A brief introduction to DREDGING>

1. DREDGING

2. 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.

3. 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.

4. DREGING REQUIRED AREAS
1. 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.

5. 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

6. OPERATION AND DREDGER CLASSIFICATION
Dredgers 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.

7. TYPES OF DREDGERS
1. 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.

8. CLASSIFICAION OF DREDGERS
DREDGE
DREDGE
MECHANICAL HYDRAULIC
DIPPER BUCKET LADDER HOPPER
SIDE
CASTIN
G
AGITATIONPIPE
LINE
CLAM
SHELL
DRAG
LINE
BACK
HOE
BUCKET
LADDER
PLAIN
SUCTION
CUTTER
HEAD
BUCKET
WHEEL
DUST
PAN

9. 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.

10. 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.

11. HOPPER DREDGE
 Self 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.

12. 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.

13. SIDE CASTING DREDGE
 Side 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.

14. 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.

15. 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.

16.  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…..

17. 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.

18. 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.