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Semi-Submersible Crane Vessel [ SSCV ]

When making heavy lifts during the building or refurbishment of offshore oil and gas platforms, the enormous crane vessels employed need to work close to the platforms in areas where there are pipelines and cables on the seabed, or simply in very deep water. In neither situation is it possible to use anchors. Instead, the vessels need to be dynamically positioned (DP) using a number of azimuthing thrusters to manoeuvre or to remain accurately in one fixed position.

The core business of Heerema Fabrication Group (HFG) are installation projects, from initial feasibility studies through soil and foundation analysis to final platform hook-up. The installation division of Heerema Fabrication Group operates three of the world's largest semi-submersible crane vessels, with lifting capacities of up to 14,200 tons and lifting heights of almost 100 metres, as well as a wide variety of marine support equipment.

The SSCV Thialf, built in 1985, has twin cranes with a combined lifting capacity of 14,200 tonnes. It is the largest in the crane vessel fleet of Heerema Marine Contractors (HMC) and is employed in all types of heavy lifting at offshore projects around the world. Thialf has an overall length of 201.6 m and a breadth of 88.4 m. In operation, it can accommodate hundreds of workers.

In 1978 Heerema introduced the world's first two semi-submersible crane vessels, Balder and Hermod, each carrying two cranes (3,000 and 2,000 tons lifting capacity respectively). HeereMac, a joint venture between Heerema and McDermott, was formed in 1989 for offshore installation services, employing semi-submersible crane vessels. Heerema received the 'Offshore Technology Conference Distinguished Achievement Award' in 1991 for the concept, design and successful operation of the first semi-submersible crane vessels. The year 1997 saw the acquisition of the world's largest SSCV "Thialf" with a lifting capacity of 14,000 tons.

During the decades in which the platforms have been installed, HMC was key player in the market with its large semi submersible crane vessels. Now, with the upcoming removal market other parties consider to enter this market with new concepts and compete with HMC and other contractors.

With its state-of-the-art J-lay tower, upgraded dynamic positioning capability and fast ballastinthe Saipem 7000 semisubmersible crane vessel has the capacity to handle the entire workscope of offshore construction developments worldwide, encompassing pipelaying in water depths greater than 2,000 metres and heavy lift operations up to 14,000 tonnes. The vessel's Class 3 DP system of 12 thrusters ensures that the vessel maintains good station keeping in the most difficult of weather conditions. Additionally, because the Saipem 7000 has accommodation facilities up to 725 people, the vessel can assist hook-up and commissioning as well as initial platform life support activities.

When a semi-submersible crane vessel lifts a load extra submergence takes place and when the load is located eccentrically of the vessel large pitch and roll angles will appear. These angles are, for heavy loads, outside the allowed inclinations for the crane boom and perpendicular to the crane boom. It is thus desirable to keep the vessel as close to its original position during load variations or changes and to make compensation for such variations or changes.

Marine structures of this type are less affected by the movement of the sea surface in rough water than vessels floating on the surface and the former have therefore been developed for drilling operation in open sea. However, they present the disadvantage of relatively small stability. Apart from the drilling derrick which is a centrally positioned fixed structure only small cranes for handling light loads can be used on a vessel of this type.

Still it has been proposed in the known art to adapt a vessel of this kind for outboard handling of loads, for instance, by means of a gantry crane mounted near the front of the vessel and to stabilize it by means of water transport to and from ballast tanks in the submersed hulls. According to this known system a list is imparted to the vessel before hoisting an outboard load opposite the list which is expected to be imparted by hoisting the load. However, the varying tilts imparted to the vessel during load operation are troublesome for the work and for living on the platform.

Also the maximum admissible preliminary list is limited and therefore only loads up to about 250 tons can be handled and that at a limited distance from the centerline of the vessel. For handling heavier loads the vessel has to be brought in a floating position with the buoyancy hulls at sea level, but there they are exposed to the water movement at the surface and therefore no loads can be handled when the sea is rough.



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