Design of Dredging Equipment(TUDelft)

Chapter 1 Introduction

1. Introduction to Dredging Equipment 1.

Introduction to Dredging Equipment .......................................................................... 1 1.1. Introduction......................................................................................................... 1 1.2. Types of dredging equipment ............................................................................. 2 1.3. Mechanical dredgers ........................................................................................... 3 1.3.1. The bucket ladder dredge................................................................................ 3 1.3.1.1. General........................................................................................................ 3 1.3.1.2. Working method ......................................................................................... 5 1.3.1.3. Area of application...................................................................................... 6 1.3.2. Grab or Clamshell dredger.............................................................................. 7 1.3.2.1. General........................................................................................................ 7 1.3.2.2. Working method ......................................................................................... 7 1.3.2.3. Area of application...................................................................................... 9 1.3.3. Hydraulic cranes (Backhoe and front shovel)............................................... 10 1.3.3.1. Working method ....................................................................................... 11 1.3.3.2. Area of application.................................................................................... 12 1.4. Hydraulic dredgers............................................................................................ 13 1.4.1. Plain suction dredger..................................................................................... 13 1.4.1.1. General...................................................................................................... 13 1.4.1.2. Working method ....................................................................................... 15 1.4.1.3. Area of application.................................................................................... 16 1.4.2. Barge unloading dredger............................................................................... 17 1.4.2.1. General...................................................................................................... 17 1.4.3. The cutter suction dredger ............................................................................ 18 1.4.3.1. General...................................................................................................... 18 1.4.3.2. Working Method....................................................................................... 19 1.4.3.3. Applied working area................................................................................ 21 1.4.4. The bucket wheel dredger............................................................................. 22 1.4.5. Trailing Suction Hopper Dredger ................................................................. 23 1.4.5.1. General...................................................................................................... 23 1.4.5.2. Working method ....................................................................................... 24 1.4.5.3. Applied working area................................................................................ 26 1.5. Conclusion ........................................................................................................ 27

1.1. Introduction Definition: A dredgers is a piece of equipment which can dig, transport and dump a certain amount of under water laying soil in a certain time. The quantity of soil moved per unit of time is called Production. Dredgers can dig hydraulically or mechanically. Hydraulic digging make use of the erosive working of a water flow. For instance, a water flow generated by a dredge pump is lead via suction mouth over a sand bed. The flow will erode the sand bed and forms a sand-water mixture before it enters the suction pipe. Hydraulic digging is Prof.Ir. W.J.Vlasblom

Pagina 1 van 27

May 2003

Wb3408b

Designing Dredging Equipment

mostly done with special water jets. Hydraulic digging is mostly done in cohesionless soils such as silt, sand and gravel. Mechanical digging by knives, teeth or cutting edges of dredging equipment is apply to cohesive soils. The transport of the dredged soil can be done hydraulically or mechanically too, ether continuously or discontinuously. Continuously

Hydraulically Transport via pipeline

Discontinuously

Mechanically Transport via conveyor belts Transport via grab, ship, car

Deposition of soil can be done in simple ways fi by opening the grab, turning the bucket or opening the bottom doors in a ship. Hydraulic deposition happens when the mixture is flowing over the reclamation area. The sand will settle while the water flows back to sea or river. Dredging equipment can have these three functions integrated or separated. The choice of the dredger for executing a dredging operation depends not only on the above mentioned functions but also from other conditions such as the accessibility to the site, weather and wave conditions, anchoring conditions, required accuracy and so on.

1.2. Types of dredging equipment Dredging equipment can be divided in Mechanical Dredgers and Hydraulic Dredgers. The differences between these two types are the way that the soil is excavated; either mechanical or hydraulic.

Mechanical dredgers are

Bucket ladder dredge

Prof.Ir. W.J.Vlasblom

Pagina 2 van 27

Grab dredge

May 2003


Dipper and backhoe dredge Hydraulic dredgers are:

Cutter dredge

Plain suction dredge

Trailing suction hopper dredge All dredgers except the trailing suction hopper dredgers are stationary dredgers, which means that they are anchored by wires or (spud)poles.

1.3. Mechanical dredgers 1.3.1.

The bucket ladder dredge

1.3.1.1.

General


Pagina 3 van 27

May 2003

Wb3408b


The bucket ladder dredge “Big Dalton” The bucket ladder dredge or bucket chain dredger is a stationary dredger, which has an endless chain of buckets carried by the so-called ladder, positioned in the well of a Ushape pontoon. The chain is driven by the upper tumbler, a pentogonal, at the upper part of the ladder and fixed at the bottom with lower tumbler, mostly a hectagonal. Under the ladder the chain hangs freely, while on the upper site of the ladder the chain is supported and guided by rollers. The buckets filled during their rotation over the lower tumbler are emptied by the rotation over upper tumbler. The soil from there guided via shutes to an alongside layer barge. Bucket sizes vary from 30 liters to 1200 liters. Rock bucket dredgers do have a double set of buckets; a small rock bucket and a bigger soft soil bucket.


Pagina 4 van 27

May 2003


1.3.1.2. Working method The bucket ladder dredge is positioned on 6 wires. Under working conditions the dredge swings around her bow anchor. The bow anchor line or headline can have length longer than 1000 m. In order to avoid dragging of the wire over the soil, which results in a smaller radius, the wire is supported by a headline pontoon. As a result of this long headline the cut width can be large as well (200 m or more). The sideline winches take care of the swinging of the dredge as well as the power necessary for the cutting process. The swing speed depends on the spoil condition, the layer thickness cut and forward step (pawl length)


Pagina 5 van 27

May 2003

Wb3408b

Designing Dredging Equipment Bow anchor

Headwire Forward ground anchor PS Headwire pontoon

Forward side anchor SB

"Pawl" length

Swing over

Aft ground anchor PS Aft side anchor SB

Groundlevel

Dry excavation

Dredge profile

Spillage

Cutwidth

Stern anchor

1.3.1.3. Area of application A bucket dredgers can be applied in almost all soils, from soft silt and clays to soft rock depending on the power on and the strength of the bucket chain. They are use in blasted rock as well. The maximum dredging depth depends on the size of the dredger. Bucket ladder dredgers with a maximum dredging depth of over the 30 m are built. However for such dredgers the minimum dredging depth is almost 8 m. Prof.Ir. W.J.Vlasblom

Pagina 6 van 27

May 2003

Chapter 1 Introduction Nowadays they are often used for dredging contaminated mud, because the can dig the soil under in situ density conditions. The bucket ladder dredge can not applied under offshore conditions and is certainly an obstruction for shipping. Compared to hydraulic dredgers he production is rather low.

1.3.2.

Grab or Clamshell dredger

1.3.2.1. General The grab dredger is the most common used dredger in the world, especially in North America and the Far East. It is a rather simple and easy to understand stationary dredger with and without propulsion. In the latter the ship has a hold (hopper) in which it can store the dredge material, otherwise the material is transported by barges. The dredgers can be moored by anchors or by poles (spuds) The capacity of a grab dredger is expressed in the volume of the grab. Grab sizes varies between less than 1 m3 up to 200 m3. The opening of the grab is controlled by the closing and hoisting wire or by hydraulic cylinders. 1.3.2.2. Working method For grab dredgers the method of anchoring and the positioning system plays an important role for the effectiveness of the dredger. At every pontoon position an area as wide as possible will be dredged. Looking from the centerline the volume to be dredged at the position decreases with the angle to the centerline. The positioning is important to localize the bit of the grab. This helps the dredge master to place the next bit after the fore going. Prof.Ir. W.J.Vlasblom

Pagina 7 van 27

May 2003

Wb3408b


Releasing the aft wires and pulling the fore wires does the movement of the pontoon. When the dredgers have spud poles, this movement is done by a spud operation, which is more accurate than executed by wires. 15 %

37 %

48 %

1* step

ste

Center line

tep 7s 0.8

0.5

p

30 60

o

o

Dredge pattern

The dredging process is discontinuously and cyclic. 1. Lowering of the grab to the bottom 2. Closing of the grab by pulling the hoisting wire 3. Hoisting starts when the bucket is complete closed 4. Swinging to the barge or hopper 5. Lowering the filled bucket into the barge or hopper 6. Opening the bucket by releasing the closing wire. The principle of this hoisting operation is given in the figure below. In order to avoid spinning of the clamshell a so-called taught wire is connected to the clamshell.


Pagina 8 van 27

May 2003

Chapter 1 Introduction Hoist winch Top shieves

Hoist wires

Gear segments

Upper sheave block

Closing winch Closing wires

Bucket

Gear segments

Lower sheave block

1.3.2.3. Area of application The large grab dredgers are used for bulk dredging. While the smaller ones are mostly used for special jobs, such as: • Difficult accessible places in harbors • Small quantities with strongly varying depth. • Along quay walls where the soil is spoiled by wires and debris • Borrowing sand and gravel in deep pits • Etc. The production of a grab depends strongly on the soil. Suitable materials are soft clay, sand and gravel. Though, boulder clay is dredged as well by this type of dredger. In soft soils light big grabs are used while in more cohesive soils heavy small grabs are favorable. The dredging depth depends only on the length of the wire on the winches. However the accuracy decreases with depth.


Pagina 9 van 27

May 2003

Wb3408b

1.3.3.


Hydraulic cranes (Backhoe and front shovel)

Hydraulic cranes are available in two models the backhoe and the front shovel. The first is used most. The difference between those two is the working method. The backhoe pulls the bucket to the dredger, while the front shovel pushes. The last method is only used when the water depth is insufficient for the pontoon. These stationary dredgers are anchored by three spud poles; two fixes to the front side of the pontoon and one movable at the aft side. This means that the dredging depth is limited to about 15 m. (maximum 25 m). At the front of the pontoon is normally a standard cranes mounted. Here pontoon deck is lower to increase the dredging depth. Bucket sizes vary from a few m3 to 20 m3.

Backhoe dredge


Pagina 10 van 27

May 2003


Front shovel 1.3.3.1. Working method During dredging the pontoon is lifted a few out of the water by wires running over the spud poles. A part of the weight of the dredger is now transferred via the spuds to the bottom, resulting a sufficient anchoring to deliver the required reaction for the digging forces. Besides that the dredger is in this case less sensible for waves. The bucket is placed and filled by hydraulic cylinders on the boom and the bucket arm. Due to the small radius of the boom and arm is the cut width limited to 10 to 20 m, see figure below. 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0


Pagina 11 van 27

May 2003

Wb3408b


The effective dredging area depends on the swing angle and the forward step per pontoon position. A small step results in a large width and a large step in a small width, however the total area is almost the same.

1.3.3.2. Area of application This is roughly the same as for the clamshell dredgers with the exception dredging depth over the 25 m


Pagina 12 van 27

May 2003


1.4. Hydraulic dredgers 1.4.1.

Plain suction dredger

1.4.1.1. General A plain suction dredger is a stationary dredger that position on one ore more wires, with at least one dredge pump, which is connected to the suction pipe and the delivery pipe. The suction pipe is situated in a well in front of the pontoon. Good production can only achieved by this kind of dredgers either the soil is free running sand or the cut or breach height is sufficient (at least 10 m) The discharge of the soil sucked is done either by pipeline or by barges. Most suction dredgers are equipped with jet water pump(s) to assist either the beaching process or to improve the mixture forming process near the suction mouth. Types of plain suction dredgers There are different types to be distinguished. 1.

Barge Loading suction dredger


Pagina 13 van 27

May 2003

Wb3408b


28

18 24

16

22

11

31

13

25 19

10

23

17

26

21

20

27

14 15

Used when the transport distances are too large for direct pumping 2. Standard plain suction dredger

Discharged the material direct via pipeline to the reclamation area. 3.

Deep suction dredger


Pagina 14 van 27

May 2003


6

22 23 24

5 1

17/18

11

28

6

27

19/20

7

16

10

9

30

29

34

11

31

14

12

12

35

33

35

33

13/15

32

32

2 2

3

8

8

This dredger is equipped with an underwater pump and have two appearances; the standard or from the barge loading type. When dredging depth exceeds the 30 m this dredgers is more appropriate than the standard one. 4.

Dustpan dredger

A suction dredger with a wide suction mouth, which makes it possible to dredge with reasonable productions low cut heights. 1.4.1.2. Working method The working method is based on the “breaching process” and the erosion created by the flow near the suction mouth, generated by the dredge pump. Breaching is a process of soil shearing on a slope caused by local instabilities or by erosion of the density current running along the slope to the suction mouth


Pagina 15 van 27

May 2003

Wb3408b

Designing Dredging Equipment z

x

Instabilities

H br

Sand-water mixture (density current) Suction tube Vz

Breach This process is essential for this type of dredger and is fully determined by the soil conditions of the slope, from which the permeability and the relative density re the most important parameters. The dredge patron made by a plain suction dredger is shown below.

The length of the cut depends, inside the borrow area, on the position of the anchors. Mostly the anchors are laid down in such a way that more cuts can be made without repositioning the anchors. However this depends not only of the length of the anchoring wires but also from the “breachebility” of the soil. 1.4.1.3. Area of application Due to the lack of cutting devices this type of dredger is only suitable in non-cohesive soils. Further more this method exclude accurate dredging work. Dredging under Prof.Ir. W.J.Vlasblom

Pagina 16 van 27

May 2003

Chapter 1 Introduction offshore conditions is possible with special equipment. As already said borrowing in deep pits of over 100 m depth is possible. These types of dredgers are frequently used in borrow pits for reclamation areas as well as for the borrowing of sand for the concrete industry.

1.4.2.

Barge unloading dredger

1.4.2.1. General Barge unloading dredgers are used for emptying loaded barges either by suction dredgers or by bucket ladder dredgers and cranes. The barge-unloading dredger is a stationary special suction dredger anchored by spuds near the shore, where the water depth is sufficient for the loading barges to come along side the dredger. The water for the unloading and the transport is supplied into the barge by a jet.


Pagina 17 van 27

May 2003

Wb3408b

1.4.3.


The cutter suction dredger

1.4.3.1. General The cutter suction dredger is a stationary dredger equipped with a cutter device (cutter head) which excavate the soil before it is sucked up by the flow of the dredge pump(s). During operation the dredger moves around a spud pole by pulling and slacking on the two fore sideline wires. This type of dredger is capable to dredge all kind of material and is accurate due to their movement around the spud. The spoil is mostly hydraulically transported via pipeline, but some dredgers do have barge-loading facilities as well. Sea going cutter suction dredgers have their own propulsion, however this is only used during (de) mobilization. Cutter power ranges from 50 kW up to 5000 kW, depending on the type of soil to be cut.

Custom build dredger Prof.Ir. W.J.Vlasblom

Pagina 18 van 27

May 2003

Chapter 1 Introduction The more powerful dredgers are capable to dredge rock The small and medium size cutter suction dredgers are deliverable in a demountable application. In that case the hull consists out of five or more pontoons. The central pontoon contains the machinery.

Standard Beaver dredger 1.4.3.2. Working Method The rotating cutter excavates the soil during their movement, generated by the side winches, form port side to starboard and vise versa. The necessary side winch force depends not only on the type of soil but also on: • The rotation direction of the cutter head; (over cutting) rotation in the direction of the swing movement or (under cutting) opposite to that.

Ds

Ds Under cutting mode

Over cutting mode

In the over cutting mode the cutter head tries to drag the cutter dredger in the direction of the pulling winch. Braking with the opposite winch may be necessary. • The position of the anchors in relation to the path of the cutter head. The more the anchor lies in the direction of the moving cutter head the less the required side winch force will be. • External forces, such as wind, current and waves. Prof.Ir. W.J.Vlasblom

Pagina 19 van 27

May 2003

Wb3408b


The thickness of the layer, which can be cut in one swing, depends besides on the soil conditions also on the size of the cutter head. At the end of the swing will either the ladder be lowered and the dredger is swung in the opposite direction or the dredger will make a “step” forwards.

As said earlier the dredgers swings around a pole the working spud, which is positioned mostly in a carriage. The spud carriage can be moved over a distance of 4 to 6 m. by a hydraulic cylinder. When the working spud is set on the ground the dredger is pushed forward when the cylinder pushes against the carriage. This forward movement is called step and depends also on the soil conditions and the size of the cutter head. During a step the breach is cut in one or more cuts. Cut width

Workspud in carriage

Spud carriage length Auxilary spud

Vertical swing pattern


Pagina 20 van 27

May 2003

Chapter 1 Introduction Because the spud stays on the same spot the dredger makes concentric circles during swinging. Is the stroke of the hydraulic cylinder is maximum the dredger is moved to the centerline of the cut where a second spud at the aft side of the pontoon, the step spud, is lowered. Where after the working spud is hoisted and the carriage is pulled back, the working spud lowered to the ground and the step spud hoisted again. The dredger can make a new cycle again. 1.4.3.3. Applied working area Cutter suction dredgers are applied for dredging harbors, channels, reclamation areas and so on. The transport distance of the mixture is limited to maximum 10 km. She is very useful when the accuracy of the works is important. As said already the cutter dredger can dredge all kinds of soil.

Rock Cutter

clay cutter

For dredging under offshore conditions is this dredger less suitable.


Pagina 21 van 27

May 2003

Wb3408b

1.4.4.


The bucket wheel dredger This dredger is, with the exception of the cutter head, is comparable with the cutter suction dredger. The rotation axe of the bucket wheel is perpendicular with the ship axe. The wheel contains 10 – 14 open or closed buckets. Due to the construction of the drive the wheel is difficult to replace and therefore less universal than the cutter suction dredger. Is application area is the same as the cutter dredger with the exception of hard rock. This dredger is often used in areas with constant conditions, such as the sea mining.


Pagina 22 van 27

May 2003


1.4.5.

Trailing Suction Hopper Dredger

1.4.5.1. General A Trailing Suction Hopper Dredger (TSHD) is a self-propelled sea-going or inland vessel equipped with a hold, called hopper, and a dredging installation by which it can fill and/or empty the hopper. The basic options of a THSD are: • One or more suction tubes provided with suction mouths (dragheads) which are dragged over the seabed during dredging. • One or more dredge pumps to suck the material from the seabed. • A hopper in which the dredged material can settle. • Easy operational bottom doors or valves in the hopper to dump the dredge material • Gantries and winches to operate the suction tubes. • A swell compensator to control the contact between the suction mouth and the seabed when dredging in waves. The size of a TSHD is expressed in the hopper volume and varies between a few hundred m3 up to 33000 m3


Pagina 23 van 27

May 2003

Wb3408b


A 23350 and 700 m3 hopper dredger 1.4.5.2.

Working method When arrived at the dredging area, the speed of the vessel is reduced to about 2 to 3 knots (1 to 1.5 m/s), where after the suction tubes are lowered till the seabed and the dredge pumps started. When the suction tubes reach the seabed the swell compensator reacts, easy to see by the movement of the hydraulic cylinder. Nowadays electronic charts and screens shows where and how much there is to dredge. During dredging a mixture of soil and water is dumped into the hopper. When dredging non-settling slurries dredging is stopped when the mixture reach the overflow; a device to discharge fluids from the hopper above a certain level.

When dredging settling slurries dredging is continue after the mixture has reached the top of the overflow. Now the majority of the soil will settle in the hopper, while the fine particles together with the water will leave the hopper via the overflow. Prof.Ir. W.J.Vlasblom

Pagina 24 van 27

May 2003


Overflows After the overflow is reached, the dredging procedure depends either the overflow level is fixed or variable. • With a fixed overflow level the loading is continued till the ship has reached the allowed draught. The mixture volume in the hopper stays constant during this part of the loading process. Depending on the bulk density of the settled material there will be a certain volume of water above the settled material. (constant volume system) • Is de THSD provided with a variable overflow system, the overflow may be lowered when the ship has reached the allowed draught, on order to replace the water volume by settled material. (constant tonnage system)

Rods for opening and closing Suction channel for self-discharching Upper door Rubber seal

Rubber seal

Pivot

Bottom door

Bottom door Prof.Ir. W.J.Vlasblom

Pagina 25 van 27

May 2003

Wb3408b


When the hopper is filled, dredging is stopped and the suction tubes placed on the deck of the ship, where after she is ready to sail to the unloading area. The THSD can be unloaded either by opening the bottom doors or to pump the load via a pump ashore equipment to the reclamation area.

Pumping ashore (rain bowing)

1.4.5.3. Applied working area The THSD is a free sailing vessel and does not hinder other shipping during dredging and is therefore ideal for dredging in harbors and shipping channels inshore as well as offshore. The seagoing vessels are very suitable for borrowing sand under offshore conditions (wind and waves) and large sailing distances. The dredged material is dredged, transported and discharged by the vessel without any help from other equipment. (De)mobilization is very easy for this type of dredger. It can sail under its own power to every place in the world. Suitable materials for the THSD to dredge are soft clays, silt sand and gravel. Firm and stiff clays are also possible but can give either blocking problem in the draghead and/or track forming in the clay. In that case the draghead slips into foregoing tracks, resulting in a very irregular clay surface. Dredging rock with a TSHD is in most cases not profitable. It requires very heavy dragheads with rippers and the productions are rather low.


Pagina 26 van 27

May 2003


Modern 9000 m3 Hopper dredger with one dredge pipe

1.5. Conclusion Summarized it can be stated that every type of dredger has its own applied working area in which its production is optimal in a technical way as well as in an economical way. It will be clear that the boundaries of these applied working areas are not strictly determined, but are also determined by other working conditions, which can differ from lob to job. In the table below the possibilities of the different types are shortly summarized. Bucket Grab Backhoe Dredger Dredger Dredger

Suction Cutter Trailer Hopper Dredger Dredger Dredger Dredger

Dredging sandy materials

yes

yes

yes

yes

yes

yes

yes

Dredging clayey materials

yes

yes

yes

no

yes

yes

no

Dredging rocky materials

yes

no

yes

no

yes

no

no

anchoring wires

yes

yes

no

yes

yes

no

yes

Maximum dredging depth [m]

30

> 100

20

70

25

100

50

accurated dredging possible

yes

no

yes

no

yes

no

no yes

working under offshore conditions possible

no

yes

no

yes

no

yes

Transport via pipeline

no

no

no

yes

yes

no

no

Dredging in situ densities possible

yes

yes

yes

no

limited

no

no


Pagina 27 van 27

May 2003

Chapter 2 Trailing suction hopper dredger 2

Trailing suction hopper dredger............................................................................................ 10 2.1 General description .................................................................................................. 10 2.1.1 Characteristics ............................................................................................. 10 2.1.2 Application area .......................................................................................... 11 2.1.3 History......................................................................................................... 11 2.1.4 Work method............................................................................................... 13 2.2 The design ................................................................................................................ 16 2.2.1 The productive capacity .............................................................................. 16 2.2.2 The main dimensions .................................................................................. 18 2.2.3 The dredge installation ................................................................................ 23 2.2.4 The propulsion power ................................................................................. 40 2.2.5 Power balance ............................................................................................. 46 2.2.6 Main layout ................................................................................................. 49 2.3 Technical Construction ............................................................................................ 55 2.3.1 The dredge installation ................................................................................ 55 2.3.2 The hopper .................................................................................................. 71 2.3.3 The propulsion ............................................................................................ 83 2.3.4 The maneuverability.................................................................................... 83 2.4 Strength and stability ............................................................................................... 85 2.4.1 Strength ....................................................................................................... 85 2.4.2 Stability ....................................................................................................... 86 2.5 The dredging process ............................................................................................... 88 2.5.1 The loading process..................................................................................... 88 2.5.2 Sailing from and to the discharging area..................................................... 107 2.5.3 The discharge .............................................................................................. 108 2.5.4 The cycle production................................................................................... 110 2.5.5 The instrumentation .................................................................................... 111 2.6 Special designs of trailing suction hopper dredgers................................................. 112 2.6.1 The gravel suction dredger.......................................................................... 112 2.6.2 The stationary suction hopper dredger ........................................................ 114 2.6.3 Boom dredgers ............................................................................................ 115 2.7 Literature.................................................................................................................. 117


Pagina 9 van 109

May 2005

wb3408B


2 Trailing suction hopper dredger

Figure 2-1 Trailing Suction Hopper Dredger (TSHD)

2.1 General description 2.1.1

Characteristics

The characteristics of the trailing suction hopper dredger are that it is a self-propelled sea or inland waterway vessel, equipped with a hold (hopper) and a dredge installation to load and unload itself. In a standard design the trailing suction hopper dredger is equipped with: • • • • • • •

One or more suction pipes with suction mouths, called dragheads that are dragged over the seabed while dredging. One or more dredge pumps to suck up the loosened soil by the dragheads. A hold (hopper) in which the material sucked up is dumped. An overflow system to discharge the redundant water. Closable doors or valves in the hold to unload the cargo. Suction pipe gantries to hoist the suction pipes on board. An installation, called the swell compensator, to compensate for the vertical movement of the ship in relation with the sea-bed.


Pagina 10 van 109

March 2003

Chapter 2 Trailing suction hopper dredger

2.1.2

Application area

The trailing suction hopper dredger has a very wide application area and is therefore called the workhorse of the dredging industry. Because it needs no anchorage system to position the vessel when dredging, which can be an obstacle for passing ships, in the early days the trailing suction hopper dredger (TSHD) was mainly used for the deepening and maintaining of waterways. Nowadays the trailing suction hopper dredger is also used for land reclamation. Examples of that type of jobs are the large reclamation works executed in the Far East. Here the non-bearing soil was first removed by the trailing suction hopper dredger, after which the same area was filled again with sand. The reason for a preference of the trailing suction hopper dredger above other types of equipment for this type of work is mainly the fact that the distances to the dump areas for the non-suitable material and distance from the sand pits are too large for a direct discharge and supply with pipelines. The main advantages of a trailing suction hopper dredger are: • •

The ship does not dredge on a fixed position. It has no anchors and cables, but it moves freely, which is especially important in harbor areas. The trailing suction hopper dredger is quite able to work under offshore conditions.

The materials that can be sucked are mainly silt and sand. Clay is also well possible, but can give some trouble with congestions in the draghead and rutting. Rutting is the slipping back of the dragheads in their old rut or trail. Dredging rock with a trailing suction hopper dredger is in most cases not economical. It requires very heavy dragheads, also called ripper-heads, and the productions are usually very low.

2.1.3

History

The first TSHD “General Moultry” with a hopper size of 155 cu yard (118.5 m3) was built in 1855 in the United States. Few years later 1959 a trailing suction hopper dredger was build in France for maintenance work in the harbor of St. Nazaire.

Figure 2-2 French trailing suction hopper dredger from 1859 Prof.Ir. W.J.Vlasblom

Pagina 11 van 109

May 2005

wb3408B


The ship had two drag suction pipes, which were connected at the bottom by a tube with holes (Figure 2.2). The dredging material, silt, was sucked through the holes in the connection tube by a steam-driven centrifugal pump. The size of the hopper was 240 m3. In 1962 a dredger was built according to this layout at the yard Fijenoord at Rotterdam, Netherlands. Those types were able to dredge only very light silty material. The real development of the trailing suction hopper dredger emanated from the stationary suction hopper dredger, one of the few Dutch dredge inventions. This self-propelled ship has a hopper and a forward pointing suction pipe. The dredge method is like a stationary suction dredger, working stationary on anchors and cables. At first with a pipe in the well, but the suction pipe was mounted on the side during the excavation of the Nieuwe Waterweg as it appeared not the right solution in waves. The change from an anchored to a self-propelled dredging ship was a big step ahead. At first the suction pipe on board of a trailing suction hopper dredger was placed in a well behind the ship, but was soon moved to the side. The trailing suction hopper dredger has mainly developed in the USA and reintroduced in the Netherlands in the fifties and improved till it state of today.

Figure 2-3 Artist impression of TSHD


Pagina 12 van 109

March 2003


2.1.4

Work method

When arriving on the dredging area the speed of the trailing suction hopper dredger is reduced to approximately 3 knots (± 1.5 m/s) and the suction pipes are swung outboard. The suction pipes are initially lowered approximately horizontally until the trunnion slide is positioned in front of the suction intake (Figure 2.4).

Main deck

Middle gantry wire Base of ship

Draghead wire

Next the intermediate gantry and the draghead winch gantry are lowered such that the pipe rotates like a straight line around the trunnion.

Figure 2-4 Suction pipe lowered

When the suction mouth arrives a few meters above the sea bottom the sand pumps are started, the dragheads are lowered onto the seabed (which can be seen by the rise of the swell compensators cylinders (Figure 2.5) and the dredging can start. Where and how much needs to be dredged is nowadays shown on electronic maps (computer screens). It also shows the position, direction and course of the ship.

Figure 2-5 The swell compensator

The trailing suction hopper dredger sucks the soil from the seabed at a sailing speed of 1 to 1.5 m/s (2 to 3 knots) and deposits it in the hopper. For non- or bad-settling soils the dredging is stopped when the surface of the mixture in the hopper reaches the upper edge of the overflow (Figure 2.6).

Adjustable overflow

Dredging mark

Figure 2-6 Justable overflow Prof.Ir. W.J.Vlasblom

Pagina 13 van 109

May 2005

wb3408B


The hopper filling is at maximum or the fill rate is 100%. Usually pumping continues for five minutes more to remove floating water on the mixture through the overflow. When dredging settling soils the dredging continues when the maximum level of the overflow is reached. Most of the solids will settle and the remainder is discharged with the water through the overflow.

Fixed overflow

Fixed overflow This water is not removable

Dredging mark

Constant Volume hopper

If the trailing suction hopper dredger is equipped with a fixed overflow (not adjustable) than the ship is loaded until it reaches its dredge mark (a fixed allowed draught) after which the suction is stopped.

Figure 2-7

That case it is said that the ship is designed as a Constant Volume System (CVS).

Adjustable overflow

Dredging mark

Constant Tonnage system

Figure 2-8

If the ship however has a height adjustable overflow system, than it is possible, when the hopper is full and the ship is on its mark, to lower the overflow level such that the total weight of the in the hopper present water and soil remains constant.

This is called a Constant Tonnage System (CTS).

The dredging is stopped when: • • •

The hopper is full. Overflow not allowed. The maximum allowable draught is reached and the overflow can not be lowered usefully anymore. The economical filling rate is reached.

When dredging stops, the suction pipes are pumped clean to prevent settling of the sand or gravel during the hoisting of the pipes causing an extra load for the winches. When the pipes Prof.Ir. W.J.Vlasblom

Pagina 14 van 109

March 2003

Chapter 2 Trailing suction hopper dredger are cleaned the pumping stops and the pipes are raised. When the dragheads are out of the water the ships velocity is increased to sail to the discharge area. The discharge area can: •

• • •

Be in its most simple shape a natural deepening of the seabed, the dumping area (shortly dump), to store redundant material. If the storage capacity is large, there is no concern about the way of dumping. This hardly happens nowadays. The client demands usually a dump plan to fill the dump as efficiently as possible. At all times the draught on the dump needs to be sufficient to open the bottom doors or valves (Figure 2.9). Be a storage location for contaminated silt, like for instance the Slufter (Rotterdam harbor). Here the material is pumped ashore using a pump ashore discharge system. An area that has to be reclaimed. An oil or gas pipe that has to be covered.

Rods for opening and closing Suction channel for self-discharching Upper door Rubber seal

Rubber seal

Pivot

Bottom door

Figure 2-9 Bottoms doors operated by rods

In case of the discharge area is a dump, opening the doors or valves in the base of the hopper does the unloading. This is usually done with an almost non-moving ship, certainly when accurate dumping is required. During the dumping water is pumped onto the load by means of the sand pumps. The eroding water stimulates the dumping process. If the trailing suction hopper dredger is equipped with jet pumps connected to a jet nozzle system in the hopper, those will be used too. The jets more or less fluidize the load and improve the dumping process.

If the load is pumped ashore using the sand pumps than only these jets are available to fluidize or erode the load. The shore connection, being the connection between the board pipeline and the shore pipeline is currently mostly positioned just above the bow (Figure 2.10). The connection between the ship and the shore piping is this case a rubber pipeline. The ship remains in position by maneuvering with its main propellers and bow thruster(s).

Figure 2-10 Pump ashore connection

.


Pagina 15 van 109

May 2005

wb3408B


When the load is either dumped or pumped ashore the ship will return to its suction area and a new cycle starts. In general the ship sails empty, in a non-ballast way, back to its suction section. There is only some residual water and/or load left in the hopper Figure 2-11 TSHD J.J.F. de NUL picking up the floating pipeline to the shore connection

2.2 The design 2.2.1

The productive capacity

When a dredging company wants to order a new trailing suction hopper dredger usually a market study is performed that about the required production capacity of the new dredger. The required production capacity is expressed in m3/week or m3/month or even cubic meters per year. Besides that insight required about the expected average cycle time of the trailing suction hopper dredger on the different jobs, as well as the type of soils to be dredged. Then the production capacity can be translated to: • •

The required payload in ton mass. The maximum hopper volume in m3.

If the ship is used for a single purpose, for instance the maintenance of a harbor area, than the required production capacity is usually known and therefore the above mentioned ship data. For an international operating dredging contractor this is different and far more complicated. Answers have to be given to the question how the average cycle and the required production capacity will evolve in the future. For these contractors there is in fact only one requirement and that is dredging cheaper than their competitors. This leads quickly to a demand for large dredgers, which dredge cheaper and therefore more competitive.


Pagina 16 van 109

March 2003


y = 3.0656Ln(x) - 19.711

Load - Draught relation

2

R = 0.8888

Draught [m]

14 12 10 8 6 4 2 0 0

5000

10000

15000

20000

25000

30000

35000

Payload [ton]

Figure 2-12 Displacement - draught relation

The only decelerator on the building of larger vessels is the draught of the ship. When the draught increases the usability of the ship decreases. The contractor can, dependent on the expected amount of work as function of the (initial) dredging depth, determine the availability of the ship for a certain draught.

Cumulative frequency [%]

Cumulative frequency distribution of initial dredging depth 120 100 80 60 40 20 0 0

10

20

30

40

50

Initial dredging depth [m] Figure 2-13

Unfortunately it is possible that market expectations of today are totally out-of-date in 5 years. The management chooses for a certain production capacity and later one wills just if this choice was right. The design is usually made a co-operation between the builder and the client is often scaled-up from successful ships. Of course the proper scale rules have to be obeyed when scaling-up. At this moment five classes of trailing suction hopper dredgers can be distinguished: Small hoppers Medium size hoppers Prof.Ir. W.J.Vlasblom

deadweight capacity to deadweight capacity Pagina 17 van 109

± 50 MN (to 5000 ton mass) 50-100 MN (5000-10000 ton mass) May 2005

wb3408B


Large hoppers Jumbo hoppers Mega hoppers

deadweight capacity deadweight capacity deadweight capacity

100-150 MN (10000-15000 ton mass) 150 250 MN (15000-25000 ton mass) >250 MN (above 25000 ton mass)

Figure 2-14 Different scales Fairway (23.347 m3) and the Sospan (700 m3)

2.2.2

The main dimensions

When the choice for the production capacity of the trailing suction hopper dredger to be built is made, the hopper volume is known too. The main dimensions of the trailing suction hopper dredger are determined, as by other ships, by the required payload, draught and speed. It will be clear that a straight correlation exists between these quantities to satisfy the shipbuilding demands. After all a large hopper volume with a limited draught gives wide long ships with possible disadvantages like a poor behavior in swell or problems to obtain the required speed. Trailing suction hopper dredgers are therefore build according to certain ship ratio, such as L/B, B/H and B/T ratio's (L=length, B=width, H=depth and T=draught). Those ratios’s depend on market requirements too and therefore change in time (Figure 2.15) With the remark that a large B/T ratio: • •

Results in a large initial stability, resulting in heavy ship motions in swell. Has an adverse effect on the resistance of the ship.

With a large L/B ratio a lean ship is obtained with the advantages of: • •

A simple construction as a result of the long equal mid-section (cheap). A relative low resistance, therefore a higher velocity with the same installed propulsion power.


Pagina 18 van 109

March 2003


L/B, B/H, B/T

Ships Numbers 8 7 6 5 4 3 2 1 0 1965

L/B B/H B/T

1970

1975

1980

1985

1990

1995

2000

Year of Construction Figure 2-15

On the other hand a small L/B gives a good stability and longitude strength and demands therefore less material, which is also cheaper. In general a smaller B/H and a larger L/B result in less building costs. So demands for the draught (smaller T) will cost extra money and will have to be earned with a higher usability.

T L

Cb =

B

∇ LBT

Figure 2-16 Definition Block coefficient

Definition Block coefficient Of course the required block coefficient Cb =

∇ displacement is involved too. = L ⋅ B ⋅T L ⋅ B ⋅T

Displacement = In m3 B = Width of ship at the main section I m L = Length between perpendiculars in m T = Draught at International mark in m The lower Cb, the longer the ship will be with the same displacement. For trailing suction hopper dredger Cb lies between 0,78 and 0,85. Prof.Ir. W.J.Vlasblom

Pagina 19 van 109

May 2005

wb3408B


Also the required maximum dredging depth can have an influence on the length of the ship. Naturally, the long suction pipe has to be stored on the deck and that requires length. A good measure to see if the trailing suction hopper dredger is well placed in the market is to compare its specific weight with that of its competitors. The specific weight can be defined as the ratio between the ships weight and payload. The weight is directly related to the costs and the payload to the profits. In Figure 2.17 the specific weight for a large number of ships is given.

Specific Ships Weight 1

W_spec

0.8 0.6 0.4 0.2 0 0

10000

20000

30000

40000

50000

60000

Displacement [t]

Figure 2-17

2.2.2.1 The load As aid, the payload in tons and the maximum hopper volume in m3 determine the amount of soil that a trailing suction hopper dredger is able to carry each voyage. These are of great importance. The payload is the weight of the paying load that the ship may carry on the maximum allowed draught. The payload is often a cause for misunderstandings. As a definition the payload is the ship weight of the loaded ship subtracted with the weight of the empty ship ready for service. This is shown in the hereunder shown chart. 1

Dutch term Scheepsgewicht

2

Toegevoegde gewichten Added weights

1+2 3

Gewicht leeg schip Toelading


English term Ships weight

Weight “light” ship Dead weight Pagina 20 van 109

Explanation Construction weight and necessary equipment like: anchors, chains, moor cables, rescue equipment, nautical equipment and inventory of the cabins, galley, engine-room and tool-room of the boatswain This is the liquid filling of all systems on board including the water in the inlets. Also the outside water situated above the bottom deck for instance under and around the bottom doors is included. Weights of: Crew and their possessions, March 2003

Chapter 2 Trailing suction hopper dredger consumer goods, spare parts, and ballast water and load. 1+2+3 Gewicht van het “geladen” schip 4 Gewicht lading 1+2+3 Gewicht bedrijfsklaar +4 schip

Weight of “loaded” Vessel Weight cargo Ships weight ready for Service

Weight of the paying load.

Figures below gives some information about ”light weight” and “dead weight” of TSHD’s 70,000 60,000

y = 0.6827x R2 = 0.9929

Weight [t]

50,000 40,000

G Light weight

30,000

Dead weight

20,000 y = 0.3173x R2 = 0.9622

10,000 0 0

20,000 40,000 60,000 80,000 100,000 Displacement [t]

Figure 2-18 Light weight as function of deadweight

Light weight [t]

25,000 20,000 15,000

y = -3E-06x 2 + 0.5586x R2 = 0.9607

10,000 5,000 0 0

10,000

20,000 30,000 40,000 50,000

60,000 70,000

Deadweight [t[

Figure 2-19

Except that there are different names for the payload, it is also apparent that it varies in time and often decreases. The reason is that when the ship has been in use for a while things will be added or reinforced, which causes an increase in the ships weight. Spare parts also tend to remain on board that should be stored onshore. In fact there is only one way to determine the payload correctly: 1. Clear the hopper such that no remaining soil is present. 2. Determine the displacement of the ship with the draught and the trim of the ship, the displacement is the weight of the ship including the water in the hopper. 3. Determine the weight of the water present in the hopper by determining its volume and the specific gravity 4. Subtract the weight of this water the ships weight determined under point 2. This is the weight of the ship ready for service. Prof.Ir. W.J.Vlasblom

Pagina 21 van 109

May 2005

wb3408B


5. The payload is obtained by subtracting the ships mass (displacement x water density) in tons on the maximum allowed draught with the weight of the ship ready for service. It will b clear that the payload is never constant, but varies with the weight of the consumer goods like fuel, lubricants, drinking water etc. In case of light soils, such as silt and soft clay, the maximum hopper volume can be decisive for production instead of the payload. 2.2.2.2 The hopper density. As mentioned earlier, the production capacity of a trailing suction hopper dredger is indicated with the quantities: • •

Pay-load Maximum hopper volume

The quotient

pay − load [kg / m3 ] is called the hopper density and is a maximum hopper volume

measure for the average density that a dredging contractor expects to dredge during the economical lifetime of the ship. It also says something over the purpose for which the dredger is designed. Is this for instance maintenance of a fairway in a sandy soil, than the dredges sand in the hopper will have a density of approximately 1900 kg/m3. Unfortunately no hopper can be filled to a 100% but approximately to maximum 90%. The maximum hopper density required is 1900 * 0.9 = 1710 kg/m3 For a gravel trailing suction hopper dredger this is for instance: 2000 * 0,9 = 1800 kg/m3. And for a silt trailing suction hopper dredger this could be even 1300 kg/m3. In Figure 2.20 the hopper density of international operating dredging contractors is shown as function of time. It stabilizes at the end of the eighties and early nineties around 1500 kg/m3, but due to the big reclamation works it is increasing again.


Pagina 22 van 109

March 2003


Hopper denisty as function of time 2.50

Hopper density [t/m3]

2.00 1.50 1.00 0.50 0.00 1950

1960

1970

1980

1990

2000

2010

Construction year Figure 2-20

2.2.3

The dredge installation

The design of a dredge installation includes the determination of the required main dimensions and required powers of the following dredging components: • • • • • • • • •

Number of suction pipes Pump capacity [m³/s] Suction and discharge pipe diameter [m] Type dredge pump Sand pump drive and power [W] Type and size of the draghead(s) Hopper shape Jet pump power and drive [W] Discharge systems

For the subjects the production should be corrected in a certain way from the average cycle production of the dredger. For instant, assume that the dredger is designed for a payload of 16000 ton and a hopper volume of 10000 m3 and a average loading time in sand with a d50 of 200 μ of 90 minutes. De density of the soil in the hopper is 1900 kg/m3. When the hopper is loaded the volume of sand will be 8421 m3. The average load rate is in this case 8421/90=93 m3/min=1.56 m3/s. When cumulative overflow losses of 20% are to be expected, then the dragheads should excavate 1.56/0.8=1.95 m3/s as an average. Every m3 of sand contains (1900-1025)/(26501025)= 1-0.538=0.462 m3 water in the pores. (ρwater=1025 kg/m3, ρsand is 2650 kg/m3). So a production of 1.95 m3/s equals a sand mass of 1.95*0.538*2650=2780 kg/s 2.2.3.1 Number of suction pipes A trailing suction hopper dredger is usually equipped with two suction pipes. For smaller and medium size trailing suction hopper dredgers it is cheaper to use only one suction pipe. With Prof.Ir. W.J.Vlasblom

Pagina 23 van 109

May 2005

wb3408B


two suction pipes the total efficiency is often better because it is still possible to dredge when one of the pipes fails. There are also examples of large trailing suction hopper dredger with one suction pipe: the ANTIGOON of Dredging International with a hopper volume of 8.400 m3 and the VOLVOX TERRA NOVA of Van Oord ACZ with 18.000 m3 hopper volume. In principal it is an economical consideration, but looking from the process technical side there are some questions. For example: is one draghead as efficient as two dragheads with the same width?

Figure 2-21 Volvox Terra Nova and HAM 316, both with one suction pipe

2.2.3.2 Pump capacity The sand pump capacity can be determined using several criteria:

1. In a particular type of soil a certain load time is demanded. (for instance 1 hour for sand with a d50 of 200-300 μm) T ⎡T ⎤ ⎢ ⎥ The load as function of time is: L mass =ρvs ⎢ ∫ CiQ i - ∫ Co Qo ⎥ dt 0 ⎣⎢ 0 ⎦⎥ C0 Cin Q0 Cin T ρvs

= = = = = =

Volumetric concentration at overflow Volumetric concentration at intake Discharge at overflow Flowrate at intake Loading time Volumetric density of sand in the hopper

[-] [-] [m3/s] [m3/s] [s] [kg/m3]

For TSHD’s having a constant volume system Q=Qi=Qo and the above formula become: T

L mass =ρvsQ ∫ (Ci -Co ) dt= ρvsQ ⋅ Ci (1 − ov) ⋅ T 0


Pagina 24 van 109

March 2003

Chapter 2 Trailing suction hopper dredger T

∫CQ

0

∫ CQ

i

o

With ov being the cumulative overflow losses defines as

ov=

0 T

i

0

For 1 hour loading the flow rate becomes: L mass Vsand = Q= (1-ov ) Ci ⋅ ρvs 3600 (1-ov ) Ci ⋅ 3600 The relation between Ci and Cvd is as follows Ci =

Cvd =

ρm − ρw ρs − ρw so: Ci = C vd ρs − ρw ρvs − ρw

ρm − ρw and ρvs − ρw

The expected Cvd depends on the particle size, the permeability of the soil and the available jetwater momentum. (see 2.5.5.1.3) If the TSHD is designed as a constant tonnage dredger the incoming mass equals the outgoing mass; so m=mi=mo. mi = Qi ρmi

and mo = Qo ρmo so Qi ρmi = Qo ρmo or Qo = Qi T

The load becomes now : L mass =ρvsQ i ∫ 0

ρmi ρmo

⎛ ⎞ ⎜⎜C -C ρmi ⎟⎟dt=ρ Q ⋅ C (1 − ov) ⋅ T i o i vs i ⎜⎝ ρmo ⎠⎟⎟

Although the formula is the same as for the constant volume system hopper dredger it doesn’t mean that the cumulative overflow losses are the same for both types of hopper dredgers. 2.

In an ascertain type of sand the load rate in m³/s or in t/s must have a minimum value. If there would be no overflow losses than the load rate is directly proportional to the flow rate. However, the overflow losses increase with an increasing flow rate, which result in an increasing deviation from the linear relation. (Figure 2.22& 2.23)


Pagina 25 van 109

May 2005

wb3408B


Loadrate=F{Q} d50=.15 mm ρ=1100

ρ=1200

ρ=1300

10

12

180 160

Loadrate [m3/min]

140 120 100 80 60 40 20 0 0

2

4

6

8

14

16

18

20

Capacity [m3/s]

Figure 2-22 Loadrate as function of pump capacity

It can be proven that for certain particle sizes there is an optimum loadrate.

Loadrate=F{Q} d50=.1 mm ρ=1.1

3

ρ=1.2

ρ=1.3 [t/m3]

Load rate m /s 350 300 250 200 150 100 50 0 0

5

10

15

20

Capacity [m3/s] Figure 2-23

The increase of a higher suction production (load rate) must be considered against the higher sand and water pump power, larger suction pipe diameter and dragheads etc. Prof.Ir. W.J.Vlasblom

Pagina 26 van 109

March 2003

Chapter 2 Trailing suction hopper dredger Remark: In Figure 2.23 the step in the load rate is caused by the fact that for high densities and high flow rates the loading after the overflow is not necessary since the optimal production for the dredge cycle has been reached. 3.

When apart from the soil the cycle time is known too, than the flow rate can be chosen such that the cycle production is maximal. The cycle production is defined as the quotient between loading and cycle time, so: Pc =

load tsuction + tnon − suction

If there are no overflow losses than this formula can be written as:

Pc =

load ⎛ ⎞ load + tnon − suction ⎟ ⎜ ⎝ Q ⋅ Cvd ⋅ ρ k ⋅ g ⎠

=

Q ⋅ load

⎛ load ⎞ + Q ⋅ tnon − suction ⎟ ⎜ ⎝ Cvd ⋅ ρ k ⋅ g ⎠

This is a monotone ascending function. However the overflow losses cause an optimal flow rate for which the cycle production has a maximum. (Figure 2.24)

Cycle Production d50=.15 mm ρ=1.1

Pcycle [m3/c]

ρ=1.2

ρ=1.3 [t/m3]

2000 1500 1000 500 0 0

5

10

15

20

Capacity [m3/s] Figure 2-24

4.

Also the pump capacity can be scaled from existing "well working" trailing suction hopper dredgers, by using the scale rule from Froude. However overflow losses will not be on scale when using this scale rule.

Above mentioned criterions lead to a design flow rate and a design density. 2.2.3.3 Suction pipe diameters Old trailing suction hopper dredgers are equipped with relatively large suction pipe diameters. In the past the size of the diameter was mainly based on minimizing the pressure loss in the suction pipe to avoid cavitation of the dredge pump. However it was understood that the concentration distribution was homogeneous over the diameter, which is not always the case.


Pagina 27 van 109

May 2005

wb3408B


For a homogenous flow it can be shown that the suction production is maximum for a certain suction velocity. This is done with the so-called suction formula, a force balance over the suction pipe. For a pump that is positioned k meters under the surface The pressure at the suction mouth is ρmgH. The pressure in front of the pump p is equal to the allowable underpressure, vacuum, so p=-VAC. The pressure difference over the suction pipe equals the weight of the mixture and the losses in the pipe.

hz

Mixture velocity vs Mixture density ρm

Figure 2-25

ρ water ⋅ g ⋅ H + Vac = ρ mixture ⋅ g ⋅ hz + ξ ⋅ 12 ρ mixture ⋅ v 2 = ρ mixture ⋅ g ⋅ ( H − k ) + ξ ⋅ 12 ρ mixture ⋅ v 2 ρ ⋅ g ⋅ H + Vac ρ mixture = water ξ 2 g ⋅(H − k ) +

Pr = Q ⋅ Cvd ⋅ ρ k = v ⋅ A

2

⋅v

ρ mixture − ρ water ⋅ ρ grain ρ grain − ρ water

This function appears to have, dependent on H, k, Vac and ξ, an optimum for a certain suction velocity v, which is independent of the suction pipe diameter. ξ can be written as ξ = β + λ L with; D

β=entrée loss coefficient [-] λ=Darcy-Weisbach resistance coefficient [-] L=length of suction pipe in m D=suction pipe diameter in m


Pagina 28 van 109

March 2003


D=750 mm

D=1000 mm

1350

1200

1300

1000

1250

800

1200

600

1150

400

1100 1050

200

1000

0 0

2 4 6 Suction velocity [m/s] vacuum=80kPa

Production [kg/s]

Mixture density [kgm3]

rho_m

8

Figure 2-26

Application of the suction formula has several disadvantages: 1. The mixture density, the resistance factor ξ and the suction velocity are not independent of each other, but are determined by the erosion process and the pump characteristics. 2. The flow is only homogeneous for sand types with a d50 < 0.15 mm. For coarser materials the flow becomes heterogeneous. As a result the volumetric concentration (the amount of sand in the pipe) increases and therefore also the pressure loss in the pipe. In other words the decrease of the pressure loss by the lower velocity is cancelled out by the increase as a result of the higher volumetric concentration. Therefore the pressure loss in the pipe does no longer behave according: Δp = 12 ⋅ ξ ⋅ v 2 . For this reason modern trailing suction hopper dredgers do have relative smaller suction pipe diameter then in the past. Besides that heavier pipes demand heavier winches, gantries and their foundations. This leads to a lower useful deadweight capacity and more investment cost. Figure 2.27 below shows the relation between the maximum hopper volume and the suction pipes diameters for trailing suction hopper dredgers with two suction pipes. (diameters above 800 mm are round off to 100 mm and under 800 mm to 50 mm) As can be seen in the Figure 2.27 the spread in the used suction pipe diameters is considerable. This could lead to the conclusion that design process is not yet unambiguous. At present however modern TSHD’s have smaller in suction pipe diameter at the same flow rate. This is especially affected by the better insights in the two-phase flow at relative low velocities for inclined pipes.


Pagina 29 van 109

May 2005

wb3408B


Two pipe vessels

One pipe vessels

1.20

Pipe diameter [m]

Pipe diameter [m]

1.40 1.00 0.80 0.60 0.40 0.20 0.00 0

5,000

10,000

15,000

20,000

1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00

25,000

0

5000

10000

15000

20000

25000

30000

35000

Hopper volume [m3]

Hopper volume [m3]

Figure 2-27

From many researches it appears that the velocity for which all soil particles in the pipe are still in motion is dependent on the Froude-value:

v2 . (v=velocity and D pipe diameter) g⋅D

Depending on the grain size and concentration the Froude-value may not become less than a certain value FI,H. Adding the maximum average velocities for which no stationary bed is

formed in a horizontal pipeline can be calculated using Vsm = Fl 2 ⋅ g ⋅ ( S s − 1) ⋅ D or with the

demi-McDonald of Wilson, which can be estimated with the formula:

⎡ μs ( Ss − S f ) ⎤ 1.75 8.8 ⋅ ⎢ ⎥ ⋅ D 0.7 ⋅ d50 0.66 ⎢⎣ ⎥⎦ Vsm = 2 d50 + 0.11 ⋅ D 0.7 0.55

With d50 in mm and the diameter D in meters.

In Figure 2.28 both formulas are drawn (Durant, Fl=1.4). For inclined suction pipes Vsm has to be raised with a value ΔD dependent of the incline. According Wilson and Tse ΔD reaches a maximum for approximately 30° and is then ΔD=0.333 (Matousek, 1997). In the design of trailing suction hopper dredgers usually Fl = 1.00 is assumed and ΔD is not considered. This implies that the dredger is designed for materials with a d50 between 100 and 300 μm and that for coarser materials a stationary bed is accepted.


Pagina 30 van 109

March 2003


V_stationary deposition for horizontal transport mm Wilson

Durant

d50=.5

Practice

V_deposit [m/s]

10 8 6 4 2 0 0

0.2

0.4

0.6

0.8

1

1.2

Pipe diameter [m]

Figure 2-28

The use of suction pipe with a submerged pump (Figure 2.29) has a direct influence on the choice of the diameter of the suction pipe. Is this the case then it is possible to choose the suction pipe diameter a little smaller and so lighter and cheaper, against the disadvantage of a little additional pressure loss in the pipeline..

Figure 2-29 Dredge pump incorperate in the suction pipe

2.2.3.4 The pressure pipe diameter The diameter of the pressure pipe should have a larger diameter than the suction pipe, because the factor 0.333 for the inclined transport. However often, depending of the value of the factor Prof.Ir. W.J.Vlasblom

Pagina 31 van 109

May 2005

wb3408B


Fl,H, the pressure pipe diameter is chosen 50-100 mm smaller for costs reasons. Particular when the casted elbows and valves are used. The diameter of the pump ashore installation will generally be chosen smaller than the suction pipe. Normally the hopper is unloaded with considerable higher concentrations than loaded. This allow for a lower flow rate when discharge time equals the suction time. 2.2.3.5

The dredge pump

The main dimensions of the ship and the dredge installation are now known, so an estimate can be made to the required manometric head of the dredge pump for the different (un)loading conditions. The required pump pressure during loading is determined by the static head from hart pump to the discharge in the hopper and the losses in the discharge line. The manometric head is the sum of required pressure and the allowable vacuum at the suction side of the pump.

Figure 2-30 Pump room with 2 pumps

Because the impeller diameter is approximately known ( minimum 2 times suction pipe diameter) and there is a relation between the required manometric pressure and the peripheral velocity of the pump impeller, also the specific pump speed is approximately known. The dimensionless specific pump speed is defined as: 1

Ns =

Φ2 3

Ψ4

With:

Φ≈ Ψ=

Q = ωπ Db

p p = = 2 ρu ρω 2 r 2

dimensionless capacity

dimensionless pressure

In these is: Q = flow rate p = pressure D = diameter pump impeller b = width pump impeller r = ½D Prof.Ir. W.J.Vlasblom

[m3/s] [Pa] [m] [m] [m] Pagina 32 van 109

March 2003

Chapter 2 Trailing suction hopper dredger ρ = density fluid ω = angular velocity pump impeller

[kg/m3] [rad/s]

Filling in Φ and Ψ results in 1

Ns =

Φ2 Ψ

3 4

=

3 4

ρω Q p

3 4

⋅

D 4π b

(1)

Figure 2-31

The specific speed is assessed to the maximum efficiency point and is a characteristic number to compare pumps with their dimensions like the b/D ratio, inlet and outlet diameter ratio Di/Du and impeller shapes (Figure 2.31). Equation (1) shows that for a constant number of revolutions (ω) the specific number of revolutions increases with an increasing flow rate and decreasing pressure. Since the pressure is proportional to the square of the peripheral velocity, the pressure will decrease at a constant number of revolutions with a decreasing diameter. A higher flow rate requires a larger diameter in the impeller, therefore a larger b/D ratio. Besides the b/D ratio especially a wider passage in the impeller has a large influence. Figure 2.32 shows the relation between the dimensionless capacity and pressure as function of the number of revolutions for all types of hydraulic suction dredgers. Left in the chart are the standard centrifugal pumps and on the right the modern half-axial or mixed flow pumps, usually used as submerged pump in the suction pipe pump of trailing suction hopper dredgers and cutter suction dredgers. In general the dimensionless pressure for hopper pumps is slightly higher for the same specific flow rate than for the pressure pumps of cutter suction dredgers and suction dredgers. From formula (1) it follows that when Q, p, and Ns are known, the pump speed can be determined, so that the pump and impeller type can also be chosen. (note: When the dredger will be equipped with a pump ashore installation, there will be two pump speeds.) For relative small trailing suction hopper dredgers and suction depths a fixed pump speed for the dredging mode (suction) is often sufficient. When the difference between minimum and maximum dredging depth is large, a variable pump speed may be required. Prof.Ir. W.J.Vlasblom

Pagina 33 van 109

May 2005

wb3408B


All Dredgers

Specific Capacity

Ca p a c ity

0.8

0.14

0.7

0.12

0.6

0.1

0.5

0.08

0.4

0.06

0.3

0.04

0.2

0.02

0.1

0

Specific Head

Hea d

0.16

0 0

0.2

0.4

0.6

0.8

1

Specific Speed

Figure 2-32

With increasing size and particular for increasing depth the question may rise if this can lead to large flow rate variations during the dredging process. Large flow rate variations often lead to water-hammer problems in the pipelines. If this risk exists than an adjustable pumpspeed is necessary. There are more factors involved in the choice of a pump, such as: • • •

•

3, 4 or 5 impeller vanes. Dependent on the required minimal opening area between the blades. Single- or double-walled pump (wear considerations). Inboard or submerged pump or both. If great suction depths are expected, it has to be considered if the installation of submerged pumps is more economical. The limit where this economical point is reached is closely connected with depth of the inboard pump below water level under service conditions, so roughly with the draught of the ship. This break point is therefore different for every ship. The operation of the pump during pumping ashore (if necessary).

When the dredger is provided with a pump ashore installation attention shall be given to the pumps working under both conditions. During pumping ashore it becomes more and more a custom that all available power of the main engines are used. This implies that the maximum pump speed when pumping ashore differs significantly from the pump speed during dredging. As a consequence the best efficiency point of the pump when pumping ashore shifts to a considerable higher flow rate than during dredging. This shift is in reality even larger because the pump ashore capacity is usually smaller than the flow rate during dredging (why?). It has to be realized however that a pump working under conditions far above or below the best efficiency point, will wear faster. A good research of the position of the best efficiency points under the different service conditions is therefore necessary to obtain the optimal installation. Also the required pump power for both modes can now be calculated. However, the maximum available pump power during pumping ashore is with a combined drive (one engine for pump + propulsion) determined by the required propulsion power. Prof.Ir. W.J.Vlasblom

Pagina 34 van 109

March 2003


Pumpcharacteristics for dredging and pump ashore Q-p/165 rpm

Eff/280 rpm

Eff/165 rpm

1200

120

1000

100

800

80

600

60

400

40

200

20

0

Efficiency [%]

Manometric pressure [kPa]

Q-p/280 rpm

0 0

1

2

3

4

5

6

7

capacity [m3/s]

Figure 2-33

2.2.3.6 The dredge pump drive Before choosing a drive the question should be answered whether continuous pump speed control is required or speed control by a gearbox is sufficient.

The following factors are involved: •

• •

The expected range of the flow rate variation between the pumping of the water and of the slurry. This range is larger with an increasing suction depth, provided no cavitation takes place. Limitation of this variation can be necessary to reduce the risk of water-hammer. In that case a constant pump speed or a stepped control is insufficient. When a constant flow rate control is desired. The flow rate is regulated by a variation of the pump speed. An electric drive is necessary. A constant flow rate control by varying the number of revolutions is not suitable to prevent water-hammer (too slow). If the ship is equipped with a pump ashore installation and the propulsion power can be used totally or partly when pumping ashore. To use this additional power a higher pump speed than use in the dredging mode is required.

Dependent on these demands the sand pump can be driven directly by the main engine through a, if necessary, a stepped gearbox or directly by an electric engine through a generator. Of course there are several intermediate solutions that are treated in the chapter "Main arrangement".


Pagina 35 van 109

May 2005

wb3408B


2.2.3.7 The dragheads Dragheads are designed to excavated the soil and mix it with water for hydraulic transport. Excavation can be done hydraulically or mechanically or combined. Hydraulic excavation is either by erosion of the dredge pump flow, by pressurized water jets or both

Pure mechanical excavation is mainly done in cohesive soils, such as clays and very soft rock. For that case teeth or blades are mounted in the draghead (Figure 2.34). The width of the draghead is now dependent on the expected cutting forces in the particular soil in relation to the available cutting force from the propulsion. The length of the visor of the draghead should be chosen such the flow pattern for the transport of the excavated material suites the excavation process.

Visor

Figure 2-34 Draghead with blade

Modern dragheads have water jets assisted with knives or teeth. A reasonable assumption is that the jetproduction is linear with the total momentum flux of the jet system and independent of the trail speed. The momentum I=ρwQu.

M sand = α ⋅ I = αρ w ⋅ Qu = αρ w ⋅ Q

2 p jet ρw

Figure 2-35 Draghead with jets (not working)

With: I Msand pjet Q u α

= = = = = =

ρw

=

Momentum in N Eroded sand mass in kg/s per jet Jet pressure at the nozzle in Pa Jet capacity in m3/s Jet velocity at the nozzle in m/s Coefficient depending on the particle size, jet pressure, jet capacity and trailspeed. A reasonable assumption for alpha is α=0.1 Water density in kg/m3.

When the nozzle are divided well over the width of the draghead the mass M should fulfill the relation:

∑M

sand

= B ⋅ d ⋅ v trail

all jets


ρ situ − ρ water ρ particle ρ particle − ρ water Pagina 36 van 109

March 2003

Chapter 2 Trailing suction hopper dredger B D vtrail ρsitu ρparticle

= = = = =

Width draghead in m. Eroded layer thickness in m Trailspeed in m/s Density soil in situ kg/m3 Particle density in kg/m3

length/width [mm]

When the trailspeed is said to 1.5 m/s, which equals 3 knots and using the relation between pipe diameter and draghead width of Figure 2.36, d can be calculated. In general the effective of the jet decreases somewhat with increasing pressure at constant momentum. This means that low pressure- high capacity jets are more effective than high pressure-low capacity jets. They use more specific energy too. On the other hand however, much jetwater dilutes the mixture density (Figure 2.128). So the designer has to search for the optimum solution between cost (power) en production

4000 3500 3000 2500 2000 1500 1000 500 0

width

Length

0

500

1000

1500

Suctionpipe diameter [mm] Figure 2-36 Dimensions Dutch draghead

2.2.3.8 The water pumps Jet-water is used for loosening the soil within the dragheads, as well as to assist the process during discharging the load, either by dumping or by pumping ashore. The flow rate of the water pump is between 20 to 30 % of the sand pump flow rate and the pressure is usually between 5 and 15 bar. The required pressure can be calculated using the same basic formula’s as mention in the forgoing chapter.

M sand = C vd Q mρ sand = αρ w Q jet

LM MN

1 ρ sand C vd Q m p= 2 ρ w α Q jet

OP PQ

2p ρw

2

In general there is no requirement for speed control of the type of pump


Pagina 37 van 109

May 2005

wb3408B


2.2.3.9 The hopper As mentioned before ships are built according certain L/B, B/T and B/H ratios. This also accounts for trailing suction hopper dredgers.

Some insight in the effect of these ratio’s on the overflow losses is got from the Camps Diagram (Figure 2.132) The removal Ratio R, the percentage of the incoming material that settles in the hopper is een function of:

R=f

FG S , S IJ = R = f FG SbBLg , SbBHgIJ HS V K H Q Q K 0

0

The following conclusion from Figure 2.132 can now be drawn when keeping the hopper volume constant: 1. The width B is kept constant and L→2L and H→0.5H 1st term of the removal ratio shall increase and 2e term shall decrease. This results in the conclusion: More sedimentation at long shallow hoppers or less in short deep hoppers 2. The height H is kept constant and L→2L and B→0.5B 1st term of the removal ratio stays constant and 2e term shall decrease. This results in: A little less sedimentation at long small hoppers or little better sedimentation in short wide hoppers. 3. The length L is constant and H→2H and B→0.5B1st term of the removal ratio shall decrease and 2e term stays constant. This results in: Less sedimentation in small deep hoppers or better sedimentation in wide shallow hoppers. 4. The height H and the width B are kept constant, while L→0.5L and Q→0.5Q 1st term of the removal ratio stays constant and 2e term shall increase. This results in: Central intake or a TSHD with 2 hoppers is a little better. From the theory of the overflow losses (chapter 2.5.1.3) can be derived that long, shallow hoppers are favorable for the settlement process. Unfortunately such a shape leads to long relatively narrow ship with a limited depth that result in certain design problems for engine room en deckhouse. Therefore a compromise has to be found between the price and the performance.

When scaling-up the hopper shape to larger dimensions one should be aware for an undesirable increase of the overflow losses. After all for all new to build trailing suction hopper dredgers it is often demanded that the load time, independent of the size of the hopper, has to be 1 hour for a sand type with a d50 of 250 μm. This implies that the flow rate will be proportional to the volume of the hopper when the concentration is assumed constant. Therefore the capacity scale is: ηQ = (η L )

b g

b g

Q

Q

3

Both the terms S BL and S BH shall decrease and this implies that the overflow loss for larger trailing suction hopper dredgers will be higher than for smaller trailing suction hopper dredgers, even if the hoppers are similar. Dependent on the magnitude of this increase this Prof.Ir. W.J.Vlasblom

Pagina 38 van 109

March 2003

Chapter 2 Trailing suction hopper dredger could still be acceptable, since the cycle production can still be higher with higher overflow losses. A design requirement directly related to the hopper shape is that the sand level at restricted loads needs to be higher than the sealevel. Such a requirement is of importance in situations where it is not possible to dredge to the dredge mark because of the waterdepth. If the sea level is higher than the sand level, the water cannot flow out and the ship can’t be loaded economically.

This water is not removable Adjustable overflow

Dredging mark

Constant Tonnage system

Figure 2-37

For modern ships this requirement can be satisfied for a 50-60% of the maximal load. 2.2.3.10 The discharge system From the theory of the flow of bulk material from silos follows that a plane symmetrical flow will occur for discharge openings where length L ≥ 3B (width) and that this flow type, is preferred above an axial symmetrical flow. Unfortunately most discharge systems, except for the split hopper (Figure 2.38) don't satisfy this requirement, while the building of a split hopper suction dredgers is considerably more expensive than "single hull" ships.

Figure 2-38 The split TSHD

As a rule of thumb the following ratios between the discharge opening and the well surface are used, dependent on the discharge material:


Pagina 39 van 109

May 2005

wb3408B • • •


for silt for clay average

10% 50% 30%

Instead of a large door or valve surface there are also systems that discharge the load with a limited amount of doors or valves by partly fluidizing or eroding the load. Experience showed that these systems function usually well for the fine sand types. A design requirement for discharge system may be the necessity of dumping in shallow water. Is this the case than sliding doors or a splithopper are options. Also cone valves function well when discharging in shallow water. With a small opening they already provide a good discharge. If doors are used shallow dumping doors have to be considered

Figure 2-39

2.2.4

The propulsion power

Except for the propulsion there are also requirements for the maneuverability of the trailing suction hopper dredger. For this purpose extra bow thrusters are often used. 2.2.4.1

The propulsion power

Trailing suction hopper dredgers are real workships. They have a high block coefficient, no high ship velocities and they often sail in shallow waters, which make them "feel" the bottom. The velocities in knots do not exceed 1.4√L (Figure 2.40).


Pagina 40 van 109

March 2003

Chapter 2 Trailing suction hopper dredger 1.4*L^1/2

20

Loaded speed [kn]

18 1.22*L^1/2

16 14 12 10 8 6 8.5

9.5

10.5

11.5

12.5

13.5

SQRT(L) [m^1/2]

Figure 2-40 Maximum speed TSHD's

The ships resistance is composed of a number of components:

Rtotal = R f (1 + k ) + Rapp + Rw + RTR + RA with

Rfl 1+k Rw Rapp Rb

Further is: Rf =

friction resistance according the ITTC-1957 formula [N] shape factor for the hull [-] wave resistance [N] resistance as a result of the appendage [N] resistance as a result of the additional pressure difference [N]

1 ρV 2 C f S total 2

with Cf =

b

0.075 log 10 R n − 2

g

2

Determination of the resistance demands a lot of experience. The average sailing speed in knots for TSHD’s is 1.22√Length (0.63√L for v in m/s) Figure 2.40. That means that the wave resistance part is small and the total resistance can be estimated by a polynomial of the second order. Nevertheless the ships resistance of a trailing suction hopper dredger is considerably higher under sailing conditions compared to normal ships with the same block coefficient. This is caused by the bottom valves or doors and the suction pipe guides in the hull.


Pagina 41 van 109

May 2005

wb3408B


R2

V

Fimpuls

R3

Rship G1

Δ pdraghead

R1

Rpipe

Fcutting+Rfriction G2

Rdraghead

Figure 2-41 Forces working on a TSHD

The required propulsion power appears to be decisive under the trailing condition, in particular when a combined drive is used. For this condition requirements are set regarding the trail speed, expected counter current and effective cutting forces at the draghead. For the trail speeds a normal value is 1.5 m/s with a counter current of 1 m/s. At these velocities the resistance of the hull, as could be expected, is little. The largest resistance arises from the dragging of the suction pipes over the seabed. This suction pipe resistance is composed of several components: The first, the hydro-visco components. In the direction perpendicular of the pipe: R pipe ↵ = C D

1 ρ w v sin β ⋅ v sin β ⋅ LD 2

In the direction parallel with the pipe:

R pipe6 = C L In which: CD = CL = D = L = R pipe↵ =

1 ρ w v cos β ⋅ v cos β ⋅ LD 2 Drag coefficient Lift coefficient Pipe diameter Pipe length Drag force


[-] [-] [m] [m] [N] Pagina 42 van 109

March 2003

Chapter 2 Trailing suction hopper dredger R pipe6

v β w

= = = =

Lift force Relative water velocity to the ship Pipe angle Density water

[N] [m/s] [°] [kg/m3]

The dimensionless coefficients CD and CL are apart from dependent on Reynolds number, also dependent on the appendages on the suction pipe. For a more accurate calculation it is better to divide the pipeline in different section with different projected areas. This has the advantage that the relative velocity of the water can be dependant of the waterdepth Another force that the propulsion has to generate, which is often forgotten, is the force needed to accelerate the dredge mixture to the trail velocity of the ship, this momentum force.

FMom = Q ⋅ ρ mix ⋅ v trail with: FMom Q vtrail mix

= = = =

Momentum force Pump capacity Trail speed Density mixture

[N] [m3/s] [m/s] [kg/m3]

The resistance of the draghead over the seabed. This force is more difficult to determine, but it can be derived as follows: During dredging erosion water shall enter the draghead at the backside and the sides. (See chapter 2.5.1.1.3) This pressure difference depends on the type of soil and the amount of jetwater used to loosen the soil (chapter 2.5.1.1). An average value for this pressure difference is 50 kPa. Multiplying the suction area of the draghead with the pressure difference gives the force that push the draghead to the seabed. Additional to this is the weight of the draghead on the bottom, which can be determined with a simple equilibrium equation. The coefficient of friction of steel on wet sand is 0.3 to 0.5. Additionally it is known that the draghead "bulldozers". Therefore a coefficient of friction of at least 0.5 must be used. Teeth or blades mounted in the draghead with intension to cut a significant part of the soil do increase the trail force significant. Effective trailing forces of 250 to 500 kN per pipe are common for the big dredgers If the total resistance of the suction pipe is known than this can be roughly converted to other diameters using: α

W1 ⎛ D1 ⎞ =⎜ ⎟ W2 ⎝ D2 ⎠

with α = 2.2 – 2.4

In conclusion the required effective trail force(s) are strongly dependent on the expected type of the dredging work and therefore to consider in detail during design. The above consideration can be visually clarified in the resistance-propulsion power chart:


Pagina 43 van 109

May 2005

wb3408B


Thrust-Resistance Diagram Thrust / Resistance [kN]

Opeation point when trailing 2000 Operation point when sailing 1500 Pipe only

1000

Sailing 500

Trailing

0 0

2

4

6

Speed [m/s]

8

10

Figure 2-42

In Figure 2.42 the effective propulsion force (trust), T_sailing (corrected for wake) as the ships resistance, R_sailing, are shown as a function of the ships speed. In the operating point "sailing" the supplied power is equal to the ships resistance. Under this condition the main engines are usually only driving the screws and the thrust curve is determined by the power of the main engines. This propulsion force curve can be described by a second-order polynomial:

Tsailing = a 0 + a 1 v s + a 2 v 2s During dredging the main engines usually drive, besides the screws, also the sand-pump installation (sand- and water-pump) either directly or through a generator/electric motor set. This means that less propulsion is available for the propulsion in this mode. Because the propulsion force is proportional to the propulsion power as:

T2 = constant, the propulsion P3

force curve is approximated under dredging (trailing) conditions by:

Ttrailing

FP I =a G H P JK trailing

0

2 3

+ a 1 v s + a 2 v s2

sailing

The sum of the ships resistance (R_ship) and the suction pipe resistance (R_pipe) has to be equal with this propulsion force curve (operating point "trailing"). Usually this condition appears to be decisive for the to be installed power of the main engines. If no combined drive is used than the "sailing" condition is normative for the required propulsion power. 2.2.4.2

The bow thruster power


Pagina 44 van 109

March 2003

Chapter 2 Trailing suction hopper dredger Maneuverability of THSD’s has improved much compare to the past. In the sixties and the seventies the socalled bow jets (Figure 2.43) were used. These made it possible to generate a transverse force with the sand-pumps. But for practical reasons this was done only when the pumproom was positioned in the bow. The effectiveness of these jets is pretty good, certainly for 2 to 3 knots. The construction costs are only a fraction of those for a bow thruster. Figure 2-43 Bow jet

However continuous use during dredging is not possible and so not economical. Therefore this idea is abandoned and one or more bow thrusters are used. However bow thrusters have the disadvantage of hardly any transverse force above 3 knots. There are different types on the market. A propeller mounted in a tunnel with a speed or pitch control, which means that the flow direction and capacity is control by the revolutions and speed direction or by changing the pitch of the propeller vanes. A axial flow pump by which the direction of the flow is control by valves and the capacity by the speed of the impeller.

Figure 2-44 Thruster types

With the increase of the jet-pump power one could consider to use these, totally o partly, for the bow jets. The required bow thruster power depends strongly on the expected type of work for which the trailing suction hopper dredger has to be designed. Prof.Ir. W.J.Vlasblom

Pagina 45 van 109

May 2005

wb3408B

2.2.5


Power balance

From the above mentioned it shows that a lot of power is installed in a trailing suction hopper dredger, that is: • • • •

the dredge-pump power the jet-pump power the propulsion power the bow thruster power

and of course the power for the electrical circuit on board. After all the suction pipes have to be lowered and raised. The valves and other auxiliary equipment must operate, etc. Powers of 15000 kW or more are no exception. Therefore it makes sense to take a close look to the power balance. For instance, separate drives for the propulsion and the sand-pumps are not always necessary or desirable. Most of the time several objects can be combined. The following will show that this is strongly related to the suction pipe configuration.

Figure 2-45 Direct drive

The most common combination is to drive both the propeller as well as the dredge-pump with one engine (Figure 2.45). The total installed power will not be much less than these units are separate as shown in Figure 2.46 but during sailing more power is available for a higher sailing speed and resulting in a higher production. If the units are driven directly, there will be no loss in generators, cables and electric motors. The speed control of the sand-pump is however poor. The engines run on constant speed, while adjustable propellers control the speed of the vessel, while the configuration of Figure 2.46 has fixed propellers (Why?).

Figure 2-46 Separate propulsion and dredge pump engines

When the trailing suction hopper dredger needs pump ashore installation than generally an extra transmission is installed in the gear-box to use the total available power for this installation. The same engine supplies the jet-pump power usually. In that case the gear box is fitted with an extra axis. The only disadvantage for this arrangement is the limitation in the suction pipe length. Of course this is not totally black-and-white. Extending of the inboard Prof.Ir. W.J.Vlasblom

Pagina 46 van 109

March 2003

Chapter 2 Trailing suction hopper dredger suction pipe offers the possibility to place a longer pipe on the deck, but this results in a lower production when dredging at large depths. Such a ship is put into service in 1992 and the concerned company (J.F.J. de Nul) took this decision intentionally.

Figure 2-47 TSHD with dredge pumps in the fore ship

If limitation of the suction pipe length is not desired both powers can be combined with the arrangement of Figure 2.47. In the engine room the main engines drive the adjustable screw, but on the other side a generator is placed that supplies the dredge-pump placed in the fore ship with energy. This is attended by an energy loss of 10 to 15 % of the power required. So for a sand-pump power of 2000 kW times two, there is a loss of approximately 400 to 500 kW! This also accounts for jet-pumps installed in the fore ship too. If the pump ashore installation needs the total power of the main engines this solution will require a considerable larger investment than the previous case. The speed control of the dredge pump can of course be well adjusted with an electrical drive. Between these two solutions there are of course all kinds of variants possible, which have been built in the past too. (See chapter 2.26 Main Layout) 25000

y = 0.4641x - 510.11 2

R = 0.8741

Pp [kW]

20000 15000 10000 5000 0 0

10000

20000

30000

40000

50000

Displacement [t]

Figure 2-48 Propulsion power


Pagina 47 van 109

May 2005

wb3408B


Bow trust power [kW]

3,500 3,000

y = 0.1758x - 19.495 R2 = 0.8036

2,500 2,000 1,500 1,000 500 0 0

2,000

4,000

6,000

8,000 10,000 12,000 14,000 16,000

Propulsion power during trailing [kW]

Figure 2-49 Bow thrust power

30000

y = 0.5806x

25000

2

R = 0.8931

Pi [kW]

20000 15000 10000 5000 0 0

10000

20000

30000

40000

50000

Displacement [t]

Figure 2-50 Total installed power


Pagina 48 van 109

March 2003


2.2.6

Main layout

Now the main dimensions of the ship and the dredging equipment are known, the layout of the ship has to be determined. 2.2.6.1 Single well ships Most currently built trailing suction hopper dredgers are of the single cargo-hold type. The hopper, also called well, is positioned somewhat forward of the middle of the ship. This is also the case when the bridge is on the foreship. The engine-room is always positioned in the stern. The trailing suction hopper dredgers used by the dredging industry are usually equipped with two adjustable screws.

The position of the pump-room, a with watertight bulkheads sealed space in which the sandpumps are located, also has a large influence on the layout of the trailing suction hopper dredger. The simplest and most efficient layout is the one where the pump-room is positioned just before the engine-room (Figure 2.45). In this case the main engines drive both the adjustable screws as the sand-pumps. Adjustable screws are necessary in this case because if the sailing velocity of the trailing suction hopper dredger is controlled by varying the number of revolutions of the engine then also the production of the pump changes which can lead to production loss. Since the sand-pumps on a trailing suction hopper dredger usually run on a fixed number of revolutions (variation of the suction depth has only a limited influence on the required head) the ships velocity can be easily adjusted by varying the pitch angle of the adjustable screws. Of course adjustable screws are more expensive and vulnerable than fixed screws. If fixed screws are desired than the layout shown in Figure 2.46 is appropriate with different engines for the sailing and dredging. An alternative for Figure 2.46 is Figure 2.51

Figure 2-51

It will be clear that in the first solution the total installed power is better used. After all during sailing the full power of the engines is available for the propulsion. However these solutions are also seen with adjustable screws. In both cases the limitation of the arrangement is the suction pipe length and therefore the suction depth. After all the suction pipes still need to be stored on board. If large dredging depths are also required (until ±70 m) than the layout of Figure 2.47 and 2.52 are automatically Prof.Ir. W.J.Vlasblom

Pagina 49 van 109

May 2005

wb3408B


obtained. Figure 2.52 is called the All Electric Ship, an development of nowadays. All power needed is delivered by the main engines via high efficient generators and motors.

Figure 2-52 The all electric ship

Of course there are may combinations possible with of these main layouts. The number of suction pipes may have some influence. Many smaller trailing suction hopper dredgers have only one suction pipe. Nevertheless these small trailing suction hopper dredgers are equipped with twin screws for two reasons: 1. The empty draught determines the maximum allowed propeller-diameter. Transferring a certain amount of power to one screw leads to a high revolutions, heavy loaded propeller with a relatively low efficiency. 2. A twin screw ship has a much higher maneuverability than a single screw ship Nevertheless, special trailing suction hopper dredgers such as gravel dredgers, are equipped with a single screw (see special applications) 2.2.6.2 Twin Hopper Trailers In the end of the sixties and starting seventies several trailing suction hopper dredgers were build with two separate hoppers. In these ships the engine-room and/or pump-room is positioned between the two hoppers. The main advantage of the twin hopper type is the smaller longitudinal ships bending moment that arises from the mid-ships connection of the engineroom and/or pump-room bulkheads.

Figure 2-53


Pagina 50 van 109

March 2003


Figure 2-54

The disadvantage of such ships that on one hand the hopper ratios are unfavorable for the settling process and to the other hand the total capacity is dived over both hoppers which will improve the sedimentation process somewhat. Besides several extra valves are needed to trim the ship sufficiently. These layouts are shown in Figure 2.53 and Figure 2.54. The accommodation is also positioned amidships. In both cases the main engines drive propellers and dredge-pumps. Besides the longer pipes for large dredging depth can be installed. Of course an electrical driven dredging installation is possible too. 2.2.6.3 Single well ships with a submerged-pump For larger suction depths, more than 50 m, the installation of a submerged-pump becomes economical. The submerged-pump, also called the suction pipe pump, can be driven electrical or hydraulically. The hydraulic drive exists on smaller trailing suction hopper dredgers.

On larger trailing suction hopper dredgers the pump and the electrical drive with bearings are accommodated in a compact compartment, directly mounted in the suction pipe. The number of revolutions of the electrical drive is chosen such that it corresponds with the required number of revolutions of the submerged-pump. This solution provides a compact and relative light construction. The submerged-pump related possible layouts of the engine rooms and/or pump-rooms are shown in Figure 2.55.

Figure 2-55 TSHD with inboard (direct driven) and submerged pumps


Pagina 51 van 109

May 2005

wb3408B


Figure 2-56 TSHD with inboard en submerged electrical driven pumps

Figure 2-57 Electrical driven pumps and direct driven propulsion

For smaller, simpler trailing suction hopper dredgers and converted barges submerged pumps can be used to. For such ships the dredge installation is composed of modules (Figure 2.58). The drive unit of a dredge installation is now positioned on the fore-deck. The (existing) engine room is located in the stern. Therefore adjustable propellers are not necessary..


Pagina 52 van 109

March 2003


Figure 2-58 Pump module on barges

2.2.6.4 Split hopper suction dredgers Split hopper suction dredgers can in principle also be divided as shown in Figure 2.59 and 2.60.

Figure 2-59

Figure 2-60

With the observation that both the engine-room and the pump-room are divided in the longitudinal direction (Figure 2.50 and 2.60).


Pagina 53 van 109

May 2005

wb3408B


The engine driver has to ascend to a height higher than sea level when he wants to go from starboard to portside.

Figure 2-61 Split TSHD

2.2.6.5 The position of the pump-room Positioning of the pumproom near the engine-room instead of in the fore-ship has the following advantages:

• • • • •

the control and the maintenance of the pump installation can be done in a simple way by the engine-room personnel. with an empty ship the suction intake is submerged deeper than in the fore-ship, as a result of the trim. as a result of the shape of the stern the dragheads will move less frequently under the ship base, when working in shallow waters or on slopes. the direct drive of the sand pump by the main engine is considerable more efficient than the transport of energy from the stern to the fore-ship. the total propulsion power can used easily for the pump ashore installation. With a foreship pumproom this requires considerable investments.

Of course there are also disadvantages: • the main disadvantage of the pump-room near the engine-room in the stern is the limitation in the dredging depth of the suction pipe, something that has become more important in the last few years. • the distribution of the weight is less ideal than with a pump-room in the front. For this reason the bridge is positioned on the bow nowadays. • because the dragheads are nearer to the screws there is an increased chance for cables picked up to get entangled in the propellers.


Pagina 54 van 109

March 2003


2.3 Technical Construction The technical construction of the trailing suction hopper dredger will be discussed in the flow direction of the dredging process.

2.3.1

The dredge installation

2.3.1.1

The dragheads

Figure 2-62 Modern draghead (Vasco da Gamma)

The draghead is the suction mouth of the trailing suction hopper dredger and is, with the sandpump, one of the most important components of the dredge installation. Looking at the amount of patent applications on the area of dragheads the conclusion can be made that there is a lot of knowledge of the operation of this device. Unfortunately this is not the case, the last 5 years the remarkable progress made about the understanding of excavation process in the draghead. Dragheads must be able to break up the coherence of varied soil types. The excavation process is done erosive, mechanical or by both methods. Dragheads are designed to resist the forces, needed to loosen and suck up the soil. They also need to be strong enough to withstand collisions with unknown objects in the dredge area. This especially gives high demands on the reliability of the equipment mounted on the draghead to control the water supply and/or cutting blade depth. Prof.Ir. W.J.Vlasblom

Pagina 55 van 109

May 2005

wb3408B


In general draghead consist of a fixed part that is connected to the suction pipe, sometimes helmet mentioned and a one or two pivoting part(s), the visor, which is mounted in the fixed part. The last part is (self) adjusting to keep in fully touch with the seabed. In the dredging industry different types of dragheads are used. The most known dragheads are: • the Hollandse (Dutch) draghead, also called IHC draghead (Figure 2.63 and 2.65) • the Californian draghead (Figure 2.64 and 2.66)

Figure 2-63 Dutch draghead

Figure 2-64 Californian draghead

Figure 2-65

Figure 2-66

Both type are developed based on the principal of erosion generated by the dredge pump flow. Nowadays these dragheads can be equipped with water jets too (Fig 2.65 and 2.66) In addition to the excavation of the soil, the jets are also important for the forming of the mixture in the draghead. The dragheads rest on the seabed by means of replaceable, so-called, heel-pads of wear resistant material. When dredging cohesive materials the dragheads are provided with blades or cutting teeth mounted in the visors. The position of the visor is fixed relative to the helmet corresponding with the average dredging depth Sometimes this position is controlled by hydraulic cylinders. Prof.Ir. W.J.Vlasblom

Pagina 56 van 109

March 2003

Chapter 2 Trailing suction hopper dredger When using tooth and/or blades one has to realize that different items can hook on to the draghead, causing high longitudinal forces in the suction pipe. This can be prevented by dividing the fixed part, the helmet, (Figure 2.67) in two parts, connected with a hinge on the top and breaker bolts at the bottom. The strength of the breaker bolts has to be slightly weaker than the weakest link of the several components of the suction pipe.

Figure 2-67


Pagina 57 van 109

May 2005

wb3408B


However, if they are designed such that they fail regularly than soon the two parts are welded together with the danger that the next link fails. Modern dragheads have one visor with jet nozzles over full width. At the backside of the visor replaceable teeth are fitted. The purposes of these teeth are to remove not eroded sand bands and to guide the flow in the direction of the suction pipe. Some of those dragheads do have movable water flaps to control the diluting water to the draghead. Visors can be adjusted either by bars or by hydraulic cylinders.

Figure 2-68 Modern dragheads

Furthermore fenders are mounted on the draghead, to prevent damage caused by the bumping of the draghead against the hull. By mounting these fenders on both sides, the draghead can be used both on starboard and port.

Figure 2.69 fenders of the draghead of the One piper TSHD Volvox Terra Nova Figure 2-69 Fender for protection

The connection between the movable visors and the fixed helmet is usually sealed with a rubber strip. This prevents the entering of "strange" water and it decreases the wear caused by the sand picked up by this "strange" water. 2.3.1.1.1 Other types of dragheads In the last 25 years a lot of experiments are performed with several types of dragheads, like: Prof.Ir. W.J.Vlasblom

Pagina 58 van 109

March 2003


Figure 2-70 Silt draghead

The silt head (Figure 2.70). A draghead specially designed for dredging silt and soft clays. The silt is pushed in the draghead, while the propulsion delivers the required force. The active draghead (Figure 2.71) A draghead with a hydraulic driven roller with cutting tools, able to cut firm clay or compact sand. The disadvantage of this dragheads was the ability to pick up cables and wires

Figure 2-71 Active draghead

And The venturi head (2.72). A draghead that would be hydraulically better shaped than the Hollandse and the Californian draghead and therefore would reach higher productions. The advantage of this draghead was the high trailing force due to the pressure difference over the draghead .

Figure 2-72 Venturi draghead

All these dragheads were not successful. Mostly the idea behind was good, but secondary reasons. like wear, sensitive for dirt, difficult to handle, etc. etc. Resulting in lower average productions than the earlier mentioned dragheads. Prof.Ir. W.J.Vlasblom

Pagina 59 van 109

May 2005

wb3408B


Aside from IHC there are also other companies that supplies dragheads. Usually these draghead are named after the company since they differ somehow from the standard dragheads. Examples are the "Van de Graaf-heads" and “VOSTA.” heads Furthermore every dredging company with self-respect has developed its own draghead, whether or not used. 2.3.1.2

The suction pipe

Figure 2-73 Suction pipe

The purpose of the suction pipe (Figure 2.73) is to make a connection between the seabed and the ship in order to make transport of dredge slurry possible. Because a fixed connection is not possible due to a varying water depth and the forces in size and direction, they have to comply with a number of important requirements: • • • • •

the dredging depth must be adjustable. there must be enough freedom of movement to maintain the connection with the seabed as good as possible. the bending moments due to the forces acting on the pipe should be kept as small as possible for reasons of strength and weight hit- and shock load resistant. a small pipeline resistance for the mixture flow.


Pagina 60 van 109

March 2003

Chapter 2 Trailing suction hopper dredger The trunnion slide (Figure 2.74) that slides between the hull guides during the raising and lowering of the suction pipe, is fitted with tapered cams that push the trunnion slide against the hull when the suction pipe is in front of the suction intake.

Figure 2-74 Trunnion slide with elbow

Mounted on this trunnion piece is a casted elbow, which can rotate around a horizontal axis, perpendicular to the hull. This hinge construction allows the suction pipe to be lowered to the desired depth. The elbow has two arms, positioned in the vertical plane of the suction pipe. On these arms, the upper or short piece pipe is mounted with hinges.This upper hinge makes the bending moments small, for example for the case

where the ship is swayed aside by the current. Between the elbow and the upper pipe a rubber suction hose is mounted that can move 40° to both sides. Steel rings are vulcanized in this suction sack to prevent a collapse of the suction sack by the subpressure as a result of the suction. The upper pipe is connected with the lower pipe by Figure 2-75 Universal joint the gimbal (Figure 2.75) and a second suction sack. This gimbal allows the two pipes to move independently, which is necessary in heavy weather and/or an irregular sea bottom. A turning gland (Figure 2.76) is mounted, usually directly behind the gimbal, in the lower pipe. This allows the lower pipe to rotate around its longitudinal axis, so that the draghead can also follow the bottom profile in the transverse direction.


Pagina 61 van 109

May 2005

wb3408B

Inner pipe

Wear ring


Outer pipe

Wearing ring

Wearing ring

Inner pipe

Lip seal

For small diameters (<900 mm)

Outer pipe For large diameters (>900 mm)

Figure 2-76 Turning glands

If the draghead is fitted for jet-water, a jet-water pipeline is mounted along the suction pipe (Figure 2.77).

Figure 2-77 Suction pipe with a jet water pipe

Because this pipeline also needs to follow all suction pipe motions, a lot of pressure hoses and elbows are needed, causing additional pressure losses in the jet-pipeline. The connection of the suction pipe with the ship becomes now more complicated.

Figure 2-78 Jet pipeline passing the universal joint Prof.Ir. W.J.Vlasblom

Pagina 62 van 109

March 2003


It becomes even more complicated when a submerged pump is mounted together with the suction pipe (Figure 2.79). Except the pipelines, a lot of cables for power supply and to control the pump speed are necessary. For the powerful pumps a special frame is necessary to carry the loads.

Figure 2-79 Submerged sand pump frame

2.3.1.3

The suction pipe gantries

Figure 2-80 Suction pipe gantries

The three suction pipe gantries serve to move the suction pipe either inboard or outboard.

The draghead gantry and the middle gantry are carried out mostly as an A-frame, connected with the main deck by a hinge-construction (Figure 2.81 and 2.82). A hydraulic cylinder or the hoisting wires controls the motion when moving the suction pipe in- or outboard.


Pagina 63 van 109

May 2005

wb3408B


Figure 2-81 3 different types of suction pipe gantries

Figure 2-82 Suction pipe elbow gantry

The suction elbow gantry consists of a fixed and a moveable part. The fixed part is welded to the main deck and is fitted with tracks for the wheels of the moveable part. (Figure 2.82). When the moveable part has reached the lowest [position than the trunnion slide can be lowered into the guides in the hull


Pagina 64 van 109

March 2003

Chapter 2 Trailing suction hopper dredger 2.3.1.4

The swell compensator

The swell compensator has contributed to the success of the trailing suction hopper dredger too. The most important goal of the swell compensator is to maintain the contact between the seabed and the ship, due to either both ship motions or the irregularities of the bottom contour. The swell compensator is positioned in the hoist-cable system of the draghead winch gantry. The swell compensator prevents the uncontrolled slackening and re-tensioning of the hoist cables. (Figure 2.83): Furthermore it maintains almost a constant pressure of the draghead on the seabed. A swell compensator system consists of the following components: An hydraulic cylinder, of which the head is fitted with one or two pulleys that guide the hoist cable of the draghead. One or more pressure vessels, of which the lower part is filled with oil and the upper part with air. A oil pump and reservoir. An air compressor. A pipeline system that connects the hydraulic or pneumatic components.

Figure 2-83 Swelll compensator

During an ascending motion of the ship the piston rod of the compensator is pushed downward as a result of the increasing force in the cable. The plunger then compresses the air in the pressure vessel. During the following descending motion of the ship the piston is pushed out again as a result of the increased pressure in the pressure vessels. This assures a tight cable at all times.

Elektrical driven draghead winch

Switching relays

Draghead winch controller

Air-oil vessel

Swelll compensator

Suction pipe Dragheadp

The average pressure in the pressure vessels is determined by the weight with which the draghead may rest on the bottom, or better: how much the swell compensator has to compensate this weight. It will be clear that the compensation in silt will be higher than in sand. In table 1 values are given as a guideline by IHC for a certain configuration.

Figure 2-84 Swell compensator with draghead winch controller


Pagina 65 van 109

May 2005

wb3408B


Table 1.

Suction depth

25 m

17.5 m

10 m

2.3.1.5

Draghead weight on bottom Fill air pressure P in P midstroke P out Draghead weight on bottom Fill air pressure P in P midstroke P out Draghead weight on bottom Fill air pressure P in P midstroke P out

kg bar bar bar bar kg bar bar bar bar kg bar bar bar bar

Compensation Mud Sand 80% 50% 50% 20% 1800 4500 4500 7200 15.0 15.0 8.0 8.0 26.2 17.9 18.6 9.8 24.7 17.1 17.1 9.4 23.3 16.4 15.8 9.0 2080 5200 5200 8320 15.0 15.0 8.0 8.0 30.0 20.1 21.0 10.8 27.9 19.1 19.1 10.3 26.1 18.2 17.5 9.8 2190 5475 5475 8760 15.0 15.0 8.0 8.0 31.4 20.8 21.8 11.2 29.1 19.8 19.8 10.6 27.1 18.9 18.1 10.1

The suction pipe winches Suction pipe winches have a grooved winding drum, with a length and /or diameter such that the there are 5 windings left on the drum (Figure 2.85) when the suction pipe is in its lowest position. When the suction pipe is out of the water. The load of the winches becomes heavier. To overcome this problem the wire is transport to a drum with a smaller diameter, which results in a lower torque for the winch drive.

The winch drives is either electrical or hydraulically.

Figure 2-85 Suction pipe winch


Pagina 66 van 109

March 2003


2.3.1.6 The dredge pump The dredge pump is the heart of the trailing suction hopper dredger.

The position of the dredge or sand pump has to meet certain requirements, certainly for the case without a suction pipe pump:

Figure 2-86 Dredge pump

1. The inboard placed dredge pump must be installed as low as possible. The deeper the pump is under the water level, the higher the concentration of the mixture can be. 2. The resistance of the pipeline must as low as possible. So short suction pipes, wide bends and no constrictions. 3. The direction of rotation of the pump has to comply with the rotation direction of the mixture caused by the bends in the piping system.

The second requirement cannot always be met because of demands for maintenance or the accessibility for inspection or removal of debris. There are also some practical objections concerning the third requirement. To comply with it the direction of rotation of the starboard and port pumps has to be opposite. This means more different spare parts like pump casings, impellers etc. Speed control of the dredge pumps is highly dependent on the type of drive. If the main engine directly drives the sand pump then speed regulation is not possible or only by stepwise control using a gearbox. Is the dredgepump driven by a separate diesel engine then speed control is possible, but the best control is obtained by an electric drive. It has to be mentioned that currently new developments in variable transmissions come available for diesel engine driven pumps. In most cases the requirements regarding the cavitation properties of the dredgepump are more important than the pressure properties. After all, even if the trailing suction hopper dredger has a pump ashore system, operations in dredging mode are considerably more frequent than the pump ashore mode. Both single walled and double walled pumps (Figures 2.87 and 2.88) are used in trailing suction hopper dredgers, dependent on the view and strategy of the dredging company. Double walled pumps have a separate inner pump casing that can be worn out without necessary repairs. This is achieved by pressure compensation. The pressure in a running pump is equal inside and outside the inner pump casing. To do this the space between inner and outer pump casing is filled with water and pressurized. Besides the advantage of a longer lifetime for the inner pump, this type of pumps gives a higher security in case of explosions.


Pagina 67 van 109

May 2005

wb3408B


Figure 2-87 Single wall dredge pump

Figure 2-88 Double wall dredge pump


Pagina 68 van 109

March 2003

Chapter 2 Trailing suction hopper dredger 2.3.1.7 The jet-water pump The water- or jet pumps are usually also positioned in the pump room. If these pumps are implemented with "clean-water pumps" than attention has to be paid to the position of the water inlet. After all contaminated water causes extra wear. Because the water surrounding the trailing suction hopper dredger is usually very muddy due to the overflow water, nowadays dredge pumps or weir resistant water pumps are used jet pumps. 2.3.1.8 The discharge pipeline The discharge pipeline connects the dredge pump and the hopper loading system, or the dredge pump and the shore pipeline. Every trailing suction hopper dredger has the possibility to discharge the dredge mixture directly. Previously this was done above the waterlevel, but with increasing environmental protection demands, the so-called poor mixture installation (Figure 2.89) is connected with an always submerged pipe-end. FLOW

doorsnede

doorsnede

FLOW

Figure 2-89 Poor mixture overboard systems


Pagina 69 van 109

May 2005

wb3408B

Designing Dredging Equipment Trailing suction hopper dredgers with one suction pipe do have one delivery pipe constructed over the middle of the hopper and connected the discharge side of the dredge pump. Trailing suction hopper dredgers with two suction pipes can also have one central delivery pipe (Figure 2.90) on which the discharges of both dredgepumps are connected, or two separate delivery pipes (Figure 2.91).

Figure 2-90 TSHD with one delivery line

In this last configuration it must be possible to use both delivery pipes with both pumps. When one of the suction pipes cuts of, whatever the cause may be, the ship still must be loaded equally athwart-ships to prevent listing. This requires more valves than for one central loading gully. Figure 2-91 TSHD with 2 delivery pipes

A similar complexity of the piping system arises also when shore pumping must be possible over starboard, port and over the bow. In a shore pumping installation the pressure pipe usually ends in a ball on which the shore piping can be connected. The bends are usually from cast steel for maintenance reasons. Prof.Ir. W.J.Vlasblom

Pagina 70 van 109

March 2003

Chapter 2 Trailing suction hopper dredger On every trailing suction hopper dredger it must be possible, whether or not it is equipped with a shore pump installation, to suck the water from the well. If poor settling material is sucked than it is strongly recommended to discard the water that is left in the well when dumping, before suction to prevent dilution of the sucked up mixture.

2.3.2

The hopper

2.3.2.1 The loading system The goal of the loading system is to dump the sucked up sand-water mixture as quiet and even as possible in the well. Three systems can be distinguished: • the diffuser system (Figure 2.92). • the central loading system (Figure 2.93). • the deep loading system (Figure 2.94).

All with several variants on which many have explored their creativity. In the diffuser system an open diffuser is positioned at the end of the delivery pipe, which discharges just under the highest overflow level. With such a system a good width distribution can be achieved. A disadvantage of the open diffuser is the reasonable amount of air that is taken in, which can obstruct the settling. Therefore closed diffusers are used sometimes that always discharge under the overflow level. The system is maintenance friendly of the system, compare to deep loading systems

overvloei

Waterniveau

Figure 2-92 Diffuser system

Via closed diffuser the mixture is dumped through a distribution box in the middle of the hopper. The mixture flows to both sides of the hopper, where adjustable overflows are fitted. Theoretically the hopper load remains equal, if the flow remains 2D. The turbulence degree will decrease due to the distribution of the flow rate to two sides. An additional advantage of this system is that due to the overflows on both sides of the hopper the ship can achieve even keel more easely. Prof.Ir. W.J.Vlasblom

overflow Discharge pipe

centrale Discharge

Figure 2-93 Central loading system Pagina 71 van 109

May 2005

wb3408B


overvloei

Figure 2-94 deep loading system

In a deep loading system the mixture is discharged deep in the well, whether or not with a vertical diffuser. The advantage of such a system is the energy reduction that is achieved as a result of the contact of the mixture with the already settled material. Another advantage mentioned the energy profit as a result of the siphon effect. In principle this is true, but there are quite a number of trailing suction hopper dredgers with a deep loading system for which it doesn't count because the delivery pipe is not airtight. Fitting of a simple kind of heavy loading or distribution valves in the delivery line causes this. These valves are necessary dredging coarse sand coarse or gravel. Than the settlement is that good that when these valves are not fitted the material settles immediately at the inlet and it becomes impossible to fill the hopper evenly (Figure 2.95). This results in a uneven trim vessel with water on their load

Diffusor

Distribution valves

Delivery pipeline

water Sand

Figure 2-95 Distribution valves in the delivery pipeline, necessary for coarse material

Apart from that the take-in of air largely reduces the advantage of the deep loading system. Another disadvantage is that it is very hard to discharge the mixture evenly distributed over the width of the hopper. This causes jets with turbulence production with as a result possible disturbance of the already settled material. A combination of the diffuser system and the deep load system is the diffuser box, which is placed half way the hopper height

Water level

Figure 2-96 Box diffuser system Prof.Ir. W.J.Vlasblom

Pagina 72 van 109

March 2003

Chapter 2 Trailing suction hopper dredger 2.3.2.2 The well shape The well shape has to comply with the following requirements:

• • • •

the least as possible obstructions in the well to keep the turbulence degree as low as possible in connection with the settling. as straight as possible side walls, preferably angling inward to improve the discharge of the load. easy accessible for maintenance. sand level above outside water level at least when the ship is in maximal draught, but preferable also at restricted draught (50-60% of maximum pay load).

The goal of the well or hopper is that the dredged material settles while the surplus water leaves the hopper through the overflow. These overflow losses are largely dependent on the parameter Q/(L*B)/w and less on Q/(B*H)/w. The first parameter is the ratio between the time a particle needs to settle and the time it stays in the hopper. The second parameter is the ratio between the horizontal velocity in the well and settle velocity of the particle and is a measure for the turbulence degree in the hopper. For a good settling a long narrow and shallow hopper shape is therefore favorable. A danger is however that no equal distributed load over the length of the hopper can be obtained which results in a need for distribution valves in the delivery pipe. These valves decrease the settle length the final result can become worse. Besides, long small ships with a limited depth results in small engine room(s). A compromise between price and performance has to be found. In the years past the obstructions in the hopper became less and less, as can be seen in the following cross sections (Figure 2.97):


Pagina 73 van 109

May 2005

wb3408B


a

b

c

d

e

f Figure 2-97 Different hopper cross sections


Pagina 74 van 109

March 2003


The last years hoppers with a V-shape become more and more popular

Figure 2-98 V-shape cross section

A well-shaped hopper (Figure 2.99) without any obstacle is formed by the split hopper suction dredger. There are no bars or obstacles, because the ship has no doors or valves but splits in two parts. The largest split hopper suction dredger built, has a deadweight of 7000 ton.

Figure 2-99 Split TSHD


Pagina 75 van 109

May 2005

wb3408B


The installation of pump ashore systems, as well as the requirement for easy maintenance have caused that, in general, closed hoppers hardly build, although they have certain advantages. (Figure 2.97e) •

In heavy seas rolling and pitching of the ship with a open hopper causes water movements and splashing over the deck of the mixture. A ship having with a closed hopper and a small overpressure, the water displacements during the rolling and pitching will be much less, which improves settling.

•

The free space on the deck of a closed hopper is also seen as an advantage. Especially during mobilization, the trip from one job to another, when all kinds of equipment can be stored on the deck. During dredging these have to be removed to increase the deadweight of the ship.

2.3.2.3 The overflow type At present almost all trailing suction hopper dredgers are built with a continuous adjustable overflow (Figure 2.101 & 2.102). Besides that most trailing suction hopper dredgers are of the so-called Constant Tonnage System, which requires a continuous adjustable overflow system.

Figure 2-101 Adjustable overflow over the full width of the hopper

Figure 2-100 Overflow with environmental valve


Figure 2-102 Standard adjustable overflow.

Pagina 76 van 109

March 2003


There are however differences in the shape and place of the overflow to in order to increase the effective settling length (Figure 2.103 and 2.)

Figure 2-103 Flow of two round overflows on the side

Figure 2-104 Flow of straight overflow at the end

A requirement that gets increasing attention is the environmental friendly overflow. Environmentalists do not liked a “beautiful” silt-jet behind the dredger. Dredging is often associated with polluted silt, so everything visible behind the dredger is “polluted”. A method to reduce the visibility of the overflow losses is to prevent the intake of air by the flow. This means that the overflow has to work as a non-free fall spillway instead of a free fall spillway. This can be done by building a so-called environmental valve (Figure 2.100) in the overflow. However, it is of course much better to design the overflow such that it works as an imperfect weir. This leads to a higher head (the height of the fluid surface above the upper side of the overflow).

2.3.2.4 The discharge system As said earlier, discharging the load can be done in two completely different ways, either by dumping or by pumping. 2.3.2.4.1 Dumping systems The goal of the dumping system is to discharge as quick as possible the material dredged with great effort.

All kind of systems are available. Expensive conical valves (Figure 2.105a), simple bottom doors (Figure 2.105b), horizontal sliding doors or valves (Figure 2.105c) or a ship that splits totally in two halves (fig 2.105d). There are also several exotic systems (fig 2.105f to 2.105h) all with their specific advantages and disadvantages. The lijster valve (Figure 2.105f) is very Prof.Ir. W.J.Vlasblom

Pagina 77 van 109

May 2005

wb3408B


expensive and takes a significant loss of hopper space. Recesses valves (Figure 2.105g) influence the stability unfavorable and necessitate a larger hull.

A

B

C

D

E

F

G

H

Figure 2-105 Different discharge systems

Requirements for his dumping systems •

• •

First of all the ship has to be able to discharge the load in a short time, as completely as possible (so without any load left) and for all types of soil. This means that the discharge area has to be large enough. Dependent on the dredged material the discharge-area ratio (the ratio total discharge-area/ horizontal hopper area) increases from 10% for slurries to 50% for the cohesive soil types. For general useable ships this will be about 30% of the hopper area. As already mentioned in chapter 2.2.3.10 the discharge is better as the outflow behaves like a plane symmetrical flow. The length/width ratio of the discharge opening has to apply to L ≥ 3B. Furthermore as few as possible protruding parts are allowed in the hopper, they can cause bridging of the material. Additionally they have the disadvantage of forming an obstruction for the settling too. An proper sealing under all circumstances. This demand increases in importance when (polluted) silt is dredged.


Pagina 78 van 109

March 2003

Chapter 2 Trailing suction hopper dredger • • •

Little or no influence on the ships resistance. Maintenance friendly. Places where wear can occur have to be easily accessible. Possibility for discharging the load in shallow waters and grounded ship. Regarding the first requirement the doors have the advantage over the others and for the last four demands the conical valve or the split-hopper. Dumping in shallow water can also be achieved with so-called shallow dumping doors. (Figure 2.106). Figure 2-106 Shallow dumping doors

The operation of the dumping system is mainly done by a hydraulic system. For the doors and the valves the cylinders are positioned vertical. The doors or valves in this system can be operated in groups, usually three. In every group the hydraulic system controls both the starboard and the corresponding port cylinder.

For the horizontal sliding bottom valves two cylinders positioned in the longitudinal direction of the ship activate those. Both cylinders move simultaneously, so all doors are open at the same moment.

Hopper

Cilinder for closing the valves valves

Bottom plating

Cilinder for closing the valves

closed open Discharge apertures

Figure 2-107 Sliding bottom valves

The split hopper dredger has a hopper without obstacles and in opened position one large discharge opening (plane symmetrical flow) and therefore a high discharge velocity, especially Prof.Ir. W.J.Vlasblom

Pagina 79 van 109

May 2005

wb3408B


useful to dump submerged dams. The split hopper dredger can under grounded conditions discharge well. The frequently mentioned advantage of well discharging cohesive soils is disappointing in practice. Usually the bottom plates in the hopper, even in opened position are insufficiently steep to be assured of a good discharge (Figure 2.108). For a split hopper dredger dumping is done by the splitting of the ship in the longitudinal direction. The two halves are connected with hydraulic cylinders and hinges. Of course the deckhouse and the accommodation remain upright during the splitting, because it is connected with the deck by hinges and hinge rods. A

B

Figure 2-108 Split hopper dredger C

Figure 2-109 Different mechanism

2.3.2.4.2 The pump ashore system Except for direct discharge or dumping, it can be desirable for certain works to pump straight to shore, not only for technical reasons but sometimes also for financial reasons. In principal direct discharge and re-handling with a cutter suction dredger is cheaper, however several important financial conditions have to be met:

• •

The work must have a sufficient size to earn back the mobilization costs of an extra cutter suction dredger. This also counts for the re-handling pit, from which the cutter suction dredger pumps the dumped sand to the reclamation area. This can be positive if such a dump can be situated within the work.

If the work is done with more trailing suction hopper dredgers it is in many cases beneficial to discharge directly and re-handle the sand. Because, even having two identical trailing suction hopper dredgers on the job, the stochastic behavior of the dredging process causes that at a certain time that the two ships arrive at the same time at the connection point for pumping ashore, causing waiting for one of the dredgers.


Pagina 80 van 109

March 2003

Chapter 2 Trailing suction hopper dredger But there are also works where the direct pumping ashore or so-called rainbowing has large advantages. For example works at sea like beach nourishments. For that goal small trailing suction hopper dredgers are equipped with pump ashore equipment. There are also jobs without space for a re-handling pit.

Figure 2-110 Rainbowing

Besides, there are jobs requiring controlled dumping of their load at a certain depth and in a relative small area. Then the material is pumped back through the suction pipe. This has been the case at the Oosterschelde works and is done too when covering pipelines.

Upper doors Valve Valve

Dredge pump

Self discharge channel Water intake

Flow direction

Bottum valves

Figure 2-111 Longitude cross section pump ashore system

The decision to equip a ship with a pump ashore system is not taken just before the work needs it. Except for the fact that the preparation and the fitting time can be more than half a year, it is also much more expensive than when it is fitted directly during the construction of the ship. Ships initially not fitted with a pump ashore system don’t have mostly today. Nowadays the European dredging contractor usually chooses for a pump ashore system. A pump ashore discharge system consists of one or two suction channels, situated at both sides of the center-keelson (Figure 2.112 under) or a pipe centrally placed in the center-keelson (Figure 2.112 upper). In the first case the top of this suction or self-emptying channel is fitted with so-called top-doors, by which the sand can be supplied into the channel. Transport water is mostly supplied in two ways, first through the channel, which is connected in some way with Prof.Ir. W.J.Vlasblom

Pagina 81 van 109

May 2005

wb3408B


the outside water and second by the jetpumps that fluidize or erode the sand in the surrounding where the sand has to enter the channel. The mixtures pumped ashore with a well-designed installation do have very high densities. For example 7500 m3/h in a 800 mm pipe. Of course this is also dependent on the type of sand. The rest load, the load that cannot or hardly be removed, is a measure for the design of the shore pump discharge system. For the mono-hull ships it may not be more than 5% of the total load.

Rods for opening and closing Upper door Suction channel for self-discharching

Rubber seal

Rubber seal

Pivot

Bottom door

Figure 2-112 central discharge pipe line (above) and channels on both sides of the keelson (under)

In split hopper dredger the self discharge channel(Figure 2.113) is situated exactly in the middle, between the connection of the two halves. For split hopper dredgers this rest load is zero, except for cohesive materials.

Self empty channel Figure 2-113


Pagina 82 van 109

March 2003

Chapter 2 Trailing suction hopper dredger Except for trailing suction hopper dredgers having besides bottom doors or valves, a pump ashore discharge system, there are also trailing suction hopper dredgers without a bottom discharge system, but with a self-discharge system. This is usually seen on aggregates hopper dredgers. The self-discharging happens mechanically, either with a dredging wheel (Figure 2.114) or with a clamshell that grabs over the full width of the hopper.

Figure 2-114 Dredging wheel unloader (Left) and clamshell unloader (right)

2.3.3

The propulsion

Trailing suction hopper dredgers in general two controllable pits propellers. (see also chapter 2.25) Only in the sea mining industry trailing suction hopper dredgers with only one screw can be found, whether or not controllable pitch. The advantage of controllable pitch propellers has to do with the method of operation of the ships. On one hand the ship needs enough propulsion power at relative slow speed of 2 to 3 knots to drag the suction pipes over the seabed. On the other side the sailing speed from and to the borrow area should be as high as possible, normally between 12 and 15 knots. TO fulfill both requirements the propellers are placed within nozzles. Additionally the concept of double and adjustable screws strongly improves the maneuverability. A trailing suction hopper dredger needs surely good maneuverability. For instance dredging along a quay wall with a ship with a length of 100m or more on a distance of less than 10m. When maintaining harbors trailer dredgers always moves in shipping lanes. This in contrast with merchant shipping stays in the harbor as short as possible. The maneuverability has strongly improved over the years. Not only by installing more powerful bow thrusters and in some cases even aft thrusters, but also by (special) rudders with large angles

2.3.4

The maneuverability

The trailing speed of trailing suction hopper dredger dredges is 2 to 3 knots (1 to 1.5 m/s). At this velocity the maneuverability needs to be high. After all the higher the maneuverability the less the over-dredging (outside the tolerances) and the less a chance on collisions there will be. Therefore most trailing suction hopper dredgers are equipped with double propellers and one or more bow thrusters. If Dynamic Positioning/Tracking (DP/DT) is stern thrusters are sometimes installed too. To maneuver the following options are available on a trailing suction hopper dredger: • • •

Just rudders Just the adjustable screws Just the bow screw and/or stern screw


Pagina 83 van 109

May 2005

wb3408B •


A combination of these

Which possibility will be used depends strongly on the direction in which the ship has to sail and the effectiveness of the various options under certain circumstances. The thrusters are only effective for very slow forward velocities. Above 2 to 3 knots the effect is mostly gone, the combination of propellers and the rudders are in that case a better option. However, the maneuverability is also strongly dependent on the center position of the rudders in relation to the propellers. On trailing suction hopper dredgers these are usually positioned more inboard in relation to the direction of the propeller shafts to be able to exchange the propellers without removing the rudders. Turning with one propeller forward (port) and one backward (starboard) with both rudders fully starboard is now less effective than the starboard propeller full ahead. After all in the first case the port propeller will hardly exert any force on the rudder. Is a transverse movement desired and the ship is equipped with both a bow and stern thruster than it is logical to use these. If there is no stern thruster available the transverse movement can be generated by rotating the adjustable screws opposite (Figure 2.115).

S

Also the effects of flow during dredging have to be compensated either by the bow thruster or delivering more power to one of the propellers than the other.

-

+

S

a

b

Figure 2-115 Opetration of adjustable screws


Pagina 84 van 109

March 2003


2.4 Strength and stability 2.4.1

Strength

Every sea-going vessel longer than 24 m, and therefore also a trailing suction hopper dredger must have a load line assigned according international agreed rules. The free board is the distance from the load line to the top of the main deck. The size of this free board is indicated on the vessel both on port and starboard by the Plimsoll-line1 (Figure 2.116) (Samuel Plimson let the English Parliament approve an act in 1876 that had to prevent the overloading of ships). This line indicates, except for the allowed loading level in several different waters, also the initials of the registering agency of the ship.

Top of main deck

TFW FW L

T

R

S W

or:

B

V

G

L

N

V

A

B

WNA

Figure 2-116 Plimsoll mark

Every seaship loaded to the International Free Board Line, has to comply with certain demands for strength. In principal there are two demands: 1. demands of strength concerning the loading of the ship until the allowed draught on flat water. 2. demands of strength concerning the wave forces on the ship For this last condition a distinction is made of the working areas of the ship. The so-called classification: 1. Deep sea ( haute mer). Is assigned to ships capable for transoceanic navigation. 2. Great coasting trade (grand cobotage). Assigned to ships deemed suitable to perform deep sea voyages but not transoceanic navigation. 3. Small coasting trade (petit cabotage). Assigned to ships that may not sail further from the coast than a distance from the coast that they can reach a save harbor or mooring place within six hours. 4. sheltered waters (eaux arbitrées). This class is assigned to ships that are allowed to sail, usually under good circumstances, at most at a small distance from the coast (mostly less than 15 miles). Above mentioned classification, of the Bureau Veritas, is international acknowledged, as well as those of other classification bureaus (Lloyd’s Register, Germanische Lloyd, Norske Veritas, American Bureau of Shipping and others). In the dredging industry there is a by local authorities allowed draught, known as the dredging mark. That is the allowed draught that is usually set in the middle between the international free Prof.Ir. W.J.Vlasblom

Pagina 85 van 109

May 2005

wb3408B


board and the top of the main deck of the ship. The ship must of course be able to carry the loads that can arise under such circumstances. Trailing suction hopper dredgers that are loaded to the dredging mark are not allowed to make international trips. Except for classifications there are also notations that are related to the rules for building specialty ships. Both the trailing suction hopper dredger as the stationary suction dredgers are assigned to those rules.

2.4.2

Stability

Except demands regarding the strength, a ship has to comply too with a minimum stability. For sea-going ships the international demands apply, dependent on the type of the ship. For trailing suction hopper dredger in principal the same rules apply as for sea-going cargo vessels. Definition:

Stability is the ability of a floating construction (ship) to return to its original equilibrium position when it is disturbed from its equilibrium position by external effects.

The stability of a ship is determined by a lot of factors, like the shape, the weight, the weight distribution and particular for a trailing suction hopper dredger all so-called free liquid surfaces in relation with the "wet surface". Wind, waves, movement of the cargo, movements of liquid cargo, sharp turns, etc can cause forces or moments that can bring the ship out of equilibrium. When a ship tilts, the position of the mass center of gravity doesn't change as long as the cargo doesn't move. This is in contradiction with the center of buoyancy that shifts to the side to which the ship tilts (Figure 2.117). M0 , N

FB G B

a B FG

K

Figure 2-117 Recovering moment

The upward force remains, of course, the same but opposite to the weight, but their worklines are now shifted apart over a distance a. They form a moment that tries to bring the ship back in equilibrium. This moment is called the static stability. The work-line of the upward force cuts the symmetry plane in a point that is called the meta-center M. For small angles of heel (<6°) this point can be considered as fixed (initial meta-center). The distance between the center of gravity and the meta-center is also called meta-center height MG. For larger angles of heel the meta-center is dependent on the angle of heel (false meta-center).


Pagina 86 van 109

March 2003

Chapter 2 Trailing suction hopper dredger From the Figure 2.117 can be directly derived that : • •

The arm of the static stability is equal to MG*sin ϕ. There is only an equilibrium recovering moment when the meta-center is above the center of gravity of the ship.

If the arm of the static stability is set out as a function of the angle of heel ϕ than a curve is obtained that looks globally like Figure 2.118.

B C A

0

10

D

20

30

40

50

60

70

80

angles of heel (degrees)

Figure 2-118

Every ship has to comply with the minimum stability curve (Figure 2.119).

0,30 0,20 MG = 0,15 0,10 0 10

20

30

40

50

angle of heel (degrees)

Figure 2-119

This is determined with the following requirements: • The surface under the curve to a angle of heel of 30° has to be at least 0.055 radial. • The surface under the curve to a angle of heel of x° has to be at least 0.09 radial. • The surface under the curve between the angles of heel of 30° to x° has to be at least 0.03 radial. • The arm of the static stability has to be at least 0.2 m. • The initial meta-center height has to be at least 0.15 m. In the above mentioned requirements x° is equal to 40° or a smaller angle that is indicated by openings in the hull or deckhouse that cannot be closed watertight. With the above mentioned Prof.Ir. W.J.Vlasblom

Pagina 87 van 109

May 2005

wb3408B


stability curves it has been assumed that the mass center of gravity does not shift but remains in the symmetry plane. If a fuel or water tank is not completely filled, the fluid will try to maintain a horizontal level independent of the tilt of the ship. This so-called free water surface is the cause, however, of a shift of the mass center of gravity outside of the symmetry plane. As a result the arm of the raising couple becomes smaller. It is clear that the effects of a free liquid surface in all possible storage tanks have to be taken into account in a stability calculation. The free liquid surface is not only important for the tanks of common ships, but particular important for ships with a relative large free liquid surface like a trailing suction hopper dredger.

2.5 The dredging process As already described in paragraph 2.1.4, the dredging process of a trailing suction hopper dredger consists of the cycle of dredging, sailing to the discharge area, discharging and sailing back to the dredging area. Every part of this cycle contributes more or less to the production. So the less malfunctions occur in the separate processes the higher is the cycle production. In the following chapter these cycle parts and the connected dredging processes are discussed.

2.5.1

The loading process

The loading process can be divided in excavation, the transport and the deposit of the material in the hopper. 2.5.1.1 The excavation Though other working methods exist, in principal the trailing suction hopper dredger deepens a large area entirely. The different layers of soil are removed horizontally. This in contrast to the cutter suction dredger and surely the suction dredger, that first deepen locally and than slowly expand horizontally. This working method has consequences for the determination of the material to be removed. Usually the horizontal variation, for instance the grain size or the chance of soil type, is considerably less than the vertical variation. This also implies that the mixture of the several layers is considerably less, which gives less meaning to an average material in the dredging area.

The trailing suction hopper dredger can in principal be deployed in nearly all soil types. Only the efficiency is strongly dependent on the soil type and the power and means to break up the coherency of the soil type. When excavating with dragheads the soil type is very important. In the excavating process the following materials can be distinguished: • • •

Liquid soil types (silt and soft clay). Cohesive soil types (firm clay, soft rock). Non-cohesive soil types (sand and gravel).

2.5.1.1.1 Excavating of liquid soil types When dredging silt or soft clay the Attenbergs limits (plasticity-index and the liquid-index) are important. The first index determines if the soil type behaves clayey or sandy. For a plasticityindex < 7 the material behaves sandy. The second index determines if the material behaves like


Pagina 88 van 109

March 2003

Chapter 2 Trailing suction hopper dredger a fluid and thus easy to dredge or firm and has to be cut. A soil type behaves like a fluid when the water content is close to the liquid limit. For a fluid-like behavior the liquid-index must be like: Water content

0 Firm

w − wp Ip

> 0.9 100 % LiquidWater

Plastic Plastic limit

Liquid limit

Plasticity index Figure 2-120

When dredging a liquid-like soil the volumetric concentration, Cv =

ρ mixture − ρ water , is almost ρ situ − ρ water

independent of the in situ density. Also the dimensions and type of the draghead have hardly matters. This means that the fill rate also is almost constant. For virginal fluid silt this is around 70 to 75 %. Then the ship is loaded "until overflow". The nett suction time is totally determined by the rheological behavior of the silt. If there is a lot of contamination, like stones, wires, old bikes, etc. in the silt or if the length of the dredging area is small, requiring frequently turning, the fill rate will reasonably decrease. When debris clogs the draghead, the dredge-master will dilute the mixture. Besides that regular stops for removing the debris in the draghead as well the restarts of the process, dilutes the mixture too. Fill rates of 40 % or less are easily reached. When the silt gets a more consistent behavior, thus a lower liquid-index, the fill rate to the overflow decreases. But because the silt is more consistent it will behave less like a homogeneous fluid and more like a mixture of pieces silt/clay in a heavy transport fluid. The loading after the overflow is reached, with a lot of overflow losses, becomes interesting again; therefore the fill rate can still be reasonable. However the suction time will increase. In silt, as a result of the decay processes of organic material, gas can exist in the form of bubbles. Besides it is possible too that this gas is dissolved in the pore-water. When dredging silt, the gas-bubbles will grow when moving upwards caused by the pressure drop in the suction-pipe. (p*V=constant) Regarding physics this situation is almost equal to the forming of vapor bubbles in water during a pressure drop, however than it is called cavitation. Because cavitation decreases the performance of the dredge pump, this will also be the case with gas bubbles. The advantage with gas bubbles is that it happens in the pipeline system before the pump. This creates the possibility to take away a part of the gas bubbles before they implode in the pump. For this reasons a de-gassing installation is mounted in the pipeline just before the pump. A well-designed de-gassings installation does not or hardly decrease the performance of the pump. Two systems are used: a de-gassings installation with accumulator (Figure 2.121) or a de-gassings installation with a gas-extractor tank (Figure 2.122).


Pagina 89 van 109

May 2005

wb3408B

Designing Dredging Equipment Atm air

B

A

CV Waterlevel at empty ship

Valve

ejector

accumulator High

Valve

low water pump

Remote controled valve

Water intake

filter

(B goes open when A is closed) (CV = controlable valve)

Figure 2-121 Degassing installation with accumulator

Gas discharge VW = water supply pump BT = buffertank GA = gas-suction mouth VA = vacuüm-control valve RA = control valve LP = drain pump WRP = water-ring pump MRA = mixture return-valve MRP = mixture return-pump

WRP

BT

RA

Water discharge to drain MRP MRA

VA

LP VW GA

max min

Figure 2-122 Degasssing installation with gas-extractor tank

2.5.1.1.2 Excavating in cohesive soil types In cohesive soil types, like very soft rocks, clay and to a less extend in silt, the cutting dominates the excavating process. In the dragheads blades, chisels or teeth are mounted (Figure 2.123). A well-shaped design is important to prevent clogging. Besides this improves the mixture forming too.


Pagina 90 van 109

March 2003


Figure 2-123 Modular draghead with a “teeth beam”

The linear cutting theories for rock cutting and undrained clay cutting apply here. In this case the cutting forces for the applied trail-velocities are only slightly speed-dependent. Besides the cutting forces increase linear with the depth. This means that the specific energy is almost constant for this cutting process. The pressure difference over the draghead plays not or hardly a role for the cutting forces. To keep the blades pushed into the soil the pressure difference over the draghead is usually insufficient and the visor has to be fixed to the helmet. The cutting depth is adjusted either by placing a stopper on the helmet related to the dredging depth or by hydraulic cylinders. As described in chapter 2.2.5.1 these cutting forces has to be provided by the propulsion. For the calculation of the cutting forces for design purposes it is the custom to use the specific energy concept. The specific energy Es is the energy needed to cut one m3. In formula:

Es = Es Ns Ps

= = =

Ns Ps Specific energy cutting power cutting production

[J] [W] [m3/s]

For the force applies:

Ps = v ⋅ d ⋅ b and for the power:

N s = v ⋅ Fs

Fs = with: v Fs d B

= = = =

Es ⋅ v ⋅ d ⋅ B = Es ⋅ d ⋅ B v drag velocity cutting force cutting depth draghead width


[m/s] [N] [m] [m] Pagina 91 van 109

May 2005

wb3408B


The specific energy of different soil types is known within the dredging companies, but can be calculated also the linear cutting theories. From the available thrust of the propellers the maximal available pulling force can be determined. For the calculation of the excavation production of the draghead, however, the average available force must be used. This depends among other things on the variation in the cutting depth.

Fpiek Faverage

is usually between 1.25 and 1.5 and sometimes even 2.

The production is totally determined by the cutting process and is independent of the pump flow rate, if it does not interfere too much with the mixture forming. 2.5.1.1.3 Excavating in non-cohesive soil types In non-cohesive soils, like sand and gravel, the excavation process within the draghead is physically complicated. If no jets are used to excavate the soil, the working of the draghead is totally based upon the erosion by the flow underneath the rims of the draghead generated by the dredgepump. The pressure difference over the draghead generated by this flow causes a groundwater flow underneath the draghead (Figure 2.124 and 2.125).

V

t

Excavating profile and grondwater flow underneath a draghead without jets

Groundwater flow underneath a draghead in longitudal direction

Figure 2-124

1/2b

1/2b

For the 2-D stationary situation this groundwater potentials can be describe accoding to :

LM MM MN

H arctan φ= π

F x + 1 bI F 1 IO GG 2 JJ − arctanGG x − 2 b JJ PP GH y JK GH y JK PPQ

Figure 2-125

The vertical groundwater flow under the draghead generated by this pressure difference causes a decrease of the effective stress in the sand. The critical hydraulic gradient for moving the particles follows from the equilibrium of the flow force with submerged weight of the particles. This leads to the equation: Prof.Ir. W.J.Vlasblom

Pagina 92 van 109

March 2003


LM OP 1 1 x+ b x− b 2 2 MM PP = − 100 − n ρ − ρ ≈ 1 _ MN y + FGH x + 21 bIJK y + FGH x − 21 bIJK PQ 100 ρ L OP dφ H M b Hb y H 1 1 = M >1 ⇒ >y + b ⇒ < − x=0 ⇒ 1 PP dy π M π b bπ 4 4 y + b N 4 Q

dφ H =− dy π

For

p

2

w

2

2

2

For y = 0 ⇒

dφ H = dy π

w

2

2

2

2

b Hb 1 >1 ⇒ > x2 − b2 1 2 π 4 x − b 4

⇒

2

x < b

H 1 + bπ 4

For y=0 this condition is always fulfilled. The term (100-n)/100 is the ratio sand particles over the total volume. For Y=0 the condition is always fulfilled because X/b is always smaller than or equal to ½ Critical depth Pressure differance H/b [-] 0

1

2

3

4

5

6

Critical depth for X=0 is shown in the Figure next and shows relatively very high critical depth!

0 0.2 y/b [-]

0.4 0.6 0.8 1 1.2 1.4

Figure 2-126

However, by the erosive action of the water entraining into the draghead, the grains want to move from each other (dilatancy) and a pore pressure drop, which increases the effective stresses of the grains. Which process is dominant depends on a number of factors. The question is if the ground water flow is able to keep up with the increase of pore volume of the sand. If that is not the case than a further decrease of the water pressures arises, with a decreased erosion process as a result. The ratio between the mixture flow rate Qm and the erosion flow rate Qe as function of the Cvd is:

Qmixture = Qerosion + Q pores + Qsand ⇒ 1 =

With:

Qerosion Q pores Q + + sand Qmixture Qmixture Qmixture

1=

Qerosion n Qsand Q Q n + 0 + sand ⇒ 1 = erosion + 0 Cvd + Cvd Qmixture 1 − n0 Qmixture Qmixture Qmixture 1 − n0

or

Qerosion C = 1 − vd 1 − n0 Qmixture

Qmixture = The mixture or suction pump flow rate. [m3/s] Qerosion = the erosion flow rate, sucked from underneath the rims of the [m3/s] draghead


Pagina 93 van 109

May 2005

wb3408B Qsand Qpores Cvd n0

Designing Dredging Equipment [m3/s] [m3/s] [-] [-]

= The sand flow rate. = The flow rate of the pore water present in the sand. = transport concentration pore ratio

This volume balance is shown in Figure 2.127. From a physical point of view, the concentration will increase as well when the erosion or crack velocity underneath the draghead increases (erosion line in the Figure 2.127) when Qmixture remains constant). From experience it is known that for a certain type of draghead without jets, the concentration Cvd is only slightly dependent on the mixture flow rate, which points out that the quotient

Qerosion remains almost Qmixture

constant. As a rule of thumb for the average erosion depth can be written: d = α

k 0.3 . vt0.9

In this k is the water permeability of the sand and vt the trail speed of the draghead, both in m/s. The factor α is dependent on the dimensions of the draghead.

Draghead without jet water 0.6 Volume balance 0.5

Cvd

0.4 Erosion 0.3

0.2

0.1

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Qe/Qm

Figure 2-127

With increasing width of the draghead the average depth will decrease, looking to the erosion process around the draghead. Unfortunately there is yet insufficient knowledge of this process to determine an optimum width of the draghead. The maximum concentration Cvd for the dragheads without jets remains limited to 15 % in loose sand. In a lot of cases however Cvd is smaller than 10 %. If jets are used to excavate the sand, this decreases the erosion flow rate, because the volume balance should be fulfilled:

Qmixture = Qerosion + Q jet + Qsand + Q pores

[m3/s]

With:


Pagina 94 van 109

March 2003

Chapter 2 Trailing suction hopper dredger Qmixture = the mixture or suction pump flow rate. [m3/s] Qerosion = the erosion flow rate, sucked from underneath the rims of the [m3/s] draghead Qjet = the jet pump flow rate. [m3/s] Qsand = the sand flow rate. [m3/s] Qpores = the flow rate of the pore water present in the sand. [m3/s] Cvd = transport concentration [-] n pore ratio [-] Furthermore:

Qsand = Cvd Qmixture n Q pores = Qsand 1− n With:

n

=

(transport concentration) and:

pore ratio [-]

From the above mentioned continuity condition now follows:

1−

Q jet Cvd Q = erosion + 1 − n Q mixture Q mixture

This is a bundle of lines under 45° in a

C Qerosion Q jet , diagram for constant values of vd Qmixture Qmixture 1− n

(Figure 2.128).

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

Qjet / Qmixture

0.7

0.8

0.9

1

Cvd/(1-n)=0 Cvd/(1-n)=0.2 Cvd/(1-n)=0.4 Cvd/(1-n)=0.6 Cvd/(1-n)=0.8

Figure 2-128 Relation between capacities to fulfill the volume balance in the draghead

This picture shows that high concentration or mixture densities can be reached only for low Q jet Q values of erosion and Q mixture Q mixture Prof.Ir. W.J.Vlasblom

Pagina 95 van 109

May 2005

wb3408B


In case of a large jetpump capacity the erosion flowrate can get negative value resulting in spillage behind the draghead. With jets well devided over the width of the draghead an erosion-profile can reached with an almost constant depth over the full width

Erosion profile for a draghead with a well designed jetsystem

Figure 2-129

As said earlier, a reasonable assumption is that the jetproduction is linear with the total momentum flux of the jet system and independent of the trail speed. The momentum I=ρwQu.

M sand = α ⋅ I = αρ w ⋅ Qu = αρ w ⋅ Q I Msand pjet Q U α

= = = = = =

ρw

=

2 p jet ρw

Momentum in N Eroded sand mass in kg/s per jet Jet pressure at the nozzle in Pa Jet capacity in m3/s Jet velocity at the nozzle in m/s Coefficient depending on the particle size, jet pressure, jet capacity and trailspeed. A reasonable assumption for alpha is α=0.1 Water density in kg/m3.

When the nozzle are divided well over the width of the draghead the mass M should fulfill the relation:

∑M

sand

= B ⋅ d ⋅ v trail

all jets

B D vtrail ρsitu ρparticle

= = = = =

ρ situ − ρ water ρ particle ρ particle − ρ water

Width draghead in m. Eroded layer thickness in m Trailspeed in m/s Density soil in situ kg/m3 Particle density in kg/m3

When the trailspeed is said to 1.5 m/s, which equals 3 knots and the product B.d can be calculated. In general the effective of the jet decreases somewhat with increasing pressure at constant momentum. This means that low pressure- high capacity jets are more effective than high pressure-low capacity jets. They use more specific energy too. On the other hand however, much jetwater dilutes the mixture density (Figure 2.128). So the designer has to search for the optimum solution between cost (power) and production Prof.Ir. W.J.Vlasblom

Pagina 96 van 109

March 2003

Chapter 2 Trailing suction hopper dredger Jet-water is used for loosening the soil within the dragheads, as well as to assist the process during discharging the load, either by dumping or by pumping ashore. The flow rate of the water pump is between 20 to 30 % of the sand pump flow rate and the pressure is usually between 5 and 15 bar.

1 Lρ p= M 2 MN ρ

sand w

C vd Q m α Q jet

OP PQ

2p ρw

2

The results are give in fig 2-130

2 1.8

10000 9000

1.6 1.4 1.2

8000 7000 6000

1 0.8 0.6 0.4

5000 4000 3000 2000

0.2 0

1000 0 0

0.2

0.4

0.6

0.8

Cvd p [kPa]

M sand = C vd Q mρ sand = αρ w Q jet

Density [t/m2], Qj/Qm,Cvd

The required pressure can be calculated using the same basic formula’s as mention in the forgoing chapter.

density Qjet/Qm p {kPa}

1

Cvd/(1-n)

Figure 2-130

The breaking up of the coherence of the soil, which is done in the draghead either by the erosion or by jets, can also be done by the gravity under certain circumstances. When the sand layer has sufficient thickness a narrow path is deepened to full depth as quickly as possible. Next the trailing suction hopper dredger keeps on dredging at the base of the embankment. By the breaches process the embankment will slowly move perpendicular to the trail direction (Figure 2.131). Besides the breach causes the sand to be looser packed at the bottom of the embankment. Also mixing of various materials takes place.

Movement of slope

Figure 2-131

The disadvantage of this method is, of course, that the material has to be obtained at greater depth. If the "horizontal" or "vertical" method is preferred depends therefore on the grain distribution of the various layers, the suction depth and how far the pump of the trailing suction hopper dredger is below the waterlevel. The dredging soft rock by trailing suction hopper dredgers is only done in exceptional cases. In fact only in those cases where the operating hours of a cutter suction dredger are so limited by the weather conditions that it is not profitable or where the amounts to be dredged are so limited that the mobilization of a cutter suction dredger is not profitable. Dredging rock with a trailing suction hopper dredger is not just done. The dredger has to be equipped for that. This means that the dragheads, the suction pipes and hull attachments able to resist the forces that during the ripping of rock.


Pagina 97 van 109

May 2005

wb3408B


2.5.1.2 The transport of the slurry In the course Dredging Processes II (Wb 3414) the pumping of sand-water mixtures will be discussed extensively, so that only specific cases will be discussed here with regard to the transport and deposition in the hopper of the dredged material.

If the trailing suction hopper dredger is limited for its dredging depth to a dredging depth of 30 m than one fixed pump-speed is sufficient. If the ship has to dredge over a deeper range of depth or equipped with an additional submerged pump, than the question rises whether the flow rate variations are not too high between the suction in shallow waters and at the maximum dredging depth. The maximum suction depth determines the highest pump speed, if the pump is sufficiently under water. If this pump-speed is fixed than the flow rate when dredging in shallow water will significantly larger than dredging at the maximum depth. Since overflow losses increase linear with the flow rate it must be considered if it is economical to equip the dredgepump with a speed control to keep the flow rate constant at different depth. Furthermore the pump will have to be optimized for either the dredging operations or pumping ashore, depending on the total expected time of operations under these modes. When no submerged pump is fitted, it might better to pursuit for straight a piping system in the suction line, even if lead to an extra elbow in the discharge line. 2.5.1.3 The loading In order to obtain the highest possible fill rate during the loading the hopper with nonsettling slurries, the poor mixture (mixture with a too little density) van be pumped straight overboard. An automated valve controller can easily do this. However, with the increase of environmental requirements this is banned nowadays.

For settling mixtures like pieces of clay, sand and gravel, a part will settle and a part will leave the hopper through the overflow. A rule of thumb sometimes followed is that all with a d50 < 75 μm flows overboard. A measure for the quality of the settling process is the relative cumulative overflow loss. This is defined as the ratio between the total amount of solids that leave the hopper through the overflow and the total amount of solids pumped in the hopper. This relative cumulative overflow loss is, except for the material properties as grain size, the grain distribution, shape and specific mass, also dependent on the loading conditions like the flow rate, concentration, turbulence intensity, temperature and the hopper geometry. These overflow losses are, like mentioned above, largely dependent on the parameter

( s Q) = s B⋅L

term

s0

and less of

( s Q) = s B⋅H

v0

(see reader: Dredging Processes I (Wb3413). The

Q is called the surface load. B⋅L

In these: Q L B H v0

= = = = =

the total in-going mixture flow rate the length of the hopper the width of the hopper the settling height in the hopper the drop velocity


Pagina 98 van 109

[m3/s] [m] [m] [m] [m/s]

March 2003

Chapter 2 Trailing suction hopper dredger The first parameter is the ratio between the time the particle needs to settle and the time that it stays in the hopper. The second parameter is the ratio between the horizontal velocity in the well and the settle velocity of the particle and is a measure for the degree of turbulence in the hopper.

The overflow losses as function of the earlier mentioned terms:

( s Q) = s B⋅H

v0

( s Q) = s B⋅L

s0

and

are “reasonably” approximated by the theory of Camp, although the

sedimentation process in the hopper is quite different as assumed by Camp. For a real understanding of the sedimentation process the reader is referred to the thesis of Dr.Ir. C. van Rhee . In Figure 2.132 the settled part (removal ratio), so Rt = (1-overflow losses), is shown as function of these two parameters.


Pagina 99 van 109

May 2005

wb3408B

Designing Dredging Equipment 2

1,0

3

4

6

8

2

3

4

6

8

2

3

4

6

8

S So 0,9

2,0 1,5

0,8

1,2 1,1

0,7

1,0 0,9 0,6

0,8 0,7

0,5

0,6 0,5

0,4 0,4 0,3 0,3

0,2

0,2

0,1

0,1

0 0,001

0,01

0,1

S 1 Vo

Figure 2-132 Camps diagram

By calculating the settling process in a number of steps the relative cumulative overflow losses can be determined as function of time or load rate. From the theory of Camp can be de derived that the influence of the bed height is marginally. This implies that during the loading process the overflow losses are almost constant. Although in practice loading curves are almost straight. The overflow rate is not. 2.5.1.3.1 Loading curve Dependent on the way of payment, in cubic meters or in Tons Dry Solids (TDS), the contractor will like to know the development of the volume in m3 or of the TDS in the hopper during loading. To do this it is necessary to measure the volume of the total load (sand and water). Acoustic silo indicators usually do this. The weight of the (useful) load is measured by determining the development of the draught as function of the time (chapter 2.2.2.1). From the volume and the weight of the useful load the volume in m3 or the TDS can be determined if the volume weight γz of the sand and the specific weight ρk of the sand and the water ρw are known.

The loading curve can be divided in three phases: Prof.Ir. W.J.Vlasblom

Pagina 100 van 109

March 2003

Chapter 2 Trailing suction hopper dredger 1. Before the overflow is reached:

Vload ( t ) = Qi t Gload ( t ) = Vload ( t ) γ i = Qiγ i t

γi −γ w t γ z −γ w γ −γ w γ −γ γ −γ γ −γ Gsand ( t ) = Vsand ( t ) z γ k t = Qi i w ⋅ z w γ k t = Qi i w γ k t γ k −γ w γ z −γ w γ k −γ w γ k −γ w

Vsand ( t ) = Qi

In this: Gload and Vload, the weight and the volume of the total load, so sand and water. Vsand the sand volume (including the pores) in the hopper and Gsand the weight of the sand (excluding the pore water), so TDS. Qi and Qu are the in- and out-going flow rate. γi, γk, γz and γw are the volume weights (γ = ρg) of the mixture, the sand grains, the sand volume with the pores and the water. In this it is silently assumed that the hopper is totally empty before the start of the suction. If this is not the case than volume must be increased with the value V0 and the weight with G0. 2. When the overflow is reached tov, but the ship is not yet on its dredge mark, the hopper volume remains constant (constant volume loading).

Qi = Qu Gi = Qiγ i and Gu = Quγ u with γ i > γ u > γ w and therefore:

Vload ( t ) = Vhopper ( t ) = constant = Vload ( tov ) Gload ( t ) = Gov + Qi (γ i − γ u )( t − tov )

(γ i − γ u ) t − t ( ov ) (γ z − γ w ) (γ − γ ) ov Gsand ( t ) = Gsand + Qi i u ( t − tov ) (γ k − γ w )

ov Vsand ( t ) = Vsand + Qi

ov ov Vsand and Gsand are the volume of the sand and the weight of the grains at the moment the

overflow is reached. 3. The overflow is reached and the ship is on the dredge mark. In this case the weight of the total load (water and sand) remains constant (constant tonnage loading).

Gi = GU = Qiγ i = Quγ u and therefore Qu = Qi


γi γu

Pagina 101 van 109

May 2005

wb3408B


mark mark Vload ( t ) = Vload − Qu ( t − tmark ) = Vload − Qi

Gload ( t ) = Gmark = constant

γi ( t − tmark ) γu

γi −γu ( t − tmark ) γ z −γ w γ −γ + Qi i u γ k ( t − tmark ) γ k −γ w

mark Vsand ( t ) = Vsand + Qi mark Gsand ( t ) = Gsand

mark mark Vsand and Gsand are the volume of the sand with pores and the weight of the sand grains

(TDS) on the moment the hopper reaches the valid dredge mark. The total load curve is now known in mass and volume if Qi, γi, γu, γk, γh and γw are known. γu can be determined from the overflow losses and γv depends on the type of soil.

Loading curve for hopper density =1450 kg/m3 V_mixture

V_sand

Load

W_sand

16000

Volume [m3] / Load [ton]

14000 12000 10000 8000 6000 4000 2000 0 0

20

40

60

80

100

120

Loading time [min]

Figure 2-133

For pure constant volume hoppers the weight of the load is proportional to the draught of the ship. This increases in time, though the mixture-volume in the hopper remains constant.


Pagina 102 van 109

March 2003


Loading curves for constant volume hopper V_mixture

V_sand

Load

W_sand

20000


18000 16000 14000 12000 10000 8000 6000 4000 2000 0 0

20

40

60

80 Loading time [min]

100

120

140

160

Figure 2-134

This does not account for the pure constant tonnage hoppers. Then the draught remains constant after reaching the overflow (Figure 2.135).

Loading curves for constant tonnage hopper V_mixture

V_sand

Load

W_sand

12000


10000 8000 6000 4000 2000 0 0

10

20

30

40 50 Loading time [min]

60

70

80

90

Figure 2-135

To calculate the weight of the load extra data is needed: the volume of the mixture and the volume-weight (or density) of the sand in the hopper. The first quantity is measured with silo indicators and the second by probing on several trips the volume of the sand. Now the determination of the load during the dredging process is done as follows: Prof.Ir. W.J.Vlasblom

Pagina 103 van 109

May 2005

wb3408B •


Before the start of the dredging the displacement and the weight of the water in the hopper is determined. The displacement by measuring the draught of the vessel and the watervolume by the silo indicators.

displacement empty ship = • •

displacement ⋅ ρ water ⋅ g volume water in hopper

During dredging the fore and aft draught of the ship is measured continuously and so the displacement as well as the mixture volume by means of silo indicators. By subtracting the start values from the momentary values of the displacement and the mixture volume, the weight of the dry load (TDS) can be determined with the following formula.

Gload −γ w Vload γ kVload TDS = γ k −γ w Gload = γ load is the volume weight of the mixture in the hopper. Vload Though the load nowadays usually is expressed in TDS, it does not imply that payment is also dependent on the amount of TDS. This can be: 1. ton dry solid (TDS) 2. m3 in the hopper (means of transport) 3. m3 in the excavation The mutual relation between these quantities is: TDS with volume load in the hopper:

Vload =

TDS ⎛ γ grains − γ water ⎞ ⎜ ⎟ γ grains ⎝ γ load − γ water ⎠

Therefore the conversion factor of TDS to m3:

fv =

Vload 1 = TDS γ grains

⎛ γ grains − γ water ⎞ ⋅⎜ ⎟ ⎝ γ load − γ water ⎠

And for m3 to TDS:

fTDS =

⎛ γ − γ water TDS = γ grains ⎜ load ⎜ γ grains − γ water Vload ⎝

⎞ ⎟⎟ ⎠

Shown in Figure 2.136.


Pagina 104 van 109

March 2003


Multiplica tion fa ctors for TDS to m 3 2

25

1.8 1.6

20

1.2

15

1 0.8

10

m3 to TDS

TDS to m3

1.4

0.6 0.4

5

0.2 0 1000

1200

1400

1600

1800

2000

0 2400

2200

Situ density [kg/m3]

Figure 2-136

An aspect that also takes place during loading is the change in the volume weight of the dredged material, the bulking, which can be positive, so more, as well as negative, so less. The production unit in the dredging industry is the cubic meter per time unit. Unfortunately this is not an unambiguous unit. A m3 in excavation appears to be a "different" m3 after settlement in the well. Because sand grains in the hopper are usually stacked looser than in situ. The volume weight in the hopper is lower than the situ volume weight. Also, as a result of overflow losses, more fine sand particles will flow overboard than coarse particles. If these particles are located in a matrix of coarser particles than the volume weight will decrease even if the stacking of the matrix remains the same. If this phenomena happens in the dredged material can be simply shown by comparing the sand curve with the Füller-distribution (Figure 2.137).

FÜLLER'S METHOD

FÜLLER'S METHOD 100

% by weight passing

overmaat fijn

overmaat grof

Füller

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

% by weight passing

overmaat fijn

overmaat grof

Füller

0 0

.1

.2

.3

.5 .6 .4 SQRT (d/dmax)

.7

.8

.9

1

0.01

0.1

10

1 SQRT (d/dmax)

Figure 2-137

In a Füller-distribution the cumulative grain distribution, given as function of

d d max

, is a pure

straight line. Such a distribution appears to give a maximum volume weight, which implies that the pores are constantly filled with the smaller particles. If the gradient of the smaller particles is above the Füller-distribution than there is a surplus of fine material and the above mentioned phenomenon would not show. If the gradient of the fine material is below the FüllerProf.Ir. W.J.Vlasblom

Pagina 105 van 109

May 2005

wb3408B


distribution than the fine material is embedded in the coarser material and the phenomenon shows. The volume weight in the hopper is usually lower than in situ. Dependent on the grain distribution, a situ m3 takes the same or more space in the hopper, caused by the increase of the ratio, which are filled with water. So the water takes this larger volume. Example:

Assume the in situ density of the sand ρ1 and the density in the hopper ρ2. The specific weight of the sand is ρk and of the water ρw. The cumulative overflow losses are ov and according the Füller distribution there is a surplus of fine material. If the situ volume is V1, then the volume in the hopper with in-situ density (1-ov) V1. The weight of solids of this volume must be equal to the solid weight of the volume V2. Weight of the volume V1 for ρ1:

G1 = (1 − ov ) ⋅ V1 ⋅

γ1 − γ w ⋅ γ k with γ = ρ g γ k −γ w

Weight of the volume V2:

G2 = V2 ⋅

γ 2 −γ w ⋅γ k γ k −γ w

Since G1 = G2 :

V2 γ −γ ρ − ρw = (1 − ov ) ⋅ 1 w = (1 − ov ) ⋅ 1 V1 γ 2 −γ w ρ2 − ρw Example: ρ1 ρ2 ρwater ov

→

= = = =

2000 kg/m3 1900 kg/m3 1020 kg/m3 10 %

V1 2000 − 1020 = (1 − ov ) ⋅ = 0.9 ⋅1.11 = 1.0 1900 − 1020 V2

So the volume in the hopper occupies the same space as the in the excavation. It has been silently assumed that the overflow losses do not flow back into the winning area. If that is the case than the term (1-ov) is discarded and the delivery becomes 11 %. If the fine sand particles are situated in a matrix of coarser particles than, for a similar stack of the coarser particles, G2 = 0.9 G1 with V1 = V2. This leads to:

(1 − ov ) ⋅V1 ⋅

γ1 − γ w γ −γ ⋅ γ k = V2 ⋅ 2 w ⋅ γ k ⇒ (1 − ov ) ⋅ ( ρ1 − ρ w ) = ρ 2 − ρ w γ k −γ w γ k −γ w

This gives in the example:


Pagina 106 van 109

March 2003


ρ 2 = 0.9 ⋅ ρ1 + 0.1⋅ ρ w = 1800 + 102 = 1902 kg/m3 If all overflow losses remain in the winning area than this still holds but as a result the original layer will be covered with 10% fine material at the end of the work. When sucking very loose sand the bulking can be smaller than 1. The bulking is than called negative. When dredging firm clay the bulking in the hopper is substantial, as is proven in the following example: Assume the situ density of the clay as 2000 kg/m3. After cutting the pore percentage of the clay fragments is 40 %. The volume weight is than ρ2 = 0.6*2000 + 0.4*1020 = 1608 kg/m3. And the bulking than will be: V1 2000 − 1020 = = 1.67 V2 1608 − 1020

This can be seen directly as the new volume is only 60 % of the original. During pumping ashore to a reclamation area, usually a negative bulking takes place, since the volume weight of the dump material is often higher than the volume weight of the material in the hopper and losses can occur at the reclamation.

2.5.2

Sailing from and to the discharging area

It will be clear that the sailing speed determined during the sea trials, for an empty as well as for a fully loaded ship, cannot be used as the average speed during the lifespan of the trailing suction hopper dredger. Between the dry dock periods the hull of the ship becomes overgrown with barnacles and seaweed and the propulsion engines and propellers are subjected to wear. This leads to a 5 to 10 percent lower average or operational speed in deep water than the sea trial speed. In general the trailing suction hopper dredger sails in seaways with a depth which gives the ship extra resistance. The trailing suction hopper dredger "feels" the bottom. The influence of the less deep seaway on the operational velocity is calculated with Lackenby's formula (Figure 2.138).

v shallow = v deep

R| L F A |S1 − MM01242 . || MM GH bd + Dg T N

2

I − 0.05J + 1 − K

1 c 1 c+ c c−

OPU| PP|V PQ||W

with:

c=e

g FGH v 4 IJK

b

g d+D ⋅

deep

in this: d D A

= = =

keel clearance draught of the ship wet cross-section of mid ship


[m] [m] [m2]

Pagina 107 van 109

May 2005

wb3408B


Sailingspeed according Lackerby Loaded

Empty

15 14.5

Sailing speed [knots]

14 13.5 13 12.5 12 11.5 11 10.5 10 0

5

10

15

20 25 keel clearance [m]

30

35

40

Figure 2-138

The sail time can now be determines with: N

Tvh = ∑ n +1 N

Tvt = ∑ n +1

sn

with draugth full

sn

with draugth empty

( vvol )n

(v )

leeg n

Another facet that has to be accounted for, are the sail-limitations in certain areas like harbors and narrow fairways. Furthermore the fairway has always to be checked for sufficient depth. In case of doubt it might even be wise to carry out a hydrographic survey

2.5.3

The discharge

As described in the chapter Technical Construction the trailing suction hopper dredger may be able to discharge its load in two ways, either by direct dumping or by means of the selfemptying installation by rainbowing or pumping to the shore. If the load can be dumped directly it has to be known if the depth of the dump area is always sufficient to sail with opened doors or valves, even with extremely low water. The increasing lack of dump areas it happens regularly that the depth of the dump is limited. In such a case it is advised to make a dump plan to use the dump as efficient as possible. For land reclamation works for which the first layer of the sand body can be dumped directly, a dump plan has to be made too, in order to dump directly as much material as possible, so that less material needs to be pumped ashore. The discharge of the load through the bottom doors or valves usually costs little time. For free flowing soils this is done within several minutes. The discharge time increases when the material becomes finer and more cohesive. For plastic clays this can increase to half an hour. For such a material it has to be checked that no load, the rest load, remains in the hopper. There is a possibility that this rest load increases with the number of trips. It appears that the longer the clay remains in the hopper the more difficult it is to flush it out. Prof.Ir. W.J.Vlasblom

Pagina 108 van 109

March 2003


Discharge through the hopper self-emptying installation is done to: • pump the load, through pressure piping to the shore. • to heighten, for example, submerged dumps that are too shallow to dump; the so-called rainbowing (Figure 2.139). • to accurate fill submerged dumps or to cover pipelines with the use of pipe dumping. After the pumps are started and the water comes out of the pipe the discharge of the load is started on the side of the hopper that is the furthest away from the pump. This assures that the pump is always as deep under water as possible. Because the material in the hopper is in general pretty loose packed, the process looks a lot like the process of a stationary suction dredger. The sand breaches to the opening of the suction pipe.

Figure 2-139 Rainbowing

If the hopper is not equipped with an installation that improves the breaching by means of water-jets, than, as a rule of thumb, the discharge time is equal to the suction time. If the hopper is equipped with water-jets to fluidize or loosen the load, than the discharge time can be shortened considerably. The discharge process through the hopper self-emptying installation behaves clearly like an Scurve. The discharge process is started usually slowly, because a quick start often leads to a blocked suction pipe. After that there is for 75 to 80 % of the time an almost constant high production. At the end of the unloading process the decreases almost linear zero (Figure 2.140).


Pagina 109 van 109

May 2005

wb3408B


Unloading proces with time Pr_time

Pr_ave

100 90 Prod/max. production

80 70 60 50 40 30 20 10 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

t/t_unloading

Figure 2-140 Production of the unloading process

In almost all self-emptying installation a rest load remains of around 5 %. By the fluidization process the rest loads of rocks and dirt accumulate, so that regularly the rest-load needs to be dumped on a dump.

2.5.4

The cycle production

The cycle consisting of: loading, sailing to, discharging, sailing back can be optimized simply. The cycle production is defined as:

Pcycle =

L (t ) tsuction + tsail + tdischarge

If tsuction and tdischarge are considered constant than this production is optimal when the following is condition is met:

dPcycle dtsuction

=0

This is the tangent to loading curve L(t) that also crosses the negative y-axis in the point tsailing + tdischarge (Figure 2.141).


Pagina 110 van 109

March 2003


Load [m3] max. cycle production

Loading time [minutes]

no loading time

Figure 2-141 Optimal cycle production

This loading process can be made visible on board of the dredgers to determine the optimal load. However it should be noticed that the overflow losses increase sufficient at the end of the loading process to determine the optimal point.

2.5.5

The instrumentation

To support the dredge master instruments are available. Modern trailing suction hopper dredgers are equipped with suction pipe position indicators both in the longitudinal as in the transverse direction. Not only the position in relation to the bottom is indicated but also the position of the suction pipe and the draghead in relation to the ship and sometimes even the soil. Furthermore the dredge master has a direct view on the swell-compensators to judge if the dragheads are on the bottom. If this is not the case than indicators are necessary. For the suction process there are besides the vacuum and pressure indicators, also velocity and concentration indicators. With the aid of these instruments the suction chief will optimize the suction process by trial and error.

Figure 2-142 Instrumentation panels


Pagina 111 van 109

May 2005

wb3408B


2.6 Special designs of trailing suction hopper dredgers 2.6.1

The gravel suction dredger

Trailing suction hopper dredgers that collect aggregates for the concrete industry and road construction differ in several aspects from the "standard" trailing suction hopper dredger. These differences usually arise from economical considerations. Items that are of less use are left out, while others are added. Figure 2-143 Gravel dredger Charlemagne

These include: •

The maneuverability. A lot of gravel suction dredgers are built to collect aggregates at sea. These are relative wide concessions where accurate dredging is of no or small concern. Furthermore there are long transportation distances. Therefore the requirements for the maneuverability are less strict than for the trailing suction hopper dredger that has to dredge frequently in busy fairways or ports.For this reason the gravel suction hopper dredger is equipped with only one screw.

•

The longer dredge cycle. The longer sail distances mean that the suction time is only a small percentage of the total cycle time. Therefore it is much more economical to equip the ship with only one suction pipe and one dredge pump.

•

Since the quality of the material determines the price, these ships are equipped with a creening installation. The "bad" material can than be put overboard. Of course it is also possible to load all the material (called all-in or tout-venant).

•

A discharge installation with which it is possible to unload "dry" in every arbitrary port. Seldom a gravel suction hopper dredger has bottom doors or valves.


Pagina 112 van 109

March 2003


Figure 2-144 Screening installation

Since the concessions are increasingly further away from the land and therefore in deeper waters, submerged pumps on the suction pipe are also used on modern gravel suction hopper dredgers. The discharge systems are of the drag system, clamshell or excavation wheel (Figure 2.114) principle that delivers the material from the hopper to a silo from which the material is distributed further via a conveyor belt. The way of operation does not differ much from the "classical" trailing suction hopper dredger. Instead of pumping the material straight into the hopper, it is now pumped into the screening installation, where it is separated into the required class(es). When sailing to the discharge area the drain installation is turned on to bring the load as dry as possible ashore.

Trailing suction hopper dredger for inland waters provides also sand and gravel to the concrete industry as well as sand for reclamation purposes. They do also maintenance dredging in river harbours Their design is much simpler than ordinary trailer suction hopper dredgers (Figure 2.145).

Figure 2-145 Trailing suction hopper dredger for inland waters


Pagina 113 van 109

May 2005

wb3408B

2.6.2


The stationary suction hopper dredger

The stationary suction hopper dredger is the predecessor of the trailing suction hopper dredger. In the most well known design the stationary suction hopper dredger has a hopper and behind it the pump room with one dredge pump. The suction pipe is directed however forward. Stationary suction hopper dredgers are single-screw ships. The propulsion engine directly drives the dredge pump. Figure 2-146 Stationary suction hopper dredger

The method of operation differs significantly from the trailing suction hopper dredger and is in principle equal to the suction dredger. When dredging the vessel anchored in its borrow area. The amount of anchors needed depends strongly on the operational circumstances, like current and wind velocity, current and wind direction and shipping. If the circumstances are well than one or two front anchors are sufficient. If the dredging takes place in a tidal area where the current change direction depending on the tide, than also one or two aft anchors are placed. A second anchor is needed if the ship must be hauled frequently. As with suction dredgers the stationary hopper dredger is used in free running sand. Dependent on the breach height the ship is slowly hauled in the direction of the suction direction. The loading of the hopper is similar to the process of the trailing suction hopper dredger.


Pagina 114 van 109

March 2003


Figure 2-147 Trailing suction hopper dredger for stationary dredging

Sometimes the trailing suction hopper dredger is used as a “stationary dredgers" for certain works. To do this the dragheads are removed and if not already present an aft anchor is mounted. When arriving at the winning area first the aft anchor is placed. Dependent on the weather conditions the front anchor is also placed. Since the pipes put backwards the trailing suction hopper dredger works itself while dredging backwards. There are also trailing suction hopper dredgers that have the possibility to bring their suction pipe forward and are than able to work on the bow anchor (Figure 2.147). With well-breaching sand trailing suction hopper dredgers can also suck profiles with the drag suction method. The embankment must than be at all times more gentle than the suction pipes of the trailing suction hopper dredger. The trailing suction hopper dredger forces its way into the embankment with a velocity of 0.25 to 0.5 knots. The main advantage of this method is that no anchors are needed which gives more freedom of movement and a quicker leave in case of an emergency.

TSHD working as PS Dredger TSHD dredging to the Face

Figure 2-148 Trailer suction hopper dredger working in a plain suction mode

2.6.3

Boom dredgers

The boom dredger (Figure 2.149) is a special design of the trailing suction hopper dredger.


Pagina 115 van 109

May 2005

wb3408B


Figure 2-149 Boom dredger

It is equipped with a 50 to 60 meter long construction, the boom, that makes it possible to pump the dredged material immediately sideways back (side casting). This method of dredging is used in silt rich fairways, where it is cheaper to spray the material to the side, a hundred meters from the bank of the fairway instead of bringing it to a dump far away. Approach channels at the lake of Maricaibo in Venezuela are dredged in this manner


Pagina 116 van 109

March 2003


2.7 Literature 1. Trailing Suction Hopper Dredging Handbook. Issued by The Training's Institute for Dredging. 2. Coastal and Deep Ocean Dredging, John B. Herbich, Gulf Publishing Company, Houston, Texas, USA, 1975. 3. Dredging and Dredging Equipment, R.J. de Heer and Rochmanhadi, part 1 and 2, IHE, Delft, 1989. 4. Baggertechniek, collegedictaat f14, G.L.M. van der Schrieck, TU Delft, Civiele Techniek, 1996 (in Dutch). 5. Constant Tonnage Loading System of Trailing Suction Hopper Dredgers, J. de Koning, Proceedings International Course Modern Dredging, 1977. 6. Nassbaggertechnik, A. Welte, Institut für Machinenwessen in Baubetrieb, Universität Fridericiana, Karlsruhe, 1993. 7. Proceedings of the dredging days, Europort 1980, CEDA, 1980. 8. Technical aspects of large Trailing Suction Hopper Dredgers, P.J. Koert, IHC Holland. 9. Further development of loading and unloading processes for Trailing Suction Hopper Dredgers, S. Steinkühler, 14 World Dredging Congress, Amsterdam, 1995. 10. Several articles from Port & Dredging of IHC Holland.

P&D

Split trailer suction hopper dredgers 106 + 107 + 110 128 VOLVOX SCALDIA, Trailing Dredgers with built-in booster unit CORONAUT, the sixth IHC Eurotrail 130 AGRONAUT, the seventh IHC Eurotrail 134 New Trailing Suction Hopper Dredger for Dredging International 134 3 Trailer VOLVOX IBERIA, 5700 m 140 142 TSHD J.F.J. DE NUL, Versatile Leviathan Trailing Dredger, HAM 311 143 Trailing Dredger, CRISTOFORO COLOMBO 143 PEARL RIVER, Trailing Dredger of 17000 m3 144 TSHD Ham 311 and Ham 312 148 151 TSHD Queen of Penta Ocean TSHD Ham 317 153 TSHD Rotterdam 155+156 TSHD Ham 318 157 Gravel Dredger Cambeck and Charlemange 133 + 157 Dragheads 124 + 137+157


Pagina 117 van 109

May 2005

Chapter 3: Cutter Suction Dredger

3.

The Cutter Suction Dredger .........................................................................2 3.1. General description........................................................................................3 3.1.1. Areas of application......................................................................................4 3.1.2. History ..........................................................................................................5 3.1.3. Working method...........................................................................................6 3.2. The design.......................................................................................................8 3.2.1. The production capacity ...............................................................................9 3.2.2. The dredging depth.......................................................................................9 3.2.2.1. The maximum dredging depth..................................................................9 3.2.2.2. The minimum dredging depth ..................................................................10 3.2.3. The width of the cut......................................................................................12 3.2.4. The type of soil.............................................................................................14 3.2.5. The transport distance...................................................................................14 3.2.6. Access to the dredging site ...........................................................................15 3.3. The dredging equipment ...............................................................................15 3.3.1. The cutter head .............................................................................................16 3.3.1.1. The dimensions of the cutter head ............................................................16 3.3.1.2. The cutting power.....................................................................................16 3.3.1.3. The cutter speed........................................................................................17 3.3.2. The reaction forces on the cutter ..................................................................18 3.3.2.1. The horizontal and vertical cutting force..................................................18 3.3.2.2. The axial force..........................................................................................20 3.3.2.3. The ladder weight .....................................................................................21 3.3.3. The side-winch power and speed..................................................................21 3.3.4. The ladder winch speed and power ..............................................................24 3.3.5. The dredge pumps ........................................................................................24 3.3.6. The jet pump.................................................................................................25 3.4. The drives .......................................................................................................25 3.4.1. The cutter head drive ....................................................................................25 3.4.2. The side winch drives...................................................................................27 3.4.3. The ladder drive............................................................................................27 3.4.4. The sand pump drives...................................................................................27 3.5. Spudsytems .....................................................................................................28 3.5.1. The spud carriage system .............................................................................28 3.5.2. The fixed spud system ..................................................................................30 3.5.3. The spud door system...................................................................................32 3.5.4. The walking spud system .............................................................................32 3.5.5. The rotor spud system ..................................................................................33 3.5.6. The Christmas tree........................................................................................34 3.6. The general layout .........................................................................................35 3.7. Technical construction Error! Bookmark not defined. 3.7.1. The Hull........................................................................................................40 3.7.2. The cutter head ladder ..................................................................................41 3.7.3. The cutter head .............................................................................................43 3.7.4. Tooth and cutting edge systems ...................................................................46 3.7.5. The side wires...............................................................................................50 3.7.6. The anchor booms ........................................................................................51 3.7.7. The spuds......................................................................................................52 3.7.8. The spud lifting system ................................................................................52 3.7.9. Pumps and pipelines .....................................................................................53 3.7.9.1. The suction pipeline .................................................................................53 3.7.9.2. The pumps ................................................................................................54


Pagina 1 van 79

May 2003

Wb3408b


3.7.10. The winches..............................................................................................55 3.7.10.1. The ladder winch ..................................................................................55 3.7.10.2. The side winces ....................................................................................55 3.7.10.3. Other winces.........................................................................................56 3.7.11. Hoisting equipment ..................................................................................56 3.7.12. Auxiliary equipment.................................................................................56 3.8. The dredging process.....................................................................................57 3.8.1. The spillage ..................................................................................................57 3.8.2. The production in breach-forming soils .......................................................59 3.8.3. The production by non-breach forming soils ...............................................61 3.8.4. Specific energy .............................................................................................63 3.8.5. The cutting production .................................................................................65 3.8.6. The spillage ..................................................................................................67 3.9. Enclosures.......................................................................................................68 3.9.1. The relation between swing speed and side winch speed.............................68 3.9.2. The side winch force and power...................................................................69 3.9.3. The shape and cutting geometry of cutter heads ..........................................70 3.9.4. Cutting by teeth or chisels ............................................................................74 3.9.5. Conditions for cutting clearance...................................................................75 3.9.5.1. The effect of warping on the clearance angles .........................................77 3.10. References.......................................................................................................79

3. The Cutter Suction Dredger

Figure 3. 1

The “Mashhour”, at present the biggest cutter suction dredger in the world,


Pagina 2 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.1. General Considerations

Discharge pipe Dredge pump Suction pipe

Cutter ladder

Cutter head Dredge pump

Ladder winch

Auxiliary spud

Port side winch.

Spud carriage Working spud Starboard winch

Figure 3. 2 Lay-out

snijkopzuiger.

The cutter suction dredger is a stationary dredger equipped with a cutter device (cutter head) which excavate the soil before it is sucked up by the flow of the dredge pump(s). During operation the dredger moves around a spud pole by pulling and slacking on the two fore sideline wires. This type of dredger is capable to dredge all kind of material and is accurate due to their movement around the spud pole. The stationary cutter suction dredger is to distinguished easily from the plain suction dredger by its spud poles, which the last don’t have. The spoil is mostly hydraulically transported via pipeline, but some dredgers do have barge-loading facilities as well. Cutter power ranges from 50 kW up to 5000 kW, depending on the type of soil to be cut.

Cut width


Spud carriage length Auxilary spud


Figure 3. 3 Swing pattern The ladder, the construction upon which the cutter head, cutter drive and the suction pipe are mounted, is suspended by the pontoon and the ladder gantry wire. Seagoing cutter suction dredgers have their own propulsion that is used only during mobilization. The propulsion is situated either on the cutter head side or on the spud poles side.


Pagina 3 van 79

May 2003

Wb3408b


Figure 3. 4 Seagoing CSD Aquarius sailing in the Beaufort Sea

3.1.1. Areas of application Cutter suction dredgers are largely used in the dredging of harbours and fairways as well as for land reclamation projects. In such cases the distance between the dredging and disposal areas is usually smaller than the distances covered by trailing suction hopper dredgers. The cutter suction dredger also has the advantage when an accurate profile has to be dredged. The cutter suction dredger can tackle almost all types of soil, although of course this depends on the installed cutting power. Cutter suction dredgers are built in a wide range of types and sizes, the cutting head power ranges between 20 kW for the smallest to around 4,000 kW for the largest. The dredging depth is usually limited; the biggest suction dredger can reach depths between 25 and 30 m. The minimum dredging depth is usually determined by the draught of the pontoon. In the late seventies and early eighties of the previous century two offshore cutter suction dredgers have been build for applications offshore. The All Wassl (Figure3.5) build by Mishubitsi, Japan for Gulf Cobla Ltd. Has dredged the approach channel to the harbour Jebel Ali in Dubai, Unit Arab Emirates. Figure 3. 5 All Wassl Bay


Pagina 4 van 79

May 2003

Chapter 3: Cutter Suction Dredger After 2 years working the dredger is sold and scrapped. The Simon Stevin (Figure 3.6) build for Volker Stevin Dredging has even never worked. Boths dredgers appeared too specialised to be economical.

Figure 3. 6 Simon Stevin As said the cutter suction dredger is a stationary dredger with at least two side anchors that are necessary for the dredging process. Because of these anchors they may obstruct shipping movements. Self propelled cutter suction dredgers uses their propulsion system no only during mobilisation but also during shifting from one place to the other or when the dredging area has to be left, “breaking up” when bad weather is expected. The small to medium sized cutter suction dredgers can be supplied in a demountable form. This makes them suitable for transport by road to inland sites that are not accessible by water, for example to lay a sand foundation for a road or to dredge sand and gravel for the building industry. When working under offshore condition with waves or swell cutter suction dredgers clearly have more limitations than trailing suction hopper dredgers even if equipped with swell compensators 3.1.2. History The cradle of the cutter suction dredgers stood in the United States. In 1884 a cutter suction dredger was used in the port of Oakland, California. This dredger had a cylindrical cutter head and was used to dredge layers of sandstone. It had a pipeline of 500 mm diameter and a pump with an impeller of 1.8 m! The disadvantage of this design was that the suction mouth was frequently blocked. At the end of the 19th century and beginning of the 20th century there was a major development in suction dredgers


Pagina 5 van 79

May 2003

Wb3408b


Figure 3. 7 Layout

of the cutter suction dredger “RAM”

For example, in the fall of 1893 the cutter suction dredger “RAM” was built by the Bucyrus Steam Shovel and dredged company for use on the lower Mississippi river. This dredger was already equipped with an rotating cutter head. (Figure 3.7). The cutter suction dredger became the workhorse of the dredging industry in America, as did the bucket dredger in Europe at that time.

3.1.3. Working method After the ladder of the cutter suction dredger has been lowered under water, the dredge pump(s) started and the cutter head set in motion. The ladder is then moved down until it touches the bottom, or until it reaches the maximum depth. The movement of the dredger round the spud pole is initiated by slacking the starboard anchor cable and pulling in the port side anchor cable or reverse. These anchor cables are connected via sheaves close to the cutter head to winches (dredging side winches) on deck. The pulling winch is called the hauling winch. The paying out winch ensures the correct tension in both cables, this being particularly important when dredging in hard rock.

Ds

Ds Over cutting mode

Under cutting mode


Pagina 6 van 79

May 2003

Chapter 3: Cutter Suction Dredger Figure 3. 8 Different cutting modes

In addition to the type of soil, the required side winch force also depends on: • Whether the rotation of the cutter head is in the same direction as or the opposite direction to that of the swing movement. In the first case the reaction force of the cutter head on the soil will pull the dredger with it, as a result of which the side winch forces are smaller than when rotation is in the opposite direction It is also necessary to ensure the correct pre-tensioning of the cables when the cutter head rotates in the same direction as swing. If the cutter head forces propel the cutter head more quickly than the hauling winch does there is a very real danger that the cable of the hauling winch will be picked up and cut through by the cutter head. • The position of the anchors has a big influence on the force needed to swing the dredger. The closer the path of the cutter head is to the direction of the side cable, the smaller the required force. • Naturally the side winch force is also affected by external influences such as wind, current and waves.

Figure 3. 9 Steps and cuts

Of course, the thickness of the layer that can be removed by one swing (cut thickness Figure3.9) depends on both the diameter of the cutter head and the type of soil. When the required dredging depth has not been reached at the end of a swing, the ladder is set more deeply and the ship will move in the opposite direction. As previously mentioned, the cutter suction dredger describes an arc round a fixed point, the spud pole or working pole. In many cutter suction dredgers this pole is mounted on a movable carriage, the spud carriage. A second pole, the auxiliary spud, is set out of the centreline, usually on the starboard side of the stern of the pontoon. The spud carriage can be moved over a distance of 4 – 6 m by means of a hydraulic cylinder. Because the spud is standing on the bottom, pressing the spud carriage towards the stern can move the cutter suction dredger forward. The size of the cutter head and the hardness of the soil determine the size of this ‘step’. During each step one or more layers of the face are cut away by lowering the ladder one cutting thickness at the end of the swing.


Pagina 7 van 79

May 2003

Wb3408b With each step the cutter head describes the arc of a concentric circle round the spud, the radius of which increases with the step length. (Figure 3.10) a) = step length b) = length of carriage If the spud carriage cylinder has reached the end of its path the spuds must be moved. Before stepping, the cutter moves to the centre line of the cut.


D D a

a = steplength b = length of carriage

B

C

a b


Figure 3. 10Vertical swing pattern

The auxiliary spud is then placed on the bottom, the working spud is lifted and the spud carriage is moved forward. After this the work spud is again lowered and the auxiliary spud is lifted. The dredger can then resume working. The first cut made after stepping is not an arc of a concentric circle!

3.2. The design When designing cutter suction dredgers, the following basic design criteria are important: • Production capacity • Dredging depth • Working conditions which affect the size of the dredger • Type of soil • Transport distance(s) • Access to the side As mentioned earlier, the cutter suction dredger can be used in all types of soil, from soft clay to hard rock. The soil to be dredged has a great influence on the design and construction. Considerable forces are generated when working in rock. They are generated by the cutter head and returned to the ground partly via the ladder and side winches and partly via the pontoon and the spud pole. The design of cutter suction dredgers is also determined by the required amount of installed cutting power.


Pagina 8 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.2.1. The production capacity As in the case of other types of dredger, the production capacity is determined by the market demand with regard to the projects for which the dredger can be used. Because many cutter suction dredgers must dredge various types of soil during their lifetime, design parameters are set with regard to the types of soil the dredger must be able to dredge. A dredger designed to dredge rock will also be able to dredge sand, but a ‘sand” cutter suction dredger will not be able to dredge rock. On the other hand a ‘sand’ cutter suction dredger will be able to dredge sand more cheaply than a ‘rock’ cutter suction dredger. In other words the design production capacity of a cutter suction dredger is related to the hardness of the material that it must be able to dredge. For example, 100 m3/hr in a rock of 10 MPa. It is important that the production capacity is defined m3 per week, hour or second. The smaller the unit of time chosen, the greater the production capacity. (As a result of averaging the long term production capacity is less.) When the requirement with regard to the production capacity in the design-soil is known, this can be translated into a production to be cut by the cutter head. This so called cutter production is considerably higher than the dredged production because not all the material that has been cut enters the suction mouth. Often 20 – 30 % remains behind as spillage. This must be taken into account when determining the production to be cut. The maximum cutter production is also higher for reasons such those described above as a result of the unit of time. With a cutter suction dredger this appears primarily in the mode of work employed. Production is usually highest in the middle of a cut. In the corners of the cut where manoeuvres are often carried out with the ladder or spud carriage, the production is low or zero. This results in the fact that the cutter production when expressed in m3/s is 20 – 30% higher than the cutter production in m3/hr. In order to maintain a high degree of usability cutter suction dredgers designed for rock dredging should be equally as good in other types of soil. This implies that although the cutting equipment is designed for rock dredging with regard to the other parts of the dredging equipment, the other types of soil must not be forgotten. 3.2.2. The dredging depth When designing cutter suction dredgers both the maximum and the minimum dredging depths must be taken into consideration, since these both influence the usability of the dredger. Often the need for a greater dredging depth leads to a pontoon with deeper draught and thus to a reduction in the minimum dredging depth. So on one hand the usability of the dredger increases with increasing dredging depth, while on the other hand it decreases as a result of the related smaller minimum dredging depth. Here too the market demand plays a role in the best choice.

3.2.2.1. The maximum dredging depth The maximum dredging depth is an important design parameter. Because in a cutter suction dredger the pontoon and the spud pole transfer part of the interplay of forces to the soil, the magnitudes of the moments that occur are proportional to the dredging depth. Thus with increasing dredging depth, not only is the dredger larger and broader (for stability), it must also have a heavier construction. Moreover the dredging depth has a great influence on the design of the ladder construction and thus on the pontoon. After all it must be possible to raise the ladder above water for inspection.


Pagina 9 van 79

May 2003

Wb3408b


Figure 3. 11 Different cutter dredgers with ladder above waterlevel

From the point of view of production, the suction depth determines whether an underwater pump is needed to obtain the required production capacity. It is obvious that mounting an underwater pump will increase the weight of the ladder. If no underwater pump is considered, the diameter of the suction pipe and the head of the pump must be increased and the concentration of the mixture reduced in order to avoid creating a vacuum. This may lead to the pumping of low concentrations and thus much water, which is uneconomic. With the aid of the vacuum formula (see also lecture notes ‘Dredging processes’), from a given limiting vacuum and the maximum concentration to be dredged it is possible to determine whether or not an underwater pump is necessary, and if so how far under water it must be placed. Whether or not an underwater pump is fitted is, of course, also a question of economics, since cost of the fitting of an underwater pump is considerable.

8000

Light weight [t]

7000 2

y = 9.0577x - 101.29x

6000

2

R = 0.757

5000 4000 3000 2000 1000 0 0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Maximum dredging depth [m]

Figure 3. 12

3.2.2.2. The minimum dredging depth The minimum dredging depth makes demands with regard to the draught of the pontoon, the position of the cooling water inlet and the shape and construction of the cutter ladder. It will be clear that even when dredging at minimum depths the pontoon must have sufficient bottom clearance. For heavy duty cutter suction dredgers this leads to deep draughts or wide vessels (Figure3.13). The minimum dredging depth must be at least 1 m deeper than the maximum draught of the vessel. The design of the cooling water inlet must be adapted to prevent the intake of material from the bottom Prof.Ir. W.J.Vlasblom

Pagina 10 van 79

May 2003


6

Maximum draught [m]

5

4

3

2

1

0 0

500

1000

1500

2000

2500

3000

3500

4000

Cutterpower [kW]

Figure 3. 13

When dredging at depths, which are shallow in comparison to the draught of the vessel, the shape of the ladder must also be adapted to avoid dragging of the ladder. To prevent dragging the angle γ between the underside of the ladder and the horizontal must be at least 50 (Figure 3.14).

Removable wedge

Figure 3. 14

In order to obtain a better rate of filling when dredging free running material is desirable that the axis of the cutter head shaft should make a steeper angle with the horizontal than the ladder. The filling of the cutter is determined by the sum of the angles of the slope gradient and the ladder (α+ β) (Figure 3.15).

θ θ+ β

β

Figure 3. 15 Prof.Ir. W.J.Vlasblom

Pagina 11 van 79

May 2003

Wb3408b


3.2.3. The width of the cut The usefulness of a cutter suction dredger is also determined by the minimum width of cut that the equipment can dredge, and to a lesser degree on the maximum width of the cut. ‘Minimum width’ of cut is taken to mean the width that the dredger needs to dredge a channel for itself in an area where the surface of the ground is higher than the water level; a problem that occurs is during dredging the onshore end of pipeline trenches.

Figure 3. 16 Minimum cut width

The minimum width of the cut is determined by the line that meets the contour surface of the cutter head at the front of the pontoon (Figure 3.16) or at the outer side of the side winch sheaves. To reduce the minimum cutting width each side of the front of the pontoon is often chamfered as shown in Figure 3.17 and 3.19. Figure 3.18 also shows that the further the cutter head projects in front of the pontoon, the smaller is the minimum cutting width. Such a solution is particularly common in American and Japanese dredgers.

Ballast tank Lubricating olie Ballast tank

Fuel

ballast tank ballast tank

Spare parts

Spare parts

Dry tank

ballast tank

Drinking water Engine room Ballast tank Ballast tank

Fuel


Spare parts

Spare parts

Dry tank

ballast tank

Figure 3. 17 Chamferred pontoon


Pagina 12 van 79

May 2003


Figure 3. 18

Figure 3. 19

The distance between the spud and the cutter head determines the maximum cutting width. To ensure the efficiency of the side winches the maximum swing angle is restricted to 450 ; so that the maximum width B = 2L*sin(450) +Dcutter, in which L is the distance between the spud and the cutter head. The length L depends on the depth of the water and the position of the spud pole. From the point of view of production a broad cutting width is desirable, since per m3 dredged the downtime for stepping, anchoring and other q manoeuvres is shorter. However long cutter suction T S dredgers have a big minimum cutting width, so the advantages must be weighed L=S+Tcosq against the disadvantages. The maximum cutwidth depends on the maximum side winch force too. This will be explained in chapter 3.2.2.3

B=2Lsina+Dcutter

L

a

Figure 3. 20 Cut width


Pagina 13 van 79

May 2003

Wb3408b


3.2.4. The type of soil The type of soil to be dredged has a strong influence on the installed cutter head and side winch power, the strength of the ladder, pontoon and spuds. To some degree the type of soil also influences the choice of suction pipe and discharge pipeline diameters. With the same cutting power a cutter suction dredger dredging rock will have a lower production rate than when dredging sand. In view of this, a rock-cutting cutter suction dredger should have pipelines of smaller diameter, because it becomes more economical to pump solids with higher concentrations. With the same production rate it is possible to increase the concentration by reducing the pump flow. Because a minimum velocity is required to transport solids this can only be achieved by reducing the diameter of the pipelines. It must be noted that reduction of the pump flow may lead to a higher percentage of spillage resulting caused by a bad mixture forming-process in the cutter head. (See Dredging Processes, Spill.) 3.2.5. The transport distance The transport distance makes demands in relation to the installed sand pump power and the need to load barges. The requirement to load barges is determined by the question of whether the required transport distance is too great to be economically bridged by using a hydraulic pipeline. It is also possible that the use of a hydraulic pipeline is impossible from the point of view of hindrance to navigation. Cutter suction dredgers are seldom equipped to load barges only. Figure 3.21 shows the CD Marco Polo barge loading in the busy waters of Singapore If the cutter suction dredger is equipped with an underwater pump, the pump power can be such that during the loading of barges this pump is used only. The pipeline system and valves should be designed to fulfil this requirement.

Figure 3. 21 CD Marco Polo

It is also possible to choose an underwater pump with a higher power than is needed for barge loading. The surplus capacity can then be used during discharging. The grain size and the discharge length of the pipeline determine the required pump pressure, while this determines the number of dredgepumps required. The maximum allowable pump pressure that a dredger can supply depends on the quality of the shaft sealing of the last pump. Often values exceeding 25 - 30- bar are not permitted. Prof.Ir. W.J.Vlasblom

Pagina 14 van 79

May 2003


3.2.6. Access to the dredging site Dredging sites are not always easy accessible via water. The access can be very shallow and have to be dredges deeper before the actual dredging can start. If there is no access via water at all, the dredger have to be mobilised to the site by road. This is only possible with small demountable dredgers. In case of long contracts, such as for the tin and gold mining the dredgers can be constructed on the dredging site. Both cases do influence the design of the dredger. Figure 3.17 shows a general plan of a demountable dredger consisting of one main middle pontoon and two side pontoons.

Figure 3. 22 Another point in relation with access to the site is the possible restriction height of the dredger. High ladder and spud gantries can be a problem by passing bridges or electrical cables. Compare the different designs of the dredgers in Figure3.22 and Figure 3.23

Figure 3. 23

3.3. The dredging equipment For the design of the dredging equipment the following dredging parts will be considered: • The cutter head • The bow side-winch power • The axial cutting force • The vertical cutting force • The ladder winch power • The drives • The dredge pump • The sand pump drive • The water pump • The spud system


Pagina 15 van 79

May 2003

Wb3408b


3.3.1. The cutter head The cutter head is the most important part for this type of dredger, because it determines the production in may cases that shall be excavated and transported. For the production is besides the required cutting power also the cutter head speed and the dimensions important. The cutting power to be able to cut the soil. The cutter head speed is important for the mixture forming process and the dimensions should be in relation to the cutting power and the production. Further it is important to know the reaction of the cutting process working on the cutter head for determining the side winch forces, speed and power; the ladder weight, ladder inch forces, ect. 3.3.1.1. The dimensions of the cutter head The production capacity is affected not only by the cutting power, the side winch power and the velocity, but may also depend on the diameter of the cutter head. This is the case when the side winch force, the side winch velocity and the cutting torque are not limiting factors. Production can only be increased by increasing the cut thickness and step size, thus increasing the cutter head dimension. The dimensions should be in relation with the theory described in chapter 3.3.2 3.3.1.2. The cutting power The required cutting power can be determined either from the cutting theories (Lecture notes Wb 3413) or from the required specific energy that is needed to cut the design-soil. The specific cutting energy SPE is defined as the work that is needed to cut m3 of soil, that is the power P that is needed to cut a production Qcutter of m3/s, thus

SPE =

PCutter Q Cutter

[N/m²]

The cutting power is therefore:

PCutter = Q Cutter ∗ SPE

[W]

When cutting soil the cutting force is seldom constant due to the inconstancy of the soil. Therefore the terms ‘average cutting force’ and ‘ peak forces’ are used. The peak forces for rock may well be a factor 2 higher than the average forces (Verhoef, 1997) The following may be used as rules of thumb: . for rock; depending whether the cutting process is ductile or F peak = 1.5 − 2 brittle.

Fmean F peak Fmean F peak Fmean

= 1.25 − 1.5 = 1.1 − 1.5

for sand for clay, depending whether the cutting process is flow, tear or shear type.

The theoretical cutting power must also be multiplied by these factors. The revolution velocity of the cutter head is also dependent on the type of soil. Note: This factor should be included in the work coefficient as mentioned in chapter1

1

Reference to be made


Pagina 16 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.3.1.3. The cutter speed Specific energy decreases as the rock size increases. In rock a nominal cutter head speed of 30 revolutions per minute is often used. Lower nominal revolution rates leads to bigger rock pieces and so to lower specific energy but also to higher torques and cutting forces. Higher cutting torques and forces can also be achieved by reducing the diameter of the cutter head. Except that the rock size does not increases in this case the maximum thickness of the cut decreases and thus the maximum production will reduce. Both cutter head speed and pump capacity have big influence on the spillage of the cutter. Spillage is the material that is cut but no sucked up by the dredged pump. Den Burger (1999) showed from his research on laboratory scale that the optimal cutter head speed in rock depends a little with the pump capacity (Figure 3. 24) Translation of the optimum results for the different mixture velocities or pump capacities to prototype values leads to Figure 3. 25 when using the scale laws as describe by den Burger. It should be noticed that for a cutter head with a diameter of 3 m the pump capacity should be more than 5 m3/s (mixture speed 5m/s) to get a relative production of a little more than 70% (30% spillage). Reducing the cutter head diameter with a half a meter results in more acceptable practical values for the pump capacity with a cutter head speed of a little less than 40 rpm. Higher speed will give in rock smaller particles and therefore less spillage.

Figure 3. 24 Vm=4 m/s

Vm=5 m/s

120

12

100

10

80

8

60

6

40

4

20

2

0

Pump capacity [m3/s

Vm=2.67 m/s

Cutter head speed [rpm]

As could be expected the results for dredging sand are quite different from dredging rock. In Figure 3. 25. The results for rock and sand are plotted against the dimensionless flow number: Q . The difference ωR 3 between two soil types is tremendously.

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Cutter diameter [m]

Figure 3. 25


Pagina 17 van 79

May 2003

Wb3408b

RELATIVE PRODUCTION Gravel 10 mm

Gravel 15 mm

Ladder 25 deg.

Sand

80 70 60 Pr [%]

The productivity depends except on the capacity and the cutter head speed on the particle size and the ladder angle too (Figure 3. 26) The flow numbers with the same productivity for sand at the (ladder angle also 25°) are a factor 1.5 smaller than for gravel (10 mm). This allows the use of cutter heads with a large diameters and with higher production results.


50 40 30 20 10 0 0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Flow number [-]

Figure 3. 26 If the cutter suction dredger is designed for dredging sand a speed of 20 revolutions per minute is adequate (see also Figure 3. 26). In silt or soft clay even lower revolutions are sufficient, provided that the cutter head does not become blocked. 3.3.2. The reaction forces on the cutter Forces acting on the cutter suction dredge are shown in Figure 3.27. All reaction forces from the cutter head have to be transferred in a certain way the surroundings, either by the side winch forces or the spud poles to the soil or via the ladder wires and the pontoon to water. Besides that these cutting forces determines the weight of the dredger, while the forces to move the dredger through the water can have influences on the design of the dredging parts. In a ladder related co-ordinate system he cutting forces can be decomposed in the 3 dimensions; horizontal, vertical and axial. There is a general linear relation between the 3D-cutting forces and the cutting power (Vlasblom, 1998). Furthermore the cutting forces in cavitating sand, clay and rock are almost independent for the cutting speed. Therfore: F R F R Fhor.R cutter = c h , vert cutter = c v , axial cutter = c a can be taken as constant for a specific soil M cutter M cutter M cutter type and relative cutting thickness D s . 2⋅Rc 3.3.2.1. The horizontal and vertical cutting force .

M cutter is the tangential force T as R cutter

shown in Figure 3. 28; Both the cutting force as well as the normal force can be decomposed in the horizontal force Fh and the vertical force Fv. Fh is delivered by the side winch and Fv by the weight of the ladder or the extra draught of the pontoon. The axial force is partly taken up sideline forces, depending on the directions of those wires and partly via the thrust bearing of the cutter Prof.Ir. W.J.Vlasblom

Pagina 18 van 79

May 2003

Chapter 3: Cutter Suction Dredger shaft via the ladder trunnion transferred to the seabed via the spud pole. Design values are for cv=0.9, ca=0.4 and ch=1for under cutting and ch=0.6 for over cutting. The relative thickness of the cut (d/Dc) has a considerably greater influence on the hauling force than on the vertical and axial forces.)

Rsh

β

Gs

Mc

Fl

Fv

Wp WL

Rs

Fa

Fpsw

Rs

Fa+Fv

ω Fh Fsbw Rw

W

Figure 3. 27

The horizontal component of the cutting force changes in direction when it passes the rotation centre of the cutter head. (Figure 3. 28, Left Center of Cutter

Tangential Force T Ra

diu s

r

Vertical Force V

Cutting Force C

Tooth

θ − ϕ

θ+ ϕ

Cutting Force C

R

Tooth Normal Force Ncosκ Path of Tooth

Horizontal Force H θ − ϕ Radial Force R Normal Force Ncosκ

Forces in a plane perpendicular to the cutter shaft

Decomposition of the Forces working on a Tooth

Figure 3. 28


Pagina 19 van 79

May 2003

Wb3408b


3.3.2.2. The axial force Generally cutter heads have profiles as given in Figure 3. 29. This profile is determined by a plane through the cutter axes and the surface of revolution shaped by the teeth positions. Cutter teeth are positioned such that the centerline of the tooth is perpendicular to the contour line. This can easily be understand when the break out pattern is considered. (Figure 3. 30,right) The normal force N can be de-composed in 2 perpendicular forces : Nsinκ and Ncosκ ,which are respectively parallel and perpendicular with the cutter axes.

Nsinκ κ

Ncosκ

Axial and Normal Force Figure 3. 29

H

Minimum distance = cut depth Break out pattern

v

Fv

Fa

Fh

ι

R Break out Pattern Figure 3. 30

Cutter heads with plain or serrated edges (Chapter 3.4.4) develop axial force by the helix. angle ι of the cutter head blade, which causes the so-called snow plough effect (Miedema. 1995).

α

In that case is the leading edge of the knife not perpendicular to direction of the movement (Figure 3. 30, left) The cutting process have to be considered in 2 perpendicular directions; one perpendicular with the cutting edge and the other parallel with it. The last one takes care for the transport of the soil in the direction of the knife. Furthermore the component of the side winch forces also gives a force in the axial direction (Figure 3. Prof.Ir. W.J.Vlasblom

N

Figure 3. 31 Pagina 20 van 80

May 2003

Chapter 3: Cutter Suction Dredger 31),depending on the position of the anchor. As with the cutting force, the maximum forces are higher than the average forces. 3.3.2.3. The ladder weight

Fvert R cutter = c v = 0.9 the minimum weight of the ladder can M cutter be determined in order to fulfil the requirement that over cutting have to be possible. 0.9 Pcutter Rewriting the condition and multiplying with the rotational speed ω gives Fvert = ; ω Rcutter ωR is in the order of 4 m/s, which means that Fvert ≥ 0.225 Pcutter Following from de condition that

If the load on and the weight of the ladder are divided equally over the length of the ladder than the weight of the ladder W≥ 0.45 Pcutter The mass of existing ladders is somewhat lower as shown in figure 3.39. This might be caused by an uneven distribution of the load.

Mladder/Pcutter

Ladder mass over cutter power 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

1000

2000

3000

4000

5000

Cutter power [kW]

Figure 3. 32

3.3.3. The side-winch power and speed If the relation between the horizontal force and the tangential force is assumed to be constant, then for the net side winch power:

Pc Fc = Ps Fh Symbol Fc Fh Pc Ps Rc N vh

= = = = = = =

2π nRc 60 = Fc ⋅ π nRc vw Fh 30vw Parameter Tangential force Swing force Cutter power Swing Power Radius Cutter Cutter head speed Swing speed


Dimension [N] [N] [W] [W] [m] [rpm] [m/s]

Pagina 21 van 80

May 2003

Wb3408b


For a dredger with a cutter head of radius Rc=1 m, a swing speed v of 20 m/min (.333m/s) and a cutter speed of 30 revolutions per minute, this gives a relation between the capacities of:

Pc Fc π nRc Fc 30π ⋅1 F P = = = 9.4 c with ch=1 follows c = 9.4 Ph Fh 30vh Fh 30 ⋅ 0.333 Fh Ph For a cutter head of half this size the relation is:

Pc Fc π nRc Fc 30π ⋅ 0.5 = = = 4.7 × 1 = 4.7 Ph Fh 30vh Fh 30 ⋅ 0.333 Ds

is the same for both cutter heads. 2R c This relative increase in side winch power with reducing cutter head radius is also shown in the installed power in existing cutter suction dredgers (Figure 3. 33.) Small dredgers have small cutter head radius and less cutter power.

Here it is assumed that the relative cut thickness

14

ratio CP/SW

12 10 8 6 4 2 0 0

1000

2000

3000

4000

5000

Cutter power [kW] Figure 3. 33.

Ratio Cutter power over Swing Power

In (Vlasblom, 1998) it is shown that the ratio of the normal force to the cutting force influences the required ratio of cutter power over sidewinch power too. For sharp teeth this ratio is 33 but decreases rapidly with increasing wear flat to a ratio of 5 for worn cutter teeth. In addition to the soil type and the revolutions of the cutter head, both the side winch power and the side winch (wire) speed depend on the dimensions of the dredger and the position of the anchor. It should be noted that the swing force is not equal to the side winch force and the swing velocity not to sideline velocity. If Fh is the horizontal swing force to move the cutter with a speed vc and the force in the sideline wire is Fw and de speed vw It can be proven that in a horizontal plane the power needed to swing the cutter head

Pswing = Fh vh = Fw vw = Pwinch under the assumption that the friction in the winches, blocks and motors are small.


Pagina 22 van 79

May 2003

Chapter 3: Cutter Suction Dredger Moreover the power required to swing the dredger around its spud depends not only on the cutting forces but also on the ladder angle α and the resistance force W to rotate the pontoon. The influence of the ladder angle is because the torque on the cutter has a de-component in the horizontal plane (Figure 3. 27). The moment to swing the dredger around the spud pole is:

M h = Fh ⋅ Rsp − M c sin β + W ⋅ Rw in which Rsp and Rw are respectively the distance from the spud to the working point of Fh and W. Mc may be either positive or negative, depending on the direction in which the cutter head is turning. Therefore the swing power is: Ps = M h

vh v = ( Fh Rsp − M c sin β + W ⋅ Rw ) h Rsp Rsp

For dredging rock the influence of the force W is in order smaller than that of the cutting reaction forces. The swingspeed vh should be taken in relation to the production Q, because Q = S ⋅ Dc ⋅ vh , with S the stepsize in m. and Dc the layer thickness in m. In the position of the side winch sheave on the ladder (Figure 3. 34, Left) , the relation band velocity Vz to warping direction of the side winch sheave Vp is equal to: k vz vp

•

= lϕ

l

sin ϕ − 2

b l

cosϕ

k  b   − cos ϕ +  − sin ϕ l  l 

2

(Figure 3. 34, right)

Figure 3. 34

For the cutting of rock the maximum wire velocity is 20 tot 25 m/minute. For cutting sand values of 30 tot 35 m/minute are taken.


Pagina 23 van 79

May 2003

Wb3408b


3.3.4. The ladder winch speed and power If it is necessary for the cutter suction dredger to dredge slopes completely automatically the ladder winch speed must be in accordance with the nominal side winch velocity. If this is not necessary the ladder winch speed may be chosen freely, bearing in mind that at low ladder winch speeds the production may be significantly affected. When, for example, the teeth of the cutter head must be frequently changed it will be necessary to raise the ladder many times. For medium large cutter suction dredgers a value of 10 m/minutes is often used. The required power is determined by the weight of the ladder and the vertical reaction forces during slope dredging in the under cutting mode. 3.3.5. The dredge pumps To decide which pump type is appropriate for the dredger the working range of the pumpcapacity and pump pressure have to be assessed. Therefore the production capacity in various types of soil must be translated into: 1. The mixture capacity 2. The mixture concentration Because:

Q = Q mixture ⋅

C vd 1− n

with: Q Qmixtur Cvd N

= = = =

Production Pumpcapacity Transport concentration Void ratio

[m3/s] [m3/s] [-] [-]

The mixture capacity is determined by the mixture forming process in the cutter (see chapter 3.3.1.1) The critical velocity required to keep the material in motion determines the minimum flow velocity and thus the pipe diameter. v crit = Fl ,H 2 ⋅ g ⋅ (S s − 1) ⋅ D in which the value of Fl,H is determined by the material to be pumped (see Section 2.2.3.3. Suction pipe diameters of lecture notes “Dredging Processes). Ss is the relative density of the solids and D the pipe diameter in m

Figure Figure 3. 35 from MTI shows practical values used in the dredging industry for the critical velocity in horizontal pipelines The expected production is determined by the cutting power, the side winch power or the side winch velocities, depending on which is the limiting factor in the various types of soil. Using the equation Q = Q mixture ⋅

C vd 1− n

together with vcr gives the pipe diameter and Cvd Figure 3. 35


Pagina 24 van 79

May 2003


Jet Pump Power [kW]

3.3.6. The jet pump To promote mixture forming when dredging sand some cutter suction dredgers are equipped with water jet installations. One or more jets are mounted on the sides of the ladder close to the cutter ring.

2500 2000 1500 1000 500 0 0

500

1000

1500

2000

2500

Cutter Power [kW] Figure 3. 36.Jetpump

power versus cutter power

The power needed for the jets depends strongly on the insight of the designer as.Figure 3. 36 shows. For more theoretical insight into this phenomena the chapter jet pumps for plain suction dredgers should be consulted.

3.4. The drives The drives of the cutter head, the side winches and the ladder winch are either electric or hydraulic drives. Formerly the ladder winch and the side winches were combined to form a tree drum winch with one drive, which made simultaneous operation of the ladder and side winches impossible. With hydraulic systems various drives can run on the same hydraulic circuit and for this reason they can influence each other. The best choice of what may or may not run on the same circuit is important for the operation and thus finally for the production of the dredger. 3.4.1. The cutter head drive The cutter head drive is mounted on the ladder either near the hinge side (the trunnion) or close to the cutter head. In the first case the drive and the gearbox are above water and in the second case these may be in a box under water.


Pagina 25 van 79

May 2003

Wb3408b


Figure 3. 37

If the drive of the cutter head is mounted near the hinge the shaft must be both long and heavy because of the high torque. This long shaft needs several ladder bearings. When the drive is mounted close to the cutter head there is more freedom to adapt the direction of the cutter head axle to the required angle, especially when dredging in shallow water. Figure 3. 38

The choice between hydraulic and electric drive depends primarily on the expected relation between the average load and the peak load. Electric drives are especially suitable because they can take overloading up to 150% without stalling (Figure 3. 39, right). This is possible because of the considerable rotation energy of the rapidly turning electric motor. As a result a flywheel effect is created. The long driving shaft also plays a role in this. However, due to the strong dynamic character of the dredging process, gearboxes for cutter drives have to resist heavier loads than gearboxes for the all drives on board of the dredge. The dynamic cutting process and as consequence the torsion vibrations cause remarkable increase of the torque. It is even possible that due to these vibrations negative torques occur in the shaft and gearboxes with a result “hammering” of the gears. Such situation decrease the live time of the gears. Therefore gearboxes for heavy duty cutter dredgers are designed to resist a torque of 3.5 of the nominal torque. (Hiersig, 1981)


Pagina 26 van 79

May 2003


Speed [%]

Speed [%]

TORQUE - SPEED CHARACTERISTIC (Simple Hydraulic Drive)

TORQUE - SPEED CHARACTERISTIC (Electric Drive)

100

100

100

Torque [%]

100

Torque [%]

150

Figure 3. 39

With hydraulic drives the torque is determined by the piston displacement of the engine and the pressure in the system. When overloading occurs a safety valve which limits the pressure operates, stopping the engine. This means that the average pressure c.q torque is usually considerably lower than the maximum in the order of 60-70 % (Figure 3. 39, left). Hydraulic drives do have the advantages of being completely watertight and of driving the cutter head directly without a gearbox. Often several hydraulic drives are used simultaneously to provide the cutter head with the desired power. 3.4.2. The side winch drives Here too, the drives may be electric or hydraulic. This choice is based on the same line of reasoning as that followed for the cutter head drive. It is not necessary that when the cutter head drive is electric the side winch drives must also be electric. The required power for the side winch drives is roughly a factor 5-10 smaller, so often secondary matters such a standardisation and price play a different role 3.4.3. The ladder drive Because the depth of the cutter head is set with the aid of the ladder winches, the drives must be easy to regulate and must not slip when the ladder drive is not activated. The latter happens frequently with hydraulic drives as a result of leakage of the hydraulic fluid, resulting in changes of the cutting depth the dredging operation. To prevent this slipping the winch must be equipped with a break or ratchet. 3.4.4. The sand pump drives Underwater pumps are often electrically driven. If barge loading is required with the underwater pump, it is necessary to use drives with speed control. With a fixed rate of revolution, f.i. an asynchrony ac-current motor, the variations in flow resulting from differences in concentration and grain size are often too big for the efficient loading of the barges or leads to overload of the motor. Nowadays underwater pumps for small dredgers can also be driven by diesel engines via a pivoting gearbox. (Figure 3. 40)


Pagina 27 van 79

May 2003

Wb3408b

Designing Dredging Equipment Figure 3. 40

Diesel drives are most suitable for the discharge pumps. The choice between one or more pumps and thus diesels depends on the total required pump pressure and the requirements in relation to the speed control of the diesel engines. It will be clear that when only one large pump is installed it is not so easy to control the pumping system for long and short pumping distances. Very important when using diesel drives is the type of governor. Modern governors limit the fuel injection at low revolution to avoid incomplete burning of the fuel. These governors increase increases the speed control of the diesel engines. For jet pumps diesel engines or an asynchrony ac-current motor are used often. Speed control is less important for jet pumps than for dredge pumps, because of the almost fixed layout of the pipeline and the constant fluid density.

3.5. Spudsytems The choice of the spud system plays an important part in the design of the cutter suction dredger. The spud system influences not only the layout of the pontoon, but also the efficiency of the cutter suction dredger. The most frequently used systems are the spud carriage system and fixed spuds (several other systems have been mentioned in the section on technical construction). 3.5.1. The spud carriage system With the spud carriage system the work spud is placed in a carriage which, with the aid of a hydraulic cylinder, can travel over several metres (4 - 6 m) (Figure 3. 41) in longitudinal direction in a well at the stern of the dredger. The carriage is generally positioned in the centre of the dredger (Figure 3. 42) and is support by four wheels on rails for the vertical forces and by guide rollers or bearing strips for the lateral forces. The cylinder is a double acting hydraulic ram. Spud carriage

Figure 3. 41. Spud carriage

A second spud, the auxiliary spud is mounted at the stern of the pontoon, which is used to move the carriage back to its start position.


Pagina 28 van 79

May 2003

Chapter 3: Cutter Suction Dredger The initiation of a new cut is obtained by moving the spud carriage one step forwards. After stepping, the cutter head describes concentric circles until the spud carriage reaches the end of the stroke of the hydraulic cylinder. The return of the carriage usually takes place in the middle of a cut in the following sequence of actions. The auxiliary spud is lowered and the work spud is lifted, the carriage is moved back and then the spuds again changed. After each single swing the dredge master is “ free either to step forwards or to lower the ladder till the final is reached.

Cut width


Spud carriage length

Auxilary spud


Figure 3. 42

In addition to the spud carriage in the stern well of the main pontoon of the dredger, it is also possible to have a separate spud carriage pontoon. This pontoon is fixed to the cutter suction dredger by a stiff link, usually by making use of the existing auxiliary spud carriage . This is done to change the existing, less efficient spud system or to make a wider swing (Figure 3. 43and Figure 3. 44). It is also necessary to move the pivoting bend on the stern of the dredger to the rear of the spud pontoon.

Figure 3. 43


Pagina 29 van 79

May 2003

Wb3408b


Spud carriage

Auxiliary spud

Spud carriage pontoon

Figure 3. 44

3.5.2. The fixed spud system When using fixed spuds both the work spud and the auxiliary spud are in fixed positions on the stern of the pontoon at equal distance from the centre line of the dredger (Figure 3. 45).

Figure 3. 45

The step or start of the cut is now initiated by letting the dredger make an angle from the centre line, then lowering the auxiliary spud and lifting the work spud. The dredger is then swung into a symmetrical position with regard to the centre line where both spuds are changed again (Figure 3. 46). After each single swing the ladder is lowered till the final depth is reached. It will be clear that stepping with fixed spuds takes considerably longer than with a spud carriage, due to the down time of the swing movements. Figure 3. 46

Note that the arc is not symmetrical with regards to the centre line of the cut. Prof.Ir. W.J.Vlasblom

Pagina 30 van 79

May 2003


As an example the difference in effective dredging time has been worked out for a spud system with fixed spuds and one with a spud carriage. Both dredgers are the same with regard to size and power. The following boundary conditions are taken for the work: B vs

Width of cut Swing velocity

75 15

S Lsc

Step size Effective spud carriage length Distance between fixed spud and cutter head Distance between fixed spuds Number of cut layers Number of steps per carriage movement

1 5

Nc Ns

[m] [m/ s] [m]

80

[m] [m]

10

[m]

Time Spud carriage travel

Spud changing 2 min. Change in swing direction incl. lifting 2 min. and lowering ladder ¾ minute

[-] [-]

Fixed spuds

2 min.

=Lsc/Ns

Spud Carriage

Fixed / carriage

0.9

Efficiency [-]

0.8 0.7

0.7

0.6 0.5

0.5

0.4 0.3

E_fixed/E_carr

0.9

0.3 0

2

4

6

8

10

12

Number of cuts [-]

Figure 3. 47 Effectiveness of spud systems

The above example (Figure 3. 47) clearly shows the superiority of the spud carriage system over a fixed spud system.


Pagina 31 van 79

May 2003

Wb3408b


3.5.3. The spud door system For small dredger a cheaper system than the spud carriage is developed by IHC-Holland; the so called “Spud Door” In A heavily constructed door, pivoting around the auxiliary spud, is placed the working spud. The dredge pattern is the same as for the spud-carriage system, however spuds have to be changed more frequently and the accuracy is less because the working spud stays not exactly in the centerline of the dredger. The system is much cheaper than the spud carriage system.

Figure 3. 48 3.5.4. The walking spud system The walking spud system is similar to the spud carriage system with regard to the movement of the cutter head during swinging and stepping. The working spud is not in a carriage but swivels round a horizontal axis (Figure 3. 49). The step is now taken by allowing the spud to tilt to the requisite angle. The disadvantage is immediately apparent; the maximum step depends on the depth of the water and so walking spuds are difficult to use in shallow water. The disadvantage is that it is very little or not at all cheaper than a spud carriage. The dredging pattern is similar to that with a spud carriage, while the number of spud movements is considerably larger.

B

A

Walking spud

Figure 3. 49


Pagina 32 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.5.5. The rotor spud system This system was already invented in the early years of 20th century.With the rotor spud system both spuds are in a rotor and stand on the ground diametrically opposite each other. (Figure 3. 50 ).

Figure 3. 50 Rotor spuds

During dredging the midpoint of the rotor remains in the centreline of the cut, so the dredger turns round the rotor. Stepping is accomplished by lifting the rear spud and turning the rotor until the rear spud becomes the front spud. The step S=2*L*sin(2α), in which L is the distance between the spuds and α the angle through which the rotor turns. Using this system the dredger makes a pattern of concentric circles. The advantage of this type of system is that when stepping, only one spud has to be raised and lowered. This disadvantage is that it is very expensive, certainly for the large cutter suction dredgers. Moreover the spuds cannot be placed horizontally.

Figure 3. 51

From the point of view of efficiency, here defined as the actual dredging time in relation to total time per spud cycle, the spud wagon is the best. The number of spud changes per metre of progress is minimal. With a well-chosen cutting pattern no partly or entirely unproductive swings (warping without cutting) are needed. Likewise the rotor spud and tilting spud systems have advantages over the fixed spud systems.


Pagina 33 van 79

May 2003

Wb3408b


3.5.6. The Christmas tree There are situations in which anchoring by means of spuds is not possible. Such a situation arises when working at sea if the forces that waves or swell can exert on the spuds are too large. In that case one changes to working on wires. For this a Christmas tree (Figure 3. 52), a construction with wire leads, is mounted in one of the auxiliary spud carriages. With this the anchor wires meet at one point under the under the hull. However, in order to keep the cutter head well into the face throughout the entire swing the laterally directed anchors of the Christmas tree must stand well forward. with the disadvantage that they must be moved frequently. For this reason a bow anchor is often used. One of the advantages is the possibly to work in deep water, but this can only be done in special cases. In a well designed cutter suction dredger the spuds are so long that they can reach the maximum dredging depth at all times, so dredging in deep water is only possible with an extension by means of a special ladder construction. A very real advantage of working on anchors is that a considerably bigger cutting width can be achieved.. Obviously the disadvantages overweigh the advantages, otherwise the system would be more widely used. These are: • At least three anchors must be moved. • The freedom of movement when working on anchors is so great that it is almost impossible to dredge accurately • This is equally true for dredging in hard soil. A star system is needed for this.

middelste lier "Christmas tree"

BB lier "Christmas tree"

SB lier "Christmas tree" acht

SB achterzijanker

erzijd e

zuig e r

50°

50°

75° 30°

BB achterzijanker achteranker

Figure 3. 52


Pagina 34 van 79

May 2003


3.6. The general layout

Figure 3. 53 CD EDAX

Depending on the spud system the hull may consist of a simple U-shapes pontoon (with fixed spuds) or an H-shaped pontoon (with a spud carriage system). The main dimensions; length, beam and draught of the pontoon derive from the requirements in relation to the above mentioned design parameters and the associated requirements in relation to stability and strength. Figures 3.54 and 3.55 gives design information for the pontoon.


Pagina 35 van 79

May 2003

Wb3408b


10,000 y = 0.4664x R2 = 0.9597

9,000

8,000

y = 0.3485x

8,000

2

R = 0.925

6,000

Light weight [t]

Light weight [t]

10,000

4,000 2,000

7,000 6,000 5,000 4,000 3,000 2,000

0

1,000

0

5,000

10,000

15,000

20,000

0

25,000

0

Total installed power [kW]

5,000

10,000

15,000

20,000

25,000

BLD [m3]

Figure 3. 54 L/B

B/T

12.00

L/B & B/T

10.00 8.00 6.00 4.00 2.00 0.00 0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Light weight [t]

Figure 3. 55

The engine room, the pump room and sometimes in larger cutter suction dredgers, also the control room for the machinery, are located in the pontoon. In smaller cutter suction dredgers the sand pump is sometimes located on the engine room directly in front of the engine, with all the well-known disadvantages of such an arrangement.

Figure 3. 56


Pagina 36 van 79

May 2003

Chapter 3: Cutter Suction Dredger A frequently used layout is shown in Figure 3. 56. Here the pump room is directly aft of the bow well; aft of which is the engine room. The fuel and ballast tanks are located in the side pontoons of the fore and aft wells. The storerooms are located in the side pontoons of the forward well. The hydraulic system drives, workshops and a galley for the *local crew are often located in the side pontoon next to the well for the spud carriage. Mess rooms, toilet facilities and possibly also crew quarters are above deck.

Figure 3. 57

If the cutter suction dredger has been designed to work in the tropics the generators are separated from the engine room to assist in the cooling of these machines (Figure 3. 57).

Figure 3. 58

Figure 3.56 shows a dredger with the spud carriage out of the centre line of the dredger, while the cutter lead axes is the the centre line . This means that the teeth position is not optimal for both sides and as a consequence this will result in more teeth wear. Self propelled cutter suction dredgers have a more complicated layout resulting from the two possible modes of working; dredging and sailing. The propulsion mechanism can be located at the ladder end (CD Taurus, CD Marco Polo, CD da Vinci) or at the spud end (CD Ursa, CD Oranje). In the second case the dredger sails with the ladder at the front and port and starboard is the same for both sailing and dredging. Moreover the propellers are directly driven by the main engines. This is not possible in the first case, so the propellers are powered by electric motors. The layouts described are therefore self explanatory (Figure 3. 59).


Pagina 37 van 79

May 2003

Wb3408b


Figure 3. 59

Self propelled cutter suction dredger “Ursa”, built in 1986

Small to medium sized (to 3500 kW) cutter suction dredgers are often used to make roadbeds. To permit overland transport to the sand extraction area these dredgers are demountable. Because of the need for strength, the main pontoon in which the pump and diesel engine are located is usually constructed as a single unit. When designing demountable dredgers it is necessary to consider how the parts of the dredger will be transported by road or over water. In the first case the maximum size of the pontoons is determined by the permitted size and weight for road transport. For smaller dredgers the pontoons are made up of 40 or 20-foot containers, while the other parts are of such size that they can be carried in containers.


Pagina 38 van 79

May 2003


Figure 3. 60 Beaver 1600 on 5 trucks

Ballast tank Lubricating olie Ballast tank

Fuel


Spare parts

Spare parts

Dry tank

ballast tank

Drinking water Engine room Ballast tank Ballast tank

Fuel


Spare parts

Spare parts

Dry tank

ballast tank

Figure 3. 61

In demountable dredgers also, the pump room and the engine room are located one behind the other in the main pontoon and the ballast tanks and storerooms are in the side pontoons (Figure 3. 61). With containerized dredgers the entire vessel is built up out of containers. In this case the pump and motor are often in a container “on deck” (Figure 3. 62).


Pagina 39 van 79

May 2003

Wb3408b


Figure 3. 62 Containerised Dredger

3.6.1. The Hull The floating capacity of a stationary cutter suction dredger derives from the pontoon that is constructed as a single unit (mono-hull or mono-pontoon) for most large cutter suction dredgers and, for demountable cutter suction dredgers, consists of several pontoons. The pontoons beside the ladder well are often chamfered to form trapezoids in order to limit the minimum width of cut.

It is essential that there is a separate pump room: if the pumps were located in the engine room a leakage or an error during inspection of pumps might result in the flooding of the engine room with a good chance of the dredger sinking. The pump room should be designed in such a way that, when flooded, the dredger doesn’t sink. Furthermore the pipeline system must be designed in such a way that the flooding of the pump room can be kept to a minimum. Consider therefore: • a remote controlled valve behind the well bulkhead. This is necessary for the changing of the rubber suction hose • a bilge alarm.


Pagina 40 van 79

May 2003

Chapter 3: Cutter Suction Dredger In designing the hull it is necessary to take into account that a part of the reaction forces from the dredging process must be transferred to the work spud via the hull. For this reason the main pontoon of demountable dredgers is constructed as a single unit. This means that the ladder hinge and spuds are mounted on the main pontoon, so the side pontoons as well as the links to the main pontoon are not so heavily loaded. The ladder gantry spans over the forward well as a simple A-frame, a frame construction or a frame in the form of a box girder construction. When dredging in ‘undercut’ the vertical forces are transferred to the pontoon via the gantry.

Figure 3. 63 Different ladder gantries

3.6.2. The cutter head ladder Originally the cutter ladder, or cutter ladder was constructed as a frame girder with two longitudinal girders consisting of steel beams connected to each other by many transverse beams and struts. The name cutter ladder derives from this structure. The transverse beams were used as supports for the cutter shaft bearings. The ladder that is located in the forward well is hinged (the trunnion) on one end to the pontoon and a tackle and ladder wire to the ladder gantry suspends the other end. The ladder wire runs via the ladder gantry and various sheaves to the ladder winch to adjust the desired depth.


Pagina 41 van 79

May 2003

Wb3408b


Because owing to the transverse forces it is essential for the ladder of a cutter suction dredger to be stiff, for the large cutter suction dredgers a double box construction is used, strengthened by longitudinal and transverse links. Furthermore this has the advantage that the ladder is given sufficient weight. This weight is needed in order to swing the cutter head to both sides. If the ladder is not heavy, as in the case of small cutter suction dredgers, extra arrangements must be made. For example the cutter head drive can be mounted as close as possible to the cutter head. Lead is often added close to the cutter head. For very heavy cutter suction dredgers the requirement of the stiffness may exceed the demand for sufficient underwater weight. In this case the ladder is equipped with floats.

Figure 3. 64 Boxtype cutter ladder

In small cutter suction dredgers the ladder is often built up from basic elements. The ladder is supported by pins that are fixed to the ladder and rest in bearing houses that are rigidly fixed to the pontoon. The drive of the cutter head is either at the top of the ladder, thus at the hinge side or below near the cutter head. In the first case the drive and the gearbox remain above water and the cutter head is driven by a long shaft, sometimes tens of metres long. Because of the high torque demanded by the cutter head this shaft has a considerable diameter. The shaft has supported at various points and must, especially in the case of heavy cutter suction dredgers, be on the centreline of the ship. The end bearing, (Figure 3. 66 and Figure 3. 66) close to the cutter head is made of rubber and lubricated by water. The axial forces are taken up by a pressure bearing that is mounted in the gearbox.


Pagina 42 van 79

May 2003


Figure 3. 65 Rubber end Bearing

Cutter ring Gland water

Cutter shaft Bearing bush

Release ring Rubber bearing Suction mouth Cutter hub Cutter blade Figure 3. 66

3.6.3. The cutter head The production of the cutter suction dredger is largely determined by the cutter head. Its type and size depend not only on the technical specifications of the cutter suction dredger, including cutting and side winch power, cutter revolutions and the weight of the ladder, but also on type of soil to be dredged. With relatively high side winch forces and a small cutter diameter, higher cutting forces can be generated and thus harder soil can be cut. In contrast, with the same cutter power in soft ground it is necessary to use a bigger cutter diameter and exchange the high side winch forces for a higher speed by changing the gears of the side winch drive. When cohesive soil is being cut different boundary conditions play a role, for example, the need to avoid blocking the cutter head.

General guidelines for cutter heads for various types of soil.(Figure 3. 68): • for hard soil. Suitable to withstand impact forces on one or more teeth, thus heavy and robust. Small in contour with replaceable teeth. Can withstand extreme wear on both the cutter head itself and on the teeth and adapters. Good, accurate tooth positions. The size of the fragments may not exceed the minimum passage of the pump. • for non-cohesive soil. Suitable for very high production rates Good mixture formation required. Many replaceable chisels (wide or narrow) or cutting edges. Wide though flattened contour (little pumping action). Well able to withstand wear, especially of the cutting elements. Here also good, accurate tooth positions are needed.


Pagina 43 van 79

May 2003

Wb3408b •


for cohesive soil. The cutter head may not become blocked, so is ample and round in contour. Open near the hub. Often with one less blade (thus 5 blades). Good cutting properties in clay, small fragments. Plain or serrated edges or many small teeth.

HUB

Ring

Figure 3. 67

Sticky soils Non sticky soils Rock

Elements of a cutter head

Contours

- open to prevent blockage - multi purpose - high torque

Figure 3. 68 Cutter head contours

Although it is better to use a different type of cutter head for each type of soil, cutter heads are marketed that can be used in more than one type of soil. The so-called ‘multipurpose cutter’ is a compromise with regard to contour. A cutter head is comprised of the following parts (Figure 3. 67). • The back ring, that is the ring on the underside of the cutter head. The inside diameter of the ring is such that this fits the suction mouth and or the cutter shield (Figure 3.66). • The hub by which the cutter head is mounted via an ‘Acme” or three threaded screw onto the cutter shaft. The distance between the underside of the ring and the underside of the hub is termed the set height.

Figure 3. 69


Pagina 44 van 79

May 2003

Chapter 3: Cutter Suction Dredger •

•

• •

The cutter arms or blades, usually 5 or 6. The number is related to the required strength and/or space between the arms. The cutter arms form a screw shape and link the ring to the hub. The cutter head is termed a normal helical cutter head if the chosen screw shape is such that the dredged material is transported to the ring. (Figure 3. 69 left) If the thread of the screw runs in the other direction the cutter head is termed a reverse helical cutter (Figure 3. 69 right). Edges (knives) or replaceable teeth or chisels are mounted on the cutter arms. The tooth is attached by means of a locking pin to an adapter that is fastened to one of the blades. In hard soil a six bladed cutter head is often used with teeth on the even blades that are offset in relation to those on the uneven blades. This is termed ‘staggered mounting’. The turning direction of a cutter head is defined when looking from the control cabin towards the cutter head; that is against the underside of the ring. The passage through the cutter head increases towards the ring. This may cause blockages in the pump if fragments that are too large for the pump can be taken up. The passage through the cutter head is sometimes reduced by the addition of skirts, which are welded onto the blades to extend the cutter arms(Figure 3. 70). The passage can also be reduced by the welding of plates perpendicular to the blades (Figure 3. 70).

Figure 3. 70

Besides the turning direction the height H between the under side of the hub and underside of the ring, the internal ring diameter Di and the type of tread in the hb are the important data for mounting the cutter well on the shaft and ladder.(Figure 3. 71)

Cutter (teeth) contour

Protection plate Double ACME Tread

Hub

Cutter blade H Cutter ring Inner diameter Di


Pagina 45 van 79

May 2003

Wb3408b


3.6.4. Tooth and cutting edge systems There are various tooth and cutting edge systems on the market, each with its own advantages and disadvantages. They are all based on the principle that it must be possible to quickly replace the parts that are subject to heavy wear. In addition to the property mentioned above, a tooth must satisfy the following requirements: • There must be a good transfer of the cutting force to the cutter arm. • The positioning of the teeth and adapters must be such that there is little or no wear on the cutter arms. The blades must therefore run freely. • Mixture formation in the cutter head is promoted. .

Figure 3. 72

As shown in Figure 3. 72, there is a wide range of types of tooth and chisel. The use of the specific type of tooth depends on the strength of the soil. •

pick points short


: hard rock Pagina 46 van 79

May 2003

Chapter 3: Cutter Suction Dredger • • • • •

“ “ “ “ chisels “ “

long trapezoid narrow wide flared

: rock : soft rock :cemented sand :sand and loose soil : clay

Figure 3. 73 Tooth Systems

A*

A

Cutting angle Cutting angle

Rake angle

CONVENTIONAL

Rake angle

VOSTA D

Figure 3. 74 Vosta tooth System


Pagina 47 van 79

May 2003

Wb3408b


The best known systems are: • Esco (Figure 3. 73 left) • Florida (Figure 3. 73 right) • Vosta (Figure 3. 74) The first two types are very similar to each other. 3.48 Verschillende adapter typen.

3.49, Spherilok systeem.

Figure 3. 75 Adapter systems

The difference lies in the fitting of the tooth and the adapter (Figure 3. 73 Four types of adapter can be distinguished of both systems, these being: • • • •

the weld-on adapter the single-leg adapter the double-leg adapter the Spherilock adapter

From above downwards these adapters have a reduced grade of freedom in positioning. On the other hand the chance of incorrect positioning during repairs also decreases.


Pagina 48 van 79

May 2003

Chapter 3: Cutter Suction Dredger There is a wide variation in the types of teeth and chisels used by these systems, depending on the material to be dredged. The adapters take up the cutting force, which implies that there must be a good fit between the tooth and the adapter, in other words the tooth must not be loose. The joint is secured with a locking pin, which is prevented from falling out by a flexible rubber locking keeper. The Vosta system is clearly different from the Esco and Florida systems (Figure 3.73).

ADAPTER EDGE

TOOTHED EDGE

SERRATED EDGE

PLAIN EDGE

Types of cutter knives.

Figure 3. 76

In addition to cutter heads with replaceable teeth or chisels there are also cutter heads with cutting edges. The edges welded directly onto the cutter arm of the cutter head, with or without a fitting lip (see Figure 3. 76) Such types of cutting edge are suitable for various types of. edges. The main shapes are : • plain edges : for various types of soil • serrated edges : for clay • toothed edges : for hard clay • adapter edges : for hard clay These edges can also be obtained as projecting offset edges. In this case the plane of the edge forms an angle with the cutter head arm. This prevents material such as clay from sticking to the arm.


Pagina 49 van 79

May 2003

Wb3408b


3.6.5. The side wires As said, the dredger is moved over the width of the cut by hauling on one of the side wires while at the same time paying out the other. The side wires run from the side winches via the side wire sheaves to the anchors

The side wire sheaves, which are fastened at the lower end of the ladder must be able to adjust to the angle that the side wire makes with the plane of the horizontal, because the anchor is not usually at the same level as the point of attachment of the side wire to the ladder. The position of the side wire sheaves and the anchor determines not only the force in the side wire, but also the speed at which the cutter head moves. (Figure 3. 77)

Figure 3. 77Side wire sheaved in upwards position

The side line winches can either be placed on the ladder or on the pontoon. Some heavy duty cutter suction dredgers have double drum winches (Figure 3. 78). The side line wire is first laid over a grooved drum with a relative small diameter to a drum with a bigger diameter. On the grooved drum sufficient wire length can be stored to swing over a full cut width On the big drum additional wire can be stored. Figure 3. 78

Figure 3. 79

Figure 3. 80

Figure 3. 79shows the sheaves on the ladder to guide the side wires to the winches on the pontoon and Figure 3. 80hydraulic winches on a Beaver Dredger.


Pagina 50 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.6.6. The anchor booms Anchors can be moved by a floating crane, assisted by a flatboat. To keep anchoring movements to the minimum, they are dropped as far as possible from the dredger. Modern cutter suction dredgers are often equipped with anchor booms, which makes it possible for the skipper to move the anchors without outside assistance. Top wire

y iliar Aux

wire

Buoy wire Ancher boom

Figure 3. 81 Anchor boom

The anchor booms are placed on the bow pontoons at the point where the chamfering starts (Figure 3. 82) and fastened to the deck by a pivoting construction. Each anchor boom is fastened by one or more wires to a frame or, as if often seen, to the ladder gantry.

Figure 3. 82 Al Mirfa changing her anchor position


Pagina 51 van 79

May 2003

Wb3408b


The anchor boom can turn on its pivoting construction by means of the anchor wires which are fixed to the top of the anchor boom and which run via a series of sheaves to the anchor winches. The anchor wire, which is used to pull up the anchor, runs from the anchor to the top of the anchor boom via the anchor boom downward and then via a set of sheaves to the anchor winch. 3.6.7. The spuds The spuds are fastened via spud doors to the spud carriage or the pontoon. Because the spuds are loaded on a bending moment the wall thickness increases with the stress level (Figure 3. 83 right). To obtain a good penetration into the soil, the lower ends of the spuds are pointed. In hard soil the spud is often dropped in free fall and needs therefore a massive point (Figure 3. 83 left)

Figure 3. 83

In soft ground, on the other hand, the spuds are set down to prevent them from sagging too far into the ground. During transport the spuds must be carried horizontally, so most cutter suction dredgers have special equipment for this purpose.

3.6.8. The spud lifting system In order to move the dredger, the spuds must be lifted and various systems for this are in use. The simplest method is one in which the spud is hoisted by means of a wire attached to the upper end(Figure 3. 84 .a). This method is often used by American cutter suction dredgers and has the advantage of simplicity and accessibility when wires break. .


Pagina 52 van 79

May 2003


sling

b

c

a Figure 3. 84 Spud Lifting systems

The great disadvantage is the high construction height needed to lift the spud in this way. It is also difficult to extend the spuds, should this be necessary. In order to avoid this disadvantage the spud can be hoisted on a wire that runs through a pulley mounted on the underside of the spud (Figure 3. 84.b). Although this is still a simple construction it has the disadvantage that when a wire breaks it is not easy to thread the new wire through the pulley and it is necessary to use either a diver or a crane. Many cutter suction dredgers lift their spuds by means of a sling, which is clamped round the spud by the tension in the hoisting wire. The hoisting wire runs over a sheave that is attached to a double action cylinder above and which runs down to a fixed position on deck. The spud is then hoisted by extending the cylinder (Figure 3. 84.c). This construction has the advantage that all the parts are easily accessible and it is not a high structure. Moreover the spud can fall freely because the sling is self releasing. The disadvantage is that the lifting height is restricted by the stroke of the cylinder. In that case the spud must be taken over. For this reason the spud has holes through which pins can be pushed so that the spud remains suspended on the auxiliary carriage.

3.6.9. Pumps and pipelines 3.6.9.1. The suction pipeline The suction mouth is mounted under the end bearing and opens into the cutter plate/shield (Figure 3. 85). The area of the suction mouth is usually a little bigger than the area of the suction pipe (1.2/suction pipe). In some cases the suction mouth is not symmetrical but somewhat turned in the turning direction of the cutting head. This gives less spillage when over-cutting (cutter head turning in the direction of swing). The suction pipe must be mounted in or under the ladder in such a way that parts can be easily changed. Figure 3. 85 view on suction mouth of CSD Ursa Prof.Ir. W.J.Vlasblom

Pagina 53 van 79

May 2003

Wb3408b


The connection of the suction pipe on the ladder to the pipeline in the ship must be flexible because of the pivoting movements of the ship. Often a suction hose is used. This is a heavy cylindrical rubber hose with steel rings embedded in the rubber to prevent it from collapsing when under pressure occurs. When dredging in coral or coral-like types of rock, suction hoses cannot be used owing to the sharpness of the fragments of coral that cut the rubber. In such cases a ball joint from a floating pipeline forms the link. The angle through which the ladder rotates is then usually more restricted than when a suction hose is used. It is also recommended that an extra suction pipe be placed in front of the first on board pump through the bottom of the hull. When using long discharge pipelines this extra suction pipeline makes it possible to raise the ladder, for example to inspect the teeth, while the pumps are still being used to clean out the discharge pipeline. 3.6.9.2. The pumps For cutter suction dredgers without an underwater pump the suction pipelines should be kept as short as possible and the position of the first pump should be as low as possible under the waterline. Where the suction pipe emerges above water the chance of air being sucked into must be minimized. (The taking in of air has the same effect as cavitation.) Besides good discharge characteristics the first pump must also have good suction characteristics. In other words a high vacuum limit and/or low NPSH-value.

If the dredger is equipped with an underwater pump the layout is less critical and factors such as accessibility for inspection and repair play a more important role. The inboard pump requires only good discharge characteristics. If there is more than one inboard pump on board the layout must be such that, if desired, the ladder pump and one of the inboard pump can be used. All pumps must have an inspection hatch so that the pump and impeller can be inspected and, if necessary, to remove debris. 3.4.4.1 The discharge pipeline The pipeline runs from the pump room high above the deck to the stern (Figure 3.57). In the pipeline on board are: • an expansion joint to take up possible changes in length. • a gate valve in case it is necessary to prevent water from running back from a higher-level disposal site. • an air release valve • a suspension bracket from which lower bend can be suspended and still rotate. • a lower bend with a ball joint to which the floating pipeline can be attached. A suction hose may be used instead of a ball joint.


Pagina 54 van 79

May 2003


Figure 3. 86 Pipeline layout on a dredger

3.6.10. The winches 3.6.10.1. The ladder winch As previously stated, the depth of the cutter head is adjusted by means of the ladder winch. This variable speed winch may be an electric or a hydraulic drive. For heavy ladder constructions, with consequent high forces on the wires, the winch drums are grooved to prevent wire weir. The size of the drums needs a diameter to accommodate the entire wire in the groove. During repairs and transport the ladder is kept in a fixed position (Figure 3.87), often by slings or rods that are directly fastened to the ladder gantry.

Figure 3. 87

3.6.10.2. The side winces The dredging process is controlled with the aid of the side winches. To a large extent the production of a cutter suction dredger is determined by the swing speed. The hauling winch takes care of the feeding of the cutter head, while the paying out winch ensures that wire remains taught. The side winches may also have electric or hydraulic drives.


Pagina 55 van 79

May 2003

Wb3408b


Modern cutter suction dredgers are often equipped with an automated cutter control system which controls the side winch speed on a number of values such as the cutting power, side winch force (amps), the concentration and the velocity of the mixture. Older cutter suction dredgers sometimes have side winches that are combined with the ladder winch to form one central winch, thus three drums and one drive. The paying out of the side winch then takes place by freeing it from the drive shaft. Braking is then entirely mechanical. It will be clear that in this case the ladder winch and the side winch cannot be operated independently of each other, which is necessary when dredging slopes.

3.6.10.3. Other winces If the dredger is equipped with anchor booms, it needs anchor winches and buoy line winches. Depending on the spud hoist system there may also be spud winches and if the cutter suction dredger must be able to work on a Christmas tree, stern winches and perhaps also a bow winch will be needed. All these winches may be found in either electric or hydraulic form. 3.6.11. Hoisting equipment On board cutter suction dredgers cranes are necessary to lift heavy parts such as pump houses, impellers and cutter heads. On large dredgers they can often travel over the length of the pontoon.

Figure 3. 88 Mobile and fixed cranes

3.6.12. Auxiliary equipment Cutter suction dredgers require the following auxiliary equipment: • A flatboat to move the dredger. By this it is understood the towing of the dredger from dredging point to dredging point. • A work barge with a crane to carry supplies to the dredger. This can also be used to move anchors if there are no anchor booms and to set out or move parts of a floating pipeline. It may also be used to change the cutter head. • Some cutter suction dredgers even have a special cutter head pontoon. The cutter head rests on this support. The pontoon sails under the raised ladder. (There are also special cutter suction dredgers equipped with cutter manipulators with which the cutter can be removed from the shaft in an easy way and placed on deck, after which a new cutter head can be fitted.)


Pagina 56 van 79

May 2003


3.7. The dredging process When dredging with the cutter suction dredger the three main phases of excavation, transport and disposal can be distinguished too, however in this chapter only the excavation will be considered. In the process of excavation by cutter suction dredgers an important part is played by the breach-forming characteristics of the soil to be dredged. In good breach-forming soil, which will be defined later, the flow of soil to the underside of the breach is so good that little or no further cutting is required. With soil that does not breach easily, the cutter head must cut the entire face of the bank. This takes more time and thus the production rate will be lower. In addition to the type of soil and its properties, it appears that the cutter production also depends on a number of the ship’s characteristics such as the cutting power, the swing speed and swing force, the spud system, and the position of the anchors during the cutting process. The boundary conditions set by the work, such as the cutting pattern, possible slopes that must be dredged, hydraulic pipeline transport distances, weather conditions and shipping movements also have a big influence on the production. 3.7.1. The spillage In both breach-forming and non-breach-forming soil, spillage plays an important role. Spillage is defined as the material in the dredging area that comes to rest above the cutting area of the cutter head. In other words spillage is the material that is not taken up by the suction mouth.. (Figure 3. 89) g Lowest cutting level Spillage

Figure 3. 89

There are two reasons why such material is not recovered by the dredger 1. The method of working is such that not all the material comes into contact with the cutter head and thus it cannot be taken up. Such a situation arises when the thickness of the material that the cutter head removes with one cut is greater than the diameter of the cutter head. The material which lies above the cutter head falls behind it and thus cannot be taken up. (Figure 3. 90). This phenomenon occurs mainly in cohesive soils such as clay and in rock.

spillage

Figure 3. 90


Pagina 57 van 79

May 2003

Wb3408b


2. All the dredged ground is not taken up. The reason for this is more complex. Owing to its shape a cutter head has some pumping power. It pumps water in an axial direction to the rear. When the dredge pump is out of action the water taken in by the cutter head leaves the pump close to the ring. As in the case of dredge pumps, the size of the flow that is sucked in by the cutter head is proportional to the revolution speed of the cutter head. Figure 3. 91

If the dredge pump is also running, the amount of water that leaves the cutter head close to the ring is reduced. In principle it is possible to use such a pump flow rate that no outflow takes place near the ring. It appears that the percentage of the material cut by the cutter head that is taken up is linearly dependent on the relation::

Production=1-Spillage=

Q pump Pump capacity  v  = F ⋅ z  = θ Cutterhead capacity ω ⋅ RR3cutter ω ⋅R 

The value of the angle θ depends on the direction of rotation of the cutter head, swing direction and on the material to be dredged. For sand with a d50 < 500µ, θ can be taken as 0.4. For soils such as clay and rock the process is much more complicated because the interaction of the separate soil particles with the cutter head play an important role. As stated in chapter 3.2.2.2. θ may be a factor 3 higher in that case. Often in this type of case a constant spillage factor of 0.3 - 0.4 is used. As mentioned earlier, the spillage also depends on the work method. When breach-forming soil (Figure 3. 92) that forms an angle of slope α with the horizontal is cut by a cutter head, the spillage depends only on the above I mentioned relation of the velocity as long as the underside of the slope passes through the cutter and area I equals area II q The maximum cutter head filling by an q+b unchanging spillage factor is obtained if II the cutter head is at right angles to the b slope. That is when β + θ = 900, in which For no addtional spillage Arae I = Area II θ is the angle that the cutter ladder makes with the horizontal. Figure 3. 92 If the underside of the slope runs behind the cutter ring the material will not be cut but will be transported further from the cutter head by the action of the pump. Moreover there is now a good chance that that part have to be shifted by the ladder. See chapter 3.2.2.2 minimum dredging depth. The further the underside of the ladder comes behind the slope, the greater will be the chance of a dragging ladder. On the other hand the filling of the cutter head is better. Prof.Ir. W.J.Vlasblom

Pagina 58 van 79

May 2003


Whether the underside of the slope passes through the cutter ring depends on the breach forming behaviour of the sand, the swing velocity and the size of the step of the cutter head. 3.7.2. The production in breach-forming soils The breach-forming characteristic of a slope depends on the permeability, thus the grain size and pore volume of the sand layer. If a suction pipe is quickly lowered vertically Suction Tube into the sand, a pit with almost vertical sides 240 is formed. The dimension of the pit increases 210 Slope 180 with time because the sand grains and 150 Vz fragments of sand slide from the slope and 120 100 0 flow to the suction pipe. The bank of the 80 15 60 slope moves away from the suction mouth at 50 20 an almost constant velocity. The velocity is 40 30 also called the bank velocity Vwal. This Vwal is roughly 30 * the permeability. Time in seconds B

Suction Velocity Vz = 2.5 m/s

Figure 3. 93

In the lecture notes lecture of Wb3413, part “the Breaching Process” the following theoretical k γ k − γ w 1− n 1 value for Vwal is derived: v wal = which leads to the above-mentioned n tan φ ∆n γ w value of vwal≈30k . The angle of slope β in front of the suction pipe follows directly from the relation between the bank velocity Vw and the velocity Vh at which the suction pipe moves forward (Figure 3. 94.).  tan α   v h = v w 1 −  tan β 

D

C

vh

vw a

b B

A

Figure 3. 94

(3.12) From this relation it follows that β is equal to 90° when Vh = Vw. The maximum angle of slope α, the angle at which no more soil runs down to the suction mouth, is for small breach heights the angle of internal friction. In most cases however, and certainly with deep extraction pits, this angle is smaller. With bank heights of 15 m or more, angles of slope of 1:10 to 1:20 occur. The erosion of the sand flowing over the slope causes these.


Pagina 59 van 79

May 2003

Wb3408b


When dredging good breach-forming soil, with a permeability 1*10-4 en α = 10° at such a depth that the axis of the cutter head makes an angle of 30° with the horizontal, the maximum cutter head filling β = 60°. The maximum progress of the dredger is then:  tan 10 o  −4 −4 v h = 30 ⋅ 10 1 − m/s o  = 27 ⋅ 0  tan 60  The breach production is: Pb = v h ⋅ B ⋅ H [m3/s] In which: B = width of the cut H = height of the face

[m] [m]

The bank production for a width of 80 m and a face height of 5 m is now:

Q b =27*10-4 *80*5=1.08

[m3/s]

For an average cutter head radius of 1 m, a cutter head speed of 30 revolutions per minute and a suction velocity of 4 m/s in an 800 mm suction pipe, the percentage that can be taken up is: v 4 Pf = θ⋅ z = 0.4 ⋅ = 0.51 [-] ω⋅ R π The suction production is therefore:

Qs =0.51×1.08=0.55

[m3/s]

The spillage is thus 49 % of the face height, that is 2.45 m. If the revolution of the cutter head is reduced from 30 to 15 revolutions per minute because no cutting process develops in breach forming soil, then : Pf = 0.4 ⋅

4 = 1.02 0.5 ⋅ π

[m3/s]

Because there is always some loss, for example due to the variation in the permeability of the sand layer, Qs is given an upper threshold Pf = 0.9 The suction production is then:

Qs =0.9×1.08=0.97

[m3/s]

The spillage is now only 45 cm. In breach-forming soil the ladder is almost at maximum depth and only swings from port to starboard and back.

If a specified depth must be dredged it is always necessary to make a clean-up sweep: a final swing, which removes all irregularities. The question that now arises is how quickly must the cutter head swing in order to remove this material.. If the area of the cutter contour is assumed to be Ac = 3 m2 , the cutter head must move at a swing velocity of:

vt =

Q b 1.08 = =0.36 m/s = 21.6 m/min Ac 3

Whether or not the side winches are able to deliver this velocity in one way or another must be ascertained. (see chapter 3.2.2.3) Prof.Ir. W.J.Vlasblom

Pagina 60 van 79

May 2003


The area Ac that the cutter head cuts while swinging across the face also determines the step size that the dredger must make in the corners. After all the face production must be equal to the cutting production, thus: A A c ⋅ v t = H ⋅ S ⋅ v t ⇒ S = c [m/s] H vt = translation velocity of the cutter in [m/s] The average production reached during a full dredging cycle, that is the time between two movements of the spuds, is in fact lower. This is because stepping, moving the spuds and, if necessary, raising the ladder, all take time. These factors are entirely dependent on the spud system and the time needed to perform the various procedures.

3.7.3. The production by non-breach forming soils If the soil forms an inadequate breach or does not breach at all, as is the case with cohesive soils such as clay and rock, and to a lesser degree fine sand, the cutter head must do what it is designed for, that is cut the soil loose.

Depending on the type of soil, the spud system, the suction depth and the insight of the dredge master, the breach may be cut in various ways. Figure 3. 95 gives an example for a cutter suction dredger with fixed spuds.

Dredging in cohesive soil

2

Swing number

3

4

5+6

10

11+12

17+18

13

7

1

8

14

79

15

21

16

22

23+24

29+30

28

25

19

20

27

26

cut 1

cut 2

cut 3

cut 4

cleaning up

Figure 3. 95


Pagina 61 van 79

May 2003

Wb3408b


If the dredger has a spud carriage the variety of ways in which the breach can be cut is even greater (Figure 3. 96). This pattern is used when the cut is to be made to the desired depth in a single cut. The numbering gives the order of cutting.

Dredging in cohesive soil

6

Swing number

11

16

17

22

21

3

2

1

8

7

13

18

19

10

cut 2

cut 3

15

20

cut 4

cleaning up

24

23

9

14

12

cut 1

5

4

Figure 3. 96 Dredging in cohesive soil

If the breach rises above the water level, in order to prevent a spillage problem. The pattern shown in Figure 3.97 or Figure 3.98 must be used.

1

7

13

20

28

8

14

21

29

cut 3

4+ 9+ 15

+ 22 5+ + 30 10 + 16 + 23 + 31 11 + 17 + 24 + 32 18 + 25 + 33

3

19

12

2

26 + 34

Swing number

6

35

27

cut 1

cut 2

cleaning up

Figure 3. 97 Dredging in cohesive soil

14+15

1+2

24

33

10

17

25

34

cut 3

39

16

30 +

37

9

21 + 29 +

36

20 + 28 +

13 +

19 + 27 +

6+ 12 +

5+

11 + 18 +

26 + 35

4

3

38

Swing number

22+23 31+32 cut 1

35

cut 2

cleaning up


Pagina 62 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.7.4. Specific energy The number of layers over which the breach is cut, the step size and the swing velocity are closely related to the specific energy that is required to cut the soil. The energy consumption per unit of production is called the specific energy and is thus, by definition, the energy that is needed to cut loose one m3 of soil. Although it is often thought that the specific energy is independent of the cutting process, it is certainly not, since the finer the material that must be cut, the greater the energy consumption. The cutting method also exerts a big influence. When cutting rock, the specific energy increases strongly as the teeth are worn away. Furthermore the influences of the radius and the revolutions of the cutter head are limited, so no account can be taken of the possible dependence of cutting force on the velocity or of the permissible torque.

To obtain some insight into this subject, the specific energy is calculated from a general cutting theory or a straight cutting edge on a rotating cutter head. With a linear movement the cutting force of a straight cutting edge can be characterised by the following power equation:

Fc =c×d α ×vβt ×W in which: c

d Vt W

=

= = =

[N]

a constant that is dependent on the soil type and on the boundary conditions such as water depth, cutting edge angle, cutting edge height, etc the cutting depth or slice thickness the cutting velocity the width of the cutting edge

[m] [m/s] [m]

The production of a straight cutting edge is: [m3/s] Q = d·Vt·W Therefore the specific energy is:

Es =

Fc ×v t c×d α ×vβt ×W×v t = =c×d ¶-1×vβt Q d×v t ×W

[J/m3]

From this it follows that the specific energy is only constant if the cutting process is entirely linear, thus when:

Fc =c×d×v t ×W If this theory is applied to cutting with a cutter the chip thickness is:

 2π×v t d=   ω×z d=p×sinθ

p

  ×sinθ 

Tooth path

Ra d iu sr

p d q -j

Tooth path

Figure 3. 99

ω = the angular velocity of the cutter head z = the number of cutter arms Prof.Ir. W.J.Vlasblom

Pagina 63 van 79

[rad/s] [-] May 2003

Wb3408b


vt = the swing velocity [m/s] θ = the angle between the cutter radius and the tooth path [radian]  2π ⋅ v t  The maximum chip thickness is: d max =   ω⋅ z  Because the peripheral velocity of the cutter is equal to ω·R, the cutting force of a cutter

 2π×v t is: Fc =c   ω×z

α

 β   ×sinθ  ( ω×R ) ×L   α

 2π×v t   β L is proportional to the step size S thus: Fc =c   ×sinθ  ( ω×R ) ×S  ω×z   Moreover the cutting power is equal to: Pc =Fc ×v t =Fc ×ω×R '

α

 2π×v t Pc =c   ω×z

 β+1   ×sinθ  ( ω×R ) ×S  

'

With increasing step size the average radius of the cutter head increases; thus R = f ( S) = Sδ . α

 2π×v t   δ β+1 From this the cutting force can be reduced to: Pc =c   ×sinθ  ( ω×S ) ×S  ω×z   '

The cutting production is: Q c =S×v t ×D and thus the specific power: α

α

 2π×v t    δ β+1 '  2π×v t  δ β+1 c   ×sinθ  ( ω×S ) ×S c   ×sinθ  ( ω×S )  ω×z   ω×z    Es =  =  S×v t ×D v t ×D '

From this equation it follows directly that the specific cutting power is constant only under very exceptional conditions. These conditions are: •

A cylindrical cutter head δ = 0

•

The cutting force must increase linearly with increasing chip thickness.

•

This gives

v αt vt

= v αt −1 is constant

[sinθ ]

α

•

The average chip thickness must be linear with the layer thickness. Thus

D

is

constant •

The cutting force must be independent if the constant β = 0

Then: Es = c''

R z

From this it follows that the specific cutting energy is always dependent on the type of cutter head.


Pagina 64 van 79

May 2003

Chapter 3: Cutter Suction Dredger Because there are often big variations in the types and strength of the soil and many factors that cannot be determined in advance play a part in the cutting process, the specific energy appears to be a good parameter for estimating the production of cutter suction dredgers.

3.7.5. The cutting production The specific energy required for a particular type of soil can be estimated with the aid of existing cutting theories or from production estimates from previous work with the same type of soil. If the specific energy Esp, is known, it follows from the definition of the cutting process: w ⋅ Nc Pc = Es

in which Nc is the cutter power. The value w, a work coefficient, gives an indication of the average maximum percentage of the installed cutting power that can be used. This value is dependent, not only on the type of soil (relation between peak forces and average forces), but also on the man-machine relation. The dredge master and the automated cutter control regulate the cutting speed on the basis of the amperage (torque) of the cutter head engine. Types of soil the hardness or strength of which vary greatly from place to place will give a torque or amperage signal that varies greatly over time in which Nc is the cutter power (Figure 3. 100.)

TORQUE SIGNAL

mean value 150

torque [%]

120

90

60

30

0 0

5

10

15

20

25

30

35

40

45

50

Time [s]

Figure 3. 100.

This may quickly lead to overloading of the cutter head engine, with the result that, for example, for the torque-revolution characteristic shown below, the cutter head will stall at a torque of 150% (Figure 3.101)


Pagina 65 van 79

May 2003

Wb3408b


Speed [%]

100

100

Torque [%]

150

Figure 3. 101 Torque speed Characteristics of an electrical drive

If this occurs frequently the dredge master will reduce the swing speed of the dredger to ensure that the peak loads do not cause the cutter to cease turning. It will be clear that the type of drive plays a big part in this. An electric drive can take up the variation in torque better than a hydraulic drive. (See chapter 3.4.2.) The skill of the dredge master also plays a part. Dependence on his skill can be reduced to some extent by the use of an automated cutter control. This regulates the swing velocity, for example in relation to the torque of the cutter head. In many cases such an automated control system can react more quickly than the dredge master can, certainly at times when his watch is almost over. It will also be clear that only rough estimates can be given for such a factor as the work coefficient. For rock :

w = 0.5 - 0.65

For sand :

w = 0.65 - 0.8

For clay :

w = 0.8 - 0.9

An automated cutter control increases these values by 10% to 20%. With the information given above, the cutting process can be found and also the warping speed of the cutter head. Because:

Qc =S×v t ×D With Pc = D = S = Vh =

[m3/s]

cutting production layer thickness step size swing velocity


[m3/s] [m] [m] [m/s]

Pagina 66 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.7.6. The spillage The face is cut away layer by layer, the spillage of one layer will be entirely or partly cleared away during the cutting of the following layers. For this reason the cutting of layers over the length of the spud carriage (Figure 3. 102 left) is preferable to the pattern shown in Figure 3. 102 right Dredging in cohesive soil

Dredging in cohesive soil 3

2

1

cut 1

5

4

7

8

9

10

12

13

18

19

14

15

2

21

22

17

20

14

79

15

21

16

22

23+24

29+30

20

26

cut 2

27

cut 3

cut 4

4

10

cleaning up

24

23

8

cut 3

3 16

25

19

cut 2

Swing number 11

13

7

1 6

Swing number

5+6

11+12

17+18

28

cut 4

cleaning up

Figure 3. 102

In this case the spillage can be calculated as follows: Assume that the spillage is M % of the cut surface. (M can be determined in the same way as in breach forming soil.). If the thickness of the layer and the step do not greatly exceed the dimensions of the cutter head, the spillage is M % of the layer thickness. Thus: -

for layer 1:

-

for layer 2:

-

for layer k:

Z1 = M ⋅ D

D = layer thickness

(

(

2

Zk = M ⋅ D

3

Z k = M + M + M +.......... M

After simplification it follows that: 1− Mk

) )D

Z 2 = M ( D + M ⋅ D )1 = M + M 2 D

=

(

M 1− Mk

1− M k (1 − M ) The part taken up is thus:  M 1− Mk Sk = H − Z k = H1 − k (1 − M ) 

(

k

)H

)   

Clearly, when the thickness of the layer or the size of the step exceeds the dimensions of the cutter head the part of the material that has no chance of entering the cutter head must immediately be considered as spillage. Figure 3. 103 shows a breach, which projects above water.

Figure 3. 103

Because the suction mouth must remain sufficiently under water to prevent the taking in of air, the dredge master must make the first cut thicker than the diameter of the cutter head. Prof.Ir. W.J.Vlasblom

Pagina 67 van 79

May 2003

cut 1

Wb3408b


The direction in which the bank is stripped now affects the spillage, although not in the cut, which is being dredged, but in the cut that has already been dredged. If the first cut has been made with a reverse turning cutter working towards the already dredged cut, because of the failure to raise the necessary reaction force, it is possible that at the end of the cut, some of the material from this new cut is pushed into the already dredged area.

Swing direction

Former cut

Over cutting mode Figure 3. 104

The result is that a ridge of soil is formed on the boundary between the cuts. In such a case it is better to make the uppermost cut in the same direction as the rotation of the cutter head. If the spillage is known the average dredging production over one spud cycles is:

Qs =

Sk ⋅W ⋅ L t s + ∑ ta

[m3/s]

in which: Sk = the thickness of the layer which has been taken up W = the width of the cut L = effective advance of the spud carriage ts = net cutting time during a spud cycle Σta =the sum of the times during the spud cycle when no cutting occurs, such as ladder raising, stepping, spud moving ,etc.

[m] [m] [m] [s] [s]

In non-breach forming soil, if a specified depth has to be delivered a clean-up swing must also be made. The production of this swing is calculated separately. The cutting energy that is required in this layer can only be determined from the part that has not been cut. It is therefore possible that because of a thin layer, the clean-up production is high.

3.8. Enclosures 3.8.1. The relation between swing speed and side winch speed. The swing speed of the cutter head must not be confused with the side wire speed. The latter is the speed with which the side wire is hauled in and which controls the swing velocity. Although there is a clear relation between these two velocities, they are certainly not equal. The position of the anchors in relation to the cut plays an important part in this. By the correct positioning of the anchors it is possible to reach a high swing velocity with a small side winch velocity.


Pagina 68 van 79

May 2003

Chapter 3: Cutter Suction Dredger Figure 3. 105

In Figure 3.105 the distance between the work spud and the sheaves of the side winch on the ladder is equal to L and the distance between the sheaves and the anchor is equal to S. If the angle between the centreline of the cut and the line linking the spud-side winch sheaves is equal to θ, then:

x = l ⋅ cos ϕ y = l ⋅ sin ϕ z = k − x = k − l ⋅ cos ϕ t = b − y = b − l ⋅ sin ϕ s = z2 + t2 =

( k − l ⋅ cos ϕ ) + ( b − l ⋅ sin ϕ ) 2

2

⋅

ds ds dϕ 2 ⋅ ( k − l ⋅ cos ϕ ) ⋅ ( l ⋅ sin ϕ ) ϕ + ( b − l ⋅ sin ϕ ) ⋅ ( −l ⋅ cos ϕ ) ⋅ ϕ = = 2 2 dt dϕ dt 2 ⋅ ( k − l ⋅ cos ϕ ) + ( b − l ⋅ sin ϕ ) ⋅ Since l ⋅ϕ is the swing velocity, the previous equation can also be written: ⋅ ds k ⋅ sin ϕ − l ⋅ sin ϕ ⋅ cos ϕ − b ⋅ cos ϕ + l ⋅ sin ϕ ⋅ cos ϕ = l ⋅ϕ dt k 2 − 2 ⋅ k ⋅ l ⋅ cos ϕ + l 2 ⋅ cos 2 ϕ + b 2 − 2 ⋅ b ⋅ l ⋅ sin ϕ + l 2 ⋅ sin 2 ϕ

of ds k b ⋅ sin ϕ − ⋅ cos ϕ dt = l l 2 2 lϕ k  b   − cos ϕ  +  − sin ϕ  l  l 

Since the side winch force do not act on the ladder at the same distance from the spud as the cutter head, the swingspeed have to be corrected according:

vs l = vc lc

3.8.2. The side winch force and power The swing force Fh takes effect at right angles to the centreline of the dredger, thus in the direction of the movement of the cutter head. The chance that the anchor is positioned in exactly the same direction as this track of the cutter head is valid for only one point. If the angle made by the tangent at one point of the track of the cutter head with the line joining this point to the anchor position is α, the required side winch power is Fz = Fh/cos (α).

Cos(α) can also be expressed in the units given in.   b π  − sin ϕ    cos α = cos + ϕ − arctan  l  k − cos ϕ   2  l 

(3.45)

The side winch force is thus: Prof.Ir. W.J.Vlasblom

Pagina 69 van 79

May 2003

Wb3408b

Fz =


Fh cos α

=

Fh ⋅ R c M c cos α

=

Fh ⋅ R c

 b  π  − sin ϕ    M c cos + ϕ − arctan  l  k − cos ϕ   2 l  

(3.46)

If the diameter of the side winch drum is equal to Dw, the required side winch torque is:

Dw D Fh ⋅ Rc w 2 = 2 = Mw = M c cos α cos α Fh

Fh ⋅ Rc

Dw 2

 b  π  l − sin ϕ   M c cos  + ϕ − arctan   k 2  − cos ϕ    l  

Both the side winch velocity and the side winch torque are now known as functions of the position of the anchors and the position of the cutter head in the cut. Neither the necessary side winch velocity, nor the necessary torque may exceed the maximum value of the side winch characteristic. If this does happen, the side winch velocity must be reduced until this condition is met. Because during the progress of the dredger the positions of the anchors in relation to the track of the cutter head must be continually changed, if the side winch velocity or the side winch force is the limiting factor for the dredging process, the dredge master must continuously adjust the side winch velocity until the point is reached where it seems wiser to move the anchors. From the above it will be clear that the further away the anchors are positioned from the ship, the longer the force will be effective, thus the anchors will have to be moved less often. On the other hand the longer the side wires, the weaker the system will be. This is a disadvantage when dredging hard soil such as rock. From the relation between the swing velocity vh or the angular velocity ϕ, together with required side winch electric current, dredge master can see whether or not the anchor is holding or dragging. 3.8.3. The shape and cutting geometry of cutter heads Because the cutting process plays an important role in excavation, this section will give more detailed consideration to the shape and cutting geometry of cutter heads. Definitions:

The base plane is the plane that passes through the underside of the cutter ring. The cutting point P may be a point on a cutting edge of a plain edge, the cutting point of a serrated edge or the edge or point of a tooth. The position of the cutting point determined by the cylinder coordinates Rp, Hp, and φp. Here: Rp = the radius from the cutting point to the cutter axis. Hp = the distance between the cutting point and the base plane. φp = the angle between the projection of the cutting point onto the base plane and the cutting point (Rp,0,0)


Pagina 70 van 79

May 2003

Chapter 3: Cutter Suction Dredger The cutting edge of a cutter blade is the smooth curve passing through the cutting points. The contour or outline of the cutter head is the section made by the cutting edge (the contour plane) though a plane perpendicular to the axis of the cutter head. The contour tangent touches point P on the contour. The contour angle κ is defined as the angle between the line in the contour plane passing through P at right angles to the contour tangent and the line through P parallel to the base plane. The cutting plane is at right angles to the contour plane and the contour tangent.. In the dredging world both Florida and Esco cutters are used. The positions of the tooth points of both systems are determined by using cylinder coordinates. The direction of the tooth axis given by Esco differs from that given by Florida. Tooth axis direction according to ESCO ESCO gives the direction of the tooth axis in two ways:

1.

By giving the tooth point and the tooth base of the tooth axis in cylinder coordinates.

2e. By giving the tooth point and two angles of the tooth axis. These angles are defined as follows: •

The pitch out angle θ . This is the angle between the tooth axis projection in the plane parallel to the base plane and the tangent on the circle passing through the tooth point projection.

The pitch up angle φ this is the angle between the tooth axis and its projection in the plane parallel to the base plane.. Thus in Figure 3.77.:  P' B'  θ = arctan   BB'  •

en  PP'  φ = arctan   P' B'  In addition ESCO give the roll angle ρ (rho) of a tooth. This is the position of a tooth in relation to the tooth axis. The roll angle is the angle between the edge of a chisel (flared or chisel leading edge) and the line parallel to the cutter axis as seen along the tooth axis. This angle is equal to the centreline of the locking pin and the line parallel to the base plane seen along the tooth axis. Tooth axis direction according to FLORIDA. FLORIDA gives the tooth axis by the giving coordinates of the tooth point with two angles. FLORIDA defines these angles as follows: •

The tooth axis angle α(tooth angle).This is the angle between the tooth axis and the tangent on the circle passing through the tooth pint. This is the tangent to the line of the movement during rotation.


Pagina 71 van 79

May 2003

Wb3408b


•

The contour angle κt (Kappa=Profile angle) of the tooth. This is the angle between the tooth axis projection in the contour plane and the line parallel to the base plane (P'B').

•

FLORIDA has a fixed roll angle ρ (rho) because the cutting edge or blade edge of the tooth always lies in the contour plane. This makes the roll angle a function of the tooth axis angle α and the contour angle κt ρFlorida = arctan[ tan κt ⋅ cosα]

When working, in most cases a piece of auxiliary equipment, the so-called ALFE is used in order to ensure that adapters are correctly positioned on the cutter head arm when these have to be replaced owing to breakage or loss (Figure 3. 106.). The plane of the ALFE is thus a contour plane. In that case the FLORIDA instruction is more simple than the ESCO. With ESCO cutter heads the angles must be recalculated to the FLORIDA instruction.


Pagina 72 van 79

May 2003


Figure 3. 106

Tooth axis angle α α = arccos( cos θ ⋅ cos φ ) Contour angle κt  tan φ  κt = arctan   sin θ  Roll angle ρFlorida = ρEsco − ρmal Prof.Ir. W.J.Vlasblom

Pagina 73 van 79

May 2003

Wb3408b


Here ρmal is the angle over which the adapter must be turned on its axis to get the cutting edge in the contour plane, thus against the ALFE. ρ_mal may be positive or negative. 3.8.4. Cutting by teeth or chisels

CUTTERAXIS

Hp

P

(Rp ,Hp, p) CUTTING EDGE

R p

(Rp,0,0)

OUTLINE OF THE CUTTER

Hc CUTTING EDGE

(HELIX ANGLE) BASE

Figure 3. 107

For the definitions of the various angles see Figure 3.107. -

Cutting edge/rake angle

-

Tooth axis angle

-

Clearance angle

Wedge angle In addition to a clearance angle on the rear of a chisel there are also side clearance angles.


Pagina 74 van 79

May 2003

Chapter 3: Cutter Suction Dredger 3.8.5. Conditions for cutting clearance The front and rear edges of the arms of cutter heads, edges, teeth and chisels follow different tacks during the cutting process (Figure 3.108). The most unfavourable point for the cutting clearance is the point where the velocity vector s of both the front and rear edges are parallel. In that case there is a maximum and minimum distance between the two paths. This happens when the velocity component in the X-direction is vx=0.

l φ

φ

Figure 3. 108

The path of a point on a cutter head can be described by the two following equations in parameter form (Figure 3.81.): x p = v h ⋅ t + R p ⋅ cos ωt y t = R p ⋅ sin ωt ϕ = ω⋅ t Here: Xp, Yp = the coordinates of the point P with regard to the cutter head axis. vh = the swing velocity of the cutter head = the angular velocity of the cutter head Rp = the radius of the cutter head t = the the time of passagede The direction of the velocity is the tangent to the path: R p ⋅ ω ⋅ cos ωt dy dy dt = ⋅ = dx dt dx v h − R p ⋅ ω ⋅ sin ωt The velocity in the x-direction is zero when the deriviative is infinite, thus as: v h − R p ⋅ ω ⋅ sin ωt = 0 Further: y = R p ⋅ sin ωt so that: vh − y⋅ ω = 0 ∴y=

vh

ω and the associated angle ϕ:


Pagina 75 van 79

May 2003

Wb3408b

Designing Dredging Equipment vh

ϕ p = arcsin

ω⋅ Rp

Now when: l = distance between the front of the tooth and the rear of the arm Rv = the radius of the tooth point and Ra, the radius of the rear of the arm . then: vh

ϕ v = arcsin

ω⋅ Rv

and ϕ a = arcsin

vh ω⋅ Ra

Furthermore if l is the distance between the front of the tooth and the rear of the arm, it follows from Figure 3.80 with ϕ=0 that the angle between the two pointy mentioned is equal to:

( R v ⋅ cos ϕ0 − R a ) 2 + ( R v ⋅ sin ϕ0 ) 2 2 2 l 2 = ( R v ⋅ cos ϕ 0 − R a ) + ( R v ⋅ sin ϕ 0 ) l=

l 2 = R 2v + R 2a − 2 ⋅ R v ⋅ R a ⋅ cos ϕ 0  R 2 + R 2 − l2  a ∴ ϕ 0 = arccos v   2 ⋅ Rv ⋅ Ra  The tooth and arm now run clear if the horizontal distance between the paths at the distance y is greater than the distance the cutter head moves as a result of the following the swing velocity round the ϕ 0 + ϕ a − ϕ v . Thus when v R v ⋅ cos ϕ v − R a ⋅ cos ϕ a ≥ ( ϕ 0 + ϕ a −ϕ v ) h ω Example: Rv = 1.50 m, Ra=1.45 m, l=0.7 m vh=0.3 m/s en ω = π (n=30 t/min) then: . R v ⋅ cos ϕ v = 1497 R a ⋅ cos ϕ a = 1.447 ϕ v = 0.064 ϕ a = 0.066 ϕ 0 = 0.478 y = 0.095 The maximum side winch velocity may then be: vh ≤

( R v ⋅ cos ϕ v − R a ⋅ cos ϕ a ) ⋅ ω ϕ 0 + ϕ a −ϕ v

thus v h ≤ 0.33 m/s Prof.Ir. W.J.Vlasblom

Pagina 76 van 79

May 2003


It will be clear that when designing cutter heads this exercise must be carried out for a number of points on the cutter head, since cutter arm length and radius are a function of the height of the cutter head, measured from the ring. This also determines the maximum thickness of the cut. When the rear of the arm touches the path of the front of the tooth, the maximum cut thickness is equal to: d max =

60 ⋅ ( v h ) max

n⋅z in which z is the number of arms. From the example it thus follows that: d max =

60 ⋅ ( v h ) max n⋅z

=

60 ⋅ 0.33 30 ⋅ 6

. = 011m

A c h te rzijd e arm

T an d p u n t

1.5 1 0.5 0 -1.5

-1

-0.5

-0.5

0

0.5

1

1.5

2

-1 -1.5 Xp Figure 3. 109

Finally the same example, but now with n=10 t/m and Ra=1.36 m. dmax=0.30 m and vmax=0.30 m/s. The path of the two points is shown in Figure 3. 109.. If parts of the tooth or arm project through the line passing between the tooth point and the rear of the arm, it is necessary to carry out a check for more points. 3.8.5.1. The effect of warping on the clearance angles The direction of the movement of the tooth point is (see Figure 3. 110): R p ⋅ ω ⋅ cos ωt R p ⋅ ω ⋅ cos ϕ  dy  =   =  dx baan v h − R p ⋅ ω ⋅ sin ωt v h − R p ⋅ ω ⋅ sin ϕ


Pagina 77 van 79

May 2003

Wb3408b


VH VT

nC

corr.

RP VH V A

A

A

corr.

VH VT + VH

Figure 3. 110

The rear plane of the tooth makes an angle βA with the circumference of the cutter head, thus with the tangent on the circle: R p ⋅ ω ⋅ cos ωt R p ⋅ ω ⋅ cos ϕ  dy  −1 = = =    dx circel − R p ⋅ ω ⋅ sin ωt − R p ⋅ ω ⋅ sin ϕ tan ϕ The clearance angle between the path of the tooth and the back of the tooth thus varies with the rotation. The difference between the two tangents is the varying clearance angle:  R p ⋅ ω ⋅ cos ϕ   R p ⋅ ω ⋅ cos ϕ  π  −1  β corr = arctan − arctan − − ϕ  = arctan  tan ϕ   v h − ω ⋅ sin ϕ   v h − ω ⋅ sin ϕ  2

For Rp = 1.0m,

= ,

=0 and vh = 0.3 m/s it follows that:

 π  π β corr = arctan − = −0.0095 rad=-5°.27'  0.3 − 0  2 In other words, the cutting angle is 5° 27' smaller.


Pagina 78 van 79

May 2003


3.9. References 1. calculation of the cutting forces when cutting in fully saturated sand, S.A. Miedema, Thesis TU-Delft, 1987 (in Dutch) 2. Coastal and Deep Ocean Dredging, John B. Herbich, Gulf Publisching Company, Houston, Texas, U.S.A., 1975 3. Dredging and Dredging Equipment, R.J. de Heer and Rochmanhadi, Parts 1 and 2, IHE, Delft, 1989 4. Dredging technology, lecture notes, G.L.M. van der Schrieck, TU-Delft, Civiele techniek, 1996 (in Dutch) 5. Concept, design and construction of the World's first self elevating offshore heavy cutter suction dredger: "Al Wassl Bay", D.A. Gaasterland, Proceedings 3e International Symposium on Dredging Technology, BHRA 198? 6. Nassbaggertechnik, A. Welte, Institut für Machinenwesen in Baubetrieb, Universität Fridericiana, Karlsruhe, 1993. 7. Proceedings of the CEDA Dredging Days, Europort 1980, CEDA, 1980 8. Technical aspects of large cutter suction dredgers, P.J. Koert, IHC Holland 9. Dredgers of the World, 3rd edition, Oilfield Publications Ltd (OPL). England, 2001 10. Various articles from Port & Dredging from IHC Holland ARTICLE Spudsystemen van cutterzuigers Demonteerbare cutterzuiger/baggerwielzuiger SCORPIO IHC Beaver cutterzuigers Cutterzuiger NOORDZEE Automatisering van cutterzuigers Zelfvarende cutterzuiger van 27000 PK LEONARDO DA VINCI: een nieuw record Nieuwe serie IHC Beaver cutterzuigers The IHC Beaver container dredger Cutter suction dredger ABU AL ABYADH for NMDC Sensative environmental cutter dredger for Samsung Mighty MASHHOUR for Suez Canal Dismountabe IHC Beaver dredgers CD Al Mirfa CD Kattouf


Pagina 79 van 79

P & D no 108 108 109 118 119 119 124 126 134 145 146 147 153 154 157

May 2003

Chapter 4 Plain Suction Dredgers

4.

The Plain Suction Dredger ...........................................................................................2 4.1 General considerations ........................................................................................2 4.2 Areas of application ............................................................................................3 4.3 Types of plain suction dredgers ..........................................................................3 4.4 History.................................................................................................................5 4.5 Working method .................................................................................................6 4.6 The design ...........................................................................................................8 4.6.1 The production capacity.............................................................................9 4.6.2 The suction depth.......................................................................................10 4.6.3 The transport distance ................................................................................12 4.6.4 The dredging installation ...........................................................................12 4.6.4.1 Suction and discharge pipe diameter .....................................................12 4.6.5 The dredge pump .......................................................................................13 4.6.5.1 Pump types..........................................................................................13 4.6.5.2 The sand pump drives .........................................................................14 4.6.6 Jetpumps ....................................................................................................14 4.6.6.1 Pump type ...........................................................................................14 4.6.6.2 Jetpump drives. ...................................................................................17 4.7 General layout .....................................................................................................18 4.8 Technical construction ........................................................................................20 4.8.1 The hull ......................................................................................................20 4.8.2 The dredging equipment ............................................................................21 4.8.2.1 The suction mouth...............................................................................21 4.8.2.2 The suction pipe..................................................................................22 4.8.2.3 The sand pumps ..................................................................................23 4.8.2.4 The sandpump drives ..........................................................................25 4.8.2.5 The discharge pipeline ........................................................................25 4.8.2.6 Sprayers...............................................................................................25 4.8.2.7 Jet-pipeline and pump .........................................................................26 4.8.2.8 The winches ........................................................................................26 4.8.2.10 The bow winch .................................................................................27 4.8.2.11 The side winches ..............................................................................27 4.8.2.12 The stern winch ................................................................................27 4.8.2.13 The auxiliary winches.......................................................................27 4.8.2.14 The fairlead.......................................................................................27 4.9 The dredging process ..........................................................................................29 4.9.1 The production of the breach .....................................................................29 4.9.2 The production of the pumps .....................................................................32 4.9.3 The production of the barges .....................................................................33 4.10 The dustpan dredger............................................................................................34 4.11 References ...........................................................................................................35

Page 1 van 35

Wb w3408b

4.


The Plain Suction Dredger

Figure 4. 1 A plain Suction Dredger

4.1 General considerations The characteristic of a plain suction dredger is that it is a stationary dredger, consisting of a pontoon anchored by one or more wires and with at least one sand pump, that is connected to a suction pipe. The discharge of the dredged material can take place via a pipeline or via a barge-loading installation. The suction tube is positioned in a well in the bows of the pontoon to which it is hinged. The other end of the suction pipe is suspended from a gantry or Aframe by the ladder hoist. The ladder hoist is connected to the ladder winch in order to suspend the suction pipe at the desired depth. Excavation of material to dredge is by the erosion of a jetstream and/or the suction flow of the dredge pump and the breaching process (see lecture notes wb3413 the Braching process)During sand dredging the dredger is moved slowly forwards by a set of winches. To increase the amount of sand flowing towards the suction mouth, a water jet is often directed onto the breach/bank. In this case the jet-pipe is often mounted above the suction pipe.

Figure 4. 2 Plan view of a PSD

Pagina 2 van 35 prof. W.J.Vlasblom

April 2003


4.2 Areas of application Plain suction dredgers are only used to extract non-cohesive material. Moreover these dredgers are less suitable for accurate work such as the making of specified profiles. Suction dredgers are very suitable for the extraction of sand, certainly when this occurs in thick layers. Suction dredgers can be seen in working in many sandpits. If the dredger is equipped with an underwater pump, it is possible to dredge at depths exceedin g 80 m. Depending on the pumping capacity; it is possible to transport material over considerable distances via hydraulic pipelines. Because suction dredgers are often demountable they can also be used in excavation pits which are not on navigable waterways. In general, suction dredgers are relatively light vessels and, although anchored on wires, are usually unsuitable for dredging in open waters (unless specially adapted).

4.3 Types of plain suction dredgers Different type of plain suction dredgers can be distinguished. 1. The barge loading plain suction dredger A dredger which loads the barges which lie alongside it by means of a spraying system. This type is used when the transport distance is too long for hydraulic transport to be economic (Error! Reference source not found.).

28

18 24

16

22

11

23

17

31

26

21 27

13

25 19

10

20

14 15

Figure 4. 3 Barge loading PSD

2. The reclamation dredger This dredger pumps the sand ashore via a pipeline and, if necessary, further away to a disposal site or treatment plant (Figure 4. 4). .

Page 3 van 35

Wb w3408b


Figure 4. 4 Reclamation dredger

3. The deep suction dredger The deep suction dredger. A dredger equipped with an underwater pump. It may take the form of a barge loader or a reclamation dredger. (Figure 4. 5)

6

22 23 24

5

1

17/18

11

28

6 27

19/20

7

16

10

9

30

29

34

11

31

14

12

12

35 33

35

33

13/15

32

32

2 2

3

8

8

Figure 4. 5 The deep suction dredger

4. The dustpan dredger A suction dredger with a relatively wide suction mouth. This dredger is suitable for extracting sand at a reasonably high production rate with a low breach or bank height. With regard to production the cutter suction dredger (Figure 4. 6) has superseded this type.


April 2003


Figure 4. 6 Dustpan dredger

In many cases these types can easily be transformed to another type. The barge loading dredger shown in figure 4.2 can be transformed to a reclamation dredger by connecting a booster just behind this dredger. The same might be possible with reclamation dredgers by placing a sprayer pontoon after the dredger.

4.4 History In 1851, more than a century after their invention, the first centrifugal pumps were used to excavate sand with hopper dredgers. A few years later (1856) the first attempts were already being made to transport the material onshore via pipelines. Ten years later this idea was demonstrated in the Netherlands during the excavation of the North Sea Canal. (Figure 4.7)

Figure 4. 7 The wooden Hutton Dredger dredging the North Sea Canal

Meanwhile, in 1864, Freeman and Burt patented a flexible floating-pipeline.

Page 5 van 35

Wb w3408b


From this history it appears clear that the development of the suction dredger was closely linked with the development of the dredge pump. Because at that time little power was available to drive the dredge pump, the reclamation dredger was only used when the distances to the disposal site were short. In the other cases barges were used or the dredger was modified. As the sand pumps became able to withstand higher pressures, the transport distances and pump capacities were increased.

4.5 Working method The working method of the suction dredger depends on both the progressive collapsing of the breach/bank and the loosening of the sand near the suction mouth by eddies created by the flow of water caused by the sand pump (Figure 4. 8). The progressive collapse of the breach/bank resulting from the dislodgement of particles of soil or of masses of soil as a result of localised instabilities is termed “breaching”.

z

x

Instabilities

H br

Sand-water mixture (density current) Suction tube Vz

Figure 4. 8 Breaching

This process is essential for the production of a suction dredger and is entirely determined by the soil mechanical properties of the slope, the most important factors being its permeability to water and relative density. When a suction dredger starts on a new work there is no dredge pit, slope or breach and the angle between the suction pipe and the horizontal is usually very small. The sand that is carried towards the suction pipe lies entirely within the area influenced by the water flowing to the suction mouth. This process causes a small pit to develop in the soil. The dredger is now drawn forwards a little by means of the bow winch and the suction pipe is set deeper, after which the process is repeated. As the small pit becomes deeper and the angle of the suction tube becomes steeper (more effective for the swirling up and transporting of the sand) the production increases. (Figure 4. 9) This process is continued until the suction mouth is deep enough or until the production is so high that the pump can no longer cope with a further increase. This slow forward movement with the dredger, with simultaneous lowering of the suction pipe is termed ‘breaking in’ or ’commencing’.

Figure 4. 9 “Breaking in”


April 2003


The time that is needed to reach a state of equilibrium thus depends on the previously mentioned soil mechanical properties, the height of the slope and the pump capacity of the dredger. When a state of equilibrium has been reached it is the task of the dredge master to maintain this situation by letting the dredger follow the breach/bank, by regularly hauling the dredger forwards and by continuing to lower the suction pipe for as long as this remains possible. If the movement of the dredger is too slow, a less steep slope forms and the production is reduced. If, on the other hand, the forward movement is faster than the transport of the sand, the angle of slope will increase and there is an increasing chance that large scale shearing will occur. The sand concentration may then become so high that the pump cannot cope with it and the mixture ceases to flow. The shearing can be so great that even the suction pipe becomes fast/firmly embedded and, if it cannot be pulled free, another dredger must be used to free it by using suction or must cut it free. The dredging pattern that is made with a suction dredger generally appears like that shown in Figure 4. 10. As long as it lies within the dredging area, the length of the cut is determined by the positions of the anchors. The anchors are usually placed in such a way that more cuts can be made beside each other from the same position. In addition to the length of the anchor wires, this possibility also depends on the width over which the sand is being excavated. This, in turn, depends on the shear characteristics of the sand layers.

Figure 4. 10 Dredge pattern of a PSD

For suction dredgers equipped with an underwater pump the excavation depth no longer determines the production. This also makes it possible to exploit the dredging area in the vertical sense. In other words, production can be maintained by continuing to lower the suction pipe until the maximum suction depth has been reached. If the production falls below an economic minimum, the pit is abandoned and dredging recommences ½ to ¾ pit diameter away from it. It will be clear that this dredging method produces a pockmarked excavation area and that considerable amount of sand that cannot be economically excavated remain behind in the dredging area. This is a situation that the managers of the dredging sites prefer not to see.

Page 7 van 35

Wb w3408b


This method of dredging does provide the possibility to obtain sand from directly beneath a clay layer, but it must be realised that the removal of the sand will cause the clay to lose its stability. In the most favourable case the clay will fall onto the slope in fragments that will be taken up with the sand. If the clay falls in large pieces there is a good chance that these will become fast and block the suction pipe, with all the disadvantages that this can bring. It is difficult for the water needed for mixture formation to flow, especially in the beginning phase when the clay layer has not yet been penetrated.

Figure 4. 11 PSD with suction pipe of 2 sections

Water must be brought to the suction pipe via the jet pipe. For the above described excavation method the suction pipe is made in two parts, (Figure 4.11) the lowest section being hinged onto the upper section so that the lowest part is always first suspended almost vertically. With such a suction pipe, moments that occur during horizontal movements can be taken up only to a small extent.

4.6 The design When designing suction dredgers the following parameters are important: • • • •

Production capacity Suction depth Transport distance Type of soil

Because suction dredgers are only suitable for the dredging of non-cohesive material, the last parameter plays an important role only in the determination of the diameters of the suction pipe and hydraulic pipeline and the required sand pump capacity.


April 2003


4.6.1 The production capacity As in other dredgers, the market forces in relation to the sites where the dredger can be used determine the production capacity. As mentioned earlier, the plain suction dredger is much used in the extraction of sand for landfill sites and for the concrete industry. For this too, it is important to know the production capacity per week or per hour. In the Netherlands, to a limited extent, the labour agreements between the trade unions and industry permit a working week of 168 hrs, thus an entirely continuous operation. Often this is restricted to only four nine-hour days (36 hrs). The percentage of hours during which effective dredging can take place, however, is not equal. With a 36-hr week, major repairs are often carried out during overtime. When using barge transport, for example, the percentage of downtime resulting from the absence of barges is lower during a 36-hr week than during a continuous working week, since part of the downtime is made up when the dredger has stopped work at the end of the day. If, during a 168 hr working week, the number of effective working hours is 0.75*168=126 and during a 36 hr working week the effective hours are 0.86*36 = 30.6, the production ratio is 126/36.6 = 4.1 instead of 168/36 = 4.7. For the design of the dredging installation, and thus for the vessel also, the production per hour is more important than the daily, weekly or monthly production. In many cases, in order to prevent overloading of the drives, even shorter time intervals are considered. If the production capacity is known, this requirement can be translated into: 1. A sand flow rate 2. A sand concentration Since: Q = Qmixture ∗

Cvd 1− n

(4.2)

with Symbol Q Qmixture Cvd n

= = = =

describtion Production Flow rate Delivered concentration Porosity

dimension [m3/s] [m3/s] [-] [-]

The anticipated average concentration depends on the behaviour of the soil in the breach/bank (see lecture notes ‘Dredging Processes”). The maximum suction concentration is determined on the basis of the types of soil and the insight of the designer. The maximum average concentration that can be transported by a pipeline depends on the ratio maximum grain diameter/pipe diameter and the length of the pipeline. In long pipelines aggregation (increased concentration) may occur as a result of density variations during dredging (Matousek, 1995). As rule of thumb, a maximum average density of 1500 kg/m3 (Cvd = 30%) is often used for sand. On the basis of this assumption the flow rate is now fixed because the production capacity is taken as a given value.

Page 9 van 35

Wb w3408b


4.6.2 The suction depth A second important design parameter is the suction depth. This determines whether an extra underwater pump is needed to achieve the required production. When the suction depth increases, if the use of an underwater pump is not considered the suction pipe diameter and also the pump flow must be increased. At the same time the concentration must be reduced to avoid reaching the vacuum limit (under-pressure at which cavitation occurs). This can lead to the pumping of low concentrations and thus much water, which is uneconomic. With the aid of the suction formula one can determine if a submerged pump is useful and hoe deep below the waterlevel the pump has to be fitted on the suction tube. The suction formula is a force balance over the suction tube. The pressure difference over the suction tube equals the weight of the mixture in the suction tube and the friction due to the flow.

k rw

H rm

hz

b

rp

hp

Figure 4. 12 scheme for suction formula

Lö 1 æ ρ w g (H − h p ) + ρ p gh p − p pomp = ρ m ghz + ρ m v 2 ç1 + ζ + λ ÷ Dø 2 è with ρw = density water ρp = density suspended sand in the pit ρm = mixture density in the suction tube H = waterdepth hp = depth of pit hx = suction height ppump = pressure in front of the pump v = mixture velocity ξ = entree loss factor λ = Darcy Weisbach headloss factor L = total length suction tube D = diameter suction tube

[kg/m3] [kg/m3] [kg/m3] [m] [m] [m] [N/m2] [m/s] [-] [-] [m] [m]

Because hz = H − k the equation can be written as:

1 Lö æ ρ w g (H − h p ) + ρ p ghp − p pomp = ρ m g (H − k ) + ρ m v 2 ç1 + ζ + λ ÷ 2 Dø è This results in:

ρm =

ρ w g (H − hp ) + ρ p gh p − p pomp Lö 1 æ g (H − k ) + v 2 ç1 + ζ + λ ÷ Dø 2 è

For the boundaries given in Figure 4.13 the maximum dredgeable mixture density is calculated for different depth of the dredge pump below thw waterlevel Pagina 10 van 35 prof. W.J.Vlasblom

April 2003


Mixture density as funktion depth pump below water line hp=3 m, Vac=75 kPa, Vz=5 m/s, rho_water=1000 km/m3, G_p=1600 km/m3, Zeta=2 , Lambda=0.02, L/(H-k)=1.5, D=0.8 m

1800

Mixture density [kN/m3]

1700 1600 1500

k=0 m

1400

k=5 m

1300

k=10 m

1200 1100 1000 0

10

20

30

40

50

60

70

80

90

100

Dredging depth [m]

Figure 4. 13

The above graph (Figure 4. 13) is derived from this equation In order to dredge, from a depth of 30 m, a density of 1500 kg/m3 the dredge should be place almost 8.5 m below the waterline. A pump on the waterline can pump a density of1120 kg/m3. In the second case, if the same production is required, the flow should be:

Q0 ρ5 − ρ w 1500 − 1000 = = = 4.17 as great. Q5 ρ 0 − ρ w 1120 − 1000

With the same pumping velocity this leads to a suction pipe of a diameter that is 2 times as big. For a given decisive vacuum and a maximum suction concentration it is possible to determine whether an underwater pump is necessary and, if so, how far under water this pump must be positioned, as a function of the required suction depth.

Rho_mixutre=1500 kg/m3 hp=0 m, Vac=75 kPa, Vz=5 m/s, rho_water=1000 km/m3, G_p=1600 km/m3, Zeta=2 , Lambda=0.02, L/(H-k)=1.5, D=0.8 m

Depht pump below water level [m]

35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 0

10

20

30

40

50

60

70

80

90

Dredging depth [m]

Figure 4. 14

From the above graph (Figure 4.14) it appears that to pump a mixture density of 1500 kg/m3 at a depth of 50 metres the pump must be positioned 17 metres under water.

Page 11 van 35

Wb w3408b


Of course whether or not an underwater pump is mounted is a question of economics. The cost of fitting an underwater pump is considerable and, moreover, the suction depth can have a great influence on the ladder construction and thus on the pontoon construction. It is also necessary to hoist the suction pipe above water for inspection. 4.6.3 The transport distance The transport distance makes demands with regard to the installed sand pump capacity and/or the need to load barges. The need for barge loading depends whether the required transport distance is too long to be economically covered by the use of a hydraulic pipeline. It is also possible that the use of a pipeline may not be feasible from the point of view of hindrance to shipping. Suction dredgers may also be designed exclusively for barge unloading. In general, if material does have to be transported by a hydraulic pipeline there is still the option to place a booster station with the necessary capacity behind the plain suction dredger. If the suction dredger is equipped with an underwater pump the chosen discharge pressure (and thus capacity) can be such that during the loading of barges only the underwater pump is used. The pipeline system and valves can also be designed for this. The grain size and the distance over which the material must be transported determine the required manometric pressure for the discharge pump(s). It is also possible to choose an underwater pump of higher capacity than is needed to unload the barges. The surplus capacity can then be used during discharging. The maximum discharge pressure that a dredger can supply depends on the quality of the shaft sealing of the last pump. Often values exceeding 25 - 30 bar are not permitted.

4.6.4 The dredging installation Under the dredging the following components are included • Suction and discharge pipe • The dredge pumps • The dredge pumps drives • The jet pumps • The jet pump drives 4.6.4.1 Suction and discharge pipe diameter The critical velocity that is necessary to keep the dredged material in motion determines the maximum suction and pressure pipe diameters.

(

Thus: v kritiek = Fl ,h + Fl ,v

)

2 g ( S s − 1) D in which the value of Fl,h is determined by the

material to be pumped. (See lecture notes “Dredging Processes) Fl,v is the correction for sloping transport and has a maximum value of .333 (See also the relevant Section 2.2.4.3. of Hopper dredgers). If both the critical velocity and the average concentration have been determined, the relation between pipeline diameters and production is:

Q = Qmixture ∗

Cvd π D 2 Cvd π D 2 Cvd C = vkrit = Fl 2 g ( S s − 1) D ≈ 1.5π D 2.5 vd [m³/s] 1− n 4 1− n 4 1− n 1− n

with Pagina 12 van 35 prof. W.J.Vlasblom

April 2003


Symbol

describtion

Q Qmixture D Cvd Ss n g vcr

= = = = = = = =

Production Flow rate Pipe diameter Delivered concentration Relative density of the solids=ρs/ρw Porosity Gravity Critical velocity

dimensio n [m3/s] [m3/s] [m] [-] [-] [-] [m/s2] [m/s]

Figure below give the results of the equation for Cvd=30%

Discharge diameter [m[

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

1.1 1.2 1.3

Production [m3/s]

Figure 4. 15 Minimum discharge diameter

4.6.5

The dredge pump

4.6.5.1 Pump types Now that the capacity, the required pressures on both sides of the pump and the power are known under the various transport conditions, the type(s) of pump can be selected. The pump types, centrifugal, semi axial or axial are determined by the specific speed of the pump; defined as:

ns =

ω Q

( gH )

3 4

3

=

ρ 4ω Q

( p)

3 4

For discharge pumps the specific speed ns is in the interval between 0.25 and 0.50 (Figure 4.16). With the aid of this figure the type of pump and impeller can be chosen.

Page 13 van 35

Wb w3408b


0.08

0.8

0.07

0.7

0.06

0.6

0.05

0.5

0.04

0.4

0.03

0.3

0.02

0.2

0.01

0.1 0

0 0.15

Specific Head

Specific Capacity

Inboard Pumps

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Specific Speed

Figure 4. 16

For the underwater pump usually a higher specific speed is taken than for the discharge pumps, but for the sake of standardisation the same pump is often selected. One should ask oneself whether the position of the maximum efficiency point could still reasonably satisfy the stipulated demands with regard to the flow. This is also valid when no underwater pump is fitted. In such a case stipulations must be made with regard to the suction properties (NPSH value) of the inboard pump. Other factors also play a part in the selection of a pump and impeller: • •

A three, four or five blade impeller. Depending on the required minimum passage between the blades. Single or double walled pump. (considerations relating to wear.)

If long transport distances have to be covered the question arises of whether one large pump or two smaller ones will be needed. In addition to the specific revolution speed the peripheral velocity of the impeller also plays a part. To limit wear, the peripheral velocity of the impeller is limited to 35 to 40 m/s. This also limits the maximum manometric pressure. Whether or not one or more delivery pumps are needed depends on the total require delivery pressure and delivery pump power.

4.6.5.2 The sand pump drives Underwater pumps often have electric drives, but hydraulic drives and even direct diesel drives may be encountered. If barge loading is required, a controllable drive is necessary. With a fixed revolution speed the variations in flow resulting from differences in concentration and grain size are often too big for the efficient loading of the barges. Diesel drives are often used for the delivery pumps, but of coarse electrical drives are possible too

4.6.6

Jetpumps

4.6.6.1 Pump type The flow of the water pumps depends on the required functions of these pumps. Two functions can be distinguished: Pagina 14 van 35 prof. W.J.Vlasblom

April 2003


1. The activation of the breach process of the bank. Suction dredgers are usually equipped with a water jet for this purpose. The speed of the jet flowing from the water jet decreases hyperbolically with the distance from the water jet in accordance with:

vL =

6D v L 0

See Figure 4. 17

Here:

v L = Velocity of the jet at distance L in m/s. D = Diameter of the jet nozzle in m. L = Distance to the jet nozzle in m. v 0 = Velocity of the jet at the nozzle in m/s.

Vr r D Jet

VL

V0 L

Figure 4. 17 Flow establishment of a jet

Example. If the pressure at the nozzle is 500 kPa and the jet nozzle has a diameter of 0.3 m e and a minimum velocity in the centre of the jet **at the breach/bank of 3 m/s is needed to activate the breach/bank, the maximum distance to the breach/bank is:

v0 L = 6D = 6D vL

µ

2p ρ vL

0.6 = 6∗0.3

2∗500 1 = 11 m 3 æ r ö2

−90ç ÷ vr The decrease in velocity towards the edge of the jet can be calculated with: = e è Lø . vL Here v r = the velocity of the jet at distance r from the centre.

Page 15 van 35

Wb w3408b


v_r=v_L*exp(-90*(r/L)^2) 0.25

r/L

0.2 0.15 0.1 0.05 0 0

0.2

0.4

0.6

0.8

1

1.2

V_r/V_L Figure 4. 18 jet velocity as function of the radius r.

At a distance of 11 m and with a relation of vr = 0.4 the diameter of the jet is as shown in the vL

r graph below D = 2 L = 2∗01 . ∗11 = 2.2 m L In other words, the influence of the water jet is only very local.

The jet flow is: Q j =

πD 2 π ⋅ 0.32 18.9 = 134 . m³/s v0 = 4 4

and the power at the water pump: Pj =

Qj ⋅ p

η

=

134 . ∗500 = 838 KWatt .8

2. The maintenance and control of mixture forming. In this case, when it is assumed that no water from the environment can be sucked in because the suction mouth is completely embedded in the soil, it is necessary to satisfy the volume balance:

Qj Qm

= 1−

Cvd 1− n

Here:

Q j = the jet flow m³/s Qm = the sand flow in m³/s Cvd = the transport concentration [-]

n

= the pore number [-]

Figure 4. 19 gives a graphical representation of the equations.


April 2003


Verband Qj/Qm - Cvd 0.8 n=.35 n=.4 n=.45 n=.5

Cvd

0.6 0.4 0.2 0 0

0.5

1

1.5

Qj/Qm Figure 4. 19

Example: If Cvd = 0.25 and n=0.5 (loose packed sand), then

Qj Qm

=.5

The area of influence by the jet is now less important, as long as the water that is added benefits mixture formation. The water pumps are chosen in the same way as the sand pump 4.6.6.2 Jetpump drives. In case of activation the breaching process required pressure and capacity will always be constant. So separate diesel engines are frequently used. In the other case, the mixture forming process a speed control engine is required to control the density.

Page 17 van 35

Wb w3408b


4.7 General layout

3,000

3000

2,500

2500

Light weight [t]

Light weight [t]

The hull consists of a simple U-shape pontoon. De width of the pontoon is determined by stability and sometimes by the distribution of the loads. (Figure 3.1.7) The length of the pontoon is in certain way determined by the length of the suction pipe, the number dredge inboard pumps or by the requirements for mooring barges along side. Loads on the suction pipe resulting from the dredging process are relatively small, so are the loads on the pontoon. For small plain suction dredgers the dredgepump is situated in the engine-room, however a separate pump room is certainly advisable from safety point of view, in particular for the bigger dredgers. Nowadays even small dredgers do have a submerged pump.

2,000 1,500

y = 0.2712x

1,000

R = 0.712

2

y = 0.4074x 2

2000

R = 0.8715

1500 1000 500

500

0

0 0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

0

8,000

1000

2000

3000

4000

5000

6000

LBD [m3]


Figure 4. 21

Figure 4. 20

The lightweight of the plain suction dredgers depend on the total power installed. (Figure 4.20), while the volume of the pontoon is 2.5 times the light weight (Figure 4.21). The main ships parameters vary widely; L/B between 3 and 8 and B/T between 7 and 3.5, because the length is mainly determined by the factors mentioned above. (Figure 4.22)

L/B & B/T

L/B

B/T

9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0

500

1000

1500

2000

2500

3000

Light weight [t]

Figure 4. 22

Figure 4.23 shows the dredger Seeland, with a total installed power of 3200 kW and a maximum dredging depth of 40 m. The dredger is build under the classification of the Germanische Lloyd GL + 100 A 4 dredger. The length of the suction pipe often determines the length of the well. With very long suction pipes or two-part suction pipes the catamaran principle is often used. The suction pipe is then hinged onto the stern of the pontoon (Figure 4.2) This is certainly not essential. Sometimes special gantries are designed to carry the long suction tube (figure 2.23).


April 2003


Figure 4. 23

In deep dredgers with an articulated pipe, the lower pipe is fastened to the upper pipe by hydraulic cylinders, in which case it is not necessary to have a long well (Figure 4.24).

Figure 4. 24 Plain Suction _Dredge Seeland, Yard Orestein and Koppel Lübeck Germany

Page 19 van 35

Wb w3408b


In other cases an additional pontoon is connected to the main pontoon by means of a special construction (Figure 4.24 PSD Weesperkaspel). The engine room, pump room, fuel tanks, water tanks and storeroom are all located in the pontoon. On small suction dredgers the sand pump is located in the engine room, while large suction dredgers have a separate pump room. The control cabin, and if required, crew quarters are above deck. The anchor winches are also on deck

Figure 4. 25

Figure 4.25 shows an offshore plain suction dredgers designed for significant wave heights of 2.75 m and a total installed power of 7425 kW. The coupling with the floating pipeline is in the middle of the port side where the movements of the pontoon are minimum when working in waves. This is in contradiction with dredgers for inland waters. They do have the connection on the aft of the pontoon.

Figure 4. 26 General arrangement of an offshore plain suction dredger, Yard IHC Holland

4.8 Technical construction 4.8.1 The hull As previously mentioned, the hull usually consists of a simple U-shaped pontoon. The width of the pontoon is determined by stability considerations and varies from 6 m for small to 20 m for large deep dredgers. The length of the dredger is usually determined by the requirements relating to the length of the suction pipe and/or the need to accommodate barges alongside and by the warping of the barges.


April 2003


The ladder gantry, which usually takes the form of an A-frame, provides the link between the pontoons, which are separated by the well. By deep dredgers, having a suction pipe in the raised position pointed very far ahead of the pontoon, the gantry is a relatively heavy structure (Figure 4.23 and 4.27).

Figure 4. 27

4.8.2 The dredging equipment The dredging equipment will be discussed according the flow o f the mixture. 4.8.2.1 The suction mouth Suction mouths of plain suction dredgers are in many cases very simple. The end of the pipe is just covered by a screen to avoid pump blockage by boulders and debris (Figure 4.1, 4.28 and 4.29)

Figure 4. 28

Figure 4. 29

In many cases jet nozzle are situated around the suction mouth to activated either the breaching process and/or the mixture forming (Figure 4.30) When the suction mouth is fully penetrated in the sand, water jets are necessary the fulfil the requirements for the mixture forming. In that case jets are situated around the suction mouth (figure 4.31)

Page 21 van 35

Wb w3408b


Figure 4. 30

Figure 4. 31 Suction mouth of the sea going PSD DECIMA

4.8.2.2 The suction pipe For many suction dredgers the suction pipe, together with the jet water pipe, forms a strong construction (Figure 4.32). To strengthen the suction pipe this it also equipped with a jacket pipe through which the jet water flows to the suction mouth. If this jacket pipe is divided into sections, these can also be used as float tanks to reduce the underwater weight of the suction pipe.

Figure 4. 32

With bigger dredgers, and certainly at greater suction depths, these constructions are too weak and it is necessary to turn to the use of a ladder (Figure 4.19). If an underwater pump is used, the upper part of the suction pipe must certainly be constructed as a ladder in order to transfer the heavy weight to the hull.

H

Figure 4. 33


April 2003


On the suction pipe there is often a water admitting valve or breaching valve. If, as a result of irregular shearing of the breach/bank the vacuum becomes so high that the pump starts to cavitate and threatens to cut out, water can be admitted through this valve to keep the process going. This valve, which was formerly operated manually, is currently regulated automatically by the under pressure in front of the pump.

Valve open Valve closed

Cylinder

to pump

From suction mouth

Figure 4. 34

To ensure good control it is advisable to provide the valve with two openings, a big one for sudden emergencies and a second smaller valve that can be used for fine control with a continuously high vacuum. A rubber suction hose forms the link between the suction pipe and the pipelines on board. This rubber hose is equipped with vulcanised steel rings, which prevent it from collapsing when under pressure occurs in it. The centreline of the suction hose is at the same height as the hinge and often lies beneath the waterline (Figure 4.35).

Figure 4. 35

To prevent water from flowing in during pump inspections a so-called “outboard valve” must be fitted onboard before the pump PSD’s without a submerged pumps have to be designed in such away that the suction pipeline is as short as possible. Where the suction pipeline comes above water, the chance of taking in air must be reduced to the minimum. (Taking in air has the same effect as cavitation.) 4.8.2.3 The sand pumps Barge-loading suction dredgers usually have only one pump, even when the dredger is equipped with an underwater pump, while reclamation dredgers have one or more inboard pumps independent if provided with an underwater pump.

Page 23 van 35

Wb w3408b


Figure 4. 36 View on ladder with pump for a PSD

When suction dredgers do not have an underwater pump, efforts must be made the position of the first pump must be as deep as possible below the water line. This means on the base of the pontoon. As well as good discharge characteristics, the first pump must also have good suction characteristics, thus a high decisive vacuum and/or a low NPSH value. If the dredger is equipped with an underwater pump the layout is less critical. In that case aspects such as accessibility for inspection and repairs play a more important role. The onboard pump is then only required to possess discharge characteristics. For the required specific speed for these pumps referred is to chapter 2.2.3.5 Dredge pump. Submerged pumps have mainly a single wall, while inboard pumps have either a single or a double wall. If there is more than one inboard pump the layout must be chosen in such a way that, if desired, it is also possible to work with the ladder pump and one inboard pump. An inspection hatch must be provided for every pump, so that the pump and the impeller can be inspected and, if necessary, debris can be removed. .

Figure 4. 37 Double wall pump


April 2003


4.8.2.4 The sandpump drives The underwater pump often has an electric drive while the inboard pumps are powered by diesel engines. Diesel direct driven submerged pumps is till today in use for relative low powered pumps. See also chapter 3.2.3.4 4.8.2.5 The discharge pipeline Reclamation dredgers pump the dredged material ashore by means of a floating pipeline and, if necessary, to a more distant disposal site via the land pipeline. Because the movement of the suction dredger is considerably less than that of a cutter suction dredger, it is not necessary to connect the discharge pipeline of the vessel to the floating pipeline by means of a swivel on the stern of the vessel. Often the discharge pipeline is connected to the floating pipeline by means of a delivery hose/pressure hose (a floating rubber hose). This can be mounted either on the stern of the vessel or on the port or starboard side.

Figure 4. 38 Ths sea-going PSD AURORA with the discharge pipeline connected on starboard

4.8.2.6 Sprayers If the dredged material has to be loaded into barges alongside because the transport distance is too long for pipeline transport to be economic, sprayers which are connected to the discharge pipeline are fitted on both sides of the dredger. The number of sprayers that is fitted on each side of the dredger depends on the capacity of the dredger and the size of the barges and varies between one and four per side.

Figure 4. 39 Two different types of sprayers

Page 25 van 35

Wb w3408b


To prevent barges from being unevenly loaded, the sprayers must be positioned as closely as possible to the centreline of the barge (Figure 4.39). Sometimes extra measures are necessary for this. For example, when it is necessary that to satisfy the demand that free fall of the dredged material must be prevented, the sprayers must be positioned as low as possible. The capacity of the pump and the pipeline plan must be designed in such a way that on each side a barge can be loaded simultaneously. The sprayers are moved by means of winches or by a hydraulic system.

Figure 4. 40 barge loading with movable sprayers

4.8.2.7 Jet-pipeline and pump The jet pipeline is of such a size that the pipeline loss remains within acceptable boundaries. It is advisable to design the bends, valves, crossovers etc. as large as possible in order to keep the losses within acceptable limits. Often a sand pump is used as a jet pump to keep the wear between limits. This is certainly advisable when the dredger is a barge loading suction dredger. The water surrounding the dredger due to the overflow of the barges is diluted by fine sand particles, and thus the water taken in by the water pump.

4.8.2.8 The winches Besides the ladder winch and the auxiliary winches, the Suction dredger is equipped with six winches for mooring: • one bow winch • two forward side winches • two after side winches • one stern winch to maintain tension on the bow winch 4.8.2.9 The ladder winch Pagina 26 van 35 prof. W.J.Vlasblom

April 2003


The ladder winch that serves to adjust to the correct dredging depth is usually mounted on deck. If the hoisting wire runs through one or more blocks, the lowest block is fastened to the suction pipe by a rod (Figure 4.41). This is to prevent the block from being fouled by sand when dredging an irregularly shearing breach/bank. At present slow running electric or hydraulic drives are used.

Rod

Figure 4. 41

4.8.2.10The bow winch With the aid of the bow winch the suction pipe is held against the breach or bank. For the optimum control of the suction process good control of the bow winch is essential. It must be possible to pay out the bow winch quickly when moving the bow anchor. Bow winches are mounted on or below deck. Because of the great length of the bow wire, the bow winch usually has a large drum. 4.8.2.11The side winches The side winches control the position and direction of the dredger in both the cut and in the dredging area. Side winches are usually mounted on deck and are electrically or hydraulically driven. 4.8.2.12The stern winch The stern winch has a secondary function, namely to maintain tension on the bow wire, and it does not determine the production. Like the side winches it mainly comes into action when the dredger is being moved to another cut. The stern winch is usually mounted on the stern deck and electrically or hydraulically driven. 4.8.2.13The auxiliary winches The moving of the sprayers and the warping of the barges is usually done by separate winches. One or more jib cranes may be fitted and used to lift heavy parts during repairs. 4.8.2.14The fairlead To sail the barge from and to the dredger fairleads are used to bring the side line wires on a sufficient depth below the water level that the barge can sail over the wires.

Page 27 van 35

Wb w3408b


Pin to change the height of the fairlead

Fairlead guide

Side wire

Figure 4. 42 Fairlead


April 2003


4.9 The dredging process The dredging process of a suction dredger can be subdivided into 1. The behaviour of the breach/bank during dredging also termed the breach/bank production. 2. The suction production of the dredger. 3. The discharge production of the dredger. The last two productions will not be considered in these lecture notes. They will be treated in a course on dredge pumps and pipeline transport because the calculations involved are similar for all types of dredger. 4.9.1

The production of the breach

When a vertical suction pipe is lowered into a sand layer quickly, narrow pit forms with almost vertical side slopes (Figure 4.43). The diameter of the pit decreases from the top downward with time so the sand grains and sand fragments glide down under the force of gravity. The velocity at which the instability of the slope moves depends on the permeability and the relative density of the sand layer and is roughly 20 to 40 times the permeability, depending on the slope and the angle of internal friction of the breach.

Suction tube

240 210 180

vwall

Slope 150

vs

120 100

0 80

Time in seconds

15

60 50

20 40

30

Suction velocity vs = 2.5 m/s

Figure 4. 43

Detailed information about this process can be found in the lecture note wb3413 the “Breaching process” .

When, under laboratory conditions, a 2-D suction mouth is moved forward with a constant speed at the base of a breach, a slope with an angle β will occur which is much steeper than the angle of internal friction. (Figure 4.4)

D

C

vh

vw a

b B

A

Figure 4. 44

The relation between vw and vz follows from the similarity of shape after a time ∆t.

Page 29 van 35

Wb w3408b


vw ⋅ ∆t =

H and vh ⋅ ∆t = tan α

H 1 1 − tan α tan β

From this it thus follows that:

ì tan α ü ý v h = v w í1 − î tan β þ Production per metre wide:

ì tan α ü Qsand = vh ⋅ H = vw ⋅ H í1 − ý î tan β þ Here H is the height of the breach/bank. The cause of the steeper slope is cause by the dilantancy (an increase of porosity) due to the shearing of the sand matrix. When the porosity increases pore water has to flow to the these large pores. When this happens slowly a decrease in pore pressure will occur and a increase in the effective stresses causing an more stability. When sufficient water has flowed into the pores the under pressure and additional stability will vanish. When a 3D suction pipe is moved forward horizontally at a constant speed a pit forms the slope of which is at its steepest directly in front of the suction pipe (Figure 4.45). The slope decreasing at the sides to a value α that is determined by the eroding effect of the density current flowing towards the suction mouth. The angle β between the slope just in front of the suction pipe and the horizontal can be derived according above. If all the material is removed, the production will be:

Q =W

H tanα

a)

H

FG 1 − 1 JI H tan α tan β K

H tanβ Symmetry plane

H tanα

Vh

H H2 vh = vh 2 tan α

However, due to the movement of the suction tube not all the material from the side slopes will reach the suction mouth and spillage will occur.

b)

b

Spillage from breach

H

Figure 4. 45

This spillage can be calculated with the following production balance can be set up:

(H − S) v H −S 2S S Hvh − vw = h tan α 2 tan δ tan α 2

with: Symbol

Declaration

Dimension


April 2003


H S vh Vw α

Maximum pit depth Height of spillage Horizontal velocity suction mouth Distortion (Wall) velocity Minimum slope angle angle of internal friction

M M m/s m/s °

The first term is the volume per unit of time passing through area of the plane TAR, the second term is the production from the face BAT and BRA with ½S being the average height retrogressive erosion or wall over the area considered and the term on right side of the equation is the volume per unit of time passing through a plane with the final cross section.

R

0.5b

H tan β

H H − tan α tan β

S

O

g

S tan δ

B

0.5b

T

H S

Figure 4. 46

This leads to:

Page 31 van 35

d

A

Wb w3408b


H 2 − HS S 2 vw H 2 − 2 HS + S 2 − = tan α tan δ vh tan α H 2 − HS − S 2

tan α vw = H 2 − 2 HS + S 2 tan δ vh

S = 0 and S =

H tan α vw 1+ tan δ vh

The theoretical production without spillage, according equation Q = vh

H2 , tan α

2

é ù ê ú v H v w the real production Q = h ê tan δ ú , and tan α ê vh + vw ú ë tan α û 2

the spillage production Q spillage

é tan δ ù v vh H 2 ê tan α h ú = ê ú tan α ê tan δ v + v ú h w ë tan α û

2

Laboratory measurements have shown that tan α = 4.77 . tan δ However, in practice appeared that the angle α is small too. Taking α=δ results in a production of: é ù vh H 2 é vw ù vh H 2 ê 1 ú ê ú Q= ê ú = tan α ë vh + vw û tan α ê vh + 1 ú êë vw úû

2

2

4.9.2 The production of the pumps The sand flowing towards the suction mouth will be taken up by the dredger and must be transported away by means of barges or pumped to the disposal site via a pipeline. Depending on the pipeline system and the position(s) of the sand pump(s) the following situations may occur.

More sand flows to the suction mouth than the pumps can handle. The pump is the limiting factor and this criterion can be subdivided as follows:

•

•

•

The under-pressure/vacuum in front of the pump is the limiting factor. The under pressure in front of the pump is so high that cavitation occurs, resulting in the loss of the discharge pressure. The pump then cuts out. The only good remedy is to position the underwater pump deeper. The discharge pressure is the limiting factor. The discharge distance is so long that the pressure required for the critical velocity of the mixture is higher than the pump can deliver. A stationary deposit will be formed in the pipeline, with the chance of a totally blocked pipeline. Depending on the loading on the engine, consideration can be given to the installation of a pump with a larger impeller or to changing the transmission ratio in the gearbox. If the loading of the engine is already maximal the maximum concentration has been reached. The pump torque is the limiting factor. This is the contrary situation to the above mentioned limiting pressure situation. The remedy is to use a smaller impeller. Pagina 32 van 35

prof. W.J.Vlasblom

April 2003


4.9.3 The production of the barges The pump production of a barge loading stationary suction dredger is not the same as the amount of material transported by means of barges. This is caused by the overflow losses that occur during the loading and also the bulking that occurs because the sand in the barges often has a lower density than the in situ density. These two factors must be taken into account when determining how many barges are required.

The number of barges follows from:

n=

P( 1 − ov ) β P( 1 − ov ) β P( 1 − ov ) β = = t cyclus Lbak Pbak Lbak t cyclus

(4.24)

Here: N P Ov ß Lbarge Tcycle

= number of barges = pump production = overflow loss [-] = bulking factor = load of barge [m³] = cycle time

[-] [m³/s] [-] [s]

As a rule of thumb the percentage smaller than 100 µm can be taken as overflow losses. The bulking is determined by the difference volume weight in situ and in the barge. With strongly graded material the volume weight in the barge is ± 19 kN/m³ and with uniform material this can decrease to ± 18 kN/m³. For the calculation of the bulking reference should be made to Section 2.6.3.1. The cycle time of the barge is composed of: • the loading time • the sailing time • the discharge time • the return sailing time • waiting times for bridges, locks etc. In addition to the fact that the pit or the pump can be *maatgevend, with a barge-loading dredger, a situation may occur in which the barges are *maatgevend. In other words there are not enough barges. A situation that may have a variety of causes such as: • weather and wave conditions • shipping • Bridges and lock • Unequal speeds of the barges • Loss of time by the barge • Delays on the dredger • Loss of time at the discharge site It will be clear that when using a barge-loading dredger there is always a chance of delays due to the absence of a barge.

Page 33 van 35

Wb w3408b


Because the above mentioned delays can be reasonably well estimated with regard to their average values and standard deviations, the Monte Carlo Simulation can provide insight into the probability of delay resulting from the absence of barges.

4.10

The dustpan dredger

As appears in chapter 4.9.1, the production of the suction dredger is proportional to the square of the breach height. With low breach heights the production remains lower than the discharge capacity of the pump. In order to compensate this to some extent, a broad suction mouth, the dustpan head, is mounted on the suction pipe. The width of the dustpan head is 10 - 15 times the diameter of the suction pipe. In addition a large number of spray nozzles are mounted on this suction head, which by means for water jets stimulate breaching process. Moreover they are necessary to prevent the suction head from becoming blocked. The working effect of the spray nozzles can be calculated in the same way as is given in chapter 4.5.6.1. In fact, the dustpan dredger has been superseded by the cutter suction dredger, which, with a considerable larger width of cut, can attain a much higher production on low breaches/banks.

zuigmond

Figure 4. 47 Dustpan heads

Dustpan dredgers are now only used for small projects or on special dredgers such as the “Cardium.” The “Cardium” is equipped with 6 suction pipes and suction pumps, each with two suction mouths, in order to ensure that the bottom is at the correct depth (the foremost suction mouth is in dustpan mode) and is flat and clean immediately before a block mattress is laid down (clean up model).


April 2003


Figure 4. 48 Dustpan haed with pump and pipel ine sceme of the matress laying vessel “CARDIUM”

4.11

References

1. Offshore soil mechanics, Verruit, 1992 2. Investigations to the spillage of the horizontal suction process, W.J. Vlasblom, to be published in May 2003. 3. Hydraulic excavation of sand, H.N.C. Breusers, Proceedings International course Modern Dredging, June 1977, The Hague 4. Neue Erkentnisse beim Gewinne und Transport von Sand im Spülproject Venserpolder, J. de Koning 5. Coastal & Ocean Dredging, J.B. Herbich, Gulf Publishing Company, Texas 6. Lecture notes wb 3413 “The Breaching Process” 7. Lecture notes additional to wb 3414 “ Dredge pumps”

Page 35 van 35

Chapter 5 Barge Unloading Dredgers

———————————————————————————————————————

5. The barge unloading/reclamation Dredger 5.

The barge unloading/reclamation Dredger........................................................................ 1 5.1. General considerations...................................................................................................2 5.1.1. Characteristics ............................................................................................................3 5.1.2. The areas of application..............................................................................................3 5.1.3. The history..................................................................................................................3 5.1.4. Work method ..............................................................................................................4 5.2. The design......................................................................................................................6 5.2.1. The production capacity .............................................................................................6 5.2.2. The transport distance.................................................................................................6 5.2.3. The dredge installation. ..............................................................................................7 5.3. Main layout ....................................................................................................................13 5.4. Technical construction ...................................................................................................16 5.4.1. The hull.......................................................................................................................16 5.4.2. The pipelines ..............................................................................................................18 5.4.3. The shore connection..................................................................................................18 5.5. The dredging process .....................................................................................................19

Figure 5-1 Barge unlading dredger “HOLLAND”

A specialized dredging tool that can be categorized in the section of stationary plane suction dredgers is the barge unloading/reclamation suction dredger.

Page 1 of 1

Wb3408b

5.1.


General considerations

Barges that are used for the transport of dredged material can be divided in self-unloading and non-self-unloading. The self-unloading barges, called hopper dump barges or bottom unloaders, are usually equipped with doors (valves) that one way or the other can be opened to dump the dredged material under water. Non-self-unloading barges need to be unloaded either mechanically or hydraulically. Mechanical unloading can be done with a grab, backhoe, excavating wheel or bucket elevator. Non-selfunloading barges are therefore often called elevator barges.

Figure 5-2 A Japanese BUD with backhoe’s and belt conveyors.

Hydraulic unloading can be done using a shore pump discharge system, usually installed in trailing suction hopper dredgers or by means of a barge unloading suction dredger. For the last 20 years the transport with barges is strongly reduced and because, as mentioned, the barge unloader is a specialized dredge tool, it is hard to use the tool for other purposes. Hence the amount of barge unloading suction dredgers has decreased considerably in this period. At present many barge unloading suction dredgers are in service that can also be used as plain suction dredger or cutter suction dredger.

Page 2 of 2 Prof.Ir. W.J. Vlasblom

May 2003


——————————————————————————————————————— 5.1.1.

Characteristics 28

28

13a

15

17

14

22 18

8 20 22

4 3

16 2

13

7

2

12 6

3

13a

5

1

10

11

8

4

6 3

7

7

9 25

5

19

3

13

10

24

13

11 9 9 25

25 7 8 13a 16

13

19

15

14

17 30 31 31

2

18 21

20

13 3

4

13a

7

18

29

30

22

31

3

31

23

Figure 5-3

The barge unloading suction dredger is a stationary dredge tool, moored along mooring piles or anchored with spuds. (Figure 5-3) The barges are moored along the tool for unloading. The tool is equipped with one or more sand pumps and a jet pump. The suction pipe sticks out at the side of the tool and can be lowered in the barge lying next to the dredger. The water needed for the mixture and the transport is jetted into the barge using one or more nozzles.

5.1.2. The areas of application The barge unloading suction dredger is able to unload barges hydraulically. These barges are filled one way or the other, for instance with a plain suction dredger or a bucket ladder dredger. The material in the barge is diluted with water and sucked up (figure 5.1). This immediately implies that the barge unloading suction dredger can only handle materials that fluidize quickly like silt and sand. Cohesive materials, of which the forming of a mixture is too slow, will cause the barge unloading suction dredger a lot of problems.

5.1.3. The history The barge unloading suction dredger is a Dutch development. During the excavation of the North Sea Canal a stationary plain suction dredger was transformed to a barge unloading suction dredger (± 1875). Before the barges were unloaded using a bucket elevator. Next the material was transported to the dump with small sand trains. With the arrival of the barge unloading suction dredger these trains, which were very labor-extensive became redundant. Besides it was now possible to transport weak soils simply. The first pressure pipes were mad of wood but soon these were replaced by iron pipes.

Page 3 of 3

Wb3408b


Figure 5-4 The steam driven BUD “Sliedrecht I”

5.1.4. Work method In Figure 5-1 the working method is schematically explained At the start of unloading process, the suction pipe is lowered to the sand level in the barge, while the jet pump is connected to the suction pipe. The speed of the dredge pump on board of the dredgers is reduced in such away that the jet water flows via the suction tube on the sand in the barge, where its erodes a pit under the suction mouth. The dredge master lowers the suction mouth below the water level in this pit. When no air is released via the suction mouth, the butterfly valve between the jet pump and suction pipeline is slowly closed, causing an outflow of jet water via the jet nozzle. (Figure 5-5.A.). Meanwhile the speed of the dredge pump is increased When the dredging process is running well, the jet nozzle erodes the breach while the sand is removed via the suction mouth. During this process the pit under the suction mouth becomes larger and the suction mouth is lowering until she reaches the bottom of the barge. (Figure 5-5.B). Sand flowed behind the suction mouth has to be jetted back to the suction mouth regularly (Figure 5-5.C). Therefore modern BUD”s have either a jet installation around the suction mouth or additional jet pipe to overcome this problem The concentration in the discharge line is controlled by hauling the barge (Figure 5-5.D).


May 2003


——————————————————————————————————————— Jet pjpe Suction pipe

Figure 5-5 Working method of barge unloading

During the exchange of the barges the pressure side of the jet pump is connected with the suction side of the sand pump. This keeps the sand pump moving in the discharge line. The more the sand-water mixture is exchanged for clean water in the discharge pipeline, the velocity increases and if necessary the number of revolutions of the sand pump can be reduced. Apart from the continuation of the dredging process, this construction is necessary to prevent the suction in of air through the suction mouth of the suction pipe, with all consequences (think of submerged pipelines). When the next barge is moored along the barge unloading suction dredger, the number of revolutions of the sand pump is decreased such that it just can handle the flow rate of the jet pump. The surplus water is run away through the jet piping and the suction pipe and a new dredge cycle can start.

Page 5 of 5

Wb3408b


Figure 5-6 Unloading a barge

5.2.

The design

The barge unloading suction dredger has to fulfill in principal two functions: 1. the material in the barges must be diluted such that a mixture develops that can be sucked up in high concentrations. 2. the dredge pumps in the dredger have to take care that the sucked up material can be pumped to the reclamation area with enough velocity and production. 5.2.1. The production capacity Like with the other tools the required production capacity plays a crucial role in the design. The production capacity is however determined by the supply of the sand by barges and therefore by the tool that loads the barges. This can be, for instance, a barge loading plain suction dredger, a backhoe dredger or a bucket dredger. For the design of the barge unloading suction dredger the required production for each barge is the criterion, so the required discharge time for each barge. After all the non-presence of barges by external causes has nothing to do with the required production capacity. Besides that the size of the barges is of course of influence on the required production capacity. 5.2.2. The transport distance The transport distance gives requirements for the installed dredge pump power and the necessity for the installation of one or more pumps. For further details with regard to the choice of the pumps see Chapter 4 Plain suction dredgers.


May 2003


——————————————————————————————————————— 5.2.3.

The dredge installation.

5.1.1.1 General When the dredge capacity is known, this requirement, like with the plain suction dredger, is translated in: 1. a sand flow rate Q 2. a sand concentration Cvd After all:

P = Q⋅

Cvd 1− n

with:

= = = =

production flow rate transport concentration porosity

P Q Cvd n

[m3/s] [m3/s] [-] [-]

The minimum flow rate is determined by the critical velocity that is required to keep the material in motion. So vcritical = Fl , h 2 g ( S s −1) D in which the value of Fl,h is determined by the to be pumped material (see wb3414, Dredging processes). The maximum concentration that can be sucked depends on many factors, like: • • • •

the breach behavior of the soil. the design of the suction mouth in comparison with the width of the barge. the maximum mixture forming that can be reached with the water nozzles and the jets at the suction mouth and the flow rate of the jet pump. the height of the suction pipeline. Because the maximum under pressure is created here, it determines for a large part the maximum concentration. As a value a concentration of 1400 kg/m3 is maintained.

This last factor can be checked with the vacuum formula (see also Figure 5-7):

  H + k  v2 H ρb g + vac =  µ + λ + H + k  ρm g  D ⋅ sin β  2 g   H ρb g + vac ρm =   H + k  v2 µ λ + + H + kg   D ⋅ sin β  2 g   In which: H k vac ρw ρm

Depth suction mouth below water level in barge height discharge piping above the water in the barge maximum allowable vacuum in the discharge piping density water density mixture

Page 7 of 7

[m] [m] [kPa] [k/m3] [k/m3]

Wb3408b

[k/m3] [°] [-] [-] [m] [m/s] [m/s2]

density mixture of the water in the barge angle of suction pipe with horizontal. loss coefficient friction coefficient Darcy Weissbach diameter suction pipe suction velocity Gravity

H

k

ρb β µ λ D v g


Figure 5-7



For H=2.5 m; ρb=1050 [k/m3]; vac = 90 kPa;  µ + λ



H +k D ⋅ sin β

  = 1.5 + 0.01( hz + k ) and v= 4 

m/s the below shown graph is obtained.

19 18 17 16 15 14 13 12 11 10 0

2

4

6

8

H e ig h t su c tio n lin e a b o ve w a te rle ve l in b a rg e [m ]

Figure 5-8

This graph shows (Figure 5-8) that the upper side of the suction pipe may lay hardly more than 3 m above the water level in the barge to meet the earlier mentioned requirement of γm=1400 [kN/m3]. This height needs than to be sufficient to haul the barge underneath the suction pipe. Page 8 of 8 Prof.Ir. W.J. Vlasblom

May 2003


——————————————————————————————————————— The expected average concentration during the suction of the barge is dependent on: • the time necessary to start the process, see the chapter the dredging process 5.4. • the availability of a barge hauling installation. The production is mainly determined by the haul speed of the barge. • the whether or not present of additional bulkheads in the barge, for which extra breaking in necessary. When both the critical velocity as the average and maximal concentration are determined, both the pump flow rate and the diameter of the pressure piping are also fixed (see chapter 4.2.1). 5.1.1.2 The suction mouth and pipe Nowadays the suction mouth of a BUD is provided with jets to improve the mixture forming and to hindered the settling of material behind the suction mouth (Figure 5-9) The width of the suction mouth is based on the smallest hopper width of the barge. Are barges used with different sizes it is advisable to design a flexible suction pipe (Figure 5-9). Sometimes the suction mouth is provided with bars to avoid debris and boulders entering the suction mouth.

Figure 5-9 Suction mouth and pipe

5.1.1.3 The jet pumps All the water necessary to transport the sand over the required distance must be supplied to the barge by the jet pump. The flow rate of the jet pumps depends on the functions of these pumps. Usually two functions are considered: 1. The activation of the breach. By way of a water nozzle before the suction mouth the breach is activated. Usually a second water nozzle is present that jets loose the sand behind the suction mouth so that it still is sucked up by the suction mouth. 2. The mixture forming. The flow rate of the jet pump must be related to the average concentration that can be sucked. Here also that the following condition must be met:

Qj Qm

= 1−

Cvd 1− n

In this: Qj Qm Cvd n

= = = =

the jet flow rate the sand flow rate the transport concentration pore percentage

Page 9 of 9

[m3/s] [m3/s] [-] [-]

Wb3408b


Relation Qj/Qm - Cvd 0. 0.

n=.

0. 0.

n=.

0.

n=.

0. n=.

0. 0 0

0.

0.

0.

0.

1

Qj/Q

Figure 5-10

Looking at the above mentioned boundary conditions (Figure 5-10) the flow rate of the jet pump needs to be 0.4 to 0.5 times the flow rate of the sand pump. With a decrease in the concentration, like when the suction mouth reaches the end of the barge, the flow rate of the jet pump will have to increase to maintain the desired velocity in the pressure piping. If this is not possible the water level in the barge will drop. If there is however enough water in the barge to maintain the velocity there is no problem. If this is not the case water have be supplied in another way to maintain the velocity in the discharge line. F.i. an additional water inlet connected to the suction side of the discharge pump

Jetpump suction strainer

jetpump engine

2 1 Nozzle

3 4

dredgepump engine

Dredgepump

Turning gland

Suction mouth valve

Figure 5-11 Pump-pipeline layout on board of a barge unloading dredger

This is possible by installing a pipe from the suction side of the pump to the bottom of the pontoon or the suction strainer or weed box (Figure 5-11). In such a design enough water can be sucked up at all times to maintain the dredge pump process , also when the unloading of the barge is stopped completely.


May 2003


——————————————————————————————————————— 5.1.1.4 The jet pump drive The drive of the jet pump may be electrical or diesel driven. The dredge master controls the process visually by keep the water level in the barge at a constant height. Increasing or decreasing of the water level determines that there is no equilibrium between the volumes water pump into the barge and the mixture pump out of the barge. Therefore speed control is necessary to control the unloading process well. 5.1.1.5 The sand pump. The dredge pump should be chosen on basis of discharge properties and less on suction properties, because the last properties are mainly determined by the highest point of suction pipeline. The required manometric pressure of the pump is determined by the transport distance. When large pumping distance is large, more than one dredge pump may be necessary. The use of submerged pumps close to the suction mouth to increase to design density of the mixture is also possible but expensive. For Dutch dredging environment it seems not useful due to the shallow and relatively small barges. However in Japan where large sea-going barges are frequently use, there is a need for a submerged pump as shown inFigure 5-12.

Figure 5-12 Japanese BUD

5.1.1.6 The sand pump drive In the process of barge unloading suction dredging the control of the sand pump(s) plays an important part. After all, when the sand pump is not connected to a suction strainer, the flow rate must drop to the value of the jet pump when exchanging the barges. This is done by decreasing the number of revolutions of the dredge pump drive. By the decrease in flow rate this will usually not cause any trouble for the allowable couple of the drive.

Page 11 of 11

Wb3408b


5.1.1.7 The barge hauling installation Modern BUD”S have an installation to move the barge along the dredger by means of a so-called barge hauling installation. The installation consist of a steel wire or rail along the full length of the mooring side of the BUD. (Figure 5-13) On a pulley or a movable part on the rail two slings are connected. These sling are on the side connected to the bollards on the barge. (Figure 5-14) This construction has the advantage that the barge is kept along side of the BUD, The pulley or slide is connected via a wire to a winch, which makes it possible for the dredge master to control the haul speed by himself.

Figure 5-13 Sliding part of the Barge Hauling installation

Figure 5-14 Barge hauling installation with pulley and wire Page 12 of 12 Prof.Ir. W.J. Vlasblom

May 2003


———————————————————————————————————————

5.3. Main layout The layout of the barge unloading suction dredgers is quite simple. The hull consists of a simple rectangular pontoon, usually anchored by spuds at the ends (Figure 5-16Figure 5-16). Centrally in the pontoon the pumps (dredge and jet pump) and engine room are located. Furthermore fuel and water tanks and storage rooms are situated in the pontoon. The control of the dredger is done from a cabin at the side of the deck from which the suction operator has a good view on the alongside moored barge. Present accommodations are also situated above decks (Figure 5-15 and Figure 5-16). Instead of spuds the barge unloading dredger might be moored on wires. Suction pipe, discharge pipe are supported by booms or A-frames. The jet pipe or nozzle by hydraulic cylinders to control the direction of the jet water.

Figure 5-15 BUD Rozkolec

Figure 5-16 View of the BUD Rozkolec

Page 13 of 13

Wb3408b


Figure Figure 5-17 shows the top view of the BUD Sliedrecht 14 and Figure 5-18 the side view of the same dredger.

Figure 5-17 Top view of BUD Sliedrecht 14

Figure 5-18 Side view of BUD Sliedrecht 14

Figure Figure 5-19 shows a barge unloading dredger that can be used as a plain suction dredger too.


May 2003


———————————————————————————————————————

Figure 5-19 BUD Hercules

Page 15 of 15

Wb3408b


5.4. Technical construction

The pontoon volume in cubic meters is almost 2.5 times the light weight in tons (Figure 5.13). Length of width have values between 4 and 4.5 while width over draught have values between 3 and 6.

1400 y = 0.2496x R2 = 0.7486

Light wieight [ton]

1200 1000 800 600 400 200 0 0

1,000

2,000

3,000

4,000

5,000

6,000


Figure 5-20 Light weight versus installed power. 3500 y = 2.4534x

3000

2

R = 0.8951

2500 BLD [m3]

5.4.1. The hull The main dimensions length, width and depth of the pontoon depend totally on the requirements for the above mentioned design parameters and the from these following demands for stability and strength. The light weight of the pontoon in tons is roughly 25 % of the total power installed (Figure 5.12)

2000 1500 1000 500 0 0

200

400

600

800

1000

1200

1400

Light weigth [tons]

Figure 5-21 pontoon volume versus light weight L/B

B/T

7 6 L/B and B/T

The fuel and water tanks are distributed such over the pontoon that a good trim of the ship is obtained. The winches for hauling the barges during the suction process are located on the deck. The barge unloading suction dredger is in general equipped with spuds for anchorage.

5 4 3 2 1 0 0

200

400

600

800

1000

1200

1400

Light weight [tons]

Figure 5-22


May 2003


———————————————————————————————————————

Figure 5-23 Plain Suction and BargeUunloading Dredge Seeland Besides plain suction dredgers Figure 5-1 and Figure 5-23 also cutter suction dredgers can be converted into a barge unloading dredger. ( Figure 5-24), although the last conversion will be more expensive.

Figure 5-24 The CSD “VICKSBURG” converted to a Barge Unloading Dredger

Page 17 of 17

Wb3408b


5.4.2. The pipelines The suction pipe that sticks out of the construction on the side where the barges are moored, must on the one hand be located as low as possible for the pump process and on the other hand be high enough to let the empty barges through underneath. The lower part of the suction pipe, the haul pipe, runs parallel and approximately in the centerline of the barge. This part can rotate around a horizontal axis by way of a rotation gland mounted in the horizontal part of the suction pipe. Since this construction causes a under pressure in the suction pipe during dredging, a lot of attention must be given to the air tightness of the piping. The necessary movability of the suction pipe is obtained by hanging this pipe in a boom with a hoist cable. For good movability the suction pipe can swing in a horizontal plane by a hinge mounted in the suction tube. (Figure 5-25) The suction mouth is in general widened to obtain a lower height of the suction mouth with a similar opening surface. This reduces the chance of sucking in air. (Figure 5-9)

Figure 5-25 Movable suction tube

The supply of the necessary dilution water to the barge is done with one or two water nozzles. In case of one nozzle the suction mouth is usually equipped with jets, while the movability of the main nozzle is than so large that it can also spray behind the suction mouth. To present sand well to the suction mouth it is necessary to have moveable water nozzles. This is done using hydraulic cylinders. For the dredging process the pressure side of the jet pump is, except for the water nozzles, also connected with suction side of the dredge pump.

5.4.3. The shore connection The connection of the dredger to the shore needs to be flexible at all times, due to the movements of the barge unloading suction dredger by: • • • •

trim during dredging difference in draught by supplies tides or water levels hits of the barges against the dredger


May 2003


———————————————————————————————————————

Figure 5-26 Shore connection for a barge unloading suction dredger

The shore connection must therefore consist of enough hinges. A flexible hose can also possibly give enough flexibility, if this doesn't get stuck on the slope of the embankment. For large differences extra attention must be paid to this movement (Figure 5-26).

5.5.

The dredging process

The dredge process is a hydraulic transport process with a clear nonstationary character as a result of the exchange of the barges. After all this results that on regular intervals the production reduces to nil. In Figure 5-27 the concentration and the sand pump speed and jet pump flow rate are shown as function of time. The first phase is characterized by an increasing concentration during the process to bring the suction mouth to the bottom of the barge. During the second phase the concentration is approximately constant. The barge is hauled under the suction pipe with constant velocity. The last phase consists of a decreasing concentration because the suction mouth reaches the end of the barge, resulting in a decreasing face height

Conc.

Time

Speed dredge pump

Time

Qjet

To dredge pump

To jet nozzle

To dredge pump

Time

Figure 5-27

This phase is lengthened if the barge have to be cleaned. (The barge is pulled back and the remaining sand is dredged.) Such a process might be necessary when the barge is relative wide compare to the suction mouth and the suction mouth can’t swing in the horizontal plane.

Page 19 of 19

Wb3408b


The production is determined by the breachebility of the sand in the barge and the erosion by the jet water. This dredging process is mainly determined by the minimum NPSH value on top of the suction pipeline and the time necessary to change the barge and to start the dredging process again, as mentioned above. A complication however is that during the emptying of the barge the sand pump flow rate corresponds to the jet pump flow rate and the amount of sucked up sand. If this is not the case than the flow rate in the barge will raise or drop. In a good tuned up process the suction operator maintains the water level in the barge by hauling the barge slower or faster underneath the suction mouth. If there is a continuous increase or decrease of the water level in the barge than the number of revolutions of the sand pump must be adjusted. To obtain the highest possible concentration the water level in the barge must be as high as possible. Unfortunately the breaching of the sand behaves different under water than above water. If the water level in the barge is high the dredge master can’t see if sand flows behind the suction mouth and prefers a low water level in the barge. During the exchange of the barges the velocity in the pressure piping needs to be maintained to avoid sanding up. For this the suction side of the sand pump can be connected to the weed box (figure 5.7). This is not directly necessary. Since the pressure side of the jet pump is in connection with the suction side of the dredge pump a situation with two pumps in series is obtained. The required sand pump flow rate can now be reached by the control of the number of revolutions of the sand pump engine.

pressure Pipeline resistance for mixture

Dredge pump curve for mixture at high rpm

W D

Dredge pump curve for water at high rpm

E Jet pump curve II

Jet pump curve I Pipeline resistance for water

Dredge pump curve for water at low rpm

C

QC

B

A

QA QB

Capacity

Figure 5-28 Pump –pipeline interaction for a barge unloading system


May 2003


——————————————————————————————————————— In Figure 5-28 the pipe and pump characteristics are drawn for the pumping of water and mixture. If it is desired to maintain the minimal flow rate QA during the exchange of the barges, than this is possible, when the suction side of the sand pump is connected with the weed box, by reducing the number of revolutions regularly. This makes the operating point W shift to A. Without a reduction of the number of revolutions of the sand pump, in the last phase of the emptying process, the operating point W will shift over the dotted line to point E, so to a reasonable higher flow rate. If the suction side of the sand pump is connected to the pressure side of the jet pump than the operating point will be in A or B for the same low number of revolutions of the sand pump and dependent on the pump characteristics of the jet pump. For the calculation of the hydraulic process one can refer to the course Wb3414 Dredging Processes 2.

Page 21 of 21

6. The bucket dredger

Figure 6- 1

6.1.

General Considerations

The bucket dredger is one of the mechanical dredgers. A bucket dredger is a stationary dredger that is equipped with a continuous chain of buckets, which are carried through a structure, the ladder (Figure 6- 2. This ladder is mounted in a U-shaped pontoon. The drive of the bucket chain is on the upper side. The bucket dredger is anchored on six anchors. During dredging, the dredger swings round the bow anchor by taking in or paying out the winches on board. The buckets, which are filled on the underside, are emptied on the upper side by tipping their contents into a chute along which the dredged material can slide into the barges moored alongside. The chain is driven by the so called upper tumbler at top of ladder frame, which is connected either via a belt to the diesel or directly to an electro motor or hydro-motor.

5.1

Figure 6- 2 Since 1960, bucket dredgers ( also called bucket line dredge(r) or bucket chain dredge(r)) that were much used before the Second World War, have been almost entirely replaced by Backhoe dredgers or trailing suction hopper dredgers and cutter suction dredgers. The reason for this is that the bucket dredger, with its six anchors, is a big obstacle to shipping. Moreover maintenance costs are high and the bucket dredger requires many highly skilled operatives. But above all, their production has not kept pace with the increase in scale that has taken place in the suction dredgers.

6.2. Area of application Bucket dredgers are only used in new or maintenance dredging projects when the initial depth of the area to be dredged is too shallow for trailing suction hopper dredgers and the distances involved are too long for hydraulic transport. For environmental projects, which require the dredging of ‘in situ densities’, the bucket dredger is suitable peace of equipment. When dredging for construction materials such as sand and gravel, or for minerals such as gold and tin ores, bucket dredgers are still frequently used. Bucket dredgers also come in a variety of types. For example: • Dredgers with or without the means of propulsion (Figure 6- 3) •

Dredgers with a conveyor belt system (Figure 6- 4)

•

Dredgers with equipped with pumps

Figure 6- 3

5.2

Figure 6- 4 The maximum dredging depth is highly dependent on the size of the dredger. There are dredgers with a maximum dredging depth of more than 30 metres. Such large dredgers the minimum dredging depth is often 8 metres. Dredging in shallow water is certainly not the strongest point of the bucket dredger. Bucket dredgers can be used in almost every type of soil, from mud to soft rock. When rock has been fragmented by blasting, bucket dredgers are often used, because of their relative lack of sensitivity to variations in the size of the stones. Bucket dredgers cannot be used in areas with waves and swell. Furthermore, because of the amount of noise they produce, in urban areas they are often subject to restrictions in relation to the working time or the permitted number of decibels measured at a specific distance from them. The capacity of a bucket dredger is expressed in terms of the content of the buckets. The capacity of a bucket can vary between 50 and 1200 litres. Rock bucket dredgers often have a double set of buckets, the small rock buckets and the large mud buckets. This is in order to make better use of the power of the dredger and to widen the range of its use.

6.3. The history From a historical point of view, the bucket dredger derives from the ‘mud mill‘ that was invented in the Netherlands in 1589. In the earliest days this ‘mill’ was powered by a treadmill driven by manpower. () In 1622 the drive system was improved and horses could replace the men. Around the beginning of the 19th century the first steam driven bucket dredgers came into existence.

Figure 6- 5 MUD Mill Dredging Museum at Sliedrecht

5.3

Still, it was not until the second half of the century that steam dredgers had replaced those powered by horses. Over the course of the years preceding 1915, both the power of the dredgers and the capacity of the buckets increased. There was no further increase after that time. The great advantage of the bucket dredger is that it can attain a reasonable production in most types of soil from soft clay to soft rock. For this reason, by about 1900 the bucket dredger had grown to be the most important type of dredger in Europe; a position that it maintained until just after the Second World War. The two last steam powered bucket dredgers were built in the Netherlands in 1956. At the end of the fifties and beginning of the sixties, because of the big increase in the tonnage of oil and ore tankers, large deep-water ports were needed. This led to large dredging contracts, which created a need for bigger production units that, moreover, could dredge to a greater depth. Increasing the capacity of bucket dredgers is no longer the solution because deeper dredging with larger buckets leads to a very heavy bucket chain. Stationary suction dredgers and cutter suction dredgers could solve this problem in a considerably less expensive way. Besides their bigger production capacity, these suction dredgers also have the advantage that their maintenance costs are much lower. For these reasons buckets dredgers are now only used for the types of work mentioned above.

6.4. The method of working When a bucket dredger is working the anchoring system plays an important role in both positioning the dredger in the cut and in the excavation by the buckets.

5.4

Figure 6- 6 Positioning of the dredger in the cut As mentioned previously, the dredger swings round the bow anchors (Figure 6- 6) The bow wire has a length of 1 to 2 times the bucket capacity in litres. This means that for large dredgers it may be 1 to 2 km long. It will be clear that with such great lengths, measures must be taken to prevent the radius of the swing circle from being reduced by the bow wire being dragged over the bottom. Over water, therefore, one or more pontoons/floats/bow barges are positioned under the bow wire. If the bow wire runs mainly over land it is placed on a drum roller. The swinging of the dredger and the provision of the excavation forces is mainly carried out by the side winches. The side winch velocity used depends on the type of soil and also on the step length and the height of the cut. For the most effective possible transition of forces the side wires must make an angle with the bow wire that is a little smaller than 90° .

5.5

When swinging round the bow anchor the swing angle (β) that the dredger makes with the swing circle (Figure 6- 6), must be kept as constant as possible. The choice of the swing angle is related to the clearance between the buckets on the lower part of the chain over the bottom or the slope. If this is not done it is possible that the bucket chain will run off the bottom tumbler as a result of the lateral forces that act on it. At the beginning of a new cut the swing angle is brought to the desired value as quickly s possible. If there is a current in the dredging area the swing angle must be kept as large as possible, that is at 90°. The stern winch controls the swing angle. The stern anchor is used to obtain the required tension in the bow wire. When dredging in tidal waters the stern anchor is usually used as a flood anchor if the winch and the wire are strong enough for this. The step length, the cut thickness and the swing velocity along the cut determine the amount of soil that is cut per unit of time. This amount must be at least in balance with the number of buckets per unit of time multiplied by the capacity of the buckets. In other words the bucket capacity and the bucket speed are related to the factors mentioned above, Some dredgers have more than one type of bucket, so that, depending on the soil type, the capacity can be adapted to the expected production. Because with high excavation forces the dredger will not be able to completely fill the buckets, so that they are partly filled with water. This is of course not economical. The position of the ladder, particular the ladder angle, also affects the maximum filling degree of the buckets. If the bucket rim is not horizontal, fluid soil will partly flow out of the bucket. After being carried upwards, the buckets are turned upside-down as they pass over the upper tumbler or the pentagon and, depending on the time, the material will fall out of the buckets. In order to accommodate to this time effect the discharge chute into which the dredged material falls, is adjustable in relation to the upper tumbler. Depending on the type of soil, extra measures may be necessary to promote the emptying of the buckets. From the discharge chute the material slides directly into the barge that is moored alongside the dredger or it is transported to it via conveyor belts. To obtain the most even possible filling of the barge it must be frequently warped along the side of the dredger.

6.5. The design When designing bucket dredgers the following design parameters are important: •

Production capacity

•

Dredging depth (minimum and maximum)

•

Soil type

•

The discharge of the dredged material (barges or via pipeline)

As previously mentioned, the bucket dredger can be used in all types of soil from clay to soft rock which has not been blasted and hard rock which has been fragmented by blasting. The type of soil to be dredged has a big influence on the design and the construction of the dredger. Considerable forces arise during the dredging of rock. For all types of soil it is necessary to know the required cutting capacity and the energy that is needed to transport the dredged material via the bucket chain to the upper tumbler.

6.5.1.

The production capacity

The production capacity of a bucket dredger cannot be increased indefinitely. Increasing the production capacity of bucket dredgers implies increasing the bucket capacity. This means that the forces in the bucket chain resulting from the weight of the buckets and links themselves is also greatly increased. This in turn demands an even heavier construction. The production capacity of bucket dredgers therefore seldom rises above 100.000 m³/week. The same goes, to an even greater degree, for the dredging depth, because greater dredging depths demand longer bucket ladders and thus more buckets.

5.6

In principle, the product of the bucket capacity and the bucket velocity determines the production capacity, thus: Qb = I b ⋅ vb ; with: Q the production capacity in m3/s, Ib the effective volume of the bucket and vb the bucket speed in buckets per second. The maximum bucket size is 1200 litres and the maximum bucket velocity approximately 30 buckets per minute or .5 buckets per second. Often this bucket velocity can only be reached with empty buckets. With full buckets and when some excavation force is needed, the bucket velocity is quickly reduced to values of 15 to 20 buckets per minute. Moreover factors such as the filling rate of the bucket and the bulking factor of the soil play a part. For a bank height h [m], a step size s [m] and a lateral or swing speed vs [m/s], the insitu production Qs dredged is:

Qs = h ⋅ s ⋅ vs [m³/s] This insitu production must be in balance with the bucket production Qb corrected for the filling degree FDb and the bulking factor B, thus:

Qs = h ⋅ s ⋅ vz =

I b ⋅FDb ⋅ vb B

Note: The filling degree FDb<1 and B>1 Because it is impossible to fill every bucket for 100% it is advisable to take as first assumption the filling degree a value of 0.85 and bulking factor depending on the soil to be dredge: Type of soil Very soft silts and clay Clay sand Rock

6.5.2.

Bulking factor 1.05 1.3-1.5 1.05- 1.25 1.3-1.4

The dredging depth

As with other dredgers both the maximum and minimum dredging depths are very important in relation to the use of the dredger. Requirements in relation to these values are closely related to market demands. The difference between the maximum and minimum dredging depth determine the change of the angle of the bucket rim with the horizon.

6.5.2.1.

De maximum dredging depth

For large bucket dredgers the maximum dredging depth is about 25 m. and exceptional 30 m. By adjusting the height of the mounting of the ladder on the ladder gantry or by lengthening the ladder, it is possible to dredge to a maximum depth of 35 m (see 5.4.3). It will be apparent that by adjusting the setting of the ladder or lengthening it, the number of buckets will increase. The figure below gives a general view of the dredging depths used. For the smaller bucket dredgers the dredging depth is around 10 m.

5.7

maximum ladder depth [m]

Dredging depth 40 35 30 25 20 15 10 5 0

Normal extended

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Bucket capacity [m3] Figure 6- 7

5.2.2.2. The minimum dredging depth The minimum draught is, on one hand, determined by the required clearance including *navigational/keel clearance and, on the other hand, by the *filling degree of the buckets at the minimum dredging depth. In Figure 5.4 below, the maximum draught of the bucket dredger is shown as a function of the bucket capacity. From the graph it can be seen that for bucket dredgers with a bucket capacity of 300 litres the minimum dredging depth must lie between 3 and 4 metres.

5.8

4 3.5 3 2.5 2 1.5 1 0.5 0 0.2

0.4

0.6

0.8

1

1.2

1.4

Bu c ke t c a p ac ity [m 3 ]

5. 1 With small dredging depths, depending on the ladder angle, because the buckets are tilted so far back *the filling degree may well be so low so that dredging in this situation becomes uneconomic. In the figure below (Figure 5.5), the *filling degree of the buckets is given as a function of the maximum dredging depths. The shape of the buckets is such that the maximum filling degree is obtained at the maximum dredging depth. Naturally the buckets can also be designed for the average dredging depth.

5. 2

5.2.3. The soil The influence of the soil to be dredged is seen in the power of the upper tumbler, the strength of the ladder, links and buckets and also in the bucket capacity and shape. If a bucket dredger is equipped with buckets for both soft soil and rock, the capacity of the rock buckets is roughly 60 to 70% of that of the soft soil buckets. Naturally, the length of the links must be the same for both types of bucket. The length of the link must be the same. (Why?) Moreover rock buckets are usually cast and soft soil buckets are often welded.

5.9

5.2.4. The transport of the dredged material Usually barges that are loaded while moored alongside the dredger are used to transport the dredged material. The height of the main gantry must be such that the soil falling from the buckets can slide down into the barges moored alongside via the chute. 5.2.4.1. The bucket dredger with a pipeline discharge system Sometimes the dredged material is carried away directly. In these cases it is collected in a hopper and mixed with the right amount of water to be transported by means of a dredged pump and pipeline. As in the case of a cutter suction dredger, the floating pipeline is attached to the stern of the dredger. Naturally a barge with a dredge pump can also be moored alongside the dredger for this purpose. This option is increasingly rarely used; indeed, unless the work stipulates the use of a bucket dredger the contractor will employ the much cheaper cutter suction dredger.

5.2.4.2. Discharge by conveyor belts

5. 3 Conveyor belts are frequently used to discharge the dredged material when excavating sand and gravel for the cement industry. This type of discharge system can be easily fitted to the normal bucket dredger. The conveyor belts are mounted on floats that are attached to the stern of the dredge. Because no discharge chutes are used the main gantry can be lower. 5.2.5. The main drive The choice of the source of power for the drive of the bucket chain is now limited to a diesel with a direct belt drive, a diesel-electric drive or a diesel-hydraulic drive. When electricity can be obtained from landlines, for example during sand or gravel dredging, it is also possible to use an electric drive. The power/energy needed for the excavation, lifting of the soil, the friction of the buckets over the guiding rollers and the tumblers, the friction of the tumblers, resulting from tension in the bucket chain are transferred to the upper tumbler via the bucket chain. The required cutting power can be determined in a way similar to that described for the cutter suction dredger. Thus with the aid of the specific energy. If the desired cutting production is Qs and the specific cutting energy Es, the required cutting power is:

Ps = Q s ⋅E s

(5.1)

The required cutting power must be multiplied by a factor the represents the relation between the average and peak loads. When lifting the soil the number of buckets under or above water plays a role. Since:

[

P0 = Q e ⋅ g (ρ e − ρ w )H bw + ρ z ⋅ H ow

]

(5.2)

With:

5.10

Qe g e How Haw

= = = = =

the bucket production acceleration due to gravity the density of the dredged material in the bucket the dredging depth the height above water that the soil must be lifted.

[m3/s] [m/s2] [kg/m3] [m] [m]

In principle, the cutting production cannot exceed the production of the bucket chain, thus: I ⋅E ⋅ v Qs ≤ e v e = Qe (5.3) B Here: Ev = the bucket *filling ve = the bucket velocity Ie = the bucket capacity B = the bulking factor If it is assumed that the quotient Ev is equal to 1 and Qs=Qe, the power required to lift the soil, is known. With a filling degree lower than 1 the weight of the water above the soil must also be included. Because the number of buckets that goes upwards is equal to the number of buckets that goes downwards it is not necessary to take into account lifting the weight of the buckets themselves. Naturally the friction of the guide rollers over which the buckets slide must be taken into account. The effect of the tensile forces also makes an extra contribution to the required drive power, with the exception of the friction in the bearings of the lower tumblers. To calculate the reactions and the tensile forces see Section 5.7 The total power required is thus:

Pt = Ps + Po + Pwl + PwT Pt Ps Po Pwl PwT

= = = = =

(5.4)

the power to be installed the cutting power the lifting power the friction power/work of the guide rollers/pulleys the friction power/work of the tumblers

The friction forces that, as described above, can arise are the cause of the fact that the gross energy requirement to lift the soil with a ladder angle of 45°, are roughly two times as high as the nett energy requirement. At small dredging depths this can increase to a factor 4! So the relation between the length of the lower/under-bend of the bucket chain and the length of the ladder has a big influence on the horizontal force (Figure 5.7). For small dredging depths this may increase to a factor 4! Thus the relation between the length of the lower bend and the ladder S/L a big influence upon the horizontal tensile force (Figure 5. 4)

5.11

5. 4 As a guideline it can be assumed that the installed power in kW for the drive of the chain in soft soil is roughly 1/2 and for heavy soil at 2/3 of the bucket capacity in litres. (Figure 5. 5)

1200 1000 800 600 400 200 0 0

0.2

0.4

0.6

0.8

Bu c ke t c a p a c ity [m 3 ]

5. 5

5.12

1

1.2

1.4

5.2.6. The winches

5. 6 Simplified diagram of a barge loading bucket dredger The winches on a bucket dredger have various functions and therefore various requirements with regard to the power, the forces and band velocity, which differ from winch to winch.

5.2.6.1. The ladder winch The ladder winch (letter i in Figure 5. 6), which is used to adjust the required dredging depth is usually mounted on the ladder gantry of the larger bucket dredgers, while the smaller demountable dredgers usually have the ladder winch mounted on deck. Owing to the great weight of the ladder and the buckets this is the strongest winch on the bucket dredger. The installed power is often in the order of magnitude of ¼ of the bucket drive. The ladder winch velocity is roughly between 6 and 10 m/min. Currently the drive is usually a slow running electric or hydraulic engine. Because of the need to set the dredging depth it is necessary to have an adjustable winch. 5.2.6.2 The bow side winches As in the cutter suction dredger, the side winches (see Figure 5. 6) make a major contribution to the excavation process. The installed bow side winch power is between 10% and 20% of the main drive. The side winch velocity of the bucket dredger is generally lower than that of the cutter suction dredger. Nominal side winch velocities lie between 10 and 15 m/min. It will be clear that the excavation process requires a winch that can be well controlled and adjusted. The control must be such that any desired velocity can be set and remain as constant as possible, even when side winch forces vary. As in the cutter suction dredger, when paying out, the wire being loosened must be kept under control by braking while paying out. The winches are mounted on the fore deck.

5.13

. 5.2.6.3 The stern side winches The stern side winches have a secondary function and do not determine the production. The stern winches control the dredger with regard to the cut (swing angle β, (see Figure 5.2). The requirements relating to the control and force are thus considerably less than for the bow side winches. The power is roughly half that of the bow side winches. The nominal side winch velocities are of course equal. The stern side winches are usually mounted on the afterdeck. To avoid hindering the arrival and departure of barges, as well as the warping of the barges alongside the dredger, the side wires are led down to a sufficient depth directly beside the dredger in vertical guides, also called wire spuds (Figure 5. 7).

5. 7 The wire spud construction

5.2.6.4 The bow winch The bow winch is used to pull the dredger forwards when a new cut is started. The required force for this lies in the same order of magnitude as for the side winch. The required velocity, however, is considerably lower (nominally 2 - 3 m/min). Higher speeds are, of course, necessary when positioning the bow anchor.

5.2.6.5. The stern winch The function of the stern winch is to ensure the required tension in the bow wire. This consideration demands that the required force is roughly equal to that of the bow wire, however, the need to move the bucket dredger backward quickly to the adjacent cut places higher demands on the velocity (5-10 m/min).

5.14

6.6.

5.3. The general layout

5. 8 The hull consists of a U-shaped pontoon with long forward pontoons. The dimensions of the pontoon are primarily determined by the required dredging depth and the necessary stability. The well is rather long compared to that of a cutter suction dredger, roughly 60 % *of the length of the dredger. The pontoon is divided into a number of compartments for the engine room, crew accommodation, stores, and fuel and ballast tanks. The latter are often located for and aft in the pontoon. The engine room is located in the pontoon aft of the main gantry and its layout depends on the type of main drive. To satisfy the need for longitudinal stability the bottom of the dredger slopes upward at the stern or the forward end may be wider (Figure

5. 8.). The main gantry is roughly in the middle of the pontoon. Although formerly the crew quarters were often located in the pontoon, in modern dredgers they are now often situated on deck.

5. 9 Tekening IHC

6.7.

5.4. The technical construction

5.4.1. The hull The hull consists of a U-shaped pontoon with almost horizontal deck and bottom plates. Often the bottom plate slopes up at the stern to ensure the correct longitudinal weight distribution of the ship. The corners of the pontoon are rounded off to make it easier for the barges to come alongside.

5.15

5.4.2. The main gantry Because of the way in which the buckets are emptied and the need to load barges that are moored alongside, the main gantry is high and heavy. The construction of the main gantry is often carried through to the bottom ribs. In modern bucket dredgers the drive of the tumblers is mounted on the main gantry. The *stort wagons/fixed chutes are located on each side of the main gantry. They catch the dredged material from the buckets that have been turned over by the tumbler and convey it to the movable chutes, which discharge into the barges, 5.4.3. The bucket ladder

5. 10 Bucket ladder of the demountable bucket dredger “Big Dalton” The vertically rotating upper end of the ladder is suspended from two axle boxes which are mounted on the sloping legs of the main gantry (Figure 5. 10). If necessary, these axle boxes, which are attached by bolts, can be moved along the legs of the main gantry in order to dredge more deeply. When they are in the lowest position it is necessary to add an auxiliary ladder to support the bucket guides. /If they were in the lowest position the upper part of the bucket guides would come to be suspended in the air. To prevent this from happening an auxiliary ladder is added. The shape of the auxiliary ladder is such that the bucket chain is also carried over the upper part. (Figure 5. 11) and is suspended at the lower end via the ladder wire which runs from the ladder gantry.

5.16

5. 11 The weight of the full buckets is transferred to the ladder by rollers. These rollers are mounted at a distance of twice the link length apart. To guide the buckets these rollers are fitted with flanges, hence the name *ladder rollers/guide rollers De bucket *chain/leiding is driven by the upper tumbler (often five-sided) and pulled round the underside by the lower tumbler (often six-sided). As a rule of thumb the total tensile force exercised by the upper tumbler on the bucket chain is 700 kN per 100 litre bucket capacity. The weight of the descending buckets that form a chain provides the tensile force in the tumblers. *These tensile forces, are dependent not only on the ladder angle, but also on the relation between the arc and the chord, which generally amount to 1.1 to 1.15 and if necessary can be changed by adding or removing buckets. /These tensile forces, excepting the ladder angle are dependent on the relation between the arc and the chord, which generally amount to 1.1 to 1.15 and if necessary can be changed by adding or removing buckets. See Section 5.7. **NB not included in Dutch version). Summarising, the following forces act on the ladder: 1. The weight of the ladder itself, including the guide rollers. 2.

The weight of the bucket chain, including the links and bolts.

3.

The weight of the contents of the buckets.

4.

The tensile forces generated in the under bend.

5.

The excavation forces in both longitudinal and transverse directions if necessary multiplied by a factor for impact loading.

5.17

5.4.4. Dredge buckets Dredge buckets may be either welded or cast. Welded buckets are most often used on small dredgers or dredgers that are suitable only for soft types of soil. The buckets are either welded onto the links or cast as one unit with the links. The weight is then very high; 30 to 40 times the bucket capacity in kN. For welded buckets the weight is 13 to 15 times the bucket capacity. The front of the upper edge of the buckets is equipped with a cutting edge or with cutting teeth (Figure 5. 13). The latter are most often found on rock buckets. The shape of the bucket is always a compromise. Because a good shape for excavation and the required strength do not give the optimum content.

• •

The shape of the buckets is also determined by the required swing force (Figure 5. 12).

•

The theoretical filling degree, the amount of water that the bucket can contain in relation to the total bucket capacity, is highly dependent on the dredging depth (Figure 5. 4).

•

A bucket shape from which the soil readily falls is equally difficult to combine with a good excavation shape.

•

The price of the bucket.

5. 12 Rock buckets are small heavy buckets, somewhat egg-shaped, which must be able to resist impact loads. Soft soil buckets, termed mud buckets, are much bigger and lighter. The relation rock bucket capacity to mud bucket capacity lies between 60 and 70 %. The so-called *pan buckets have good soil discharging properties; their disadvantage is that the *filling degree is very sensitive to the angle of the bucket.

5.18

5. 13 The links are fastened to each other by bucket bolts. The holes in the links, through which the bucket bolts pass are equipped with wearing bushes, termed, bucket bushes. These are forged steel *bushes/sleeves that are hydraulically pressed into the link. This simple means of attachment makes these bucket bushes very prone to wear and so they must be frequently replaced. (Figure…). *The lubrication of the guide rollers and tumblers is now carried out centrally. Nowadays *caterpillar tracks are sometimes used instead of links and bushes (Figure 5. 14).

5.19

5. 14 Undercarriage van Caterpillar

5.4.5 The ladder gantry The ladder gantry straddles the outer end of the well. On it are found: • The ladder winch that is used to set the dredging depth. •

The control cabin of the dredge master. From this it is now possible to operate all the winches.

•

The crane. The free height of the ladder gantry is determined by the height required to rotate the entire ladder above water. Because of the large well, in order to give sufficient stiffness to the dredger the ladder gantry construction must be very heavy. 5.4.6 The main drive Although in the past many steam powered dredgers were built, nowadays the choice is limited to: • Diesel-direct driven via belt •

Diesel-electric drives.

•

Diesel-hydraulic drive.

•

Direct power supplies from the shore; sometimes used for sand and gravel extraction.

This means that the upper tumbler may be electric or driven by a hydraulic engine. In steam powered dredgers or those powered by diesel engines with a direct drive the energy is transferred to the upper tumbler by driving belts. The control of the revolutions of the upper tumbler and thus of the bucket velocity is simple when using the above mentioned modern control systems. With an upper tumbler that is directly driven by a diesel engine control is limited and switchable or hydrodynamic gears are needed. The drives of auxiliary equipment such as winches and chutes present no problems when modern drives are used.

5.20

5.4.7 The winches 5.4.7.1 The ladder winch Because of the great weight of the ladder two wires are usually used to hoist it. For this purpose the winch drum is grooved on both sides in such a way that when the ladder is raised the wires are on the outer sides of the drum (Figure 5. 15).

5. 15

5.4.7.2 The bow winch With the aid of the bow winch the dredger is held against the cut. This winch also serves to pull the dredger forward to the following cut during stepping. The revolution speed of this winch is very important. When moving the bow anchor this winch is paid out. Bow winches may be mounted above or below the deck. Because of the great length of the bow wire the bow winch has a very large drum.

5.4.7.5 The auxiliary winches Separate winches are used to operate the discharge chutes and for the warping of the barges. A jib crane is needed to lift out stones and debris that has been dredged, and also when changing the buckets during repairs. The winches used by this crane must satisfy the stipulations that apply to lifting cranes.

6.8. 5.5 The stability Under working conditions the stability of the bucket dredger is seldom in question. After all, the greatest weight is always under water. If the ladder is raised, however, the situation is entirely different. The great weight of the ladder is then entirely above water. For this reason, when a bucket dredger is being towed at sea it must be unrigged. The entire bucket chain must be dismantled and, if possible, stowed below deck.

6.9. 5.6. The dredging process The dredging process of the bucket dredger includes only the excavation and lifting of the dredged material. Barges carry out the transport.

5.21

As previously mentioned, the bucket dredger swings on the bow anchor along the arc of a circle *following a curving path. The axis of the dredger makes an angle β, the swing angle with the tangent to this arcuate path. The size of the swing angle depends primarily on the clearance between the lower bend and the bottom and on the slope of the breach/bank. At the end of the cut the dredge master will allow the swing angle to slowly increase to 90°. After this a step will be taken or, if necessary, the cutting of the following layer will be started. By means of this movement back and forth, the bucket dredger makes concentric arcs/curves that lie at a distance of one step length from each other. During this swinging back and forth the dredge master closely observes/keeps an eye on the tension in the bow wire and the loading of the bucket chain. The tension of the bow wire is controlled with the aid of the stern winch. The amount of soil that is cut per unit of time depends on: • The thickness of the cut. This is the thickness of the layer that can be dredged in one swing. •

The step length; the forward motion of the dredger during one swing.

•

The warping velocity of the dredger along the cut.

To prevent spillage, the cutting production must be less than or equal to the product of the bucket velocity and the bucket capacity. The cutting thickness depends on the total thickness of the layer to be dredged. If this is not too thick, generally less than 5 m, the dredge master will try to dredge it in a single cut. If the layer exceeds 5 m thick the entire breach/bank will be dredged by making several cuts. In any case the first cut must be so thick that the dredger can create sufficient draught for itself. The step length is roughly equal to the length of the links. As rule of thumb, 0.6 to 0.8 times the cube root of the bucket capacity may also be taken. For both cases the swing velocity must be sufficiently high (> 5m/min). The warping velocity selected is such that either the buckets are full with a minimum spillage or that the loading on the bucket chain is the limiting factor. If possible, a width of the cut is selected that is so wide that the total width of the work can be covered in one swing. The wider the cut the fewer the anchor movements. If that is not possible the total width is divided into a number of equal cutting widths. There is also a minimum cutting width for every bucket dredger. The required depth for the dredger and the space for manoeuvring the barges play a role in determining this (Figure 5.2). This is roughly 1.5 times the length of the bucket dredger. The dredging depth also determines the position of the buckets on the ladder and thus for the *filling degree. The available excavation energy of a bucket dredger is highly dependent on the energy needed to carry/lift up the dredged material. This depends on: 1. The nett weight of the bucket contents. Part of this is under water and part is above water. The weight of the buckets themselves plays no role because there is an equal number of buckets under and above the ladder. 2.

The friction resistance in the ladder/guide rollers results from the weight of the buckets and their contents.

3.

The friction resistance in the axles of the tumblers results from the tensile forces of the bucket chain.

4.

The impact loads that develop as a result of the bumping of the buckets.

5.22

The cutting production of the buckets is:

Qs = h ⋅ s ⋅ v

[m³/s]

with: h = cutting thickness usually < 5m s = step length v = swinging velocity

(5.5)

[m] [m] [m/s]

The cutting production must balance with the amount that can be transported by the buckets per unit of time thus: I ⋅E ⋅ v Q Q s = hsv = e v e = e [m3/s] (5.6) 60 ⋅ B B Ie = bucket capacity [m³] ve = bucket velocity ev [buckets/min] Ev = filling degree [-] B = bulking factor [-] [m³/s] Qe = bucket production On the basis of the specific energy concept, the cutting energy for this production is: I ⋅E ⋅v Psnij = Q s ⋅ E sp = e v e ⋅ E sp (5.7) 60 ⋅ B The energy needed to lift sand and water is: I E v Popv = e v e g (ρ e − ρ w )H ow + ρ e + (1 − E v )ρ w ) H bw 60B

[

ρe ρw Ee How Hbw

= = = = =

(

]

(5.8)

density of the soil in the bucket density of water bucket filling lifting height under water lifting height above water

[kg/m³] [kg/m³] [-] [m] [m]

If the friction in the ladder/guide rollers and tumblers is assumed to be a linear function of the weight and the velocity then: Pwr =

Q e ⋅ A( n e , α )v e 60

=

IeE vve v A( n e , α ) e 60B 60

(5.9)

Here ⋅A( n e ,α ) is the influence of the friction force on the ladder/guide rollers and the tumblers. Thus here the influence of the tensions is *taken into account /verdisconteerd.

The total power required is thus:

Ptot = Psnij + Popv + Pwr

Ptot =

Ie E v ve 60B

(5.10)

[

]

ve ⎫ ⎧ ⎨E sp + gB (ρ e − ρ w )H b + ρ e + (1 − E v )ρ w ) H o + BA( n e , α ) ⋅ ⎬ 60 ⎭ ⎩ (5.11)

(

Because the installed power must be higher than the average required power, it must be true that:

5.23

Ptot = Pinst ⋅ w

(5.12)

Here w is the relation between the average and the peak power. The relation between installed power and production is therefore:

[

]

ve ⎫ ⎧ ⎨E sp + gB (ρ e − ρ w )H b + ρ e + (1 − E v )ρ w ) H o + BA( n e , α ) ⋅ ⎬ 60 ⎭ ⎩ (5.13) If the bucket chain is driven by a top tumbler the relation between ω and ve is: Pinst =

IeE v ve 60wB

v e = 5n = 5∗

Mω = M

M=

(

60ω 150ω = 2π π

(5.14)

[

]

veπ Ie E v ve ⎧ ve ⎫ = ⎨E sp B + g (ρ e − ρ w )H b + ρ e + (1 − E v )ρ w ) H o + A( n e , α ) ⋅ ⎬ 150 60B ⎩ 60 ⎭

2.5I e E v πB

[

(

]

ve ⎫ ⎧ ⎨E sp B + g (ρ e − ρ w )H b + ρ e + (1 − E v )ρ w ) H o + A( n e , α ) ⋅ ⎬ 60 ⎭ ⎩

(

(5.15) This is the machine characteristic. When the drive characteristic is known, the bucket velocity and the associated torque are known and thus the production.

5. 16

The filling degree is determined by the equation: 60hvsB Ev = Ie ve

(5.16)

So, for a given step length and cutting thickness the desired warping velocity is also known. As long as Qe>=Qs is valid the spillage during cutting will be limited. The spillage that occurs during the turning of the buckets is an entirely different question. Here factors such as cohesion, adhesion, the shape of the buckets and the position of the fixed chute all play a part.

5.24

Cohesive soil and also fine sands can give great problems on this point. In principle, this is a problem of timing. Although the fixed chute is indeed adjustable, the range over which it is adjustable is closely linked with the dredging depth and the shape of the lower bend. With soil that is not easily loosened the bucket velocity must be reduced, as otherwise there will be too much spillage behind the dredger. Measures are also taken to get rid of the under-pressure, which develop in the buckets when discharging cohesive soils. As with the barge-loading dredger/reclamation dredger, a situation may also arise in which the supply of barges is the limiting factor. This situation may be caused by many different factors, such as: • Weather and wave conditions •

Shipping movement

•

Bridges and locks

•

Differences in the speed of the barges.

•

Differences in the size of the barges.

•

Delays of the barge

•

Delays of the *reclamation dredger/barge unloading dredger

•

Delays at the discharge site

Clearly, with a bucket dredger, there is always a chance that sometimes there will be no barge available. Because the above mentioned delays can be reasonably well estimated with regard to their average values and standard deviations, the Monte Carlo Simulation can provide insight into the probability of delay resulting from the absence of barges. Clearly, when using a barge-loading dredger there is always a chance of delays due to the absence of a barge.

5.25

7. Grab or Clamshell dredger

Figure 7- 1:Large grab in the world (200 m3)

7.1. General The grab dredger is the most common used dredger in the world, especially in North America and the Far East. It is a rather simple and easy to understand stationary dredger with and without propulsion.

±7 ½

m m 0½ ±1

½ ±7

m

Figure 7- 2 Self propelled grab hopper dredge

In the latter the ship has a hold (Figure 7- 2) in which it stores the dredge material, otherwise barges transport the material. The dredgers can be moored by anchors or by poles (spuds)

Figure 7- 3 Grab bucket reclaimer

The most common types are boom type clamshell dredgers with a boom that can swing around a vertical axis. Beside these, but considerably less in number, are the overhead cranes (Figure 7- 3), with the trolleys, like the ones used for the transshipment of bulk goods in ports. The capacity of a grab dredger is expressed in the volume of the grab. Grab sizes varies between less than 1 m3 up to 200 m3.(Figure 7- 1)

25

120

20

100 80

15

60 10

40

5

20

0

Cumulative frequency [%]

Frequency [%]

Frequency grabsizes

0 200

20

14

12

10

8

6

4

2

0

Grabsize [m3]

Figure 7- 4 Figure 7- 4 shows a rough overview of the most common grab sizes.

The opening of the grab is controlled by the closing and hoisting wires or by hydraulic cylinders. To ensure that the grab does not spin during hoisting and lowering many crane are equipped with a tag line, running from half way the boom straight to the grab.

7.2. Working method For clamshell dredgers the method of anchoring and the positioning system plays an important role for the effectiveness of the dredger. The volume to be dredged at a position decreases with the angle from the centerline. (Figure 7- 5). So dredging areas from -90° to +90 ° from the centerline is not always effective. R·sinζ

A eff = R sinς ⋅ L = ς

Step

2π RL ′ 360°

L=S

Effective Area

S

Average width cut

End last cut Effective Width

Top view cut

L'

1

S

End this cut

Cut projection

R 2

R

ζ

R ζ ζ

S

Figure 7- 5 Effective dredging area

In figure 8.5 a top view and a projection of the dredging area area is shown. The width of the dredging area is R sin V and the width of the cut is L, so the surface of the effective dredging area is Aeff = L ×R sin V which equals: Aeff

p = V 2 R L ¢. 90

The mean dredging efficiency as function of the swing angle of the crane being follows from equalization of both equations:

L ¢ sin V 90 . See Fig.8.6 = L V p 2

L¢ L

1.2

Efficiency

1.0 0.8 0.6

L ′ sin ς 360° = ς 2π L

0.4 0.2 0.0 0

20

40

60

80

100

Swing angle [deg]

Figure 7- 6 Swing efficiency

It is important to localize every bite of the grab by means of a positioning system. This helps the dredge master to place the next bit after the foregoing. The dredging process is discontinuously and cyclic. • Lowering of the grab to the bottom • Closing of the grab by pulling the hoisting wire • Hoisting starts when the bucket is complete closed • Swinging to the barge or hopper • Lowering the filled bucket into the barge or hopper • Opening the bucket by releasing the closing wire. Releasing the aft wires and pulling the fore wires does the movement of the pontoon. When the dredgers have spud poles, this movement is done by a spud operation, which is more accurate than executed by wires. The principle of this hoisting operation is given in the Figure 7- 7 below. For a good crane-working behavior the cable cranes have two motors: • The hoisting motor, which drives the hoisting winch and • The closing motor, which controls the closing and the opening the grab. In order to avoid spinning of the clamshell a so-called tag wire is connected to the clamshell.

Hoist winch Top shieves

Hoist wires

Gear segments

Upper sheave block

Closing winch Closing wires

Bucket

Gear segments

Lower sheave block

Figure 7- 7 Hoisting system of cable cranes

The crane-working behavior is than as follows: no. 1 2 3 4 5 6 7 8

Cycle part ease dig hoist swing ease dump hoist swing

position yaws open closing closed closed closed opening open open

Hoisting winch eases hoists hoists rest eases eases hoists rest

Closing winch eases hoists hoists rest eases rest hoists rest

7.3. Area of application The large grab dredgers are used for bulk dredging. While the smaller ones are mostly used for special jobs, such as: • Difficult accessible places in harbors • Small quantities with strongly varying depth. • Along quay walls where the soil is spoiled by wires and debris • Borrowing sand and gravel in deep pits • Sand and gravel mining • Dredging in moraine areas where big stones can be expected. . The production of a grab depends strongly on the soil. Suitable materials are soft clay, sand and gravel. Though, boulder clay is dredged as well by this type of dredger. In soft soils light big grabs are used while in more cohesive soils heavy small grabs are favorable. The dredging depth depends only on the length of the wire on the winches. However the accuracy decreases with depth. For mining of minerals dredging depths can reach more than 100 m.

7.4. Important design aspects 7.4.1. Type of grabs

Figure 7- 8 The clamshell

The clamshell most common and is used in silty, clayey and sandy materials. In mud the yaws in general have flat plates without teeth. In sand, clay and gravel, the yaws are fitted with in each other grabbing teeth. The two halves, shells, rotate around a hinge in the lower sheave block and are connected with the upper sheave block by rods. The closure/hoist cable is reefed several times between the head and the disc block to generate enough closing force. In mud the yaws in general have flat plates without teeth. In sand, clay and gravel, the yaws are fitted with in each other grabbing teeth. For the removal of contaminated soil closed clamshells are used to avoid spillage.

Figure 7- 9 Orange peel grab

Figure 7- 10 Cactus grab

The orange peel grab (Figure 7- 9) is often used for the removal of large irregular pieces of rock and other irregular pieces. This type of grab has 8 yaws that in general do not close very well.

The cactus bucket (Figure 7- 10) is used in the occurrence of both coarse and fine material at the same time. This grab has 3 or 4 yaws that close well in the closed position and form a proper bucket. The size of the bucket depends on the required production capacity of the crane. 7.4.2. Size and weight of the clamshell The size of the grab depends on the capacity of the crane. The construction weight is determined, besides by the size also by the required strength and therefore by the type of soil to be dredged. So a grab suitable for the dredging of silt will be relatively large in volume and light in weight, while for the dredging of heavy clay or rocks a relative small but heavy bucket will be used. However, because the hoist force remains constant, with increasing weight of the grab the load weight must decrease. For this reason the efficiency of the grab is expressed as:

η=

paying load in tons paying load + grab weight

Research done in Japan has found the following relation between the ratio of the B .Figure 7- 11 2M bucket

mass of the material in and the mass of the bucket: K g = L With • B = Width of grab • Mbucket = mass of grab • L = length of fully opened grab • Mf = mass of grab fill

[m] [kg] [m] [kg]

2 san d Sand

G rav e

Mf Mg

l

1 G ra vel C lay

0 0

0.02

0.04

0.06

0.08

Kg

Figure 7- 11 Fill mass and bucket mass ratio

7.4.3.

Main winch drive The winch drive systems are mainly electric (direct current or thyristor-controlled d-c motor connect to the 3 phase board net system) and has the 4 quadrants system. (Figure 7- 12)

Speed

T

T

T

2 1 34

n

+

Torque

T

Figure 7- 12 Four quadrants system

7.5. Main Layout for pontoon type floating dredgers Non self-propelled grab dredgers consist of simple pontoons on which the crane is positioned. The deck is heavy reinforced no only for foundation of the crane but also where heavy loads can be expected, in particular where the grabs are stored. Winces for the movement of the pontoon are placed on deck as well as the accommodation for the crew when necessary In many cases a standard crane is placed on the pontoon. The boom of the crane is movable with a simple wire system. During dredging the boom is kept in a fixed position as much as possible. This avoids the need for a horizontal load path. The length of the pontoon is in many cases longer than necessary in order to keep barges along side. The poisoning of the pontoon is either by anchors (4 to 6) or by 2 or 3 spud poles.(Figure 7- 13) In the last case 2 fixed spuds are situated at on the sides of the pontoon and one walking spud aft.

Figure 7- 13 Plan view of Grab crane Eendracht, BOSKALIS

An idea about the lightweight in relation to grab size is given in (Figure 7- 14) and is in the order of 100 times the grabsize. 12.00

Grab size [m3]

10.00 8.00 6.00 4.00 2.00 0.00 0

200

400

600

800

1000

Ligth Weigth [t]

Figure 7- 14 Light weight of grab dredge pontoons

1200

1400

The lightweight of the pontoon is low compared to that of the other dredgers. The relation between light weight and pontoon volume is shown in Figure 7- 1

2500

Ligth weigth [t]

2000 1500

y = 0.3259x 2

R = 0.6118

1000 500 0 0

500

1000

1500

2000

2500

3000

3500

BLD [m3]

Figure 7- 15 Pontoon volume

The L/B and B/T ratios f the pontoons are respectively between 2 and 3 and 4 to 6. ()

L/B ; B/T

L/B

B/T

9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0

500

1000

1500

2000

2500

Ligth Weigth [t]

Figure 7- 16 Pontoon numbers

Special attention needs the stability of the dredge because of the varying and eccentrics loads. Free fluid levels should be avoided. 7.6. The theory of excavation The most interesting part of the dredging process takes place during the digging in the soil of the closing grab.

When the grab falls on the soil the yaws penetrate vertically into the soil. This is called the initial penetration. If the closing cable is pulled up, the lower sheave block and the upper sheave block are pulled together and as a result the grab closes. During this process the hoisting cable is kept slack to allow the grab penetrate deeper into the soil. In very soft soil, like silt or soft clay, the hoisting cable is kept tight to prevent a too large penetration. The movement and the accompanying forces are described in the proceedings of the Wodcon 1992 in India of Steven Becker: The Closing Process of Clamshell Dredgers in Water-Saturated Sand. The calculation of the path of the grab and the occurring forces is done by solving the equations of motion with the aid of the cutting theory for sand and/or clay. The friction forces on the sides of the yaws must however be taken into account. During the excavation the cutting edge follows a certain path through the soil (the digging curve) due to the weight of the grab. During this movement the lower sheave block moves upward and the upper sheave block downward (see also figure 7.8). If the cable grab is outlined as shown in figure 7.9, than the closing curve of the grab, not being the digging curve, can be determined as function of the opening angle λ.

Figure 7- 17 Geometry of cable clamshell Engels!!! In the above shown scheme (Figure 7- 17) can be distinguished:

xc = d + b sin λ = e + l sin φ − a sin (γ + λ ) yc = g + b cos λ = l cos φ − a cos(γ + λ )

and

xa = xc + b sin λ ya = yc + b cos λ Furthermore, xc = e , with which the relation between λ and φ can be determined:

xc = d = e + l sin φ − a sin (λ + γ ) , so that xa and ya can be calculated as function of λ.

The closing curve shows the path of the grab yaws compared to the grab head.() Yc [m]

18 16 14 12 10 8 6 4 2 0

4 3.8 3.6 3.4 3.2 3 2.8 2.6 2.4 2.2 2 0

10

20

30

40

50

60

70

Yc [m]

Fie [graden]

fie [grad]

80

Lambda [graden]

Figure 7- 18 Rotation and vertical movement of the grab and rod

Closing curve Xa [m]

Ya [m]

0

0.2

0.4

0.6

3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3

Figure 7- 19 Closing curve

0.8

1

Xa [m]

D

0

1

1.5

2

2.5

3

3.5

0

0

1

1

2

2 C

3

3

Ya [m]

Such a curve is fully dependent of the dimensions of the rods and the grab. For a good working of the grab the underside of the grab, the line AE, may not cross the path of the closing curve. In the below figure also the path of the point E is shown, while in the movement of the grab is shown when closed, half closed and open. Also the closing curve is drawn. Typical for cable grabs is that, when they are hanging in the hoist cable, the cutting edge first moves downwards, followed by the upward movement caused by of the movement of the lower sheave block

0.5

Y_closed Y_open Y_halfway wire_curve

B

4

4 E

5

5

A

6

6

7

7

Figure 7- 20 comment: wire curve is closing curve

Closing curve for point A and E X [m]

Y [m]

0 3 3.2 3.4 3.6 3.8 4 4.2 4.4

0.2

0.4 Y_A

0.6

0.8

1

1.2

1.4

Y_E

Figure 7- 21 Closing curve for the points A and E

. For grabs where the rods are replaced with hydraulic cylinders the closing curve is like the one in the below picture.(Figure 7- 22) Closing curve for hydraulic clamshell Xa [m]

Ya [m]

0

0.2

0.4

0.6

0.8

1

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

Figure 7- 22

The cutting edge of the grab continues a downward motion when the grab hangs in the hoist cable. The closing curve is very important when dredging contaminated soils. Storage of these soils is very expensive, so digging uncontaminated soils have to be avoided as much as possible. There for grab dredgers dredging these soil types use horizontal closing grabs as shown in

Figure 7- 23Horizontal closing hydraulic grab (Boskalis)

For production purposes the determination of the closing curve is insufficient. For this the excavating of digging curve is necessary. This is determined by calculating the forces on the grab yaws and the disc blocks at every moment of the closing process. Therefore the following forces have to be known, both in magnitude and size. • • • •

the weight of the grab parts the cutting forces of the material to be dredged the forces needed to transport the soil backwards in the grab-shells the inertia forces of the grab shells

ad 1. The determination of the weights and the inertia forces of the different grab parts will not give many problems. ad 2. For the determination of the cutting forces the linear cutting theories can be used, with the remark that the cutting angles change during the closing. By dividing the excavating process in a large number of discrete steps these theories can be used well. ad 3. This problem is the most difficult one. As long as the soil is shoved backwards in the grab shell, the forces can be calculated with the passive soil theory, dependent on the way the shear planes run. If however, except for backwards, the soil is also pushed above than this happens from two sides. As far as is known there has not been any research of the than acting deformations and therefore there has not been developed (yet) a theory. For the calculation of the excavation curve can be referred to the earlier mentioned article of Steven Becker that is realized in cooperation with the section dredging technology. The calculation of the excavation curve is as follows: Due to the weight and the drop speed the grab will penetrate initially into the soil. To calculate this penetration the empirical formula of Gebhart can be used:

Fc = 0.14 ⋅ e 0.0019 d m ⋅ K f ⋅1.26( ρ s −1) + 0.21⋅10 −3 e(0.0175 d m ) ⋅ (B − 900) + 1.21 ⋅10 −3 ⋅ e(0.0145 d m ) ⋅ (h − 300)

in which: dm = Kf = B = h = ρs =

average grain diameter the grain shape factor the width of the grab mouth the initial penetration the situ density of material to be dredged

calculate the position, velocity, and acceleration of the grab. determine the shear planes. determine the passive earth pressures. determine the horizontal and the vertical cutting forces. calculate the acceleration of the grab from the equilibrium of the forces. The remaining part of the dredging process like hoisting, opening and easing are totally determined by the winch characteristics and the swing velocity of the crane.

7.7. The production capacity 7.7.1. Influence of hoisting power The production capacity of a grab dredge crane depends strongly on the size and the weight of the grab as shown in the previous section. When however, it is assumed that the weight of the grab is not decisive during the closing process, than the specific energy concept can be used for the calculation of the required closing energy. When the grab volume is equal to Vg, with an average efficiency of w, the closing time is Tc, the average fill rate of the grab is F and the specific energy of the soil equals SPE, than the required closing power equals:

Pc =

Vg ⋅ SPE F ⋅ w ⋅ Tc

= Fc ⋅ vc

With Fc and vc the closing force and the closing speed of the closing wire. The closing force Fc should be smaller than the closing force that can be delivered by the closing winch. 7.7.2.

Influence of the soil type The to be dredged soil type determines for a required production capacity, the required excavation energy and therefore grab weight in relation to the grab volume, the necessary closing force and the hoist force. As mentioned before the application area of grab dredge cranes lies mainly in the non-cohesive soils and soft clays. Nevertheless boulder clay is sometimes also dredged, although with low productions.

7.7.3.

The dredging depth

200 180 160 140 120 100 80 60 40 20 0 0

50

100

150

900 800 700 600 500 400 300 200 100 0 200

Grab T1 =volume 40sec. 10 m^3 Hoisting speed 100 m/min. Production [m3/hr]

Hosting time [s]

The maximal dredge depth For a certain production capacity the grab volume has to increase with increasing depth, since the total cycle time increases. If this is not the case the production will decrease hyperbolically.

H_time Production

Dredging depth [m]

Figure 7- 24 Influence of dredging depth on hoisting time

Figure 7- 24 is an example of the production decrease as a result of increasing dredging depth of a 10 m3 grab with a hoist velocity of 100 m/min and a nonhoist time of 40 seconds.

The maximum dredge depth also determines of course the size of the winch drums for the hoist and closing cables of the grab. The minimal dredge depth The minimal dredge depth is determined by the required draught of the pontoon and the related keel clearance. However it could well be that the minimal dredging depth is not determined by the pontoon, but by the barges that transport the dredged material. 7.7.4.

The discharge of the material The dredged material is usually transported with barges. But as already mentioned in the general considerations there are dredge cranes placed on selfpropelled hoppers, so the material is transported by the dredger. For grab dredge cranes that are used for the winning of sand and gravel, the discharge of the material to the separator installation is done with conveyor belts.

8.

The backhoe or Dipper dredger

Figure 8. 1 BHD IJZEREN HEIN,

8.

8.1. 8.2. 8.3. 8.4. 8.5. 8.6.

The backhoe or Dipper dredger ..............................................................................1 General considerations .......................................................................................1 Working method ................................................................................................2 Area of application .............................................................................................4 Main Layout.......................................................................................................5 Production capacity ............................................................................................9 Cylinder forces................................................................................................. 10

8.1. General considerations A backhoe dredge is a stationary tool, anchored by three spuds: two fixed spuds at the front (starboard and portside) and a moveable spud at the back of the pontoon (see Figure 8. 1and Figure 8. 10). Hydraulic dredgers are available in two models, the backhoe (Figure 8. 1) and the dipper or front shovel (Figure 8. 1 ). The first is used most. The difference between those two is the working method. The backhoe pulls the bucket to the dredger, while the front shovel pushes. The last method is only used when the water depth is insufficient for the pontoon.

Figure 8. 2 Backhoe dredger

Figure 8. 3 front shovel or dipper dredger

Due to the anchoring by spud poles and the fixed boom and stick the dredging depth is limited (maximum 25 m). Some of these type of dredgers are self propelled. In 1999, the biggest Backhoe dredger in the world was delivered by Shipyard "De Donge" to "Great Lakes Dredge & Dock Co". This dredger is equipped with a Liebherr P996 excavator and can dredge with a 13 m3 bucket till an approx. 17 m. depth. The dredge can however dredge till a maximum depth of 30 m. in case the boom / stick configuration is changed. The maximum penetration/ breakout capacity is 170 tons! The weight of the excavator 470 tons!

8.2. Working method

Figure 8. 4 Cylinders on boom and stick

During dredging the pontoon is lifted partly out of the water to create sufficient anchoring. Besides that the dredger is in that case less sensible for waves. The bucket is positioned and excavates the soil by means hydraulic cylinders on the boom and stick (Figure 8. 4). The effective dredging area depends on the swing angle and the forward step per pontoon position, which on his turn depends on the length of the boom and stick. On the mooring side for the barges the swing angle is restricted. Swinging over the other side is mostly restricted 60° Larger angles are less effective (Figure 8. 5). The method is the same as for cable cranes.

R·sinζ

A eff = R sinς ⋅ L = ς

Step

2π RL ′ 360°

L=S

Effective Area

S

Average width cut

End last cut

End this cut

Top view cut

L'

1

S

Effective Width

Cut projection

R 2

R

ζ

R ζ ζ

S

Figure 8. 5 Effective dredging area

The forward step per pontoon positions can be sub-divided in bucket forward positions (Step) and bucket swing positions (width) (Figure 8. 1). A small step results in a large width and a large step in a small width to fill the bucket, however the total volume is almost the same.

Volume V

Width

W

Step

D

Figure 8. 6 bucket forward (step) & bucket swing (width) positions

Due to the radius of the boom and arm the cut width is limited to 10 to 30 m, see (Figure 8. 7). The dredge has sometimes more than one boom and/or sticks. A shorter boom and / or stick result in higher excavating forces.

19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Figure 8. 7 The reach of the dredger for different booms & sticks 1.

8.3. Area of application Backhoes are used in soil types like firm clay, soft rock, blasted rock and when large stones can be expected, like the removal waterside protections. The length of the stick and the boom determines the dredging depth. Some backhoes have more than one bucket to be able to dredge well at several depths. The lack of anchorage cables limits the hindrance for other ships and there is also no delay for anchorage. Hydraulic backhoes are especially suitable for accurate dredge work, due to the construction of the stick and the boom. In general this dredge tool cannot be used under offshore conditions, due to the limited pontoon width. Since there are several ways of defining the volume of the buckets one has to be aware when ordering one. The definitions are (Figure 8. 8): • struck capacity (water volume): this is the amount of water that the bucket can hold at maximum when the upper bucket rim is held horizontal. • •

heaped capacity (SAE volume (SAE = Society of Automotive Engineers)): in this an extra amount of soil with embankment slopes of 1:1 is calculated in. heaped capacity CECE volume (CECE = Committee of European Construction Equipment): same as above but with embankment slopes of 1:2.

afstrijkhoogte

“Water” Capacity

1

1

1

1 1

1

1

1

SAE Capacity

1

2

2

1

1

2

2

1

CECE Capacity Figure 8. 8 Different capacities

Mainly the type of soil determines the filling degree of the bucket. In soft and sticky soils the bucket is heaped, while in rock due to the shape of the boulders only a part of the bucket is filled. Besides, the bulking (volume increase) from the soil plays a role too. Soil type Soft clay Hard clay Sand & Gravel Rock; well blasted Rock, unblasted

Filling degree 1.5 1.1 1 0.7 0.5

Bulking factor 1.1 1.3 1.05 1.5 1.7

8.4. Main Layout The crane is positioned on the front side of the pontoon on the “turning table”, which situated just above water level. This part is a compromise between the required freeboard and the maximum available excavating force. The required reaction forces for excavations are delivered by the spud-poles. The crane on the turning table is mostly from a well-known brand (Demag, Liebherr, O&K Poclain, etc.), which can be delivered in modules (Figure 8. 1). The boom and stick are constructed more heavy duty than those for land operations. Marine operations results in higher and more dynamic loads due to deep excavation depths. Bucket sizes vary from several cubic meters to 20 m3. The spud are provided with a hoisting system to hoist the spuds from the sea bed as well as to lift the pontoon partly out of the water to increase the transfer of the reaction forces to the Figure 8. 9 Shovel modules soil The aft spud is either placed in a carriage (Figure 8. 10) or is executed as a walking spud (Figure 8. 11).

The engine room and the accommodation is place at the stern. .

Figure 8. 10 General plan BHD IJZEREN HEIN

The backhoe dredge IJzeren Hein is equipped with a Liebherr P 984 crane and is build under the classification of Burea Veritas I 3/3 (-) ✠ Pontoon NP/Deep Sea.

Figure 8. 11 Plan view BHD ROCKY, Owner BOSKALIS

The BHD Rocky, one of the most powerful backhoes, is provided with a DEMAG H 286S excavator with 1230 kW and can be equipped with bucket varying in size between3 and 16 m3. She has a dredging depth of 25 m. The aft spud is executed as a walking spud. Data from existing backhoe dredgers shows that there is hardly a relation between bucket size and installed diesel power as well as between diesel power and lightweight (Figure 8. 12 and Figure 8. 13). 1200.00

1800 1600 1400

800.00

Light weight [t]

Installed power [kW]

1000.00

600.00 400.00

1200 1000 800 600 400

200.00

200 0.00 0.00

5.00

10.00

15.00

20.00

Bucket size [m3]

Figure 8. 12Relation bucket capacity versus installed diesel power

0 0

250

500

750

1,000

1,250


Figure 8. 13 Relation bucket installed diesel power versus light weight of the pontoon

Lightweight of the pontoon is some what related to the total power installed (, while lightweight is roughly 47 % of the pontoon volume (Figure 8. 16 and Figure 8. 17).

Data from excavator suppliers shows a better relation. Liebherr Excavators

30 2

y = -7E-06x + 0.0494x + 1.5486

2500

2

R = 0.9778

20

2000 Power [kW]

Bucket size [m3]

25

15 10 5

1500 y = 4.4679x R2 = 0.9936

1000 500

0

0

0

100

200

300

400

500

600

0

100

200

Crane weight [ton]

300

400

500

600

Crane weight [tons]

Figure 8. 14

Figure 8. 15

Data from Liebherr Excavators With Figure 8. 14 and Figure 8. 15 a better estimate of the installed power is possible then from Figure 8. 12. 1800 1600

y = 0.4713x

Light weight [t]

1400

2

R = 0.6122

1200 1000 800 600 400 200 0 0

500

1000

1500

2000

2500

3000

LBD [m3]

Figure 8. 16 Pontoon volume versus lightweight

Length-width ratio and width-draught ratio are almost the same as for the pontoons of the grab dredgers (Figure 8. 17).

L/B & B/t

L/B

B/T

9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0

200

400

600

800

1,000

1,200

1,400

1,600

Light weight [t]

Figure 8. 17 Lightweight versus pontoon dimensions.

1,800

8.5. Production capacity When dredging soft soils (free running sand, silt and soft clay) the volume per bite of the bucket is determined by the bucket capacity. For harder materials the cylinder forces can be the decisive factor. If the cylinder force is Fc and the cutting speed vc and the specific energy of the soil is SPE then:

Fc v c ⋅ SPE = Qs =

Vbucket d layer ⋅ step ⋅ Wbucket = t digging t digging

With: Qs Vbucket Tdigging dlayer Step Wbucket

Production Bucket capacity Excavating time Thickness layer Step size Width of bucket

m3/s m3 s m m m

The cutting speed can be calculated either by rotating the bucket or the stick. Cycle times of the bucket depends on the dredging depth and soil type, but are in the order between 20 and 40 seconds. The cycle consists of: • Digging • Lifting and swinging • Dumping • Swinging and lowering • Positioning. The step procedure takes more time, 5 to 10 minutes. No. 1 2 3 4 5 6 7 8 9

STEP PROCEDURE FOR BACKHOE DREDGERS Spud carriage Walking spud Lower pontoon into the floating position Lower pontoon into the floating position Put the bucket into the soil Put the bucket into the soil Lift front spuds Lift front spuds Move pontoon one step forward by Move pontoon one step forward by tilting moving the carriage and the stick. the walking spud and pulling the stick. Set front spud into the soil Set front spud into the soil Lift walking spud Lift movable spud Tilt waling spud back into its middle Move carriage one step forwards position Lower walking spud Set the movable spud into the soil Lift pontoon in working position Lift pontoon in working position

Points 6, 7 and 8 for the spud carriage system are only necessary when the stroke of the cylinder to move the carriage is used.

8.6. Cylinder forces The cutting forces are calculated either by the specific energy concept or by the cutting theories for sand, clay or rock. The cutting theories give the normal forces too, however for sharp knives or teeth only. For design purposes the average normal forces (between sharp and blunt cutting tools) are assumed to be a ratio of the cutting forces. For sand and clay Fcutting/Fnormal =10 and for rock Fcutting/Fnormal =2 If the ratio is known, the cylinder forces can be calculated by taking the moments around the suspension points. The cylinder force to move the boom follows from the equation (Figure 8. 18):

Fcylinder =

Fc ⋅ d + Fp ⋅ l − Wboom zboom − Wstick zstick − Wbucket zbucket a

l a zboom Wboom

zstick Wstick zbucket Wbucket

Figure 8. 18 Forces on the boom and stick

Finally, the moments and shear forces can be calculated in the boom and stick to depend the dimensions of the boom and stick under dynamic conditions.

Design of Dredging Equipment(TUDelft)

Recommend Documents