The excavation carried out in either shallow or fresh waters with the aim to gather up the sediments located in the bottom to dispose them off at another place is called Dredging. Find out about different types of dredgers used in the maritime industry.
The sediments might be gathered for purposes like:
- making the water navigation or fishing easier in shallow waters
- for replenishing the sand on public beaches which might have undergone severe coastal erosion
- Gold and coal mining
- Removal of contaminants from the sea bed
- Reclamation of areas damaged by oil spills or natural calamities
- Creation of new harbors
Although dredging can have very harmful effects on the marine and aquatic environment, in some situations it may be the only option available. The device used for excavation and scraping of the sea bed is called the Dredge and the ship or vessel a dredge is fitted to is known as a Dredger, although these terms now days are used interchangeably.
Broadly the types of dredgers are classified into three categories; Mechanical dredgers which are suited for working in confined areas and are useful for removing the hand-packed material or debris, Hydraulic dredgers which work on the principle of adding large amounts of process water to change the original structure of the sediments, and Other dredgers which do not fit in to the above two categories.
Whether mechanical or hydraulic, the different types of dredgers that help in removal of the seabed sediments are:
Trailer Suction Hopper Dredgers
Suitable mostly for harbor maintenance and pipe trenching, a hopper dredger is a self-propelling vessel that holds its load in a large onboard hold knows as the hopper. They can carry the load over large distances and can empty it by opening the bottom doors or by pumping the load offshore. Hopper dredges mostly dredge the soft non-rock soils and because of their high production rates can carry out land reclamation projects easily.
Cutter Suction Dredger
CSD, as they are normally called, have a cutter head at the suction inlet which helps to loosen the earth and take it to the suction mouth. Used for hard surfaces like rock, CSDs suck up the dredged soil with the help of wear-resistant pump and then discharge it through a pipeline or a barge.
Bucket Ladder Dredgers
The bucket ladder dredgers use a series of buckets that are mounted to a wheel, which then using mechanical means pick up the sediments. They can be used for wide variety of materials including soft rock material and are powerful enough to rip out the corals as well. But because of their low production, high level of noise and the need for anchor lines, their use has hugely diminished in the recent times.
Like some onshore excavators, Backhoe dredgers have a digging bucket attached to it which digs through a wide range of materials and when it is excavated it’s brought out and placed on the onboard barges. Although they have few limitations where deep dredging is concerned but with some recent modern dredgers, deeper excavation is made quite easy.
A revolving crane, fitted with a grab, placed on a hopper vessel or pontoon is known as a grab dredger. As the name suggests, it picks up the sediments at the seabed with a clam grabbing motion and discharges the contents. Often used for excavating bay mud it also is useful to pick up clays and loose sand.
Water Injection Dredger
Often used for environmentally sensitive projects, water injection dredgers work by fluidizing the material by pumping water into the bed material. Once it is fluidized it is either moved by a second burst of water or is carried away by natural current.
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References: ritchiewiki, marine-knowledge, iadc-dredging, maoso3ah