Semi-submersible

A semi-submersible (semisubmerged ship) is a specialised marine vessel used in a number of specific offshore roles such as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They are designed with good stability and seakeeping characteristics. Other terms include semisubmersible, semi-sub, or simply semi.

Offshore drilling in water depth greater than around 520 meters requires that operations be carried out from a floating vessel, as fixed structures are not practical. Initially in the early 1950s monohull ships such as CUSS I were used, but these were found to have significant heave, pitch and yaw motions in large waves, and the industry needed more stable drilling platforms.

A semi-submersible obtains most of its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. Structural columns connect the pontoons and operating deck. The operating deck can be located high above the sea level owing to the good stability of the design, and therefore is kept well away from the waves.

With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a normal ship. With a small water-plane area, however, the semi-submersible is sensitive to load changes, and therefore must be carefully to maintain stability. Unlike a submersible, a semi-submersible vessel is not supported by resting on the seabed.

Semi-submersible vessels are able to transform from a deep to a shallow draft by deballasting (removing ballast water from the hull), thereby becoming surface vessels. Usually they are moved from location to location in this configuration. The heavy lift vessels use this capability to submerge the majority of their structure, locate beneath another floating vessel, and then deballast to pick up the other vessel as a cargo.

The semi-submersible design was first developed for offshore drilling activities. Bruce Collipp of Shell is regarded as the inventor. But Edward Robert Armstrong may have paved the way with his idea of "seadrome" landing strips for airplanes in the late 1920s, since his idea involved the same use of columns on ballast tanks below the surface and anchored to the ocean floor by steel cables.

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