semi-submersible

A semi-submersible (semi-sub) is a column-stabilised mobile offshore unit consisting of an upper working deck supported by vertical columns connected to two or more submerged horizontal pontoons. When on station, the pontoons are ballasted down so that the waterplane area is reduced to the narrow cross-sections of the columns; this dramatically lowers the unit's response to wave action and provides a stable working platform in harsh sea conditions that would make a monohull impractical. Semi-submersibles serve several roles: drilling semi-submersibles are used for mid- and deepwater exploration and development drilling (typically 200–3 600 m water depth) using either mooring spread or DP-2/DP-3 dynamic positioning; accommodation semi-submersibles (floatels) provide large temporary accommodation capacity near platforms during construction or maintenance campaigns; semi-submersible crane vessels (SSCVs) carry very large cranes for platform installation; and some semi-submersibles are converted or designed as floating production, storage, and offloading (FPSO/FPS) units. All semi-submersibles used for drilling are governed by the IMO MODU Code. SOLAS Chapter II-1 applies to stability in the transit (floating) condition. DP systems must meet IMO MSC/Circ.645 requirements. MARPOL Annex I governs oily discharges, and the ISM Code (SOLAS Chapter IX) mandates a documented Safety Management System. Classification is by ABS, DNV, Lloyd's Register, or Bureau Veritas. See the /reference/ship-types page for further comparison.