DREDGINGDREDGING

Definition:

• Dredging is an excavation activity or operation usually carried out at least partly underwater, in shallow seas or fresh water areas with the purpose of gathering up bottom sediments and disposing of them at a different location.different location.

• In all situations the operation is undertaken by special floating plant, known as a dredger.

Dredger:

• 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 • The quantity of soil moved per unit of time is called Production.

• 4 phases are: - Excavation - Lifting- Transportation-Disposal

Why dredging?

• Capital works:Creation of new facilities, such as harbour

basins,deeper navigation channels etc

Navigation Navigation

Infrastructure Infrastructure

coastal engineeringcoastal engineering

mining industry mining industry

offshore industryoffshore industry

• Maintenance works : Maintain design depth of navigation

channel and ports.

• Remedial works: Careful removal of contaminated

material ,reuse or relocation of the material

Types Of Dredgers:`• Mechanical Dredger

• Hydraulic Dredger

Hydraulically Mechanically

continuously Transport Via pipeline Transport via conveyor belt

Discontinuously Transport via grab,ship,truck

Mechanical Dredger:

Mechanical Dredger:

In Mechanical dredgers materials conveyed by mechanical contravenes.

• Bucket Ladder Dredger

• Grab Dredger• Grab Dredger

• Dipper/Back Hoe Dredger

Bucket Ladder Dredger:

Bucket Ladder Dredger:

Working method :

• The bucket ladder dredge is positioned on 6 wires. Under working conditions the dredge swings around the 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 side-line 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 soil condition, the layer thickness cut.

• This type of dredge is suitable for excavating soft ground, sand or gravel.sand or gravel.

Specification:

• The size of these vessel is about 60m in lengthand 15m in width, having a draft of about 2.5 to 3m.

• The capacity of bucket is about 1cubic meter.

• The digging depth commonly extend to 1 cubic metre

• The output is the range of 250cubic meter/hr

Grab Dredger

Working method:

• 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.• 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.

• 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.

• The positioning is important to localize the bit of the grab.

Specification:

• The Hull is generally 30m to 60m long and 15m cross, the boom is being 15m long.

• 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.

Dipper or Black Hoe Dredger:

Dipper or Back Hoe Dredger:

• This is similar to the land operator crawler shovel.

• It mounted on a Barge.

• It carries a lever outer end of which, a steel bucket is attached.bucket is attached.

• The lever can rotate both about the vertical and horizontal axis.

•The bottom of bucket can be opened by operating wire rope from the barge.

• The bucket scoops out the material and after slewing, the contents of the bucket are dropped on to the barge by opening bottom plate of the bucket.

Specification:Specification:

• This type of dredge can dig in depths up to 15m of water.

• Dipper capacity varies from 1 to 4 cubic meter and heavy capacities of about 12m3 are used in panama canal.

• Profile or plain suction dredger

Hydraulic Dredger:

Hydraulic dredgers use hydraulic pumps to produce a slurry of the dredged material.

• Profile or plain suction dredger

• Cutter suction dredger

• Trailing suction hopper dredger

Profile or plain suction dredger:

Plain Suction Dredger:

• Dredgers are equipped with a centrifugal pump for raising mixture of soil and water to deliver the material into the transport system.

• There is a plain end suction pipe, the suction • There is a plain end suction pipe, the suction head buried to material to be excavated.

• These type of dredgers are well suited for free running granular material.

Working method:

• These operate by sucking through a long tube, like some vaccum cleaners but on a larger scale.

• A plain suction dredger has no tool at the end • A plain suction dredger has no tool at the end of the suction pipe to disturb the material. This is often the most commonly used form of dredging

Specification:

• The Dredging depth is about 15m.

• Bucket capacity is about 6cubic meter, then its used for soft material.for soft material.

• If a smaller capacity bucket of about 4 cubic meter, then its used for dredging hard material.

Cutter Suction Dredger:

• A cutter-suction dredger's (CSD) suction tube has a cutting mechanism at the suction inlet.

• The cutting mechanism loosens the bed material and transports it to the suction

Cutter suction Dredger:

.

material and transports it to the suction mouth.

• The dredged material is usually sucked up by a wear-resistant centrifugal pump and discharged either through a pipe line or to a barge.

• Cutter-suction dredgers are most often used in geological areas consisting of hard surface materials (for example gravel deposits or surface bedrock) where a standard suction dredger would be

Cutter suction Dredger (cont…)

.

standard suction dredger would be ineffective.

• In recent years, dredgers with more powerful cutters have been built in order to excavate harder rock without the need for blasting.

Trailing suction Hopper Dredger:

• A trailing suction hopper dredger (TSHD) trails its suction pipe when working, and loads the dredge spoil into one or more hoppers in the vessel.

• When the hoppers are full, the TSHD sails to a disposal area and either dumps the material through disposal area and either dumps the material through doors in the hull or pumps the material out of the hoppers.

• Some dredges also self-offload using drag buckets and conveyors

Specification:

•The normal dredging vary from 10 to 30m

•The hopper capacities vary from 300-11000cubic meter.

•Majority of these dredgers are in the range of 500-3500cubic meter.

IMPACTS OF DREDGING:

• Dredging can create disturbance to aquatic ecosystem.

• This activities have the potential to change the environment.

• Depending upon the nature of the dredged material, its disturbance from the sea bed may lead to changes in the chemical composition of the water.

• As well as toxicants, the nutrient elements, particularly nitrogen and phosphorus, which control the rate of marine plant growth can also be released from sediments during dredging.

Thank You.