Characteristics
Offshore drilling in water depth greater than around 120 meters requires that operations be carried out from a floating vessel, as fixed structures are not practical. Initially in the early 1950s monohull ships were used like CUSS I, 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 its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. The operating deck can be located high above the sea level due to the good stability of the design, and therefore the operating deck is kept well away from the waves. Structural columns connect the pontoons and operating deck.
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 trimmed to maintain stability. Unlike a submarine or submersible, during normal operations, a semi-submersible vessel is never entirely underwater.
A semi-submersible vessel is able to transform from a deep to a shallow draft by deballasting (removing ballast water from the hull), and thereby become a surface vessel. 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.
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