Semisubmersibles

Semisubmersibles are a class of floating offshore platforms used primarily for drilling in deepwater and rough seas. They consist of a pontoon-shaped, partially submerged hull with several vertical columns (typically three or four) that support a large deck on top. The hull is held upright by ballast tanks in the columns and submerged pontoons, allowing the unit to be ballasted deeper for stabilization.

During operation, ballast systems adjust the draft so the platform sits partly below the surface, reducing motion from waves. The deck and drilling equipment are located above the waterline, while the hull remains submerged. Semisubmersibles are typically moored to the seabed using moorings or kept on station by dynamic positioning systems, and they can be relocated to different locations as required.

Because of their stability in rough seas and their ability to operate in deep water, semisubmersibles have become a dominant design for offshore drilling and, in some cases, for conversion to production or storage units. They generally offer a large deck area suitable for drilling rigs and supporting equipment, and they can perform heavy-lift operations when equipped with appropriate cranes.

History and context: Semisubmersible rigs were developed in the 1960s and 1970s to address the need for stable platforms in deep water. They have since become a standard solution for offshore exploration and development in deepwater regions such as the Gulf of Mexico, the North Sea, Brazil, and West Africa. They are distinct from other offshore platforms such as jack-ups and drillships in their buoyant, floating, partially submerged design.