Hydraulic and earth engineering – Marine structure or fabrication thereof – With work deck vertically adjustable relative to floor
Reexamination Certificate
1998-05-01
2001-02-20
Lillis, Eileen D. (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With work deck vertically adjustable relative to floor
C114S256000, C114S264000, C405S195100
Reexamination Certificate
active
06190089
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a semi-submersible floating offshore structure suitable for use in deep waters for supporting, for instance oil and gas drilling and production operations.
In recent years, various types of structures have been developed for use in oil and gas exploration in waters more and more distant from shore, demanding vessel designs that can operate in deep waters, preferably free standing offshore platforms and floating vessels. Some of the known designs use fixed bottom supported structures, with legs embedded into the ocean floor, while others provide for drilling from floating structures, particularly in deeper water.
Currently available floating designs include drill ships, conventional semi-submersible units, as well as tension leg platforms. All of these design approaches have advantages, as well as apparent disadvantages. For instance, drill ships have poor motion characteristics, particularly in quartering or beam sea conditions; conventional semi-submersible vessels are expensive to fabricate and are sensitive to topside changes in weights, wave and wind forces and direction; fixed platforms cannot be moved from one location to another and therefore are considered a poor economic investment for small field production in deep waters. Tension leg platforms are very sensitive to changes in topside weights and area and require permanent type footings on the sea floor, which cannot be easily moved.
All structures are exposed to ocean waves, and it is common knowledge that conventional vessels exhibit substantial amounts of vertical heave, horizontal surge and angular roll and pitch motions. The heave and roll motion is partially alleviated by semi-submersible vessels, which are supported by buoyant floatation columns, tanks and damping action of the submerged pontoons. However, a conventional semi-submersible vessel requires a larger water plane area at the water line in order to maintain positive stability, thus making it more responsive to surface waves and winds. Tension leg platforms reduce heave and roll motions but require strong mooring tendons to resist the vertical and lateral forces acting upon them.
The present invention contemplates reducing or eliminating many of the drawbacks associated with other conventional deep water structures, and providing a mobile, stable structure, having superior motion characteristics and reduced response to wind and wave forces.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a deep draft semi-submersible offshore structure with inherent absolute stability and minimum movement.
It is another object of the present invention to provide a deep water semi-submersible vessel for offshore drilling and production operations.
A further object of the present invention is to provide a mobile offshore structure that can be conveniently relocated to a different location, as may be required.
These and other objects of the invention are achieved through a provision of a deep draft semi-submersible offshore structure for conducting offshore operations, such as drilling, production or thermal energy recovery operations. The offshore structure comprises a plurality of buoyant columns, which can be three or more in number, with permanent ballast or water ballast in the lower portion of the structure so as to place, its center of gravity below center of buoyancy. The structure has a deep draft, with the major volume thereof being located below the influence of the surface wave action. The water plane is spread into several distinct segments substantially increasing the metacentric height. As a result, the structure has an inherent absolute stability and minimal response to surface waves.
A plurality of horizontal bracing members fixedly joins the columns at selected vertical locations along the columns. The bracing members retain the columns in a spaced apart relationship in a geometric configuration, such as for example a triangle in a three-column structure. Each column has a cylindrical outer shell with reinforced walls. In the preferred embodiment, the reinforcement is provided by corrugated panels inside the shells or, alternatively, by stiffeners mounted in the vertical direction about the inner circumference of each column. Ring frames are used to reduce the spans of the corrugations or stiffeners and maintain the cylindrical shape of the shell. The columns are divided into a plurality of watertight independent compartments.
The spaced columns provide transparency to surface wave motions, making the structure less affected by surge and drift motions induced by ocean waves. Additionally, the spread relationship of the columns increases the metacentric height and renders the structure less responsive to wave forces inducing roll and pitch motions.
The columns and the bracing members, which can be either horizontal or a combination of horizontal and diagonal, provide sufficient moment of inertia of the water plane, or water plane inertia, for short, to minimize motions of the platform and make it a suitable support for operations thereon. The buoyancy of the structure can be adjusted by liquid ballast to maintain the deck above the level of maximum wave height expected at a particular location. The independent columns provide ballast compartments at large lever arms from the center of buoyancy; water ballast can be transferred to accommodate eccentricities of topside (upper deck) center of gravity and/or operational loads.
In order to deploy the structure on site, the deck is towed separately to the selected location, while the hull, column array, is floated horizontally, with columns on their sides, to a selected location from a fabrication yard. The columns are towed using their own buoyancy, without the need for a cargo derrick or barge, which provides significant cost savings to the platform owner. Once delivered on site, the columns are ballasted and upended in a fashion similar to the upending of deepwater jackets. The deck is then positioned atop the hull (columns), secured to the columns, and the hull is deballasted, elevating the deck to the operational level.
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Oil & Gas Journal—Jun. 22, 1998—3 pages “Patents protect deepwater platform concepts”.
Bennett, Jr. William T.
Laborde Alden J.
Keaty Professional Law Corporation
Lagman Frederick L.
Lillis Eileen D.
Mindoc, LLC
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