Semi-submersible rigs are used for the establishing of stable platforms that suit the needs of drilling offshore oil and gas. They can either be moved and kept in their respective position on account of their very own azimuth thrusters or towed to the desired location via the use of a tug boat and then anchored.

A semi-submersible (semisubmerged vessel) is a specially designed marine vessel that can be employed for carrying out various offshore roles like that of an offshore drilling rig, safety vessel, oil production platform and heavy lift crane. They offer good stability and have viable see keeping characteristics. Other terms used to refer to this type of vessels include semi-sub, semisubmersible or just semi.

If one is to perform offshore drilling in water depth levels reaching beyond 520 meters, he would have to carry out the appointed procedures from a floating vessel, due to the fact that fixed structures are merely not practical in such a scenario. Back in the early 1950s monohull vessels such as CUSS I were employed for the task but they were later on determined to demonstrate a great level of heave, pitch and yaw motions in big waves, thus the industry was forced to come up with a more reliable and stable type of drilling platform.

A semi-sub is able to obtain its buoyancy via its ballasted, watertight pontoons that are positioned below the water surface and waves’ effect. It is possible for the operating deck to be positioned at a higher level than that of the sea surface thanks to the design’s sufficient stability and thus the operating deck is not affected by any wave action. The connection between the operating deck and the pontoons is established by using structural columns.

Due to its hull structure being submerged at a deep draft, the semi-sub is affected to a lesser degree by waves than a typical ship would naturally be. Although, due to its small water-plane area, the semi is rather sensitive to load changes, thus it has to be carefully trimmed in order to maintain the required level of stability. A submersible unlike a submarine is never entirely under the water during standard operations.

A semi has the ability to turn from a deep to a shallow draft via deballasting (ballast water is removed from the vessel’s hull), and thus transform into a surface vessel. Heavy lift vessels typically use this method to submerge the larger portion of their structure, position beneath another floating unit, and then perform a deballast procedure in order to pick up the other unit as a form of cargo.

Eirik Raude (IMO number 8765266 and MMSI 308463000) was built in 2002 and is registered in the Bahamas.