Semi-submersible platform

[1] Offshore drilling in water depth greater than around 520 metres (1,710 ft) requires that operations be carried out from a floating vessel, since 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.

[2] The operating deck can be located high above the sea level owing to the good ship stability of the design, and therefore is kept well away from the waves.

With a small water-plane area, however, the semi-submersible is sensitive to load changes, and therefore must have their onboard mass trimmed to maintain stability.

It was observed that the motions at this draught were very small, and Blue Water Drilling and Shell jointly decided that the rig could be operated in the floating mode.

Offshore drilling rigs have been loosely classified in nominal "generations" depending upon the year built and water depth capability as follows:[citation needed][original research?]

The International Maritime Organization MODU Code is an accredited design and operational guideline for mobile offshore drilling units of the semi-submersible type.

As the oil industry progressed into deeper water and harsh environments, purpose-built production semi-submersible platforms were designed.