Acknowledgements 1 wish to lhank the many firms, organisations and individuals who have provided me with assistance and malcrial during the writing of lhis book. For guidance provided in their specialist areas I would like (0 thank Me W. Cole, Welding Manager and Mr I. Waugh, Ship Manager, both of Swan Hunter Shipbuilders.
To the firm of Swan Hunter Shipbuilders, now a member of British Shipbuilders, I wish 10 extend my thanks for their permission to use drawings and information based on their current shipbuilding practices. The following firms and organisations contributed drawings and information for various sections of this hook, for which I thank them: AGA Welding Ltd Austin and Pickergill Ltd Blohm and Voss, A.G.
DOC CUlling Machines Brown Brothers & Co. Ltd Cammcll Laird Shipbuilders Cape Boards and Panels Ltd Clarke Chapman Ltd Dookin and Co. Ltd F.A. Hughes and Co. Ltd Flakt Ltd (S.F. Review) Glacier Metal Co. Ltd Hempel's Marine Paints Hugh Smith (Glasgow) Ltd International Maritime Organisaton Uoyd's Register of Shipping
MacGregor Centrex Ltd Moss Rosenberg Verft, AS. Odense Steel Shipyard ltd Philips Welding Industries Phoceenne Sous-Marine, S.A. Power Blast ltd Rockwool Co. (UK) ltd Sigma Coatings Ltd Stone Manganese Marine LId Stone Vickers Strommen Staal, A.S. Taylor Pallister and Co. ltd The DeVilbiss Co_ Ltd The Ntn'aJ Architect Voith GmbH Wilson Walton International ltd
Contents The ship - its functions, featutes and types 2 Ship stresses and shipbuilding materials
13
3 Shipbuilding 4 Welding and cutting processes 5 Major structural items A Keel and bottom construction 72 B Shell plating, framing systems and decks 77 C Bulkheads and pillars 85 D Fore end construction 92 E Aft end construction 101 F Superstructures and accommodation 116 6 Minor structural items 7 Outfit 8 Oil tankers, liquefied gas carriers and bulk carriers 9 Ventilation 10 Organisations and regulations II Corrosion and its prevention 12 Surveys and maintenance 13 Principal ship dimensions and glossary of terms
"
72
125 134
165 185 197 213 223
229
Index 235
1
The Ship- its Functions, Features and Types Merchant ships exist 10 carry cargoes across the waterways of the world safely, speedily and economically. Since a large pan of the world's surface. approximately three·fifths. is covered by water. it is reasonable 10 consider that the merchant ship will continue to perform its funclion for many centuries 10 come. The worldwide nalure of this function involves the ship, its cargo and its crew in many aspects of inll~rnational life. Some It:atures of this international transportation. such as weather and climatic changes, availability of cargohandling facilities and international regulations. will be considered in later chapters. The ship, in its various forms. has ~volved to accomplish its function dqxnding upon three main factors - the type of cargo carried. the type of construction and materials used. and the area of operation. Three principal cargo-carrying Iypes of ship exisl loday: Ihe general cargo ftSSel, !he lanker and the passen~r ve~1. The general cargo ship functions today as a general carrier and also, in several particular forms, for unit-based or unitised cargo carrying. Examples include container ships, pallet ships and 'roll· on, roll-ofr ships. The tanker has its spedalised forms for the carria~ of crude oil, refined oil products, liquefied gases, etc. The passenger ship includes, senerally speaking, the cruise liner and some ferries. The type of construction will affect the cargo carried and, in some generally internal aspects, !he characteristics of !he ship. The principal types of construction refer to the framing arrangement for stiffening the outer shell plating, the three types being longitudinal, transverse and combined framing. The use of mild steel, special steels, aluminium and other materials also influences the characteristics of a ship. General cargo ships are usually of transverse or COmbined framing construction using mild steel sections and plating. Most tankers employ longitudinal or combined framing systems and the larger vessels utilise high tensile steels in their construction. Passenger ships, with their large areas of superstructure, employ lighter metals and alloys such as aluminium to reduce the weight of the upper regions of the ship. The area of trade, the cruising ra.nge, the climatic extremes experienced, must all be borne in mind in the design of a particular ship. Ocean-going vessels reqUire several tanks for fresh water and oil fuel storage. Stability and trim Irrangemenu musl be satisfactory for the weather conditions prevailing in the
2
Th~
Ship - 111 Functions, Features and Types
3
area of operation. The strength of the structure. its ability to resist the effects of waves, heavy sus. etc., must be much greater for an ocean-going \'essel than for an inland waterway vessel. Considerations of safety in all aspects of ship design and operation must be paramoun!. so The ship must be seaworthy. This term relates to many aspects of the ship: it must be capable of remaining anoat in all conditions of wC3ther; it Inust remain alloat following all but the most serious damage: and it musl remain stable and behave well in the various sea states encountered. Some of the constructional and regulatory aspects of seaworthiness will be dealt with in later chapters. Stability and other design aspects are explaincd in dt'uil III .VOl'o/ Architecture {or Marine Enginccrs. by W, Muckle (Bulterworths, 1975). The development of ship types will continue as long as there IS a sufficient demand to be met in a particular area of trade. Recent ynrs ha\'c seen such developments as very large crude carriers (VlCCs) for the transport of oil. and the liquefied natural gas and liquefied petroleum gas tankers for the bulk carriage of liquid gases. Can tainer ships and \ roriou5 barge o.:arriers have developed for general cargo transportation. Bulk carriers and combination bulk cargo carriers are also relatively modern developments. Seveul basic ship types wiJI now be conSidered in further detail. The particular features of appearance. construction. layout. size. etc. will be examined for the following ship types:
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(I) General cargo ships. (2) Tankers. (3) Bulk carriers. (4) Container ships. (5) Passenger ships.
Many other types and minor \~.triations exist. but the above selection is considered to be representative of the major part (If the world's merchant neeL
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J General cargo ships The general cargo ship is the 'maid of all work', operating a worldwide 'go anywhere' service of cargo transportation. II consists of as large a clear open carga
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cargo·handling limes and manpower requiremenls. A special heavy.lift derrick may also be fitted. covering one or two holds. Since full cargoes cannot be guaranteed with this type of ship. ballast
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The accommodation and machinery spaces are USUally located with one hold between them and th'e aft peak bulkhead. This arrangement improves the vessel's trim when it is partially loaded and reduces the lost cargo space for shafting tunnels compared with the cenltaJ machinery space arrangement. The Current range of sizes for general cargo ships is from 2000 to 15000 displacemenl lonnes with speeds of 12-18 knots.
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The filling of refrigeration planls for the cooling of cargo holds enables the carriage of perishable foodstuffs by sea. Refrigerated ships vary lillie from general cargo ships. They may have more than one tween deck, and all hold spaces will be insulaled to reduce heal transfer. Cargo may be carried frozen or chilled depending upon its nature. Refrigerated ships are usually faster than general cargo ships, offen having speeds up to 22 knots, and they may also cater for up 10 12 passengers.
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The tanker is used to carry bulk liquid cargoes, the most common type being Ihe oil tanker. Many other liquids are carried in tankers and specially constructed vessels are used for chemicals, liquefied petroleum gas. liquefied natural gas. etc. The oil tanker has fhe cargo
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,c<:onltnodatiOn and machinery spaces 3re located aft in modem tankers. The range of sizes fOf oil tankers at present is enorrnous. from small to 700000 deadweight tonnes. S~eds range from 12 to 16 knolS. Oil lanke", 3rc dealt W'ith in more detail in Chapter 8.
Liquefied gas tankers (jquefied gas tankers are used to carry. usually at low
tem~ratufe. liquefied gas (LPG) or liquefied natural gas (LNG). A ~parate inner tank is 1eum ~IY employed to contain the liquid and this lank is supported by the outer .uall which has a double bottom (Figure 1.3). LNG tankers carry methane and other paramn products obtained as a by' product of petroleum drilling operations. The gas is carried at atmospheric pressure and temperalUres 3S low as _164°C in tanks of special materials (see T.b/~ 2.3). which can accept the low temperature. The tanks used may be prismatic. cylindrical or spherical in shape and self·supporting or of membrane ~RStruction. The containing tank is separated from the hull by insulation which also acts as a secondary barriet in the event of leakage. LPG tankers carry propane. butane. propylene, etc .• which are extracted (rom natural gas. The gases are carried either fully pressurised. part pressurisedpart refrigerated or fully refrigerated. The fully pressurised tank operates at 18 bat and ambient temperature. the fully refrigerated tank at 0.25 bar and -saoc. Separate containment tanks within the hull are used and are surrounded by insulation where low temperatures are employed. Tank shapes are either prismatic, spherical or cylindrical. Low temperature steels may be used on the hull where it acts as a secondary barrier. Displacement siltS for gas carrie", range up to 60 000 tonnes, with speeds of 12-16 knots. liquefied gas carrie", are dealt with in mme detail in Chapter 8 . uo
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Bulk C8rrtel'$ Bulk carriers are single-deck vessels which transport single-commodity cargoes such as grain. sugar and ores in bulk. The cargo-carrying section of the ship is divided into holds or tanks which may have any number of arrangements, depending upon the range of cargoes to be carried. Combination carriers are bulk carriers designed for flexibility of operation and able to transport anyone of sew:ra1 bulk cargoes on anyone voyage. e.g. ore or crude oil or dry bulk cargo. The general.purpose bulk carrier. in which usually the central hold section onJr is used for cargo. is shown in Figurcs 1.4 and 1.5(a). The partitioned tanks which sunound it are used for ballast purposes either on ballast voyages or. in the ~ of the saddle tanks, to raise the ship's centre of gravity when a low denSIty cargo is carried. Some of the double·bottom tanks may be used for fuel oil and fresh water. The saddle tanks also serve to shape the upper region of the c~go hold and trim the cargo. Large hatchways are a feature of bulk carriers, SUIte they reduce cargo-handling time during loading and unloading. . An ore carrier has two longitudinal bulkheads which divide the cargo section tnto wing tanks port and starboard, and the centre hold which is used for ore. The high double bottom is a feature of ore carriers. On baUast voyages the wing
The Ship _ Its Functioru, Frorures and Types
9
8 pnlc.s and double bottoms provide baUast capacity. On loaded voyages the ore is (;&fried in the central hold. and the high double bottom serves to raise the centre of pvity of this very dense cargo. The vessel's behaviour at sea is thus much iJnproved. The cross·section is similar to that of the ore/oil carrier shown in Frpre J.5fbJ. Two longitudinal bulkheads are employed to divide the ship into ~nue and wing lanks which are used for the carriage of oil cargoes. When orc is carried, only the centre tank section is used for cargo. A double bottom is fitted beneath the centre tank but is used only for water ballast. The bulkheads
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10
Thf! Ship - Its Functions. F~Qtuns lUTd Types \I
it is sometimf!S fitted. Combination carriers handling oil cargoes have their own cargo pumps, piping systems. f!tc., for discharging oil. Bulk carriers are dealt with in more detail in Chapter 8. Deadweight capacities range from small to 150000 tonnes depending upon type of cargo, etc. Speeds arc in the range of 12- J6 knots.
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The cargo--earrying section of the ship is divided into several holds which have hatch openings the full width and length of the hold (Figure 1.6). The COntaincrs arc racked in special frameworks and stacked one upon the other within the hold space. Cargo handling therefore consists only of vertical movement of the cargo in the hold. Containers can also be stachd on the hatch covers where a low deruily cargo is carried. Special lashing arrangements exist for this purpose and this deck cargo to SOme extent compensates for the loss of underdeck capaCity. The various cargo holds are separated by a deep web-framed structure to provide the ship with transverse strength. The ship section Outboard of the Containers on each side is a box-like arrangement of wing tanks which provides longitudinal strength to the structure. These wing tanks may be utilised for water ballast and can be arranged to COunter the heeling of the ship when discharging containers. A double bottom is also fitted which adds to the longitUdinal strength and provides additional ballast space. Accommodation and machinery spaces are usually lOCated aft to provide the maximum length of full·bodied ship for container stowage. Cargo-handling gear is rarely fitted, as these ships travel between speciaUy equipped terminals for rapid loading and discharge_ Container ship sizes vary considerably with container-earrying capacities from 100 to 2000 or more. As specialist carriers they are designed for rapid transits and are high powered, high speed vessels with speeds up to 30 knots. Somc of the larger vessels have triple-screw propulsion arrangements.
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Ship Stresses and Shipbuilding Materials The ship at sea or lying in still water is being constant!)' subjected \0 :I wide variety of sueues and strains. which result from the action of (orces from outside and within the ship. Forces within the ship result from structural weight. cargo. machinery weight and the ef(ects of operating machinery. Exterior forces include the hydrostatic pressure of the water on the hull and the action of the wind and waves. The ship must at all times be able to resist and withstand these messes and strains throughout its struClUre. II mUSI therefore be conslrucled in a manner, and of such materials, that will provide the necessary strength. The ship must also be able to function efficiently as a cargo-carrying vessel. The various forces acting on a ship are constantly varying as to their degree and frequency. For simplicity. however. they will be considered individually and the particular measures adopted to counter each type of force will be outlined.
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The forces may initially be classified as static and dynamic, Static forces are due to the differences in weight and buoyancy which occur at various points along the length of the ship. Dynamic forces result from the ship's motion in the sea and the action of the wind and waves. A ship is free to move with six degrees of freedom - three linear and three rotational. These motions are described by the terms Ylown in Figure 2. J. These static and dynamic forces create longitudinal, transverse and local stres.ses in the ship's structure. Longitudinal stresses are greatest in magnitude and result in bending of the ship along its length.
13
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Ship Stresses and Shipbuilding MateriDls 8u.ouncy
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Longitudinal stresses Static loading If the 1h.ip is considered floating in stm water, two different (orctS will be acting upon it along ils length. The weight of the ship and its contents will be acting vertically downwards. The buoyancy or vertical component of hydrostatic pressure will be acting upwards. In total. the two forces exactly equal and balance one another such that the ship floats at some particular draught. The centre of the buoyancy force and the centre of the weight will be vertically in line. However. at particular points along the ship's length the net effect may be an excess of buoyancy or an excess of weight. This net effect produces a loading of the structure, as with a beam. This loading results in shearing forces and bending moments being set up in the ship's struCture which tend to bend it. The stalic forces acting on a ship's structure are shown in Figure 2.2(Q). This distribution of weight and buoyancy will a.Iso result in a variation of load, shear forces and bending moments along the length of the ship. as shown in FlgU.res 221bj-(d}. Depending upon the direction in which the bending moment acts, the ship will bend in a longitudinal vertical plane. This bending moment is known as the still water bending moment (SWBM). Special terms are used to describe the two extreme cases: where the buoyancy amidships exceeds the weight, the ship is said to 'hog', and this condition is shown in Figure 2.3; where the weight amidships exceeds the buoyancy. the ship is said to 'sag', and this condition is shown in Figure 2.4.
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Ship Stressf?s and Shipbuilding Materials
Dynamic loading If th\: ship is now considered to be moving among W
The traditional approach to solving this problem is to convert this dynamic situation into an equivalent statk une. To do this, the ship is assumed to be balanced un a static wave of trochoidal form and length equal to the ship. The profile of a wave'll: sea is considered to be 1I trochoid. This gives waves where the creslS are sharper than the troughs. The wave crest is considered initially at midships and then at the ends of the ship. The maximumqagging and sagging moments will thus occur in the structure far the particular loaded condition considered, as shown in h'gure 2.5.
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Shipbuilding Materials
17
decks and compressive stresses in lh-e bottom shell. This stressing, whether compressive aT tensile. reduc('s ill magniwuc lowuds a position kn'J'wn
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The total -shear force and bending moment are thus obtained and these will ndude the still water bending moment t;onsidered preViously. If actual IoadJng ~onditions for the ship aTe considered which will make the above conditions lIorse, e.g. heavy loads amidships when the- wave trough is amidships, then the naximum bending moments in normal operating servic-c can be found. The ship's stmcture will thus be subjected 10 constantly fluctuating streSSes esulling from these shear forces and bending moments as the waves move along he ship's lengtll.
;tressing of the structure :he bending of a ship causes stresses to be set up within its structure. When a hip sags, tensile stresses are set up in the bottom shell plating and compressive tresses are set up in the deck. When the ship hogs, tensile stresses occur in the
is obtained for the stress in the material at some distance y from the neutral axis. The values M. I and y can be determined for the ship, and the resulting stresses in the deck and bottom -shell can be fuund. The ratio f!y is known as tne section modulus. Z, when y is measured to the extreme edge of the section. The values are d-etermineJ for the midship scction, since the greatest moment will occur at or near midships (see Figure 2.2). A mme detailed explanation of this process is given in Muckle's work, Nrrval Architecture jar Marine Engineers, previously cited. The stmctura.l materil:l1 induded in the calculation for the second moment J will be all the lungitudinal material which extends fm a considerable proportion of the ship's length. This material will include side and bottom shell platin;g, inner bottom plating (where fitted). centre girders and decks. The material forms what is known as the hull girder, whose dimensions arc very large compared to its thicknesS.
Transverse stresses Static loading A transverse section uf a ship is subjected to static pressure fmrn the surrounding water in addition to -the loading resulting from the weight of the structure, cargo, etc. Although transverse stresses are oflesser magnLtude than longitudinal stresses, considerable distortion of the structure ~ould ocellI, in the absence of adequate stiffening (Figure 2.6). The parts of the structure which resist transverse stre-sses are transverse bulkheads, Ooors'in the double bottom (where fitted), deck beams, side frames and the brackets between them and -adjacent structure such as tank top flooring or margin -p13tes.
18
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Ship Stresses and Shipbuilding Mat~rillh Def'e<;110~
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, Slamming or pounding
In heavy weather, when the ship is heaving and pitching, the forward end leaves and re-enters the water with a slamming effect (Figure 2.8). This slamming down of the forward region on to the water is known as pounding. Additional
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effect is felt when the ship is in the- light or ballast condition (Figure 2. 7). The brackets and beam knees. joining horizontal and vertical items of structure are used to resist this distortion.
The movement of waves along a ship causes fluctuations in water pressme on the ,plating. Th~ tends to create an in-and-out movement of the shell plating, known as panting. The effect is particularly evident at the bows as the ship pushes its way through the water. The pitching motion of the ship produces additional ·..ariatiuns in water pressure, particularly at the bow and stem, which also cause panting of the plating. Additional stiffening is provided in the form of panting beams and stringers. This is discussed further in Section 0 of Chapter 5
Localised loading Localised stresses The movement of a ship ill a: seaway results in forces being generated which aJe largely of a local nature. These forces are, however, liable to cause the structme to vibrate and thus tranunit stresses to other pans of the structure.
Beary weights, such as equipment in the mac-hinery spa<:cs or particular items of general cargo, can give rise to localised distOItiOfl. of the transverse section (Figure 2.9). Arrangements fm sp-r-c-adil1g HLC load, addilional stiffening and thicker plating are methods used in de.aling with this problem.
20
Ship Stresses and Shipbuilding Materials
Ship srresses {Jtld Shipbuilding MateriaA Ddlec,;"" n' , "Id·jng
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usually exceeding ilbout 2%. Special steeh of high tensile strength are used on certain highly stressed parts of Ihe shi~'s structure. Aluminium alloys have particular applications in the constructlOn of superstructures, espeCIally on passenger ships.
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Superstructures and discontinuities The ends of superslructures represent major discontinuities in the ship's structllTe where a considerabJ.e change in section modulus occurs. Loc.aJised st,resses. will. occur wllich may result in cracking of adjacent structure. Sharp (flscontmuitle, are therefore to be avoided by gradual tapers being introJuced. Thicker strakes of deck and shell plating may also be fitted at these points. Any aoles or -openings cut in decks create similar areas of hig.lllocal stre,s. Well-rounded comers musl be used where openings are necessary, and doubling plates may also be filled. In the case of hatchways the bulk of the longitudinal strength matl;rial is concentrated outboard of the hatch openings on either side to reduce the change in sccHan modulliS al tile openings, This is discussed further in Sections Band F of Chapter 5.
Vibr.ations Vibrations- set up in a: ship due to reciprocating machinery, propellers, etc., can result in the setting up of stresses in the structure. These are cyclic stresses which could result in fatigue failure of local items of structure leading to mme general collapse, Balancing of machinery and adequate propeller tip clearances c-an rcduoCC the effects of vibration to acceptable proportions. Aplirt fmm possible damage to equipment and structure, the presence of vibration can be most uncomfortable to any passengers and the crew. The design of the slmcture is outside the scope of this book. The various shipbuilding materials used to provide the structure will now hI; considered.
Steel Ste~l is the basic shipbuilding material in use today. Steel may be regarded as an lron-carbon alloy, usually containing other elements, the carbon content flot
'Acid' OJ 'basic' are terms often use-d when referring to -steels. The re~erence is t.o the production process and the type of furnace lining, e.g. an alkaline or baSlc lining i-s used to produce basic ~teel. The choke of furnace linmg IS dictated by the raw mateJials used in the manufacture of the steet There are three partlcular esses currently used for lne m-3TIufacture of carbon steeL namely the open ~:th process, the oxygen or basic oxygen steel proces~ and the elec:tric furnace racess. In all these processes the het molten metal is exposed to alT o.r oxygen p hich oxidises the impurities to refine the pig iron into high quality steeL W In the open hearth process a long shallow furnace is used which is fired from both ends. A high proportion of steel scrap may be used in this process.. Htgh qua!jty steel is produced whose properties can be controlled by the addltlOn of suitable alloying elements. . .. In the oxygen or basic oxygen steel proce-s~ the molten metal IS contamed m a basic lined furnace. A jet of oxygen is injected into the molten metal by an overhead lance. Alloying elemcnts can be introduced into the rno1ten metal and a high quality steel is produced. In the electric furnace process. an electric arc is struck between carbon electrodes and the steel charge in the fum-ace. Accurate control of tne final composition of the steel and a high standard of purity are possible with this process,
Finishing treatment Steels from the above·mentioned processes will all contain an excess of oxygen, usually in the form of iron oxide. Several finishing treatment~ are possible in thermal casting of the steel. Rimmed ~teeJ is produced as ~ resuli of little ur no treatment to remove Oxygen. In the molten state the oxygen combines with the carbon in the steel. roCleasing carbon monoxide gas. On solidifying, an almost pure iron ollter surface is formed. The central core of the ingot is, however, a mass of blow holes. Hot rolling of the ingot usually 'welds up' thes.e holes but tnick plates of this material are prone to laminations. Killed steel is produced by fixing tlw OXygCll hy the addition of aluminium or &ilicon before pouring. the steel into the mould. The uluminium or s.llicon. produoCes oxide~ reducing the iron oxides to iron. A homogeneous rnatenal ot , , superior quality 10 rimmed s(cd is thus produced. Balanced or sl;mi·kilkd steels arc an intermediate form ot steel ThIS resulls from the beginning of the rimming process in the mould and its termination by tne use of deoxidi~ers. VaoJ.U1ll degassecl steels are produced hy reducing the atmospheric pressure WhCli the steel is ill the llloltc-n state. The equilibrium between carb~)n and
22
Ship Stresses and Shipbuilding MateriD/J
Ship Stresses and Shipbuilding MaleriDls
oxygen is thus obtained at a much lower level and the oxygen content becomes very small. Final residual deoxidation can be achieved with the minimum additions of aluminium or silicon. A very 'clean' steel is produttd with good notch toughness properties and freedom from lameUar tearing problems (lamellar tearing is explained in Chapter 4). The composition of steel has a major influence on its properties and this will be discussed in the next subsection. The properties of steel are further improved by various forms of heat treatment which will now be outlined. In simplified terms the heat treatment of steels results in a change in the grain structure which alters the mechanical properties of the material.
23
sund ard steel sections ..nety of standard sections are produced ~ith vatting scantlings to suit their ~tion. The stiffenin~ of plates and sections utihses one or more of these ~. which are shown m Figure 2. J O.
Norma/u;ng. The steel is heated to a temperature of 850-950°C depending upon its carbon content and then allowed to cool in air. A hard strong steel with a refined grain structure is produttd.
,.,
I",
'"
Annazling. Again the steel is heated to around 850-950°C. but is cooled slowly either in the furnace or in an insulated space. A softer more ductile steel than that in the normalised condition is produced. Hardening. The Sleel is heated to 8S0-9S0°C and then rapidly cooled by quenching in oil or water. The hardest possible condition for the particular steel is thus produced and the tensile strength is increased. Tempuing. This process follows the quenching of steel and involves reheating to some temperature up to about 680°C. The higher the tempering temperature the lower the tensile properties of the steel. Ontt tempered, the metal is rapidly cooled by quench;-:g.
J[ ,,,
,.,.... 1.10 St.,.d4ud
,.,
,,,
Jtt~l UCtiOllJ: (tI)
fltlr plllr~: (b) Dffut bulb pllllt: (cJ tqutzl tln,le; (d) unequal tlfIIlt; (t) chtlMtI: tnla
Composition and properties Various terms are used with reference to steel and other materials to describe their properties. These terms will now be explained in more detail.
lIIlipbuiding .-Is The steel used in ship construction is mild steel with a 0.15-0.23% carbon
Tensile strOlgrh. This is the main single criterion with reference to metals. It is a measure of the material's ability to withstand the loads upon it in service. Terms such as stress, strain, ultimate tensile strength, yield stress and proof stress are all different methods of quantifying the tensile strength of the material. The two main facton affecting tensile strength are the carbon content of the steel and its heat treatment following manufacture. Ductility. This is the ability of a material to undergo permanent changes in shape without rupture or loss of strength. It is particularly important where metals undergo forming processes during manufacture. 1fIlrdnen This is a measure of the workability of the material. It is used as an assessment of the machinability of the material and its resistance to abrasion.
Toughness. This is a condition midway between briulenm and softness. It is often quantified by the value oblained in a notched bar test.
ctJDtent. The properties required of a good shipbuilding steel are: (I) (2) (3) (4) (5) (6)
Reasonable cost. Easily welded with simple techniques and equipment. Ductility and homogeneity. Yield point to be a high proportion of ultimate tensile strength. Chemical composition suitable for flame cutting without hardening. Resistance to corrosion.
These features are provided by the five grades of mild steel (A-E) designated by the C~fication societies (see Chapter 10). To be classed, the steel for ship co:;urucli?n must bt manufactured under approved conditions, and inspected, :' prescnbed tesu must be carried out on selected specimens. Finished material ...;~ped Wi.~ the ~ety's brand. a symbol with L superimposed on R being of ~OYd s ~gLSter. The chemical composition and mechanical properties a Ie eClion of mild steel grades are given in Table 2./.
r
Tlble 2.1 PROPERTIES AND COMPOSITION OF SOME MILD STI::r:LS
'" ~
UlIImult'
C
N,
51
S
P
(%)
,%)
(%)
(%)
(%)
LR 'A' mUd ncel
0.23 mu.
2.5)(. C
min.
0.50 mlUl.
0.05 mu.
0.05 mux.
LR '0' mUd liCe!
0.21 mix.
0.701.40
0.10-
0,"
0.04 mu.
0.04 m",.
0.015 min.
LR 'E' mild stcel
0.18 mIX.
0.70-
0.100.50
0.04 mIX.
0.04 mIX.
0.015 min.
!.SO
A' (%)
M/"Imllm yield SIfI'U
SI'f!U
(N/mm')
IN/mm')
MiltlmulII f'/ollxallm, (%)
230
400-490
22
'" '"
400-490
22
-
Tlble 2.2 I'ROPERTIES AND COMPOSITION OF SOME HIGHER
/1'I1S1l1i
(1I/1"J)'
(J)
413\
O·C 400 490
"
27 DI
-41f('
TI~NSILE sn:I~LS
Millimum
Uf,i",/I/t 1('II,IIt
MI,,/"',m'
C
N,
51
S
P
AI
)'/t/d 1/"'11
I/,rll
"/WllfulltN'
(7I/UP)'
(%)
(%)
("')
(%)
(%)
(%)
(N/mm')
(N/mm'l
f%'
(J)
0.18 m",.
0.701.60
0.50 mu.
0.04
0.04 max.
0.015 min.
"'
440-590
22
mu.
LR AIU6
0.18 max.
0.701.60
0.50 max.
0.04 max.
0.04 mix.
0.015 min.
'"
490-62U
LR Eli 36
0.18 mu.
0.701.60
0.100.50
U.04 max.
0.04
0,0]5 min.
'55
490-620
LR 11.1132
mu.
)1 a1 UaC
.l4 al O"C
" "
)4 III
-40·C
Table 2.3 PROPERTIES AND COMPOSITION OF TYPICAL LOW TEMPERATURE CONSTRUCTIONAL MAT1~RIALS
C
N,
Sf
S
("')
(%)
("')
(%)
Low carbon staWeu steel 0.03 m.... AISI304
1.2
0.75
36% Nlalloy
0.09
0.3
0.20
'')
(%)
,"'"')
C,
0.02
0.02
-
10.7
18.5
-
0.'
om
0.02
-
3.5.8
-
-
0.300.60
0.150.35
0.035 mix.
0.035 mIX.
-
4.5S.O
-
-
0.13 m....
0.'
0.150.)0
0.040 mIX.
0.035 mu.
8.5-
-
-
,"'C.)
,"')
0.10 mu.
0.40-
P
((nvu)
51' Nillul
m....
91' Ni steel
Aluminium Illoy S083
m....
N,
1.0
AI
-
•.s
N.
''''' "'»
51
N,
I:e
AI
Z,
C,
TI
("')
,%)
("')
("')
(%)
(%)
(%)
0.40 max.
4.0-
0.40 m....
Rem.
0.2.5 max.
0.050.25
0.15 max.
•••
Mlllimum
U1/fm/l/e
y/tld or proof
Itlullt "rtnx,h
(N/mm')
(N/mm' )
Minimum e/onpl/on (%)
'"
560
SO
103 al -196"C
11S
..,
480
"
147 II -196"C
.590-740
20
881\
587
690-830
22
".
m
16
fl""
OI/1rpy (l)
-120'C 34 II -196'C
26
Ship Streue$ and Shipbuilding Materillls
Developments in steel production and alloying techniques have resulted in the availability of higher strength steels for ship construction. These higher tensUe strength (HTS) steels, as they are caJled, have adequate notch toughness, ductility and weldability, in addition to their increased strength. The increased strength results from the addition of alloying elements such as vanadium, chromium, nickel and niobium. Niobium in particular improves the mechanical properties of tensile strength and notch ductility. Particular care must be taken in the choice of electrodes and welding processes for these steels. Low hydrogen electrodes and welding processes must be used. Table 2.2 indicates the chemical composition and mechanical properties of several high tensile steel grades. A special grade mark, H, is uSt'd by the classification societies to denote higher tensile steel. Benefits arising from the use of these steels in ship construction inclUde reduced structural weight, since smaller sections may be used; larger unit fabrications are possible for the same weight and less welding time, although a more specialised process, is needed for the reduced material scantlings. Cryogenic or low temperature materials are being increasingly used as a consequenct of the carriage of liquefied gases in bulk tankers. Table 2.3 details the properties and composition of several of these cryogenic materials. The main aiterion of selection is an adequate amount of notch toughness at the operating temperature to be encountered_ Variow alloys are principally used for the very low temperature situations. although special quality carbon/manganese steels have been used satisfactorily down to -50°C.
-" <-
N~ 0.
N~
0.
Castings and forgings ~~ o.
The larger castings used in ship construction are usually manufactured from carbon or carbon manganese steels. Table 2.4 details the composition and properties of these materials. Examples of large castings are the sternframe, bossings, A-brackets and parts of the rudder. The examples mentioned may also be manufactured as forgings. Table 2.4 also details the composition and properties of materials used for forgings.
~~
0. ~~
~~
0.
C (':I)
M. (':10)
Si
S
P
(':I)
(':10)
(")
Sleet castings 0.23 1.6 max. 0.60 0.04 0.04 max. but nOI max. max. max.
lIlrimtue unsile Yilld
Minimum
stTerJrtlt stms tlO#Igarion (Nfmm') (Nfmm ' ) (%)
'00
'00
less IhVl
"
,X C Slee) forgings 0.23 nwt.
0.301.70
Sleel forliDas 0.30 0.30(not UlleDded 1.50 forwddind
0.45 0.045 0.045 mu. max. max. 0.45
0.045 0.045 max. max.
0.
24 longitudinal 18 tnnsveue
'30
2I5
24 Iongiludinal 18 tnnsverle
2~
•
0.
28
Ship Srreue! and Shipbuilding Marerial!
Ship Srreues and Shipbuilding Marerials
AJuminium alloys The increasin8 use of aluminium alloy has resulted from its several advantages over steel. Aluminium is about one·third the weighcoLstee1 for~uivalent volume of material. Ibe use or-aluminium alloys in a structure can result ill reaucllons or~of the weight of an equivalent stul structure. This reduction in weight. particularly in the upper regions of the structure, can improve the stabi't 0 e vessel. This follows from the lowering of the vissel's centri:or. ~ resu.!!ing...in.-arLiJ:lcr ase me en nc ~Sta6ility is discussed in detaIfliirluckle's Naval Archir«tu~ for MQrin~ Engineen. The corrosion resistance_of aluminium is very good but careful maintenance and insulation from the adjoining steel structure are necessary. The properties required of an aluminium alloy to be used in ship construction are much the same as for steel. namely strength, resistance to corrosion, workability and weldabililY. These requirements are adequately met. the main disadvantage being the high cost of aluminium. The chemical composition and mechanical properties of the common shipbuilding alloys are shown in Table 2.5. Again these are classilication society gradings where the material must be manufactured and tested to the satisfaction of the society.
• When the force applied tends to lengthen the material the stress is termed ..-:. tress'. When the force tends to cause the various paru of the material ....; ~ver one another the stress is termed 'shear stress', ..,. tensile test is used to determin.e the ~ehaviour of a material up to ,its 'fhe point. A specially shaped specimen piece (Figure 2. J 2) of standard sue ~ in the jaws of a testing machine. A load is gradually applied to draw •
......
1---- 5.65,S",----I
7.65"'S,,_o.._~-_.1
1_'
,.,
";i
1 J 11
I
1---- t" • 511-----1
1_.--,.", ·._.·.--_·1 101 ~ 2.12 Ttrfsilt tnt sp«imt1U: (tI) [or pitltn. !rripl tI1Id ItcriOiU Itl • tJrickntlf o[
mrHtrial): (b) [or hot-ToIftd btlr
IeIt length L 1 of the specimen is known and for each applied load the new
leasth L 2
can be measured. The specimen will be found to have extended by
SOIne small amount L 2 -L 1 • This deformation, expressed as
Extension Original length
Materials testing Various qualities of the materials discussed so far have been mentioned. These qualities are determined by a variety of tests which are carried out on samples of the metal. The terms 'stress' and 'strain' are used most frequently. Stress or intensity of stress, its correct name, is the force acting on a unit area of the material. Strain is the deforming ofa material due to stress. When the force applied to a material tends to shorten or compress the material Ote stress is termed 'compressive
10 ......
•
FiKu" 211 Aluminium tll/oy J«riOllS
Aluminium alloys are available as plate and section. and a selection of aluminium alloy sections is shown in FiguTf! 2.1 J. These sections are formed by extrusion. which is the forcing of a billet of the hot material through a suitably shaped die. Intricate or unusual shapes to suit particular applications are therefore possible. Where aluminium alloys join Ote stul structure, special insulating arrangements must be employed to avoid galvanic corrosion where the metals meet (see Chapter II). Where rivets are used, they should be manufactured from a corrosion-resistant alloy (see Table 2.5).
29
is known as the linear strain.
Additional loading of Ote specimen will produce results which show a ~orm increase of extension until the yield point is reached. Up to the yield ~t the removal of load would have resulted in the specimen returning to its ~~. Stress and strain are therefore proportional up to the yield point, or e ~ limit as it is also known. The stress and strain values for various loads CIn shown on a graph such as Figure 2. J3.
30
Ship Strene3 and Shipbuilding Materill/3
Ship Stresses and Shipbuilding Materloli
Slr~,s
F ,aClU'~
----==~----/
Ultimate ton,il. lIre \5
Yiel-d
"
Yi~ld
€laS;i~
PC"''' limi,
31
'['his coMtan t is known as the 'modulus of elasticity' (E) of the material and has
the same units as stress. The yield stress. is the value of stress at the yield point. Where a clearly defmed yield point is not obtained a proof stress value is given. nus is obtained by a line parallel to the stress-strain line being drawn at some percentage of the strain, such as 0.1%, The interse~tion of this line willi the s.treSS-strain line is considered the proof stress (see FIgure 2.14). The bend test is used to determine the ductility of a material. A piece of material is bent over a radiused fanner. sometimes through 180 degrees. No cracks or surface laminations should a.ppear in the material. Impact tests can have a number of forms but the Charpy vee-notch test is usuallY spedfied. The test specimen is a 10 mm square cross-section, S5 rnm in Length. A vee-notch is cut in the centre of one face, as shown in Figure 2.15. The St,iker
Speci",en
Figure 2.13 Srre~~-st/1lin gNIph for mild steel Slress Ultimate temiie ,lre.1 P'u"f
/
F-raclure
/
f
,tr~ss
NotCh
/"
Figure 2.15 Chllrpy impilet test
St,ain
-I
'_O.'%I\ra'n
Figure 2.14 Stre!r-mairz graph /0' higher leftl'ile steel
[1' thor testing worriO c()nlinued bey()nu 'neck' or reduce in clUs:;-'edlon. Th-o speClInen CIOSS-5Cc!lIJnal ale:> would give highest value of stress is known a~ the material.
Ilw ),jeld point the spcdml;."n wouLd loau value, divided by the original the ~harc shown in Figure 1.13. The ultimate tensile ,tress (llTS} of th~
Within the clastic limit, stre~:; i:; proportiumJ tll strain. and theretore SIres:; Shain
==
constant
specimen is mounted horizontally with the notch axis vertical. The test involves the specimen being struck opposite the notch and fractured. A striker or hammer on the end of a swinging pendulum provides the blow which breaks the specimen. The energy absorbed by the material in fracturing is measured by the machine. A particular value of average impact energy must be obtained for the DUlterial at the test temperature. This test is particulariy important for materials to be used in low temperature regions. For low temperature testing the specimen ia cooled by immersion in a bath of liquid nitrogen or dry ice and acetone for about 15 minutes. The specimen is then handled and tested rapid.y to minUnlse any temperature changes. The impact test, in effect, measures a material's resistance to fracture when shock loaded. A dump test is used on a specimen length of ba.r from which rivets are to be made. The bar is compressed to half its original length and no surface cracks must .appear. Other rivet material tests include bending the shank until the two ends touch without any cracks or fractures appearing. The head must also accept flattening until it reaches two and a half times the shank diameter.
33
,
,
-
3
e
e
•
•
•
t-................. r--...
Shipbuilding
-."
-. b9
........
~
r; \ ~
'\
Building I ship is a complex prottSS involving the many departments of the shipbuilding organisation. the arrangement and use of shipyard facilities and the many skills of the various personnel involved. Those departments directly involved in the construction, the shipyard layout, material movement and the equipment used will be examined in tum.
e
•
e ~
e
e
•
•
L
Drawing office The main function of the shipyard's design and drawing offices is to produce the working d~wings to satisfy the owner's requirements. the rules of the c1assifiation societies and the shipyard's usual building practices. A secondary, but nevertheless important, function is to provide information 10 the production planning and conlrol departments, the purchasing departments. etc .. to enable steelwork outfitting and machinery items to be ordered and delivered to satisfy the building programme for the ship. Closely follOWing the basic design drawings will be the production of the lines plan. This plan (Figu" 3.1) is a scale drawing of the moulded dimensions of the ship in plan, profile and section. The ship's length between the forward and after perpendiculars is divided into ten equally spaced divisions or stations numbered 1 to 10. Transverse sections of the ship at the various stations are drawn to give a drawing known as the body plan. Since the vessel is symmelTical. half·sections are given. The stations 0 to 5 representing the after half of the ship are shown on the left side of the body plan with the forward sections shown on the right. The profile or sheer plan shows the general outline of the ship, any sheer of the decks, the deck positions and all the waterlines. For clarity, the deck positions have been omitted from Figure J.1 and only three waterlines are shown. The various stations are also drawn on this view. Additional stations may be used at the fore and aft ends, where the section change is considerable. The half-breadth plan shows Ihe shape of the waterlines and the decks formed by horizontal planes at the various waterline heights from the keel. This plan is usually superimposed upon the profile or sheer plan. as shown in Figure 3.1. The initial lines plan is drawn for the design and then checked for 'fairness'. To be 'fair' all the curved lines must run evenly and smoothly. There must also be exact correspondence between dimensions shown for a particular point in all the three different views. The fairing operation, once the exclusive province: of a skilled loftsman, is now largely accomplished by computer programs. 32
......... L
•
I) 1/
/"-7 1/V
~ I) ~
~•
"
•o
.
e
•
t'-
I':::
·v
"" l / ' /
V
V V l/
.
e
•
34
Shipbuilding
Once fairtd. the final lines plan is prtpartd and a tablt of offsets is compiled for usc in producing the ship's plates and frames. The traditional practice of drawing plans according to structural areas such as the shell, the deck, the double-bottom framing, elC., is inconvenient in many cases since the ship is nowadays buill up of large prefabricated units. A unit may consist of shell plating, some framing and part of a deck. An expansion of a ship's shell is given in Figure 3.2, sho.....ing the positions of the various units. Plans are therefore drawn in relation to units and contain all tht information required 10 build a particular unit. A number of traditional plans arc still produced for classification society purposes, future maintenance and reference, but without the wealth of manufaclUring information which is only needed on the unit plans. The planning and production control departments require drawing information to compile charts for monitoring progreSS. compiling programmes, producing programmes for material delivery, parts production and assembly and finally unit production and erection.
fI
fI--
••
1• l
~
~
•
,....-
The ordering of steel to ensure availability in line with programmed requiremenU is essential. It must therefore begin at the earliest opportunity,
~
li ~
..;
_c-
~
f-
"!. ~
i
i
vi
f--
Plan issue
Steel ordering
~
.II
With plan approval the ordering of equipment, machinery, steel section and plate, etc., will begin and the plans will be issued to the various production departments in the shipyard. The classification society, the owners and their representatives in the shipyard also receive copies of the plans. During the manufacturing processes, as a result of problems encountered, feedback from previous designs, modifications reque5ted by the owner, etc., amendments may be made to plans. A system of plan recall, replacement or modification in the production departments must be available. This ensures that any future ships in a series do not carl)' the same faults and that co"ective action has been taken.
/
/
Plan approval
The fundamental design plans and basic constructional details must all receive classification society approval and, of course, the shipowner's approval. Unwuai aspects of design and innovations in coostructional methods will receive special attention, as will any depanures from standard practice. Progress is not hindered by the classification societies, whose main concern is the production of a sound and safe structure. The shipowner will normally have clearly indicated his requirements from the design inception and his approval of plans is usually straightforward. Most large shipowning companie5 have a technical staff who utilise their practical experience in developing as near perfect and functional a design as possible.
~
tr>
i
i,
,
(
\\\\-
\ h
\UJ
-
'Q~
~ ~
..;
•
~
36
37
Shipbuilding
occasionally before plan approval where delivery problems may occur. The steel ordering is a key function in the production process. requiring involnmenl with the drawing office. planning departments. production departments and the st~1 supplier. The monitoring and control of stock is also important. since the steel material for a ship is a substantial part of the ship's final cost. Stock held by a ~ipyard represents a considerable capital investment.
Loh work Loft work takes place in a mould loft. The mould loft is a large covered area with a wooden floor upon which the ship's details are drawn to full size or some smaller more convenient scale. Much of the traditional loft work is now done by computer but some specialist areas still require wooden templates to be made, mock-ups to be constructed, etc. In the traditional mould loft operation the lines plan and working drawing information is converted into full-scale lines drawn on the 10ft noor. From these lines the (airing or smoothness of the ship's lines is checked and a scrieve board produced. A scrieve board is a wooden board with the body sections at every frame spacing drawn in. Once the ship's lines arc checked and fair, a half-block model is constructed by joiners usually to about 1!50th scale. This model has the exact lines of the ship and is used to mark out the actual plates on the shell, giving all the positions of the butts and seams. The loftsman can now produce templates for marking. cutting and bending the actual plates using the full·size scrieve board markinp.s in conjunction with the plate positions from the model. A table of offsets is produced finally for the various frames and plates, giving manufacturing information for the various tndes involved in production.
On~Unth
scale lofting
•
•i
••, .,
.", . i
<
"
"!
g
"• •'1
H
!t i.
t
! r~\
~\
~
•
~
, ,~
v
~
.;:
~
~
•;
~
, ~
-~
,,
I
,
~
-g ~
l
~
<:
•- ;
,
With one-tenth scale lofting the mould loft becomes more of a drawing office with long tables. Fairing is achieved using the one-tenth scale drawings. The scrieve board is made to one-tenth scale, perhaps on white-painted plywood. One-tenth scale drawings are then made of the ship's individual plates. These dnwings may then be photographed and reduced in scale to one-hundredth of full size for optical projection and marlting of the plates. Alternatively, the one-tenth scale dnwings may be tnced directly by a cutting machine head.
t L
Numerical control A numerical control system is one where a machine is operated and controlled by the insertion of numerical data. The numerical data is a sequence of numbers which fully describe a part to be produced. In addition, the use of certain code numbers enables instructions to be fed into the machine to enable it to operate automatically. A reading device on the machine converts the numbers into electrical impulses which become control signals for the various parts of the machine which produce the finished part.
i<
"
1
:~ ~
38
Shipbuilding
Shipbuilding
. The i~pul data for the machine is initially produced from drawings and offset Informal.lon. !he various parIS to be produced 3rc programmed and then coded ~r deSCribed In numeri~~ terms. Punched card, punched tape or magnetic tape IS then p~oduced contalrung I~e numerical data. The card or tape information is then fe~ mlo a compuler neSlang program. The various pariS are then 'nested' or econo~lcally fi."ed into a standard plate size (Figurr 3.1). A final punched or magnetIC tape IS. produ~ed Which. is. used for the operation of the numerically controlled machine. ThIS process IS illustrated by a simple now chart in Figu
~4.
rr
sler before, during and after the various
pro~esses
in
shipbuildin~
39
utilises
uan handling appliances. such as overhead travelling cranes, vacuum 11ft cranes ~l8JIetic cranes, roller conveyors, fork.li~t equipment, etc. . .
o The various steel parts in plate and sectlon form are now Jomed to~ether by lding to produce subassemblies, assemblies and units. A subassembly IS several ~ of steel making up a tw
;;-;n
Shipyard layout The sh.i~yard layout is arranged to provide a logical ordered now of materials an~ equlpme~l towuds the final unit build-up, erection and outfitting of the ship_ The vanous production stages are arranged in work areas or 'shops' and as far as .pracli~ble. in modem yards, take place under cover. The sequence' of eve,U$ IS outlllled In Figu~ 3.5.
SVl'I\ITIeltd
tauUal
---I
o
Prdiminafy ship desip Dra, oC delaikd plans
• Slet! ordered
ApproVal oC plans and iuue
•
Stet! dtlivUed
LoClwork and production oC
taMe
0+ oirK'ls
~
_______
~
lillie oC stet! and production begun
Inpplng
t
Material prtpantion - lhot4:llutinl and priminl
•
Manuearure oC plates and sections - marltins. euttinl. macb.in.in,: and shapina
o
Notch tor~11 ~m
Subassemblies and assemblies produced
Figt.lrt 3.6 SubotsrmbJ)' - "'rb fr(Jmr Units CiriC&'ed and delivered to the berth
V nits erected, Caired and wdded · • Figuft 3.' Ship/NUdill1 ttqutnct Olt~nrlt
~t~el ~I~tes and sections are usually stored in separate stockyards and fed into theu ,indIVIdUal sh~t-blasting and priming machines. The plates are cleaned by abrasl,ve .shot o.r gnt and then coated with a suitable prefabrication priming paint to a lumted thickness for ease of welding, The major areas of steel are therefore protected from corrosion dUring the various manufacturing processes which follow. 'J?te plates. and sect,ions follow their individual paths to the marking or direct. cutting ma~hiner;: w~ch produces the suitably dimensioned item, Flame cutting or mechamcal. guillotmes ~ay be u~d. Edge preparation for welding may also be d~ne at this stage. Vanous shaping operations now take place using platebendmg rolls, presses, cold frame benders, etc., as necessary. The material
of plating and sections weighing up to 20 tonnes. Flat panels and bulkheads are examples and consist of various pieces of shell plating with stiffeners and perhaps deep webs crossing the stiffeners (Figure 3.7). The flat or perhaps curved panel may form part of the shell, deck or side plating of, for instance, a tanker. Units are complex built·up sections of a ship, perhaps the comolete fore end forward of the collision bulkhead, and can weigh more than 100 tonnes (Figure 3.8), their size being limited by the transportation capacity of the yard's eqUipment. The various subassemblies, assemblies or units are moved on to the building berth or storage area until required for eJl!ction at the ship. At this stage, or perhaps earlier, items of pipework and machinery may be fitted into the unit in what is known as pre-outfitting, Once erected at the berth the units are cut to size, where necessary, by the removal of excess or 'green' material, The units are faired and tack welded one to another and fmally welded into place to form the hull of the ship,
Shipbuilding 40
41
Shipbu.ilding l(}ng',tud'ln~1 bulk"~6rl
Inn~r
/,,-_-1
bottom plating
SI,IFene,
--r-"-
- -._--.,.
FiliUrc 1.8 lJmr
Materials preparation
Plates and sections received from the stcel mill are shot·blasted to remove scale, primed with a temporary protectivc pai'llt and finally straightened by rolling to remove any curvature.
Shot.blasting and priming Fij?Ufe], 7 Assembly
Materials handling The layout ,of .a shi~yard should aim tu reduce materials handling to.a minimum by appro~nate locatIOn of -",ark s~ations Dr areas. The building of large uni ts and the <:apaclty to transport them will reduce the number of items handled but will re-quue greater care and more- sophisticated equipment. The building of a ship is as ~uch gov.erned by the shipyard layout as the materials handling equipment and Its capacity. An. actual shipyard ~yout is shown in Figure 3.9. The progression of ~ate~a1S through the .vanous production stages can deaIly be seen. The various , arkinE pro~esses which the plates and sections undergo will naw be ex.amined m more detail.
A typical m.achine will first water-wasb then heat·dry the plates hefore descalil1g. The plates are then simultaneously shot-blasted both siJ-e-s with metallic abrasive. The plate is fed in horizontally at speeds of llP to 5 mfmin, and around 300 trh of shot are projected on to it Blowers and suction deYi.ces remove the shot which is cleaned and recycled. The clean plates are immediately cove rca with a coat of primmg paint and dried in an automatic spraying machine (Fi/(Ure 3.10). A thickness of about 1 mm of compatib-le priming paint is applied to avoid problems with fillet weLds on to the plating.
Straightening Plate straightening or levelling is achieved by using a plate rolls machine (Fi;;UTe 3.]1). This consists basically of five large rollers, the b(lHom two being driven
..
42
Shipbuilding
43
.",,. w.
],
~0
f'late ,...."ing._ d~i~"
, RQII"(
,"''' --
. -. ,•
~
C()nUmina~V
",
.r Plale
-
tain
11
ai' e,,-~.ust
-
M
Filto
-
r
conveyor
,
Spray
/iilt
I
"---l
• T ra"e,..ng
•
,•
carriage
Figure ).10 Automatic paint-spraying plant
!
l
v -
/
Bottom roller
Bottom sUPp<>rting rolle.~
Secti(Jn th,origh rQlle-r
~
1- --
r;,.
Bent r;lale ~
~~
~i
Pla~ •
-:;='1"~':'~5,,~,;,~o~"~"~"~~=I-t- __ '~""'-- __J_.~~~ ~ r luawlRaJl Jallaj,ll!;
pJRl.lPOll Jaua,I!1-S
::;: Ii;
Straightlmed plate
Figure 3.11 Platestroighrcning
:i
/..eq AIQwi'lSRqnS
000 00
and the top ones idling. The top roners can be adjus.ted for height independently at each end and the bottom rollers nave adjustable centres. A number of smaller SUpporting rollers are positioned around the five main rollers. The plate is feci
through with the upper and lower rollers spaced at its thickness and is subsequently straightened. This machine is also capable of bending and flanging plate.
----------------------,Cuning .and shaping Various machines and equipment are used for cutting.and shaping the steel parts which ronn the subassemblies, assemblies and units.
Shipbuilding 44
45
ShipbuildillK C"m"~o IrTlovabl~
Contour or proftle-cutting machine
tix~d
Driven
This machine is made up of a robust portal frame for longitudinal travel which is traversed by several burner carriages, .some of which are motorised (Figure 3.ll). A motorised carriage can pull one or more slave carriages for congruent or miIIor-image operation. The bUffin carriages may be equipped with single bumeTs or up to three heads which. l;an be angled and rotated for edge preparation in addition to cutting, as shown in Figure 3.12. Fully automatic Mmmi,oo cama""
Sl",,~
MDlOrised
Slav~
ca"iage~
-cam"g~"
carr;"'l~ ' "
but
when CU!!In9'
ganlr~
'\ Plale
Trip~ ..·n"ule
Rail
bead
\
-'--
--.J__.,,:-~
Swp-port ' I Colwrn:"_l._.l._'L'
~..L_...;._
, Cutt'ng
l~bl.
Porlal frame Triple nOHle
Track Sid~
,up'port
\
Cwtting
I
Ii
Rail
/
IranS\l~rS!!ly
r
labj~
Figure 3.12 Profile-rurting mllrhine
GanlrV'IJtior>ary
Co"iage rnmmg
•
f""I
I
l
-..
I'j ,
-......,..
Gantry rn"ving longitudinally
operation is possible with punched paper tape input under numerical control. Semi-automatic operation can be acmelled by a photoelectric tradng table using }:1, 1:2.5, 1:5 or 1:10 scale drawings. Complex shapes such as floor plates in double bottoms can be cut with these machines, and also plate edge preparation may be carried out while cutting shell plates to the required shape.
C~((i3ge 5t~ti(",ary
rl
II
Carriage
I
'tation~ry
II
l ---.J
Flame pkmer
A typical flame planer can have up to three gantries whi.ch run on supporting carriages. The gantries are traversed by one or two burner heads - Figure 3_13(a). With triple-nozzle heads., cutting to size and edge preparation of one or more edges of a plate can take place simultaneously. The operation of the machine is largely automatic, although initial setting up is by manual adjustment. With a three·gantry machine, the longitudinal plate edges can he rut to size and also the transverse edges -- Figure 3.13(b). The transversely -cutting gantries wilt operate once the longitudinal gantry is clear. The flame planer can split or cut plates to a desired length or width by straight-line cuts. The use of a compound or triple-nozzle head enables simultaneous cutting and edge preparation of plates. All straight-line edge preparations, such as. V, X, Y or K, are possible with this machine. Mechanical planer
Steel plate l;an also be planed or cut to size using roller shears, as in the mechanical planer. The plates are held by hydraulic clamps. Setting-up time is
Carriage rn"~lng tran,versely
I
G/¥ 'tation~,y
I w
"-J
L-
I" Figure 3. /3 Flame plani"/{ mac/1in,., (aj flame plant"; (b) rhre.e-gl1.ntry operaricm of flame planer
somewhat longer than for name planing, although the act~a.l mechanical cutting operation is much quicker. Modem machines use m.l1lmg heads. for edge preparation to produce an accurate high standard of finish far s.uperl~r to gas.cuttillg techniques _ Figure 3.14(a). These machines. can also achieve high s~ed shearing on the lighter gauges [}f plating. The mosl l;omplex edge prepa.rahons can be obtained by the use of the rotatable head and assorted cutter shapes Figure 3.14(bJ,
Shipbuilding
46
47
Gap or rillg press The gap or ring press is a hydraulically-powered press which cold works steel plait. The operations of bending, straighteni~g. dishin~ and swedging of Sleel pl:lIcS can all be achieved by the use of Ihe dlffcrenl die blocks on the bed and the Tam (Figure J. J 5). The gap press provides better access all round and is more versatile than the plaIt rolls.
Hvdr • .,l,t clamp
I /
R.m
0i• blockl
7
M,llIng
"''''
''''\
L
lo'
'Ol
,., I
'"
til 0_3S0~
r;v;wflid
H
{;iil
(vi]
'0)
,.,
FIf(Ure 3.15 Gap ptess operatiOn!: fa} edge curving: (bi plate f1auening: (ci platt flanging or bending; (d) plale straightening; (eJ p/Qle swaging
'Ol Plale ro/ls Figure 3.14 MechQniCilI ed,,: pUlfItr: M tUStmbly; (b) meclumiCi1Qy cur edge preptUtlrio1l - (i) sinde M~tl withoot nOlt, SlIitJlb/e for btJrchn of pl;ztn: Iii) rinpe bevel with shNf'td IIOU J.5 mm (5/8 in) l7l4l'imum or milled lIOU; (iii) doubk ~tl tmd "0#; (Il'l J plTportftion tmd "au uring 'ciTculll,' cutter: (v) doubiN plTptITtltion; (vi) [acinp 0If flange,
o!,lnlcturrsl,tcn"OIlS
This machine has already been described with reference to plate straightening. It is also used for roUing shell plates to the curvature required. By adjusting the height of the top roUer and the centre distance of the bottom rollers. large or Small radius bends can be made. Bulkhead flanging is also possible when the
48
ShiplJuiJdjnK
Shipbuilding
.~"
i, -
49
Guillotines
I
Hydr:lUJically-powercd shc-aril1g machines or guillotines are u~c:d for s.mall jobbing work. The plates are fed. pusitioned and often held by hand. Small itemS, such as brackets aJld machmery space nUOT plates, may bc produced in this manner Frame br:nder
Ships' frames are shaped by cold bending on a hydraulicalJy,powCIcd inilchine. Three initially in-line cLamps hold part of the frame in position. The main rams then move the outer two clamps forward O[ ba\:kwards to ben\l the frame to the desired shape (Figure ],17) The damps are then released
RDIler C€nlrfs tor V;W;I"~ rad" "dJlJSl~t,l~
,
~··;i
fd,
, __ 1:
,.,
HyrlroUlcram
'0'
FigulV! 3./7 Frllme bender operol[on, (a) bow f/OJre bmd: (b) initial posirion; «:) bilge /urn bend Ffl[1UC j, 16 Roll pres~ opera/fans_ (OJ) slicer Hrake rc>liinJ,. (h) half·round rollin/! ./Or durtclc posrs, etc, (rj 9rJ- cjfr('cf/rm!{liIK. /d) DJAf/dread [langing
JJla5rl
u
machine is fltted with a tllln,ging ment~ are shown in Figure 3.16. and operations carried out frOIll wooden lathe is used to check the
Llm] bottom block.. The~e variuus arrangeControl or lhe Illachine is by manu
bar
a comole located nearby.
A shaped metal or
finishe-J. shape.
advanced through the machine by a motorised drive. The next portion is then similarly bent. Offset bulb and angle bar plate'S can be bent two at a time, placed back to back. In this way, port and starboa.rd frames are produced simultaneously. The machine can be controlled by hand :md the frame bent to match a template made of wood or stee1 strip. Modem machines arc now equipped for ~e lIumericall;ontrol of frame bending which enables fUlly automatic operation "'.thout the use of templates.
Punchrngand notching pre,ls
Air holes and drain holes requireu in many plates and ~ections can be cut un a profile burner or by a punching press A fully automated press can be used to pUllch round and elliptical holes, as well as rect:mgular and semicircular notches, at preset pitches a![)ng a plate or section. The machine is hydraulkally powered and fed. Stolting up is against datum rollers on the 1I1'-lchine. Manual operation is possible. in addition to the automatic mode.
Materials handling equipment
Be!ween the various machines and during build-up of the plates and sections into Ilnits. numerous items. of materials-handling equipment are used.
Cranes. of various types are used in shipyards. The overhead electric iravelling crane (OETe) win be found in burning halls and fabrication shops. 11ris crane
50
Shipbuilding
Shipbuilding
traverses a gantry which is itself motorised to travel along rails mounted high on the w~ls of the .hall or shop. Using this type of crane the sorting. loading and unload~~ o~rations can be combined and maximum use is made of the ground area. !.:ifting 15 usually accomplished by magnet beams, vacuum devices or grabs. Goliath cranes are. to be seen spanning the building docks of most new shipyards. Although of hIgh first cost, this type of crane is flexible in use and covers the groun~ area .very efficiently. Some degree of care is necessary in the region of the rails which run along the ground. Mobile cranes are used for internal materials movement, usually of a minor nature.
'-'
P,ne-I comple-It
'-'
Stlllt'llt... welded 10 p,nel
..,
-
Slifftnf'f cl,mping ''''' welding 9Inlry
,
5I
Special motorised heavy-lift trailers or transporters are used to transfer unils and large items of steelwork around the shipyard and to the berth or building dock. Fork-lift trucks. trailer.pulling trucks. roUer conveyor lines and various other devices are also used for male rials movement of one kind or another.
Panel lines Most modern shipyards use panel lines for the production of flat stiffened panels. A number of specialist work stations are arranged for the production of these panels. The plates are first fed into the line, aligned, clamped and manually tack welded together. The plate seams are then welded on one side and the plate turned over. The second side welding of the plate seams then takes place. Some panel lines use a one-sided welding technique which removes the plate-turning operation. The panel is now flame planed to size and marked out for the webs and stiffeners which are to be fitted. The stiffeners are now injected from the side, positioned. clamped and welded on to the panel one after another. The stiffened panel is then transferred to the fabrication area if further build up is required, or despatched directly to the ship for erection. The process is shown in Figure 3.18.
Shipyard welding equipment St. 3 P,nel CUI 10 finished tIlt ,nd mlrked off tlifftfle-...
V'
'Of
./
-
'-'
PIlle- st,ml welded one IIde-. p,ntl turned OVtr Ind s«ond lide welded P,ne-I Ilhing eflne
-
Wtlding 9I n llV
-
St. 1 PI.tn POI\tioned ,no mlnu,lIV tICk Wllldc
V Figu" 3.18 PilnellUle
... .---",m, w
The equipment required for the manual welding of a ship's hull should enable the operator to usc high amperages with large-gauge electrodes and yet still have adequate control of current for the various welding positions adopted and lhe plate thicknesses being welded. It should also be robust in construction and safe in operation_ Multi-operator systems, in which a three-phase transformer supplies up to six operators, are favoured in shipyard. Each operator has his own regulator and a supply of up to ISO A. The regulator is fed from an earthed distribution box on the transformer and provides a range of current selections. The regulawr should be positioned fairly close to the welder both to reduce power losses and the time laken when changing current sellings. Remote-controlled transformers, whose current can be altered by the welder through his electrode holder cable, are now fitted in some shipyards. The various welding processes are described in Chapter 4.
, )'
Weldin}? a/ld Culling Processes
53
,"
k:: arc welding
':1i1 III'1 I!
,,
1" I
I
4
Welding and Cutting Processes
" ' d . d between two metals in an electric circuit when tney , "3JClsprOIJ(;e -F 42Th " _ , e]ed nc hort distance. The basic cin:ui! is shown 1Il 19Uie . . e -';\~'. separated b~;e~ (orms one electrode in the Cllcuit and the welding rod_or ,,,,_tp.taJ to be W Othe r . The electric arc produ.eed creates a reglOn of 11l~h ..::jtIe foflTl S the. h elts and enables fusion of the metals to take place. ElectriC .~ tute" whic ill h' h i , \;~P"" voltage J.e. transformers W Ie may supp y on :, is supp I'Ie d VI"a vari3ble , ,':'" -', -
-'.;;..
e welding operations.
;
~:;./i!f.I"
l'/elrJ,ny,el
"'---
"
In shiP?uilding, welding is now the. a-ccepted method of juining metal. Welding is the fUSUlg of two metals by heatmg to produce a joint which is as ,trung or stronger than the parent metal. All metals may be welded, but the degree of simplicity and the me~hods used v.a:ry considerably. All shipyaId welding processes are of the fUSIOn type. where the edges of the joint afC melted and fuse with the molten weld metal. The heat source for fusion welding may be pro.. . ided by gas torch, electric aTe or electric resistance
Gas welding
Figure 4.2 Elef'lric arc welding l:lrcuj[
A gas flame pmduced by the bu.rning. of oxygen and 8;cetylene is llsed in this process. A hand-held torch is used to direct the flame around the parent metal
"':'1IIe actu.al welding operation the welding rod and plate are fust touehee!
and: quickly dr3wn ap3Jt some 4-5 mm to produce the arc across tne
:·,'.The temperature produced ls in the region of 4000°C and current !low . the metals. may be from 10 to 600 A. The current flow must be preset or
• depending upon the metal type and thickness and ttle 'Supply vo\1age.
Yoltage across Ihe arc affects the amount of penetration and the profile or ,'j• .
of the metal depos-ited. The c.urrent to a larg~ extent detCJ~ncs t~e
~ of we1d metal dcpD'3i\eJ.. A \ugh quality weld \s proo'.lced With 'Se:enl
Or
•
\'II~ld
Figurl' 4. I Gos wefdinK with an o-xy-acetylene torch
and filler rods proVide the metal f(lf the joint (Figure 4.1). Gas welding is little used, having been superseded by the faster process of electric 3rc welding. OutfiT tr.ades, sud1 as plumbers, m.ay employ g.as welding 01 use the gas flame foo brazing or silver soldering.
52
- . . . layers weld metal, but it is less costly to usc a single heavy depOSIt 01 '_IIIClaI, :j.lf excessive current is used weld s-p
"ocesses usi.ng flux Manual welding
....
In the manual welding process a consumable electrode or welding rod is held iJ
,. ..
~der
and fed
011:
to the parent metal by the operator. The welding rod is
54
Welding and CUtting Proce$$C$
WeJdingandCUttingPtocesses
flux
55
• ~---\\
., ....
Direction
1.1
l
'"
the electrode to break the arc. One man is able to operate several of these devices simultaneously.
Automatic welding
'"
,.,
Fi,un 4.J Wtldinl positions; M hori:onta! or dOM11/hand: (b) horilonta/lnrtiaJ!: (c) )lutica!; (d) overlrad; (e) inclined
Manual welding may be accomplished in any direction, the three basic modes being downhand, vertical and overhead, and some combinations of these modes are shown in Figure 4.3. The correct type of electrode must be used, together with considerable ~, in particular for the overhead and vertical welding positions. As far as possible, welding is arranged in the downhand mode. The gravity welder is a device consisting of a tripod, one leg of which acts as a rail for a sliding electrode holder (Figure 4.4). Once positioned and the arc struck, the weight of the electrode and holder cause it to slide down the rail and deposit weld metal along a joint. The angle of the sliding rail will determine the amount of metal deposited. At the bottom of the rail a trip mechanism moves
In the automatic machine welding process, travel along the metal takes place at a fixed speed with a flux
56
Wdd{n~aml
Welding and Cu rring Processes
Culting Processes
57
Bare ",ie. e'e<:trode
~b/~~";;t;~: (;or""m~hl,,
/
f-Iu~
/
Long,tudinal
Welding Luree-c,
I
I Di'~1iDn
II
of weld ~
I,
~U1d~
, ,
Figure 4.5 Automutic /lu.r-c[){]led dt'Ctnxlf' IVI'lding uSing a twin·hEaded macJlinc
~
W"ld
' r--
Di'eeti,m of weld ing
r(>ll,
~C:) /
GranulMed
filM
t !
r,,",o~e-ry
P
Welding CUfrent
tra~elling
,h-oe
_ -, Starting
!:=======:::':':::!==:':::::!:'="::"""======::'0,,,/
• Wice f-eed
-~
WalW cooled liv,;or
Ngur.e 4. 7 EieClroslag wefdin;2,'
Bare wire elecHooe
7,J (J,'.nulated flux
\ Coppe, 'lfIp
I.' Figure 4.6 Submerged arc welding,' Ill) submerged arc IWlding; (b) l)(/cking plate iJ"ange men! for one·sided welding
of the joint may be necessary for the fmal run. In tae one-sided process various forms of backing plate can be used, of which one example is shown in Figtm: 4.6(b). Any defects in the- weld will then have 10 be repaired by veeing out and welding from the other side. This process is limited to indoor undercover use and is unsuitable for use on the berth.
Run-on and fun·off plates are required at the beginning and end of the weld and no ~toppage must occur during the p-rocess. The arrangement is shown diagrammatically in Figure 4. 7.
Electrogas welding 1llis process is parHcularly suited 10 shipbuilding since vertkal plates of thickneues in the range 13-40 mm are efficiently joined. Cooled shoes are again used but a flux·coated electrode is now employed. Fusion is achieved by an arc between the electrode and the metal, and a carbon diQxide gas shield is supplied through the upper region of the saoes. The arrangement is similar to Figure 4. 7, for e1ectroslag welding, with the carbon dioxide supplied through the top of the
oboes. Stud welding .A macltine OJ gun as part of the electric circuit is used in stud welding. In one Illethod the stud is fed into the clutch and a ceramic ferrule is placed over the end. The stud is placed against the metal surface and the operation of tae gun trigger withdraws the stud to create an arc (Fi!?UTe 4.8). After a period of lilrcing,
Electroslag welding
I II
The vertical welding of plate thicknesses In excess of 13 mm is efficiently achieved by this process. Initially an arc is struck but the process continues by electrical resistance heating through the slag_ The weld pool is contajned by cooled shoes placed either s~de of the plate which may be moved up the plate mechanic.a.lly or manually in separate sections. Alternatively, shoes the height of the weld may be fIxed in place either side. The bare wire electrode is usually fed from the top through a consumable guide and acts as the electrode of the circuit.
I"
1"
10>
Figure 4.8 Stud weldin;:_ (a) ~rud and ferrule pkzud on plate; (v) arr drawn; (c) weld completed
58
Welding arid Cutting Pro-r:esses
the ~tud .i:s driven into the molten meta] I ' ferrule concentrates the are, reduces the ac~e~~ o~n: we11mg ~akes lace . The metal area. Fhm is contained ill the end ofrhe stud. It an con mes t e rna ten Another method uses a fUSible coUar over the d h . electricity to crea te the arc and then coil /n .of t e srud :-Vhich condUcts metal pool and fanning the weld Weld dapse~, Dlelllg the stud m~o the molten
b
~~:~s~~a~~~~r~u~[~~::~~~~~~ings. ~th:~u()~p~~eo~~~:'o~se~~~~:min~~~;~~;
Welding (lnd Cutting ProceSses
59
suPPly source is usually d.c::, and the proces; may be fully or semi-automatic:: in opention. In steel welding using this ptoc:;ess. carbon dioxide may be the shielding gas and plating of any thickness may be welded. Controls within the wire feed unit enable a range of constant wire feeds related to the current to be selected. With carbon dioxide gas, the arc characteristic changes with the current from a shortcircuiting (dip transfer) arc at low currents to a spray arc at high currents. Dip FIe-"bIE
{"bing
Processes usin g gas
,-~---------
The~...,aIl'd W elding processes employing a bare electrode or welding wire with gas :>lue utom f ' . a .' A a IC or sem.l-automatlC operation is usual. With automati ~perat~on, once, set the. process is controlled by the machine. In semi-automati~ peratl~n cert.am machine settings are made but the torch js h!lTld held and th proces!3-IS to some extent controlled by the opefif.tor, e
G~.con"ols
€ lect
-ccnlinuou.I','
Tungsten inert gas (TlG)
""
This is a process for thin sheet metal such as steel or aluminium. A water-cooled non-consumable tungsten electrode and the plate material have an arc cre.ated
• Pial...
Ga~ shield
-"~===~~b==""======7~~I
tnnsfer allows all positions of welding, but the -spray arc is downhand only. Dip tnnsfer is ideaUy suited to thinner materials, since it produces less distortion effects.. This process is being used increasingly in s.ltipbuilding.
Plasma metal inert gas
Fille, ,,,d
-
"
O"-e<:tlon of w~ld
Plal~;.,:::::;:",:,~J~l2rL,~~~?~;~"O~'Zd='O"'Z'''210'2'Z'ZltZ'OI,?,?Z'Z'ZUZ' Z? 'N~ld Figure 4.9 Tungsten inert gas prOCeJt
~etw~~ them by a high frequency discharge across the gap. The inert gas shield
JS USlJ
Y argon gas. The process is shown in Figure 4,9.
Metal inert gas (MIG)
~ ~:nsutnable
metal wire electrode is used in this process and is fed through the t:rc~rt~r:rl~h from a feed unit (Figure 4.10). An inert gas is fed through the e the arc and the tOrch and plate are part of an ele<:tric circuit. The
1bis is..a further development of the metal inert gas process which incorporates a plasma arc around the MIG arc. The plasma is an ionised stream of gas which surrounds the MIG arc and concentrates its effect on to the metal. The plasma arc has its own set of controls fOJ its electric circuit. It is initially ignited by the MIG arc and with both arcs indiridually controlled the process can be finely 'lUlled' to the material requirements. Automatic and semi·au tomatic versions are lV3.i1able. The semi-automatic version uses a dual-flow noule arrangement. as di()Wn in Figure 4,11, with a single supply of gas, usually argon, as the shielding and the plasma gases. The torch used i-s no heavier than a conventionaJ MrG torch .and the process has the advantages of higher weld metal deposition rates and tne use- of a narrower vee preparation which may be as small as 30 degrees.
Thermit welding
lhis is a fusion process taking place as a result of the hea t released in a chemical ~tion
between powdered aluminium and iron oxide ignited by barium
60
Welding and Cutting Processes
Welding and Cutting Proces:ses
61
C""sum •.,Ie- electrode- ",i~
/
Pla.ma (~rrent
,,'
Weldi~g ourre~1
Wate
---101
~ollie
. 1\
ArgOn
f----
I
(Ia'
/
v - - - -.
'"
Sh-i.. ldi~g gal
+,,--f*\ i
I
r "
f>lasma gas
iel
£Iil'C~
al~e,
G •• 'SI1ield
'"
-gou9" iniual we~d
E!oc~-qoug"
atter ,n'lial weld
Figure 4.12 Hu.tt wdd preparatiOI1,: fa) l./lIi'Qre Inm join 1, (b; si~lIk,-V burr joint. (l:) doubfe· V butt {oin!.- (d) dOI/Me- U hu rl joint
-
Sr.ieldir>9 gas
5ecri()n thff.)(lg/l MouJe
Figlue 4.
Pla,ma 9'"
II Plasma mef.al inert gao pmce.s
peroxide. The parts to be welded are usually large sections, such. as a stemframe, and they are positioned together in .a sand or graphite mould. The molten steel and slag from th.e chemic..l reaction is first formed in .a cmcible and then run into the mould_
Types of weld
Fillet welds are used for righ.t·angled plate joints and lapped jOilltS. as showlJ. in Figure 4.13. Two paJticular terms are lIsed in relation to ftIlet welds - the leg length L ;Ind the throat thickness T - as shown in Figure 4. 13(a). The leg length is related to the thickness of the abutting plate and the throat thickness must be at least 7cYJ'<; of L. A full penetration type of fillet weld may be used where special strength is required. A full penetration joint is shown ill Figure4.J3(c). The abutting plate is of V or J preparation to ensure full penetration wh.en welding. The fillet welds descrihed may be arranged in a number of ways, depending on structural requirements. Fully continuous welds are used in important
Wel~
Leg length
,
-i-
i
'-. (a)
(\,-, , Throa, Th,c~ne<;,
A number of different welded joints are used, depending upon their situation, material thickness, re-quired strength, etc. The depth of weld may require more than one- pass or run of weld 10 build up to the workpiece thickness, Reversing the workpiece, gouging out and a final back-run will also be necessary unless a one·sided technique is employed. The butt weld is the strongest joint when subjected to tension and is illustrated in Fif?Ure 4.12. The single--V type of preparation is used for the butt weld for plate thicknesses in excess of 6 mm up to a maximum of 20 mm. Below 6 mm, a square- edge preparation may be employed and for very thick plates a double-V preparation is used. A U·weld preparation is also used which requires less weld metal and gives a better quality joint in return for a more expensive edge preparation_
r
'"
Figure 4.13 HUe! welds,' (a) flUei w.-ld, fb) lap weld; Ie) filler weld WIth ft
62
Welding and Cutting Processes
We-Iding and CutUng Processes Welc\
_
S,dfe"pr
(=~/1~"'" W~ld
§§32§J~' , , "
\
...
St;ff~n.r
W.ld
Figure 4.14 Nan-COnnll1.lOUi fillet ....'flds· ffl) inrermit/ent WJ?/ding: (b-J ch4ill wdding
strength connections and for oiltight and watertight connections_ Chain and intermittent welds are spaced sections of welding and are shown in Figure 4. ]4. &lme savings in weight and distortion are pOS5ible for lightly stressed material which does not require watertight jQints. Tack welds are short runs of weld on any joint to be welded. They are used to initially align and hold the material prior to the finished joint. They are assembly welds and must be subject to a full welding procedure. They should not be less than 75 mm in length to ensure a sufficient heat input and should not be welded ovu.
Welding practice Tile welding of the metal, because of the localised concentration of heat, gives rise to areas of plating which first -expand .and later contract on cooling. The lo"g,ludin~1
t1.:l
effect of thh, -afId the differell~e in deposited weld metal and parent metlkl properties, results in dbtortion of the wor.kpie-ce. The appea~ance of dJ.stortion may be in onc or more of the followmg forms - longttudlnal shnnk.age, transverse shrinkage. and angular di.stortiDn. Figure 4.15 muatratts these vanous effects. . _ The cause of di"torti-on may be attributable to s~veral pos.s.lble factors actUlg individually or together. fhe con-centrated heating of the welded area ~d i~s ubseqtlent later con-traction will affect the weld metal and the workpIece III ~jffereJ1t ways. As a consequence, streSSes will be set up ill the weld, the two joined workpieces and the overall structure. ." ,.. The degree of fe-stain t pe-rmiJ;ted to the welded Jomt Will affect its dIstOrtion. Where welded joints are unrestrained their subsequent weld shrinkage will Jelie-ve any stresses se, up. Restrained jointS, by virtue of the rigidity of the structure or some applied form of damping, induce high. stn:sses to the weld and cracking may occur if the COrrect welding sequences are not a.dopted. The properties of the wmkpiece- and tne -pm.\ible stresses 'locked in' it due to manufacturing processes may b-e altered or affected by welding and lead to dis-tnrtioI\.
mstortion prevenflon Good design mould ensUTe as few welded joints as possible in a structure, particularly wilen it is. made up of thin section plate. Where they exist, welded ]atn.u should be accessible, preferably for downhand welding. The edge preparation of joints. can be arranged to reduce distortIon, as shown in FIgUre 4.16. A single-V preparation joint with four runs of welding will distort as shown. A double-V pTepal
! ,~ - .hrlnl
/,: i
'I "
.-'///~ ,,",
L"n9;I
, ,
//<'>
!
///
/
//"
.-'
FilJUre 4.16 Edge prl!pl1TtItio/'l to redUce dirlOrriQl1: (8) ringle· Y preplJJdn'O n ,Mng cO/'lsiderob/e distort/em (1 fiNt welding ron, 2 Jecond we/ding nm. J third welding run, 4 final weldin!,: run), (b) double- V pre{Xlration givt,.,g only slIght Ihril1~
/ /
Ffgure
4. 1,5 DistOltfon
effect:
Restraint is the usual method of distortion prevention in shipbuilding. Where UOIts are faired ready for welding they ue tack welded to hold them in place dUring welding. The parts will then remain dimensionally correct and the rig.id1ty of the structur-e will usually restrain any distortion. Strongba-cks or clampin~ \tlmgements are also used on bu1t and fl11e1 weld" as shown in Figur.e 4.17. All welds 'shrink', so the use of the correct procedure in welding can de bJ,ueb to red~ dUi.tmti
Welding Qnd Oltting Processes
64
65
preparalio n will ploduce a dislortion·frce weld. Simultaneous welding by 1'010'0 operal ors is Iherefore ;I useful Icchniqut which should be practised whenever possible. Welding should always take place IOwards Ihe free or unrestrained end of a joint. For long welding runs several techniques are used to minimise diSl0l1ion. Th." b~ck-5te~ metJ.1od is iUustral.cd in Figllrt 4.18" Her,t the operator lWeids the jOint In sections In IhC' numenC
.....
proglesses in the numerical order and direction shown. Distortion may then be controlled by balancing the welding as much as possible and allowing the weld shrinkage to occur rreely. Welding sequences taking Ihis into accounl should be well thought out berore welding commences.
Distortion correction Despite the most stringenl methodS to e1iminale il. distortion can still occur. Where the distortion in a joinl is considered unacceptable the joint must be gouged. grooved or completely split. and then rewelded. Strongbacks may be placed across the joint to restrain distortion during rewelding. Straightrorward mechanical means may be used. such as hydraulic jacks or hammering on localised areas or distortion or buckling. Where such methodS inv~lve straining the welds, they should be examined ror cracks arter correction. Every errort should be made to avoid mechanically straightening structures ror this reason.
Figuft 4.17 QampitlX 1Irr/11lltmttffS
_---:;-,• .-,_-;-_'+'_-:-_'.>I.'__~I••__.... ~
3
1
D,'ftlionOI
-t<
Ie
4
,.
6
_Id
.......01 ===::J:===C=::::::I==i==C:::!~:::I=~=:c=~=o..der _ _ _ _ _ _ _ Joonl
pr;..eu>on,-------_ ,
c:=
I.,
c===t
Fi,utr 4. III B«k·$ttp _ldinK t~niqllt
1<1
___••.-, 4
'.>I 2
I.---;,---~
6"·
",
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,.
O;"llon
..."-
I
Fiprt 4.19 Skip OT ...._dm"6 ....ddilt,'mniqllt
c===t t
::::::J
101
==:J
(
'"
Fiprt 4.20 SpOf hftlti",: (aj cu"'ed pltue: (b) h~ted: (e) exptJIuiofl: (dJ kvelled pfafe
o..... eld
===C=::::::I=~=C=I ====~=:J::=~J=::J:=~'~=Otdef _______ Jo'nl Pfc:.,eu,OI',--------
=:J
01
The application of concentrated heat from a gas·burning torch may be used for correcting distortion in steels other than the higher tensile. quenched and tempered types. The process is shown in Figure 4.20. A small area is heated on the side where the contraction would bring about an improvement. The steel is heated to a 'red heat' and the torch slowly moved along a previously drawn tine, at such a speed that the 'red heat' does not pass right through the material. The area heated wants to expand. but is resisted by the surrounding material. The fecrystallisation absorbs the expansion and, on cooling, contraction occurs
66
Welding and Curting Processes
67
which brings about a favourable distortion. thus correcting the original distorted 1I JL
L ~.;.
II...
~_J
__
~
- - --'1- - - - - --- - -- -, 1- - -U :'
It "
I:I. I II
,
) HUlf'O
I:
I
The weld
I.
II
II It
Weld faults
, I 11
!eng""
II
! II"
I
I'
II
II
" n II U
n U
I'
I' ...Jl
____ JL
SlfUclure (Figure 4.21).
_
----'r------------·r---II II
Faults may occur in welding as in any other process. These faults may arise from bad workman.ship, incorrect procedures, wrong materials used, etc. A good weld is illustrated in Figtlrt! 4.22fa). In such a weld a degree of fusion should have taken place at the sides of the weld. There should be no overlap or undercut at the toe of the weld. A slight reinforcement or build·up of material should be present at the top surface and there should be root penetration along the bottom surface. A bad weld is shown in Figure 4.22(b). The absence of reinforcement and root penetration are the result of incorrect procedure or bad workmanship. Overlap is infused metal lying over the parent metal. Undercut is the wastage of parent metal, probably caused by too high a welding current. Porosity is caused by gases trapped in the weld. Slag indusion is the result of inadequate cleaning between weld runs. Poor fusion or penetration between runs may be due to poor cleaning or incorrect voltage or current settings. The result of a bad weld is a weak or faulty joint. A bad weld can also be the starting point for a crack.
Figurt 4.2/ Distortion co".~tion
lamellar tearing
Oegoree 01 lU$"'"
Weld Ih,"" ill loe
Relnlo,c""""
j
I t
\ _
\_
O'_1I;01l0n
\
,I
I
"
t
Rool _trillIOn
101 Lillie
V""~"', St.,
Of
no
.. ".IOtC~1
PorCl"!Y
umellar tearing around welded joints has become a problem as plate thicknesses have increased and structures have become more rigid. Lamellar tearing is a brittle cracking in steel plate as a result of tensile stresses at right·angles to the plate. It is caused by the contraction of weld metal when cooling. Lamellar tearing is most likely to occur when thick plates, large weldments and high internal connection restraint are all present. The characteristic 'tear' occurs in the cross-plate of a T-configuration and may begin at the toe or rool of a weld or at some point below the weld (Figurt! 4.21). One method of reducing the problem of lamellar tearing is the use of 'clean' steels such as those produced by the vacuum degassed process. Other measures indude the use of joint configurations which avoid right angle tensile stressing of the plate, or preheating the plate before welding.
/ /""'''~
\
.........
;ncl",ion-
No inlf"""
,}
No,ic!'e fusion
penetrl';on
' " Lack 01 '001 _trillion
lbl
Fftr,Ire 4.22 Ezilmpiet of_Ids: ra) 1I' tood wtld: (bJ 1I' bfld 'oIt>tld
Tutong
68
Wdding and Cutting Proce,~ses 69
We!dingarld Cutting Processe~'
Weld testing Several non·de~\luctive technique" a,e ul>ed 11, the eX:lminati"n 01 wehJed joints. These include yjsuul examim;tion, dye pcnelr.aJlts, magnetic p
Classification societY weld testing The classification societies reqUire various tes.ts, Some of them destructive, in order to approve weld materials and electrodc&. Joints. made between the materials and the electro-des are then subjected to various strength. metallurgical and other tests.
and then oxidls-ed by a ~tream of lligh preSSUre ox.ygen whi.ch carries away the ox.idised metal. A narrow g.ap with parallel sides remains along the line {:f the t Small amounts of al10ying elcm('\lts in the steel plale call be removed In the ~. ' ~t clJ1tiag process, but large amount& o~ elements ~uc h as ch rn~llum may prev cutting. Tile introduction of an irorHICh pOWder mto the cutting area. OVeH;omes this llwb\em, p-,nticu\arly with stainless steel. .. _ . Acetylene or propane is usually used as the preheat.mg gas. m conjunction with oxygen. A typical cutting torch is. shown in Flgu~e 4.24. Automated ;auangementS of cutting torches are used Ln ..anou" machl11es for edge pre?il' :ration. flame planing, etc., a.Ji ~en~ioned In_ Chapter 3. An edge preparation .arraogewent of ton::hes is shown In FIglJ. ye 4.2,;,.
r=' ),-<)11
r>'e"UT~ ~"\\\'VJ O~"I'l"·n
I~----
-
I .1 I '
=L-_~
Pr~I'ea\ln'Jo>\qenanr
In'lylp,,,
,II! II I
~~A",B
~
/
~
A,~lVI~'W
m'~lur~
P,ehea\
h(ll~,
CUlling
o.y~en h"Io'
~~I~e
Q~Y'J,",,~~i~~
Pi.,,, Vjew (l{ noul" Figl
pI",,,
\
Cutting processes \
The majo-rity of metal .eu tting in shipyards utilises gas cutting techniques. Plasma arc and gouging cutting technique. are also being increasingly used.
====3
·.-·di
,~-
Gas cutting During gas. cutting the mC'tal is, in effect, "cut' by oxidising and blowing away a narrow band of material. The metal is heated by the preheat :section of the flame
I
," Figure 4.25 Edge preparation: (tJ) triple-nozzle head; (b) pLIIl new of m)n(f3 showing order cf Ocperllrl/)"n
Welding and o,ttingPrtxesu1 70
W~lding and
CUtting Proc~ne:s
71
at~a~hed. stre~m ~ec;~~;~S::t~ra~ ~~;~~o~ ~~::;:
Plasma arc cutting The cutting torch consists of a lungsten electrode located in a waler-cooled nozzle which acts as one eleclrode in the circuit (FIgU~ 4.26). The materia! 10 be cut is the other electrode and the circuit is completed by a stream of ionised gas which will conduct electricity. This 'plasma gas' is supplied around the tungsten electrode and constricts the arc formed between it and the metal plale.
I./L------"~. cu.."nl Flt:Urr 4.26 l'IIlunll IUC currin, t~h
A very high temperature region is created at the arc which melts the metal and cuts through it. The gas is initially ionised by a short electrical discharge between the electrode and the nozzle. Inert gases such as argon have been used bUI modem developments have enabled air or oxygen to be used_ This is an automated cutting process which is much faster than other methods. Gouging Gouging steel plate by 'arc-air' or by a special cutter fined to a gas torch is a way of removing metal for the 'back-runs' of a butt weld. Gas or arc welding processes may be modified for gouging purposes. Arc-air gouging consists of a solid copper-dad carbon graphite electrode in a special holder which has a
,
Plllie
i
compressed a;ir pipe A to the workpiece 10 Oludtse and remo e cd 'de the high temperature Another arrangement useds tubthular. el~d",r ott~~ ~I~~~ode. The electrodes are The air is blown own e my . h . arc.. The solid electrode anangement 15 s own In consumed In both these processes_ FJ&Ure 4.27.
Major Stmctu"a/ Items
Some double bottoms have a duct keel filled along ttIe cenLreline. Tnis is an internal passage of watertight i,;onstTUction rUlJning some distance along the length (l\ the sh.ip, (;fH~n (mm the forepeak tQ the f~Hwafd machinery space bulkhead. Use is made of this pilssage to carry the pipework along the length of the ship to the various holds or tanks_ An entrance- is usually prQvided at the forward end of the machinery space via Ol watertight manhole. }iu dUd keel is nece-ssary ill the machinery spOlce or aft of it, SiflC'~ pipework will run Llbove the engine room douiJle bot/om and along the shaft tunnel, where one is fitt-ed.
5
Major Structural Items
C"nli"uou~
KEEL AND BOTTOM CONSTRUCTION
,,
The ~otlom shell constI~c1ion consists of the central .keel of the ship. with the floormg ~trUl;tllre and sure shell platIng on either side. Almost all """ssel, bUilt :oday , With the ex.ception oftankels, are fiued with a double botlom. This is an ntcconal ~kin fitted about I rn above the outer shell pJalin 6 and supported by' the oorffig structure.
Solid floo,
0
InnglhJd,nal gifuer
S.l;ffe~e'
Brac:k~t
SECTION A
73
"V
0 1:'')... i
,
"
4
IJ
~ ,long,tUrl,n~1 ,
<1lif~ncc
!
11
~~--
"I-Iat bar
"i'iencr
, I("~I
plo1e
I nmr,osto= 1,,~g·,nJ6,~,1 >t;tl~ner
Keel The construction of the duet keel uses two longituuinal girders spaceJ not more than 2,0 m apart. This r-c,tril:!jull is to enSUfe that the lon~itlldinal girders rest on the ducking blocb wJlen the ~Itip is in ilryduck. Stiffeners are fitted to shell and bottom platillg at alternate frame space-S and arc bracketed to tlte longitudinal girders (Figure 5.2). The kce-l plale and the tank tCJ-P above the duct keel must have their sc:alltling~ inneaseJ 10 ..:ompensate for the reduced strength ()f the transverse floors.
Th~ keel runs along the centreline of the bottom plating of the ship and for the maJoflty of merchant ships is of a fiat plate construction. At right-angles to the flat plate keel, r~nning alo~g the s~p's centreline from the fore peak to Ihe aft peak bulkhead, IS a watertIght longItudinal division knowfl as the centre girder or vertical keel. WhcYC a double-bottom constructIOn is employed, th.e centreline Centre lone
Centre Y;rder
'lrak~
of
ct~
Double-bottom s.tructure Longilu<:tinal ~ljffen~r
501,.0
:--
floor
FI~tba'
• Keer plole
Figure
5,}
Hilt plale keel
~t~.ake of, tank t.op plating results in the furmation of .an I·seclion
keel (Figure . )'. ThIS prOVIdes conSlderable- strength to the structure- and resistance t b~ndmg. The flat plate keel or 'middle hne strake of plating' is increased i.~ ~/kness fDr st:engt~ purposes and for a corrosiDn allowance, because of the l ~culty In mamtrunmg paint pIOteetion systems in way of the docking blocks d unng the vessel's life, 72
Where a, double bottom ur inn(':r ~hcll is fitted it is watertight up to the bilges tnus proViding complete watertight integrity' s}\Ould the outer snell be pieree{ in way of the double bottom The minimum depth is determined by rule require ments for the size of vessel but the- adual depth is sometimes increaseo in place to suit double-bottom tank capacities. The douhle bottom may have a slopin: margin Leading to the bilge radiused plating or a l:ontimlOUS double botton extending to tne side shell. The sloping margin CUtls-tructiol1 requires the use 0 margin plates to connect up with the sirle hami"g and pTovirles",i collecting ba~ or well fur bilge water (Figure 5.3). The continuous tank top -or Oat margin mus have bUge water collecting points or drain 'hats' fitted into it (Figure 5.4: The flat margin is connected to the side framing by a flanged bracket. The l1a margin type of consttuction is much used in modern COllstruction. The struclure is made up of vertical noms wnieh may be watertight, solid c of bracket constructicJn. The floor Structure is continuous from the centre girde to the side shell and supports the inner bottom shell. Side girders are fitted i
74 Major StrncturaI Items
. I nter<:ostlll
.io;legirder
Flange
,
LorogitudiMI
Continuou,! centre-line
Fla",!"
!
~ird8r
\
lt
oi
~;::t,:",_,~'-_J!._J~_!L~J~_t_l~_f~)
Suck'"
Ship
t.
Conlinuou. centreline
8racket
I.,
.girder
Longitudinal
./
'.:' "
75
. the longitudinal direction, their number depending on the width of th.e ~hip, These side girde[~ are bwken either side of the floors and are therefore termed wtercos-tal girders. Watertight or oiltight floors are fitted beneath the main bulkhe.a.
Transversely framed double bottom Ship l
Flat ba,
niffenoer
\
In tercoswl
"
,ide girder
Air hole
FIgure 5.3 wngltudtJrlIlly framed dOUble bottom: (a) bracket /1-00',- (b) solid floor
When tramversely fr.a.med, rhe double-bottom structure L:onsists of solid plalc Roors and bracket floors with transver~e frames. The bracket flom is fiued between the Widely spa-ccd solid noors. It consists of transverse bulb
Lonyitudinally framed double bottom lJ.t~il of dr.-in
Continu,Ju, cen"e 9irder
I
Angle bar \lru I
Upper frame
!
IntereoSt.' ,ide girde.
/
'h'-I'
Flange
/
I
jI
Bracket
,0
,
Brack.. t
Flange
Bottom
fr~me
Flat bar
",
'tiffen..r
This is the system favoured as a resul.! of tests and it provides adequal:e resistance to distortion on ships of 120 ll\ in length or greater. Off~et bulb plates are used as longitudinal stiffeners on the shell and inner hottom plating, at intervals of about I m. Soiid floors provide support at transverse bulklleads and at intervals not exceeding 3.8 m along the length of the ship. Brackets are fitte-d at tnc centre girder and side shell at intermediate frame spaces between solid noors. The1;.e brackets arc flanged at the free edge and extend to the tlrst longitudinal. Channel bar or angle bar struts are provided to give support at intervals of not more than 25 m wnerc solid floors arc widely spaced. Intercostal side girder~ are again fitted, their number depending upon classification society rules. Solid and br.a.cket fioors for a longitudinaUy framed vessel are shown in Figure 5.3.
Flat bar .tiffener
Machinery space double bottom
ContinuDIJ$ Centre !l"der
I
Droin hole~
, 1'11&'<:0'181
,ide girde,
Figure 5..4 TnllfSl'f!"eJy framed double b(Jtt~-. "''' (a) IJrodei floor; (b) ~Qiid floor
The construction of the double bottom in the machinery space regardless 01 framing system has solid plate noors at every frame space under the m..ir engine. Additional side girders are fitted outhoard of the maUl engine seating, a! required. The double-bottom height is usually increased 10 provi.de fuel oil lubricating oil and fresh water tanks of suitable capacities. Shaft alignment als( requires an increase in the double·bottom height or a raised ~eating, the forme
76
Major Structunz/ Items
Mflj11 .. " ...... , ...... _
met~od usually ~ing adopted. Continuity of strength is ensured and maintained by .~dualJy ~~pU1g the tank lop height and internal structure to the required po~tion. Additional. sup~rl ~d stiffening is necessary for the main engines, bolkrs, etc., 10 proVIde a VlbratJon-resistanl solid platform capable of supporting ~~ concentrated loads. ~n slow-speed diesel..engined ships lhe tank top pialing IS Increased to 40 mm lhlck or thereabouls in wlY of the engine bedplate. This CO,,,inuou5 Cftlue!irw
..
HNvy ~It ~It
Girder
gi«lItr
, ""
o o
their capacity determined. All double-bottom tanks are tested on completion by the maximum service pressure head of waler or an eqUivalent air test.
Structure to resist pounding Pounding or slamming results from the ship heaving or pitching, Ihus causing the forward region to 'slam' down on to the water. Additional structural slrength must be provided from the forward perpendicular afl for 25-30% of the ship's length. The shell plaling either side of lhe keel i:. increased in thickness, depending upon the ship's minimum draught. The frame spacing is reduced, full· and half-height intercostal side girders are fined and solid floors Ire installed at every frame space. With longitudinal framing the longitudinal spacing is reduced. intercostal side girders are fitted and tran$Ye~ floors are installed at alternate frames.
Single-bottom construction
lubtic,ling oil d~;n 'lnt -I--+_o~oil tint Colf~
Figurt 5.S MQdlillU)' rpDU doubl~ bottom
~ ach~eved by using a sp~cia.1 insert plate which is the length of the engine rncluding the ~rust block In SIZe (Figure 5.5). Additional heavy girders are also fitte.d undet ~IS plate a~d i~ other positions under heavy machinery as required. Plating and guder maten.al In the machinery spaces is of increased scantlings in the order of 10%.
In oil. tankers particularly, and some smaller vessels, a single-boltom construction is employed. The oil tanker bottom structure is detailed in Chapter 8. The
construction of the single bottom in smaller ships is similar to double-bottom construction but without the inner skin of plating. The upper edge of all plate floors must therefore be stiffened to improve their rigidity.
SECTION B SHELL PLATING, fRAMING SYSTEMS AND DECKS SheU plating
Double-bottom tanks Access to the doUble-b~ttom tanks is usually by manholes cut in the tank top. These ma~holes are SUitably jointed and bolted to be completely watertight when not In use. Docking plugs are fitted in all double-bottom tanks and are a means ~f c.ompletely draining these tanks for inspection in drydock (Fi,grJTt! 5.~). Air pipeS are fitted to all double·bottom tanks to release the air when filling. Sounding pipes are also fitted to enable the tanks to be sounded and
I
M,td-51H'l PId Bouomllhtll
ed.r c1!r;z 4"~ DockIng plug. btl'S or m"nleu 51eet
T.....'ne
Ino:l ted
lti
The side and bottom shell plating provides the watertight skin of the ship. The shell plating also makes the greatest contribution to the longitudinal strenglh of the ship's structure. As a result of its huge area the shell plating is composed of many strakes or plales arranged in a fore and aft direction and welded together. The horizontal welds are termed 'seams' and the vertical welds are termed 'butts'. Several strakes of plating are usually joined together as part of a unit. A shell expansion by units was shown in Figure 3.2. The thickness of shell plating is largely dependent upon ship length and frame spacing. The fmal structure must be capable of withstanding the many dynamic and static loads upon Ihe hull, as discussed in C1l.apter 2. Some tapering off of .shell plate thickness towards the ends of lhe ship is usual, since the bending moments are reduced in this region. The strake of side plating nearesl to the deck is known as the 'sheerstrake'. The sheerstrake is increased in thickness or a high tensile steel is used. This is because this section of plating is furthest from the neutral axis and subject to Ihe greatest bending stress, as discussed in Chapter 2. The region where the sheerslrake meets the deck plating is known as lhe gunwale. Two particular arrangements in this region are used and are shown in Figure 5.7. With the rounded gunwale arrangement no welding is permined on the sheerstrake
16
MaiorStl'Uctura/ items
Maior Structural items
metho
Drai"3go! ..ran9<'mem
st,lf"noer
Conti""")LJ' centreline gircfer
/
o
i
o
~
Flat Heavy pl..t. seal
\ \
0 I .
I
I Cot1erdam
""
I
I--OI'''I'''ltank-+-_~LUbncalln~O'ld,amti'''k' I +---r-r
Girder
"
Girder
1
0 'ese-
,,' 01
tank
--jI
Cottoerdam
Figure 5,5 Machinery space dvul1l" borlom
~s ach.!eved by using a special insert plate which is the length of the engine mcludmg the thrust block in size (Figure 5.5). Additional heavy girders are also fiae.d under :his plate a~d i~ other posi:ions under heavy machinery as required. Platmg and glrder matenalm the machinery spaces is of increased scantlings in the order of 100/0.
77
their capao;;ity determined. All double-bottom tanks are tested on completion by the maximum service pressure head of water or an equivalent air test.
Structure to resist pounding pounding or slamming results from the ship heaving or pitching, thus causing the forward region to 'slam' duwn on to the water- Additional structural strength must be proVided from the forward perpendicular aft for 2S·-30'YL of the ship's length. The shell plating either side of the keel I:' increased in thickness, depending upon the ship-'s minimum draugllt. The fra'me spacing is reduced, full- and half·height intercostal side girders are fitted and solid floors are installed at e'lery frame space. With longitudinal framing the longitudinal spacing is reduced, intercmtal side girders are filted and tranwerse floors are installed at alternate frames. Single-bottom construction
In oil tankers particularly, and some smaller vessels, a single-battom construction is employed. The oil tanker oottOlll structure is detailed in Chapter 8. The construction of the single boltom in smaller ships is similar to double-bottom construction but without the Inner skin I)f pla.ting. The uppel edge of all plate floors must therefore be stiffened to improve their rigidity.
SECTION B SHELL PLATING, FRAMING SYSTEMS AND DECKS Shell plating
Double-bottom tanks Access to the double-bottom tanks is usually by manholes cut in the t.ank top. These manholes are sUitably jointed and holted to be completely watertight when not -in use. Docking plugs are fitted in all double-bottom t:mks and are a means of c:ompletely draining these tanks for inspection in drydock (Figure 5.6). Air pipes are fitted to aU double-bottom tanks to release the air when filling. Sounding pipes are also fitted to enable the tanks. to be sounded and
Botoarn sh.11
Docking plug, bra"s Or 'lainl""lt".,
Figure 5.6 Dockin,; pfug and pad
The side and bottom shell plating provides the watertight skin of the ship. The shell plating also makes tne greatest contribution to the longitudinal strength of the ship's structure. As a result of its huge area the shell plating is composed of many strakes or plates arranged in a fore and aft direction and welded together. The horizontal welds are termed 'seams' and the vertical welds are termed 'butts'. Several strakes of plating are usually joined together as part of a unit. A shell expansion by unin was shown in Figure 3.2. The thickness of shell pLating is largely dependent upon ship length and frame spacing. The final structure must be capable of withstanding the many dynamic and static loads upun the hull, as discussed in Chapter 2. Some tapering off of shell plate thickness towards the ends of the ship is usual, since the bending moments are reduced in this region. The strake of side plating nearest to the deck is knl)wn as the 'sheerstrake'. The sheerstrake is increas.ed in thickne~s or a high tensile steel is used. This is because this section of plating is furthest from the neutral axis. and subject to the greatest bending stress, as discussed in Chapter 2. The region where the sheerstrake meets the deck plating is known as the gunwale. Two partkuLar arrangements in this region are used and are shown in Figure 5.7. With the rounded gunwale arrangement no welding is permitted on the sheerstrake
MlliorSlructurul Items
79
Deck beam lleam knee -......
I
i
1
,
I
i Knee
-
Deck Tr ,,'s~e rse
T'","w~··,e
I,,,mc
ha~e
,
,
~
,
,
-, ......
,
~i'der
I, centrel g"d~r ,
Figt./n' 5.7 Gunwale- arrangement,
'l:ause of the high stressing which could re~uJt in crack~ emanating from the oes' of fillet welds. Sucn welds reduce the resistance of components to acking. Where such structure is butt wclde-d the welding must blend into the m:nt plate. Towards the ends of the ship, as the CJOss·section reduces, the niolls strakes of plating will taper in width. Where these plate widths become nall, a stealer plate or strake is fitted (hgure 5.8).
Tran.v"""
~.
fr~me
., I. ,
Margin _ bra"ket
V
0
°1 0°'1 000 I 0
0
0
0
0" o~,
girder - __ Si,le
lon~itudinal
Cal
Side lonyitudi"al,
t7:r::':Jr'"';::]I'r
// ~"rTle,
, ", /"
i P .11;~~ ! ,: ,alo.~
Deck
----
,
Longitudinal 't.... lkhead
c.nrr~
~ird.r
I P'.'ll"~ , "rdk~
S:~dl~r
"r"H
.
rr I
IPIJt;ng
Centre gird~r
Bo-nom
IOrlgitudinal5
_'S:.kbl,LLU-lClLLJCILLU-llLLI...llLUY "
PIJllng
----- - -' ,
O~,
"' -
fbi
De-ck I,a,,;\, eroe "-
gi,-der
--
!,
1
1
L
De;;kl
""gitudinals
T,anS\l''''" beams ~t"""'>\
(hilt.... """
T Figure 5.8 Steala strake arrangement
,
All openings in shell plating must have rounded edges to avoid stress con· entr~t-;oll~ and usually some form of -compensation to avoid a discontinuiiy of
: '-0
lie bottom shell and side plating are framed, i.e. stiffened along their length, gainst the compressing forces of the sea. Two different types of framing are in
kn~e
I
1
lrength. :raming systems
l
I
8eam
00 0
q.ldo'QTojot
/ Side 9irde,
/
C'l'nl'.
I
.... Bonom
-
j
0
longitudinal,
giroe,
101 Figure 5.9 FnIming tystel1lJJ; fa) traml'eT:l'€ framing; (b) /ollgtrudillsl framing; (c) c()mbine~ framing
80
Majo, Stntctural hems
MajorStrncturai Items
use, or a combination of the tW{) may he employed. These are known. respectively, as transverse, !-ongitudinal
81
, :JCllJ~1
Transverse framing
,,'q
1,IoLe
Transverse fr.aming of the shell plating consists of vertical stiffeners. either of bulb plate or deep-flanged web frames, which are attached by brackets to the deck beams and the flooring Structure. The sca:ntlings of the frames are- to some extent dependent upon their depth and also on the natu.re of their end connections. Particular locations, such as at the ends of hatches, require frames of increased scantlings. Very deep web frames are often fitted in the machinery space. Framc spacing is generally not mOle than 1000 mm but is always reduced in the pounding (cgion and at the fore and aft ends in the peak tank regions. Longitudinal framing
Longitudinal framing o-f the side shell employs horizontal offset bulb plates with increased scantlings towards the lower side shell. Transverse webs are used to support the longitudinal frames, their spacing being dependent upon the type llf ship and the section modulus of the longitudinal,. This constructiun is described and illustrated in Chapter 8 with reference to oil tanker construction.
Bilge keel l-olf",\ \lulb platel
• Ibl
With a flat keel construction there is litde resistance to rolling of the ship. A bilge keel is fitted along the bilge radius either side of the ship to damp any tendency the ship Itas t() wll {Figure S.lO} Some improvement in longitudinal strength at the bilge radius is a1so provided. The bilge keel must be arranged to penetrate the boundary layer of water along the hull but not too deep to have large forces acting on it. The bilge keel is fitted at right·angles to the bilge radiused plating but does not extend beyond the extreme breadth line. It runs the extent of the midship section of the ship and is positioned, after model tests, to ensure the minimum resistance to forward motion of the ship. Construction is of steel plate with a stiffened free edge or a section such as a bulb pla!e. A means of fastening to the hull is employed which will break off the bilge ked witllout damage to the hull in the event of fouling or collision. The ends are fastened to a doubling plate on the shell, since the bilge plating is in a highly stressed region of the ship.
lee navigation strengthening lee class no lations l·, I, 2 or 3 are assigned to ships which have additional strengthening as required by classification society rules. Various means of
Figure 510 Bilge keeL (a) pian view shO\l,1-ng ammgemellr at ends; rbj section rlJrough t/ilge keel
-additional stiffening by increased frame scantlings, reduced frame spacing and increased plate thickness. are required. The extent and nature of the stiffening reduces from 1*, which is the highest classific-ation, to 3, which is the lowest. Some modifications to the stem -and stem region~ may also be required.
Decks The deck of a ~hip is the horizontal platform which completes the enclosure of the hull. It must provide a solid working platfonn capable of supporting any load~ resting upon it, and also a watertight top cover to the hull structure. The deck with its various forms of stiffening and its plating provides a considerable contribution to the strength of the ship. Where tile deck is pierced by hatches, ~pecial warnings or surrounds to the openings must be provided. These large openings require special compensation to offset their effect on the structural strength of the ship.
82
Major StTucrural /tems
Major Strnctural Items
Deck plating The deck plating is made up of longitudinal strakes of plating across its width. The plates or strakes nearest to the deck edges are termed 'stringer plates'. They are of thicker material than the remaining deck plating since they form the iJriportant join between the side shell and deck plating. Towards the ends of the ship the deck plating,like the shelJ plating, is reduced in thickness. The large openings in the deck for hatchways, engine casings, pump room entrances, etc., require compensation to maintain the section modulus of the material. The deck plating abreast of such openings is therefore increased in thickness. The plating between the hatches of a cargo ship is thinner than the rest of the deck plating and contributes little to longitudinal strength. The plating of the weather decks is cambered towards the ship's side to assist drainage of any water falling on the deck. This camber is wually of the order of one·fiftieth of the breadth of the ship at midships.
Deck stiffening The deck plating is supported from below in a manner determined by the framing system of the ship. With longitudinal framing, a series of closely spaced longitudinals are used in addition to deep web lIansverses. With transverse fnming, transverse deck beams art used at every frame space. Where hatches are fitted to a ship, continuow longitudinal girders are fitted over the length of the ship running alongside the hatches.
Deck beams and transverses
83
fitted on the ship which run alongside the hatchways and the beams are bracketed to these girders. In this way the unsupported span is reduced. Deck beams arc usually offset bulb plates. For the length of the open hatch space the beams are broken and bracketed to the longitudinal girder or hatch side coaming. The beams are likewise broken and br~cket~d to the. longitudinal girders in way of the engine casing. A beam broken In th15 manner IS known as a 'half·beam'. Deck transverses support the longitudinally framed deck. These arc deep plate webs with a facing flat or a flanged edge. They are bracketed to the side frames by beam knees. Small tripping brackets are fitted between alternate longitudinals and the transverse (Figurr 5.JJ).
Deck girders Deck girders exist in a number of forms, depending upon their location. A flanged girder with tripping brackets will often be used ~ part of a hatch coaming. Such a flanged girder is referred to as unsymmetncal and mwt have tripping brackets fitted at alternate frame spaces. The symmet~ca1 girder is often used, particularly as a centreline girder. Brackets join the girder to the. deck bums and are fitted at every fourth frame space. At hatch comers these guders must be additionally supported either by pillars or uansverse girders. The symmetrical and unsymmetrical types of girder are shown in 5.1!. The combination of longitudinal girders with transverse beams IS much In use in modem ships. The deck longitudinal girders extend as far as possible along
n,gure
Deck beams are fitted across the width of the ship and are joined to the side frames by brackets known as 'beam knees'. Continuous longitudinal girders are
OKk~.m
G..""
/
fxe flal (.1
FXIII'l II~I
Fifun 5.12 Girdef amlllrtmtnn: (a)
Ibl lym~tricaJ;
fb) unsymmttrical
... the fulJ length of the ship on the outside of the hatches. This continuous longitudinal material permits a reduction in deck plate thickness, in terms of classification society requirements. The deck between the hatches must be supported by longitudinal or transverse beams. Where side girders join transverse beams, particularly beneath hatch openings, gusset plates are fitted (Figures 5.13 and 5.14).
Local loading Fxing
flat
Fifure 5.11 Deck bum
On the deck where concentrated loads are situated or likely, additional stiffening mus~ be provided. M2chinery such as winches, windlasses, etc., will
l4
Majo, Stmcfllral Irems
85
also require seatings which arc discussed in detail in Chaplcr 6. Also. any bums fitted in way of deep tanks. bunktr l:mks. etc.. must ha\"c increa~d scanllings and perhaps reduced spans to be at leaSl equal in strength to Ihe boundary bulk· heads.
,,",, ,
=~~~~t~~~_"_=_"_=_"_~=~,~
" ---- ....,-,r-------r-, " : , I
Discontinuities
11
II II
,"
A discontinuity. as discussed here, refers 10 any break or change in section. tttickness or amount of plating materi31. Greal cart must be 13k~n 10 compensale for any discontinuities in shell or deck plaling resulting from doors, hatchways, etc. Where lhe loss of longitudinal material results. this compensation is of particular importance. Where changes in the alllount of plating material occur. such as at bulwarks, the change should be gradual and well radiused. Well·radiused corners must be used and sometimes the filling of doubling plates or thicker insert plates, al Ihe corners of all openings. Any sharp corner can produce a notch which. after stressing, could result in a crack. Figure 5./5 shows an insert plate filled 3t the comer of a hatch opening.
... GU\OO!I ;>1~lt
Figuf't j. / J H(uch cOnl,r KUsur plillt. ~it ..V!d {rom ~lo ..'
l ...... 01 1141
--"",1",,;"--
"'I,,
----- :
Gn(1t.
I
Fi,un' J.14 GUSSf!t plott' uud in mKhinf!!ry SplIU COnlIn/trion
=L
F•• meop«:1
f,_'I>oa!
0."
P~I;ng
/
Hatch coamings The edges of all hatch openings are framed by halch coamings. On Ihe weather deck the coarnings must be at a minimum height of 600 mm according to the load line regulations. This is to reduce the risk of waler entry to the hoids. Internal coamings, e.g. those within the superstructure or holds. have no height specified and in tween-
Well
e1li' 01 ,1td'UI
Ra
","" I
.Bult 10 be .f
t
SECTION C
BULKHEADS AND PILLARS
Idfofined in rules}
Bulkheads
eoaming bun
FiJ$lrt.5. J.s I,Ut" p/4tf!! fittm at hatch r:omer
The vertical divisions arranged in the ship's structure are known as bulkheads. Three basic types are found, namely watertight, non·watertight and oiltight or tank bulkheads. Oihight or W1Ic. bulkheads are watertight in their construction but are subjected to more rigorous testing than a simply watertight bulkhead.
Major Structurtllltems
86 Ramp for "ate" c"".. wtleels ,~m
b'''''k
;eke! c"",er
r\
Side
'"'
~'"
,
,, ,
,, ,, ,,
~
t
I I
J
, ~:, , ,,
I
I
/
Hateh. gi rde.
t.
'"
Gusset plate
bracket
\
Watertight bu Ikheads '~
~
I'.
~
Tripping
".~
"" " - - - - - - - - r - -- +" r---r .... ---+
Coaming ,tiff ene'
\
Hateh end
:I
The transverse watertight bulkheads subdivide the ship inlo a number of watertight compartments and their number is -dictated by das~ification sadety regulations. Oiltight bulkheads form the boundaries of tanks used for the carriage of liquid cargoes or fuels. Non-watertight bulkheads are any other bulkheads such as engine casings, accommodation partitions or stores compartments.
Coamir>g
brll(;ke'l
,.,
-In addition to subdividing the ship, transverse bulkheads also provide considerable structural strength as support for the decks and to resist deformation caused by broadside waves (racking). The spacing of watertight bulkheads, whicn is known as the watertight subdivision of the ship, is governed by rules -dependent upon ship type, size, etc. All ships must have:
D~'
stiffener
(1) A collision or fore peak bulkhead which is to be positioned not less than 0.05 X length of the ship, nor more than 0.08 X length of the ship. from the forward end of the load waterline. (2) An after peak bulkhead which encloses the sterntube(s) and rudder trunk in a watertight compartment. (3) A bulkhead at each end of the machinery space; the after bulkhead may, fOI an aft engjne room, be the after peak. bulkhead.
"
I C,
I (
--_-I.:
',-
)
Elliptical
Half.rOUrl<:l
COfnBf
Daf at
""
I II
--"-tT,,..... _ '" -_-_-_ '-';~'5E:S=:::!5=':':'::i="=5":
"Ir
----1----
---r ·L.--.i~~,T~~~=-=b HateMgirdM tH!low
Hat<:n
'"' ""m lbelowl
-----t
Additional bulkheads are to be fitted according to the vessel's length, e.g. a ship between 145 and 165 m long must have 8- bulkheads with machinery midships and 7 bulkheads with machinery aft. Fitting less than the standard number of bulkheads is permitted in approved circums.tances where additional structural compen~tion is provided. Water· tight bulkheads must extend to the freeboard deck but may rise to the uppermost continuous decle. The aft peak bulkhead may extend only to the next deck above the load waterline, where the construction aft of thi-s deck is fully watertight to the sheIL The purpose of watertight subdivision and tne spacing of the bulkheads is to provide an arrangement such that if one compartment is flooded between bulkheads tne ship's w£terline will not rise above the margin line. The margin line is a line drawn parallel 10 and 76 mm below tlte upper surface of the bulkhead deck at the ship's side. The subdivision of passenger ships is regulated by statutory requirements which are in excess of classification society rules for cargo ships, but the objects of confIning flooding and avoiding sinking are the same.
CoiIming cOt,~r
'I
br~ket
'"
Figure 5.16 Hareh cOflming: (a) elevation of hatch coammg (~teel hQtch Cov~TSJ: (b) pkm ~ie\tl t:>fhatch cOflming Isteel hatch CQ~~) Offw1 0011> pla1e
Offllel Itlgle
"..
Hatch
c""ming
--t"~
87
Coaminog
-If---
Bracket
..
flU
"
D~'
plating
Figure- 5.17 COilming bnJt::ket
J
Constrnction oj watertight bulkheads
I
w.atertight bulkheads, because of their Large area, are formed of several strake1 of plating. They are welded to tne shell, deck and tank top. The plating strakes are horiz()ntal and the stiffening is vertical. Since water pressure in a tank increases with depth and the watertigh.'t bulkhead must withstand such loading, the bulkhead must have increasingly greater strength towards the base. This is
l
j
Major Structural Items
89
achieved by increasing the thickness of the horizontal sirakes of plating towards the bottom. The collision bulkhead must have pia ling some 12% thicker than other watertight bulkheads. Also, plating in the aft peak bulkhead around the stemtube must be: doubled or iJlcreased in thickness to reduce vibration. The bulkhead is stiffened by vertical bulb plates or toe-welded angle bar stiffeners spaced about 760 mm apart. This spacing is reduced to 610 mm for coUision and oillight bulkheads. The ends of Ihe stiffeners are bracketed to the lanktop and Ihe deck beams. In twecn decks. where the loading is less. Ihe stiffeners may have no end connections. A watertight bulkhead 3rrangtment is shown in Figure j. J8.
,
• c
Corrugated watertight bulkheads
•,
The use of corrugations or swedges in a plate instead or welded stiffeners produces as strong a structure with a reduction in weight. The troughs are vertical on transverse bulkheads but on longitudinal bulkheads they must be horizontal in order (0 add to the longitudinal strength of the ship.
•
-c
~
, •,
I I,
r----t!+--+--+--f-----+---! •~ I-
!+-+--+---+-1-- !A
•
..
~lel...k
I . I I ! I I .l'-Jr.., ,I " I' , I ,
I ' I~'~'~I I ' I' ,I .I I"·"' I' I' I . . I I I I i Ii I I II I' I l/ IAa~
..-.~ -+--++-+-+---1-+- -~.I!
.....te.. rght
• l)ulknuo:l
~
>
Upper ,tool
!
•
Lo- 'lool
r
J
t.,
i
Happe'l... k
Double bOil""'
'"'
Welded se.m
• td
FifUrt 5.19 Coffllpud _l~rtitht bulkhrlld_' (tl) WCriOll throut/lt:OfflIprion: (b) d~~llrion o{ bu/khtfld: (t) plDn ,ir.., oftorruprions
91
90 Major Structural Items The corrugations or swedges are made in the plating strikes prior to fabrication of the complete bulkhead. As a consequence, the strues run vertically and the plating must be of unifonn thickness and adequate to support the greater loads at the bottom of the bulkhead. nus greater thickness of plate offsets to some extent the saving in weight through not adding stiffeners to the bulkhead. The edges of the corrugated bulkhead which join to the shell plating may have a stiffened fiat plate fitted to increase transverse strength and simplify fitting the bulkhead to the shell. On high bulkheads with vertical corrugations, diaplrn.gm plates are fitted across the troughs. This prevents any possible collapse of the corrugations. A corrugated bulkhead arrangement is shown in Figure 5.19. A watertight noor is fitted in the double bottom directly below every main transverse bulkhead. Where a watertight bulkhead is penetrated, e.g. by pipework, a watertight closure around the penetration must be ensured by a collar fully welded to the pipe and the bulkhead.
/ ,:
" "
"
,
" ""
,.,
'"
Testing o{watertight bulkheads The main fore and aft peak bulkheads must be tested by filling with water to the load waterline. Subdividing watertight bulkheads ate tested by hosing down. Oiltight and tank bulkheads must be tested by a head of water not less than 2.45 m above the highest point of the tank.
Stiffener
Non-watertight bulkheads
-----------
Any bulkheads other than those used as main subdivisions and tank boundaries may be non-watertight. Examples of these are engine room casing bulkheads, accommodation partitions, store room divisions, etc. Wash bulkheads fitted in deep tanks or in the fore end of a ship are also examples of non-watertight bulkheads. Where a non-watertight bulkhead perfonns the supporting function similar to a pillar, its stiffeners must be adequate for the load carried. In all other situations the non-watertight bulkhead is stiffened by bulb plates or simply fiat plates welded edge on. Corrugated and swedged bulkheads can also be used for non-watertight bulkheads.
Pillars Pillars provide a means of transferring loads between decks and fastening together the structure in a vertical direction. The pillars which transfer loads, as in the cargo holds or beneath items of machinery. are largely in compression and require little or no bracketing to the surrounding structure. Pillars which tie structure together and are subjected (0 tensile forces are adequately bracketed at the head or top and the heel or bottom. Hold pillars are usually large in section and few in number to reduce interference with cargo stowage to a minimum. Pillars are provided to reduce the need for heavy webs to support the hatch girders or end beams. The use of piJlan also enables a reduction in size of the hatch girders and beams. since their
BrICkel
f----Pill¥
,., Slif!ffief
Girder
,O! Fip'" .5.21 lIIbulu piJiJI, llffllll'~mtnU: (a) pi11tzf hud COIlJl«POI"I: Ib) pillJu h«l COftntctiOlt
Maior Structuralltf!rns
92
_-----------7
Major Stnl("t/lralllC'ms
unsupported span is reduced. Where pIllars are filled between a number of venical decks they should be in line below one another 10 efficiently transfer the loads. Hold pillar sections arc usually a hollow fabricated shape manufaclured from steel plale. Typical sections are round. square and somelimes octagonal. Machinery space pillars are usually fabricatcd from sections and. while slllalicr in dimensions than hold pillars. a greatcr number are tined (FigllrC' 5.20). Additional slruclural matcrial must be pro\'ided at the head and heel of pillars to evenly disuibuu: the load. AI the head a plate is used. orten with tripping brackets to surrounding structure. At the heel an insert plate or doubling plate is used, wilh or wilhoul bracken depending upon Ihc type of loading (Figur(' 5.21). Solid pillars may be fined in accommodation spaces or under poinls of concenuated loading. Solid round bar up to abolll 100 111111 diameter is filled. again with head and heel plates to spread the lead.
SECTION D
93
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FORE END CONSTRUCTION FlKUtr 5.22 F(H'f! end ronStlllction
The forward end of a ship refers to the struClUre forward of the collision bulk· head. TIle forward end is designed to provide a smooth enlfY 10 Ihe water and a sneamlined flow along the ship. As a result. resistance to motion is reduced to a minimum. The stem is the most forward pari of the ship and runs down to the keel. It is constructed in two parIS - a bar stem from the keel to the load waterline and a plate stem up to the deck. TIle plate slcm usually rakes well forward providing pleasing lines 10 the s.hip. an increased deck area and a readily coUapsible region in the event of a collision. The side shell plating is narl~d OUI to fUrl her increase the de-ck are-a. This arrange-me", also serves to deflect sea waler and spray away from Ihe- ship in heavy weJ,the-r. The forward deck area or fore-castle houses the windlasses and winches required for anchor and mooring duties. The anchor chain is housed in a chain locke-r benealh the forecastle. A bulbous bow may be filled. which is a protrusion below the waterline designed 10 reduce fhe ship's resistance to motion.
Fixutr 5.23 Sutton IhTOCiP plaIt' sum sho..inf brusthook
tDf .. ~
Panting structure Stem The stem is the terminating point of the forward shell plating. It is made up of a stem bar from the keel 10 the load waterline and a stiffened plate struclUre up to the forecastle deck (Figure 5.22). The stem bar is a solid round bar which is welded 10 the inside of the keel plate at the lower end. At its upper end the bar joins [he slem plate. The shell plating is welded to either side of the stem bar. The stem plate conSlruction of curved plales is stiffened at intervals by breasthooks which are small flange plates fined horizontally (Figure 5.23). A continuous bulb or flat bar stiffener may be fined where the sle-m plale radius is considerable. Heavier than usual shell plating may be fitted at the stem plate region.
Panting is an in·and-out movemenl of the she-II plating resulting from the variations of water pressure as waves pass along the huU and when the vessel pitches. Special structural arrangements are necessary in the forward region of the ship to strengthen the ship's pia ling agai.nsl this action. The structure mu~t be strengthene-d for 15-20% of the ship's length from forward to t~e ste~. Thl.s stiffening is made up of horizontal side mingers, known a~ 'pantlng stringers • fitted at about 2 m intervals below the lowest deck. Panting beams are fitted across the ship at alternate frame- spaces and are bracketed .to the. panting stringer. The intermediate frames are connected to the pantm.g strm~er b~ brackets (Fjgu~J 5.24 and 5.25). A partial wash bulkhead or a senes of pillars 15 fiue-d on the centreline to further support the structure-. Perforated flats may be
94
Major Strocturalltems SIde
,,,de<
95
fitted instead of beams but these must be not more than 25 m apart. Perforations of at lust 10% of the plate area are required in order to reduce water pressure on the flats.
Bulbous bow
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The bulbous bow is fitted in an attempt to reduce lhe ship's resistance. Arrange. ments vary from a casting plated into the forward end to a fully radiused plated structure, or in some cases a cylindrical shape plated into the forward end. The erfectiven~s of the arrangement is the subject of much discussion but improved buoyancy forward is provided which will reduct the pitching of the ship. The construction shown in Figurt $.16 consists of a vertical plate web which stiffens the free edge of the breasthooks fitted right forward in the bulb. Deep frames with panting beams are fitted al every frame space with a wash bulkhead on the centreline. The panting stringers consist of perforated plates running the full width and length of the bulb. Another vertical plate web joins the bulb to the fore end structure. A small stem casting connects the top of the bulb to the plate stem above the load waterline. The numerous manholes cut into the structure permit access to all parts of the bulb. The anchor and cable arrangemenlS must ensure that the bulb is not fouled during any part of the operation. Bft
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Figun 5.26 Bulbous bow COI/Srnu:notl
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96
Major S!fUcturalltems
Major Stru,:luralltems
Anchors and cables The forecastle deck !louses the windlass or windlasses which raise and lower the anchor and cable. Variou~ items of mooring equipment such as bol1ards faide-ads, etc., ilrC also arranged around the deck edge. The a~chQrs are housed a,gains! the forward side shell, sometimes in speci'-llly recessed pockets. The anchor cable passes throlJgh the shell via the hawse pipe on to the forecastle deck. It travels over tlte <::able stopper and on to the windlass cable lifter drum. From the cable lifter it drops vertically down into the chain locker below.
Hawse pipe The hawse pipe is fitted to enable a smooth run of the anchor cable to the ~indlass and to maintain the watertight integrity of the forecastle (Fixure :>.21). It. should be of ample size to pass the cable without snagging when raising
or lowenng the anchor. Construction is usually of thick plating which is attached
97
the side shelL A rubbing OJ chafing ring is also fitted at the outside shell. A sliding pbte cover is shaped to fit over the cable and close the opening when the ~hip is at sea.
Cable sto pper Th~
ch:Jin, <:able OT bow stopper is fitted on the {(lrccastle deck in line with th~ run of the anchor cable. II is used to hold the anchor <:Jble in place while th(
ship is riding at anchor OJ the a.nchor is fully housed. In this way th~ windlas~ is freed ami isolated from :lllY shocks or vibratiollS from the cable. The c]taii stopper i~ not designed to stop the moving cable, blJt onl}' hold it in place. Om type is shown in Figure 5,28 :and wmists of a fabricated structure of he3vy plaH with 3 roller which the cable passes over, A hinged har is designed to fal between two vertical links and hold the cahle in place. The chain ~topper i~ welded Of bolted on [() a he.avy imeTt plate in the deck and i~ additionall) stiffened by h-rackets.
to a doubling pl.ate at the forecastle deck and a reinforced strake of plating:at
Windlass
Bow.l
,/
The windlass i~ the lifting devic-e for the anchm cables or chains and i~ also ll~~( for moming and winching duties. Various drums or harre-Is can be 'clutched in tliperform the diffnent duties_ For raising tl1~ anchor. the cable lifting drum i engaged. This is a barrel with specially shaped 'snugs" which tlle cable links fit into an( pass round before dropping into the chain locker vi.a the spurling pipe. Tn. anchor cable is allowed to IGwer under its own weight wiLh Lhe lifting drun decliltched. while the brake band around it is used to ;;antrol the speed 0 descent,
DotJbling plate
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Chain locker
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Figure 5.2"7 Ha \tIl'e pipe
---
--- Anchor
The chain locker is llormally fitted forward uf the collision bulkhead, It is 0 dimensions adequate- to house all the allchor cable and stil1le.ave a considerabil empty space above. Two lockers or a centrally divided single locker will be fit tel for the port an-d starboard anchor cables. The chain locker should be as Iowa practicable to reduce Ihe height of the centre of gravity of the considerable mas of the cables, A perforated false floor or ,grating is tilted at the hottom tl provide a drainage well and keep the cable out of mud and wate-l. Figure 5.29 shows an arrangcmcnt of a chain locker. It consists of a plat, stru-cture with vertical stiffeners armmd the outside. Plate webs which forn part of the ship's internal structure are also utilised for stiffening. A raisel perforated false floor is fitted allli supported by so-lid floors. The well thu form-ed is connected to the bilge system and should be emptied every time th anchor is raised. The forecastle deck forms the top of the lucker wilh th ~purling pipe at the centre. The spurling pipe is manufactured of heavy plat with a solid round bar .as a chaffln,g ring on the lower edge. Brackets radiate fror the spurling pipe to lhe chain locker sides to strengthen the forecastle deck ani the spurling pipe. A U-section plate welded to the ,ide with footholes cut iJ provides access to the bottom of the chain locker from a watertight door at th
98
Ma;orStructural/tems Spu.ling pipe
99
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locker and can be released easily and quickly. A situation may arise where the
safety of the ship does not allow time to raise the anchor. By releasing the clench pin all the cable can quickly pass out of the chain locker. leaving the ship free to proceed out of danger. An arrangement is shown in Figure 5.30. where an P,n
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The fmallink of the anchor cable is secured to the ship's structure by a clench pin. On most modem ships this pin is positioned on the outside of the chain
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upper deck. Provis.ion is also made for ~curing the frnallink oflhe anchor cable. The chain locker illustrated is one of a pair fitted port and starboard beneath their respective windlasses.
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100 Major SlrocturaJ Items inser.t heavy pia Ie .pockel is filled into the chain locker side with a vertical pin h~ldmg Ih~ fmal Imk of anchor cable. A hand-wheel assembly on deck is used 10 ralSe the pm and release the link.
Thrusters A. thruster is .~su~y considered to be a device which assists in docking. manoeuvnng, or posllJon~g ~f a vessel which is moving at a low speed. Some fonn of propeller-type deVIce IS used 10 move water either freely or in a duct. The propeller may be fixed or. contr_o.llable pilch and the complete unit may be retractable or exposed, flXed In position or able to rotate (aZimuth). ProbabJ~ the. most common unit Iitted on merchant vessels is the tunnel t~ruster .usmg either ~ fixed pitch or a controllable pitch propeller. The flXed pilch Unit wouJd require a reversible drive. A controllable pitch type thruster is
.
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Major Structural Items
101
shown in Figun 5.3/. A non-rotating servo motor located in the gear housing is used to change the pitch of the propeller blades. The force on the servomotor piston is transmitted by a piston rod inside the propeller shaft to the crosshead and crank mechanism in the hub. Water now can thus be provided in either direclion simply by changing the blade pitch angle. Any non-reversing prime mover can therefore be used. e.g. a singie speed electric motol. The prime mover need not be stopped during manoeuvring operations since the blades can be placed at zero pitch when nO thrust is desired. The drive is obtained thsough a flexible drive shaft, couplings and bevel gears. Special seals prevent any sea water leakage into the unit. The complete assembly includes part of the athwartships tunnel through which water is directed to provide the thrust. Grids must be fitted at either end of the tunnel and this can reduce the thrust to some eXlent. The actual tunnel locatiOn is usually decided by model tests to ensure !he minimum resistance when not in usc. A tunnel construction arrangement is shown in Figure 5.32. Gill jet thrusters utilise a vertical axis propeller in a T-shaped tunnel. Water is drawn in from both sides and leaves through the bottom of the hull. Rotatable g.ilI ftns djrect the water in One of a number of flXed positions around II circle. The hydrojet thruster has a similar arrangement but draws waler in from below and discharges it at the sides with vanes directing the thrust. Steering vanes in the diverging liquid path can also be used 10 maximise the thrust to one side or the other. Ducted jet thrusters operate somewhat similarly to a tunnel thruster except that the duct is usually curved. This duct may be located either on the ship's side or the bottom shell and usually requires large openings. An azimuth or rotating thruster usually consists of a dUCled propeller which can rotate through 36(f. The propeller may be flXed or controllable pitch. This unit is particularly sujted for dynamic positioning and some propulsion duties. When fitted to ships, an azimuth thruster is usually retractable.
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1. Tunnel sulio/l Z. Motor nzOlilltillg stool J. Input dn·I'e shaft 4. InpUl drive shaft cur";dgC' 5. /'rolH""r shaft Jeal 6. Propeller blade 7. Blade palm sal 8. Illlb bod)'
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9. Ollllk pin n·", 17. Servo mOtor piston 10. Crosshead bearing llOUS;/lg 18. Sen'O motor cylinder head 1/. Toper roller t/'rust bearing 19. Feed bock IillkJlge /2. Crossheod 20. !Kn·o motor cylinder /1. Propeller shafr 21. Sul'O molar end CO~r /4. Prop~l1ershaft thrust 22 Spiral bel·el pinion betJnnl 21. Drive shofr raper roller 15. Spiral bC'1·el ...hed BearinX 16. Pirtoll rod 2-1. Gear housl/lg
Fig/Ire 5.3/ Tunnel fllruster Im;f
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FiKU'" 5.32 Bow thrusrer tunlltl
SECTION E AFT END CONSTRUCTION The aft end of a ship terminates the structure and is designed to provide a Smooth water flow into and away from the propeller. The propeller and rudder
102
MajorSttucruralltems
Major Stnu:tIlral Items
are also positioned :lnd supporled 3t the after end and require cenain structural arran!lemcnts in order 10 operale S3lisfaclorily. The after end consrruetion invoh'es an amount of overhanging structure to accept the sleering gear below deck and mooring equipment higher up on the wealher deck. This arrangemen! leads to large slamming forces in Ihis afler region. and an adequalely stiffened slruclure is therefore ri."quiled. Two main types of stern construclion have been u~d 10 dale - Ihe cluistr slern and the transom stern. The cruist'r stern is I3rely used in modem construclion but it IS slill to be $Cen in a large proportion of the ships 3t sea. The !ransom stern. "'ilh its slraig,ltt·linc form. Ii."nds itself well 10 current manufacturing techniques. It also provides a greater deck area aft and is currenlly much uSed for a variety of ship lypeS.
Cruiser Stern The COllslruclion of the cruiser stern (FigUff! 5.33) ensures adequate resisla.nce 10 any pounding stresses which may occur. Solid pIalI' floors are fitted at.every frame space and a heavy centreline girder is fitted below ~ach .of lh~ decks In the t rn A centreline web 35 a continuation of the centrelme guder IS fitted at the ~f~er' end shell plating and runs down to the centreline girder in the n~ring region. Special frames are radiused around the afler end and are ~nown as cant frames'. since they are set at an angle to the centreline of the ~up. These cant frames join cant beams which suppa" the deck at the radlUscd after e~d. Horizontal stringers may also be fitted to stiffen up the Slructure by connecung it to Ihe transverse frames further forward.
Transom stern
Upper deck
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Deep solid-plate floors are also a feature of the transom slem construction, together wilh a centreline girder (Figure 5.34) .. The ~at plate of the lIansom stem construction, however, allows use of verucal suffeners around the shell
Sft;:ond deck
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105
104
Major Structural/terns
plating. The vertical stiffeners are bracketed to the floor and to the deck heams which nm transversely across the stem. A deep hmizontal stringer can provide additional stiffening to the >hen plating if required. A deep centre girder runs beneath each of the deck:; at tlte stern and is bracketed EO the deep web at the centreline of the after shell plating. This web is likewise bracketed to the various floors in the stern and finally 10 the solid-plate floor construction below.
Elevirlion, looking afl
Rudder trunk The rudder trunk is an open section which is left in the stern for the elltry of the rudder stock into the steering flat (Figure 5.34). A horizontal platform is some_ times fitted midw.ay up the trunk to fit a watertight gland. The (runking above i, then constructed to be watertight and access to this upper section and the gland is provided bya manhole. FIMr
The shell plating at the after end is temtinated by the stemframe (Figure 5.34). This is usually a casting, but fabrications and forgings are sometimes used. In single-screw shi ps the ste-rnframe has a boss on the cen treline for the tailshaft to pass through and an adequate aperture is provided for the propeller to operate in. 1f sufficient clearance at the blade tips were not allowed then serious vibrations would be set up in the after end of the ship. The lower part of the sternframe may provide a support for the rudder post or an overh.anging section may provide gudgeons for the rudder pintles. Figures 5.34, 5.41 and 5.42 show different arrangements. Various sections of the sternframe, particularly above the arch, provide- connecting points to the individual floors of the after end construction. The transom post and vibration post are two particular connections (Figure 5.34). Sound connections at these points ensure that propeller-induced vibntions are kept to a minimum. Twin-screw ships have a sternframe which is only required to support the rudder pintles and is thus much reduced in size. Larger stemframes, particularly those of cast constmction, are manufactured in two parts with provision made for bolting together and, after careful alignment, welding at the suitably prepared joint.
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Figure 5.35 Cast sputacle [rome Tan.. tOP Palm
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A-brackets and bossings Twin-screw vessels with their shafts set away from the centreline require support for the shaft overhang as it leaves the shell. Bossings are often used to increase the vessel's width and allow the mafts to remain within the hull while still retaining a streamlined flow ofwater to the propellers. The shafting is protected and internal insp-ection is possible with this arrangement. These bossings are symmetrical about the ship's centreline and give rise to the tenn 'spectacle frame' because of their appearance from aft of the vessel (Figure 5.35). Some modern constructions make use of A-brackets set out from the huU to support the shafts (Figure 5.36). The f"mal A-bracket in addition to acting as a bearing, must support the weight of the propeller. Both bossings and A·frames are led into the stern and solidly built into the structure with additional local stiffening where required.
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Figure 5.36 A_Bracket
lW,gitudinal girder
106 Major Srructurallt~ms 107
Stemtubes
.,
The propeller shaft enters the ship through the sterntube which acts as the rll1al bearing and a watertight seal to the sea. Traditional practice saw the use of lignum vitae and certain synthetic materials as bearing surfaces within the sttrn· tube and these were lubricated by sea water. The increased loadings, as a result of slow speed shafts and heavier propellers on more modem ships, has led to the widespread use of oil.J.ubricated whitemet.al bearings. Wilh this arrangement wear down in service is much reduced but there is a need for more accurate alignment and for seals al each end of the stemtuhe. An oil-lubricated sterntube arrangement is shown in Figure 5.37. Additional details are given in Marin.e Auxiliary Machinery by D. W. Smith (6th edition, Buttcrworths Marine Engineering Series. 1983).
Propellers A propeller consists of a boss which has several helicoidal form blades. When rotated it 'screws' or thrusts its way through the water by giving momentum to the column of water passing through it. The lhrust is transmiued along the shaft· ing to the thrust block and fmally to the ship's structure. The thrust block must therefore have a rigid seating or framework which is integrated into the ship's structure to absorb the thrust. The propeller will usually be either of the fIXed pitch or controllable pitch type. In addition some special designs and arrange· ments are in use which offer particular aavantages.
Fixed pitch propeller Although described as fixed pitch. a solid single.piece cast propeller has a pitch which varie~ with increasing radius from the boss. The pitch at any particular point on a blade is however fIXed and an average value for the complete pro· peller is used in all calculations. A fIXed pitch propeller is shown in Figure 5.38, where most of the terms used in describing the geometrical features are also
A." Developed outline
Proie
COM
B~.
B''''
IeCtlonli
F~ur('
5.38 F"urd pitch propf'l/{'r
108
MajorStmcrural Item!>
Major Structural Items
ghen. 11 should be note
r~"'1
Controlla ble-pitch p rope Ilers A controllable-pitch propeller is made up of a boss with separate blades mounted into it. An internal mechanism enables the blades to be moved simultaneously
D i.tance
Connecting tu~ ,tem Seal
1-I)/.cJ
Load ing ring
-
Propeller mounting The propeller is fitted onto a t.aper on the tailshaft and a key may be inserted between the two~ alternatively a keyless arrangement may be used. A large nut is fastened and J,ocked in place on the end of the tailshaft. A cane is then bolted over the end of the tailshaft to provide a smooth flow of water from the pIOpellel. One method of ke--yless propeller fitting is the oil injection system. The propeller bore is machined with a series of axial and circumferential grooves. Highpressure oil is injected between the tapered section of the tailshaft and the propeller_ This reduces the friction between the two parts and the propeller is pushed up the shaft taper by a hydraulic jacking ring. Once the pIOpeller is positioned, the oil pressure is released and the oil runs back leaving the shaft and propeller securely fastened together. The Pilgrim Nut is a patented device which provides a predetermined frictional grip between the propeller and its shaft. With this arrangement the engine torque may be tranSJTliUed without loading the key (where fitted). The Pilgrim Nut is, in effect, a threaded hYdraulic jack which is screwed onto the tailshaft, see Ffgure 5.39. A steel ring receives thru,t from a hydraulically pressurised nitrile rubber tyre. This thrust is applied to the propeller to force it Dnto the tapered tailshaft. Propeller removal is achieved by reversing the Pilgrim Nut and us.ing a withdr:awal plate which is fastened to the propeller boss by studs. When the lyre is pressurised the pr(lpellel is drawn off the taper. Assembly and withdrawal are shown in Figure 5.38.
Se-aJ
109
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, Figure 5.39" Pilgrim nUl operl1tion
through an arc to change the pitch angle and therefore the pitch. A typical arrangement is shown in Figure 5.40. When a pitch demand signal is received, a spool valve is oper:ated which controls the supply of low pressure oil. to the aux.iliary servo-motor. This moves the sliding thrust block assembly to position the valve Tad which extends. into the pIOpt111er hub_ The valve rod admits high pJessure oil into one side or the othel of the main servO-motor cylinder. The cylinder movement is transferred by a crank-pin and ring to th~ pmpelleI blad~s. The pIopeller blade~ rotate togetheI until the feed-back signal balances the demand signal and the low pressure oil to the auxilimy servo-motor is cut off. To enable emergency control of propeller pitch. in the event ofloss ufpowe-t, the $pool val.. e~ can be opetated by hand. The oil pumps are shaft driven.
110
Majo, Stmcturalltems III The control mechanism, which is usually hydraulic, passes through the tail· shaft and operation is from the bridge. Varying the pitch will vary the thrust provided and since a zero pitch position exists the engine shaft may tum con· tinuously. The blades may rotate to provide astern thrust and therefore the engine does not require to be reserved.
Special types
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A number of specialised arrangements or types of propeller exisl and have particular advantages or applications. The Voith-Schneider propeller, the Tip Vortex Free propeller and the use of a duct or nozzle afC described here. The Voith-Schneider prop€l~ a vertically-rotating device. The blades arc vertically positioned around a disC\.!!l-d can be rotated by cams in order to change the blade angle at a particular point in each revolution. Thjs results in a lhrust whose magnitude and direction is determined by the cams. It is, therefore, in some respects(similar to a controllable-pitch propeller in that the disc is driven and the blades can be positioned independently of the main drive. This unit can effectively thrust in any direction and will respond rapidly to the pitch control mechanism. The complete assembly is unfortunately complex, noisy in operation and considerable maintenance is necessary. II is often used for main propulsion in ferries and vessels requiring considerable manoeuvrability. It may also be used as a thruster or proplusion device for drill ships or fioating cranes which require accurate positioning. . The use of a duct or nozzle around the propeller can result in an improvement of the propeller performance. Furthennore the aerofoil shape of the duct can produce a forward thrust which will offset any drag it creates. The duct also protects the propeller from damage and reduces noise. It is usually fitted on ships with heavily loaded propellers, e.g. tugs, and has been used on larger vessels. One particular patented design of duct is known as the Kort Nozzle. The Tip Vortex Free (TVF) propeller is a recent special design which results in much improved propeller efficiency. The blade tips are fitted with pieces at right angles to lhe plane of rotation. The initial impression is that the blade edges have been bent over towards the face, i.e. away from the ship. The attachments at the blade tips serve to generate thrust across the whole propeller blade and thus improve the propeller efficiency. A nozzle surrounds the propeller and a tunnel structure under the stern on either side is used to direct the incoming flow of water.
Rudders The rudder is used to steer the ship. The turning action is largely dependent on the area of the rudder, which is usually of the order of one-sixtieth to oneseventieth of the length X depth of the ship. The ratio of the depth to width of a rudder is known as the aspect ratio and is usually in the region of 2. Slrcamlined rudder of a double-plate construction are filled to all modern ships and arc further described by the ammgement about their axis. A rudder with all of its area aft of the wrning axis is known as 'unbalanced' (Figure 5.4l). A rudder with a small part of its area forward of the turning axis is known as 'semi-balanced' (Figure 5.42). When more than 2570 of the rudder area is
1
[2 Major Stnlctural Iterm
lrward of the tU01ing axis there is no torque on the rudder stock at certain rlgles and such an arrangement is therefore knoVlll as a 'balanced rudder' "~igure 5.34). Tile constructi.on of modem rudders is of steel plate sides welded to all ltemal webbed framework. Integral with the internal framework may be heavy Jrgings which form the gudgeons or bearing housings of the rudder. The upper Ice of the rudder is fonned Illto a usually horizontal nat palm which acts as the oupling. point for the ruddcI stock. A lifting hole is provided in the rudder to na.ble a vertical in-line lift of the rudder when it is berng titted or removed. A pedal lifting bar with eye plates is used to lift the rudder. On the unbalanced od semi·balanced rudders showrl. in Figures 5.41 and 5.42 can be seen a famiorl. r eddy plate at the forward edge. This is welded in place after me rudder is itted to provide a streamlined water flow into the rudder. After manufadUIe, very rudder is ai.I tested to a pre,sure equivalent to a head of 2,45m above the :lop of the rudder to ensure its watertight integrity. The internal surfa~es are sually coated with bitumen or some similar coaring to pmtect the metal shou~d b.e pl:lting leak. A drain hole 1S provided at the bottom of the rudder to check or water entry wilen the ship is examined in dry Jock.
Hurlzontol couplinq I ,,<:,'''~
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ni~tle
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~udder pintles
and bearings
B~d,ir'g
r,ntle
~he
rudder, depending on Ils type and 3fIangement, will turn on either pi.ntles ,r bearings_ The balanced rudder in Figure 5.34 has a rudder axle fitted at its turning xis. Upper and lower bearings are D.tted in the rudder, as shown in Figure .43. The bearing consists of a stainless steel bush in the rudder and a ~tainless teel liner on the axle_ The stainless steel bush i> spirally grooved to permit librication. Other materials arc in use, such as gunmetal for the liner and lignum ilae or lufnol for the bush. The uppeT and lower pair of tapered bearing rings re fitted between the rudder and the stem frame. These are fitted with a small :learance but may support the weight of the rudder should the carrier fail. The semi-balanced rudder shown in Figure 5.42 turns on pintlcs. Arrangeaents vary but the pintle (ol\sists of a bearing, length of constant ,harneter 31111 a apered l.ength which is drawn into a similarly tapered hole on the rudder or ternframe gudgeon. The pintle i5 drawn in by a laIge nut pulling on the hreaded portiOll of the pintle. The pintle nut b securely locked in place after ightening. A locking pintle has
==Iudder stock and carrier fhe ,tack. passes through a gland and a rudder carrier before entering the :teering .compartment. The gland and carrier may be cQrnbined ur separate .items )[ el.juipmen t.
'ludder ;lOll
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Figure 5Al Unbllkmced rudder
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shell ~I~ti"q
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Figure 5,42 Semi-baumced rudder
114
Ma;orStructll.rQllrems
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The rudder carrier consists of (wo halves which provide an upper and a lower beating surface (Figure 5.44). The upper pari of Ihe rudder carrier is keyed to the stock so Ihal they turn logelher. The major pan of the rudder's weight js
'-
Slock
Fiture 5.46 Combined rudder amitr and gland
J J6 Major Structural hems
transferred 10 the rudder carrier by either a shoulder. as part of the stock forging, or a collar filled between the tiller and the carrier. The rudder weight is thus transferred to the lower bearing surface of lhe carrier which is grease lubricated. A flat or conical bearing surface may be used depending on the particular design. The lower half of the carrier is bolted into a heavy insert plate in the deck of the steering flat and is chocked against fore and aft and athwart. ships movement. A separate walertight gland is often filled where the stock enters the rudder lrunk. This arrangement provides access to a grealer length of the rudder slack. removes the need for a watertight construction of the carrier bearing and reduces the unsupported length of the stock (Figure 5.45). A combined type of watertight gland and rudder carrier is shown in Figure 5.46. It is essential for ease of opera lion of the rudder lhat the pin ties and rudder stock turning axes are in the Same vertical line. Great care must be taken during installation to ensure this correct alignment.
SECTION F SUPERSTRUCTURES AND ACCOMMODATION The superstructure is that part of the ship's structure built above the uppermost complete deck and is the full width of the ship. Deckhouses are smaller structures not extending the full width and one or more storeys high. They may be built on to the superstructure or at the base of maslS, etc. The construction of Superstructures and deckhouses uses frames. plating, girders and brackets in a similar manner to the hull. but of smaller scantlings. However. superstructures extending 15% of the ship's length are considered to contribute to the longitudinal strength of the ship. As such, they must have equivalent scantlings and strength 10 the main hulL The most forward section of the superstructure is known as the 'forecastle'. Any section of the SUperstruclure around the midships region of the ship is referred to as a 'bridge structure'. The deck area aft is known as the 'poop' and any SUperstructure located aft is likewise known. A raised quarter deck is a weather deck extcnding for some portion of Ihe ship's length from aft and is pOsitioncd above the upper deck. Most modern ships have most of the superslructure and accommodation situated aft abO\'e the machinery space. The wperstructure and de<:khouses usually tOtal four or five storeys. The major part in this space. excepl for thaI losl to Ihe machinery casing, is used for crew accommodation.
Major StTUC(lIralltcms I J 7
Bridge structure Where a bridge structure exceeds 15% of Ihe ship's length. lhe side plating thi~kmust be increased by 15<;t abo\'e that of other superstructures. A I~ea\'ily n~::ed bridge front is required with the aftcr end plating somewhat hglner. ~ 'ffener scantlings will likewise be increased al the forward end and reduced at t~~ after end. Web frames or partial bulkheads must be fitted to support struc\Ure abo\'e. particularly al the corners of deckhouses abO\·e. Ho~se lOpS or decks in way of davit.unust be strengllll:,n"d and supported from belo'tl..
Poop structure The poop front must be adequately plated and stiffened as for lite bridge f~onl. The internal stiffening will include webs and partial bulkheads as reqUired, articular1~' where deckhouses are located above. The. after end of the poop. ~eing exposed. requires a more substantial constructIOn than thaI of the aft ends of olher structures.
Raised quarter deck The raised quarter deck results in a greater depth of ship over its length. Increased scamlings must therefore be provided for the. frames. shell. deck plating and beams. Structures may be built on to the raised quarter deck as already described.
Discontinuities The ends of superstructures represent major discontinuities in ~he stru:ture of the ship. Longer structures such as bridges and forecastles r~qUlre conSlden.ble strengthening at the ends. Classification society rules reqUire the upper deck sheerstrake thickness to be increased by 20%, except where Ih~ struc~ure does not extend to lhe side shell. Deck plating at superstructure en~s IS also l~creased in thickness. Side plating forming part of the superstructure IS well radluscd at the ends towards the side shell (Figure 5.47).
Forecastle Watertight opening and doors All ships must be filled whh a forecaslle or an arrangement to provide a minimum bow height. as defined in c1assificalion socielY rules. It is, usual to fit forecastles, and where this is done they must extend from the Slem a distance 0.07 L afl (where Lis lhe freeboard length). The side plaling orlhe forecastle. beint; a continualion of the shell pIa tint;, is thicker thal1the end pia ling. Adequ:lIe arrallt;ements for stiffening of the forecastle pia tjng must be provided.
Where doors are filled into structures abo\'e the fr~e~ard d:ck they must be of adequate strenglh and able to maintain the watertight mtegnty of the structure. The openings have radiused corners to reduce the str~~ effe~lS o.f the discontinuity. A substantial framing is also fitted or addltlo~al stlffenmg to retain the strength of the structure. Doors fined to the o~enmgs are of steel SUitably stiffened, with a rubber gasket filled to effect watertightness. The doors
Major Structural/tems
118
J )9
have securing dips or 'dogs' which can be operated from either side. The dogs fasten on wedges wh.ich puU Ihe frame edge into the gasket. sealing the door shut. Details of the door construction and closing arrangements are shown in
Figures 5.48-5.50.
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120
121
Major Structurul Items
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Accommodation The superstructure will comprise several storeys of cabins. public rooms, offices, navigation areas and machinery rooms. A typical arrangement of cabins and rooms is shown in Figure 5.51. Stiffened steel bulkheads are used to support the structure above and provide subdivision for fire containment (see Chapter 10). Intermediate partitions are used to create individual cabins. Plastic laminates either side of a fire-resisting material core are used for the partitions. They are set into U-section light-plate channels at the deck and the ceilings, as shown in Figure 5.52(a) and (b). Ceiling panels are fitted on to wood grounds or battens between the partitions_ Typical Ooor coverings comprise a bituminous coating with vinyl tiles fitted to provide an easily cleaned hardwearing surface,
Toilet
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FiKJU~ S.S2{t1) Panirion conltnlC'tiO/1 - htQd imd foot detaill
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Figure 5.S2(b} PuriNO" collstruction - dtckhtad bCIJm.t
124 Major SlructrJ,ral Ilt:nu
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Minor structural items are now considered which, while not contributing greatly to the strength of the vessel. can nevertheless be considerable in size and have requirements for strength in themselves.
Funnel
F~ 5.51
O'ew cabin
~--'--
COamin~ strips .are fi~ted at the edges to complete the arrangement. The cabins are proYJded WIth vanous arrangements of built-in furniture and fittings for crew comfort~ as shown in Figr,ur 5.53.
In reality the funnel is a surround and support for the variow uptakes which ensure the dispersion of exhawt gases into the atmosphere and away from the ship. The shape of the funnel is sometimes detennined by the shipowner's requirements but largely by smoke-clearing arrangements and the need for streamlining to reduce resistance. The owner's house mark or trademark is often carried on the outside of the funnel structure. The funnel is constructed of steel plaling stiffened internally by angle bars or flat plates fitted end on (Figurr 6.1). Brackets are fitted at the stiffener connections to the deck and the plating of the funnel is fuDy welded to the deck. A base plate may be fitted between the funnel plating and the deck. Internal flats are fitted 10 the funnel and are made watertight with scupper drains to coUecl ;my rainwatn. The number of flats fitted is dependent upon the height of Ihe funnel. The various main engine ;md auxiliary uptakes afl~ fitted within the funnel casing, usually on sliding feet to permit expansion. Some uptakes are arranged to stand proud of the funnel casing. In the funnel shown in Figure 6./ ventilalion louvres are fined on the after end below Ihe upper lainOat. These louvres disperse the exhausts from the various ventilators led up the funnel. Fire flaps are fitted in the airtight flat beneath these ventilators and are used to shut off the air outlet from the engine room in the event of a fire. A hinged watertight door is filled in the funnel leading out on 10 the deck upon which the funnel stands. Holes or grilles are cut into the forward face of the funnel towards the top, and the whistle is fitted on a smaJJ seat just aft of the opening. Ladders and platfonns are also provided inside the funnel for access purposes. Lugs are fitted around the outside top shell plating to permit painting of the funnel.
Engine casing The accommodation or upper deck spaces are separated from the engine room or machinery spaces by the engine casing. Access doors are provided at suitable 125
26
,
Minor Structural Irems
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Min.or Structural Items
127
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poil1ts between the engine casing and the accommodation. The volume enclosed by lhe 1:3:>ing is made as ~mal\ as possible but d ~uffkient dimemiol1:S to ..\low maintenam:e and machinery removal from the- engine- room, The casing leads lip to the up-per decks, finishing below the funneL fresh air is drawn in through jalousies OJ louvres in small fan rooms off the casing and passes down tJUnking into the engine room, The- hot air rises up the engine room into the casing. and out of the funnel at the top. The cons.truction of a typical engine c.asing is shown in Figure 6.2. The casing i.s a ligiltly plated structUIe with dosely spaced vertical stiffeners. These bulb plate or angle bar stiffeners are fitted on the machinery room side of the casing to ensure wntinuity. Swedged or corrugated bu\khcarls could aho be 1.11;.cd fOl the casing sides. Stringers and brackets are filted at wrious heights, where no flats ex.i~t, to furt her strengthen the structLJre. The casing sides are also used to support $eats for certain auxiliaries and as securing points for pipe clips or hangars. TIle casing is supported on a deep girder runnittg around tnc engine toDffi. This deep girder is in turn supported by the pillars, transverses and bulkheads of the engine room structure (see Figure 6.2).
t
/XlFe""
Brack~t
The casing top is of ~tiffene-d plate construction. with Jeep girders and bnn;kets alOtlnd \lie openings fDr the uptakes. Hea\l;l bracket, connect th-e transverse beams to the vertical stiffeners. Thi, HnmgemcI11 ensures aJeq\~atc support for the funnel which sits on tlle casing top.
Shaft tunnel Where a ship's machinny space is not right all an enclmed area or i.ulmd i' prOVided to lead the shafting to the after peak bulkhead, The tllnnel must be ()
128
Minor Structuralltcnu
Minor Structumlltem!
watertight construction to provide integrity should the shart ~al cease to operate correctly. The forward end of the tunnel is filled with a sliding water. tight door to ~al of( the tunnel if necessary. The tunnel is made of sufficient proportions to enable access for maintenance to the shafting, and an escape route is provided from the after end. Two types of construction arc used. either a curved top or roof, or a nat roof. The curved roof is monger and can therefore be made of lighter plate than the Oat-roof type. The nat.topped construction does. however. lend it~lf to more straightforward construction and provides a Oat platform in the hold above. The plating is stiffened by bulb plates usually fitted in line with the frames. A cominuous ring of stiffener bar is fitted with the curved·roof type of tunnel. The Oat-roof type has brackets connecting the roof stiffeners to the vertical stiffeners. Examples of each are shown in Figure 6.3.
- the solid type being constructed principally of are consl·dere d so "d or o""n yv
pla~h;h;u~:~kt~ak~~i~~r;~~~~b~~~~~'~~~ludinalstrength and as such_ in
;;; solid form. is of relatively thin plate supported by S~ys ~~~:" ~; ~~~~h~r~ s are RI back from the deck edge and must not .w. . . 'Uy. This avoids the high stresses, particularly al the. nudsJups secllon. bemg ~~e. ~ ng I~nsmitled to the bulwarks and possible crac g cecum . Ollwl Du'b pl.ne
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e
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129
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The structure must be capable of withstanding the water pressure should the tunnel become open to the sea. The scantlings must therefore be equivalent to th~ of a watertight bulkhead. The width of the tunnel is decided by access and maintenance considerations and will be reduced to the minimum necessary. A raised Ooor is usually fitted and pipework is run along beneath it. The shaft bearings which arc positioned at intervals along the tunnel are carried on stools or seats. These slools are welded 10 the lank top and the tunnel structure to form a rigid platform. The lunnel is opened out into a larger area at Ihe after end to provide an adequate working space for withdrawal of the tailshaft. The spare tailshaft is usually mounted on the sheU in Ihis open area or recess. Shaft tunnels must be hose tested on completion to ensure their watertight. ness.
".
ti g' bul....·ark FiKUre !SA Bul ....·arkJ: (a) ~'I bul....·uk or roili,rg: (bl arrangement 0 { ·n J'(Xt n
Wherr the solid bulwark meets the deck. freeing ports must ~e fittedf:o allohw the ~ id drainage of any water shipped. which could senou~y ~ eCI t e stabil/y of the ship. Sometimes a 'floating' type of constfUCtlOn 'h' "rsed .to • C'.. 6 4(b). The depth 0 r t e reemg provide a continuous freemg port area. rlguTe. pori must be restricted to 230 mm. hi h . o n bulwarks consist of rails and stanchions sUPporled by stays w .c ag~ are : back from the deck edge. The lower rail spa~ing must ~ a m;;~um 0 230 rom whereas the rails above may have a maxunum spacmg 0 k I ~m. Bulwa'rks of both types are usuaUy 1 m in ~cigh~. Bulwar p a~m~, particularly in the forecastle region, is increased m thickness where It IS penetraled by mooring fittings.
Deep tanks Bulwarks Bulwarks are barriers fitted to the deck edge to protect passengers and crew and avoid the loss of items overboard should the ship roU excessively. Bulwarks
Deep tanks are fined in some ships for the carriage of bunker oil, b~as~w~~r or li uid cargoes such as tallow. The entrance 10 the deep tank from t e ec IS ofte~ via a large oiltight hatch; this enables the loading of bulk or general cargoes
130
131
Minor Srructuralltf!mJ
,
if required. A deep lank is smaller than a cargo hold and of a much stronger construction. Hold bulkheads may distort under the head of water if flooded, say in a collision. However. deep tank bulkheads which may be subjected to a constant head of oil or water must nol denect at all. The deep tank construction therefore employs strong webs. nringer plates and girders. fitted as closely spaced horizontal and vertical frames. Wash bulkheads may be: fitted in larger deep tanks to reduct: surging of the liquid carried. Deep tanks used for bunker tanks must have wash bulkheads if they extend the width of the ship. to reduce free surface effects of the liquid.
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FiKurr 6.5 Dup '""k: (II) pl4n l·;tW: (b) tln'Drion lookinz ourbotl,d
The construction of a deep tank used for bunker oil is shown in Figure 6.5. The tank is one of two and extends for half the width of the ship. The strakes of plating which fonn the oiltight bulkheads of the tank increase in thickness towards the bottom of the tank where the loading is greatest. The after oiltight bulkhead is stiffened by closely spaced vertical bulb plates. The forward oiltight bulkhead is stiffened externally by II series of diaphragm plates. The diaphragm plates form a cofferdam between the bunker tank and the oiltight bulkhead orthe cargo hold forward. Three horizontal stringers are fitted across the tank, a transverse wash bulkhead and a longitudinal wash bulkhead. The stringers are bracketed to the stiffeners at the tank sides and to the wash bulkheads which they join. The whole structure is therefore stiffened by a series of deep 'ring· girders in both a horizontal and vertical direction. A very strong structure is thus formed with considerable restrictions to liquid movement within the tank. Corrugated or swedged bulkheads may be fitted to deep tanks, parlicularly those intended for liquid cargoes which require the tank to be cleaned. Conventional stiffening could be positioned on the outside of small deep tanks to similarly facilitate cleaning. Heating coiis may be fitted in tanks intended for cargoes such as tallow. Deep tanks must be tested on completion by a head of water equivalent to their maximum servict: condition or not less than 2.44 In above the crown of the tank.
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Minor Stnlctural/tems
Machinery seats Main engines, auxiliary machinery and associated items of equipm~nt ar~ fast~n~d down on a rigid fram~work known as a seating or seat. These seats ar~ of plat~_ angle and bulb construction and act as a rigid platform for th~ equipment. They are welded directly to the deck or structure beneath, usuaUy in 1in~ with the stiff~ning_ The $eat is design~d to spread the concentrated load over the supporting structure of the ship. It may be extended to the adjac~nt structure or additional stiff~ning may b~ supplied in way of the seat. Steel chocks are often fitted between the seat and th~ machinery item 10 enabl~ a c~rtain amount of fitting to tak~ place and ensure a solid 'bed'. The it~m can then be bolted down to the seat without pen~trating th~ doubl~ bottom or deck b~low.
Seats in the machinery space also serve as platforms to raise the pumps, etc.. to th~ floorplate lev~l for easi~r access and maint~nanc~. A typical pump seating as used in an engine room is shown in Figure 6.6. It is constructed of steel plale in a box-type arrangem~nl for rigidity. A shell-mounted s~aling is shown in Figure 6. 7. cool~rs,
Sea tubes and inlet boxes Mosl valves having a direct inlet or oUllel to the sea ar~ mounted on a sea tub~ which is fitled inlo the shell. A sea tube is a thick-walled steel tube with a flange
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Fip" 6.8 St'tI "''fIUT illlt'r tI"tlII~mnrts: (tI) It'tI rv/Jt'l; (b)
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i"ln box
133
., h' ed flat to form a watertight joint with the bollom shell and fully welded inside on the inboard side which IS mac In 'd valve. The tube is let into the lower SI e or and out, Figure 6.8(0). be fitted into inlet box~s which ar~ usually fit~ed A number of sea tubes m a y . bel w th~ waterline. A box-like in the forward corner.; of the englne T~~h~ s~a through one or mOTe holes structure is fitted to Ihe shell and opensb 0 let into this box. or valves can be with grids fitted. Several sea t~bes can t~e inlu box Figure 6.8Ib). mounted on to flang~s welded dU~~~IY to t strength~n the shell plating around The sea tubes or mlet boxes a~ serv~ 0 the discontinuity resulting from the hole in the shell.
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Outfit
135
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Hatch covers
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Hatch covers are used to make the cargo halch watertight, 10 protect the cargo and to stiffen up the structure of the hatch opening. Two basic types are in general use - the wooden hatch coveT fitted across halch beams and the patent sleel covers of various designs. The halch covers fit on top of the halch coarnings, which have been described in Chapter S. The weather deck coarnings are al a height set by the load line rules (see Chapter 10). The tween deck coarnings are set flush or almost flush with the deck 10 reduce interference with cargo stowage in this area.
Wooden hatch covers A combination of transverse beams and longitudinal hatch boards make up the wooden hatch cover arrangemenl (Figure 7.1). I-seclion girden, the widlh of Ihe hatch. are fitled at intervals along the length of the hatch and are known as hatchway beams, shifting beams or webs. The ends of the beams fil either inlo slols or carrien in the coaming side or lock into pos.ition on a trackway if they are of the sliding type. The beam ends are additionally stiffened by a doubling plate. The beams which lake the ends of the hatchboards have a vertical nat fined to hold the boards in place. The hatchboards 3re filled longitudinally over the hatch beams and are protected at their ends by a metal band. The boards are at least 60 mm thick and more for a span greater than 1_5 m. The roller beam arrangement of hatch· boards is the same. the roller beam simply speeding up the opening and closing of the halch. At least two tarpaulins must be fitted over the hatchboards and suitably fastened down around the hatch coaming. Bauens, cleats and wedges are used 10 'dog' down the tarpaulins. Steel locking ban or some suitable addilionallocking device are required to secure each of the hatch cover sections after battening down the tarpaulins_
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which completely enclose (he hatch opening. O~ning ~nd :Iosing arr~~e::~~ ".1", a ·singl. pull' via a winch wire or hydraubc or e ectnc power. . " ' " n wheels on a trackway along the hatc h coamlOg "oop . Th. "pante sections ~~; ~ther hinged together or joined by ch~ins to olle another. The covers finally slOW at some poinl clear of the halch opening. .. 72 The halch A MacGregor sleel halch cover arrangement is shown In F~re . . d ' gI d ned b hydraulic power or a wUlch-operale sIn e °i: Ihe hatch rollers by a on '''-'n."A venical plale is positioned each side of the coarrung ahl h "htowmg -,. " Th oUers on I e atc cover end of the halCh on the coarning trackway. e upper r
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Steel hatch covers Patented steel halch covers of a variety of designs are available from several manufacturers, Most designs employ a number of .self·supporting steel covers 134
Figurt 7,2 MacGrtlor su~1 hafch co~~r
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Outfit
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ride up on lhi> platl' .and the C()Vef the . . . h _ __ / are thus COlll"act]'w' ~LlJw~d 'I f II" hIps 1n, tD I c vntlc,1! posItIon, The covers ~ ~ car () H~ atcl opel1in..:r TI )h eccentric rollers which act as wheels' 11, "<..I <>: _ Ie lak covers rull un __ _ In le !aJSl.~ POSition and are dear of the coamin,g in the low d ere POSltlOli to <'flab-Ie tl:e covers to be fastened d ' III Thtc C()Y~rs ~~ of fabricatea' sted plate witll stiffeners or webs to ~treng~;:~' c s ru~tur('. e eads of the covers overlap in the c1()~~d l ' _ , fitted with compresSible pa'k' . P~SltlOI1. Gromes th C lIlg surround the outside edges of the -c-ovcrs Vt'he e cO,vers .HC fastened dOwn by deat~ on to a raised ed can Ih' .:oa··' n watertIght seal is formed and no tarpaulin is re'luired. Th~ athwa:tshi fmn,g. a .-
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h~twcen the,. covers have a similar sealing arrangemell t The cleating arra~S ;J~~~~ s {)W~ m hgure, 7-2 b automatic, The cover wheels -drop into slots g. th ~f~J:ll11gb plate pnOl to cleating and are raised hydr3l1lically after UJld:~tjf\ge a[~ mg, ars are fitted along the side and end coamings under the top rail Hook~ positIOned at thc cleating. pomls and L:all pivot thlougl I . h . '1 D bl 'h ' . 1 a s ot m t e COamlltg Ell. ou e·ac1lng ydraultc cylllldcrs move the bars to raise or lower the h k 00 s. Cle" lug
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In t,he raised position the hooks engage deal lugs which pull the I' t 11 sections down on to th I" 'F la c cover c sea mg stnp. or transverse deating a torsiun bar , arrangement IS used. Lever arms on the end of the torsion bar are pushed ' up as the hatch closes and rolat th e c torsIOn bar. This presses cleating lugs on to pressure pad~ on the end of the :J.djacent hatch section. A peripheral 'Ie i arrangement IS shown in Figure 7.3, c a
Minor hatch covers A number of small access openings. tank entmnces, elc., are fitted with . halch covers of steel construction. mmor A t~pica1 small hatch COWr is shawn in Figure 7.4. The coamin ed e is forced mto a r~bber gasket by a number of fastening clips or 'dogs' ar~un: the cover, a watertight seal being thus formed. The handles are .arranged for internal or e.xtental oper.atlon on accesses. A c(lunterbalance weight is sometimes fitted to ease the opemng of the cover.
Mooring equipment and arrangements The ",:,inches and windlasses positioned on the forecastle and poop decks and sometimes the upper deck perform the mooring and warping duties reqUired by
the ship when arriving and departing its various ports of call. Various fittings are provided on the deck and around the deck edge to assist in the mooring operation and provide a deaT run or lead for the mooring and warping wires. Examples of these fittings are bollards and the various types of fairlead which are found on board ship. The windlass, as mentioned in Section D of Chapter 5, has warping ends which are used when mooring the ship. One or more warping winches are fitted on the poop deck aft for similar dutie'S. Solid seatings, as mentioned in O1apter 6, transmit the loads to the deck and also stiffen the deck. Larger vessels have mooring winches fitted on the upper deck also. Bollards or ffiQoring bitts are used to moor the ship once it is alongside and are welded OJ bolted to the deck or to a box,like structure which is welded to the deck, Figure 7.5(a). Adequate structural support must always be pflJvided in way of bollards and all mo-oring fittings, usually by additional stiffening to the deck beneath. Fairleads arc used to guide the bawsers or mooring wires to the boUards Qf mooring winches. Fairleads are attached to the deck, a raised seat or the deck and the bulwarks. Several different types are to be seen, such as the multi,angled fai.rlead, the pedestal faidead, the roller fairlead and the panama £aidead. A multi.angled fairlead consists of two horizontal and two vertical rollers with the wire passIng through the hole between the rollers, Figure 7.5(0). A pedestal fairlead consists of a single horizontal or vertical roUer mounted on a raised pedestal 01 seat, Figure 7.5(c). A roller faidead is one or more vertical rollers on a steel base which may fasten directly to the deck or to the deck and bUlwarks, Figure 7.5(dj. The panama fairlead is an almost elliptical opening formed in a casting which is fitted into a suitably stiffened aperture in the bulwark, Figure 7.S( 'I" The multi-angled fairlead is fLtted at the deck edge and reduces the number of guide rollers or other faideads required to give a clear lead of wire to the winch. The pedestal faidead guides the wire across the deck to the winch clear of any obstructions. The roller fairlead is used at the deck edge to lead in the mooring
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and warping wires. A panama fairlead is lilted in the foremost position in the forecastle bulwark on the centreline of all ships which pass through the Panama Canal. Panama fairleads are also used in other positions around the deck edge as required. For the various mooring and warping arrangements possible on a ship an 'arrangement of leads' drawing is provided. This shows the runs of the various wires through and over the various fairleads and winch warping drums on the decks of the ship. Such an arrangement for the fore end of a ship is shown in Figure 7.6.
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Outfit
141
Masts, derricks and deck cranes
Samson posts
Masts
Some m3Sts on general cargo ships als11 double 3S support pOStS for the derricks used for cargo handling. 53rnson posts are also used more specifically for supporting derricks. Tied arrangements of Samson posts, or bipod masts as they are sometimes called. are :lIso used. The scantlings and construction of masts and posts used in cargo....andling work are given in the c1assifiC3tion society rules and are dependent upon the safe working load (SWL) of the derrick boom. Most masts are self-supporting by virtue of their construction and attachment to the dttk. Only special heavy·lift derricks require wire stays or prcventers between the post top and the deck. Samson post construction is of tubular steel section, stiffened internally by webs. Thicker plating or doubling plates 3re provided where atlachments are made to the post. Derrick booms are of seamless tubing usually with a greater diameter at the middle region where the bending moments are greatest. The various goosenecks and end fittings are welded inserts in the tube ends. The post attachrmnt to the deck varies but must always provide adequate stiffening and support. Mast houses are fitted at the base of some masts or samson posts and mayor may not assist in stiffening the structure. Some posts ate let into the tween decks or are attached to the comers of superstructure to obtain support. The greater the derrick load the more stiffening is required. often by fitting additional webs below decks and heavier than usual bulkhead stiffeners and brackets below the mast or post.
The. ship's mast acts. as a lookout platform and a mounting point for navigation equIpment s~ch as hghts: radat. aerials, etc. Access to the upper platform is by a ladder which. depending upon the mast size, may be fitted externally or internally. A foremast. as filled to an oil tanker, is shown in Figure 7.7. Construction is of light plate stiffened by internal webs. A O.type cross·section is often used for
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Derrick rigs The derricks used for catgo....andling work can be arranged or rigged in several different ways to provide for different manpower requirements, cargo-lifting capacities or lifting cycle times.
Union purchase The union purchase rig is a much used arrangement for cargo loading and discharging. Two derricks are used, one arranged to plumb the hatch and the
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irs streamlined, reduced-resistance form. The upper platform is additionally supported by brackets to the outer plating of the mast. The mast is fully welded to the deckhouse on the forecastle deck and to the upper deck. A solid round bar is used to stiffen each of the ftee edges of plating and before erection the mast is coated internally with a bitumen solution.
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142
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olher to plumb the qU:ly 01 over th~ ship's side. The falls or wires frol11 both derricks are shackled 10 the same cargo hook. Thus. by using the two winch controllers separately and together Ihe hook is raised or lowered O\'er Ihe hold. travels over the deck and ClIll be raised and lowered over the ship's side. This arrangement is safe. in that only the load moves. and requires two reliable operators for the winches. It is. however. only Suitable for light loads up to about 1.5 tunnes. A union purchase rig is shown in Figu11' 7.8.
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Swinging derrick The fastest and most reliable method of cargo h:lI1dling is achieved by Ihe swinging derrick rig. A long derrick boom with a deJr arc of swing is necessary for this arrangement. An adjustable Span is usu:llly arranged 10 facilitate the plumbing of the hatch and the quay over the ship's side. This is achieved by a topping wire and winch which is independent of the cargo winch. A swinging derrick rig is shown in Figurl' 7.9. Topp,ngblock
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/ Derrick ileQd filling. 2 Pendulum block {it/ing wilh guide ,oilers. ] Uppe' CQ'go bloch 4 Conn~ti"g f/Qu. 5 Lo....t:r spQn block. 6 Span s....il'el. 7 C,oss·t,ee. 8 1t,/('/ fo, rhe hauling PQrt. 9 £O ••.-C, CQrgo blocks. /0 Connecting rrQI·t,se.
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For loads heavier than Ihe safe working load of a single derrick, two derric~s coupled together by a 'yo-yo' gear arrangemenl may be us,ed, as sh?wn I~ Figure 7.10. The derrick heads must be kept c1o~ together dunng operation an
144
Outfit
Outfit
the central travelling blo~k which equalise~ the load must have a s.afe working load greater than the cargo being lifted. A special heavy·lift derrick is titted to many general cargo ships, with suitable rig and purchase gC"ur for its designed safe working load. .,various patent heavy·lift derricks .are a:vailable, one example being the Stulken derrick shown in Figure 7./1. The StUlken derrick has a safe working load up to 300 tonnes and is positioned lJ.etween two outwardly raked tapering tubular c:,lumus., Several winches are provided for the various hoisting, ~lewing and tOPPing duties. The controls are all arranged as levers in one console. which ~n be operated by one man, This heavy·lift derrick can be arranged to serve either of the hat-ches forward and aft of it. Smaller derricks are also rigged from the tubular columns for normal cargo work.
145
Deck cranes
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Derricks have been replaced on many modern l:argo ships by del:k cranes mounted on platforms between the holds (Figure 7.12). The deck crane provides an immediately operational cargo·handling de-vice with minimal rigging reqUire. ments and simple, straightforward one·man operation. The safe working load of the crane is determined by its c3rgll·handling duties, and designs are avail.ablc from 3~5 tannes and up to 10-15 tonnes as requilcd. Double gearing is a feature of~some of the larger cranes to enable speedier handling of ligh tel loads. Three basic types of cranes are available - gener.al cargo cranes, grah-bing cranes and twin-crane arrangements. The general cargQ crane is for use {In cargo ships and hulk carriers. The grabbing crane is for use with a rnechanically-operated grab when handling hulk materials. It requires a multiple-wire arrangement for the operatIon oJ the grab. Twin cranes utilise standard cranes whidl can be twinned u[iJperate-d in unison to lift heavier loads such as containers, if requireu A single operamr [, usual with this system by utilising a master and s1ave control ~ystem in the two cranes. The use of a C(JmmOIl revolvin.g platform makes this arrangement possible-
Crane platfurm The deck crane i, located on a platform pusitioned SOIlle distance from the deck to provide the crane operator with a clear uninterrupted view of the hold and the quayside (Figurf: 713). The dane also revolves around this platform. The
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seat on which the crane rests is usually circular and of steel plate construction with closely spaced vertical ribs or brackets. This seat is usually welded to or is an integral part of the raised post or plalform which is welded to the deck of the ship. Adequate structural support and stiffening should be provided both around and under the seat. ~
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pUmping and piping arrangements For the many services required on board ship. various piping and pumping systems are provided. Some systems. such as bilge drainage and fire mains. arc slatutory requirements in the event of damage or fire on board ship. Each of the various systems will be examined in turn.
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Bilge system The bilge piping system of any ship must be designed and arranged such that any compartment can be discharged of water when the ship is on an even keel or listed no more Ihan 5 degrees 10 either side. In the machinery space at least two suctions must be available. one on each side. One suction is connected to the bilge main and the other to an independent power-driven pump or ejector. An emergency bilge suction must also be provided and is usually connected to the largest capacity pump available. A diagrammatic arrangement of a bilge pumping system for a 26000 deadweight tonnes bulk carrier is shown in Figure 7.14.
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Strum boxes are filled on all bUI machinery and tunnel space suction pipes. Perforations of 10 mm maximum diameter are made in the plate to provide a suction area at least twice that of the suction pipe. In the machinery and tunnel space bilge !ines, mud boxes are fitted. The mud box fits between lengths of piping and has a perforated centreplate. The use of strum and mud boxes prevents the entry of large objects to the pipeline and safeguards the internal parts of the pump (Figure 7.15). Suction valves for the individual compartments must be of the screw-down non-return (SNDR) type to prevent reverse now. All other valves must be of the
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On·return (NR) type. The port Jnd stathOard hoLd bilge valves are usually ~ouped ill distribution chests a.t the forward end of the machinery spa.:e. Bilge iping is made up of the fore arld aft mains and suction branches to the ldividu.al compartments. Piping is arranged, where possible, in pipe tunnels. o( uct keels to ,avoid penetrating watertight double-bottom tanks. Bilge ripes are Ide-pendent of piping for any other duties such as hallasl or fre-sll water. a"se-nge-t "hip bilge main:. romt mn at lea:.t 20'% of the ,hip's beam i.nsioe of the de shell; in addition, any brandIeS further outboard must have a non-return alve fitted. Bil,gc pipe suction lines are sized according to ail empirical formula. Minimum ranch and main siz:e-s -are 50 mm and 65 mm, respectively, and the maximum ile is 100 rnm for both. Bilge piping may be constructed Qf cast iron, steel, opper or other suitable approved ma.terials. It is usual to employ galvanised teel piping in bilge s}'~tems. At \cast foU( inclepelldent puwel-dri'fe,n pump'" must be conn.ected to the lilge main. Mmt ships employ two bilge pumps and have bilge main c-onnec.1ions III the ballast and main circulating pumps. \Vhere POSSible these pumps should )e loc,Hed in separa~e- watertight compartments. One bilge system pump must )e capable of operation under reason
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{equiremenls for the hallast system of a dry cargo ship are largely similar to hose fm the bilge- system. There must be adequate protection provided against Jallast water I;'otering dry cargo or adjoining spaces. Connections between bilge md ballast lines must be by non·return valves. Locking v-alves or b-lanking. mangements musi prevent accidental ernptying of deep tanks or flooding. 'ftLere tanks are employed for oil fuel ar baltast, effeo:::tive isolating sy&tcrns mUllt ,e used. A ballast pumping arrangement for a 26000 deadweight tonnes bulk carrier s shown in FiKUre 7.16.
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W paSsenger sllips of 4000 gross tonnes and above mllst have at least three Jower-driven fire pumps. All cargo ships in excess of 1000 gross tonnes must lave at least two independently driven fire pUmps. Where these two pumps are .ceated in one area -an eme[gen.cy fiI~ pump must be pmvided and located remote from the machinery space. The emergency fire pump must be independently driven by a compression ignitiun engine or other approved means. Water mains of sufflcient diameter to provide an adequate Water supply for the ,imu.ltaneous operation {)f two fire hoses must be Connected to the fire pumps. An isolating valve :is fitted tQ the machinery space fire main to enable the ~me[gency fire pump to supply the deck lines, if the machinery spaCe main is 'rokea or the pump is out of action. A di.agIamm
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Figure 7.17. The system is designed 10 supply valves with hose connections on all the superstructure and upper decks. Relief valves are fitted at either end of the main to ensure thai working pressure is not exceeded. The water may be supplied by the machinery space lire pump. Ihe lire and tank-eleaning pump or Ihe emergency fire pump located ill the forec3stlc. Additional lines arc led (0 ,....---the hawse pipe for anchor washing and the garbage tank for nushing. The emergency lire pump in (his arrangement is supplied by a boosler pump lit I'd nc~ar the bOllom of Ihe ship. The booster pump is driven hydraulically from one end of the emergency fire pump. the other end having another sea water pump 10 furlher pressurise Ihe w:ller. A diesel engine drives the pumps filled at either end.
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Many other pumping and plpmg services are filled In ships for the various domestic. cargo and machinery requirements. For further details uf these systems reference can bt' made 10 the previously mentioned work. Marini: Auxilwry Machint:ry. by Souchotte and Smith.
Scuppers
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Direct drainage of the open decks above (he freeboard deck is adlleved by means of scuppers. A typical arrangement is shown in Figure 7.18. In enclosed spaces. such as bathrooms or galleys. the scuppers are led 10 the bilges. A scupper pot is fitted in a deck and acts as the collecting point for water. A pipe is connected to the underside to drain the water directly to the bilge (Figure 7.19).
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Sounding pipes Sounding pipes :m: fitted 10 all tanks 10 enable soundings to be taken and Ihe deplh of liquid pre~rl1 (0 be measured. Reference (0 the tank calibralion lables will then permit the quantity of liquid present in the lank to be found. Sounding pipes are made as straight as praclicable and are led above the bulkhead deck. except fOr certain machinery space tanks (Figure 7.20). A minimum bore of 32 mm is required for sounding pipes. This may be greater where a refrigerated space is passed through 10 allow for icing up. Where Lite sounding
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Cargo systems
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Cargo pumps and piping systems aTe installed on tankers 10 diSl.:harge and load Ihe liquid cargo. Separate ball:lSI-pumping systems are also provided for ballaslonly lanks which are filled during ballast voyages. System choice :md its flu;jbilily depend upon lhe range of calgces. the vessel"s uading pattern and what the owner is prepared 10 pay fOl. The standald syslem employs several ring mains along the tank length with branches off to the individual tanks. Other systems are in usc. for inslance. employing large sluice valves to empty the tanks one to another. The pump suclions arc then taken from the aftermost lank with the \'esseluirnmed by the stern. An example of a ring main system for a very large crude carrier is shown diagr:l.I11matical1) in Figure 7.2/. Three mains are employed 10 serve the various tanks. This arrangemenl also enables different grades of oil to be carried in the lanks served by each main. Branches ale led off into each of the centre and wing tanks and are fitted with isolaling valves. Cross-connections are arranged between Ihe mains. and direct-loading pipes from the deck manifolds join Ihe mains. Two stripping mains are also fitted and led forward with branches off 10 the various tanks. The stripping lines arc uscd 10 discharge the last few hundred tonnes of cargo which the main suctions cannot handle. The main cargo pumps are stt'am-driven horizontal or verlinl single·stage centrifug:lt pumps. For the system shown in Figure 7.2/ one pump is provided for ('itch main. The driving motor or lurbine is located in the machinery space and the drive p:lsses through a gastight seal in the pumproom bulkhead. The SHipping mains are connected in the pumproom to lwo stripping pumps which are usu:ll1y of the posilive-displacemcnt type.
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A particular feature of tankers is the large quantity of piping seen on deck. A typical arrangement is shown diagrammalically in Figu.re 7.22. The cargo pumps discharge inro mains which p:lSS up through the pumproOIll and along the upper deck to midships. The mains branch into crossovers to port and starboard and
Outfit
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are filled with V-pieces at the manifolds which arc grouped near to the ship's side. ProdUClJ la"kers
More complex piping arrangements with independent lines are necessary on products tankers to avoid contamination between the different cargo ·parcels·. More than onc pumproom may be fitted on such ships. or individual pumps in all tanks with no pumprooms. Anangements for flushing lines using water or a pOrtion of Ihe cargo may increa~ the flexibility of a particular sy5lcm.
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Ballasting Q"Qtlgements Many tankers operate in the ballast condition on ev:ery other voyage. A sufficient quantity of ballast sea water must therdore be loaded on board to provide the ship with satisfactory seakeeping propelties. Certain tanks 3re designated ballast only and are filled by the ballast pump and piping system. Certain cargo tanks may be loaded with sea water ballast using the cargo pumps with a sea suction.
Insulation Thermal insulation
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A ship's steel hull and structure will conduct heat very well. In way of heated tanks. refrigerated spaces and exposed accommodation spaces some form of insulation is necessary to reduce the heat flow to an acceptable le~el. Various materials such as glass fibre, cork and some foam plastics are in u~ as insulation. Glass fibre matting or sheet is used in modern ships since it is easily fitted. is fire resistant. does not rot :lnd does not support animal life. The amount of insulation filled in a compartment is decided by the temperature which is to be maintained or accepted in the compartment (Figure 7.23). Fastening is now largely by tandom pinning. using a stud gun to rut the pins to the steelwork. The pins penenate the insulation. and caps fitted on the ends of the pins hold the insulation in place. Some slab insulation may be glued to the steelwork. Joins between ~ctions of insulation are ~aled, usually with an adhesive tape. In accommodation spaces. insulation will be behind decorative panels. In places where it is exposed 10 possible damage. a protecti~e cladding or lining, such as galvanised mild steel sheeting. may be fitted. Insulation on tank· tops must likewise be protected from possible damage or be of a substantial nature in itself. ()ver oil tanks a space must be left to avoid possible contamination of the insulation. This space is not required when a bituminouS covering is placed o~er the steel surface. Plugs over manholes in cargo tanks and also hatch co~ers must be insulated to a~oid any areas through which heat might be conducted. Special scupper arrangements are necessary 10 avoid heat transfer in refrigerated holds. This is achieved by a brine seal in an S-bend trap. The bilges may thus be pumped out but the sealing liquid. although diluted. will not be removed (Figure 7.24).
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Acoustic insulation Sound results from the movement of air particles and travds in the form of waves away from the source. There arc many sources of sound on board ship. such as propulsion engines. auxiliary engines. large fans and ventilation plants. These would have a cumulative disturbing affect on p(rsonnel if allowed 10 continue unchecked. Various countries now have either codes of practice for noise levels in ships. or regulations relating to noise levels in ship spaces. Maximum noise levels are given for particular spaces using a weighted sound pressure level or db{A) value. Most ships at sea, however, would not meet these criteria. New ship designs will require consideration of noise levels in the very early stages if an acceptable noise environment is to be obtained. Two approaches are made to the solution of the problem. First, rooms and areas which are occupied for any length of time are fitted out in such a manner as to be as sound absorbing as possible. The second method is to isolate or silence the sound from occupied spaces. Increasing the sound·absorption capacity of a room is achieved by using a variety of sound absorbers. These include membrane absorbers such as lhin panels, resonant absorbers such as perforated ceiling boards and porous absorbers such as mineral wool. Sounds can be isolated by the use of nexible connections in ducting. nexible mountings on machinery, and sound insulating the surroundings of a noisy space. Air-<:onditioning plant noise can be eliminated by the use of duct and bafne silencers and sound attenuating supply and exhaust fittings. Figures 7.25(0) and 7.25(b) illustrate the problems to be found in a ship's accommodation and the various solutions that can be adopted.
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special w:ltertight doors must be fitted. On cargo ships with a shaft tunnel, the tunnel entIilflce will have a watertight door fitted. On passenger ships, Ylith their ].nge area5 ()f 3c-commodation and access requirements, a greater number of watertight doors will be I1tted. "'here ,-,penlngs are cut into bulkhead~ ,hey must be reinforced tu maintain the strerrgth of the bulkhead. This is particularly so in the lower regions of watertight bu.Jkhc:ads, where the greatest loading occurs. Where stiffeners afB cut
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or incr(.'3s_ed in spacing in way of a watertight door, adequate reinforcing is required. The watertight door has a heavy framework which further stiffens the bulkhead in w~y of (he opening. The size_of the opening is kept as small as possible. All doors fitted below the waterline are of the sliding type, either horiwntal or vertical ill operation. It is usual to use horizontal sliding doors, except where space limitations require the vertical type. The sliding door must be able to close against a list of 15 degrees to port or starboard. It must be operable hom the vicinity of the door, in addition to a point above the bulkhead deck. The remote operating point must have an indicator showing the duoT position. A hori7.0ntal sliding, watertight door of Stone Manganese Marine Ltd manufacture is shown in Figure 7.26. A stout door frame is fitted directly into the bulkhead and provides the trackway along whkh the deor slides. The door is moved by a hydrauli..:: cylinder Wl1ich may be power operated or nand pumped. A special solenoid ,-pool valve which may be remotelY or manually operated proVides the basis of the cuntrol system. Bridge operation, local manual ove-rride operation and local emergency control of the dam are possible. Operating the hand pump together with manual movement of the solenoid valve provides local or remote- emergency operation. Powered operation is possible from the bridge or by manual movement of the solenoid valves at eitner the local or remote pumping stationS. Bridge operation is only llsual on passenger ships where there may be a large number ofwat-ertight GOaTS, WatertigM duurs are pressure tested under a head of water cOrIesponding to their bulknead position in the event of the ship flooding. This usually takes place at the manufacturers' works. Ab()ve tne waterline, in certain approved positions, hinged watertight doors are permitted. TIlese will be similar in construction to the weathertighl doors descrihed in Section f of Chapter 5.
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Stabilisers The motions of a ship in a seaway can result in \'arious undesirable effects. examples of which ate cargo damage and human discomfort, Only the rolling of a ship can be elTectiYely reduced by stabilisation, Two basically different stabilis. ing systems are used on ships - the fUl and the tank. Both systems attempt to reduce rolling by producing an opposite fOtCe to that atlempting to roll the ship.
Fin stabiliser
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One or more pairs of fUlS are fitted on a ship, one on each side. see Figure 7.27. The size or afea of lhe fins is governed by ship factors such as breadth. draught, displacement. and 50 on, but is Yery small compared with the size of lhe ship. The fins may be retractable, i.e. pivoting or sliding within lhe ships fonn. or flXed. They act to apply II righting moment to the ship as it is inclined by a wave or force on one side. The angle of tilt of the fm and the resulting momenl on the ship is determined by a sensing control system. The forwlIrd speed of the ship enables lhe fins to generate the thrust which results in the righting moment.
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. The opera ling system can be compared 10 that of the sleering gear. in Ihal a signal from the control unil CilUseS a mO\'cment of the fin which. when it reaches the desir~d value, is ~r~ughl 10 r~sl. The fin movement takes place as a result of 3 hydraulic p~wer unit mcorporallng a type of variable displacemenl pump. The effectiveness of the fms as stabilisers depends upon their sp«d of mO\·e. ment. which must be rapid from one extreme point to the other. The fms are rectangular in shape and streamlined in section. The use of 3 movable nap or a fIXed and movable portlon is to provide a greater restoring moment to the ship for a i1ighdy more complicated mechanism. The control system is based upon an acceleration sensor. This unit provides a signal which after electronic integration provides a measurement of roll velocity and angle. These various parameters arc all used to bring :lbout a suitable lln movement which will oppose the roll. Fin stabilisers provide accurate and effective roll stabilisation in return for a complex installation which, in merchant vessels, is usually limited to passenge, ships. It ~ to be noted that at low ship speeds the stabilising power falls off, and when stalionary no stabilisation is possible.
Tank stabiliser A tank slabiliser provides a righting or anti.rolling force as a resull of the delayed flow of fluid in a SUitably positioned transverse tank. The system operation is independent of ship speed and will work when the ship is at rest. . Consider a mass of waler in an athwartships lank. As the ship rolls the water wil! b~ m~ved, but a moment or two afler the ship rolls. Thus, when the ship is finlshmg ItS roll and about 10 return, the still moving waler will oppose the return roll. The water mass thus acts against the roll at each ship movement. This athwartships tank is sometimes referred to as 'flume'. The system is considered passive, since the water flow is activated by gravity. A wing tank system arranged for controlled passive operation is shown in Figurc 7.28. The greater height of tank 3t the sides permits a larger water build. up and thus a greater moment to resist the roll. The rising nuid level must not however ftll the wing tank. The air duci between the two Wing tanks conlains valves which a~e operated by a roll sensing device, The differential air pressure' between tanks IS regulated to allow the fluid flow to be controlled and 'phased' for maximum roll stabilisation. A lank system must be specifically designed for a particular ship by using data from model tesls. The water level in the system is critical and must be adjusted according to the ship's loaded condition. Also there is a free surface effect .resulting from the moving water which effectively reduces the stability of the shtp. The tank system does however stab~ise at zero speed and is a much less complex installation than a fin stabiliscr.
8
Oil Tankers, Liquefied Gas Carriers and Bulk Carriers Oil tankers, because of theit sheer size and numbers at sea. are worthy of special consideration. The liqUid nature of their cargo requires special forms of construction and outfitting of these vessels. Cas carriers for the bulk transport of liquefied gases are also an increasing.ly important specialist type of ship. The bulk carrier in its many forms is increasing in its unit size and numbers such that it too is worthy of individual attention.
Oil tankers Longitudinal and transverse bulkheads divide the cargo-carrying section of t~e vessel into a number of tanks. In addition to separation of different types of oiL the individual tanks also reduce the effects of the liquid's free surface on the stability of the ship. Since oil contracts and expands with changes of temperature, tanks are rarely completely full and movement of the liquid takes place. The bulkheads, decks. etc., must therefore be oiltight even when stressed or loaded by the movement of the oil in addition to the normal static loads. Longitudinal stresses are considerable in tankers and great strength is therefore required to resist bending and stiffen the hull structure. Fire and explosion are an ever-present hazard on tankers and special systems of ventilation are necessary. Void spaces or cofferdams are also fitled in places to separate the cargo tank section from other parts of the ship, such as pump· rooms and fore peak tanks. Cargo-handling equipment is provided in the form of pumps located in a pumproom, usually positioned between the machinery space and Ihe cargo tanks. More than one pumproom may be fitted depending upon the cargo carried or the piping arrangements. Suction pipelines run through Ihe cargo tanks, and discharge lines leave the pumproom and travel along the deck to the crossover lines and manifolds situated al midships. Two main types of oil tanker are 10 be found at sea today. The very large crude carrier (VLCC) and the products carrier. The main difference is in size and the products carrier has a larger number of tanks with a more complex piping system. This enables the carriage of many different cargo 'parcels' o~ any one voyage. The various aspects of tanker construction will now be exammed. 165
166
Oil Tanker:s, Liquefied Gas Carriers and Bulk Carriers
167
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Bottom structure The bottom structure is longitudinally framed over the cargo tank length. Bulb plates and built-up T-sections are usually employed. The bottom transverses provide support and are spaced at intervals of around 3.8 m on smallet ships and up to S m on longer vessels. The longitudinals are continuous and pass through notches cut in the transverses (Figure 8..5). Aat bar make-up plates are fitted to the transverses where the longitudinals pass through. At watertight bulkheads a fully welded collar is fitted (Figure 8.6). The longitudinals are also bracketed to the transverses. The transverses are usually a plate web with a heavier flat bar flange. Horizontal stiffeners are fitted where a considerable transverse depth is employed (Figure 8.1). A centre girder is fitled. except where there is a centreline bulkhead. Various arrangements of continuous or intercostal longitudinal side girders are also sometimes fitted. The arrangements used will determine the scantlings of the memben employed in the construction. The centreline girder is stiffened and supported by vertical docking brackets fitted between each transverse (Figure
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Transverse bulkheads are similar 11\ cOllStruction to longitudinal hulkheads and may be nat with stiffeners or cmrugatcd. Vertical wc-bs must be fitted to transverse hulkheads in line with the centre girder and may be Htted in line with side girders Conugated~bulkheads may luse vertical or hori-lontal corrugations with stiffening webs l1tted ...t right·angles to the i.:UTfugations. lJJngituJinal stiffeners ·arc arranged contil1Uously through transverse bulkheads and are attach.ed by brackets Transverse bulkheads must lwt be spaced greater than one·fifth of the ship's lengtll apart. Where the tank length is greater than one-tertth Df the ship's leng.th. or 15 m. a perforated or wash bulkl1e~-d must be fit led. Wash burkheads
A wash bulkhead is similar il1 constructioll 10 a tran,ve-rse bulkhe~d but is !lot oil tight. Large holes or perforations exist in the plaling These holc~. while allowing the oil to mo-ve through. do restrict the: speed and force- of its movement and provide additional transverse strength to the ship.
Framing at ends Jnderdeck structure rhis is largely the same as that for the buttom structure, with transvcnes fitted n hac with those below. A continuuus centreline girder and perhap3 intercustal Jr continuuus side ~rders are fitted l:J.eneath the: deck.
3ulkheads three lypes of bulkhead arc to be found on tankers - longitudinal, transverse Lnd wash
.ongitudinal bufkhe.ads :13t stiffened or corrugated oiltight bulkheads may be employed. The stiffening ; largely the same as .that, of the ~ide shell, I.e. horizontal stiffeners along the ,~lkhead where- lOll gJt uJrnal shell stiffening IS used. Brackets fasten the ~Iffeners to the transverse bulkheads at the ends. Where side transverses are Itled to the snell, correspondingly positioned vcrtical webs are fitted at the ,u!khead, Horizontal stringers at the ship's si-de are matched by horizontal lrtngers on the bulkheads. A continu{lus rin,g·type structure of considerable lrength is thus builT up within the tank space. . This ring·type structure i~ further brace-d by the use of beams known as cro-ss. les fitted between the transverses or side stringers and the longitudinal bulkead!>. Where corrugated bulkheads are employed the corrugations must nm orizofltally. Vertical webs are fined at e...'ery bottom transverse, in order to .lpport the bulkhead.
Beyond the cargo tank length the -vessel may be tnmsversely or of (ombined framing consHucti{)n and must ha-ve certain additional strengthening fitted. A deep tank or tanks is often fIlted forward of the (argo tank space Where transverse franling is employed. solid floors are fitted at every frame space, Intercostal side girders of depth equal to the floors are also fitted in line with every other bottom shclliongitudinal in the deep lank space_ The deep tallk is fitted with web frames nOT mme than five frame spaces aparI. A centreline bulkhead must also be fitted, unless llle mai11 10ngitwJinal bulkheads extend through the deep tank. With longitUdinal framing, transverses are fitted in the deep tank not more than 3 m apart. Intercostal side girders are also fitted either side of the centreline, On larger vessels th,; cargo tank structure may extend into the deep tank itself. Panting and pounding arrangements are also necessary and will be similar to th{)se described ill· Chapter 5. All modern tankers now have the machinery space and accommodation located aft. Web frames arc fitted not mClre than five frame sp.aces apart in the machinery space, with fixed or portable beams acrosS the casing opening. Tranverse framing of the bottom i" u"ua\ i.ll the machinery space and construction is similar to that mentioned in Chapter 5. Transve-r~e or longitudinal framing of the sides a.nd deck may be used from the machinery space to the after end of the ship. Deck longitudinals must extend into the machinery space a distance equivalent to one-third of the ship's breadth. Panting arrangements are also fitted in the after peak. as described in Chapter 5.
Superstructu res These are of much the same construction as described in Chapter 5. The loal line rules require pJ(JtecUve housings around openings in the freeboard and othel
Oil Tankers, Liquefied Gar Carrier!: an.d Durle Carriers
171 H",~"
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decks and a furecastle extending 7% of tbe ship's length from forward. Bc-cal.l~e of a (linker's hig.h ,",ending slroes5es extra care must be taken with di~c()ntinuities at the superstructure ends.
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Cofferdams.- are filted between oil tanks and other compartments and must be at least 760 mm wide. Pumprooms l)f water ballast tanks may, subject t{) certain cOllditiom, be accepted instead of cofferdams. Special arrangements are necessary in tanker~ becallSe of the redu.ccd freeboard to clear tlle decks of water. Open rails are fitted for at least half the length of the weather deck. Solid bulwarks are usually fitted only at the forecastle and around the superstructure.
Hatches Access to the cargo t.ank spaces is by oil tight hatches. Circular or oval shapes are usually employed with cQamings at least 225 mm high. Steel covers with suitable oiltight fastening arrangements are usual, Figures 8.8(a) and 8.8(b). Patented coven -of other approved materials are also available. Other tanles and cofferdam spaces may have similar hatches or manholes for access (Figure 8.9).
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[==~=~=====:::l-
Deck
'b' F'l'tu/'e 8,8{h) Derail ofhatch dampiflg ammgemenr
Figure 8.9 MQlthole cover: j{l) plate, (b) detail of:ecu";ngQmmgement
74
175
Oil Tankers, Liquefied Gas Carriers and Bulk Carriers
lentilation 'enliJation arrangem-ellis an' fully described ill Chapter 9. nert gas plants IIcrt gas plants are being fitted to an ever·increasing rmnthcr of tankers to nprove their operational safety. The plallt provides an inert gas blanket over rlC ~llrface of the cargo to stop the build·up of flammable vapuurs which might ~ad to explosions. A typical system is ~howl1 in Figurr:13.1 O. The plant uses exhaust gas which is fawn flOm the boilu flue uptakes, where available, or frurn a separate ombustion chamber. The gas enters:a scrubbing tower via a water seal which is irculated b)' sea water. The gas is cooled, solids and unwanted gases are ~rubbed out and it then passes through a demister which removes water vapour. 'he inert gas which contains less than 5% uxygen is then pumped into the cargo mks, Using fan units to drive the gas along the supply main. A deck-mounted rat-er seal is fitted in the main to prevent the back·flow of flammable gases from le cargo tanks. During unloading the inert gas proVides a positive pressure on the cargo Jrface which assists discharging in addition w ensuring a safe operation. Inert 3S is fed into tanks prior to loading and when fu11. the fans are stopped. During Jading the ltigh velocity venting valves are opened to vent the inert gas to tmosphere. When loading is complete the valves are closed and inert gas is Jpplied to produce a slight pressure- in the- tanks. During loaded passage the lert gas pressure is monitored and maintained.
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.'3.. 0
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rn
)ther outfit items pedal circular openings with removable gas-tight covers are provided for tank. leaning operations. A number of fixed or portable tank-eleaning machines are Jwered into the cargo space through these openings. Tank sounding gauges, wltich give local and often remote readouts of liquid eptlls, arc fitted to each cargo tank usually on to a 'pot' or cylindrical seat. Heating coils are Htted in many tankers to improve the discharging of the il. Steam is passed through coils fitted on the tank bollom to heJit the cargo not to discharge. Gares will be releared dmmg heating and the ...enting system 111st therefore be open.
,
•
-iquefied gas carriers he past 25 years hJlve seen the emergence of the bulk transport Gf n.atur31 il.ses both for use as fuel and as a refrigerant. Specialist ships are now used to my the various types of gas in a variety of tank sys.tems, combined witn rrangements for pressurising or refrigerating the gas. Natural gas is found and released as a re~ult of oil·drilling operations. It is mixture of such gases as methane, ethane, propane, butane and pentane. The
o
176
Oil Tanken, Liquefied Gas CAmen and Bulk CAmen
heavier gases. propane and butane, are separated by liquefaction ,md are termed 'peuoleum gases'. The remainder consisting largely of methane are known as 'natural gas'. The properties and therefore the behaviour of these IWO basic groups vary considerably, thus requiring different means of containment and Storage during uansportation, Natural gas is, by proportion. 75-95% methane and hasla boiling point of -162°C at atmospheric pressure. Methane has a critical temperature of -82°C. The critical temperature is the temperature abo\'e wllich it cannot be liquefied by the application of pressure. A pressure of 47 bar is necessary to liquefy methane at -82°C. Thus, natural gas cannot be liquefied by pressure at normal temperatures. Liquid natural gas tanken are therefore designed to carry the gas in in liquid form at atmospheric pressure and a low service temperature in the region of -164°e. The problems encountered. therefore, deal with protecting the steel structure from the low temperatures, reducing the loss of gas and avoiding the leakage of gas into the occupied regions of the ship. Petroleum gas consists of propane, propylene and butane or mixtures of these gases, all of which have critical temperatures above normal ambient temperatures. Thus they can be transported either as a liquid at low temperature and pressure or at nonnal temperature and under pressure. The design problems for this type of ship 3re similarly protecting the steel hull where low temperatures are employed. reducing gas loss and avoiding gas leakage, with the added consideration of pressurising the tanks.
'" lIQuid llghl c~ntrd,ne
;I"head
Ca
Liquefied natural gas tankers The tank types of LNG carriers are ~If-supporting and either prismatic, cylindrical or spherical in shape or :I membrane construction which is supported by insulation, Materials used include aluminium, 9~ nickel steel or membranes composed of stainless steel or nickel iron. Tank designs are split into Ihree categories, namely self·supporting or free standing, membrane and semi-membrane. The self-supporting tank is strong enough by virtue of ils construction to accepl any loads imposed by the cargo it carries. A membrane tank requires th~ insulation between the tank and lhe hull to be load bearing, such an arrangement being termed an integrated tank design. Single or double metallic membranes can be used, with insulation separating the two membrane skins. The semi-membrane or semi·integrated design is similar to the membrane, except that the tank has no support at its comers. A double·hull type of construction is used with each of the above designs, the space between being used for water b:J1l3sl. Thl:' basic configurations are shown in Figure 8.11.
,., T"" h.,,~
Dome
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huU_
Memb<_ I....k
laIOO.. _
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Comparison 0/ tunk types
Membrane and prismatic tanks use the underdeck cubic capacity most effectively, Cylindrical and spherical tanks involve constructional probkms by penetrating the upper deck but pro\'ide greater safety in the event of collision or grounding. Membrane tanks are cheaper to build but the insulation which must be load be:lring is more expensive, The insulation of spherical tanks need not be
, W'ter tylLast
FlZUfe 8.11 Tank arTIIII~mtl lOT liqutMd lU,"lIfll p: (tl) pnmuttie ttlnk: (6) spherial.' tank: (e) c:ylindrictJ1 tank; (d) membrr'M ftlnk: (t) doublt-mtmbt"tlnt tank: sem1mtmbnmt ttulle
m
78
Oil Tankers, Liquefied G(Jj Carriers and Bulk Carrien
179
P.. n~tr~tin9 dom~
)ad bearing ~ince it is only :1 partial se\;ondary 1)arrieL if needed at all ill this ~spect The hull alld machinery cost~ arc ahout clIl.lal fDr each tyre All Ihe lfferen t types are- in se n'icc. wi th the firmly cslabli\hcd desigm heing prismatic, phcrical and membrane- types
Spl"h qu~,d
'oi!-ojf
S-e~anM~(~
t>.'"er
iquc-fic-d natural gas is continually boiling in tanks when lr;msporto.:d by S('8. be-re is therefore a lll::e-d to (eleasl:: this gas tu ;II/uid a pr~'s,ure nuild-up ill the Ink. It may be n'"nte-d dire-c!iy to atillosphere or bum( in boilers or in sT,cc'";:llly darted dual fL1ele[lgines. Barningthe hoil-(Iffg-ilsin a flare mounted on a hO(lill .:mote from the ship is another possible solution. Rc-liqud,tCiio!l is 110t ~onomical because of the- large- p'Jwer and IllJ~e cost
-iquefied petroleum gas tankers 'hrec basi( lype~ of liquefied petroleum gas tankers MOO '::lJrrcnt]y used lJle Jl1y pre~surised tank, the semi'pressuriscd partially rdri&c-Titlc.u lanK. and the lily refrigerated atmmpheric preSSlJre tank. The fully pressurised tallk areHies ~t Jhllilt 175 18.0 bar and requires eavy expensive tanks of carb-(JTT steel W11ir;h Ulf-:aCC acting 3S the secondary barrier (Figure 8.12). (2) A large dume is situated aft at the top of the tank and wing b-allast tanks are fitted (Figure 8.lJ}. The inner surface of the wing tanks acts as the second-ary barrier. (3) A large dome is situated aft at the top of the tank but no Wl[lg ballast tanks are fitted (Figure 8.14). Hopper tanks are used for ballast when necessary. The hull itself acts as the secondary barrier and must be of low temperature carbun steel in way of the cargo tanks,
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Hull
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Fi",rc 8.12 Cylhldrical rronk fanli. anan/[ement
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Water b,llla.t
Figure 11.14 Tank <1n'an,;.ell1enr with hull as scmmlary barri""
180
Oil T/lnken, Liquefied Gas Q:miers and Bulk CIlm-ers
Comporiwn of tank types
The reduction in weight of tank mat~ri
Oil Tmkers, Liquefied G/lS Carrien and Bulk Carriers
1
d'
the machinery spa\:e, the side shell in way of the \:argo tanks, the upper hopper tanks, the main deck in-side of the ~lfie of ha~che~. the forecastle deck ami the fore and aft peak tanks. LongitudInal ~rammg IS employed at the bottom shell. the tank lap and the upper det:k olltslde of the line of hatches. . h . FO A section through a typical floor in a l()wer hopper tank IS sown III 19ure 816 The longitudinal framing structure can be clearly seen. Above the ~opper t~nk can be Seel] the transversely framed hold with the bracket connectmg the
;:~I~Yt:n~: or
Construction as-pects of LNG and LPG carriers The various regulatory bodies have rules for the construction and cl:Jssiflc
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'rame
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Fi!\lm: 8,15 Bl,lk carrier rranSl'erse ~ation Transv~r5~
frame
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Bulk carriers The bulk carriage o-f single-commodity cargoes has been il continually advancing trend with the development of specialist types of ship to sUit. nil' desire for flexibllity of operation has also led to variuus desigm to enable different bulk cargoes to be carried on different voyages. Such vessels have become known as combination bulk carriers, -oil/bulk/ore (aBO) and oil/ore (00) are examples. Some particular aspects of bulk carrier construction will nowbe examined in detail. A transverse section through a general-purpose bulk carrier is shown in Figure 8.15. TIte cargo hold is Seen 10 be shaped by the upper hupper or s.addlc tanks, the lower hopper tanks and the double bottom. A wmposite framing system is. iJsed in common with most bulk carders. Transverse framing is
181
Longitudinal ._ stdfener
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Fi1[1.1l'e 8.16 SQlid·fluor- arrangemmt in
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lower hopper tank
<2
Oil Tankers, Liquefied Gas Carriers and Bulk: Carriers
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183
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ure 8.17 Tapering-off 01 hopper tan.k at a{ter end" ra) plan vie-wan hopper tank end (t;) sectiOil
01'1
hopper lank end
.
fr:am.... to the hopper hmk. At the end~ of the hupper tank region a considerable change in section lKcurs. The construction used to reduce the effect of tl1is discontinuity is snown in Fignn> 8.17. A large tapered bracket is used wnich is connected to tile surrounding transversely framed structure as shown, A section through an upper hopper t.:mJ,; or saddle tank is shown in Figure 8.18, The longitudinal framing under the deck can be seen as well as the bracket connecting the aprer edge of the transverse frame to the tanL The side shell pmtion of the tank is transversely framed by offset bulb plates with plate webs, as shown in Figure 8./8. fitted at every fourth frame. A deep-flanged bracket joins the inuer tank side to the hatch side girder. Details of a bulkhead stool are shown in Figure 8.19. With a corrugated trausverse hulkhc:ad as :shown, the stool arrangement is used to shape the forward and after lower regions of the <:3rgo hold. This !lush tapering shape permits easy discharge of bulk cargoes and simplifies cargo lIold cleanin.g. Shedder plates are fitted inside the troughs of the corrugated bulkhead for the same reason.
184
9
..
~
Ventilation
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An ucc3n-goillg ~hip i~ required 10 operate in a ~'afi['ty {le .... ery different climates. Air temperature~ m.ay range from -15°C to 50°(' and sea water temperutures from aOc to 3-8°C The moisture wnlen! of the air will vary considerahly and solar radhnion may ;lllect one or Illore of the ship's exposed surfaces. All the various forms of good lmd bad weatllcr will Jlsu be experienced. The air from the air-conditioning and ventilation plauts is therefore rcq,Jired to provide all accepl
Accommodation Most ships' air-conditioning systems employ centrally situated units. These units are s-elf-contained and supply the cabins and sraces within a particular area via tmnking. The contrDI possihle in imiividual c..bins or spaces depends upon the nature and wmp]exity of the central unit. Three ha,il: systems are in use - the single duct. the twin du~t and the twin dun with reheat. In each case the central unit will deall. supply warm or cool. humidify or lkhumidify tile air supplied to the cabins.
The single duct system In the single Juct system the central unit mixes outside air with some returned or recycled ait. This air is then filtered, heated and perhaps humidified or cooled. This conditioned air is thell distribllted along a single -duct to the indivi.Jual supply units in the different spaces. The amount of supply air can be controlled within the particular cabin lJr space. Figure 9,] shows- the arrangement of the sing.le duct system.
The twin duct system Jl,.gain, outside and returned air are mixed in the cen tral unit then filtered, preheated and perhaps humidified. Some of the air leaves the unit before it 18S
,6
Ventilation
Ventilation
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.aches the cooler, to be reheated; the amount is increased as the outside mperature tails. The remainder of the air passes over the c(loling coil. The !W(l I supplies at different conditions are passed through separate ducts to mtrolled mixing units in the individual spaces, The air temperature- and mdition can then be selected for the particular space. Figu.re 9,2 shows the rangement of the twin duct ~y~tem. Aic SUPPly cOnl,,",1 unll
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ReCiealing C"olinq P,elleJli"g
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187
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un;t
P'"",ure
9_1 The.ingle
dll£'1 ""il~ r~heat
srsrem
dischargin.g, the storage and ventilation must be suitable and satisfa<:t~TY . Inadequate, poor quality air supplies can seriously damage most cargoes. FalT~Y simple systems of cargo ventilation and attendant proce,dures can prevent sUi,;h damage. Different cargoes react to the climate 00 board III as complex: a manner as the human body, with often irreparable damage as ihe result ,, Certain general cargoes, some fruit and vegetable cargoes and hygroSCOpIc (water-absorbing or emitting) cargoes are carried in non-msulated holds: As a result they are exposed to all climatic changes which may cause c~ndens.atlOn on the hull or cargo. Ventilati.on of the holds in which they .are. carned IS therefore necessary. Refrig.erated and £Iozen cargoes are canied ill Insulated holds tut because of the living, gas-producing nature of the cargo they also reqUlre ventilatiun.
Ventilation of non-ins.ulated cargo holds ;--=-~~4-
/
\, L Fifi!:Ure
9_2
DI\llilJ"ti"n
"'ct,on
1'1". twin dun sysrem
he single duct with reheat system he central unit mixes outs.ide and return air, mters, preheats and humidifies or lOis the air to the lowest required temper.atllre for .any part of the system. The r then passes along one duct to individual units in the spaces. Within tnese rUts is a controlled healer over which the air passes. Heating may be achieved y circulating hot water or an electric heater. The air supply and its temperature lay therefore be regulated. Figure 9.3 shows the arran gement of the s.ingle duct ith reheat system.
The purpose of ventilation ill non-insulated holds is t(l remove surplus heat an,d humidity, to prevent the cundensing of rn01sture 011 cargo or hun and to remme gases pf()[lllCed in the ripening process of some fruit and vegetahle cargoes. Natural and mechanical yen tibtion systems are lIsed for thIS purpose.
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argo spaces he primary function of ships is to transport goods from place to place. The U"go must be delivered in good condition and, in addition to careful10ading and
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Fir;Lm' 9.4 Niltural vennlarion of {ween·,kcK space or workshop
188
Ventilation
Veil tiTalian
Natural ventilation is accompUshed by inlet and outlet pipe'S and trunking to each cargo sp.a-cc. Thesc inlets and outlets consist of cowls or vcntilators of various designs. Air is forced in by the action of tnc winu ordr.awll in as a result of all ejector type of c:xhaust drawing air out which is then replaced. Where the force of tlle wind is utilised the cowls must be manu:ally positioned, and are large.cumbersome fittings which must be well stayed to the deck. Figure 9.4 shows cl. natur.al ventil.ation .arrangement for a tween deck or workshop. Most modern ships utilise mech.anical ventilation for reliability, improved performan::e and the reduced size ofl;owls necessmy. . Mechanical ventilation operates in two distinct systems - the open and thedosed,
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Ine open system uses axial flow fans fitted in the inlet and exhaust trunks. The tnmks may have separate cowls or be incorporated into sampson posts or masts. The air is supplied along trunkjng and dUds to the bottom of the hold.
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Venti lation of refrigerated cargo holds - ---! <'
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11.1
FiguJ'c 9.5 Open vI'ntiwtion s)'sJem. (a) normal d,.culotior/; (b) rel'er;ed crrcuwrirm - to pre-venT underdeck drculotl'o/1 at low Outside tempaalure
The air is drawn from the top of the hold ju~t below the decks. The exhaust fans can be reversed if condensation is likely near the deckheads, for example with a low outside air temperature. Figure 9.5 shows the arrangement of the open mechanical ventilation system. The closed system recirculates air and a contfulled amount of fresh ail l:an be admitted. The ventilating air is distributed around the hold and cargo, fonning an insulating wall or curtain between the two. Ex.haust air is drawn from the boltom of the hold. This system affords every possible mode of cantm] and is widely used in somewhat varied forms. Figure 9.6 shows the c\-osed ventilation ~ystem.
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rated c;lfgo holds ret:.Juirc 3. carefully controneLl Jir-rcplacing system f~r Cooled a.ir is sllpplicQ !() the- refrigerated no~d wlle~e ~: ains he-at from ripening cargoes and entrains the ga-ses produced. ThIS 31.r I~ lod ,od a c:'teful balance must be maintained between lJllet and t en ex aus'v" exhaust gas quantities, regardles~ or the outside chm~tlc comhtlOns.", olef One s '-stem achieve~ this by drawing outmJe aIr down to a blink of" l . t ubes vi/ a central unit. The delmmidifled air thell passes lllto th.: cargo ,bold,', t· back to tne- "entra TIle exhaust gases are drawu from the h0 Id t1\fOU gh d.lIC.~, outlet "t od ,.'", returned to the outside atmosphere. The lmkmg of mlet and I uma ensures ... " a constant ;lir supply at all times . 97~now:;t1e valves to t 1If h\dF" 0 . l!!,Ur" . . arrangement of such a system.
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IypD~ of shin have their associated GlIgO ventilation problemS,e,g," P ar t "", It.:U ar .. ~ t' f d" 1 ct"n, '" li· ff shi.r~ and the vehicle exhau~t umes urlllg 03 I " rO -on, 10 0 , 1 "I t The partlcul:H discharging. Bulk. carriers umally only require natura venti a l(JIi, . , to problems for each ship type must be considered early on at the deSign stage ensure a suitable system is provid~u.
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190
Ventilation
Ventilation
191
Inlel ao,
Conl
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Hp".. 9.8 (Ie[rl ModI/'ll".'" sf>au w:mi/
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The machinery space fequires an air supply for the operation of boilers. combustion engines. compressers, etc.. and to lIlainlain a satisfactory climatc for the operating staff to work in. Certain machinery consumes or requires air for its operation and sufficient air at as Iowa temperature as practically possible should be provided. Underpressure occurring in Ihe machinery space will affect the efficiency and performance of internal combustion engines. Overpressure may lead to leakage of hOI air illlo the accommodation. Ventilalion is also necessary to remove the heiH generated within the machinery space and thus provide a reasonable climate for staff !o work in, This very difficult task is achieved by the provision of ducled supplies of fillered but uncooled air to as lIlany regions:ls possible. P:lrlkular areas such as workshops and control rooms. being sl11all. may be :lir conditioned and more feadily provided with an acceptable working c1imale. Various systems of air supply to the machinery spaces and casing are in use and are shown in Figures 9.8 9./0. Figure 9.8 utilises a medium pressure axial now fan supplying air down a lrunking. which is proportionally released at the various platfonn levels and exhausts through Ihe top of the casing. Figurr 9.9 uses a low pressure axial now fan to supply air into the casing area. Also. a high pressure cenlrifugal fan
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rovides air through ducls to outlets at the various platforms.. Figure 9.10 Pses medium pressure axial now fans to provide a through trunklllg system 10 ~he various outlels at the various platforms. This method has proved 10 be th~ best. A diagrammatic arrangement o~ m~.dium pressure a:
Control rooms The prOVISion of control rooms in most modern machinery spaces e~~ures c~os: careful control of the climate in such spaces, often with lhe pTOV1Slon 0 alT
192
Ventilation
\
193
conditioning in addition 10 ventilation. This climate control provides tht personnd wilh 3. Cum(ollable working area isolated (rom Ihe main m3chinery sp;la. Also. dchc:Jle equipment in need of careful climatic conuol is able 10 receive it. The satisfactory opcral.ion :lOd continuous per(onuanct of modern control equipment requires a carefully controlled environment which. by using 3. control room. call be achieved. A ~parate dueted supply is led inlO the control room and usually through a filtering ~iT
wilh controls located in the' control room_ A match~d exhaust will r~move stale warm air from the connol room_ FiX'IN' 9. J2 shows such an arrangemenl. E,,"••lSI oo.nltl cowling
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Figurt' 9. J 2 Control room Vt"rilan"ofl
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9./3 PlJmproom venn-/ariM
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194
Ventilation
Vemilation
195
Pumprooms Tank~1 pumprooms lequirt \'~nlil:uion to cany r~suhing from leaking glands 01 pipe join IS. T11~
away poisonous cargo fumes working climate in this spac~ well below d~ck level must also be comfortable for any pc-rsonnel present. Mechanical exhausting of air is achk\'~d by the uSC' of axial flow fans and tmnking. The trunkin& dr:Jws from th~ pumproom floor and ~metg~ncy intakes at a height of2.15 m from the working platform. These emergency intakes must be filled with dampers which can be opened or closed from the wealher deck or the working pl;lIform. The fan motors are located in the machinery space and drive the f;lns lhrough gastight seals in lhe bulkhead. Supply is through cowls or lounes at the top of the pumprOOIll. An arrangement is shown in Figure 9.1 J. Double.bottom tanks
Ventilalion of doubk-bollom tanks is provided by means of an air pipe situated rcmote from Ihe filling pipe and usually at the high~St point in the tank to avoid Slandifd A S A, ISO lIangll!
FiflUre 9. J.s lIixh "e/veir)' ~as ,'e" ri"g I'al,·t A .." ll.i$h ";'~ l'llt'
",t_,,,,,,,"nl
= P,,,onq led 10
1.,"~
FjKl.tre 9.14 Ai, pipt' ht'ad
During loading and discharging of lhe cargo t~e \'entil~t~on req~irements are considerable. Air must be drawn in or removed m quantities eqUivalent. to the cargo oil discharged or loaded. In addition, during th~ loading operaUon the hydrocarbon vapours issuing from the tank ~ust be dlS~ned well above th.e deck. This is achieved by the use of high velOCIty gas ventmg valves. One t~pe: ~s shown in Figure 9.15, The arrangement consists of a fixed cone around whIch I.S a movable orifice plate. A counlerweight holds the orifice plale closed until sufficient gas pressure builds up to lift the plate. TIle gas is throuled through the orifice and issues at high velocity, dispersing into the atmosphere well above the deck. During discharge the cover is opened and a linkage from the cover holds the orifice plate in the fully open position.
unvenlilated pockets. The air pipe is led up to the weather deck to a gooseneck or patenl type of head. Air pipes from fuel tanks arc positioned in low risk areas and have flame $Crun g3uzes fitted (Fig"re' 9.14).
Types of ventilator head
Cargo tanks
Various different Iypes and arrangements of ventilator head are in usc. Figure 9.16 shows a selection of the more common designs.
Ventilation of cargo lanks avoids overpressure or partial pressure conditions whkh could occur during loading and unloading of cargo, Temperature fluctuations during a voyage could have a similar effect. Vapour pipelines from the cargo halch arc led to pressure/vacuum relief valves which are usually mounted on a standpipe some distance above the deck. Individual vent lines are fitted for each tank on large lankers and a common venting line is led up a masl or sampson post on smaller vessels.
196
1./
10
Organisations and Regula,tions ,
-'
/
"'_. ~~~~'M~":'~I'oom'co.." Nfl A
V,",ut-to< ,n open llOSu,on Io$ed Dv ""Pw'ng
,"
/' TI'I'eilded windle
R ,I~
I I I ,
r~~~r---'-l;;;;d
own cove< pille] Gaule filled
The construction of merchant ships is considerably influenced and regulated by a number of organisations and their various requirements. Classification societies. with their rules :Ind regulations relating \0 classification, provide a set of st:lndards for sound merchant ship construction which have developed over many years. lllCse rules aTe based on experience. practical knowledge and considerable research and investigation. The International Maritime Organisation (IMO fomlerly IMCO) is an inlernational organisation which is attempting 10 develop high standards in every aspect of ship construction and operation. II is intended ultimately to apply these stanoards internationally to every ship at sea. A vast :J.moont of legislation is applied to ships and is usu:J.lly administered by Ihe appropri:J.te government department. The load line rules and tonnage measurement are twO p3rticular legislative requirements that are outlined in this chapter.
~
Ve-n!;IJI;Ofl Ifunlo;in9
-SllIM pipe
I
C'M$·ltttfH>fJl/l ........
Declo; 101..'
Sul»O
Gum~
fIlled IOCcwn
{{'-::"",,:>'\JlI\
P"le
Classification societies
I
I
'"
1,1
Figuf? 9./6 V~ntilalor hmds". (a) ""ODStflee k Iype;
Q~f"
(bl musJ"oom type; leJ jUf"d mush
A classification society exists to classify Of "3rrange in order of merit' such ships are built according to its rules or are offered for classification. A classed ship is therefore considered to have a particular standard of seaworthiness. There are classification societies within most of the major maritime nations of the world and some are listed below:
3S
~m
Uoyd's Register of Shipping (UK). Ameritan Bureau of Shipping (USA). Bureau Veritas (France). Det Norske Veritas (Norway). Germanischer Uoyd (Germany). Registro lIaliano (Italy). Register of Shipping (USSR). Nippon Kaiji Kyokai (Japan).
198
Organwuion and Reguldtion
Organiliation and Regulation
Consultation blClween the societies takiCs place on mattcn of common illterest fhrough the [nlerTl~tioJlal ASSodation of Clas~iflCatiml Societies (lACS). The classif[(.atiol1 societics oper~te by publishing rules and regulations relating to the structural eflkiency ~nd the rdiabilily of the propelling machinery and equipmenL These rules an~ the result of years of expcrience, research and investigatio[J into ship design Olnd (Qllslrudion_ They are ill fact a set of standards There is no compulsion all a shipowno ro have his ship da~sincd. Howe,'er, the insurance premiums de-pend h'ry rlluch upon the class ofa shipthe higher the standard the lower the p!.emiuTll. Also, hy being c1as~ifled a s!J.ip is shown to be of sound COllstwl:tion and a safe mean~ of transport for cargo or paS~eIlgers There is no connedioll betweeTi the insurance companies anu tlle c1.assification wcielies. The opc-ration Jnd organisatiun of LJoyd\ Register 'Of Shipping, the oldest classification society, ",ill JiUW be considered. ThmughOLlt this book all references 10 claSSification sudety rules are to thuse of LlOYd's Register of Shipping, This Socidy is run Ily J general corTlmitree composed of members of the world community and the industry which jt Snves. National committees are formed in many (llUntfies for liaison pllrpO>-C>-. A technical COmmittee advises the gener ;Rules and Rcguhltions for the Classification of Ships' in book form, which is updated as neCC-Ssary, and also 'Extr-ac1s' fmlll the~e niles and 'Guidance Notes' rdating to morc specifk stluctures and equipment. The society employs surveyors who ensure compliancc wit h the TIlJe~ hy attendancc during construction, repairs and maintellJnc(" IhrougItout the life of dassed ships. To be classed with Lloyd's. apPlIJval is n~cessary for the constIll(;tionaJ plans, the materials used and tile C011structiOilai methods and standards, as observed by the surveyor. The rules govfrnillg the :scallllings of the ship'.; structure have been deve!r.lped from theoretiC,ll Ind ('mpiric-al considerations. Llllyd's wlJect inform,Hion on the WIlUfe and <:aus-e of all ship casu~Ities. Analysis of this information uften result, in modifications to the rules to produce a structure wh.kh is wlE~id-ered tIJ be -a-dequate, Much research alld ill\'estigaliort is aIs-o carried DUt by tllC society. bIding likewise to modificatiolls
+
100 A refers to the hull when bUilt to Ow highest standards laid down ill the rules.
+
refers to the E'quipment, such as the anchors and cahles, being in good and effide-nt condition. indk:.Jtes thal the ,essel has b(en built under the supervision of the socict~/, SurveyUIs. It is also usual to name the type of ship foll owmg the classification, ego
+- I 00 AJ Oil Tanker. Machinery i, al,o sUf'leyed and the notation lMC (Uoyd's Machiaery Certificate) is U$ed where the machinery has been built a.ccording. to the society's rule, and satisfactorily proved nn sea trials This information regarding the classitkation of a ship is eJltered ift the Register of Ships. The
199
. .' inin the names, classes and general infonnation Register of Ships ~s a ~ook c~nt~IO ~'s Register of Shipping., and also particuLars COllcernmg. the ships classed y y hi - the world of 100 tons g,ro~ (a of aJI known ocean-gOlJlg merdlant s ps III
ulpacity measure) and upwanls, _ _ cd by the society in requiring, all vessels The maintaining of ,talJd~rd). IS ensm ecial surveys are also required every t.o ;mn~al tourlI,we veaTS homsur'''Yd~ tIe lJ 0,Ct 0ex, "thmel1~~~~~~;~y for classification. More detail with
regard to tbese surveys is given in Cdlttap~e~tla~·an
IMO trade has led finally to the organisation of The international llaturc of selborn~ tal c;--operation on matters ,. I body provide mtcrgovernmen a.n interna IOna ' It U ~ the aUi;,nic-e>, of tne United Natious . 1shippin, amI t e seOl I\'Uer '" ,1 concemlllg S1IPS, " ,_ ' .. I (1~'10} fOfll11Orly 1\-leO was forme", the Illterllatiolw] \1.antlll1( Orgdnlsall~1 th nrst assembly mel in London in Following its formal approval by 21 stale" e I
'0
. in out many studies, producing detailed recom[MO, IS responSIble for carry d' d o'Nding fonowing conventions, mand eloving standu ~ an pr.. , Ih. d . men a Ions, ev , clion outfitting and operation. e reqUlIe1959.,
datol}' requirements f7 ~l.~~~~~::::eIflationa1Convention for the Safety of Li~e men ts ~uch as those 0 t ~ hen ado ted by the government of the vessel s at Sea only become mandator)' W P f involvement by IMO can be seen Iegi~tered country. Some of the many areas 0 in the following list: (I) (2) (3) (4) (5) (6)
Navigationale-quipment. Ufe·saving equipment, Personnel tf-aining. . Tanker construction and equipment. Fire s-afety in ships. Radio comll1unkatiorls. (7) Search and rescue techniques. (8) Suhdivision and stability, (9) Carriage of dangerous goods. (IQ} Marine pollution.
Items (4) and (5) will now be examined in some detail, since they embody many aspects affecting the construction of merchant ships.
Tanker construction and equipment , ' o f oil tankers will continue to be a source of The constructlon i!,nd eqUipment ,",., of oil ha" h,en and are still being, , " . elargequanlts , much mvestlgatlon SlnC f d d hips Efforts are being made with the discharged fJllm damage~ o.r. oun ~:ti S of' the sea (and shore} by oil Two ohie(t of preventmg or limIting po on
200
OrganiHltion and Regulation
panicular avenues of
(1) For new crude carriers over ::!OOOO deatlwooight tonnes, segregated banast tanks (SBT}, crude ui] washing (COW) and an iller! gas system (JCS;' wi]) be required. (2) For -existing crude c:Jrrier~ O\'cr 40000 deadweigllt lonnes, CBT, SBT01 cow will be required. (3-) For existing .:rude carriers over 70000 deadw-eight ttlnnes, lCS will be mandatory, (4) For products carriers over :20000 deadweight tonnes, IGS will be required. (5} For products carriers over 30000 deadweight tOl1Oes. SBT will he required
Crude oil waJ:hillg With this system, cargo lanks are equipped with fixed washing machines through whkh crudc oil (cargo) is pumped, Thc uil spray impinges all the lank extremities and frees the sludge \.vhich has separated DUt during shipment. Crude oil washing can therefon' mean m-orc cfficient discharge of cargo, while also being a useful aid to the load·un-top cleaning system.
Fire Safety in ships I'ire [\l sea i<; an evcr-presem and much feared hazard. For pa%enger ships the ·ccommendations, rules am] r~guliltiom following the 1()74 fmemanonl11 Crm'e/'enee on the SafeD-' of Life at Sea ale extcns.ive. They cover the many aspect~ )f detection, restrictiOII awl extinguhhing of fires. Cargo ships, panicul.n]y ilJ ,he accommodation arc as, must likewise huve arrange men t:; [0 deal with I1res, The arrangcments fm fire protection, by virtue of det.ails oJ arrangemc-nt of :omtruction, a, detailed in the 1974 lmernar.ional Confernrce on Safety of Life rt Sea und Uoyd's Rules,
vertical
zones,
Organisation a.n.d Regulation.
20!
Tlle-nnal clild slrlJ..:tur
()
~\~~l:~l;~·intlamm"bk III
CHg.O
-VarlllH exi:;ts the
pos~il1ility of ils igllition IllUst be
inimis_cd.
Variuus definitions ilTt: gi"VCtl for the special telms used. Non-co)11husti'o]C matcrial m8aJlS LI material \-\ hid.l neither b urn~ nor gi'{~<;_0 E1 tJ~ tlam maol", vajJo;,,:s in a sufficient quantity trJ sdf·lgl1lte wilen heated to .1:,:>0 C III iHl approved t .. Anl-' ,,,her maleltlll is combustihle. A standard ftlc test i, "vhen sp~c:tm~n'i of t]-:e t l)I.J Ikl Ha. d''v,.... , n docks ·He expo\.\'d in a ksl furnace 10 a particular tcmperareevan , I, ture lill;] cerl:Jin peril\d of r.illle_ rhc 'A' Cl.ass division'i me those dil'isiollS fOmlc-d hy bulk.heads and dc-cks whicb comply with the following.: 1. The-v shaUne construded of steel or otller equivalent material. .., Thc;" shall be suilably stiffened. . . .. . 3. TileI' sh.all he cOll'itructcd to prevent the passlige of smoke aIld name for a onc:!1(Jur standard nrc test. . . 4 Th.e]' Illust be insulated such tl1at the unexposed side_wlllllOt nse Illorenl~n 139°C or any point more than 180°C above the ongm.al tcmperature ",~nhl~ times as foUmvs: Class /\ -60, 60 mmutes: A-JO. ],0 mtnl.JleS, A -1), l) minutes: A-O, 0 millute~. The 'W Class division~ are those divisions l"ormc:d by bulkheads which are, COll~ ., .(od co prevent tlle r>as,ag" of fi-ame for a hall-hour st:mdard fIre test. rhey sruc" ~ . I IJ90C-r side win not .nse . Illore t Lall __ ., must lIC UlSU 1a t,d ~" ~u 'h"t L '-' tJle- Ul1cxp(}~ed .. , rl'. u . ' ( ~")~o(~" ,," .,I-.ov~v the original tcmper8ttlre witfUll tlmcs as [ollnw~. lISS .any pOlll . H--15, 15 minutes and BO, 0 mlnutf5. _ . 0 The 'C' Class divisions ate made of non-<.;omh-tlstible matenals but meeL II other rcquirements. The main veltical zones an: those section, into which Lhe hull, -\Uperst~t1et~re and deckbOl.Jses me divided by ':'\' Class divisions, lhc mean \engt1-l of whiCh :;llOUld not ex.c:ccd 40 m. 1 The hull, supcrstruclure, bulkheuUs. decb und deckhouscs m~st be of stee OJ other material which has structural a.J1d firc integrity properties equlvalenl tn sted_ Pipe materials aiTe-eleli by heat must not be used for nutl~ts near the waterline. The use of combustiole matcnals should be kepi to lHl absQlute millimum. Pjints, varnishes, elC., with a nitIOcel1ulos.e base musi. not be used ... The hun, superstructure and (lcckhouscs must be SU?dlVlde~ , .1I1to matn vertical fire miles of 40 m length or less_ "A' Class fire·reSlStmg dlVl~lOns are to be u-sed from deck to deck and shell or other houndaric~. 'A' Clas~ h.ound~ry bulkheads above thc hulkhead del:k should, where pDssible, be U1 hne WIth walcrtight bulkheads below. _ _ _. . . l'A' ('loco "'uJkhead~ must Jar. tne-reSlStmg Any ()pemng~ I l, ,,-.,~ u _ . be made good ., ._ Ilumoses. Dampers must ile fitted IT! vent trunks and ducb
202
Organisation an.d Regulation
Orgnnisatiol1 and Regulation
able from c-ith;or side of lh.c l:Julkhead; irHlk:u.tors should also fl{: fi',0ct n 'A' '-'I ~ ,,'- . U()(H,,,'III '- ass :,ulkhc~ds must be as fire resistant .as tlie hulkhe.ad ant.! .,hol.ilJ IJ.C" c~~able .oj heing llpcncd from <:ithcr side by one person. Fire d()or~ must 11csdl-closlng, even ill
, ,St.airw.1Y., and Uft, ~rc 10 he sled-framed ;md "",'itllin enc!osul'e, funned by ,ft (lass c11Vlswm. Sdj·clo~lIlg dums witll positive means uf closure should be tltted.at all OP:lllllgS, and be as effective asthe bulkhead in ",..]lk:h fitted. lor fire CO!ltammcnt. C.~ll;[I)1 slatH)I.lS, slH:h as the radio foarn. bridge, etc. must b.c surruunded by. A -class dIVI,IOns. Skylights in mJchi!J()'ty SPilCc, sholllil have means of cklsmg (rom outside the spa<.:e and alsu sleel Sliii·lt I attached. , ers permunent y . Ventilation systems .other than carg{) and illdchincry spaces must haye t\\.'o Jlldcpcnde~1t control pomt: where all machinery can he stopped ill the e\{ent (Jf a fire ..Ma-elullCry ,space velllllallOll must he capable oi being stopped from ouhide ~h~ ,;pac~. All IOkts and ou,!-cts lllllst be able [0 Ile dosed from oubide the ~pace, I~lr Spaces 1Il thc accommodatio1l behind ceilings, Linings. etc. must be htt~d "', Jth draught stops nol morc than 14 m IIp
loadcd, willi adequate stability and strcngth. A nUTr.ber of terms aud dimensions are used in the -computation of frcrhoard.
Freehomd deck, This is the upperm{lSl continuous deck exposed to the we-ather and thc sea v.-hich has permanent means for the watertight closure of all exposed openings on th-e dcck and in the side shell below. This is a horlzonul tine 300 mIll Long and 2S mm wide which is positioned umidship~ port and starboard, The upper edge of the line is located level with the uppn surface of the freeblJa.rd deck plating on the outcr shell.
Dl'ck fine,
Length. The freeboard length is the greater of the following two measurements: (I) on a W4JerliIle at 8570.- of the least muulded depth. 96% of the length along the waterline; or (2) on the same W.ale-lLille, the distance from the fore side of the stem to the axis of the rudder stock. Breadth line.
Measured:l\ amidships, [his is the ma:>;lrnum hreadth to the moulded
IJepth moulded. This is the vertical distance between the upper edge of the keel and the upper edgc of the freeboard deck beam measured at the ship's side. Displacement. TlJis lli the moulcteod displacement of the ship, elle-!uding bossingg-, measured at 85* Cfthe least muulded depth. Block coefficient. This is determined using the values of ;lisplacement, length. breadth and a V'lhle of dr'lught which is 85% of the leust moulded depth, i.e. Block coefficient, Cb =
The load line rules - freeboard
~~~ebOJld is lbe distance ll1eaSl~I.cJ hom th~' watcrline
to the upper ed'i(c of the ck platll1g _at lhe slde_ of the lr('ehouru ucck 3Ifl1dslll l", TI" I d I'me ni Ie-s set _' ,,(la out ,lhc.re q unemcllt',I(H a mtnlInUtTl frcehuard wlikh tllust be indkMed 011 t.he slup s sld.c b.. ., a spc:clal 10
t""·
203
Displacement Length X Breadth X Draught
Superstrucmre. This is a structure of adequate strength on the fteeboard deck which extends transversely to at least within 0.04 times the breadth from the ship's ~idc The snperstrut:lure length, S, is taken as the mcan length of that part of Hie superstrudurc within tlte freeboard length of the ship,
Freeboard categories In order to assign freeboards. ships are divided into Types A and B, Type A ships are those -designed specifical1y for the carriage of liquid cargoes in bulk. The cargo tanks have only small openings for access which arc closed by watertight covers of adequate strength. Type B ships are al1 those wnkh are not of Type A. The greater freeboard requiled for tbe Type B ship may be reduced in certain circumstances. In ships whele steel hatch covers are fitted, special subdivision arrangements exist, improved water freeing arrangements are provided and better protection for the crew is given, a reduced freeboard is permitted. This reduction can re~ult in an almost equivalent value to that of a Type A ship. Whele this value is almost equivalent the notation Type B·IOa is used, indicating a 100'%
204
Organlsatir>n and Regulation
Orgrmisatian and Regulatiol1
reduction I~f the freeboard differencc between Type~ A and B, Tho;;: notation TYf1c B·60 is used where a 60';1c, reduction of freeboard difference is obtained. Bulk carriers particularly benellt from this reductiOll in freebOJnJ. The freeboard is detnmined from a i.::l]cu)aliol1 wl1ere iJ rabulal freeboard figure hased nn the ship's length and type is adjusted hy se'Vcr:al corrections. These corrcclions lire t{) .aecouIlt tor the v.aTiations between the actual ship and the standard ship on which the tabular freeboard is based I ""-
A Type B ship of less th:an 100 In lell~th llaving superstructures with an effe-dive length, E, of up to 35% of the freeboard length, L, may have its freeboard
increased by
7.5 (100'
,)
(, -"i r\,0.35
millimetres
where E LI the effective length of the superstructure, in metres. With the SUper. strllcture length, S, known the cffective length, F, may be found from the load
20S
:n0
an 85 m ship length and 1070 mm for all ship lengths greater than 122 Intermediate length deductions are- obtained by interpo!allon; .wlIh eHe-etlve lengths less than 1.0f. the dedudinn is a percentage of the values glven, Sheer con-cetion
The -differences be-tweeTl the actual sheer profile and a standard sh~er prof1Ie are determined. Tlle correction is Ihen the- deficie-ncy or excess mulhplled by
(075.
~L)
wht:re S is the mean length of the superstruct~rc. ,. for a defickncv of sheer, the correction IS added t{) the freeboard. ~lth an excess, a dcdwctjo~ is permitted where the superstructure covers O.lL af~ an,d 0.11. forward of midships f'm le$SeT lengths of superstructure, thc dedllctlOlllS obtained by interpolation. A maximllm deduction of 125 mlTl per 100 TIl of ship length is permitted.
line rutes.
With the tabular value amended bv the cOffections, the freeboard value will be that for the maxi-mum summer ·i1raught in sea W'..lief- This .·a/tlc may be
Block coefficient co"cction
further amended if, for instance, the bow height is insuftlcient as defincd In the rules. cargo pmts or openings are fitted in the sides below the freeboard deck or the shipowner requests a freeboard corresponding to a draught less than the maximum permissible.
'rVhere t]l1:: actual block coefficient, C", of the ship cueeds 0.6&, the freeboard ,mended by the flush deck corrcction, if relevant. is multiplied by the ratio Cb + 0,68
136 Cb is obtained as defined earlier.
Depth cOlrection The formula for the freeboard depth, D, is given in the rules. Where D is greater than the freeboard len,gth, L, divided by 15. the freeboard is increased by
where R "" L{O.48 for ships less than 120 m in length, or 250 for ships greater than 120 m in length, If D is less than 015 no deduction is made, except where there is art enclosed superstIUcture extending 0,6L at midships. ntis deduction ....auld be determined as for the flush deck correction.
Superstructure co"ecrion For an effecti'Ve length of superstructure, F, of 1.0 times the freeboard length, L, :he freeboard may be reduced by 350 mm for :a 24 m ship length, 860 mm for
load line markings The maximum summer draught, as determined above, is indicated by a load line mark. This consists of a ring of 300 mm outside: diameter and 25 mm wide, intersected by a noriwntat line 450 rnm long and 25 mm wide. The upper edge of this line passes through the centre of the ring. The ring IS pos~tioned. at midships and at a distance below the upper edge of the deck lme which corresp-onds to the assigned minimum summer freeboard. This value may not be kss than 50 mm, A series -of bad lines are situated forward of the load line mark and these denote the minimum freeboards within certain geographical zones or in fresh wateL The summer load line is level with the centre of the ring and marked S. The tropical T and winter W load lines are found by -deducting and adding, respecti'Ve1y, 1/48 of the summer moulded draught. For a shiP. of tOO ~ l:ng~ Of I.ess a Winter North Atlantic (WNA) zone load line is permttted. This lIne 1Spositioned at the winter freeboard plus 50 mm. The fresh water freeboaIds F and IF are found by deducting from the summer Or tropical freeboard the 'Value Displacement in salt water millimetres 4X TPC wh-ere TPC is the toones per centimetre immersion in salt water at the summer load waterline.
206
(Hgtmisation and Regulation
Organisation tmd Regulation
C=====~I Deck lin~ ,
207
Superstructuore end bulkheads
,
Such buLkheads for Emclosed superstructures must be adequately conslructed. Any openings. must have" minimum sill height of 380 mm above the deck,
,---300 mm----'
, , , ,
Hatchways Portable cOJ-'ers secured by tarpaulins
,c=~
,====s~ r- ~=11 ' 230
T
iL
-450mrn--_
b: "
~
,WNA
540 mmforw
Figll.re 1 (), 1 Load li"e- "'Ilrkin~ (all/rm:s 25 mm /hic-kr".s~'J
Tne~e markings arc shown in Figure IO_l. In .all cases, measurements are to the IJpper edge of tae line.
Conditions of assignment ~:;s.tion was made earlier of t ne conditions of assignment relating. to freeboard.
e a~e certaln reqUIrements which must be met to ensLJrc tbe wate t-ght of ope lungs and the ~bility of the ship to rapidly free itself of water 00 Ct' d Ok'" Refe ill b d ' I S ec s. renee w e rna e to two partIcular positions which are now defined.
Position t, E.xpose~ rreeboard, SUpers.1IUcture and raised quarter decks within one·quarter of the ShLp s length from the forward perpelldicular. Position 2, Exposed superstILIcture decks outside one-quarter af the ship's length from the forward perpendicular.
Substantial coamings. of mild steel lH equivalent material must be fitted to all hatchw
Watertight steel corers There are similar requirements forcoamings. but these may be reduced ill height or dispensed with where the safety of the snip is not affected. Again requirements must be met in respect of cover strength. construction and watertight securing arrangements.
Machinery space openings Machinery space openings in Posit ian 1 or 2 must be effidently framed and plated for strength. Openings are to have watertight doors with sill heights of 600 mm in Position I and 380 mm in Position 2, All other openings are to have attached steel covers which can be secured weathertight if required.
Other openings in freeboard and superstructure decks Manholes an-d scuttles (portholes.) must have covers fitted to efficiently secure them. All doorways are to have a minimum sill height of 600 mm in Position I and 380 mm in Position 2. All openings other than hatchways, machinery space openings, manholes and seuttles, where in an exposed position, must be enclosed by a stIu.;ture of equivalellt strength and watertightness to an enclosed superstructure.
Ventilators
Structural strength and stability Th~ ship i& required to have the necessary structural strength for the f b d assIgned. Certain criteria with regard to stability must be mot d reel·o~r e. b.' '" an an me mmg xpenment :must e ca.rned out III order to ell&Ure compliance.
Coamings 011. ventilators must be 900 mm above deck in Position 1 and 760 nun in Position 2. Where exposed to severe weather or in excess of 900 mm high, ooamings are to be suitably bracketed to the surrounding structure or deck. Some m-eans of permanent closure, either attached Of close by, is required for all ventilators except those of height in ex.;ess of 4.5 m in Position 1 or 2.3 m in Position 2.
208
(}rganisation and Regulation
Organisation and Regulation
Ai, pipes
209
IJlber means "f access 1("luiIed itl tl,e (lOllrse ,)1' tile-iT work mllst be provi(leol1 fOl till' en·...,
SpE!c~al conditions of assignment for
Tvre A
ships
Cargo ports and similar opening.,
,Had/illlY1' caS/ilKS
Ally cargo port~ mUl! be fltled with lhJOrs and tr"lInc~ ",'hid, 1\1'.' the load line deLks
An enclosed poop. bridge or >tilTldarl1 height or J dcckhouse of eqlliv.alcnt '-lrengih \\lid "eight llt\lS\ protect t\l<: l\\';l(:n(l\Cry ~asiI\.g. An cxrro~ed casing is allo\\.,'cd withoul doors or with a dDuble-door arrangemenl. provided it is of weathertight c-onstrllt:lion
<;cupp"rs, int"rs {lnd discharges
HlIu:l!wavs
All di~l;harges from above or below Ill,", freebOJrd de 'k I 'I ' ff1 " I . l rum ~JiL 11\(.",-, s]lilces are to have an e lelen lH!-n-return arrangemenl filled , , - . . , :ontrol are sp-ecificd
lide scu (ties (porrholes)
:very side s-:uttle below lhe lreeboard uec\..: is to b~ fiiteu ·'in hllate or -deadlight which may !'le securelv closed and Illa ..... \ a, mgeo,c-over.;:uttles 111.1\.·J be fitted bolow 2 5".', Ill'" , b de-00 wille-rtlght.' !\'() sIde ' - . /G I 1e S up s readth he greHter, above the load waterline or -~ min. whlchever is
;reel'lg ports
Vhe~e bulwarks on any exposed decks form wells h , fficlCnt means for Iapidly freeing the decks ofwat l ey m.llsl.be prmHled With 01 the determination of the freeing , I _ er, SpeClal formulae are gh'en s height and the sheer of the deck area In re atlOn to the l~ngth of the b-ulwark, s close to the deCK as possible. T~~~i~~:~~~~~e-f of the Jre~Jl~-PQH should be ear the lowest pOint of til" sh . 'h reemg area,s ould be IOl::ate-d _ .. eer curve \>, ere sheer eXlsts th d k lpemngs are restricted in height to "30 m,n bv b. b· I 011 e ec. /h h lars emg p .ae.ed acro h _o~ja~~~e:.s or flaps are fitted to these openings they ~hould be P::Y:l1~~d
'rOlection of the crew
11.' exposed freeb.oard and superstructure decks must have bulw k
. d iUS fitted at thelr perimeter with a minimum height of I .. ~ s or .gu,n itted the d k d I . , m.... nere ralls are il ec an ower rad spacmg must not exceed 230 rom d th 80 mm. Eff-ective protection and safety in the form of gangway~pa~s.ag:s~n:
All exposed hatchways are to h
Frecin.f!, arrangements
Opcn ,ails mU~L be fitted for at least half of the exposed length or the deck. The upper edge of the ~h-ecr strake should be kept as low as possible. Where a trunk connects parts of the Sllperstruetur~, open rails should be fitted at the perimeter
of the deck in way of the trunk.
Protectiun. oj the crew
Where separate superstructures exist they should be connected hy a raised gangway at the level of the superstructure deck. An acceptable alternative would be a passageway below deck. With a sillgle superstrut:;ture, ade4uate safe arrangements should ex.ist for access to all work areas on the ship.
Tonnage
Tonnage, as disc-ussed in this section, is a measure of cubic capacity where 1 t-on represents 100 ft3 or 2.83 m3 • Trmnage is a measure of the ship's internal capacity, with two values being used. The gross tonnage is the total internal capacity of the ship and the net tonnage is the revenue-earning capacity. Tonnage values are also used t-o determine port and canal dues, safety equipment and manning requirements and are -n statistic31 basiS for measuring the siLe of a country's merchant fleet. All ships prior to registry must be- measured according to their country's tonnage regulatiorts. The differences in the various measuring systenlS have 1ed to ships having several tonnage values and to unusual designs which exploited aspects of tonnage measurement. The 1969 ]~10 International Conference on Tonnage Measurement of Siljps led to .an international review of the subject and a r;ystem which will ultimately be universany adopted.
210
Organisation il1Jd Regulation
Rffe-rence will now be made to the British liJllnage rlIeaSllremcnt SyStem and also the 1969 (0I1ventitln mea~UreTTlent syslL:m.
Organisation and Regulation f.xem/rr<'d Th~;;~ l~e
Stich
British tonnage The current regulations governing tonnage measurcmenr ,lIe the Mereh,wt ~hipj1lng_ (T-oJlnae-'t') Regul....tions 1967(1 I). 11H~ me::t,uremenl uf tonnage talluws Irolll VarJOll.' sp<::dalist term, ~md values which will now be defined in
tunl.
Tonnaxc deck
This is the senmd deck, except In single· LIed: ships
~pat'CS
spaces wlli(h Jrc n01 rne~I~\lTed f'Jr the grllSS tonllage Illn be- ~b"H' <>r helow 1111' t
(]) C) 0) (4) (5) (6) (7)
\\-11edhousc. cllartr<1Ilm. l"adirTl'lll,n,
Tunnage kngth.
Tonnage depth
This is measured frum the upper surface ofrhe tanktop to the underSIde of the tonnage dCl;k Ci1 the centreline. with a deduuion of one-third of the camber. The height of flooring, d{)uble or single, is limited
TUNnage brmdtll.
The breadtJI of the sbi.p to the imide of the hold frame:> or
sparring,
UnJerdeck tonllage. This is the tonnage of the space bElow the tonnage deck. It is found by dividing the tOllnage iength into a specified 'number of parIs. At eilcn crosS-~ctiDll fmmed '0)' this diVision, the tonnage depth is similarly divided up. The tonnage breadtlJ, at tl,esc points are then measureu. The measured dist.1nces are then put through Simps-on's rule to provide the urlJerdeck volumc whJch is converted into a tonnage value.
":J'wss tonnage
fltis is tIle total of the underdeck tonnage and the ~onnage of the following 'paces:
(I) Any tween·deck spaces hetweel\ the second anrl upper rlecks. (2) Any 'odo"" 'poe" ,bo," Ih, opp" "eck. (3) Any excess of hatchways over 0.5% of the groflS tonnage, (4) At the shipowner's option and with the- surveyor's approval. any engine light and air s-paces- on ur above the upper deck.
[he term gross register tonn3ge (CRT) is also llsed.
~·~k\llali('n
wa.;~'\
Dcduaed
An. imaginary liM i" dmwn across Ihe :l-Ilip at the stern and st-ern un the Inside of tlte hold frames iJT spJrring. The tunnage length is the dIstance between these Imes measured along the ship's centreline on the tonnage deck.
211
.,
The [{)flnage lJf th~sc ,pa~es must fil~t he meJ~d al1d nun then b·v J<:ddcted from the g:ross tOllnage lJr" tht: sll ip tn give lITe nd tOllJlage I:::xamples uf deducted spilces arc' (1) \faster's a-ecomffiodation. (2) (reR,' accommodatioll and an ~llowal).:e for pW\'ision s!ore~, (3) Chain locke-r, steering gear 'pal,;e. anc1H)1 ge,lI and cap<;t:J1l spaL'e (4) Space -COr safet)· equipmell1 and batteries helo\\' the Lipper \~eck, (S) Workshops and stOTerooms for rLlnlplTl~n. r!t:;:tricians, carpenter. and boat-swain (6) Donkey engille "tid c1{lllke., boilc! space if tlH'se arc' oUlside the machinery space. (7) Pumprooms, where these are outside tne ma.;;hinery SpJL'e (8) 'Vater haUast tanks, w~lere tbey aro{) for the ex.clu,ive carriage of wat<,r b.a1l3st, 3 ma.\iJlturn limit of 19S" of the 2,105> tOlltuge is impos<:J. (9} Propelling power allcTIvance - this is the largest d-::dllcti{1TI and is determined aU'0rding tu cenail) nileJi:; ,15 follows: If the machinery space tonnage is bel\veel1 l:en and 20S. of the guJSS lonn:;g:e. the pfLlpelling power allowance tS 32S"t- of the gro,s IOlImgc If tbe IlI8chiner~ space tonnage is less than J 3'7 of 1he gms., tDnnage then the propdlillg. power allowance is the aJl10llnt expressed as a proportion of 32'1 of the f!ro>s !unnag:c v,'here the mal'hiTlery space- tonnage- is more than 20~" of the gross tonnag,,- the ~ropel1in!!.. power ,,"l1ow,,"nce is 1-':; time, Lhe machinery space tonnage. There j, a maximum limit of 550 of the ,HDSS ((}llIlage lell t!le ;)f(1;Jdhng pcnNer allow,lTIce If any pJrt of the light alld air space is inclu(!cd m the grms t()nlla~e rhen it may also be included itl the machinery space 1ll11lu.ge. Set forlilaxe
This i~ tile :amragc value obwined hy deducting fron; the glO" t\I[ll1Clgc lite total v::tlue of the deducted spaces. The net \(11l[lage is considered to reprcserlt lh~ earning capacity of the ship. The tt:lTll net r<:gi!':tCr t011nage (~Rn is also used. TOlllwg(' lIIarkldu'I!I<'
The- tlJTln
212
Organisation and Regulation
deck. provided a special tOllnage dr"ught Illark was not submerged. The position of this nlark on the ship's -'ide \.\ias tu ~enerally (;orrespIJnd to the draught which \\-'uuld be obtained if the freehoard had been cakuLated Cor the second
deck being Ihe freeboard deck. A special l11iUk is u~ed and is shown in Fi?:/lJ'e fU!. The position of Ihe mark on the ship·s $ide is g,i"'en in the amcndmcnt to the load line mles dealin[l: with the tonnage mark lchclllC. ___ 3-00 ,,,,n ___ _
n(J".,C"-
11
Corrosion and its Prevention
Figure) D,] 7of!{I(Jg" mark
(ailliriCS 25 mm lliitll/WSS)
When the tonnagc mark is at or above the waterline the ship i~ considered !l.ave a modified tonllage When the tonnage mark is below the waterline the ship is cOJlSidercd to be at its full tll1l11age.
10
The prevention of corrosioll on board ship is an immense onf'{)ing process demanding the attention and skills of t:omidcrable numbers of pcrsonnel The ship because of its size. its phy,ical CIlvironmcn! alld tile TTlalcri;!ls 1I.led in its construction is subjcct. to attack from the . . ariOll'; funllS or corrosion
Corrosion
1969 Tonnage Convention Two IDnnage values, the gross and the net, arc u~ed. The . . arious positiom and cxtcnt of l11eaSl1rem~nts of Icngth. breadth and depth ,lie defined and differ slightly from the British tonnage system. Excluded spat:es. a -c-argo space and other terms are clearly defilled. The gros~ ttlIlnagc b computed frum an empirical formula, the terms uf which relale to the defilled tOlln:lge distances or La conwln ts which are dete rmined from the form ulae given. N el tonnage is ~imilarly founll hy linlllhcr empiricat formula consisting of measurements and constants. The COllvention advocates the- me of thc term, 'U~S (~ross' and 'L:MS Net' as dimensionless values instead ofgro~s and net tonnages in tom. The acceptance of this c(Il1VentiOll will remove \he tonnage mark. scheme whidl has been the subiect of much con tmve rsy .
ConsequenCl:s of the 1969 Convention Gross tonnage measurements or VMS Gross are, in general, fairly close to those val\le~ determined f{Om c'J.trcnt Iegt.\latiom•. ShijJs with la(ge ex.empted spaces
will have somewhat Larger gross. tonnages under the new mles. Net tonnages do show sigllificant v-ariations between the measured values for individual ships. Ore and bulk -carriers with their high density cargoes will have a reduced net tonnage under (he new rules. Again, ships with large exempted spaces will ha.. . e larger net tonnages under the new rules. Apart from the purely quantitative aspects, the uni-....ersal adoption of !he new rules wUl provide for safer ships. This is because construdional methods and unusual design features will no longer be inOuenced hy tonnage measurement. The task of measurement will he simpler. since the necess
Co[ro~ion i~
the wasting of metals by chemical {If cleTtr-lJchemical reactions with their surroundings Erosioll is.a term often a,sodated with corrMion. and refers to the de~tru~tion of a meral by abrasion_ Emsion is therefore a mechanical wastage pw.ccss that exposes hare metal which can tlten c'-'rrode. Iron ana steel corrode in an attempt to regain their oxide forlll which i5 in a balanced state with the earth's atmosphere:, This oxidising, or rusting as it is commonly termed, will take place whenever steel is expmcd to oxygen and moi~ture, The prevention of corrosion therefore de-als with the isolation of sted from its environment in order to stop this oxidation taking pla.ce, In addition, the presence of a ship almost l,;onst-antly in sca water enables an electrochemical reaction to take place on unprotected steel surfaces. A cOlTosion cell is then si:lid to have been formed. This is often referred to as a 'galvanic cdl'. since its wnent flow is a result of a potential difference between Iwo metals (not necessarily different) in a solution such JS sea water. This current How ,CSLlltS in metal being removed from the anode metal or positi.. . e de-drude, while the cathodic metal or negati....e electrode is protected from corrosiun. Most common metals can be arranged in what is known as a gal.. . anic series. according to their electrical potential in sea water, as shown in Tablt' 11.1. A simple example of a corrosion cell would be a plate of copper and one of iron placed in a sea water solution and joined by a wire. Reference to Table 11,1 will show that .copper will becollle the cathode ()[ protected end JIld the steel will become anodic and corrode. This is shown in Figure 11J The chemical reactions taking, place and the electmIl flow occurring will result in the ar."Iodic metal combinmg with di"wlvcd (Jx.y~en to form its o;,table oxide form (mst). Corrosion .can also o.ccur as a result of stress, either set up in the mate-riat during manufacture or as a result of its 'working' in the sea. The e{feds of stress and fatigue are to provide area~ where cracking may occur. but even these sometimes minute cracks cre
'"
214
Corrosion and its Preven'ion
Corrorion and its Prevention
T.ble ILl GALVANIC SERIES OF METALS AND ALLOYS IN SEA WATER Pl;ltinun;'"
I
Go~
Graphite Silver Pusive sUlinless sU'ds Pusive h~h nickl'l alloys Pusivl' nickd Silver solders Copprr-nickl'1 alloys Bronus Gunml'lal Coppa Bran (10/30) Anivl' h;,h nickel aUoys Anivl' nickd MiU scak Naval brass and brass (60/40) Tin
etuhodit or noble m('wlg (pror«Ud marerial)
lnd Lad-lin SOkll'TS Activl' 51ainkss Srff!s: CaSI iron Iron and 51ffl Aluminium alloys Cadmium Aluminium line M:lJ:nesium alloys Magnesium
Anodic (N ipob/~ m~rllJs (<<1'7Odi"K _rerilll)
j Corrosion prevention
The prevention ~f corrosion deals in the first place with the provision of an adequate protective coating for the ship's structural steel and its continued main.tenance.. Se~ondly. a means of preventing electrochemical wastage is required. which IS known as cathodic protection. The two distinctly different
j
Curr8'l now
j
215
types of corrosion prevention are usually complementary to one another in that both are nonnally fitted on modem ships. Finally, it should be noted that a knowledge of the processes of corrosion can ensure the reduction or prevention of conosion on board ship. particularly on the internal structure, by the use of good design and arrangement of structural members.
Paint Protective coatinp refer 10 the application of a suitable paint system. Paint is a mixture of \.bree ingredients - the pigment, the binding agent or vehicle and the solvent. The pigment is responsible for the colour and covering capacity and may also refer to certain additives, depending upon the properties required of the final product. The binding agent or vehicle, depending on ils proportion in the paint. will decide the consistency and ease of application.of the paint. The solvent or thinner is added to make the paint flow easily. Most paints consist of solid pigments. usually in a fmely divided form, suspended in a liquid binder or vehicle which when spread thinly over a surface will eventually dry out. A thin dry film is then left adhering to the surface. The 'drying' process associated with ships' paints is usually the evaporation of the solvent from the vehicle. Good ventilation is therefore essential and moisture· laden atmospheres are to be avoided during the drying process. The coating applied must also be thin to ensure that it dries out correctly. The appropriate solvent is essential to ensure the correct drying time; tOO quick and blistering can occur, too slow and the paint may end up immc~ed before it is dry. The common vehle es m use are. (I) Bitumen or pitch - bitumen or pitch in a white spirit solvent, or blends of pitch with other materials. (2) Oil based - vegetable drying oils. e.g. linseed oil. dehydrated caSlor oil. (3) Oleo-resinous - natural or artificial resins mixed into drying oils. (4) Alkyd-resin - a special type of(3). (5) Chemical resistant - chlorinated rubber. epoxide resins and coal larl epoxide are examples.
All the above vehicle types are suitable for above·water use. Only types (1) and (5) and certain types of (3) are suitable for underwater use, because of the need to resist alkaline deposits formed at the anodes of corrosion cells. Anr;·folllillg painl
Figurt JJ.1 Corrosion ull
Fouling is the covering of a ship's underwater surface with marine organisms such as green slime, weeds and barnacles. Fouling occu~ usually only when the ship is at rest and is dependent on water temperature. salinity. the se:ason. the place. etc. The slower speeds of the larger tankers and bulk carriers has resulted in increased fouling problems. since some marine organisms can survive and grow:at speeds of 10-15 knots. The result of fouling is increased hull resistam::e and subsequent loss of the ship's speed or increased fuel consumption.
216
Corrorion rmd irs Prevention
Corrosion and its Prevention
Anti-fouling paints functi{)[l hy slowly rdea~ing a poisoll into the laminar sea water layer swrrounding the ship. Ti!is S~a water soilible poison is toxic to marine olganisms which rJlllst pass thmugJ.t::'is Laminar layer in order to attach them· selves to the ship. The poi'i{)1l is released al a controlled raLe, determined by the type of lOX in and also the lkgree ,,"Ild ratc (If solubility nfthe binder, Two basically dilfcrcll( tyres uf anti-fouling puint curr~ntly exist - nonpo!ishillg 1111(.1 self-polislling. Non-polishing anti-fouling may have either a solub-le or insoluble matrix, The soluble matrix: consists mainly of rosill (colophon)-') which IS slightly sea·water soluble. 111e Ilio-activc marc:ri~ls (poisons) are released in sea waler together VI'ith the binder. Ihe insoluble matrix: type uses a large proportion of po1ymeril; binders which are insoluble in sea water, The biu·a(;tive materials are released together with otlier components ",.. !licl. act as leaching aids. This leaves behind a released layer of insoluble binueJ. The release rate of 1I1e bio-active materials in each type will decrease with time in -:;ervice of the vessclThc bio-active materials will induJe cuprous oxide and organulin compounds. The amount or 'loading' of these materials is vilric:d according to tile vessel's rcquiremenb. Sm
Painting the ship The paint used must be appropriate for the degree of protection required at tlle particular area or section of the ship. The principal areas requiring. different
217
s u pc" 'cue' u,,'
Bnfll "'1'1-"
, ~:;, :,:, :, ~, -"':===-::':-=-;-;:l.:-::':::':=~=~~~=-~-~=~
•
- Top,icle,
8mlOf'l pl"l.ny
Figure ii. 2 !'nlicipa! pl1f"Ung oreaS
forms of trcatment are the underwater pl.ating and boot topping region, the topsides, the superstrue-ture and the weather deeks (Figure 11.2). Prr/lilrariorr
tlild
prill/ing
Thc sUlface preparation of the steel platt' must iirst he good 111 {)flier to ensure the- sue'eelsful operation of tile- ~rplied (laillting sy~tell1 The steel plates used in ship conslfl1ctiorL Jre fint shot-blasted to relTl(fVe all traces of rusting and mIll scale which may be present. The plating is thell immediately primed with a quick-drying prefabrication primer This all takes place as part of a continuous undercover process under contJolled conditions This coating is usually adequate to pIOtect the plate- during the various fabrication rf{)ee"e~ leading to its ine-orporation into the hull of the ship. Final painting will progress with the construction 01 the ship. ()mlr:n<,,'ater areas
The underwater and hoot copping pl
218
C01TO~ion and
Corroldon and its Prel'€ntion
its fuventiO/t
Topsides and superstrnctures
T(Jpsides and superstructures are usually adequately coated with primer, an undercoat and a finish.ing paint. Paint based un alkyd resins, modified alkyd resins and enamels arc used in this region_ Since appearance is of some importance, good colour- and gloss-retaining properties O'f the paints used on these parts i~ essential.
219
Two means of cathodic pmtectiun arc in general use on ships - tne sacriflda1 anode type .and the impresso::d current type, The sacrifldal antJde type {)f cathodic protection uses metals such as illuminium and zinc which form the anode of a C()rros~on ceB in preference to steel (see Table 11./). As a conseq,'ence. these sacrificilll a1lodes lire gradually eaten. away and requi.re repiacemcnt after a period of time. The impress-ed wrreIlt -system provides the electric:>.l potential difference from the ship's power supply Hnoug.h an anode of a 1ol'.g-life hig?ly corrosion-reSistant materi.al such as platinised titanium.
Weather dec ks
Sa(;n"!icia[ amtde system
The- paint fln the weather deck area requires excepti.onally good resistance to we-ar and abrasion and Sume non-slip quality. The deck coating should also beresistant to any oils or chemica.ls carried as cargo or fuel. Initial protective watings topped by grit-reinforced oleo-resinous p-aints have been used successfully, as have primers and chlorina ted rubber deck paints. Certain metalli.c finai coats have been tried With considerable success, more partiCUlarly on naval vessels. The constant abrasio!J on weather decks from traffic, cargo handling and seneral ship operation makes long-term protection by paint alone almost lmpossible. Self·sealing coatin,gs utili8ing epoxide resins have been used with >orne success- on top of epoxide resin paint for a hard-wearing deck covering_
Sacrificillt anodes
Tanks Ballast, cargo/ballast and fresh water tanks requiTe spedal coati.ngs, depending upon the nature of their contents. Treatments used include two coats of e-poxide resin or a three-coat phenolic resin·based paint, with care taken to enSUle compatibility with the tank contents. Fresh water tanks can be satisfacto~ily protected by bitumen or tar paint~. Drinking water tanks must have a non-taint coating such as artifu:::ial bitumen to BS 3416 Type 2.
Cathodic protection When a metal is in contact with an electrolyte, e.g. the steel of a ship's hull In lea water, small corrosion cells may be set up due to slight "'ar~ations in the electrical pOtential of the met.a1's surfac~. Electric currents flow between the iigh and low potential points, with the result that meta.! is corroded from the p-oint where the current leaves the metal (the anode). At the point wllere the :urrent re·enters the metal (the cathode) the metal is protected. Cathodic protection operates by providing a reverse current flow to that of the corrosive lystem. With current then entering the metal at every point, Le. the whole metal mrface becomes a cathode, it is tnerefore .cathodically protected. When the potential Qver the immersed hull surface is 0.SO-0 ..85 V more rlegative than .a reference silver/silver cWoride electrode in the water nearby, then the hull IS adequately protected, Current density of the order of 20-100 2 mAim h usually suffident on a pamwd hull to reverse any L"orrosiO!J current md cease further metal corrosion. Currcnt density necessarily increases for a pOO~I~ painted hull and therefore cathodic protection sllOuld be regarded as an lddlhonal protection to painting and by no means a substitute.
The impressed current system consists of a SallICe of direl.:t current, anodes, apparatus for Illeasuring and controlling, tllc current afld a hi~ yuality inert protccti,'e coating around the areas orlile hull nearesr to the anodes. Continuous con tJol of the impressed cunel1 t reguire-ll rn r adequate prntection va ries with tbe immersed area. the ship speed. the salinity of tne water and 1h-e c0l1di1ion uf the hull p
Ruooer stock
C"ntrn·ler
~rQ.,Jnd~(1
I "'",ode
,
Shalt qrou"dinq 3;s~m'olv
i\nD<1~
Refer""", onoo,
Fi;f!,llre 113 /"'wenetl run-em carlrodi<: pmu<:tioJI syflem
Be/"r"ncr anocle
220
COffosion and its Prev~tion
Corrosion and its Prevention
arise near the anodes. An irnpres)e9_~1l~ren( system for a ship is shown in Figur(! 11.3. Details of an anode and a reference anode together with the ~offerdam cable arrangements are given in Figures llA md 115 A propeller shaft bonding arrangement must be fitted with impressed current systems to ensure propeller protection (Figure 11.6).
,
n,~; "'A', .:.
Cathodi,· protection of tanks
---.
221
\,
"?
~,
The cathodic protection of ballast md cargo/ballast tanks is only ever of-..!;,he sacrificial anode type using aluminium, magnesium or zinc anodes_ The use of aluminium and magnesium anodes is restricted by height ilInd energy limitations to reduce the possibility of sparks from falling anodes. Magnesium and aluminium anodes are nDt permitted at all in cargD oil tanks or tanks adjacent to- cargD oil tanks. The anodes are arranged acrDSS the bottom of a tank and up the sides, and only those immersed in water will be active in providing protective current flow. Current density in tanks v~ries from 5 mA/m 2 for fully-wated surfaces to about 100 mAim' fOl ballast-only tanks. Deckheads cannot be
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~-Fpo"y coat;"~
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11.4 Anode
anoue
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222
CorrosiQrl and its Prevention
•
cathodically protected, since t,mks are rarely full: they are therefore given adequate additional protectiv~ coatings of a suitable paint fm the upper 1.5 m Df the tank
12
Corrosion prevention by good design The third method of corrosion prevention is by good design based on a knowledge of tht corrosion processes. Good design. therefore. should avoid the trapping of corrosive agents or the setting up of corrosion cells in places wllkh cannot be reached. are pourly ventilated. ur rarely protected ur maintained. Small pockets, crevices, etc.. where sail spray; water. etc .. can collect will result ultimately in severe ru.sting. Since this involves an inuease in volume of the material it will he followed hy distortion QT frao:;ture of tne stlllctllral members. Scaling of slJcll -crevices by welding or concrete, or their avoidance in the design stage. should he ensured. Dripping water as a r-c'Sult of poorly designed discharge, {)[ scuppers should be avoided. Condemed mQisture on the underside of encluscd ,(mctures will cause cOfIo~ioll and good design should ensure adequate ventilation ul these areas. Steel decks covered by wood will corrode unless the steel is suitably protected and the wood is 'sealed' by a bitumen coating. All joints should be sealed by a suitabk filler and any baits through the wood should have washers under the nuts to prevent the entry of water. Paint. to be an effective protection requires an adequate thickness over the metal sUlface. The surface should be made as accessIble as possible to enable good coveral:1e al1d a uniform ttry paint thickness. Welding can be used to fill small crevices. however, any welded surface must be suitably prepared prior to painting to enS-lile protection against currosion. Smooth rounded surfaces are always easier to paint and less liahlc to damage and subsequent corrosion. Tile atmosphere Qf machinery spaces and boiler rOOI11S, with the presence of heat. moisture o vibralion and fou1 air, presents ideal conditions for the corrosive process to take place. Surfaces should therefore be kept water-free and.a~ cool as possible by good drainage, insulation of steam pipes, ek" and good ventilation, Inaccessible places such as machlnery seats should 1J-e well protected by paintln,g before any machinery is fitted. Double-bottom tanks under boilers are some· times left empty and specially coated with heat-resistant paint. All double· botlum tanks shoul
.,
Surveys and Maintenance In C0J11rnC'>J1 with all machinery a ship requires regular overhaul and maintenafL(e. The particularly severe operilting conditions for an almost all·steel stll.lcture necessitate constant attention to the: ~teelwork. The operations of berthing, cargo loading ;.md discharge. [(lostant immersion ill sea water and the variety of climatic extremes encoUl1tered all take their toll on the structure and its protective coatings. Th.e dassifi;;ation societies have requirements for examination or survey of the ship at set periods lhwughout its life. The nature and extent of the survey increases as the ship- be-wrnes older.
Periodical surveys All ships must have an annual sul"\.'e'. . , which is carried out by a surveyor
"
employed by the dassific3Uon society. This survey should preferably take place in a dryduck but the period between in-dock surveys may be extended up to 2'1% years. Slich an extension is permitted where the ship is coated with a high resistance paint and an approved automatlc impressed current cathodic protection system is fitted Tn-water surveys are permitted f-or ships which are less than 10 years old and greater than 38 m in breadtll and have the paint and (:athodic protllctjOl1 systems already referred te.. Special surveys of a more rigorous nature are required every 4 years, Continuous surveys are permitted where all the various hull compllrtments are examined in rotation over a period uf 5 years betweeo consecutive examinations. During an ant~\.\~i~urvey the various closing appliances on all hatchw3ys and otller hull openings through which water might enter must be checked to be in an effident condition. Water·clearing arrangements, such as seu ppers and bulwark freeing porls. must also operate satisfactorily. Guard rails, lifelines and gangways are also examined. \\!hen surveyed in drydock the hull plating is carefully examined fer any signs of damage or corrosion. The sternframe and rudder are also examined for cracks. etc. The wear in the rudder and propeller shaft bearings is also measured. The fire prote<:tion, derection and extinguishing arrangements for passMger ships are examined every year and for cargo ship'S every two years. For a special survey, the requirements of the annu31 survey must be met together with additional examinations. A detailed examin-ation of structure by removing covers afld linings may be made. Metal thicknesses at any areas 223
224
Surveys and Maintenance
showing wast
examined for cracks of'signs of failure. All escape rOliles from occupied or working spaces must be checked, Emergency cOll1lnunio;atiOllS 10 the machinery space and the auxiliary steering position from the bridge must also be proved. For tankers. additional special survey requirements include the inspection of all cargo tanks and cofferdam spaces. Cargo tank bulkheads must be tested by filling 10 the lOp of the hatchway of all or :l!lernate 1:lflks. The greater the age of a mlp lhe grealer will be the detail of examination and testing of sus~ct or corrosion.prone spaces. Uqudied gas tankers have requirements for annual surveys. as mentioned earlier, and several additional items. All tanks, cofferdams. pipes, etc., must be gas freed before survey. Who:re the ma.'(imum vapour pressure in the tanks is 0.7 bar or less the inner tank surfaces are 10 be examined. In addition, the tanks must be water tested by a head of 1.45 m above the top of the tank. All tank level devices. gas detectors. inerting arrangements. elc .. must be proved 10 be operating satisfactorily. The special survey requirements are as previously stated. together with the examination internally and externally where possible of all lank areas. Tank mountings. supports. pipe connections and deck sealing arrangements must also be checked. Sa:mples of insulation, where filled, must be removed and the pia ling beneath examined. Pressure-relief and \'3cuum valves must be proved to be efficient. Refrigeration machinery. where fined, must be examined.
225
.
ROlaling ~~
"
''''',
ROlal
.... """"
Hull surveys of very large crude carriers The very size of these ships necessitales considerable planning and preparation prior 10 any sun·ey. large amounts of staging is necessary to provide access to the structure. Good lighting. safe access and some means of t:ommunication are also required. Surveys are often undertaken at sea. with the gas freeing of the tanks being one of the main problems. In·water surveys of the ouler hull are also done. Some thought at the design slage of Ihe ship should enable Ihe stern bush. pintle and rudder bush clearances to be measured in the water. Provision should also exist for unshipping the propeller in the water. Anodes should. be bolted to the shell and therefore easily replaced. Blanks for sealing off inlets should be carried by the ship. 10 enable the overhaul of shipside valves. The frame markings should be painted on Ihe outside of the ship at the weather deck edge to assist in identifying frames and bulkheads. An in·water survey plan should be prepared by the shipbuilder. The hull plaling surface must be clean prior 10 survey. This can be achieved by the use of rotary hand·held brushes which may be hydraulically or pneumatically powered. In·water cleaning of the hull is possible, with divel~ using Ihese brushes or specially designed boats with long rOlaling brushes altached. One particular system uses a 'Brush Karl'. This is a hydraulically·powered vehicle with three brushing heads. It is driven by a diver over the surface of the hull 10 clear the pJa!ing of all forms of marine fouling. The Brush Kart is shown in Figure 12.1. The sheU plating may then be surveyed by using an underwater survey vehicle such as the 'Scan' unit shown in figure J2. 2. The various camera
PW1O<.Jltd '.I~... ,ng pl~l.
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---
.Jock~_
35 n"n 'lOti umwa
l,ght
~ A "bollie W!>llly 10 1J
-
tank
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Figurr 12.1 'Scan' utldtr_/tr sun'l'y I·thir/f' (from 'I4If"Nlocking of IDTKt ships'. in In·k'D/tr MDinttnDnCf! Confurnrl'. 1975. b)' D.F. JonN)
!6
Surveys and Maintenance
lit5 enable dose scrutiny of all the areas of the shell plating by tne surveyor 'semng tile monitoring units. The Scan unit is fully manoeuvI
&
, \
SloIn>eys and M4jnte~~
fl ;;;'''.~ """T'
cracks or damage to the b1ade tips-. It is us-ual to O'xamine any tailshaft seals and also measure the tailshaJt weardown.
Paintwork
xamination in drydock he drydocking of a ship pTOvides a Tare opportunity for examination of the nderwater areas of a ship. Every opportunity ;hou~d th~iefQre ~ taken ~ the lip's staff, the shipowner and tile classification society to examine the ship loroughly. Some -of the more important areas are now listed.
The shell plating shOUld be examined for areas of paintwork whieh must be repaired. The whole surface of the shell will tnen be cleaned and prepared for recoatirtg with paint. In some instances- tile hull may be cleaned down to the bare metal and completely recoated; most situations, however, will only lequire preparation of the surface fDr reeoating.
ht'll plaring
Prepararion
'he snell plating must be-thoroughly examined fm any corrosion of welds, amage, distortion and cracks at openings or discontinuities. Any hull attachlents such as lugs, bllge keels, etc., must be checked for corrosion, security o-f ttacllment and any damage. All openings for grids and sea boxes must also be 'xamined.
Several methods are used for cleaning. the ~hip's hull prior to rt'coatin~ Some of the more common ones will no.. be discussed Martual wire brushing and scraping wl.th steel scrapers usually takes place on the wet surface as the water le..el drops in the dock. The finish is poor, the operation slow and the effecti..eness ..aries accQrding to the ~k.m "nd effcHt of _ the operatives involved. power discing or wire brushi.ng uses either all electrically or. pneumatt~al1y driven machine which is hand held. The method is slow but pr'lVIdes a re1atiVely good finish.. . High pressure water jelling 1; being. lncreasingly used (or hull cleamng. Water at pressures of 150-500 b3r is directed on to the hull b'y.a tubular steel. lance. The lower pressure is sufficient to remove weed fouhng growths, while ~e higher pressure will clean the hull down 'to the bare metal Th,e result~ from thlS method are excellent and very fast, although lime is lost while waiting for the hull to dry. It is, however, a skilled operation reqUiring competent tlamed personnel for efficient safe performance. _ ' Shot-blasting or abrasive-only cleaning utilises a Jet of abraslve at 5-7 bar pressure fired from a nozzle on to the ship's hulL This .method rapidly produces a clean dry surface ready for painting. The dusty, dITty nature of the work, however, stops any other activities in the area. _ Ab-rasive and water-blasting combines in effect the foregomg two methods and claims the advantages of each. The method is fast, dean and effect~ve, th~ abrasive speeding the cleaning and the water suppressing. the dust. Wtth thiS method and water jetting, corrosion inhibitors are added te the water te allow time between cleaning, drying and painting.
"Athodic protection equipment Sacrificial anodes should be checked for security of attaehment to the hull and the degree of wastage that has taken pLace. WLth impressed current systems the modes and reference anodes must be checked, again for security of attachment. The inert mields and paintwork near the anodes ShOllld be examined for any iamage or deterioration.
Rudder
The plating and vwble strUcture of the rudder should be examined for cracks :and any distortion. The dialn plugs should be removed to check fot the entry of any water. Pintle or bearing weardown and clearances should be measured and the security of the rudder stock coupling bolts and any pintle nuts should be ensured.
Sternframe The surface should be carefully checked for cracks, particularly in the areas wh-ere a change of section occurs or large bending moments are experienced. Propeller The cone should be checked for s.ecurity of attachment and also the rope guard. The blades should be examined for corrosion and cavitation damage, and any
Painting The successful application of paint requires the correct technique during painting and roii:able conditioIl-S during which the allplication. takes place_ PElinting should take place in warm dry weather but not ill dtrect sunlight. The presence of moisture in the air or on the metal surface may d~~age th~ paintwork or slow down its curing process. Where poor COndttlOn~. at unavoidable, spedally formulated paints for curing under these- conditionS
228
SuTlleys and Mainterumce
should be used The- use of shelters or awning~ perhaps supplied with warm air will greatly improve curing and adllCsinn of the paint. Any scuppel" discharges or ovcrnows which may direct wa{er on to the surfllce to be painted should be blocked or diverted herore wurk is begun. The principal methods of paint application are the aide-ss spray, th~ air· assisted spray, the roller and the brush. Brush
13
Principal Ship Dimensions and Glossary of Terms Principal ship dimensions
•
A ship is defined and described in size, shape and furm by a number of particular terms, which are listed below and some of which are shown in Figure 13,1. Forward perpendicular. An imaginary line drawn perpendicular to the waterline at the point where the forward edge of the stem intersects the summer load line. After perpendicular, An imaginary line drawn perpendicular to the- waterline, either {I) where the after edge of the rudder post meets the s.ummer load line, or (2) in cases where no rudder post is fitted, the centreline of the rudder pintles is taken. Length between perpendiculars (LBP). The distance between the forward and after perpendiculars, measured along the summer load line. Length overall (LOA). The distance be-tween the extreme point~ of the ship forward and aft. Amidships. The point midway between the forward and after perpendiculars. A special s.ymbol is used to represent this pointJFigure 13.1). Extreme breadth. TIle maximum breadth over the extreme points port and starb-oard of the ship. Extreme draugllt. The distance from the waterline to the underside of the keel. Extreme depth. The depth of the ship from the upper deck to the underside of the keel.
Moulded dimensions are measured to the inside edges of the plating, i.e. they are the frame dimensions. Base line. A hori7.0otalline drawn along the top edge of tne keel from midship&. Moulded breadth. The greatest breadth of the ship, measured to the inside edges of the shell plating. Moulded draught. The distance from the &ummer load line to the base line, measured at the rnid~hip sectiOll. Moulded depth. The depth of the ship from the upper deck to the base line, measured at the midship section. Half-breadth. At any particular section half-breadth distances may be given sinoe a ship is"'symmetricalabout the longitudinal centreline.
Principal Ship DirnemiOJIS arid Glos:sary 7'enns
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231 Freeboard. The vertical distance from tlte summer load waterline t-o the top f the freeboard deck plating, measUIed at the ship's side amidships. The upp:'. mas!: complete de4. exposed to the weather and the sea is normally the freeboard deck, The freeboard deck must have pennanent means of dosl.l re of all openings in it and below it. Sheer. The curvature of the deck in a longitudinal direction. It i~ measured between the deck height at midships and the particular point on the deck. Camber. The curvature of the deck in a transverse direction. Camber is measured b.-tween the deck height at the anlre and the deck height al the side, Rise of 11001. The height of the bottom shell plating above the base line. Rise of floor is measured at the moulded beam line" Bilge radius. The radius of the plating. jOining tne side shell to the bottom shell . It is measured at midships. Plat ot keel. The WIdth of the horizontal portion of the bottom shell, measured transversely. Tumblehome. An inward curvature of the: midship side shell in the region of the upper deck . Flare. An outward curvature of the side shell at the forward end above the waterline. Rake. A line inclined from the "Vertical or horizontal. Parallel middle body. The sfl!p's length for which the midship section is constant in area and shape. Entrance. The imntersed body of the ship forward of the parallel middle body. Run. The immersed body of the ship aft of the paralLel middle body. Displa~ement. The weight of the ship alld its contents, measured in tonnes. The value w.iJI vary accordin,g to the ship's draUght. Ughtweight. The weight of the ship, in tonnes, complete and ready fOJ sea but withoul crew, passengers, SIDres, fuel OJ cargo Dn bDard. Deadweight. The difference between the displacement and the lightweight at any given draUght, again meaSured in tonnes. Deadweight is the weight of cargo, fuel, stores, etc., that a ship can carry. Tonnage. A measure of the internal capacity of a ship where 100 ft J or 2.8:3 m 3 represents I ton. Two values are currently in use - the gross tonnage and the net tonnage. ~
Glossary of terms ,
i
Aft. In the direction of, at, OJ near the stern. Aft peak. A watertight compartment between the aftermost watertight bulkhead and Ine stern. Athwartship. ]n a direction across. the ship, at right·ang.les to the f-ore and aft centreline. Ballast. A weight of Liquid positioned in a ship- to change the trim, increase the draught or improve the seaworthinesS-. Bilge. RoundeC: region between the side and shell plating; (he space where watCJ wllects after draining down [rom cargo holds, et-::. Bitter end. The enu of the anchor cable which is secured in the chain locker by the dench pin.
232
Prlndpll1 Ship Dimensions and GloUQ1')' Temu
Bollard. A pair of short metal columns on a rigid baseplate which are used to secure the mooring ropes or wires. Bow. The forward end of a ship. Bracket. A plate which is used to rigidly connect a number of structural parts: it is often triangular in shape. Break. The paint at which a side shell plating section drops to the deck below, such as the poop or forecastle. Bulkhead, aft peak. The first major transverse watertight bulkhead forward of the sternframe. Bulkhead, collision or forepeak. The foremost major watertight bulkhead. Cofferdam. A void or empty space between two bulkheads or floors which prevents leakige from one to the other. Cowl. The shaped top of a natural ventilation trunk which may be rotated to draw air into or out of the ventilated space. IJreep tanks. Tanks which extend from the shell or double bottom up to or beyond the lowest deck. They are usually arranged for the carriage of fuel oil or water ballast but may be filled with hatches and used for cargo. DeviJ's claw. A stretching screw with two heavy hooks or claws. It is used to secure the anchor in the hav.rse pipe. Dog. A small metal fastener or clip used to secure doors. hatch covers. etc. Erection. The positioning and temporary fastening together of units or fabricated paru of a ship prior to welding. Fabrication. The various processes which lead to the manufacture of structural paru for a ship. Fair. To smoothly align lhe adjoining parts of a ship's struclUre or its design lines. F.airlead. An item of mooring equipment used to maintain or change the direction of a rope or wire in order to provide a straight lead to a winch drum. Flange. The portion of a plate or bracket bent at right-angles to the remainder: to bend over at right angles. Flat. A minor section of internal deck often without sheer or camber. also known as a platform. Forepeak. A watertight compartment between the foremost watertight bulkhead and the stem. Forward. In the direction of. at, or near the stem. Frame. A transverse structural member which acts as a stiffener to the shell and bottom plating. Gasket. A joint. usually of flexible material. which is positioned between metal surfaces to prevent leakage. Gooseneck. A fitting on the end of a boom or derrick which connects it to the mast or post and permits a swivel motion. Grommet. A ring of soft material positioned beneath a nut or bollhead to prOVide a watertight join!. Gudgeon. A solid lug on the sternframe or rudder which is drilled to take the pintle. Gusset plate. A bracket plate usually positioned in a horizontal or almost horizontal plane. Holds. The lowest cargo stowage compartments in a ship. Inboard. In a direction towards the centreline of the ship.
IJbrIensJons and GloUQ1')' o{l'erms
ljj
Intercostal ,ts. non.rd 0 ._, which replaces tWO narrow plates in adjal;ent S tlCiUer strake. A sll1g1e WI e p.. e strakes. . 'rr I" Stem. The after end of a ship. Stiffener. A nat bar. section or built-up section used to Sll en p aung. Tarpaulin. A tough waterproof canvas-type cloth cover used to cover nonwatertight hatch covers. TiUer. A casting or forging which is keyed to the rudder stock :lOd used to tum
r b d 10 the rudder. .. " Wlte. " A Wlt...-. " .......d to raise , lower or fIX the posttlon 0 a oom an Toppmg support it. . r h h" " Transverse. A direction at right-angles to the centrehne 0 t e s Ip or an Item of structure in lhis position. Tripping bracket. A nat bar or plate fitted to a deck girder, stiffener, beam. etc., to reinforce the free edge. . r Trunk. A passage extending through one or more decks to proVlde access 0 . . ventilation to a space. Tunnel. A watertight access passage surrounding ~~e propeller shaf~ w~lch IS filled on a ship where the machinery space is pOSItioned towards midships. Tween decks. The upper cargo stowage compartments or the space between any twO adjacent decks. . Uptake. A metal casing or large bore piping which carnes eud1aust gases up through the funnel to the atmosphere. . '. Web frame. A deep-section built·up frame which proVldes additional strength to the structure. WeU. A space into which bilge water drains.. . . . Winch. A machine which utiUses the wmdmg or u~wmdmg of rope or wire around a barrel for various cargo and mooring duties. Windlass. A machine used for hoisting and lowering the anchor.
232
PrinctpaJ Ship Dimen.dolU and Glossary Terms
Bollard. A pair of short melal columns on a rigid baseplate which are used to secure the mooring ropes or wires. Bow. The forward end of a ship. Bracket. A plate which is used 10 rigidly connect a number of struClural parts; it is orten triangular in shape. Break. The point at which a side shell plating section drops 10 Ihe deck below, such as the poop or forecastle. Bulkhead, aft peak. The first major transverse watertighl bulkhead forward of the stemfnme. Bulkhead, collision or forepeak. The foremost major watertight bulkhead. Cofferdam. A void or empty space between two bulkheads or floors which prevents leakage from one to the other. Cowl. The shaped top of a natural ventilation trunk which may be rotated to draw air into or out of the ventilated·spacc. Deep tanks. Tanks which extend from the shell or double bottom up to or beyond the lowest deck. They are usually arranged for the carriage of fuel oil or water ballast but may be fitted with hatches and used for cargo. Devil's claw. A stretching screw with IWO heavy hooks or claws. It is used to secure the anchor in the hawse pipe. Dog. A small metal fastener or clip used to secure doors, hatch covers, etc. Erection. The positioning and temporary fastening together of units or fabricated parts of a ship prior to welding. Fabrication. The various processes which lead to the manufacture of structural parts for a ship. Fair. To smoothly align the adjoining parts of a ship's structure or its design lines. F.air~d. An item of mooring equipment used to maintain or change the direction of a rope or wire in order to provide a straight lead to a winch drum. Flange. The portion of a plate or bracket bent at right-angles to the remainder: to bend over at right angles. flat. A minor section of inlernal deck often without sheer or camber. also known as a platform. Forepeak. A watertight compartment between the foremost watertight bulkhead and the stem. Forward. In the direction of. at, or near the stem. Frame. A transverse structural member which acts as a stiffener to the shell and bottom plating. Gasket. A joint. usually of flexible material. which is positioned between metal surfaces to prevent leakage. Gooseneck. A fitting on the end of a boom or derrick which connects it to the mast or post and permits a swivel motion. Grommet. A ring of soft material positioned beneath a nut or bolthead to provide a watertight joint. Gud~n. A solid lug on the stem frame or rudder which is drilled to take the pintle. Gusset plate. A bracket plate usually positioned in a horizontal or almost horizontal plane. Holds. The lowest cargo stowage compartments in a ship. Inboard. In a direction towards the centreline of the ship.
Principal Ship Dimensions and Glossary a/Terms
233
Intercostal. Composed ofseparale parts. non
Index
• A-brackets. 104, IDS
Bridge SlfUcture. 117
'A' class bulkhcids, 201. 202 Accommodation. 120, 121 Aft peak bulkhead, 87. 232 Afl end construction, 101-116 Au pipe. 76.194, 208 Aluminium, alloys, 27, 28 rMU, 27, 28
Bulk carrier, 7-I 0, 180-184 BulkhC3d. 85, 87-90 ',4,' cla5s. 201, 202 collision, 89, 232 conup.ted. 89-90, 126. 130, 170. 171 fonpak. 89, 232 IongitvdirW, 170 non'W:llertight,9O oiltlahl, 85, 87.89. 1]0, J 70 lesting.90 Ir.insvcrse,171 wash, 93-95, 130, 171 waterti.!hl,87-89 Bulwarks, 128, 129. 173, 209 Butts, 77
seedORS, 28
Aluminium/steel conn«tions. 28. 222 Anchon and ables, 96-99 Anodes, 219-221 ArC wdding, 53-60 Assembly, 39, 40 'S' tins divisions., 201. 202
Ballast, 231 Pipinllsystcm, 148, 149 tanks,148, ISS
"'m.
eant, 102, 103 dedi:. 82-83 half, 83
Beam kn«. 18,79 Ikndin&. eqWltion, 17
mommt, 14-16 Bil&c, 231 kC'tI.80 mud OOlt,I47 piping system, 146-148
strum box, 147 Biner end, 231 Block coefftcient, 203 BoU-ofT, 178 Bollard, 137, 138.232 Bottom ruuetufll. 73 in Iankers,167 Bo...... 232 bulbous,9S stopper. 97,98 thrust unit, 100, 101
Cabin, 124 Cable clench assembly, 99, 100 Cable Slopper, 97,98 Cargo pumping system, 153-155, 180 Cathodic protection, 218-222. 226 Impressed curren I. 219, 220, 223 of tanks, 220 sacrincial anode, 219 cavitation, 108 Chain locker. 97, 99 CbssiflClltion societies, 197-199 hull malerial teslS, 23, 26 ....dd tests, 68 Oench pin, 99,100 Coamings, 85. 86 .... CotTerdam, 165, 173, 180, 232 Container ship, 10, 11 Corrosion, 213 cell, 213, 214, 222 pnvcntion, 214, 222 C,anes, dec::k, 144, 145 shipyard,49-50 Crude oil washing (COW), 200 Cruiser slem, 102, 103 Cutting prOCC$$eS, 68-71
235
236
Index
CUlling processes (rollt.) ps.68-69 pluma arc. 70 Dead.... cipll. 7.10.231 Deck CDnt'. 144. 14S platform. 145.141 DeckhouSt'. 116. 117 [kelts.81-85 b<'ams.82 prdcQ.83 platin,.82 stiffening. 82 Derrick rip. 141_144 heavy lift. 143. 144 s..... in~n~. 142 union purchase. 141. 142 yo-yo. 142. 143 Devil's cia...... 232 Discontinuities, 20.85.117 Displacement. 4. 7. 231 Distonion. correction. 65-61 pn:..cnlion, 63-65 Doc:kin5bDckel,161,168.110 Doc:kin& p1u!- 16 ~.119.232
000•• W3ten~hl. 157,160. 161 wealhcnight.117.119 Double bollom. longitudinally fr.lmcd, 15 machincry spacc. 75-76 tanks. 76 uansvcrsdy frdmcd. 75 Drain hat. 73. 74 Dra..... in)!. office. 32 Duct keel, 73
Edge prepar.ltion, 44--46. 60. 63 Ek<:trodcs.54-56 Enpneo:asin;..I25-127 ErCl;tion. 39. 5 I. 232 Erosion. 213 Eumin;Uion in dry dock. 226-228 Fairin~. 32. 34. 36. 232 Fairlead. 137-139.232 multi~n~. 137. 138 panama. 137-139 pedestal. 137, 138 roller. 137-139 Fire main. 148. ISO Fire safety in ships. 200-202 Flame pl;aner. 44, 45 Flal marltin. 73, 74 !'Iat pbte keel. 72 Floor. bnoekel.13_15
Index Floor (oorr/.) lo.....er hopper tank. 172 solid. 73-75 "'atertil:ht, 73-75 Flux. 54 Fore end construetion. 92-95 I'orec;mle. 117. 118 Fname.232 anI. 102. 103 $pt'Ctaek. 104.105.233 ..-eb. 80. 171. 232 I'r.lmc bender. 49 FraminJ,78-80 at t:nds (oill;mkers). 171 combined. 79.80, 167.168.180 lonl;itudinal. 79. 80,166,167.183 transverse. 79. 80, 166. 167. 183 Frccbollld. 202. 205. 2JI calegorks. 203. 204 condilions of assignment. 206-209 corrections. 204. 205 Freeing POliS. 129.208 Funnel. 125, 126
Interlowernmental t.britime Consult;ative Organisation (IMeO). 20. 188. 190-193 International t.britimc: orpri'h:ation (IMOI.197.199-202 K«I.71-73 biJ&e.80.81 duet. 73 rht pbte, 72 Keel plate. n Korl noz.zJe. III L:tmellar tearing. 22. 67 Lines plan. 31,33 Liquefied gas tanken. 1. 174, 176. 180 natural ~a5. 7. 176-178 petroleum ~. 7. 178-180 Uoyd's Register of Shipping, 23. 197. 198 Lo;ad lines. markinp. 205. 206 ruks.202-209 Loading. dynamic. 16 local. IS. 19.83 Slatie.15.17
G;alvank krks. 213. 214 Gap press." 7 Gn ,,·e1dinl!. 51 Gener31 ~o ship. 1-4 Girder. cenlle, 72. 74. 167 deck,83 longitudinal. 73 side. 73. 74.167 Gouginj(. 70- 71 Gravity welder. 54-55 Gross tonnagl·. 210.21 I Gudgeon. 112.232 Guillotine. 49 Gun .....a1e.77_78
Machinery seals. 13 I. 132 Main vertical (fire) zones. 201 Manhole. 113 Marilin plate. 73. 74 Masts.140 . Materials, handling. 40 handllns equipmcnl. 49 preparation. 4 1 Meehania;1 pbner. 44--46 Mooring equipment. 1]6-139 Mould loft. 36
HaJr41readth plan, 31. 33 Halch. arlO I:Ink.171,173 coamin5. 85, 86 rowers. 134-136 minor. 136. 137 opt-nings. 84. 85 steel. 134-136. 207 wooden. 134. 135. 207 Hawse pipe. 96-97 Ho~inl:. 15. 16 Ice strenj(thening. 80-81 Inelll!asplant.I74.175 In~1I plate. 84. 85 Insulation. aeouuic,I57_159 thermal. 155.156 1n!t:rcoJtal. 73-75. 77, 133
Natural pJ. 7. 174. 176 Nt'$tc:d pbtes. ]5. 38 NOlches.167.169 Notching press. 48 Numerical conuol. 36-38 OfrSt'ts. ]4. ]6. 37. 233 Oilfbulkforc carrier, 180 Oil lanker. 4. 5. 7.165-174.199-200 Ore carrier. 7.9 Ore/oil carrier. 9. 180 O"y-Qcct)'kne cuning. 69 O"yoQCl:t)'lenc wddinll. 52
f
hints. 215-218 anti4ouun~. 215.216
237
Paints (corll.) application. 227. 228 primil'lg. 41. 217 wehides.215 I';aint.....ork. cnmin;tion.221 rtcoaling. 227 surface prep:&r.ltion. 41. 211 l:;anks. 218 toPJide and superstructures. 217 undel"'"'llter areas. 217 ..-uthcr docks. 217.218 Panclline. SO. 5 1 Panting. 19.9].233 structure 10 resist. 93-95 Passenger ship. 10. 12 Petroleum I\lls, 7. 176 Pillars. 90-92 Pin ties. I 12-114. 23] Pipes. air. 76.194. 208 $Cunding. 76_77.152.153 Pitching. 13. 19 Plans. 32-]4 Pbte. b<'ndin,. 38 cutting. ]8.43.44 eddy,lll fa,Ulion, 1I 2 prepal1ltion. ]8.4\ roUs machine. 41. 43. 41-48 S1l"3ightening.41 ~trinj(er. 82 Platinj(. deck. 82 shell. 77 Poop S1fuctUl"C. 117. 118 Pounding. 19. 77 Products tanker. ISS. 165 Prol11e-culling machine. 44 , Propeller. 107-111 controllable pitch. 108-111 <",amina-lion. 226, 221 fi"ed, pitch. 101. 108 mountinjl. 108. 109 ske..... back. 108 Tip Vorte:'O. Free. III Voilh·Sneider. III Pumpin~ and pipinJ arran~ments. 147 -ISS Pumproom ..... nIUation.193. 194 Punehinl: press. 48 Racklnjl. 18 Raisl'd. quarter deck. 117 Refri~crated t:cner.i1 cargo ship. 4 Ring press. 47 Ruddcl.III-113.226 axles. 112 babnced. 103. 112 tarrier. 112-1 16 uamin;ation.226
2:38
Index
Index
Rudder (conI.) pintle,,112,11] ~emi-h"l:mced, 111, i 12 slock 113,114 trunL,102-1I14 unha1a[l~ed, 111-113 Sacrificial anndes, "219. no Sagging, 15, 16 Samson posts, 141,233 Scantlinp, 73, 3D, 2:03 Scri"ve board, 36 Scupper" 151, 2111:1, 233 tup, 155, 1.57 Sea inkt box, 132, 133 Sea tube~, 132, 133 Seams, 77 Seats, 233 S:cawo.thy,2,-233 S:e~ond:lIY barrier, 178-181 S.cction mndu11.1" 17 Self-polislling antifouling paint, 216 Shaft tunnel, 127,128 Shear force, 14 16 Shedder plates, 11:!3, 184 Sh",er plan, 32 Sheerstrake, 77 Shell pbJing, 77-81 Shipyard. layout, 38-39,42 wddill.g equipment. 51 ~h()tblaning, 41, 227 ling1e-hottom structure, 77, 169, 170 Slammiuj';, 19 Sounding pipes, 76-77,152,153 Spectade frames. 10-4, 105.233 Spurling pip~, 9'7, 99 Stabilh~rs, 162-164 fin, 162, 164 tank, Hi3, 164 5taHcllS,31,33 ~lealcI ~trake, 78, 233 Steel, 20-26 casting>, 16 nyugcnic, 25, 2& fini~hing tre~tm~nt, 21 furgings,26 l1it:her tensile, 24, 2:6 producti{ln, 2: I propertiC.l, 22:, 24 shi-p-lJ.uilding.23 ~\aTHlald ~eoCti()m, }1 "tern, 92, 93 Stern, cruiser, 102, 103 tramom, 103,104 iternframe, Hill 104 examinati()'l,226 ,terntube, 1G6, J 07 ,tiffener, 75, 30, 233
Stiffening, bulld,,,ad,87 de~k, I'll
local luading:, 83 Still water bending mument (S-\\'BM), i5 Strain, 29, 3() Strake, 72, 77--78, 8.2: Stress,28 30 compres,iw,28 proof, 3\ shea,. 28 tenslk,28Stress-strain grl, 223, 224 Table of offset~, 36 Tank top, 7(, lank lyp~s, LNG, 176-178, 180 lPG, 178-180 Tanh.s,4 7 Tanh, balla~t, 148.155 (leep, 129, 130, 232 doub1~-bottom, 7{,-77 Clopper, 180-182 segr("gatecl b:>llast (S BT), 200 Tarpaulins, 134, 135, }07, H3 Tes!.
bencl,31 dLlmp,31 impact (Charpy), 3 \ temlle,19 Te,tin!, materials, 2!l Thruster. ]()O, 101 azimllth,IOl dllcted jet, 10 1 gill jet, 101 h;'(llCJ jet, 101 tunnel, 100, 10J
TilleI:. 116,117,233 Tonn~g~, 2()')-211 British,210-212 cOn\rcnti(ln i 969, 212 decLl10 gross. 210,211 marL, 211, 212 noel, 211 Tr
]\)3, Hl4
Transversc, 133 d.:ck,81-83 Trip-ping bacht, l:l3, 233 Tunnd,sh
Ventilation, 185-196,202 accClmmodation, 185-, 186 cargo space, 186-190 cargo tanks, 194, 195 corH.r\J1 [(loms, 191 193 double-bMtom tanks, 194 mac1lineryspaccs, 190-193,202 mcchani<:a.l, dosed, ISS, 189 open, 188 [l
VentiLation (c'.Jrlt.' ~inglcJuct wilh reheat, 18-6, lIn twin duct, 185, 186 Venlilatm, 207 hcad,195,196 Vibration pust, 1()4 Vibration, ship, 10, 1lI4, 108 Water!i~ht dorm, 157, lbll_ 161 Weathe;tight dorm, 117, 119' Web frame, 80, 171,232 Weld, bad, 66, 67 guod.66,67 t~stiflg, 68 types, 60-62 Welding, 52-58 "utom;lli~, 55--56 back-step, 64. 6:5 chain, 62 ele<:tric arc,S 3 c1eetrocle,,54-56 "lectIog;t" 5 7 electrosl"g, 56-57 g~s, 52 in termittent, 62 manual, 53 n:eM inert gas (YHGj, 58 59 plasma metal inert gas, 59, 60 po~itions, 54 practice, 62--65,kip, tiA, 65 stud, 57-58 the-rmit,:59-60 tungsten inert gas (TIG), 58 wandering, 64, 65 Windlass, 97, 1 37, 233
•
239
