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What is Dredging?
Ships are giant structures and we know that a substantial portion of the ship has to be underwater to displace equivalent weight and that is how ships float on water. At some places there might not be deep water sufficient to support all types of ships of different sizes. Hence there needs to be an artificial way to increase the depth of the seabed and this can be achieved by dredging.
Dredging is a process of digging the seabed, by scraping or sucking. It is mainly done either to increase the depth of the area to make it navigable for ships or to collect sand to dispose it to some other areas. The ships used for these purposes are known as dredgers.
Though dredging is useful it can create some adverse marine environmental problems. Dredging induces coastal erosion and destroy the marine life flourishing on the seabed.
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Dredgers are not just used for the digging purpose but for various other applications too. Water Dredgers are mainly used for excavation purposes to increase the depth of water ways to allow larger ships to access them. Apart from that they are also used in drilling, fishing and land reclamation purposes. Dredging is used to procure edible crabs and other sea organisms. Dredgers are used to replenish beaches where the sand is continuously eroded due to repetitive wave action and winds. They are also used for obtaining precious elements and minerals from the sea bed. Some dredgers are used to make new ports and artificial islands by scraping ocean beds and using that material.
Dredgers are used to remove the land from the areas which are contaminated by oil spills or other poisonous chemical leakages. They are also used to remove man made contaminants such as trash and other non degradable material from the sea bed, especially in the harbors.
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Types of Dredgers - Trailing Hopper Suction Dredger
They are generally used for increasing the depth of the water ways and for land replenishment purposes. They consist of two long and flexible pipes that drag on the seabed. These pipes are adjustable and are attached to collecting holds known as hoppers. Pipes suck a mixture of land and water from the sea floor with the help of suction pumps. The mixture is collected in the holding tanks and allowed to settled down. Water comes on the surface and heavy sediments settle down. Water is thrown out with the help of pumps. The two flexible pipes are suspended with the help of special cranes in such a way that the sucking nozzles always stay in contact with the sea surface.
The discharging of the material is done when the holds are full. This is done either by opening the bottom dumping flaps, which are opened on reaching the disposing site, or by using pumps to discharge it through the nozzles . The process of discharging the material through the nozzles is known as rainbowing. To facilitate the discharging process by opening of the discharge flaps, some dredgers are constructed using two hinged port, which separates the ship from middle, into two halves. These rails are called split rail suction dredgers.
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Cutter Suction Dredgers
Certain ocean floors cannot be sucked or scraped. They are so tough that first they are cut and broken and then sucked or scraped. The vessels used for these purposes are known as cutter suction dredgers. They are used for creating new water ways or ports.Cutters are rotating wheels which are used to cut the hard land into small pieces. These cutters are suspended with the help of movable poles that reaches up to the ocean floors. These types of dredgers do not have cutters to collect the broken materials. The broken and loosened soils are washed away with the help of dredging pumps. The soil can also be collected in barges which can be later transported to the disposing site.
These dredgers have there own propulsion system, but whenever cutting is to be done, spud poles are used to temporarily fix the vessels. This facilitates the vessel to generate a swinging motion which helps in the cutting process.
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Ship Knowledge - A Modern Encyclopedia by K. Van Dokkum