III. TYPES OF DREDGER

Dredgers may be broadly classified into two main groups or types depending upon the method used to transport loosened material from the sea-bed to the water surface. These are :

a. Mechanical dredgers;
b. Hydraulic dredgers.

Not all dredgers fall neatly into these classifications so it is convenient to include a third, which can best be described as ‘other types’. These dredgers are usually small, sometimes unique, and normally intended for quite specific dredging or disposal situations.

a. Mechanical Dredger

Mechanical dredgers come in a variety of forms, each involving the use of grab or bucket to loosen the in-situ material and raise and transport it to the surface.

BUCKET DREDGER

A stationary dredger, fixed on anchors and moved while dredging along semi-arcs by winches. The bucket dredger is one of the oldest types of dredging equipment. It has an endless chain of buckets, that fill while scraping over the bottom. The buckets are turned upside down and empty moving over the tumbler at the top. The dredged material is loaded in barges.

Bucket ladder dredgers are one of the oldest types of dredger. They usually comprise a rectangular pontoon with a central well in which a heavy steel frame or ladder is suspended. The ladder supports an endless chain of buckets, each of which is equipped with a cutting edge. By rotating the bucket chain about flat-sided wheels (known as tumblers) at each end of the ladder, material can be loosened and transported. A small proportion of the dredgers of this type are self-propelled. The propulsion machinery is used to move the vessel from site to site, but is not used in the dredging operation.

The dredging action starts when a bucket reaches the bottom of the ladder, where it loosens and scoops up a quantity of material. This material is carried in the bucket to the top of the ladder where, at the highest point of the chain, the bucket overturns and the contents are discharged. The material falls into drop chutes and into a barge moored alongside the dredger. Each bucket then returns empty on the underside of the chain to the bottom of the ladder where the cycle begins again. The size of a bucket dredger is usually described by the capacity of the buckets, which is in the range 100-900 litres.

Bucket ladder dredgers are able to dredge almost any material up to the point where blasting is required, and if fitted with ripper teeth may even be directly able to dredge weak rock. A minimal amount of water is added to the dredged material during careful use of the buckets. This is advantageous to production and costs, especially when dredging in silt and mud.

In operation, a bucket ladder dredger is held accurately in position by up to six moorings or anchors and the bucket ladder moved from side to side to excavate material. The mooring wires can obstruct other shipping, and high noise levels are a common problem if special steps are not taken to control sound emissions. Much of the power of a bucket dredger is used in turning the chain, but the high inertia of this can also assist in overcoming localized hard spots. The maximum weekly output of a bucket dredger can vary between 10.000 and 100.000 m3 (in-situ) depending upon size, location and material. Maximum dredging depths are normally around 20 m. Bucket ladder dredgers are complex and expensive machines to operate but can dredge to the required depth very accurately.

GRAB DREDGER

A stationary dredger, moored on anchors or on spudpoles. The dredging tool is a grab normally consisting of two halfshells operated by wires or (electro)-hydraulically. The grab can be mounted on a dragline or on a hydraulic excavator of the backhoe type. Many modifications of grabs have been constructed like (top) open grab, (top) closed grabs and watertight grabs. The grab dredger is used in harbours; the dragline type also in deep water. The dredged material is loaded in barges.

Grab dredgers, sometimes called clamshells, can exist in pontoon and self-propelled forms, the latter usually including a hopper within the vessel. The pontoon type grab dredger again comprises a rectangular pontoon on which is mounted a revolving crane equipped with a grab. The dredging operation consists of lowering the grab to the bottom, closing the grab, raising the filled grab to the surface and discharging the contents into a barge or, if appropriate, onto the adjoining bank. The size of this type is determined by the capacity of the grab bucket, which can vary between 1.0 and 20 m3 , depending upon the crane power.

The self-propelled grab hopper dredger is basically a ship which has one or more dredging cranes mounted around a receiving hopper. It is easily moved from site to site under its own power and also transports the dredged material to the disposal area. The size of this type of dredger is expressed in terms of the hopper capacity and can range from 100 to about 2.500 m3. The smaller vessels have a single crane, but some of the larger craft have up to four. Production depends upon crane and grab size, water depth and, in the case of the self-propelled variety, on the distance to the material disposal site.

Grab dredgers are usually held in position while working by anchors and moorings but a few are fitted with a spud, or pile, which can be dropped onto the bottom while the dredger is operating.

A wire line grab generally produces an irregular bottom profile with peaks and troughs and is thus most suited to bulk excavation. The grab is a relatively simple and inexpensive machine and performs best in consolidated silt, clays and loose sand, but the large, heavy versions are good for removing rubbish, old piles, rubble and similar obstructions. Grabs can also be used effectively for removing material from close to quay walls and in corners of docks and basins that are otherwise difficult to access.
A basic grab dredger can be quickly and economically made tip from conventional land machines securely fixed to pontoons for short term adhoc tasks, but care needs to be taken to check stability.

BACKHOE DREDGER



A stationary dredger, moved on anchors or on spudpoles. A spud is a large pole that can anchor a ship while allowing a rotating movement around the point of anchorage. Small backhoe dredgers can be track mounted and work from the banks of ditches. A backhoe dredger is a hydraulic excavator equipped with a half open shell. This shell is filled moving towards the machine. Usually the dredged material is loaded in barges. This machine is mainly used in harbours and other shallow waters.

BACKHOE/DIPPER DREDGER

Backhoe and dipper dredgers again consist of a rectangular pontoon, on which is mounted the excavator unit. The excavator can be either an integral part of the dredger or a proprietary mobile type adapted for marine working. Material is excavated using a bucket of size compatible with the in-situ strength of the material being dredged. The excavated material is either loaded into barges or placed ashore. The older form of this type of dredger, the dipper or face shovel, used a wire operated integral excavator and was very heavily built to allow for dredging of hard materials such as old masonry and unblasted rock.

The wire operated excavator unit has now been largely superseded by hydraulically operated backhoe machines. These operate more efficiently than the face shovel. The size of a backhoe dredger is described by the bucket capacity, which can vary between 0.5 and 13 m3. Production is dependent upon bucket size and the hardness of the material. Breakout forces in excess of 90 t can be exerted by the larger machines, and because of the very high horizontal loads developed by the jigging action the backhoe dredger usually works on spuds. These are heavy pile-like structures which can be dropped into the sea-bed by the dredger. Two spuds are mounted at the digging end of the backhoe pontoon to provide resistance and one backhoe excavator is very efficient and has good vertical and horizontal control; carefully worked it will produce a smooth profile. Because the bucket is heavy and relatively rigid, care needs to be taken to avoid damage to such features as quay walls and canal linings.