DREDGING

DREDGING Dredging is the removal of bottom sediments from streams, rivers, lakes, coastal waters and oceans.

The resulting dredged material is transported by ship, barge or pipeline to a designated disposal site on land or

in the water

Types of dredging: 1. capital dredging, 2. maintenance dredging

Capital dredging: The term Capital dredging involves the removal of initial dredging there by increasing the

depth.

Maintenance dredging: The term maintenance dredging involves the removal of sediments that have

accumulated since the previous dredging operation.

Dredging involves project planning, design, operation and maintenance. Dredging, dredged material disposal

and other aspects of the overall navigation project should be considered as a total project.

DREDGING REQUIREMENTS Dredges operate either mechanically or hydraulically.

Dredgers has efficient pumps, heave compensating devices, electronic equipment for automatic

controls, water jets, sophisticated navigational equipment and advanced instrumentation.

Basic dredging requirements are determined by channel design and shoaling rates.

The quantities of material to be dredged are determined from past records and planning for dredging

project should be based on long term requirements and hydrographic surveys.

Horizontal positioning and depth measurements are conducted with electronic navigation and

positioning equipment and the data are usually reduced using computers.

Accuracy and capabilities of positioning and surveying equipment are +/- 1 m or better using global

positioning systems.

DREGING REQUIRED AREAS1. Port and Harbours

2. Inland waterways

3. Maritime boards

4. Captive jetties

5. Shipyards

6. Oil and gas

7. Coastal protection

8. Recreation water bodies

9. Agriculture.

DREDGING OPERATION LAYOUT

SEDIMENT DISPOSAL SEDIMENT TRANSPORT DREDGER DREDGE SITE• Open Sea Disposal Barge transport Type Open• Confined Disposal Pipeline Size Confined• Beneficial Reuse Combi – B-P Dredging depth Sediment

Rainbowing Productivity

Fall pipe vessels

OPERATION AND DREDGER CLASSIFICATIONDredgers classified based as:

Hydraulic - Removal of loosely compacted material

Least turbidity

Preferred choice in case of enclosed water bodies

calls for water management.

Type : Cutter heads, TSHD, Hydraulic pipeline, Plain suction

Mechanical - Removal of both loose and hard compacted material.

Higher sand to water ratio.

Disposal site has a capacity limitation ( provided that the turbidity is acceptable)

Type : Clamshell, Dragline and Bucket dredgers, Hopper.

TYPES OF DREDGERS1. Mechanical dredges

2. Hydraulic dredges

3. Hopper dredges

4. Side casting dredge

5. Cutterhead dredge

6. Bucket wheel dredge

7. Dustpan dredge

8. Small hydraulic dredge.

CLASSIFICAION OF DREDGERS

DREDGE

DREDGE

MECHANICAL HYDRAULIC

DIPPER BUCKET LADDER HOPPERSIDE CASTING

AGITATIONPIPE LINE

CLAM SHELL

DRAG LINE

BACK HOE

BUCKET LADDER

PLAIN SUCTION

CUTTER HEAD

BUCKET WHEEL

DUST PAN

MECHANICAL DREDGES A mechanical dredge has limited ability to transport dredged material.

No self propulsion

Relatively low production

Advantages are : its ability to operate in restricted locations (e.g., docks, jetties and piers)

ability to treat and dewater dredged material in placer mining operations

can be used in working with contaminated sediments with special

adaptions.

HYDRAULIC DREDGE Hydraulic dredges conduct both phases of the dredging operations. (digging and disposing)

Placement or disposal is accomplished by pumping the dredged material through a floating

pipeline to the placement area or by storing the dredged material in hoppers that are emptied

over a placement area.

Hydraulic dredges are more efficient, versatile and economical

The dredged material is first loosened and mixed with ambient water by cutter heads or

water jets and pumped as fluid (slurry) through a long pipeline or to a hopper

The basic components of a hydraulic dredge are dredge pumps, digging and agitation

machinery and hoisting and hauling equipment.

HOPPER DREDGESelf propelled trailing suction hopper dredge revolutionized the dredging

industry by reducing the cost.

Extensively used in Europe and the United States

Can work in all hard materials

Hopper capacities of several 100 to 10,000m3

Usually unloaded through the bottom doors and some have pump-out

facilities

The drag arms and drag heads extend from both sides of the hull, and each

is lowered to the sea bottom.

HOPPER DREDGE CONTINUED…

The dredge moves slowly over the area to be dredged while the dredge pumps move

the sediment and water mixture through the drag arms into hopper bins.

Modern dredger have a single hopper in the mid-section.

Drag head is the important part of the hopper dredge.

A grating is used to prevent large objects entering the suction pipe

New type of drag heads uses a rotating cylinder with knives

An automatic drag head winch control system controls the movement of the suction

pipe and drag head.

It has increased ability to operate in bad weather and minimizes risk damage to

equipment.

SIDE CASTING DREDGESide casting dredges discharge the dredged

material to the side of the channel and allow

for continuous dredging.

Some hopper dredges are equipped with a side

casting boom

These dredges are used to maintain navigation

channels where there is an opportunity to place

the dredged material alongside the navigation

channel.

CUTTER HEAD DREDGE To excavate and move material hydraulically to a disposal location without

rehandling.

During a dredging operation, the floating discharge and shore pipeline are

connected to the dredge.

Additonal equipment to support the operation is required such as derrick, tugs, fuel

and pipe barges, surveying boats and other site –specific special equipment.

A cutter is connected at the forward end of the ladder and connected to the shaft of

the cutter motor.

Generally two types of cutters: straight arm or basket.

CUTTER HEAD DREDGE CONTINUED…..

Rotation of the shaft and cutter agitates soft or loose material and cuts hard material that is

then picked up by the suction.

The ladder supports the cutter, suction pipe, lubricating lines and usually the cutter motor

and reduction gear.

Forward end of the ladder is supported by an a-frame with hoisting equipment to raise and

lower the ladder.

Length of the ladder determines the dredging depth.

The diameter of the discharge line depends on the pump size and the suction pipe diameter

is usually 1.25 to 1.5 times the pump discharge diameter when a ladder pump is not

installed.

Dredge pump is located forward in the hull with its centre near the loaded

waterline.

Diesel engine, diesel electric motor or steam or gas turbine can be used to

drive the pump.

Some cases, shore electric power may be used to drive the pump.

Horse power varies from 186 to more than 11,186 KW.

Pump rotative speed varies from about 300 to 900 revolutions per minute.

Dredge is moved and held in position with spuds.

CUTTER HEAD DREDGE CONTINUED…..

BUCKET WHEEL DREDGE

• A rotating wheel equipped with

bottomless buckets is used to cut or

loosen soil that is then directed into

the interior of the wheel and

conveyed to the suction line.

• The bucket wheel is attached to the

ladder as shown in the fig.

DUSTPAN DREDGE• The dustpan dredge is a hydraulic, plain

suction vessel.

• The wide (about same as hull width)

vaccum cleaner like head is lowered to

the bottom by winches.

• It has a high velocity water jet to agitate

and loosen material that is subsequently

pumped through a floating pipeline to a

disposal area.