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Offshore construction

Offshore construction is the installation of structures and pipelines in a marine environment for the production and transmission of oil and gas. Construction in the offshore environment is a dangerous activity and where possible the construction is modular in nature with the individual modules being assembled onshore and using a crane vessel to lift the modules into place. Oil platforms are key fixed installations from which drilling and production activity is carried out. Drilling rigs are either floating vessels for deeper water or jack-up designs which are a barge with liftable legs. Both of these types of vessel are constructed in marine yards but are often involved during the construction phase to pre-drill some production wells. Specialist floating hotel vessels known as flotels are used to accommodate workers during the construction and hook-up phases.

Other key factors in offshore construction are the weather window which defines periods of relatively light weather during which continuous construction or other offshore activity can take place. Safety is another key construction parameter, the main hazard obviously being a fall into the sea from which speedy recovery in cold waters is essential.

The main types of vessels used for pipe laying are the "Derrick Barge (DB)", the "Pipelay Barge (LB)" and the "Derrick/Lay Barge (DLB)" combination. Diving Bells in offshore construction are mainly used in water depths greater that 120ft, less than that, the divers use a metal basket driven from an "A" frame from the deck. The basket is lowered to the water level, then the divers enter the water from it to a maximum of 120ft. Bells can go to 1500ft, but are normally used at 400 to 800ft.

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