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Semi-Submersible

Semisubmersible marine structures are well known in the oil and gas industries. Such structures are typically only moveable by towing. These semisubmersibles have a relatively low transit draft that allows them to be floated to a stationing location, where they can add ballast, usually by taking on seawater, to assume a relatively deep draft or semisubmerged condition for operation. Semi-submersible platforms have the principal characteristic of remaining in a substantially stable position, presenting small movements when they suffer the action environmental forces such as the wind, waves and currents.

Flotation of semisubmersibles is usually accomplished with pontoons on which an upper deck is supported by columns. The pontoons provide a relatively large waterplane area, as is desirable for transit, but when submerged for stationing, the columns connecting the pontoons to the upper deck present a lower waterplane area for operation. The low waterplane area is desirable to reduce motion characteristics from waves, especially during swell seas and storms. The upper deck from which rig activities are conducted must be maintained above the water plane at all times.

In recent years, the drilling operations have been conducted at increasingly greater distance from the shoreline, placing the offshore production or drilling facilities in often severe weather conditions. In such environments it is particularly important to have a stable floating facility for supporting the mineral exploration and production operations, as well as providing living accommodations to the crew and storage for the necessary equipment. In deep waters, over 7500 feet, it becomes particularly advantageous to deploy floating semi-submersible vessels, as opposed to fixed bottom anchored structures.

Designs of semi-submersible vessels utilize buoyant pontoons, or lower hulls which support a plurality of vertically extending columns, the upper portions of which carry a working platform. Some of the semi-submersible vessels can have a single caisson, or column, usually denoted as a buoy while others utilize three or more columns extended upwardly from buoyant pontoons.

In many such structures, vertical or diagonal braces are used between the columns, the braces contributing to the water plane area of the vessel. The braces are usually constructed with smaller diameters than that of the columns and are therefore more vulnerable to the environmental and mechanical damage. If the connecting braces are damaged, the entire structure becomes jeopardized.

One example of a single-brace structure is a two-pontoon, four-column structure, with a pair of columns being mounted on a respective underwater hull, or pontoon. One transverse horizontal stay is mounted between each pair of the columns at the ends of the underwater hulls. The object of the design is to simplify the construction and to reduce the resistance to water flow. However, the minimal number of braces may be less beneficial where spreading forces acting on the four columns are relatively high and torque imposed on the columns by the pontoon lateral bending tends to twist a column structure in the direction of the prevailing wind and wave forces.

There also exist numerous designs of semi-submersible vessels using diagonal braces in addition to horizontal stays. These tend reinforce the support structure of the platforms and resist destructive forces of the ocean waves. One of the disadvantages of the diagonal braces is increase in water plane area of the vessel, which adversely affects the weight, wave resistance and overall cost of the vessel.

Another consideration that is taken into account when designing semi-submersible vessels is resistance of the vessels to heave and roll motion induced by waves. The vessels must have sufficient stability to withstand wave motions to allow the mineral exploration and production operations to be carried out in safety. This consideration becomes particularly important in harsh environments where strong winds and waves are prevalent throughout the year.




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