Semi-Sub

Semi-submersible — an offshore drilling unit or production platform supported by large pontoons submerged below the water surface, connected to the main deck by vertical columns. The submerged pontoons reduce the vessel's response to surface wave action, making semi-submersibles more stable than ship-shaped hulls in harsh conditions. Semi-subs are suitable for deepwater drilling (up to 3,000+ metres) and are used in the North Sea, Gulf of Mexico, and West Africa. They can be moored or dynamically positioned.