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Types of Hydraulic Dredgers

Hydraulic Dredger

The principal feature of all dredgers in this category is that the loosened material is raised from its in-situ state in suspension through a pipe system connected to a centrifugal pump. Various means can be employed to achieve the initial loosening of the material. If it is naturally very loose, suction alone may be sufficient, but firmer material may require mechanical loosening or the use of water jets. Hydraulic dredging is most efficient when working with fine materials, because they can easily be held in suspension. Coarser materials – and even gravel – can be worked but with a for greater
demand on pump power and with greater wear on pumps and pipes.

Suction Dredger

Suction Dredger
A stationary dredger used to mine for sand. The suction pipe is pushed vertically into a sand deposited. If necessary water jets help to bring the sand up. It is loaded into barges or pumped via pipeline directly to the reclamation area.

Profile or Plain Suction Dredger
Plain Suction Dredger
In its most simple form this type consists of a pontoon able to support a pump and suction pipe and to make the connection to the discharge pipe. More sophisticated vessels have separate suction and delivery pumps, water jets at the suction inlet and articulated suction pipes. While working, a dredger may be held in position by one or more spuds or, in deeper water, by a complex system of moorings. Plain suction dredgers are mainly used to win fill material for reclamation, with the material being placed ashore through a floating pipeline. Very long distances can be pumped by the addition of booster pumps in the line. Material may alternatively be loaded directly into barges moored alongside. The normal measures of size are the diameter of the discharge pipe, which can vary between 100 and 1000 mm, or the installed horsepower.

Modern suction dredgers can recover material from great depths and can also extract sand from below a clay overburden. Known as a deep suction dredger, this type offers the potential to recover fill material from depths up to 100 m. Production is very dependent upon the permeability of the material dredged and is best in clean sands.

Cutter Suction Dredger

A stationary dredger which makes use of a cutter head to loosen the material to be dredged. It pumps the dredged material via a pipeline ashore or into barges. While dredging the cutter head describes arcs and is swung around the spudpole powered by winches. The cutter head can be replaced by several kinds of suction heads for special purposes, such as environmental dredging.
Cutter Suction Dredger

When the in-situ material is too compact to be removed by suction action alone, some form of mechanical loosening must be incorporated near the suction mouth. The most common method is a rotating cutter; the main feature of the cutter suction dredger. This is mounted at the lower end of the ladder used to support the cutter drive and the suction pipe. The loosened material then enters the suction mouth, passes through the suction pipe and pump (or pumps) and into the delivery line.

Cutter suction dredgers operate by swinging about a central working spud using moorings leading from the lower end of the ladder to anchors. By pulling on alternate sides the dredger clears an arc of cut, and then moves forward by pushing against the working spud using a spud carriage. A generally smooth bottom can be achieved, and modern instrumentation allows profiles and side slopes to be dredged accurately. Some of the larger cutter suction dredgers are self-propelled to allow easy movement from site to site.

Cutter suction dredgers are mainly used for capital dredging, especially when reclamation is associated with the dredging. Smaller vessels can be dismantled into sections and moved by road or rail for work in inland waterways, sludge lagoons, reservoirs and similar isolated areas. Large heavy-duty cutter dredgers are capable of dredging some types of rock which have not been pre-treated.

An alternative form of loosening is the use of a rotating bucket wheel at the suction mouth. Bucket wheel dredgers are most commonly used in mineral dredging operations and to date have not found general favour among the major international dredging contractors.

Trailing Suction Hopper Dredger

Trailing Suction Hopper Dredger
A self propelled ship which fills its hold or hopper during dredging, while following a pre-set track. The hopper can be emptied by o bottom doors or valves (dumping) or by pumping its load ashore. This kind of dredger is mainly used in open water: rivers, canals, estuaries and the open sea.

Trailing suction hopper dredgers, commonly known simply as ‘hoppers’ or ‘trailers’, have a hull in the shape of a conventional ship, and are both highly seaworthy and able to operate without any form of mooring or spud. They are equipped with either single or twin (one on each side) trailing suction pipes. Material is lifted through the trailing pipes by one or more pumps and discharged into a hopper contained within the hull of the dredger. The measure of size of a hopper or trailer dredger is the hopper capacity. This may range from a few hundred cubic metres to over 20000 m’ – increasingly larger vessels have been constructed in recent years to allow economic transport of the dredged material, especially for reclamation projects.

Reclamation Dredger

A stationary dredger used to empty hopper barges. A suction pipe is lowered into the barge. Extra water can be added by water by water jets to facilitate the suction process. The dredged material is pumped by pipeline ashore, to a reclamation area, or to a storage depot.

Barge Unloading Dredger

Barge unloading dredgers are used to transfer material from hopper barges to shore, usually for reclamation. A barge unloader is basically a pontoon supporting a suction pump for the discharge, and a high pressure water pump used to fluidize the barge contents by jetting. The mixture is then pumped through a pipeline to the point of reclamation or disposal.
Other Types of Dredgers

Specialized types of dredger are usually of small size and output. They include simple jet-lift and air-lift, auger suction, pneumatic and amphibious dredgers.

Jet-lift dredgers use the Venturi effect of a concentrated high-speed stream of water to draw the adjacent water, together with bed material, into a delivery pipe. The jet head has no moving parts so blockage by wires and other dock debris is minimized. These dredgers are relatively small units and some can be manoeuvred on spuds alone.

Air-lift dredgers are very similar to the jet-lift dredgers but the medium for inducing water and material flow is high pressure air injected at the month of the suction pipe. As with jet-lift dredgers there are no moving parts in the flow system. Hard or other difficult to loosen materials cannot be dredged.

Amphibious dredgers have the unusual feature of being able to work afloat or elevated clear of the water surface on legs. They can be equipped with grabs, buckets or a shovel installation.

All the above specialist types of dredger (and others) have been developed for specific situations and generally for small scale work such as narrow canals, industrial lagoons and reservoirs. Some types have been developed to handle contaminated sediments with minimum disturbance. They are not normally employed for large scale maintenance or capital dredging work.

A further type of dredger is the plough or bed leveller. This consists of a blade or bar which is pulled behind a suitable tug or work-boat. The method can be used for direct dredging over short distances and for levelling off the bed to the desired depth when a trailer or grab dredger is operating. It may also be used to pull material from close to quay walls and other places where a trailer cannot reach into a more accessible area. Sometimes the trailer itself operates the level1er if no tug or work-boat is available.

Water Injection Dredger

Water Injection Dredger
A self propelled dredger which brings the sediment to be excavated into suspension with waterjets. This suspension is denser than water. It will be carried away by gravity and currents. Water injection dredging is mainly used for maintenance in harbours.

A relatively recent development in dredging equipment is the water injection dredger. This can be very effective in some material in order to fluidize it and create a turbidity current of higher density than the surrounding water. The bed material then moves in its own current. The system works best in mud and fine sand beds and has been used successfully in a number of port areas. Careful assessment must be made of the likely destination of the turbid water.

About the Author

  Arslan Zulfiqar He is a student of B.Sc in Transportation Engineering at "University of Engineering and Technology, Lahore, Pakistan"

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