Dredgers may be broadly classified into these main groups or types depending upon the method used to transport loosened material from the sea-bed to the water surface. These are:
1. Mechanical dredgers;
2. Hydraulic dredgers.
3. other types
1. Mechanical Dredger
Mechanical dredgers use grab or bucket to loosen the in-situ material and raise and transport it to the surface.
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.
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.
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.
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 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.