als o is su ed escort rating ce rtificates on a number of occasio ns .
DNV found that the most important observationduring the full scale escort tests was that 'practice makes pofec:'; and that this could no t be emphasised enough. Tugs undertaking escort operations as witnessed should be purpose built and the crews need to have ample training;
Optimal manoeuvrability and high free sailing speed. High working reliability. Good sea keepin g conditions, free sailing as well as in the escort operating direction. Sufficiently high freeb oard. Good static and dynamic stability. A safe working deck for handling of towlines in rough sea conditions and at high speeds. Ability to app ly high steering and/or braking forces over the whole escort speed range and capable of assisting in different ways. A safe and effective location of the towing point with respect to heeling angle, achievable towline forces and tug engine failure. Deck equipment construction should be suitable for escort ope ra tions and b e such that it can easily withstand the high towline forces. Towlines should have a high safety factor and preferably be made of light and stro ng synthetic fibres with a p ositive buoyancy to enable safe, fast and easy handling. In case th e ship requiring assistance has no power available at the mooring stations fore and aft, it should TUG USE IN PORT 157
be possible that th e towlines can be passed manually. Good fendering, p referably all round. Go od all round visibility from the wh eelh ouse and of the towing winch. A htghly reliabl e radio communication system. O pe ni ngs in supe rs tructures , d eckhou ses an d ex pose d machin ery casings situated on the weathe r deck, which provide access to spaces below that deck, sho uld be fitted with waterti ght doors. Th ese doors should be kept closed during escort op erations . Firefighting and pollution control tasks include additional specific requirem ents.
9.6
Escort tug regulations
A selection of vario us escort regulations in force in the USA , Canada and Europe ar e sum marised h ere, starting with tho se in th e U n ited States of Am erica, where fed eral, state and local regulations are in force in a number of compulsory escort areas . Federal regulations override state and local regulations . In these regulations the term 'escort vessels' is often used, which can be normal harbour tugs used for escorting or specifically designed escort tugs. The rules and regulations described here reflect the 2002 situation and are subjec t to change du e to new developments and insights. The Oil Polluti on Act of 1990 (OPA 90) rule about escort vessels has the legislative intent of enhancing tanker navigation safety. Under Title IV (p reven tion and removal) of OPA 90, single h ull tankers of 5, 0 00 gross tons or over transporting oil in bulk in defined ar eas of Prince William Sound (State of Al aska) and Puget Sound (State of Washington) must be escorte d by at least two escort vessels with specific performance capabilities. Double hull tankers are not required to have tug escorts in these waters. The Prince William Sound and Puget Sound Federal Tanker Escort Regulations (Code of Federal Regulations ; 33 CFR 168, mandated by OPA and eff ective 17 November 1994 ) require certain performance and operational capabilities . Escort vessels must be po sitioned near the tanker such that timely response to a propulsion or steering failure can be effected. Tankers sho uld n ot exceed a speed beyond which the escort vessels can reasonably be expected safely to bring the tanker under control within th e navigational limits of the fairway. The escort vessels, acting Singly or jointly in any combination as needed (ba t not less than two escort vessels), and consid ering the appli ed force vectors on the tanker's hull, mu st me et minimum requirements to tow, stop, hold and tum a disabled tanker: Tow a tanker at four knots in calm conditions, and hold it in a steady po sition against a 45 knot h eadwind. 158 THE NAUTICAL INSTITUTE
Stop a tanker with in the same distan ce th at it could crash-stop itself from a speed of six kn ots using its own propulsion (temporarily suspen de d). Hold a tanker on a steady course against a 35 0 locked rudder at a speed of six kn ots. Tum a tan ker through 90 0 , assuming a free-swinging rudder an d a speed of six kn ots, with in th e same distan ce (advance and transfer) that it could turn itself with a hard -over ru dder. Alaska State Law requires all load ed taukers, single or double hull, to be escorted by escort tugs. There is, furth ermore, a requirement for oil spill re sponse equipme nt along the tanker route through th e Prince William Sound. This equipme nt is provid ed by, am on gst others, th e following escort tugs, the 10,192 hp VS tugs Nanuq, Tan'erlig and the 10.192 hp ASD-tu gs Alert, Aware and Attentive, being fitted with skimming and onboard storage capabilities for an initial oil spill recovery. Summarised, the following are required by th e U.S. Coas t Guard Captain of the Port for all tankers passing through Prince William Sound (PWS) regarding escorting as mentioned in the Vessel Escort & Resp on se Plan (VERP) and based on the use of available tugs ranging in size from approximately 6,000 - 10,000 hp as mentioned in the Charter Escort Vessel Fleet list in the VERP: A minimum of two escort ve ssel s fo r all loaded tankers from the terminal to sea and vice ve rsa. The primary escort vessel is on e of the 10,192 hp VS or ASD-tugs mentioned abo ve. The se con d esco rt vessel may be an y oth er tug of the Charter Escort Vessel Fleet. Sm all er tugs, th e 5,500 hp VS-tug Protectoror Guard, can be the primary tug for tankers in the 90,000 dwt class or smaller. If th at is th e case, then an Escorting Response Vessel (E RV) will be assigned, ERVs are, as the escort vessels m entioned abo ve, fitted with skimming and onboard storage cap abilities practicable for the initial oil recovery planned for a cleanup operation as identified by the oil spill removal organisation. An ERV will be either part of the escort convoy, or pre-positioned on sentinel duty during transit. The loaded tanker shall not exceed a speed beyond which the escort vessels can reasonably be expe cted to safely bring the tanker under control. Th e maximum allowable speed th rough th e water for loaded tankers is be tween six knots (Valde z Narrows) and 12 kn ots, depending on th e area. When wind in the Valdez Narrows exceeds 40 kn ots, transit is prohibited for all tanker traffic. Outbound loaded tankers will not be allowed to transit Hinchinbrook Entrance when winds exc eed 45 knots or seas exceed 15 feet. Two escort vessels shall maintain close escort within 0·25 nautical miles of a loaded tanker. In Central Prince Willi am Sound, however, the primary escort vessel shall maintain close esc ort , while the second escort vessel may be any vessel of
the Charter Escort Vessel Flee t stationed at an appropriate location underway (so-called sentinel vess els). All loaded tankers shall h ave the primary escort vessel tethe red in the Valdez Narrows and part of the Valdez Arm. The second escort vessel shall then move into a position close astern of the tethered escort vessel. M aximum allowable speed th rough th e water in Valdez Narrows for tankers in ballast is 12 knots; there is no speed limit elsewher e in Prince William Sound. Tankers in ballast are escorted by Sentinel vessels (see above). Different regulations and procedures app ly. to ice co nditions . In the VERP, furthermore, much em phasis is placed on the need to respond immediately to failur es, on th e prope r an d safe use of a teth er ed escort tu g, th e emergency towing equipme nt, and on exercises. The VERP m eets the earlier men tioned federal requirements (Cod e of Federal Regulations 33 CFR Part 168). The State of Washington regu lations on escorting (\ Vashingto n Tanker Law, September 1975) d o not requir e two tugs. The state esco rt rules require esco rt tug(s) to have an installed power equal to 5% of the dead weight of th e escorted tanker, so a 100,000 dwt tan ke r wou ld requir e a 5,000 hp tug as escort (if one tug was used). Moreo ver, according to state rules, escorting is compulsory for loaded oil tankers and gas tankers of more th an 40,000 dwt, except for tankers which meet ce rtain requirem ents, such as twin screws and double bottom s. Since fed eral rules are in force in Puget Sound, tankers of 5,000 gross ton s or over have to be escorted b y two tugs. H ow to comply with State an d Fed eral statutory provisions and performan ce obligations is worked out in the Puget Sound Tank er Escort Plan. The escort plan is to be spe cific to tankers, wate rways and weat he r conditions and suggests a team approach between tanker m aster, pilot and tug captain. The tanker Specific Escort Plan is th e final correlation of waterway and weather d ata wit h critic al tanker d at a for th e purpose of evaluatio n and selecting escort tugs and the coor dination and execution of a successful transit.
and escort speed. The unteth ered escort position for the primary tug is bow first within h alf a ship's length off, in line with the bridge. The State of California's OPA 90 legislative parallel, the Lempert-Keene-Seast rand Bill (SB2040) was passed in May 1993. Escort guidelines are developed by the H arb or Safet y C ommittees of the po rts and after approval mand ated by the Stat e. The regulations in general are the same for all ports, but each port h as specific rules that may differ. As an example the escort regulations for the San Franci sco Bay region will be add ressed briefly. For the San Francisco Bay region th e Office of Spill Prevention and Response of the California State Department of Fish and Game publish ed in · Octobe r 1993 interim escort regulations which took effect II J anuary 1994. T hese wer e amend ed in July 2001.The revised escort regulations became effective 4 • Octo ber 200 1. . T hese state regulations require both single and double hull tankers and barges carrying over 5,000 ton s of oil in bulk when und erway in defined areas of San Francisco, San Pablo and Suison Bays to be escorted. T he regulations d o not apply to double hull tankers whe n equipped wit h full y re d undant steering an d propul sion systems, which shall includ e at least the following: (I) two independ ent propulsion systems, each with a dedicated propeller; and (2) two independent rudde rs with separate steering sy st ems ; and (3) propulsion and steering components in separate spaces; and (4) a bow th ruster with an assigned power source. Regulations are give n for escor t plans and a preescort confere nce. The escort plan can be bas ed on a checklist an d should include matters such as the intended route(s) and speed(s), a communication plan, the escort tugs to be used, the respon se actions most
likely to be implem en ted in case of an emergency, the characteristics of the tanker with respect to the locations and strength of bitts and chocks to be used by escort tugs, pushing su rfaces on the hull , any pe rtinent performance characteristics of steering and propulsion system(s) and related limitations. Requireme nts for escort tugs apply to aspects such as registration, number of crew members, working
T he esco rt tu gs are selec ted fro m a fleet of conventional tugs and VS tr actor tugs, ranging in size from 3000 to 8000 hp . The 8000 hp tugs are th e large VS escort tugs Lindsay Foss and Garth Foss. As an indication of escort p ractice _the "ARC O Escort Plan Quick Reference Guide", is used. T he following is a summary:
hours, training, braking force verification, stability and equipment. The latt er should in clude a line throwi ng gun, winches, towline (with a breaking strength of two an d a half times th e cer tified braking force of the escort tug), a qui ck release d evice and appropriate fendering. Tanke rs should h ave chocks and bitts that are of sufficient size, stre ngth and nu mber for the escort tugs.
The size of the primary tu g depends on tanker size: The 8000 hp tug for tankers of9 0,000 dwt an d more and the 4000 hp VS tractor tug for smaller tank ers. Escort spee d depends on the zone. Wh eth er tethered or un tethered depends on zone
Whil e engaged in escort activity escort tugs should maintain a station keeping distan ce of no more th an 1000 feet ahead or aside, or 500 feet astern of the tank er. Depending on the zone tanker spee d should no t be in excess of eight or 10knots, howeve r, the speed or speeds TUG USE IN PORT 159
selected for transit must permit stationing the escort tug(s) to allow them effectively to influence the tanker's movement in event of a casualty. In contrast to the federal regulations , a single escort tug may be used for compliance with the California State Regulations so long as the boll ard pull (bollard pull ahead or astern - for tractor tugs and bollard pull astern for conventional tugs) meet th e criteria as given by the
tanker-escort tug(s) matching criteria. The maximum number of escort tugs to be used is three.
The required braking force depends on ship's displacement, the assisting current velocity and the zone. Required forces are given in a Default Matrix Option for Matching Tugs to Tankers.
~urrent
I
'-=::.E'-:~ Sleering force
1
2
Figure 9.24 Cantheescort tugprevent a grounding? Situation 1: A halfloaded tanker experimas an engine and rudder failure. To avoid too much drifting, the escort tugsteers the ,hip toport. Tanker speed will drop due to engine failure andsome bralringfirce of the tug. Consequently, the ,hip will driftfaster and the driftangle hasto be fUrther increased. 7'4e remit is constantly twofirces tostarboard - 'teeringforet andwindfirce. The ,hip will most probably drift onto the shoals unless a tugforward is secured intime Situation 2:].,t beftre the loaded tanker has to take a bend, an engineI rudder failure occurs. The escort tugtries tosteer the ,hip through the bend. However, inaddition to the steeringforce, the current is also pushing theship topartand counteracts the tum. Due to the decreasing ship's speed) the ,teering and currentfora, both toport, the tanker will most probably drift onto the shoals. When the underlceel clearance issmall; the ,hip will tum with even more diJficulty andthe influerue ofthe current will be much larger, resulting in a higher riskofgrounding in the case ofafailure 160 THE NAUTICAL INSTITUTE
Escorting is utilised in some othe r USA areas, for example at the Louisiana Offsho re O il Port (LO O P). In Canada loaded oil tankers while transiting to and from terminals in Placentia Bay, Newfoundland, are required to be escorted by an escort tug. The Tanker Escort Plan , as prepar ed b y Canship Ugland Limited, contains guidelin es and procedures for pilots, tanker masters and tug captains. Thr ee esco rt mod es are menti on ed in the plan : active mode (tethered), close passive escort (th e VS escort tug has then to follow the tanker stem first and close to the tanker's stern and in view of the tanke r's bridge team ) an d passive escort (esco rt tug to be positioned abeam and forward of th e tanker's bridge approximately 0·25 nautical miles from th e tanker and in view of the bridge team). De pending on the escort area, the passive, close p assiv e or active esco rt m od e has to b e utili sed . Maximum escort speed for the active esco rt mo de is eight knots , except for tankers of 160,000 dwt or less, when outbound, for which the maximum speed is 10 knots. A pre-escort conference between tanker master, pilot and tug captain is mandatory. Confere nce subjects are me ntioned in the plan.
In Europe, escort regu lati ons a re mainly local terminal regulations, agreed be tween port authority, p ilots and tug owners, except for Norway whe re escorting of tan kers is mandated by the gove rn me nt. Th er e are not yet any regulations regard ing th e required bollard pull or h orse power of tugs. This is mo re or less b ased on research. In m ost ports, but not all, escorting is usually carried out by one purpose built tug. A selection of European escort tug regulation s: H ydro, Sture Cru de Oil Terminal, Norway: Escort tugs compulsory for arriving and departing oil tankers exceeding 20,000 GRT. Statoil Terminal, Mongstad, No rway: escorting compulsory for LP G carr iers ove r 5000 m' as well. Esso Terminal, Fawley, Southampton, UK: Escorting of inbound an d outbo un d oil tank ers above 60,000 dwt. BP Terminal, Southampton, U K: all ships exporting crude oil. Visibility sh ould be n ot less than one mile. Port of Sullom Voe, UK. The following regulation s are in force for the m ain tanker route: All inbound cru d e oil and gas tankers shal l be attended by two tugs-an d when outbound by at least two tugs. One escort tug shall be secured to the ship's stem ready to apply indirect towing techniques. In escort locations with seve re swell conditions, the escort tug shall be in close attend ance, ready to pass the line at any time should an emergency occur. Th e second tug shall be at such a position that it is able to respond in time ly fashion when required.
Maximum speed in the escort zone for d eparti n g loaded tan kers is eight kno ts. A m arked difference between U S federal rul es for escorting and othe r escort rules in force is that the US federal rules apply to single hull tank ers only, while oth er escort rules, probably all, apply to sing le as well as double hull tankers.
9.7
Concluding remarks
Some remarks should be made regar ding escorting . Accordi ng to a 1993 publication by Shell International Limited, m ost studies recogni se that human error is the immediate cause of at least 80% of sh ipping casualties. Th is figure will not have changed much recentl y and it m eans that, amongst other things, improvem ents can be made by proper training. The need to train captains, mates and pilots, therefore, sh ould be emphasised. Full mission simulators can play a more important rol e in th is than has bee n th e case until now. Pollution cases in port appro aches sho uld be carefully investigated in order to establi sh what cause d the accident. \,·/ hen technical failure s on board tankers are a m ain cause , then further in sight is required into the types of failure and th eir causes. When sim ilar failures are syste matically the cause, modifications in tanker design should be proposed and agreed. Research sho uld be carried out int o whether tankers can be des igne d such that they can operate safely in port and port approaches witho ut the need of escort tugs. Good deve lopments in this field are, amongst others, the 140,000 dwt double hull Endeavour class tankers of Polar Tankers, Inc., with two independent engine rooms, twin propellers, twin ru dders and a 3,000 hp bow thruster, of which th e first one of a series of five came into service inJ uly 2001. The 315,000 doub le hu ll VLCC of the Stena V·Max design, also has two completely separate engine rooms, double rudders and double propellers, of whic h the first of this type came into service October 200 1 an d the rece ntly buil t North Sea double hull shuttle tankers of app rox ima tely 130,000 dwt have redundancy in propulsion and steering (high lift ru dde rs) and bow thrus ters . The same applies to human failures. Good insight into the type an d cause of human failures may permit th e po ssibility of prev entin g suc h failures by, for example, adapting appropriate rules and procedures. Escort tugs now have to compensate for the technical and human failures on board tankers but escorting may not and will not avoid all tanker accidents. This refers particular ly to escorting with one purpose b uilt escort tug . In figur e 9.24 two imaginary but fully no rmal situations are given, where the escort tug most probably will no t prevent a gro un ding. In situatio n 1 of figure 9.24, a grounding could probably be avoided by having
TUG USE IN PORT 161
the escort tug towing at the bow of the ship. These are just some examples . Other situations could be described, but p erhaps readers will quote from thei r own experience When full scale escort trials are carried out in deep water, they give too optimistic a view of escort tug
cap abilities. With a small underke el clearance, often the
162 THE NAUTICAL INSTITUTE
case in port approaches, the situation is far mor e difficult and complicated. The influen ce of currents is much larger, ship's rudder effective ness decreases, and m ore power is needed to turn and stop a vessel. In case of a failure, much mo re effort is required from the esco rt tug to avoid an accident, and hopefully it ca n then deliv er the required for ces.
Chapter TEN
TUG DEVELOPMENTS TuG ASSISTANCE IN PORT AND PORT APPROACHES has
bee n the subject of much resea rch over many years for safer tugs with im p roved capabilities. This has sometimes resulted in tug conce pts which have never been realised. However, the wo rld tu g fleet n owadays ge ne ra lly cons ists of a large nu mber of tugs wi th extens ive capabilities an d d evelopmen ts still continue.
10.1 Special developments in the d esign of tugs Particularly am ongs t harbour tugs with azimuth thrusters there is a continuo us deve lopme nt of ideas. For different re ason s only a very few of these becam e reality and resulted in tugs with real differences from normal tug design s. T hese alternative designs and trend s can more or less b e categorised as follows: Developmen ts in th e number and configuration of azimuth thrusters.
D ev elopm ents based on th e sys te m a tic use of hydrodynamic forces working on a tug hull. Developmen ts in tug power in relation to tug size. Several of th ese alternative designs and one spec ific tre nd in tug design will be addressed below. 10.1.1 Developments in the number and configuration of azimuth thrusters No vel New Tractor Tug Design (19 84) Thi s design has becom e reality. Tugs TP I an d TPII have been built and operate at th e coal port at Rid ley Island, Canada. They have two azimuth prop ellers in line, one forward and one aft. Th e idea of building this typ e of tug was developed after two pl atform s powered by a 3,6 00 hp diesel driving two azimuth thruster s cam e onto the market for sale. The platform s were originally built for the St. Lawrence Seaway Authority to evaluate the principle of using shunters. The original intention was to connect one powered platform, a shunter, to th e stern and one to th e bow to assist ships tra nsiting th e Weiland C an al in Ontari o (see figur e 10.1). The
if the distance between thruster centrelines \....as six time s
th e diam eter or greater, the effect would be less than 10% loss of thrust. The ratio b etween nozzle di am eter and distance between th rusters of the TP I an d TP II is I to 8. In theory this gives a loss of about 6%. So when pu lling or pushing with th rusters in line the bo llard pull is approximately 42 tons. However, in such situations the thrusters are always set at a slight angle. Th e two tugs have been a cos t effective inv estment. Their total cost was less than two thirds of the cost of on e comparable traditi on al tug with azimut h thr usters. Th ey are specific ship do cking tugs and operate for ships arriving at th e coal terminal in the push-pull m od e often parallel to th e ship with a towline from the tug's bow secured to the ship. The tugs operate very successfully at the coal terminal and, according to the owners, can safely handle vessels in the 180,000 to 200,000 dwt range.
The optimum harbo ur tug: The Supertug (1986) Th is is more or less similar to th e previous design . It has an azimuth thruster aft as main propu lsion , and one
for ward as a kind of bo w thruster. A difference from the pr evious design is that th e towing point is locat ed ab ove the main thruster. Th e underlying idea is th at when towing on a line with a ship having spee d through th e water, towing forces can be applied directly against th e towline whil e reducing th e resistance of the tug's hull through the water by steering the tug with the azimu th bow thruster more or less in line "lith the incoming water flow (see figur e 10.3). It is rec ognised that th e high
athwartships forces of the towline will have conse quences for the tug's stability and th erefore a kind of radial hook is sugges ted to reduce heeling angle . There is no indication that this conce pt has ever been realised . Tug Omni 2000 (1994) The Omni 2000 was a prop osed concep t for an omnidir ectional tug with four thrusters. The tug was fully symme trical fore and aft. Th e objective was to propo se a tug with absolutely the lowest costs and omnidirectional propulsion , which could satisfy a par ticular harbou r operation. The concept was not accepted by th e client.
exp eriment was discontinu ed becau se bulk carrier size
in creased an d because of problems with th e locking arrangem ent between sh unter and tran siting vesse l. The shunters were conv erted into the pre sent tugs with the following particulars: length ov erall 30·33m, beam 1O·97m. , en gine 3600 hp (2650 kW ), bollard pull 45 to ns. Bollard pull ah ead, astern and side ways is almost th e same . When thrusters operate in lin e there is loss of thruster efficiency. The only data the designers could find with respect to thi s related to thruster s ope rating in semi-subme rsibles. The data indic ated that
The ROTO R tug (1999) A new conce pt in tugs with azimuth thrusters is th e ROTOR escort tug . Basically it is a normal tractor tug with azimuth thrusters, but the skeg is replac ed by a third azimuth thruster arranged on the tug's centreline. T hree sm all fins are located under the stern to give course stability in transit. Gu ard plates and struts provide protection to the thrusters and when dockin g. Four tugs have been built with this conce pt, the RT Innovation, RT Pioneer, RTSpirit and RTMagicand since TUG USE IN PORT 163
Figure 10.1 Novel new tractor tug design withsketch ofthe original shunters
Figure 10.2 Taiwanese reverse tractor tug 'No 3 Tczo-Yu' (l.o.a. 33·Om, beam 71·7m, drauglu 3·7m, bollardpull4J tons) combines ship handling and oilspill recovery. The cranes can beused to deliver the towline totheship to be assisted, IlYfor oilspillrecovery. In the aJent ofan oilspill. the tugcan beconverted withinminutes into a fUlly self-sufficient oilrecovery vessel s
Rotor escort tug with three azimuth units
I e
.'1'
and forward winch
.muth bow ttvustef
I
Figure 10.3 1M optimum harbour tugconapt
I
I
I
Topview of Rotor (escort) tug with three azimuth units
Figure 10.4 ROTOR Escort Tug concept
164 THE NAUTICAL INSTITUTE
1999 they operate in the ports of Rotterdam, Hamburg and Bremerhaven and for offshore operations. The advantages of the tugs are: Excellent manoeuvrability, which includes amongst others things turning on the spot with a high rate of turn, astern spee d equal to ahea d speed and a sidestepping speed of approxi mately six kno ts. Fast positioning an d re-positioning and a large variety
The main characteristics of th e Kotug R OTO R (Esco rt) Tug are: length overall 31·6m; b eam 12m; draught 5·9m; propulsion 3 x 2,1000 bhp , bollard pull approximately 80 tons . Based on exp erien ce gained, the concept will be modifi ed in such a way tha t the aft thru ster will be located furth er aft, eithe r right below or behind the aft towing poi nt, depending on tug size. This will enhance the performance of the tug (see figure 10.6).
of assist m od es with short respon se times.
A high hollard pull or, alternatively, the same hollard pull with less draft, compared to a normal tractor tug with two azimut h thru sters. High side thrust up to 95% of bollard pull to assist vessels throu gh narrow passages, locks and bri dges. Better reliability because two units brin g sufficient man oeuvr ab ility an d bo llard pull for day to day shiphan dling wo rk; in case of breakdo wn of an engine, the tug is still operational and repairs can be postponed until a suitab le time . There is hardly any risk of damaging the azimuth thrusters on the bulb of an assisted ship as can happe n with stern dr ive tugs, due to the thruster protection. Dy nam ic positioning systems can be installed for offshore operations. Escort wo rk is possible ove r the stern as we ll as over
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Drawing: Koortn Shipbuilding(md Trading, Rouo dam
Figure 70.6 Modified ROIDR tugconcept withaft thruster located more aft, behind tlu aft towing point
the bow at relatively high speeds. Thr ee different ver sion s of this type of tug were proposed: a) Three engines and three azimuth units with a total bollard pull of 80 tons. b) Two engines, two azimuth units and a skeg like a normal tractor tug but designed in such a way that a third engine and azimuth unit can be installed at a later stage. c) Three engines, thr ee azimuth units an d an anchor! towing winch on the foredeck to escort VLCCs. Thi s is the ROTO R Escort Tug (see figure lOA).
Thr ee variants of this ROTOR tug concept will be brought on the market: a tug with a length overall of 23·2m, 25 ·8m and 27·8m, respectively 30, 45 and 60 tons bollard pull. A ROTOR escort tug of 42m length , 10,000 hp and 125 tons bollard pull, possibly with a spe cific pe rformance enhancing device, is in the ph ase
of deve lopme nt. Several advantages have been m entioned, some
additional remarks will be made below. The Rotor Escort Tug can be used for operating at the ship's side, e.g. push-pull , as well as for towing on a line in differen t ways. At speed, performance of the ROTOR escort tug differs principally from a normal tractor tug due to . rep lacing the skeg by a thruster, particularl y at the higher escorting speeds. As a stem tug in indirect mode, no use can be made of the high hy drodynamic forces gene rated .by the skeg. Additional loss of thruster effectiveness will be experienced due to the interaction of the three thrusters. High br aking forces can be achieved in the reverse arre st mode, which is possible at speeds not higher than eight kn ots due to engine overload, while at high er speeds the transverse arrest mode delivers high braking force s (see fig. 9.5 for terminology).
Photo: KOTUG, 11le Netherlands
Figure 70.5 The Rotor Escort Tug 'RT Magic' (I. o.a. 37·6m, beam 12·Om, draugh1S'9m, enginepower 6,300hhp, hp 80 tOTlJ) oftowing rompany KaTUG, Rotterdam, The Nethetlands
Tug handling with thr ee thru sters becomes more co mplicate d, wh ich can give pro bl em s in t en se situations, altho ugh basically the tug is handled like a tracto r tug, while the third thruster is used in addition to enlarge the capabilities. A pro pe r trai ning in thruster and tug handling and in the various specific assist TUG USE IN PORT 165
centreline. In the centreline at each end of the tug a skeg is placed . The main characteristics of the SOM Mark I are as follows: Length over all Maximum beam Draught Engine power Bollard pull
o
Figure 70.7 1Jpical assist modes with a ROIVR tug. 1M tugcan operate within a ship~ beam (depending onthe ship's sin}. A: Tugassist modesforpassing a hridge or entmnga lock. B: Tug assist modes while herthing. Bl: Tug captain can obseroe approar.h speed and distance toberth and can easily anticipate. B2: Tug assist mode during berthing when little berthing space is available (same manoeuvre can be
carried out Qver tug's stern).
man oeuvres that can b e performed is imp ortant. Som e
of these specific assist mod es are show n in figure 10.7. The ROTOR tugs have a 'master pilot' system, but this is seldom used . Ship Docking Module (SDM)(1997) This typ e of harb our tug has been dev elop ed by Hvide Marin e (USA), now Seabulk Towing in Tampa (U SA). Seabulk Towing has three SOMs Mark I and one SOM Mark 11. SOM Mark 11 is a follow-up of the original SOM design with the same dime nsions bu t some higher bollard pull. The first SOM was the New River, delivered in 1997, followed by the St.jahm in 1998, the Escambia in 1999 and the SOM Mark 11 Su wannee . River in 2000. Towing company Marine Towing of • Tampa acquired two SO Ms Mark 11 in 1999, named Tug Florida and E ndeavor.
The or iginal idea was to have a tug with maxim um bollard pull in all direction s, which could get in position qui ckly, stay in an optimum position without using towlin es and whi ch could work in confine d areas and in semi-she ltered waters. The SOMs ope rate in the Port of Tampa and Port Everglades. The tugs ha ve a very wid e beam com pa red to the mor e or less normal length for a harbour tug and are equipped with two azimuth thrusters and two skegs (see figure 10.9 and 10.10). O ne azimuth thruster is located at app roxim ately a quarter of the tug's length from forward and at some distance to starboar d from the tug' s centreline and on e th ruster is located at app ro ximately a quarter of the tug's length from aft and at some distan ce to p ort of th e tug's 166 THE NAUTICAL INSTITUTE
27·43m 15·24m 4·9m 4000 hp 50 tons
Engine power of the M ark II SOM has b een increased to 4,200 hp (54-55 tons bp), while the central towing staple has bee n moved to a mid ships pos ition at equal distance from bow an d stern. The tugs can produce almost full bollard pull in any direction. The tug is highly manoeuvrable. Free sailing spee d is approximately 12·5 kn ots and a sideways speed of 6·5 kn ots can be achieve d. The sides of the tugs are flared in order to provide larger righting moments when h eeling and to pr ev ent contact b etween the tug's underwater part and the ship's hull. The two skegs improve course stability and aid in dry-docking. There is a hole in the skegs to reduce the difference in pressure b etw een b oth si des of th e skegs cause d by the accelerate d water flow into the forward nozzle an d exiting from the aft no zzle. Without these h oles th e tug captains had to correct the tug's track by stee ring the aft thruster five to ten degrees to starboard. The SOMs are pure ha rbour tugs, which is included in the name, and ope rate success fully in the ports of which conditions and circumstances will have played a role in the design. The tugs can operate in certain wave conditions as well. Two men can operate th e tugs. The deckh ouse construction is well withi n the bulwarks, which enables the tug to operate under the flar e and/or ove rhanging stern of ships. Due amo ngst othe r things to the wide beam, stability of the tugs is large and consequently the tugs can operate
Photo: HansHa.fJjJm4n
Figure 70.8 SDM 'New River' ofSeabulk Towing (USA) (l.o.a. 27·43m, beam 7S·24m, draught 4·9m, engine power 4,000 Mp, hollardpull SO tons)
safely. Loss of effectiveness will be the case when one or both thrusters are operating close to the ship's hull, which often will be the case, as the tug generally operates cl ose to th e shi p's hull. With ce rtain tu g ass is t mano euvres, thruster configurations may affect tug effectiveness when pa rt of the wash of a thruster is hitting the nearest skeg and! or the nea rest skeg disturbs the in flow of water towards a thruster.
t
Assist modes utilised by th e SDNls depend on th e towin g company, circumstances in the port, tug ma ster and pilot. Assist modes used are shown in figure 10.1 1, such as the mode gene rally used, and th e assist mod e
®
for close quarter operations , w hen ro om between ships
and piers is limited. High side forces can be appli ed which makes the tug very suitable to work in narrow areas, although when ope rat ing at the ship's side the large b~am can be a disadvan tage whe n passing b ridges, in locks an d dryd o ck s, whe re the availab le width is mostly at a minimu m . The tug co uld then tow on a line, using the centre staple, which enables th e tug to apply sideways forces to th e ship within a sm aller width. The read er is invited to compar e the capabilities of th e TP llII, ROTOR tug, SDM and compact tugs.
Figure 10.71 Assist-modes SDM,. A: General ",rut mode (pulling orpushing). B: Pulling or pushing andmooing tJu ,hipforward or afi. C: Assist modefor close quarter situations
10.1.2 Developments based on systematic use of hydrodynamic forces working on a tug hull
Carrousel tug Th e basic principle of a carrousel tug is a rad ial system. New with the system as applied on the carrouse l tug is th at it is not half a circle, or less, bu t a full circle and has a diameter equal to the tug's beam.
Drawi1l:f:: Marini Towing ofTamjHJIHtJha Mariru
Figure 70.9 Silk oieia of SDMMark II
The radial system itself is not new. It has been applied for decad es on several harb our tugs and in form er tim es on tug s on th e River Rhine. The system has be en discussed in paragraph 7.2 and the advan tages of the system are dealt with in paragr aph 4.2.3. With a radial system tug' s heel due to a tran sverse towline force is limited. Performan ce and safety of several conven tional tugs has so been increase d significan tly. The carrouse l, as the system is called, is initially situated above the lateral centr e of pressure for a crosswise wate r flow. The
adva ntages of the carrouse l are: The tug can safely cope with large towlin e forces generated by the hydrodynamic forces working on the tug hull, while heeling angles are sma ller than without suc h a sys te m. Capsizi ng due to hi gh athwarts hips towline forces is not possible. It enab les the tug to turn freely, in no way restricted b y the towli n e coming in contact wi t h the superstructure.
Figure 70.70 Bow.iew afSDM
The first aspect is related to spe ed. The hi gher th e speed th e higher th e forces th at can safely be generated in the towline and applied to th e ship to be assisted. Also, high braking forces can b e achieve d b ecau se the system enables a stem tug to operate safely broadside behind the ship.
TUG USE IN PORT 167
The second aspect is not related to speed. Ii gre atly enlarges the capabilities of particularly conventional tugs and com bi-tugs, It creates the po ssibility to turn the tug freely with respect to th e direction of the towlin e, for instan ce enabling a stern tug to apply steering assistance to starboard as well as to p ort at a ship having headw ay. The lack of this capa bility is a large disadvantage of conventional tugs (see paragraph 4.3). If necessary for som e reason, the tug can turn 180 0 with the tow line attac he d. The carro usel tug is in the ph ase of development. Although the carrousel system can be applied for present tug typ es and different altern ative tug designs, model tests and full scale tests hav e b een carried out with a combi-tug. Two types of carrousel tugs are proposed .
Model tests Mod el tests ha ve b een carried out with a model of the Dutch combi-tug Multratug 72 (see figure 10.13). Right bel ow th e carrouse l two verti cal skegs were fitted , representing full scale skegs with a length of six metr es and a depth ofO·4m. Each skeg was located .at a quarter of the tug's width from the side. High towline force s were achieved, as shown in figure 10.15. The high steering for ces are of particular int ere st. The maximum forces are limited by the fact that with high towlin e forces either the tug' s stern is subme rged slowly or the turning moments on th e tug could not b e overcome by the tug's propulsion . The first (showing th e importance of a proper buoyancy, freeboard and hull shape) happ ened whe n the tug' s bow had a small angle with th e incoming water flow, whil e the latter happened when the tug's heading was more or less p erpendicular to the dir ection of the incoming water flow.
Full scale tests Full scale tests have been carri ed out with th e tug Multratug 72 fitted with a carrousel and with skegs as used for the model tests. The tests confirmed the working of the system as well as the forces measured during the model tests. Even higher towline forces could b e achi eved due to a more stable po sition of the full scale tug . The photo (figure 10.14) sh ows the carrousel tug applying steering forces. Aspects that require attention orfUrther study As the carr ousel tug is st ill in th e phase of development, some aspects require further attention. A number ofthem will b e m entioned below : The lead of the towline for all possible ships to be assist ed, assist mano euvr e s, con dit io n s and circumstan ces, needs to be con sidered .
Safety of deck operations, including the possibility of efficient and safe towline handling under all working conditions and with-a minimum of crew, require s attention.
168 THE NAUTICAL INSTITUTE
Th e possibility to install an appropriate towing winch, strong enough to withstand the high towlin e forces that can be gene rated, need s furth er study. A towing winch is of p articular imp ortan ce wh en the carro usel tug will also be used for escort op eration s. The carrousel requ ires a co nst ant ten sion in the towlin e, which also requires attention, because for mos t tug manoeuvres even a small constant tension in the towline often may create an unwanted increase
in ship'S speed and a turning moment, which should b e avoided . Carrousel tug designs may require additional model studies regarding the op timum location of the carrousel in rel ation to the location s of centre ' of pressure at
different angles of inflow, focusing on such aspects as the overall beha viour and optimum perform an ce of the carrousel tug whe n towing, while appro priate reserve bu oyancy, freeboard and hull sha pe, and in parti cular safety of operations and safe limits should be studie d as well. Further aspec ts to b e considere d are, amo ngst others, workable heel angles , safe abo rt manoe uvres and performance in wave conditions.
While high steering force s can b e ge nerate d , attention is also ne ed ed to determine whether high an d controllable braking forces can be deliver ed without giving the ship a rate ofturn ifthe latter is not wanted.
Carrousel tug applications Basically the carrousel tug makes effective use of th e hydrodynamic forc es working on a tug hull, which mean s that with increasing speed towlin e forces increase.
When speed de creases the effectiven ess of th e carrousel tug decreases . This is in contrast with the requir em ents
for tug assistanc e in many ports. In harbour operation s tug assistance is gen erally needed at spe eds b elow app roximately six knots. Full tug po wer is th en often need ed for steering, braking and controlling a ship's position.
The carro usel tug is not designed for tug operations at th e ship's side as applied in many ports ar ound the world . For this operating mode tugs with o mnidir ectional propulsion systems and a towing point at the tug's en d are m ost suitable. Ho wev er, the carrousel
system can improve the capab ilities and safety of op erations of harb our tugs to a large extent and particularly of the conventional typ e of harb our tugs and of combi-tugs. As a forward or aft tug, amongst oth ers, high steering forces can safely be handled, while the tug is not restricted by the direction of the towline. Altogether it means that basically th e carro usel tug design can mo st effectively be applied for situa tions where tug assistance is required during a transit, such as in channels, fairways and port approaches, m ore or
less as an escort tug.
The free turning capability is an advantage of the system at low as well as at higher speeds. All pro vided that the system is pr actical and safely applic able. It is not without reason that two version s of the carrousel tug are proposed, the Inner Port Design A (for operations in port areas) with focus on thrust, and the Outer Port Design B (for port approaches) with focus on thrust and highest possible hydrodyn am ic lift forces. Th e location of the carro usel on design A will be above the centre of pressure for highest drag and for design B abo ve centre
".
\Oik n -200~\D\
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Length over all beam draught ekegs propul sion
33 m 11 m 4m sho rt twin skeg twin Cpp (polSibly
steering
FPP) high lift rudders
37 m 10m 4 .5m alon g full length lingle Cpp + azimuth bow thruster steerable nozzles
- 4000 kW =85 ton s
-lOOOkW =80 tons
"'150 tons
=225 tons
engine pow er
ball ard pull
dynamic pull at 10 kn ots
Figure 10.12 Characteristics ofDesign A andDesign B ofthe carrousel tug Multratug 12 Ca rrousel Tug
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Figure 10.15 Towingforces based on model tests with a model of the 21 tons bollardpuU comhi-tug offigure 10.13
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of pressure for highest lift. Design B is shown in figure 10.16. Main characteristics are given in figure 10.12. As said, the carrousel tug is in the development phase and several aspects have still to be studied. The final design may be a combination of the two designs proposed, viz. a more multi-functional tug. 10.1.3 Devel opments in tug power in relation to tug size
Compact tugs Figure 10.14 Modifid- comhi-tug'MuIuatug 12' duriugftll scale trials
The nam e 'compact tugs' is used for small powerful harbour tugs with a length overall ofless than 24 metres. Th ere is, however, not a clear definition of what could TUG USE IN PORT 169
be called a 'compact tug'. Seeing the small powerfull tugs that are built and taking into account the needs for high er ballard pull s, in this paragraph comp act tugs will b e regarded as h arb our tugs with a len gth overall between approximately 20 and 24.metr es and a ball ard pull ahead of 40 tons or more. Such compact tugs are general ly of the ASD/ reversetractor type, mainly reverse-tractor tugs. Compact tugs of the ASD/reverse-tractor tug type will the refore be dealt with, although m any may aspects apply to othe r compact tug types as well. The tugs have a large beam , up to approximately Il ·S m, compared to a relatively small length . Length/ beam rati o vari es between approximately 2·6 and 2·0. Bollard pull can be up to 70 ton s. Th e smallest compact tugs may hav e the large st ballard pull and the smallest length/width ratio . Compact tugs are m or e or less a continuation in the development of harbour tugs, from original ly large tugs with low powered engines and a low manoeuvrability towards smaller tugs with high-powered engines and a high manoeuvrability. Small powerful tugs have been built for more than twenty years, such as severalCates ' tugs (originally twin screw tugs) of towing company C.H. Cates in the Port of Vancouver in Canada. In rec ent tim es interest in compact tugs has grown, also as a result of the low er costs and, if required the possibility to handle the tugs with two men. Most compact tugs are
design ed by naval architects Robert Allan and A.G. McIlwain, while Damen Shipyards in The Netherlands has the compact tug design ASD Tug 2477 (see figure 10.17). The tugs operate successfully in several p orts and can for instance be found in ports in Can ad a, USA, H awaii, Australia and New Zealand. Compact tugs should b e seen as pure h arb our tugs, operating in sheltered waters. Some compact tugs do operate in more exposed waters, e,g. H aw aii, but such operating conditions should be acco unted for in the design, as is the case whe n operating in ice conditions
as with the ice-reinforced com pact tugs in the Port of Montreal. Essenti al aspects of the compact tugs will be reviewed below. Lateral resistance Design of these tugs is such that lateral resistance is as low as po ssible. This means that basically th e tugs have no skegs, or just an op en docking skeg. T he centre oflater al pr essure lies forward of th e midship s. For the reverse-tractor tug op erating modes thi s results in high pushing, pulling and towing capa bilities whe n taking into account normal harbour sp ee ds, and b elow, an d in short response times. For a number of reasons compact tugs m ay have a specific skeg type, for instan ce to improve a tug's course stability when running aste rn or ahead , to increase
p erformance at higher speeds as a stern tug in th e indirect mode, and , in addition, of an ASD com pact tug wh en towing over the stern.
Damen ASD Tug 2411
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Figure 70.17 Damen ASD Tug '2411.' with an open dAJcking skeg, extending asa closed skeg forward (l:o.a. 24·55m, beam 11·49m, draught approxiamtely 4·7Om)
170 THE NAUTICAL INSTITUTE
Stability A s a re sult of, amon gst others , th e wide beam, stability of com pact tugs is large, .which is a n ec essity seeing the high trust and conse que ntly the high towline forces that can b e generated. A large stab ility is also r equir ed for effectively pu shing at a ship havin g spee d. The towing point an d pushing po int of compact tugs is low compare d to man y other ASD /reverse-tractor tugs of the same bollard pull , resulting in relatively smaller heeling moments.
Some compact tugs may h ave an aluminium wh eelhou se to bring th e centre of gravity furth er down and the sides of the tugs can b e flar ed (sponsons) in ord er to pr ovide larger righting moments wh en he eling. G M values of the small high powered tugs are around 3.0 m, amongst othe rs, because of the wid e beam required to accommodate the larg e drive units . The very high thrust, the high rates of turn that can be reached and the high towlin e forces do also require a goo d dynamic stability.
Manoeuvrability and training Th e compact tugs are extremely man oeuvrable with sh ort response times, also as a result of the relativel y low weight of the tugs, and, in particular if thru sters can be turned with high rot ational spee ds. Compact tugs are, mainly due to the large beam, inherently course unstable. However, co urse can be simply maintained because the tugs have a lot of steering power. On the oth er hand , a small steering effort can result in a quick course change, with the ten dency for tug operators then to overcompen sate in reaction. It means that a prop er and thorou gh training is a necessity for this tug type.
JIll /---''1I11 , .' '\. .' /'
!Ill llll
Whil e the powe r and quick response make them more efficient at manoeuvring and assisting ships,
train ing becomes the more important for this tug types because the high power and re sponsiveness cou ld equally cause them to get into difficulties faster if an equipment or operator failure occur s. In addition, a malfunction of the control system or an operator leaves little time for the tug master to ascertain the situation
and tak e appropriate action . The traini ng should therefore inclu de knowledge of possible system failures, how to respond to engine and control system failures and how to handle the tug with one operational azimuth unit only. (See also Refere nces for repo rt 'Waka Kum e') Deckhouse and hull shape The deckhouse construction is low and often sma ll and constructed such that the tug can operate und er the flare an d/or overha nging stern of ships.
T here is an aspect to be aware of regarding the design of compact tugs. When turning, many tugs get a lot of water on the aft deck. This can be dang erous if the tug lacks sufficient reserve buoyancy aft or when deck ope nings are left open. Therefore the shape of th e aft . sectio n of compact tugs is par ticularly important; these must be drawn to generate dynamic lift and ideally deflect water downward when turning (see References for article 'Sm all is beautiful'). Tug operations As said, the tugs ope rate very successfully in several ports. Compact tugs meet th e requirements for a goo d harbour tug (see paragraph 2.2 and Chapter 4), partic ularly for speeds be low approximately six knots, although harbour tug requirements may diffe r by port, as has been discussed in cha pter 1. The Dead Slow Ahead speed of ships is generally six to seven knots. At lower sp eeds, with engi ne stopped (or propeller pitch at zero ), ship'S cont rollability decreases fast and active tug assistance is then required. Du e to the small dimen sions, low later al resistance, high ball ard pull and stability, these highly m anoeuvrable tugs can ope rate safely an d effectively as harbour tugs, with short response times and in restricted harbour
llll "",
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Figure 10.18 Compact tugs. Common assist modes
areas. At speeds higher than approximately six knots, towing on a line as bow tug becomes more difficult for a reverse-tractor tug and effective ness decre ases fast (bow-to-bow operations). An ASD·tug towing over th e stern is then more effective .
In figure 1O.I8 common assist mod es are shown. The ship has a harbour speed up to approximately six knots.
Courtesy:MacktnZie's TugSeroia, Port ofErptranu, Australia
Figure 10.19 Example of a compact tug - 'CapePasley' (I.o.a. 22·7m, beam 10·7m, draught4'6m, engine power 5,000bhp, bollardpuU ahead 67 tons, astern 66 tons)
TUG USE IN PORT 171
Due to the low lateral resistance of the tug, compact tug 1 can push effectively. Experience with such a compact tug shows that the tug can remain pushing square to the ship up to eight knots . For the same reason tugs 2, 3 and 4 can apply pulling or steering forces mo re effectively than with a high underwater resistance. A sm all forward skeg ma y incre ase performance of tugs 2, 3 and 4, and improve safety of operations at the bow. A forward skeg also increases perform ance of tug 4 at higher speeds in the indirect mod e. As already m entioned, at sp eeds higher than approximately six knots p erformance of tug no 3 red uces. Tug 5 is a compact tug of the ASD type, capab le of working over the stern and is mo re effective at such speeds, which can furt her be improved by an appropriate skeg. For berthing, tugs 3 and 4 can easily change over to the push-pull mode. Response times of the tugs for changing positions are very low.
Note: When pulling astern with full power on a very steep towline, which sometimes might be necessary with high ships in narrow areas, the high power of the relatively light tugs will resu lt in a large stern trim.
Vs compact tug
assisting method, although installed power has in gene ral · also increased. Development as indicated refers not only to tugs towing on a line. In ports where tugs operate at th e ship'S side, such as the push-pull or similar assisting methods, tugs with omnidirectional propulsion have often been in use for many years, of which cap abilities h ave been increased as well. Tugs with omnidirectional propulsion systems are gradually replacing a growing number of ordinar y conventional tugs, although conventional tugs will still be built in the coming years, because of their gene rally greater sim p li ci ty in construction , handling o r maintenance, specific performa nce characteristics, or for
other reasons . New developments have been signalled, particularly in the USA and Canada, where a growing number of tugs with azimuth propulsion are entering service or conventional tugs are be ing modified and upgraded by installing an azimuth bow thruster, in mainly single screw tugs , by conversion of single screw tugs into conventional tugs with twin-screw propulsion , or by rep lacing conventional propulsion of twin -scr ew tugs by azimuth thrusters.
A compact tug with VS propulsion has been built for the Port of Napi er, New Zealand. The tug, named Ahu riri. with a new type of VS propulsion units of enhanced efficiency, h as a length over all of 23 .8m, a width of I J.Om, a draft of 5.0 m and a bollard pull of 69 tons . Naval arc h itec ts of Conan Wu & Associates, Singapore, des igned the tug . For the difference in pe rformance between VS tugs and other tug types, see chapter 4 and in particular paragraph 4.3.
With the deve lopment of tugs, tug safety and th e safety of operations have become governing factors. There has been a marked deve lopment in wheelhouse design, focused on ergonomic and efficient installation of control panels and instru ments and on optimum visibility. The wheelhouse of a modern tug provide s the captain with an excellent all-round view and a dir ect view of the tug's fore and aft ends, sid es, working deck and towing equipment.
10.2 D evelopments in g eneral
Deck equ ipm ent too h as evo lved. More tugs are equipp ed with towing winches, h ence improving effective ship handling an d tug safety. Modern synthetic fibres have created th e possibility to make towlines stronger, lighter an d easier to handle. There is also a growing tendency to install pollution control equipment on board tugs operating in ports and port approaches (see figure 10.2). In the 1980s an aluminium tug was built in Western Australia. The idea has proved very successful and anothe r aluminium tug has been built. Alum inium may become a material more often used for tugs because of its low maintenance and longevity. The lower weight can be compensated for by more ballast. Wheelhouses are sometimes made of aluminium to bring the centre of gravity dow n, so increasing tug' s stability.
The re have been four areas of real development during the last few decades: in normal harbour tugs, escort tugs, research and tug simulation. With h arb our tugs th er e has been a steady develop me nt towar ds high er capabilities providing safer and more efficient shiphandling. Harbour tugs have developed from being slender and low-powered to wide beam, high-p owered uni ts. Slender harbour tugs were conventional tugs and much u se was made of the hu ll form to ge ne rate hydrodynamic forces for ship assistance. In m or e recent years, often together with the increased use of azimuth thrusters, tugs have be en built with much higher power and, as a consequence, with larger be ams . More use is made of the tug's p ower than of the tug's hull, although optimum hull form , which includes differen t skeg types, re mains subject of contin uous research to imp rove a harb our tug's pe rformance. With VS tractor tugs the hydrodynamic forces, particularly those gene rated by the skeg, remai n essential over th e years, particularly for the indirect 172 THE NAUTICAL INSTITUTE
Finally, a trend can be seen towards a sm aller crew on board tugs. Several tugs have been built for a twoman crew, resulting in tugs with a high level of automation, efficient whee lhouses and easy to handle with respect to steering, engine and winch control, and towlines.
Requirem ents for escor ting have resulted in studies to find the most su itable tugs and method s of tug assistance. Research has conce ntrated on the capabilities and limitations of all typ es of tug, required ballard pull, effective tug placement, hull form, towlines, propulsion
simu lation enabling tug captai ns to handle their own tugs in in ter acti on , closely refl ecting a real world situa tion and so improving the training of pilots and tug captains and research into p ort developm en ts.
syste ms, escort speeds and so on, in order to be able to
Virtual reality (V R) technol ogy is playing an
optimise escort ope ratio ns. In particular, the need for escorting at high speeds has increased the need for model tests an d in-depth research on tugs and tug assistance, not only because of the high risk involved with tugs ope rating in indirect mo de at high speeds.
increasing role in maritime simulation. It may also be well suited to simulating interactive tugs, as tug captains len d to wo rk alone in the wheelhouse, co mmunications
Hull form has becom e imp ortant in order to gen erate high lift and steering for ces in indirect towing mode and stability has become extremely important because of high towline forces. The original tractor tug design has been modified so as to provide a better view for the captain and to improv e seawo rthines s for escort operations. It has all resulted in the construction of powerful seaworthy tugs with omnidirectional propulsion and an efficient hull/ skeg form for escort purposes. . Th e increase d towing for ces escort tugs, and also harbour tugs, can deliver pla ces additional dem ands on the deck equipment of ships for securing of tugs. This is an important aspect still requiring attention, as is the case regarding the use of fibre towlines, which are often damaged by assisted ship's fairleads and bollards . Th e consequence of the research is that much m ore
knowledge ha s b ecome availabl e on tugs and tug performance. Mod el tests and study results can be used to optimi s harbour tug design in general. In additi on, simulation programs have been developed to provide a b etter insight into the capabilities and limitations of different tug types while assisting ships. It has all created the po ssibility to make beller decisions regarding a required tug type and ballard pull. Tug simulation for training and port design can ben efit from the study results. Tug simulation on ship manoeuvring simulators has evolved from simple ve ctor tugs towards interactive tug
are by radio or intercom and the physical controls used are relatively simple. Th e tug captain need s a very wide field of view, but the objects in th e scene that are of imp ortance are relatively close. All this is within the present possibilities of VR tecbnology. So in the future this technology may play an im port ant ro le in tug sim ulation . It will b e a relatively ch ea p m ethod , surpassing existing quality. Tug simulation for training and port design can benefit from the study result s. Parallel with devel opments in research stimulated by a parti cular growing need to obta in better insight into the performance of escort tugs, studies have been carried out in J apan on the feasibili ty of automatic berthing systems. Such systems could control a ship's manoeuvring devices as well as attending tugs during an approach to a berth and while berthing. Scientists realise that berthing a ship is a most difficult ope ration wh ich becomes more com pli ca te d when tugs ar e involv ed. If an automatic berthing system was feasible, it would only be applicable to very specific situations, locations and tug assisting method s like push-pull. The practical use of sucb a system is doubtful, because a tug's crew is still needed to sail the tug ami to come alongside a ship under way and to make fast, unless this too could be done automatically. Although absolute safety with tugs will never entirely be reached, with ongoing researcb and proper training together with the input of practical experience, a situation of safe and efficient ship handling with tugs ma y be achieved.
TUG USE IN PORT 173
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Design features for ship-assist tugs. Robert G. Allan. Ship & Boat International. March 1990. Designers' Checklist No. 1. Azimuth Stern Drive Thgs (ASD. Steerprop Technical Information 1/2001). The Development of the Damen ASD 2411. Erik van der Noorda, Erik Leenders. Paper International Tug and Salvage Convention 2002.
E Elastomeric Fenders: Materials and Specification for EffectiveDesign.John E. Rector. Paper Eleventh International Tug Convention. 1990. A guide for the Emergency Towing Arrangements. Hellenic Marine Environment Protection Association in cooperation with Tsavliris Salvage (International) Ltd., Greece. 1998. Emergency Towing Equipment Solution to avoid Tanker Casualties. Pier Giorgio Torriglia, Studio Technico Torriglia, Italy. Paper ITS'96. Seattle, September 1996. Escorting Ships with Tractor Thgs. Captain Gregory Brooks, Captain S. Wallace Slough. Professional Mariner August/September 2000. Quasi-static and dynamic behaviour of Escort Tugs, A designer's viewpoint. N. Hendy, Burness, Corlett and Partners (10M) Ltd, and R. Freathy, Burness, Corlett and Partners Ltd. UK. Paper RINA International Conference on Escort Tugs, 'Defining the Technology'. London, 28, 29 October 1993. Escort Tug Computer Simulation - Standards and Advances. Duane H. Laible, PE, and David L. Gray, PE, The Glosten Associates, Inc., USA. Paper ITS'96 . Seattle, September 1996. Escort Thgs, Design, Construction and Handling - The Way Ahead. Papers RINA and NI International Conference. London, 4 February 1993. Optimised Escort Thg for Norsk Hydro's Sture Terminal. Carl]. Amundsen. Paper The 16th International Tug and Salvage Convention 2000. Escort Thgs Performance Comparisons. Gerry Banks, Clyde Consultants Ltd., UK, Paper ITS '96, Seattle, September 1996. . Escort Thg Performance Comparisons. Ship and Boat International, December 1996. Escort Thg Performance Results. Steve Scalzo, Don Hogue, Foss Maritime, USA. Paper ITS'96. Seattle, September 1996. Escort Thg Research: Towards the Industry Standard. Gerry Banks,John D. BrO\Vn. Clyde Consultants Ltd., UK. Paper 13th International Tug Salvage World Convention and Exhibition. Rotterdam, 1994. Reflections on Escort Thg Stability. N.R. Hendy and R.G. Freathy, Ship & Boat International.January/February 1994. 174 THE NAUTICAL INSTITUTE
Developments in Escort Technology. Robert G. Allan, Paper the 15th Tug and Salvage Convention 1998. The Evaluation of Escort Thg Technology: ... Fulfilling a Promis e . Robert G. Alan . Paper The Society of Naval Architects and Marine Engineers 2000 Ann ual Meeting. Standard Guide for Escort Vessel Evaluation and Selection. American Society for Testing and Materials. November 1998.
F Fibre Ro pe Technical Information and Application Manual. Th e Cordage Institute Technical Inform ation Service. Second edition,J anuary 1997. Cord age Institute, Hingh am, USA. G
Guidelines for the Design of Fender Systems: 2002. PIANC, 2002. Guidelines on the Use of High modulus Synthetic Fibre Ropes as Mooring Lines on La rge Tankers. OCIMF 2002 . Witherby, UK. H
Harbour and Marine Terminal Op erations. A New Approach. Sven O .Aarts, Managing Director of Aarts Holding B.Y. Holland . Paper International Towage and Salvage Convention & Exhibition. Southampton. 1994. On Harbour Manoeuvring and Neural Control Systems for berthing and tug operations. Kazuhiko Hasegawa, Osaka University; Takeshi Fukutomi, Mitsubishi Motors Co., Okazaki,Japan. Paper MCM C '94. Harbour Tugs. Types and Assisting Methods. Captain Henk Hensen. All Marine, Rotterdam. 1990. Hydrodynamics in Ship Design, Vol. L H. E. Saunders, SNA ME 1957. I
Full scale Ice Performance Tests of sister ships with a ducted and an open propell er. Panu Korri. Research Enginee r Raum a-Repola Oy. Rauma Shipyar d. Finland. Pekka Koskinen, Tapio Nyman, Research Scientists. Techn ical Research Centre of Finland . Ship Labatory. Espoo. Finland . 1984. Ice Seamanship. George Q, Parn ell FNL The Nautical Institute. 1986. Studies on Interaction at Sea. E.C.B. Corlett.Journal of Navigation. Volume 32. No.2. May 1979. Interaction between Ships. Merchant Shipping Notice No. M.930. Department of Trade. February 1988. K
Der KORT·Dilsenschlepper on der Seeschiffassistenz. Dipl.-Ing. C.P. Buhtz, Hamburg. Schiff & Hafe n/ Kommandobrticke, Heft 1/1983, 35.J ahrgang. L
Low Frequency Motions of Moored Vessels . Dr. !r.J.E.W. Wichers, Maritime Research Institute Nethe rlands . Schip en Werf de Zee. Novembe r 1991. M
Marine Towing in Ice-covered Waters. A Practical Guide for Dedicated and Emergency Towing in Ice-covered Water. Captain P.E. Dund erdale, P.E. Dun derdale and Associates, Inc., Newfoundland, Canada. 1997. Mooring Equipment Guidelines. OCIMF. Witherby & Co. Ltd. Lond on, England. Second Edition 1997. The Use of Tugs for Manoeuvring Large Vessels in Ports - A Preliminary Study. National Ports Council. Department of In dustry. General Coun cil of British Shipping. Sept ember 1977. Manoeuvring Technical Manual. Prepared by a grou p of experts; edited by Capt, Dipl.-Ing.J. Brix, Hamburg . Seeh afen Verlag GmbH, H amburg, 1993. N
Th e Nautical Institute on Pilotage and Shiphandling. Page 165 - 173. Pilotage and Berthing in Ice. Captain G.Q Parn ell, FNL Th e Nautical Institute. 1990.
o Performance and Effectiveness of Omni·directional Stem Drive Thgs . Paul Brandn er & Robert Tasker, Australian Maritime College, Launceston, Tasman ia, Australia. Paper Intern ational Towage and Salvage Convention & Exhibition. Southampton. October 1994. Performan ce and Effectiveness of Omni-directional Stem Drive Tugs. Paul Anthony Brandner, University of Tasmani a. November 1995. Th e Optimum Harbour Tug . Captain F.R. Mistry. Paper Ninth In ternational Tug Conv ention. London, 1986. Operational Benefits of High-speed electronic Diesel Engines, Christoffel D. Todd . Int ernational Tug & Salvage,July/August 2002.
TUG USE IN PORT 175
P Performance and Oper ations Evaluation of the Crowley VSP enhanced Tractor 'Nanuq' and 'Tan'erliq', using a radio controlled scale model. Todd Busch,john van Buskirk, David L. Gray. Paper International Tug an d Salvage Convention 2002. Prediction of Wind Load s on Large Liquefied Gas Carriers. OCIMF, SIGTTO, Witherby & Co. Ltd., London, 1995. Prince Willi am So und Disab le d Tanker Towi ng Study . Th e Glosten Associates, Inc., USA. August 1993 -j uly 1994.
R Recommendations for Ships' Fittings for Use with Tugs with Particular Referen ce to Escortin g and O ther High Load Operations. First Edition 2002. OC IMF. Witherby & Co., UK. 2002. Reducing risk of Tanker Grounding by Escort Tugs . Ship & Boat Intern ation al. March 1996. Report 00-211. H arbour Tug 'Waka Kume.' Loss of Control. Auckland H arb our. 19 November 2000. Transport Accident Investigation Commission, Wellington, New Zealand . 16 May 2001. Residual Strength Testing of Dyneema Fibre Thglines. Phil Roberts, Danielle Stenvers, Paul Smeets, Martin Vlasboom. Paper International Tug and Salvage Convention 2002. S Safety versus Performance. Gerry Bank. Paper The 16th International Tug and Salvage Convention 2000. SchotteI Rudderpropellers - the better alternative for any application. Dpl-Ing, Hans-H erbert Dimow, Schotte l. 1993. 'Scott T. Slatten'. Bisso' s latest fleet a ddition. Work Boat World. 22 j une 1995. Seamanship Notes. 5th Edition. Kemp & Young. Butterworth-Heinemann Ltd. 1992. Th e Design of Ship Assist Tugs - Towards More Cost-Effective Construction. R. Allan. Paper Int ernational Towage and Salvage Convention & Exhibition. Southampton . 1994. Ship Bridge Simulators. A Project H andbook. Captain H enk Hensen. Th e Nautical Institute, London, UK. 1999. Th e Ship Docking Module (SDM) .j. Erik H vide, Paper International Tug and Salvage Conven tion 1998. Ship Handling at Ras Tanura Sea Island. Philip F. Spaulding, M.Sc.; Life Fellow, S.N.A.M.E.; Dav id Taylor Medal , M.A.S.N.E.; President, Nickum & Spaulding Associates, In c., Seattle, \VA, USA. Paper presented at the Seventh International Tug Convention and Exhibition, London , England, 15-18 june 1982. Ship Manoeuvring Motion due to Tugboats and its Mathematical Model.junshi Takashina. From:j.S.N.A. j apan, Vo1.160, 1986. Shiphandling for th e Mariner. Daniel H . MacElrevey. Cornell Maritime Press, Centreville, Maryland, USA . 3rd Edition. 1994. Shiphandling with Thgs. George H. Reid . Corne ll Maritime Press, Inc. Centreville. Maryland. USA. Second printing. 1994. Report of the Special Committee on Large Ships. Publisher: Service de Presse Edition Information . 14, rue Drou ot, 75009 Paris, France. 1978. Small is Beautiful (and cheaper). Current trends in tug design. Robert G. Allan. Article Paper International Tug and Salvage, Novembe r/ December 2001. Squat Interaction Manoeuvring. Papers. Th e Nautical Institute Seminar. Hull. Sept emb er 1995. A Study of Standards in the Oil Tanker Industry. Shell Intern ational Marine Limited. Schip en Werf de Zee. March 1993. Current issues in the use of Synthetic Fibre Ropes. j.F. Flory, j.W.S. Hearle and M .R Parsey. Polymers in a Marine Environ ment. 23 - 24 October 1991.
T
A Tale of the Unexpected. r.w. Dand. National Maritim e Institute. Seaways. September 1980. Tanker Escort: Requirements, Assessment and Validation - Prince William Sound, Puget So und, San Francisco Bay and Europe. Sridharj agann athan, David L. Gray and Thomas Mathai (Glosten Associates, Inc ., USA), johan H. de jong (M SCN, The Netherlands). Paper Annual Meeting The Society of Naval Architects and Marine Engineers. 1995. A New Tariff for the Humber: Andrew Dalrympl e. Managing Director, Humber Tugs Ltd., UK. 12th Int ernational Tug & Salvage Convention. 1992. Team Towing. Using relatively small Tractors on Heavy Ships. Captain VJ. Schisler, Captain G.V. Brooks. Professional Mariner. August/S eptemb er 2001. Towage Tariffs for Harbour Operators. Choice of a Benchmark - Tonnage or Length. Alwyn Bauman. Queensland Tug & Salvage Company Co Pty Ltd. Australia. Th e Tenth Tug Convention. 1988. Location of the Towing Hook on a Voith Water Tractor. Dipl.-Ing. Wolfgang Bear. Voith . Intern ational Tug Conference Paper. October 1969.
176 THE NAUTICAL INSTITUTE
The Towliner. Th e Key to efficient Escort Tug Design. Aquamaster. Dec. 1994. Tow master Tug Progress. R. Clark. C.Eng., F.R.I.N.A., Burn ess, Corlett and Partners Ltd. 2nd International Tug Conference. 1971. Training - the Thgmaster/ Pilot Interface. Capt. B. Lewis, Howard Smith Industries Pty. Ltd. The 11 th International Tug Convention. 1990. Training with tugs. The Australian ship handling centre at Port Ash . Captain Cliff Beazly. Seaways. Octob er 2002. Transom Link enh ances tanker escort safety . Marine Log.January 1996. Th e Tug Book. MJ. Gaston, Patrick Stephens Limited, UK 2002. Novel Tug Design. Ship & Boat International. March 1996. Optimum Tug for Tanker Escort Duty . C.D. Dale, Aquamaste r (Propulsion) Ltd., UK, K Lindborg, AquamasterRauma Ltd., Finland. Pap er RI NA International Conference on Escort Tugs. London, Octob er 1993. Thg Masters' Training Manual on Effective Use of O mni-directional Stem Drive Tugs. The Adelaide Steamship Company Limited. 1995 Tug Operations - A Third World Experience . Com modo re P.K Nettur, Sabah Energy Corporation, Malaysia. Th e 10th International Tug Conventio n. 1988. The simulation of Tug Operations in a Multiple Simulator Envi ronment. Bent KJacobson and Eugene R. Miller. Advanced Marin e Enterprises, Inc. USA, Dr.Ir. J ohan H. Wulder, MarineSafety Rotterdam BV , Capt. H enk Hensen, The Neth erlands. Paper MARSIM '96. Kopenhagen, September 1996. Thg was h effects in co nfined waters. I.W. Dand , PhD ., C.Eng. Nation al Maritime Institute. Paper Seventh International Tug Convention . London 1982. Som e aspects of Tug-Sh ip Inter action. I.w. Dand, Bsc. Phd, CE ng, MRINA, National Physical Labatory. Paper Fourth Int ernati onal Tug Convention . 1976.
u U.S. Nav y's Synthetic Tow H aws er Pil ot Pro gram. Robert C. Wh aley, P.E. Paper Eleven th International Tug Convention. 1990.
v Vessel Escor t an d Response Plan. 2001 Prince William Sound Tanker Owners/ Op erator s. Decemb er 200 1. The potential application of Virtual Reality Bas ed Simulators to shiphandling and m arin e operations . Eugene R. Miller & Mark Fitch, Advan ced Marine Enterprises Inc., USA; Rick Castillo, Naval Air Warfare Center Training Systems Division , USA. Paper MARSIM '96. Copenhagen, Septemb er 1996. New Insight into Voith Schneide r Tractor Thg Ca pab ility. Bruce L. Hutchison , David L. Gray and Sridhar J agann athan. Th e Glosten Associates Inc., USA. Paper Society of Naval Architects and Marine Engineers. Seattle. March 1993. . W
Prediction of Wind and Current Loads on VLCCs. O CI MF. Witherby & Co. Ltd. London. England . 2nd Edition 1994. Z
Z-Drive Esco rt Tugs, Gregory E. Castleman, N.A., Aquamaste r-Rauma In c., Metairie, Louisiana, USA. August 1994. Perceived Advantages of Z-Dri ve Escort Thgs. Chris Gale, Aquam aster (Propulsion) Ltd., H arri Erone n, ILS Ltd, Matti Hellevaara, VTT Maritim e Technology, Anders Skogman, Aquam aster-Rauma Ltd. Paper International TOMng an d Salvage Convention. South ampton, October 1994.
TUG USE IN PORT 177
APPENDIX 1 The port authorities and towing companies who provided information for the first edition of this book by completing a que stionnaire and by send ing additional information like brochures and photographs are, in alphabetical order by country: AUSTRALIA Fremantle Port. Australia. Captain R.G. Howell, Shipping Services Manager, Freman tle Pilots and Towing Company. Gladstone Port Authority. Australia. Mr.J.M. Schuh, Marketing Analyst. Port Hedland Port Authority. Australia. Captain D avid Baker, H arb our Master. Port Kembl a. Australia. Captain W. H oogend orn . Port of Melbourn e Authority. Australia. Mr. A.D. H oneyborne, Man ager Port Op eration s, and Towing Company United Salvage Pty. Sydney Ports Cor poration, Australia. Mr. Reg McGee, Marin e Operations Man ager. BAHAMAS South Riding Point. Grand Bahama. Captai n D.C. McNab, Marin e Operations Manager. BELGIUM Port Authority Antwerp. Belgium. Mr.J. Burvenich, Depu ty Director-General, and Mr. P. Decock. CANADA Port of Montr eal. Canada. CaptainJean-Luc Bed ard , Harbour Master. Port of Qu ebec Cor poration . Canada. Captain Louis Riel, Harbour Master and Towing Company Quebec Tugs Limited . SaintJohn Port Corp oration. Canada. Captain A.G. Soppit! , Man ager, Operations and Harb our Ma ster. Vancouv er Port Corpo ration. Can ada. Capt ain G.B. Dr ewery, Deputy H arbour Master Operation s. Towing Companies C.R. Cates & Sons Limited, Mr. J. Claire Johnston , President & General Manager, an d Seaspan Intern ational Ltd. ENGLAND Port of Felixstowe. UK. Mr. P.S. Davey, Assistant to the Man aging Director. FINLAND Port of H elsinki and Towing Company Alfons H akans Ltd. Finland. FRANCE Port Auto nome de Dunkerque and Remorquage Towage Du nkerqu e. Franc e. Port Autonome de Marseille. France. Captai n M. Castagnera, Harbour Master. GERMANY Port Auth ority Hamburg. Germany. HONG KONG Hong Kong. Marine Department, Mr. K.W. Chan and Towing Companies South China Towing Company Ltd. and Th e Hong Kong Salvage & Towage Co. Ltd. JAPAN Nagoya Port Author ity.Japan. Mr. Yoshia Isozaki. Port Promotion Manager. Port & H arbour Bureau, Marine Affairs Division , City ofYokohama.Japan . MALAYSIA Port Klang. Malaysia. Cap tain David Padman, Marine Op erations Manager. Penang Port SDN . BHD. Malaysia. Cap tain Ahmed Husni bin H aji Zakwan, General Manager (Marine & Ferry Service). N EW ZEALAND Ports of Auckland Ltd . New Zealand . Mr. Ran Meckenzie. Port of Tauranga Ltd. New Zealand. Mr. Nigel Drake, Marine Services Man ager. 178 THE NAUTICAL INSTITUTE
NORWAY Os lo Port Authority. Norway. Mr. Harry Grytbakk, Dep. Harbour Master, Mr. Hans Chr. Gunneng, Traffic Man ager. Stat oil Mongstad Oil Harbour. Norway. Port Captain. Sture Crude Oil Terminal. Norway. Mr. Hans Schutt, Marine Supervisor. P HI LI PPI NES Philippines Ports Authority. Mr. Francisco L. Tolin, Acting Assistant General Manager for Operations. PORTUGAL Por to de Sines . Portugal. Mr. Car los Alves Botelh o, Cte, Che fe de Divisiio. RUSSIA Port of Archangelsk. Russia. Capt ain V.A. Shershner, Harbour Master, SCOTLAND Shetlan d Islands Council, Marine Operations Department. U K. Captain G. H . Sutherland, Director of Marine Operations and Captain K.J . Radley, Deputy Director of Marine Operatio ns. SOUTH AFRICA Port of C apetown. South Africa. Marin e Manager Portnet. Port of Durban . South Africa. Captai n R. van der Kro!. Port of East Lond on . South Africa. Captain B. Swemmer, Port Manager.
SRI LANKA Sri Lanka Por ts Authority, Mr. L.P.M. Wijedoru, Add!. General Manage r, Mr. Sgd. Sun draJ ayawardh ana, Chairman, a nd Harbour M aster, SPAIN Port de Barcelon a, Spain. Captain A. Perez Almoguera, O perations Manager. SWEDEN Port of G titeborg, Sweden. Captain Jtirgen Wallroth, H arb our M aster, Man ager Sea Traffic Departmen t. TOWing Company Roda Bolaget and Pilots. TAIWAN Kaoshing H arb our Bureau. Taiwan. Captain Sun Hua-Tung. Harb our Master. Keelung H arbour Bure au . Taiwan. Mr. Wang Kuo-wei, Head of Workin g Vessel Center. U.S .A. Port of Corpus Christi Authority. USA. Mr.J erry Cotter, Director of O peration s and Mr. Anthony C. Alejandro, P.E., Industrial Relations and Military Liaison Officer. Port of Houston Authority. USA. Mr. Mich ael T. Schubert, O perations Supervisor. Port of Mobile. USA. Cr escent TOWing Company. Mr. Pr entiss D. Willcutt. Port of New Orleans. USA. Mr.JohnnyJ. Cefalu, Marine Terminals Sup erint endent, Marketing & Terminal Services and Mr. Charles Andrews, Preside nt Cr escent TOWing. The Port Au thority of New York and New J ersey. USA. Mr. J oseph J. Birgeles, M anager, External Affairs Port Departm en t, and Towin g Com panies: McAllister Towing and Transportation Co ., New York City, an d Moran Towing an d Tran sportation Co ., Connecticut.
TUG USE IN PORT 179
APPENDIX 2 Departm en t of Transport Merchant Shipping Notice No. M.l531 Safety of Tugs While Towing Notice to Shipowners, Masters an d Shipbuilders
This notice Supersedes Notice No. AI. 748 Following another casualty to a tug the Departm ent wishes to again em phasise the danger of capsising which may occur when the tow rope reaches a large angle to the centre line of th e tug and the tug is unable to slip h er tow. The tug referred to above was engaged on harb our duties acting as a stem tug an d had just com menced to cant a cargo ship, prior to berthing. During the man oeuvre the tow rope reached a position at right angles to the centre line of the tug (a position commo nly referred to as "girting") causing an up setting moment on th e tug to th e exte n t tha t she capsized and sank, fortunately without loss of life. The casualty becam e inevitable when th e quick release mechan ism on th e towing hook failed to operate causing her to heel ove r to such an angle th at th e sills of th e openings were immersed , allowing rapid flooding to occur. Contributory causes to the casualty wer e: (i) (ii) (iii)
sm all freeboard poor curv e of righting levers closing appliances to spaces leading bel ow not secur ed.
In order to reduce the grave dangers associated with such conditions, particularly with sm alle r tugs engaged on harbour duti es, the Department make the following recommendations:
1. It is of the greatest imp ortance that the de sign of the towing gear should be such as to minimise the overturning moment du e to th e lead of the towlin e and that th e towing hook should have a po sitive me an s of qui ck release which can be relied upon to fun ction correctly und er ALL operating conditions. It is de sirable that th e release mechanism sho uld be controlled from th e wheelhouse, the after control position (if fitted) and at the ho ok itself. The local con tro l at the ho ok sho uld preferably be of the direct mechanical type capa ble of independent op eration . It is also essential that the greatest care should be taken in the maintenance of the towing gear to ensur e its fullefficien cy at all times. 2. O penin gs in supe rstruc tures, deckhouses and exposed m achin ery casings situa ted on th e weath er deck, wh ich provide access to spaces below that deck, should be fitted with weath ertight doors which comply with the requirements for weathertight do ors containe d in paragraph I, Sche dule 4 of the Merchant Shipping (Load Line) Rules 19 68. Such door s sho uld be kept closed during towing oper ation s. Engine roo m ventil ation sho uld be arranged by means of high coa ming ventilators and air pipes should be fitted with automatic means of closure. 3. Stability criteria for tugs not subject to th e requirements of the Merchant Shipping (Load Line) Rules 1968: (i) (ii)
In the norm al workin g condition, th e freeb oard sho uld be such that the deck-edge is not immer sed at an angle ofless th an 10°. The GM in the worst anticipated service condition sho uld be not less than
Imperial
Metric
~
4f.C. Wh ere: K = 5 + O·08L - O·45r L
=
Length of the ve ssel between perpendiculars; feet or metres
r = Length of the radial arm of towing hook; feet or metres f = Freeboard; feet or metres
CB = Block coefficient
180 THE NAUTICAL INSTITUTE
Wh ere: K = 1·524 + O·08L - O·45r
Any existing tug which cannot attain the GM calculated in accordance with sub-paragraph (ii) abo ve might nevertheless gain som e improvement in he r stability by having struc tures on the weather deck closed in acco rdance with pa ragraph 2 above. 4. In cases where compliance with the recommend ations in paragraph 2 and 3 cannot readily be attained consi deration should be given to : (a)
substitution of perman ent ballast for water ballast and conve rsion of peak ballast spaces to dry spaces.
(b)
fitting a permanent dev ice to minimise the po ssibility of the tow lead coming into the athwartship s position .
5. In the case of the tugs which proceed to sea and are subjectto th e re quirements of the Merchant Shipping (Load Line) Rul es 1968 the stability crite ria to be achieved and approved by th e Dep artment are as laid down in Schedule 4, Part I, paragraph 2 of those ru les. Department of Transport Marine Directorate Southampton SO I OZD June 1993 Note:
Merchant Shipping Notice No. M 1531 was cancelled on 30 August 1999. The con tents have be en included in th e 'Lo adlin e Instructions for th e Guida nce of Surveyors' of the Maritime and Coa stgua rd Agency, UK. References to th e Mer chant Shipping (Load Line) Rules 1968, which are now th e Mercha nt Shipping (Load Line) Regulations 1998, have been updated. Th e stability criteria m entioned apply to all non-seagoin g harbour tugs an d seago ing tugs ofless than 80 net Tons,pro vided th ey ar e engaged in coasting trad e, whi ch mean s only voyages to destinations in the U K.
TUG USE IN PORT 181
APPENDIX 3 DET NORSKE VE RlTAS Rules for Ships,January 1996 Pt.5 Ch.7 Sec.l6
i~, :
SECfION 16 ESCORT VESSELS
I I
:
I
t
I
i
Co nte nts
assiste d
A General A 100 A 200 A 300
I
Classification Definitions Docum entation
I
vesst
OJ!
\
,
B. Arrangement and De sign B 100 Arrangement
I I
I I
L-~~~
~
I I
e FS
C. Steering Force an d Manoeuvring C 100 C 2 00
FS = Steering pull fB = Braking pull P = Oblique angle e = Towline angle
Escort rating number Manoeuv ring
I
I I I
FB 't
lr-\ / ,, ''
I
1
I I I I
D. Stability D 100 Stability D 200
E 100 E 200
,,
Procedures Recordings during full scale trials
, ,,
,r
A. General A 100
Classification
The requirem ents in this Section apply to vessels
specially intended for escort service.
102
,
/
'
Stability criteria
E. Full Scal e Testing
101
•
Vessels built in compliance with the following
requireme nts may be given the class no tation Escort (n , V ), wh ere n indicates maximum transverse stee ring
pull (FS in Fig. I) exerte d by the escort tug on the stern of assisted vessel, and V , the spee d at which this pull may be atta ine d. 103 Th e esco rt ratin g num ber (n , V ) is to b e determ ined by approved full scale trials. A test certificate indi cating the escort rating number (n, V ) may be issued on com pletion of app roved full scale trials. 104 The requiremen ts for Tug notation given in Pt.5 Ch.7 Sec.2 are to b e com plied with. A 200
Definitions
201 The term Escort Service includ es stee ring, br aking and othe rwise co ntro lling the assiste d vesse l. The stee ring force is provided by the hydrodynamic forces acting on the tug' s hull. See Fig. l. 182 THE NAUTICAL INSTITUTE
escort tug
r
Fig. 1 Typical Esc ort configuration Guidance note: As the hy dro dyn ami c force s acting on the tug's hull increases approxi mately with the square of the speed, the steering ability increases more than proportionally wit h the spee d. Escort service should therefore normally be under taken in the spee d range of 8 to 10 knot s. ---e ~ n-d --~ o - f-~- G-u -i -d-a-n -c-e---n - 0 - t-e
202 By the term Escort Test Speed is understood the spee d at which the full scale measur em en ts ar e to be carried out, namely 8 kn ots and/or 10 kn ots. 203
By th e ter m Escort Tug is un derstood the tug
performin g the escort service.
204 By the term Assisted Vessel is understood the vessel being escorted. 205
The Escort Rating Numbe r (n , V ) is defined as
the stee ring force, n in tonnes dete rmine d according to
C lODacting on the stern of assisted ship in tonnes, at V kn ots. If n is determined at both 8 an d 10 knots the escort rating number will consist of 4 digits.
A 300
Do cu m en tation
Manoeuvring time in seconds from main tained
oblique po sition of tug giving maximum steering 301
The following plans and particulars are to be
force on on e side of assisted ve ssel to mirror position on the other side. Towline angle Ii need
submitted for information:
not to be taken less than 30'. towing arrangement plan including towline path and min imum breaking stren gth of t owin g line
C 200
Manoeuvring
components
201 pr eliminary calculation of steering pull at 10 knots including propulsion components for balan cing of oblique angular position of tug preliminary stability calculations.
B. Arrangement and Design B 100
Arrangement
101 The hull of the tug is to b e designed to provide adequate hydrodynamic lift and drag forces wh en in indirect towing mode . Due attention is to be paid to the balance between hydrod ynamic forces, towline pull and propulsion forces. Freeboard is to be arranged so as to avoi d excessive trim at high er heeling angl es. Bulwark is to be fitted all aro un d exposed weather deck. 102 The towing winch is to have a load reducing system in orde r to prevent ove rload cause by dynamic oscillation in the towing line . Normal escort operation is not to be based on use of brakes on the towing winch . The towing winch is to be able to payout towing line if the pull exceeds 50% of the breaking strength of towing line. The towing line is to have a breaking strength of at least 2.2 tim es the m aximum mean towing pull as measured during the test. 103 The propulsor shall be ab le to provide ample thrust for manoeuvring at higher speeds for tug being in any oblique angular position.
c.
Steering Force and Manoeuvring
C 100
Escort r ating number
101 The escort rating number, (n, V), to be based on full scale measurements at 8 and /or 10 kn ots. n
=
FS C (tonn es)
FS = steering force from tug
The vessel is to be designed so that forces are in
equilibrium "lith a minimum use of propulsive force
exce pt for pr oviding forward thrust and b alan cing transverse forces during escorting service.
202 In case of loss of propulsion, the remain ing force s are to be so balanc ed th at the resulting turning mome nt will turn the escort tug to a safer position with reduced hee l. Guida nce no te : Due attention sho uld be paid to sudden loss of thrust wh ich may be experienced beyond certain angle s of w ater infl ow to propulsion units at high er speeds. Prediction of forces acting on the tug when escorting is necessary f OT scantling, manoeuvrability and preliminary stability calcu lations. Model testing may indicate hydrodynamic forces for indirect towing.
--e- n-d ---0- f-- G-u -i-d -a-n -c-e-c-n-o-t-e
D. Stability D 100
Stability
101 Th e gene ral stability criteria in Pt.3 Ch .7 Sec.2 E are to be complied with. In addition, the stability criteria given in 201 and 203 are to be satisfied.
D 200
Stability criteri a
201 The area under the righting arm curve and heeling arm curve are to satisfy the following ratio :
RABS
~
1,25
where RABS = Ratio b etween righti ng and heeling areas between equilibr ium and 20' heeling ang le. Eq uilibrium is obtained whe n maximum steering force is applied from tug. 202 Heeling arm is to be derived from the test. The heeling arm is to be kept constant from equilibrium to 20' , See Fig. 2.
C = k28 or 1, whichever is less t
k = 1,1 (28 sees is the manoeuvring time required by Rules Pt.3 Ch .3 Sec. 2 J 100) TUG USE IN PORT 183
1,4 1.2
E. FuIl Scale Testing
1 1
Righting arm~_~
1,0 t 0.8 1
GZ
(m)
Heeling arm
> 25%
t
0.6 o!r--_--jll"--....---+-~6_-..... 0.4 0.2
1:
100%
0.0 C:f-----+----~------< 10 20 o 30
E 100
Procedures
101 A plan with documentation cov ering the full scale tri als is to be approv ed prior to th e tri al s being undertaken .
102 Th e docume n tatio n is to in clu d e a towing arrange me nt pl an showing di fferent components in towing gear including the load cell. Verification of SW L of stro ng points on board the assisted vesse l is to be submitted.
Degrees of heel 103 T he esco rt test spe ed is 8 kno ts and/or 10 kn ots. The speed should be taken relative to th e sea. Estimates of curre nt during the trials m ay be requi red.
Fig. 2 Equilibrium to 20 degr ees
Guidance note: Guid anc e not e:
The current may be estimated by logging speed by GPS
Possible mo del testing to in clude heeli ng ang le m easurement s as to predict dynamic stability margin. Th is
and relative log in sepa rate runs wh ile proc ee ding with and against the current.
requires a high degree of accuracy in determining light ship weight and centre of gravity.
E 200
---e·n -d---o· f··· G-u-i-d-a-n-c-e---n - 0- t-e
203
At lea st the following data is to be reco rde d con tinuously in real time m od e during trials for later an alysis:
201
The following requirement is to be satisfied:
A + B ~ 1,4 (B + C) wh er e A + B = area under th e C Z curve B + C = area under th e heeling moment curve The areas are taken from 0° heel to th e angle of down floodi ng or 40 °, whicheve r is less. See Fig. 3.
1.4 -: 1.2 1.0
E
N
0
0.8
position of assisted vessel and esco rt tug is to be recorded by differ ential CPS equipme nt spee d of assisted vessel by differential C PS speed of assisted vessel by log relative to th e sea head ing of both vessels from gyro compas ses rudder angle on assisted vessel hee ling angle on tug towline ten sion length oflowline angle of towline. vVeather condition and sea state are to be noted. Manual measureme nts are to be read as b ack up to continuous readings. Bearing from tug to assisted vessel is to be record ed . Suitable test forms are to be used .
.i-
~
~
Recordings during full scale trials
A
Guidance note:
0.6 0.4
B
C
0,2
Down flooding pt .
0.0 0
10
20
Degrees ofheel Fig. 3 Total Area Requirements
184 THE NAUTICAL INSTITUTE
30
Assiste d vessel is to sail on auto pil ot during trial s. Size of ves sel is to be sufficient as to withstand steering forc es from tug without using too large angles.
-, 40
--- e -n- d-o ~ f- - · G·u · i- d- a·n - c-e - - - n -o · t- e
Note: Rul es are in the phase of upd ating (200 2).
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INDEX A
B
Aarts Autohook 113 ab rasion within rop es 105 ABT S ummer 134 Ab u Dhabi __ _. 16 AC /DC {Ward- Leo na rd] dri ve 99 active escorting __ . 152, 153 adde d mass 73, 81 . 148 ad ditiona l towing point Adelaar 96 Ad steam Towage Co m pa ny 65, 98 advanc ed vector tug models 126 Aegean Sea ............................•............. 134, 135 aid s to navigation 135 aircraft carr ier s 92
Baltic ports 38 b ank sucti on 80, 81 basic tra ining 117 Bear, ""~o lfgang 21 Becker rudder 16 Belgium . 101 Bernoulli effect. .............................. . 75, 82 law ....................................... .. 86 Mr, Daniel 82 th eory 82 berth construction 68, 73 Bess ......................................•................ 65, ]48 bitts 112 bl ack fender systems 12 boll ard pull ...... 20. 30, 56. 64. 66. 67. 68. ...... 69, 70, 7], 72, 74, 75, 76, 78, 91, 99, .... 100. 108, ll O, n i, ll 8, 120. 139. 146. ....................... 154. 159. 166. 169, 170. 171 safety factor 69 bollard s 96 and fairlead s 92 Boss 65 , 148 bow cushion effects 80, 81 thruster 17, 19, 40 , 84 , 163 retractable , 9 to-bow 92 b ox keel 144 BP Terminal 160 Braa 135 braking and stee ring for ces 147 for ces 6], 13 8, 144, 146, 159 holding capacity 99, 100 power 149 mo de 156 Brandner, Dr. Paul 62 Brem erhaven 165 bridge manoeuvring sim ulators 123 Brounrd 49, 144 BrunvoU 25 Brusselle Marine Industries 10] Bukser og Berging Towing Co mp an y 99 bulb and box keel 144 bulbous bow 91 Bur chett, Ron 123
Ajax
156
Akzo Nobel 104 Al-Haunah 17 Ala ska 135 Sta te Law 158 Alert 158 Allan , Rob ert 170 Am erican Bureau of Ship ping (ABS) 51. 154 Society for Testin g an d Malerials(A STM) 154 Amoco Cadiz 134 Amsterdam 19, 113 Angola 134 Antwerp 2 Aquaduo 24 Aquamas ter 25, 57, 61, 140, 144 / KaMe\Va 24 Aramid 104. 105 Ar ea escort plan 158 ArcoIndepenthncl ]39 A rcafuneau ..............................................•... ]46 AS D compact tug 170 escort tug 94 , ]4 4 reve rse-tractor tu g
47. 55, 56, 127, 128, 140, 143, 144. 169. 170 r everse-tractor typ e
152 revers e-tractor-tug 143 tug ... 9, ]3 , 27, 29, 32 , 36, 37, 38, 45, ..... 46, 48, 49, 52 , 53, 54, 57, 58, 59, ..... 60 , 61, 63, 65, 66, 75, 80 , 92, 96,
..... 106, 123. 130, 143. 144, 146, 148. ............................................ 153. 158. 171 reverse-tr acto r 34 , 80 , 84, 85 assisting m eth od s 6, 33 Europ e 36 Atlantic Empress 134 Attentilit ]5 8 Australia 2, 17, 34, 107 Australian Maritime College 122 auto m atic be rthing system 172 release system 10] spooling ge ars 99 Aware 158 azimu th ]9, 29, 54 , ]72 bow thru ster prope llers 25, 49 , 54 , 56 , 61, 86 , 163 propulsion 120, ]44, 172 stem dri ve tugs ]43 th rusters ... 26 , 29, 32 , 61, 75, 80, ]17, ................................... 163. 165, 166. 171 tractor tug 26, 27, 120 , 148, 152
c C.H . Cat es & Sons 55 , ] 12, Ca lcutta Californi a State Regul ation s Canada 34. 38. 123, 159. 163. Canship Ugland Limite d
170 2 159
Cap. Pasley
171
Cape Town Capitol carrousel tug Inn er Port Design A O uter Port Design B Castillo De Belbur centre of pressur e centrifugal force cha fing chain chain stopp ers CharlesH Cates 7 choice pilot syste m Clyde Consultants U K
35
Coanda effect combi lev er tug
75, 132 2S
9. 17. 19, 20. 54, 65. 84. ..................................................... 138. 168
combin ation arrest mode 140, 145 combined joy stick control 26 thru ster control 25 training 119 training of p ilots and tu g captains
120. 122 com m on assist modes 170 com munications 89, 90 system s II compact tugs 169, 171 composition of towlines ]06 com pulsory escort areas 154 computer generated image (CGI) techniqu es 123 simulation ]54 con trol single leve r 86 cont rol of transverse speed 78 controllab le pitch propeller ..................................... 15, 21, 26, 61, 88, 145 control systems 14 controls logical 11 towing winc h II conve ntional fibres 104 tug 30, 36 , 38, 45, 46, 52, 54, 56 , ......... 57, 58 , 59, 65, 75, 84, 123, 127,
..................................................... 138. 158 ice strengthened 39 towing on a line 12 8 cooperation 65 between pilots and tug captains 66 Cory Towage 19 cou rse control 25 cow hitch connection 106 cranes for towline handling 112 cross lin es 36 win ds 78 C ro wley Ma rine Services 150 current coefficient 71 cyclic loading 107 cyclo idal propellers 51, 61 pr opulsion syste m 21, 120 VS propeller 22
D
170
168 134 47 67 114 91 112 156
Damen ASD Tug 24 71 170 Shipyards 59, 122, 170 dead in th e wate r 73 sbip 35 deck equipm ent 65 im m ersion 147 deckhou se construction , 170 default matrix option 159 design consequences 65 desktop computer simulation ..58, JI 9, 15 6 Det Nors ke Verita s (DNV) 30, 13 6,
31
....................... 147. 148. 150, 154. 156. 157
159
3 167, 168, 169 168
TUG USE IN PORT 187
escort tug rules 147 direct method 140 rev ersing system 14 towing 43, 44 towing m ethod 47, 143 144 to w·ing mode disc towing hoo k 97 d ouble winch 99 drift 144 angle speed of a ship 71 DSM high per formance fibers 150 Du Pont 104 Du ckpellers 25 Dunderd ale, Peter E , 42 Dutch inlan d waters , 35 dyn amic 110 forces load absorp tion 107, 150 position ing systems 165 stability 49, 170 towline pull criteria 51 Dyn eem• .............. 104. 105. 107, 114. 149, ............................................................ 150. 154 150 SK75 fib"
E eco no mic factors 1 68 , 117 p ressure effect 72 of current forces of water d epth 132 effective communic ation 119 shiph an dling with tugs 43 tug position 62 efficient employm ent of a tug fleet 79 eigh t strand plaited ropes 103 elastic limit 110, 111 eme rgency towing arr an gements 114, 149 equip ment 113, 158 pennan t 115 Endeaoor 166 endurance limit 110. 111 engine noise 129 enviro nme ntal conditions 3. 3 4, 62, 68, ................ 72, 108, 118, 119, 120, 123, 144, ........................ .......................... 146. 15 4, 156 Escambia , , 166 escort p lanning 139. 15 4. 159 regulations 139 , 157 safe speeds 147 services 13 4 spee d 147 tug 134. 135. 140. 143. 147, 151 capabilities 136, 148 class n otation 136 fre e sailing speed 147 p urp ose built 136, 157 regul ations 158 135, 157 requiremen ts suitab ility 136 tethered 146, 147. 158 escorting at h igh speeds 172 response vessel (ERV) .., 158 training ., , 139 tug positions 138 Esperanza 36 160 Esso Term inal
188 THE NAUTICAL INSTITUTE
Europe 38, 139 Eur opoort 37 experience 5 76 indispensable factor extra imp roved plow steel (X1PS) 102 Exxon Valdez I, 3 , 13 4, 135
F F(P)SO s failure scena rios Fairplay V fairway constraint s Fa w'ley fendering calculations " ex trude d profile h orizontal material pn eum atic weldable fibre differences in pr operties lin es snap-back danger pennant towlin es Finland fire fighting training fishtail rudd ers fixed pitch pr opeller towing point flanking rudders Florida flow influence aro unda ship patt ern
Flyz"ng Phantom fog
1. 5. 78. 92 145, 146, 156
23 146 160 11 74 12
,
,
12 12 12 12 105
105 106 106, 107, 111
39. 134 117 . 16 17 ,
, , 88 94, 95 16 16, 56 166 131 81, 85 19 64, 91. 119 91 122 52
condi tion s force-equilibri um-simulation forward tugs Foss 13 4, Maritime Tran som Link free sailing spe ed frequ en cy-controlled winch friction force or tracti on win ch fu ll mission br idge simulator 125, 128, 129, 130, simulator full scale escort trials
151 151
147 99 109 98 136 156 161
G Garth Foss......................•.•...•.............. 15 4, gas carrie rs gate lines Gen oa girting 14. 57. 64. 88, 89. 90. 94. 122, Glosten Associates
158 70 36
25 100 154
GM initial metacentric h eight 49 values 170 gob rope 20, 46, 8 9, 95. 96 system 46, 90 winch 54. 96 Goedkoo p H arb our Towage Company 20 Goteb org 37 Great Britain 13 4, 135 150 gromm et ,
grooved bol lar ds fairleads Guard guard plates and struts
106 106 15 8 163
H H vshaped bollard H amb urg H ann an Ring Nozzle harb our tugs choice
Hawk Horam
95. 96 165 15 7 157
17
head reach 144 heaving lin es III h eeling 129 angle moment 49 heig ht of the towin g point 50 Hendrik P. GOtd);0fJ!J 95 Hesnes Neptun Gro up 145 high bo llard pul l .. 165 104 performance fibr es Hinch inbr ook Entrance 158 hipp ed up 35 HMPE fibres 104. 149. ISO, 151 H MP E (H igh Modulus Polylithy len e] ......... ..................................................... 104. 105 h ackles 103, 106 H ong Kong 2, 9, 13, 36, 37 h ook-up points 113 passive and active 113 h orizontal tug accelerations 110 hull · 131 for ce d ata form 172 form an d effectiveness 75 side spo nsons 147 Humboldt ••..•..•............................................. 134 144, 166 H vide Marine h ydr aulically . driven winch 99 ope rated towing pins 96 hydrod yn amic forc es 44 , 52, 55, 163, 167, 168 mass 73 mom ent 44
I ice con ditions 38 ice knives , , 39 imp roved plow steel (lP S) 102 ind ep end en t wire ro pe core ([\VRC}) 102 In dia 11J indirect 140 arres t mod e me tho d 140 143, 147. 171 mod e steering m ode 15 6 towing 43, 44 47. 53 towing method towing mode 144. 148 influen ce of wind an d curren t 44 inform assisting tugs 91 informatio n ex change 15 2 pilots and tug captains Integrated Schottel Nozzle (ISN) 23 . 25 int era ction 14, 80, 122 due to tug fende ring 80 effects 83, 85, 92. 118
examp le 87 forces 88 of tug pr opellers 80 ship prope ller/ ship hull-tug 80 tug hull-ship hull 80 tug prop eller -ship hull 80 tug pr opeller-tug hull 80 tug-ship .. . 86, 92 tug-towline 80 int eractive tug 129, 130 rug simulation 127. 172 Intern ational Maritime O rganization (IM O) ....................................................... 51, 114 inward turning moment 84 h ie of Wight. 146 Italy 25
J J an Koor en Towing Comp any Janus Japan J ohn joystick
,
96 25 9, 36, 37 99 25
K KaMeWa Kawasaki Kevlar kinking Kinsman H awk , Kir sten , Professor Kart, Mr. Ludwig nozzle KOTUG Towing Com pany Kuwait :
25 25 104 103, 106 48 21 15 5. 98 17
L La Corona
134
Lam Tong..........................•.............. 10, 13, 26 lateral cen tre of gravity centre of pre ssure .. 43. 44, 45. 50. force coefficient res istance 50, 63. 170, underwater resistance win d coefficient lay cross .....................•............................... eq ual
67 167 71 171 76 70
102 102 Lang', 102 left h an d 103 of a rope 102 ordinary 102 right h and 103 right h and or left hand 102 S· 103 z103 I..e H avre , , 2 Lempert-Keen e-Seastrend Bill 159 limi tations of tug type s 118 limits of safety 68 Lindsey Foss 146, 149, 158 list 46 load reducing system , : 14 8, 149 lo ad reducing systems 50 Lon g Beach 152 longi tudi n al forces 59 Louisiana Offsho re O il Port (LO O P) 159 Lynn Marie, 140
M Maasbank
96, 148
Malaysia 34 man ipulator 113 manoeuvrability an d training 170 mano euvr ing lane width 10 8 limited space 4 panels , 10 performance of a ship ." 69 simulation programs fast-time . 123 ma nufactu re r's recom mended continuous rating (MCR) 30 marine overlay finish 105 Ma rine Towing of Tam pa 166 Mari neSafety International 122. 125 Mar itime Simu lation Centre 122 master pilot 25 system 166 Matddess , III maxim um heeling angle 56 mean towing pu ll 148 pulling capacity 100 Mcflwain, A.G 170 }'!elton 98. 106 MerUlntiu Marcia 134 Milford H aven 135 minimum br eaking strength 151 GM 51 model tests 136 in wind tunnels , 69 Mon gstad , 134. 160 mooring boats 34 op erations 34 Moran Towing Company 19 mul ti·tug 9 Multrotug 12 168, 169
N Nanuq Nau ticen nozzle
15 8 15 N"ltj' P 17 New Orlean s , , 2 New River , 166 Newfoundlan d . 159 Niigata 25 norman pins 91 North Sea Ferries 112 Norwa y 34, 134, 151, 159, 160 novel new tractor tug d esign 163 nozzle 15. 30, 166 constru ction 39 diamete r 163 pro pulsion 132 typ e 19A 15 type 37 15 number of crew members II I nylon fibre 104 loose laid 107 stretc he rs 107
o obj ectives of escorting 137 O CIMF 71, 72, 78, 92, 107, ................................................... Il l , 148, 149 O il Pollu tion Act 1990 157 oil rigs , 1, 5 omnidirectional prop ulsion 56 . 120, 138. 163 p ropulsion systems 171 prop ulsion tugs 64
stern drive tugs th rust pe rformance tugs on the hip on-job training ope n p rop eller op erating jo ice reliability op erational lim its models research safety stability optim um inform ation exchange tug placement op tiona l class notation outward turn ing m omen t
62 46 139 35 117 30 41 152 64 123 125 97. 49 119 120 154 84
p Pacific Cembi 19 paddle wheel effect 17. 56 Pan am a Can al 35, 36 Parnell. George Q 42 part task simulators 123 p assive escorting 146, 152. 153 Pegasus : 17 pennant... 107, 109, 114, 147, 150 p erforman ce enhan cing d evice 165 Petronella]. Goedkoop 18 PIAN C 70 pick up gear 114, 115 pilot 109 intenti ons 89, 118 organ isa tions 79 ship masters an d tug captai ns com munication 93 cooperation 93 in formation exchange 93 tra inin g 118 view from ship bridge 86 pit ch lever , , 23 pivot p oint 43, 44 Placenti a Bay , 159 Point Gilbert , 19 Pointe Vl.ttnle 31 polar diagrams 122 Polar Tank ers 149. 161 polluti on control training 117 polyester fibre 104 p olyester/polyprop ylene stretche rs 107 p olyprop ylene fibr e 104 Pcrsgru nn 134 Por t Everglades 166 Port of Chennai , 111 Goth enburg 134 O saka 2 Rotterd am 4 Sullom Voe 160 Tarnpa 166 Van couver 170 Yok oh am a 38 port specifi c escor t tug requiremen ts 136 Por t Stanvec 17 Por tabl e radi o-communication sets 66 por ts Australian , 38 configura tion 33, 34 conventio nal 2 de sign studies 3 dim ensions 79
TUG USE IN PORT 189
~~:u~'~'i~~d'::::::::: : ::: ::: ::::::::::: ::: : ::::::: ~: ~:::~~a~~.::::::: :::::::::::::::::::::: : ::::::.~~4,l;~
South Afri can 38 und er development 3 V{est Pacific 26 with m ainl y piers an d j etties 2 with mainly termi nals 2 pre-escort checklist 154 con ference 15 4, 15 9 Prtncewi lliarrrSound 134, 152, 153, 157, 158 propeller 9 azimuth co ntroL 26 , 26, 29 , 129 efficiency 14 fixed 14 fixed pitch 14, 18 in nozzl es 62, 75 in-turning . 17 manoeu vr ability 15 nozzle 16 sp eed 86 steerab le azimu th 6 thrus t 22 deflected 51 Voith 6 wash 30, 69, 86 propulsion sys tems diesel-ele ctric 14 protection plate 21 Protector 158
Puget Sound
134, 151, 153, 157, 158
Tanker E scort Plan pull/sp eed characteri stics pulling effectiv en ess push-pull m ode tug s . pu shing mode point
158 100 66
retr actable azim uth bow th rusters 19 r ever se ar rest mode 61, 62 tractor tug .. .... 26 , 27, 28 , 30. 36 . 37, 38, 45. 49 , ..... 52. 53, 54, 57, 61, 63, 65, 75, 92,
..................................................... 130, 171 Rexpellers
Ridley Island righting and heeling ar m curves mo ment risk assessm ent stu dy involved
River Thrra rope hawser-laid man-made fibre polyurethan e coating ropes braid-on -braid do uble braid ed ROTOR escort tug Rotterda m
RT Innovation RT Magi' RT Pioneer RTSpirit rubb er buffers rudder balanced Barke Becke r controls failure force s
108, 171 25
55
quick release controls
hook mechanism strap system to wing h ook
101
53, 91, 96, 112 100 100
29, 88, 94, 101 52
'R radial
hook
50, 65, 94, 163
system 94, 167 towing arm 54 towing h ook 51, 57, 94 , 97 real-time simulation programs 123
R"Jhridg,
65
reduced visib ility relationship b etween tug and assisting m eth od remote control tug models research reserve buoyancy residual dynamic stability r ope strength testing re sponse times
69 37 22 122 170 147 51 151 136
190 THE NAUTICAL INSTITUTE
135 119 92
104 104 165 2, 37, 165 163
163 163
98, 163 97 16 16 16 129 145 145
18 16 16
58 16 16
51, 137, 139 16 101
s SIR Benicia safety and performance durin g tug operatio ns .._ factors m argi n of operation s regu lations and m easures requirements Samson Rope Techn ologies San Francisco Bay region
San Pablo San Pedro Saona Sa udi Arabia
Sr'JYaf Schilling Mon ovec rudder rudders VecTwin Schisler, Captain Schottel
Scott T Allen Scott T. Slatten SeaEmpress
,
139 94 81, 86, 180
2,Ill,ISO 72 10 119
5 150 151 159 159 19 94 17
16 16 16 16 152 25 17
IS
135
166 148 99 14 80 138 161 13 5 171 149
]72 108 ] 11 123 66
Ship Docking Module (SDM) 103 101 lOS
high lift
Ulstein Russia
Q
51 166
J astram movable flap performance Promac Stuwa spa de
tug
43 , 44 , 50 , 59
25
163
Seab ulk Towing : seco n da ry towing point sep aration disc shaft brakes shal low wate r effects sheer She ll Int ern ation al Lim ited She tlands ship contro llab ility fittings for use with tugs m anoeuvring sim ulators m anoeuvring space mooring lines as to wlines simulation program speed
8, 166, 167
sho ck loading 150 shu nter 163 side thr ust 165 th rus ter 68 signi ficant wave height 65 simp le ve ctor tug m od el 126 simu late d esco rt tug 156 simu latio n 129 by remote-controlled models 123 programs 57 use by pilots ]23 techniques 136 simu lator ] 19 facilit ies 131 institutes 124 , 126 . 128, 131 ope rator ]26
study ,
120
tr aining 117, ]30 single or double drum winch 98 plate rudders 16 .skeg IS, 21, 25, 44 , 4-5, 47 , 49, 92 , ...................... 110, 122, 144, 163, 165 , 166,
..
168, 170, 171 effect e nd
48 148
high lift : hydrofoil-shaped long
144 148 144 82
skin fricti on slack lin e sp eed Sm it
100
Denemarken Harb our Towage Company
lerland
109 97
SaLAS
109 115 53, 130 149
Convention, 1974 South Mrica Southampton
113, 114 34, 134 65 , 160
Spain
134, 135
Safe Fast system Siberii
Spec tra
104, 105 , 107, 114, 149,
................................................... 150, 152, 154 split drum winch
SPM s sponson!! squa re braid
St.]ohns
98
I, 5, 78, 92 50. ]72 103
166
St. Lawren ce Seawa y Au thority 163 SL Petersburg 101 stab ility 5 0, 147, 171 of compact tugs 170 standard towing hook 97 vocabulary 66
static for ces 110 stability. ............................. . 49 stability curve r equi rements 51 Statoil Terminal 160 steel an d fibre towlines 101 10 1 wir e . steering assistan ce 33 force s 144, 168 nozzles 18. 56 Stena V-Max d esign 161 stopp ing forces 74 stre tchers 107 Stu re 134 Sture Crude Oil Termin al 145, 159 subm ari n es II, 13, 92 suction forces 84 Suison Bay 159 Sullom Voe 154 Supertug 163 Swannee River , , 166 Sweden 134 synthe tic fibre ropes 103 towlin es , 101 line s h eat d amage 106
T Taiwan 9. 36, 37 tandem escort towing 143 1O.n'erliq 158 ta nkers LNG or LPG 4 1O.o-Yu No 3 164 team towing 143 ten sion control 150 d ru m , 99 winches 92 Terminales Maracaibo 17 Tadbank: 97 The Neth erlands 19, 59. 122. 170 th eoretical-pr actical cour ses 118 training 117 thrust vector diagrams 30 th ruster bo w and stern 76, 118 configurations 167 controls 129 hull interaction 132 steerabl e 62 tidal re stri ctions 79 Tiger Sun 50 torque loadings 30 problems , 14 towing and pu shing force s 132 arm 50 bitts 91. 96 , 97. 100 hydr aulically locked 101 hook 91, 96 hydraulically locked 101 on a lin e 52 . 64. 171 in ice conditions 39
~~:s~~~~.:::::::::: ::::::::::::: ::::::::: :::::::::::::::: ~l: point varying location
43, 44 . 45. 94 94
staple 166 winch 53 , 96. 97. 98. ioo, 172 towline 10 and/or fend er characteri stics 129 close behind a ship 'S stern 88 elastic ity 110 force 46, 50, 53. 109. 132. 147. ............................................ 167. 168. 172 ha ndling 92. III length 85. 100, 108. 109. 119. 149 towing on a line 107 load reducing syste m 99 load s III pennant 152 proper handling 119 releasin g 91 requirem ents 101 safety factor 151 sh ort 109 steep 110 stre ngth 110 safety factor s 110 tension control , 149 Towliner 94, 144 Towmaster rudder system 16. 17 system 18. 32. 56 TP / 163
TPll
163
tractor tug 21, 25. 32. 45, 46. 49. ...........57. 63.65. 75, 84. 87. 92, 143. 152 training 117, 120 and pilo tage 156 120 for a n ew typ e of tug for specific ship s 120 in thruster and tug hand ling 165 manuals 119 objectives 117. 119, 123. 124. 130 programs definition 131 tran sit route 33 transverse approach speed 73 arrest mode 61, 6 2. 140, 15 2. 165 effect 35 for ce 62 pushin g for ce 21. 59 resistance of a tug's hull 50 thr ust 23 Triple A design 113 Triple E type 113 tripping 88, 89 tug assist in station keeping 78 assist manoeuvres 147 assistanc e in ice 38 assistance m eth od 2, 33. 68 availability 79 azimuth stem drive (ASD) 9 bo llards 97 budget , , 2 capabilitie s and limitati ons 43 configuration 66 control program 123 convention al 9. 14, 16. 17, 18 cr ew 172 critical situati ons 86 developments 163. 171 effectiveness 10 efficien cy , 109 harbour , , 1, 7 instal led engine power III Kort n ozzle 9 limitations 93 m ano euvrability 9
manoeuvring space 10 model tests 13] ope ra ting at a sh ip' s side 54 ope ra ting at th e ship's side 6 operating in pu sh-pull mod e 76 ope rati ng po sition s 43 pe rform ance 29. 43, 85 an d safety 118 calcula tion programs 122 prope llers 75 push er . 9, 26 respo nse tim e , 9 rev erse-tractor 9, 10, 13 risks 80 risks invo lved 86 ROTOR 8, 163 safet 10l safety 66. 86. 109, 172 in ice 42 salvage 115 simulation 125. l31, 132 . 171 single screw 6. 17 stability 43, 90. 172 tariffs 78 teth er ed 152 towing on aJine 6. 5 1, 85, 137. 13 8 tractor 8, 9, 21 tractor type 6 triple scr ew 17 twin screw 6. 17 type 6 ca pab ilities an d limit ation s 119 type suitability , 65 unde rwater resistanc e 110 Voith -Schneid er 9 wash effects 74 which type? 1 with azimu th propelle rs aft 6 work ing m eth od 1 Zvpeller 9 tug captain training 118. 13 0 Tug Omni 2000 163 TUGS1M performance gr aphs 61. 143 sim ulation pr ogram 59 Tugz In ternati on al LLC 28 tunnel b ow thruster 17 ture an d Mon gstad terminals 151 turning diameter 44 m oment 43. 45, 55 Twaron , 104 twel ve strand braids 103
u UHMW polyeth ylene 13 UK 19, 134. 146. 154. 160 135 UK P&I Club Ul stein 25 Ultra High Module PolyEthylene (UHMPE) . .............................................................. 104 unberthing in ice 41 underk eel clearance 71. 72, 74. B1, 118, 146 un derwater plane 55 pr ofile 46 resistance 48 . 63, 171 Urn -lev er Un iversity of Washington unsafe situations URS US Coast Guard (USCG ) 5 1. 134. U SA 14, 19. 21. 29, 34 , 38. 48.
25 21
80 101 158 58 ,
TUG USE IN PORT 191
J.M .• Gmb H propulsion _._ Schneider _ Schn eider propulsion tractor tug _ tugs (VS tugs) turbo fin (VTF) water tractor _
............ 100, 102, 122, 123, 134, 139, 15°
6
······f~d~·;;;j·;:;;i~~·::::::::::::::::::::::.I~~: . 1.5.~:. :~3 federal rules for escorting p orts
- 161 35. 36
v Valdez Arm 158 Narro ws _ 158 validati on phase 131 Vancouver , , 55 vector tug '" 130 model 125 Venezuela 17 vertical towline angle 109. 110 tug acceleration 110 vessel escort and resp onse plan 152 escort and response plan (VERP) 158 traffic services 135 set virtual mass 73, 74 reality (VR) technology 172 viscous resistance 82 Voith 57. 120
VHF
25 7, 9, 143
21 117
21 22 148 21
VS co mpact tug, _ _ _ 170 escort tug __ _ 144 de sign 14 8 p itch lever _. 61 propellers 80 propulsion " ,._.. 140 reverse-tractor tug 154 tractor tug 26, 27, 30, 46. 52, 57, .............. 64, 65, 95. 144, 158. 159. 171 tug 21. 22, 23, 25, 32, 35, 37, 38, .......- 45, 48, 54 . 59 , 61, 66, 75, 123, ................. 143, 144. 146. 154, 158. 169 design __ 65
II w
Wj. Tretter Warrington/Seale washtngton Tanker Law water depth restricted __.._
,.,
watertight doors _ wave conditions _ for ces _ p attern waves influence on tug p erformance " 'eHand Ca nal , _ __ 'Vest Indies _ _ wheelho use constru ction .._ layout wide b eam tractor tugs Wijsmuller Engineering / Ma rin , H arb our Towage Amsterdam winch " groove profile waterfall , _.., _ wind and current forces , , for ces indicator _ _ vel ocity fluctuations
92 106 158
y
4
z
yam-to-yar n friction yaw moment _
Z.pe llers .." ..,
Z·Two
192 THE NAUTICAL INSTITUTE
,
93 6472 82
"
131 161 13 410 , 10 56
,
132 ,.. 20
98 98 91 69 130 70
105 69, 71, 75
25 29
