Semi-submersible Platform
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A Semi-submersible platform or rig, is a mobile structure used for drilling for oil and natural gas in offshore environments.
Their superstructures are supported by columns sitting on hulls or pontoons which are ballasted below the water surface. They provide excellent stability in rough, deep seas. They are free floating Mobile Offshore Drilling or production units that are kept in place by either mooring systems, dynamic positioning =, or a combination of the two.
Semi-submersible platforms have leg of sufficient buoyancy to cause the structure to float, but of weight sufficient to keep the structure upright. Semi-submersible rigs can be moved from place to place; and can be ballasted up or down by altering the amount of flooding in buoyancy tanks; they are generally anchored by cable anchors during drilling operations, though they can also be kept in place by dynamic positioning. Semi-submersibles can be used in depths from 600 up to 35,000 feet (180 to more than 10,600 m).