Uncle John – Diving / Construction support vessel, built for Houlder Offshore in 1977.Semi-submersibles are particularly suited to a number of offshore support vessel roles because of their good stability, large deck areas, and variable deck load (VDL). The high stability allows them to lift extreme high loads. High stability is obtained by placing the columns far apart.

This reduces the effect of waves and swell. This way, the lower hull is well submerged. During lifting operations, the vessel will be ballasted down. Ray McDermott and Saipem also introduced SSCV's, resulting in two new enormous vessels DB-102 (now Thialf) and Saipem 7000, capable of lifting respectively 14,200 and 14,000 tons.ĭuring transit an SSCV will be de-ballasted to a draught where only part of the lower hull is submerged. These semi-submersible crane vessels (SSCV) consist of two lower hulls (pontoons), three columns on each pontoon and an upper hull. 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. The Brazilian Petrobras P-51 semi-submersible oil platform Offshore drilling rigs have been classified in nominal 'generations' depending upon the year built and water depth capability as follows Īpplications Mobile offshore drilling units (MODU) The industry quickly accepted the semi-submersible concept and the fleet increased rapidly to 30 units by 1972.ĭrilling rig construction has historically occurred in boom periods and therefore 'batches' of drilling rigs have been built. Since then, many semi-submersibles have been purpose-designed for the drilling industry mobile offshore fleet.

The first purpose built drilling semi-submersible Ocean Driller was launched in 1963. 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.

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 mid way between the top of the pontoons and the underside of the deck. 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. The first semisubmersible arrived by accident in 1961. When demands for drilling equipment was needed in water depths greater than 100 feet (30 m) in the Gulf of Mexico, the first jackup rigs were built. When oil drilling moved into offshore waters, fixed platform rigs and submersible rigs were built, but were limited to shallow waters. īut 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. Bruce Collipp of Shell is regarded as the inventor. The semi-submersible design was first developed for offshore drilling activities. 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. Unlike a submarine or submersible, during normal operations, a semi-submersible vessel is never entirely underwater.Ī 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. With a small water-plane area, however, the semi-submersible is sensitive to load changes, and therefore must be carefully trimmed to maintain stability. With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a normal ship. Structural columns connect the pontoons and operating deck. 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. 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.Ī semi-submersible obtains its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. 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.

4.2 Semi-submersible crane vessels (SSCV).

4.1 Mobile offshore drilling units (MODU).

Semi submersible crane vessel