Полупогружные платформы: стабильность и применение в морской нефтегазовой отрасли.

Marine vessel used in offshore roles with good stability and seakeeping

A semi submersible platform is a specialised marine vessel used in offshore roles including as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They have good ship stability and seakeeping, better than drillships.

Offshore drilling in water depth greater than around 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. Structural columns connect the pontoons and operating deck. 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 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 have their onboard mass trimmed 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 oil platform activities in the early 1960s. Bruce Collipp of Shell is regarded as the inventor. However, 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. The first jackup rigs, for shallow waters, was built in 1954. The first semisubmersible arrived by accident in 1961. Blue Water Drilling Company owned and operated the four column submersible drilling rig Blue Water Rig No.1 in the Gulf of Mexico for Shell Oil Company. As the pontoons were not sufficiently buoyant to support the weight of the rig and its consumables, it was towed between locations at a draught midway between the top of the pontoons and the underside of the deck. 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. By 1972, there were 30 semi submersible units.

Semi submersible rigs make stable platforms for drilling for offshore oil and gas. They can be towed into position by a tugboat and anchored, or moved by and kept in position by their own azimuth thrusters with dynamic positioning. The International Maritime Organization MODU Code is an accredited design and operational guideline for mobile offshore drilling units of the semi submersible type.

The advantages of the semi submersible vessel stability were soon recognized for offshore construction when in 1978 Heerema Marine Contractors constructed the two sister crane vessels called Balder and Hermod. These semi submersible crane vessels (SSCV) consist of two lower hulls (pontoons), three columns on each pontoon and an upper hull. Shortly after J. Ray McDermott and Saipem also introduced SSCVs, resulting in two new enormous vessels DB 102 (now Thialf) and Saipem 7000, capable of lifting respectively 14,200 and 14,000 tons. During transit an SSCV is de ballasted to a draught where only part of the lower hull is submerged. During lifting operations, the vessel is ballasted down. This way, the lower hull is well submerged. This reduces the effect of waves and swell. High stability is obtained by placing the columns far apart. The high stability allows them to lift extremely high loads safely.

Ocean Odyssey was converted from a mobile drilling rig to a launch platform in 1997. In total, 36 rockets were launched from the platform. The last launch happened in 2014, shortly before its owner Sea Launch significantly reduced all operations. The potential application of converted semi submersible oil drilling rigs is being explored by Space Exploration Technologies (SpaceX) for launching and landing of Starship. SpaceX has acquired two former offshore oil drilling rigs in 2021, similar to the ENSCO/Valaris 8506 offshore model, but later sold them in 2023 after they were considered unsuitable.