Ships – Floating platform
Reexamination Certificate
1999-10-13
2001-05-15
Avila, Stephen (Department: 3617)
Ships
Floating platform
C114S125000
Reexamination Certificate
active
06230645
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a floating offshore structure and more particularly to a floating platform used for the production and/or drilling of oil and gas.
Typically, in the oil industry, the offshore production and drilling for oil and gas has involved the use of a platform set on the ocean bottom and extending to a production or drilling platform above the water's surface. These types of operations are generally performed in water of less than 1300 feet. However, once drilling and/or production in deeper water began to be developed, the use of a solid structure stretching from the ocean surface to the bottom became impractical. Thus, alternative methods were developed for offshore drilling and production operations in deep water (over 1300 feet deep), and ultra deep water (over 2,000 feet deep).
Many different methods and devices have been proposed and used in deep water, most of which have involved some sort of floating platform. One such device is the tension leg platform, which is moored to the sea floor through the use of groups of vertically arranged high tension wires. Such arrangements, however, have not provided the control over the motion of the platform necessary for continuous, effective offshore operations. Specifically, the watch circle, defined as the circle of movement by the platform on the ocean's surface relative to the sea floor, may not be suitable for easily performing drilling and production operations. Additionally, the breakage of a high tension wire could have catastrophic effects on these operations, resulting in loss of life, platform, as well as threatening the environment.
Additional deep water offshore production and drilling apparatus include floating or semi-submersible platforms or vessels which are moored to the sea floor through the use of conventional catenary mooring lines. These types of platforms, however, while useful in deep water, can become problematic when used in ultra deep water because the vessel's watch circle can increase beyond acceptable levels when extremely lengthy catenary or other mooring lines are used. This is especially the case in high or rough seas, which can result in increased down time. Thus, such floating platforms are usually precluded from operating in ultra deep water.
One type of device that has been developed for use in deep and ultra deep water is the SPAR disclosed in U.S. Pat. No. 4,702,321 to Horton. Such prior art SPARs have had solid sides throughout their length and, thus, have allowed a substantial degree of movement both longitudinally and vertically, as well as in the pitch, roll, and yaw directions. This can cause an increased shutdown time for well production in times of bad weather or intense currents as well as safety concerns. Additionally, the risers which bring oil up from the bottom of the ocean travel through the center of the prior art SPAR with no outlet to the sea other than that at the SPAR's bottom. Thus, if a breakage or leak occurs in the risers while in the middle of the SPAR body, such leaks have no way to escape, and a dangerous situation can be created.
SUMMARY OF THE INVENTION
The disclosed floating offshore structure addresses and solves the problems that have been associated with prior art SPARs by disclosing a SPAR-type structure that has apertures throughout a portion of its body. The present invention comprises an offshore floating structure which has an outside surface that can be polygon or cylindrically shaped. The structure is comprised of sides that are welded or otherwise connected together to form a wall or outer shell. This floating structure is comprised of distinct portions, each having a centerwell wide enough to accommodate a typical riser system running longitudinally through its center. The top portion includes an operating platform located above the surface of the water, which can be used both for drilling and/or production of oil and gas. Below this operating platform are located buoyancy tanks which are sufficient to maintain the structure afloat such that the operating platform remains an acceptable level above the surface of the water. These buoyancy tanks can be placed around the wall of the structure, preferably internally, such that they define a centerwell, with enough space for a riser system to pass through the longitudinal center of the centerwell. A first portion of the offshore platform consists of only the outside wall, and contains a series of apertures in each side of the structure. These apertures allow underwater currents to freely pass laterally through the structure without buffeting its sides or causing vibration or unnecessary movement. These apertures also allow oil and gas to dissipate into the sea if a riser running up through the structure ruptures. These apertures can also comprise a coaming surrounding each aperture, which consists of a solid extension protruding laterally from the side of the structure, surrounding each aperture. These coamings reduce the movement of the structure by creating damping forces in response to the structure's attempt to move in the horizontal, vertical, roll, pitch, or yaw directions. Thus, the structure can remain much more stable than previous SPARs.
A second portion of the structure comprises a weighting section, such as a water or fluid retention tank and/or a fixed ballast. This portion lowers the center of gravity of the structure. The fluid retention tank can have two uses. It can be left empty while floating the offshore structure into place, and then filled to tip the structure into position. The tank then also provides additional weight to the structure, lowering its center of gravity. A ballast can then be added, as necessary, to the bottom of the structure in order to further lower the center of gravity of the structure to the required level. The structure, once in place, can then be moored to the sea floor by any conventional means, such as high tension mooring wires or conventional catenary mooring lines.
The primary object of the present invention is thus to provide a novel offshore floating structure for operations relating to the drilling and/or production of oil and gas.
Another object of the invention is to provide a SPAR-type floating offshore structure which is lighter weight, yet has reduced movement and high structural integrity, as compared to other types of floating platforms and SPARs.
Another object of the invention is to provide a SPAR-type floating platform which can disperse oil or gas spills resulting from a rupture in the riser system running through the center of the platform, thus resulting in higher safety and shorter shutdown time.
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Avila Stephen
Howrey Simon Arnold & White
Reinisch Morris N.
Texaco Inc.
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