dock engg

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DOCK ENGINEERING

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DOCK

Docks are enclosed areas for berthing ships, to keep them

afloat at a uniform level, to facilitate loading and unloading

for cargo and passengers.Classification of docks

1. Wet docks

2. Dry docks

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Wet docks (harbour docks)

Docks which are used for berthing of vessels to facilitate loading andunloading of passengers and cargo

Dry docksDocks which are used for repairs of vessels

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Classification of wet docks Wet dock in tidal basins

Harbours are prone to be affected by the tides, which may cause changes in the waterlevel

Thus in ports on the sea coast protected by the outlying breakwater, basins areformed within its shelter. In these basins, pier walls are protected by an right angles tothe shore alongside which vessels can lie and discharge their cargos

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Wet docks enclosed or impounded basinsDocks are enclosed and shut off by entrances by locks to maintain a fairly

uniform level of water The docks are formed by enclosures where the tidal ranges are very markedand large

The water level in the enclosures shd be maintained at constant level byproviding locks and gates

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Dock entrances Vessels can enter docks either directly or through locks. In either case,

gates are provided for dock entrances Types of gates

Wooden or iron gatesCaissons

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Caissons Two kinds of caissons

Sliding caissonIt consists of a box shaped steel structure stiffened internally with properbracing. It is provided with steel keels sliding on smooth granite floor.Instead of the keels, the caissons could be moved on rollers and rails. Theentrance is opened by hauling the caisson into recess provided in the sideof the dock. The caisson also serves as a bridge across the dock entrance.

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Ship caisson

It resembles the outline of a ship in c/s and is constructed of steel

with stiffeners at proper intervals. It is floated into position and sunk

into specially prepared grooves in the dock sides and sill. The sinking

and raising of the caisson is done by ballasting and unballasting

respectively. This type does not require any gate recess or machinery

for moving.

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Classification of repairing facilitiesRepairing facilities in fixed form

Graving dry dockMarine railway dry dock or slipwayLift docks

Repairing facilities in movable formFloating docksDepositing docks

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Floating dry dockIt may be defined as a floating vessel which can lift a ship out of water andretain it above water by means of its own buoyancyIt is a hallow structure of steel or reinforced cement concrete consisting of2 side walls and a floor, with the ends open.

To receive a ship, the structure is sunk to required depth by ballasting itsinterior chambers with water, the ship in then floated into position and

berthed. The dock is raised bodily with the berthed ship by unballasting thechambers by pumping out the water

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Types of floating docksRigid type floating docks

In this type the side walls are rigidly fixed to the pontoon or bottom section The floor portion is divided into a number of chambers, so as to assist in canting thedock if necessary to berth damaged ships, by partial unballasting of the chambers

Self docking type floating docks This type is divided into sections longitudinally, and one of which is capable of beinglifted and docked on the remainder of the dock for the purpose of cleaning, paintingor repairing

Self docking offshore type floating docks This type has no side wall on water side and has an L shaped cross section. The side walls is connected to the shore by hinged parallel booms capable of liftingor lowering the dock.

The ship to be docked, could be brought on to the dock from either end or sideways.

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Slipways/ marine railway/ slip dockIt is an inclined railway extending from the shore well into water as well asthe foreshore, to enable a ship to be drawn up clear out of the water.

The essential parts areCradle- which moves up and down on inclined track

Track- supported on an unyielding and firm foundationCradle or platform is constructed of steel and provided with keel and bilge blocks to

receive the ship. The cradle is mounted on a system of rollers which move on iron tracks carried by

longitudinal timbers, supported on cross ties or beams bearing on piles or other firmfoundations

Strong cables are attached to the shore end of the cradle to haul the cradle.Hauling is operated by strong mechanical winches built on the shore

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The way consists of heavy rails secured to longitudinal sleeperssupported on cross ties are laid at an inclination varying from 1 in 15to 1 in 20.a locking device for safety pawls under the cradle is placed in thecentre of the ways, to keep the cradle from slipping down when thehauling cable breaks.

For dry docking, the cradle is moved down into deep water and the shipto be docked is towered over the cradle and positioned to rest on keeland bilge blocks set on floor and moored to the towers on either sideof cradle. The cradle slowly emerges above the high water level,

when hauled up the ways, permitting the ship to come to rest, on the

cradle floor as the cable reaches the normal docking position.

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DredgingDraft for a ship is the vertical linear immersion of ship floating in

stable condition safely. Navigable depth of water is draft added with vertical clearance at bottom of ship. If the navigable depth requiredfor ship is greater than depth of water available at site ship cannotenter the harbour. So inorder to provide an access for sea traffic thebasin characteristics should be modified, ie excavation of bed below

water. This known as dredging.

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TYPES OF DREDGING DEVICES

Dipper dredgeGrapple dredgeContinuous bucket elevator/ ladder dredgeHydraulic or suction dredge

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Dipper dredge

It consists of floating vessel strongly constructed,carrying an inclined frame A in the bow to hold theboom B by guy wires.

Through the middle of the boom runs a dipper stick, worked by a rack and pinion arrangement and to theend of which is rigidly attached the dipper bucket K,

with a flap A hoist cable is fixed to the bucket, to move it up ordown

The vessel is fixed to the bed in position by means ofthree stakes during dredging operations.

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Operation The hoist cable is released, to enable the bucket to reachthe bed .By applying a continuous force on the stick through therack and pinion arrangement helps the teeth of the bucketto bite into soil.

The hoist cable is slowly pulled resulting in a cut beingmade in the bed.

The hoisting is continued and the bucket is hauled out of water. The boom is swung round to deposit the material in bucketin a predetermined place by opening flap

The boom is swung back and the dipper is lowered inpreparation of new cut

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AdvantagesEasy maneuverability

Suitable in confined spaces around docks andnarrow channels

Very powerful and capable of excavating in hard soil,

for removal of boulders, and breaking of heavyobjects like old piles, cribs etc.

This can dig in depths upto 15 m of waterCapacity varies from 1 to 4 m3

Can excavate soil and deposit on either on bank or onfloating barge

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Grabble dredgeConsists of a hull, to the front of which are fixed a

frame and a boom B. The frame is guyed back, by back legs. A grab bucket K is suspended by two cable lines HHcalled the opening and closing lines

The boom is fixed at the required elevation by meansof a guy wire and arranged to rotate through ahorizontal angle on a pivot at the lower end.

The bucket hoisting lines pass over two main sheavesfixed to the upper end of the boom

The dredge is moored by the help of stakes fixed foreand aft.

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Operation After positioning the dredge, a scow is brought alongsideand tied fast to it, on right side.

The bucket is lowered to the bottom, where it bites thebed and fills itself.

Tension is put on the closing cable, which closes thebucket, hoist the load and swings the boom horizontallyto the right.

When the boom reaches over to the scow, the tensions inthe cables are reversed, results in opening and discharging

of bucket. The boom swings back to the digging position and theabove procedure is repeated

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Advantages

Very efficient and suitable for dredging materials likesand, clay or mud.Not good for hard digging

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Continuous bucket elevator or ladder dredge

Consists of an endless chain of buckets, mountedand running round a ladder, formed in the middle ofthe bow of a floating vessel.

The ladder could be lowered or raised by the line N. The chain of the bucket is operated manually ormechanically by a big wheel E.

The buckets are provided with pronged cutting edges

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OperationEach bucket cuts and brings up material to the top of

the ladder as the chain moves round, where eachbucket inverts on descent and discharges its contentsinto special holes H H.

The vessel moves forward on completion of work ateach section. (size of vessel- 60 m length; 15 m width:2.5 m to 3m draft)Used for digging depths of 9 to 12 m.

Capacity 6000 to 7650 m3per day

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Advantages

Suitable for handling - Coarse gravel or sand, hardclays and even soft or broken stone, at fairly gooddepths

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Hydraulic or suction dredgeConsists of a suction pipe, carrying at lower end a

cutter of some sort and having a universal joint at thetop.

The pipe is supported on a ladder and held in positionby an A frame mounted on the bow of the dredging

vessel. The suction pipe is connected to a centrifugal pumplocated amidships having a long flexible delivery pipe

discharging into hopper barges alongside the vesselitself or on to specially selected spot on shore,needing reclaiming or filling

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Operation The hull V carries the suction pipe on a ladder in the

bow and the pumping machinery P is housed in themiddle of the ship with its delivery pipe D discharginginto hopper barges floating behind the dredging vessel.

The cutter is suspended from a frame in front and the vessels is moored by anchors. The cutter rotates, cuts and loosens the soil for quick andeasy suction by the pump.

The cut up material is well churned in the operation andmixed with water to facilitate suctionSize : Length - 45 m to 140m

Width - 9 to 30 mDigging depth 4.5 to 15 m

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Advantages Very effective in the beds of sand, silt, mud and clay

in open water and is an excellent machine to clearsand bars.

The gravels and soft rocks are easily reduced by thecutter.

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