Semi-Submersible Rig

Mobile offshore unit with the operating deck supported on columns rising from submerged pontoons, giving the vessel low natural heave and pitch responses in seaway compared with a ship-shape hull. Semi-submersibles are used as drilling rigs, accommodation/flotel platforms, heavy-lift cranes, and pipelay vessels in deep water where jack-up legs cannot reach the seabed. Station-keeping is by anchor spread or DP-3. The IMO MODU Code (Resolution A.1023(26)) sets the safety standards; class notations cover stability, structural redundancy, and the redundant power management essential for DP-3 operation.