Column-stabilized floating structures with truss pontoons

Ships – Floating platform

Reexamination Certificate

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Details

C114S265000, C405S195100

Reexamination Certificate

active

06761124

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to apparatus and installation methods for semi-submersible vessels, tension leg platforms and floating structures, and more particularly to column-stabilized floating structures having a plurality of vertical spaced apart buoyant caissons connected together by an open framework of truss-pontoon members and the method of installation and operation.
2. Brief Description of the Prior Art
The word truss, as used herein, refers to a welded or bolted open frame structure formed of slender tubular members. Horizontally disposed truss structures are known as “truss pontoons”, and vertically disposed truss structures are known as “truss columns”. The truss bridges between three or more buoyant vertical column structures that stabilize a semi-submersible vessel at the water surface when floating with respect to wind, wave, current and other horizontal loads.
Semi-submersible type offshore floating vessels were first introduced in 1950. Ever since their introduction, the semi-submersible vessels have been found to be economical for offshore drilling, production, transport, pipe laying and other shallow and deep-water applications. SPAR, TLP, FPSO's and barges are other types of floating vessels that can be used for similar applications as semi-submersible vessels. Decks, columns and pontoons are the typical components of a semi-submersible vessel. Semi-submersible vessels comprise three or more large size vertical columns that are spatially separated and connected by large size horizontal pontoons between the columns at their bottoms. The deck structure is above the water with a sufficient air gap between the still water level to the bottom of the deck to allow waves to pass across the columns without impacting on the bottom of the deck. The center of gravity of the entire semi-submersible vessel is generally high due to large deck loads above the water. The deck of a semi-submersible vessel is a very conventional structure made of boxes, trusses and or girders fixed on top of the columns. The modern semi-submersible vessel uses pleated box structures for the deck with bulkheads, frames and stiffeners, in which case, the deck is called the upper hull. The column is a shell type plated structure with stringers, bulkheads and stiffeners.
The pontoon is also a shell type plated structure with frames, bulkheads and stiffeners. The pontoon and columns have compartments for ballast, voids and storages. The column and the pontoon together is called lower hull. The lower hull provides the necessary buoyancy to the semi-submersible to take the functional structural, equipment and live loads. The vertical columns are sized and located apart to provide water plane area and to provide the area moment of inertia. Since the center of the gravity of the semi-submersible vessel is much higher above the center of buoyancy of the lower hull, the acting moments due the wind, wave and currents horizontal forces are restored by the semi column water plane areas and moment of inertia of the water plane areas. The size and the spacing of the columns provide these two stability parameters. Thus the semi-submersible design is called a “column-stabilized floating unit”. The semi-submersible vessel is a free-floating vessel compliant to wind, waves and currents. It is moored and/or dynamically positioned by powered-thrusters.
Semi-submersible vessels have good sea-keeping behavior with respect to waves and have large deck areas and carrying capacity. However the current and wind resistance results in higher positioning. A semi-submersible vessel is sensitive to the variable deck loads and its stability is limited. Semi-submersibles have large heave motions (vertical oscillation of the floating vessel) when subjected to waves and consequently a dry-tree oil production system is not feasible. In the case of a drilling semi-submersible, downtime of drilling may occur when the heave motion is not feasible for drilling.
Thomas, U.S. Pat. No. 6,024,040 discloses a mobile jack-up platform converted to a semi-submersible offshore platform. The platform includes single submerged hollow lower base at the bottom end, partially submerged elongate vertical buoyant connecting legs extending upwardly therefrom and passing through an upper barge (jack-up platform) above the level of the sea. The hollow base has a square, rectangular or triangular configuration and is filled with seawater to form the ballast for the entire platform and may include interior reservoirs in which hydrocarbons are stored. A central opening or passage in the center of the base reduces the resistive surface size of the base in the water during vertical movements of the platform. The vertical connecting legs have a hollow cylindrical upper portion with a bottom wall forming a buoyancy tank, and a lower portion formed of open frame lattice construction. The respective lengths of the hollow cylindrical buoyant upper portion and the lattice-work lower portions are dimensioned relative to one another so that a pressure force exerted by the sea on the upper portions substantially compensates for an acceleration force exerted on the base by the action of the seawater surrounding the base over a usual swell period range of the sea. The platform in operation imitates a semi-submersible, which can retract the legs with respect to the upper barge (jack-up platform). With the legs fully retracted above the upper barge and its single lower base closely adjacent to the bottom of the upper barge, the platform can be floated and transported to another location.
The tension leg platform (TLP) was introduced as a new concept based on a semi-submersible design with a deck, columns or caissons and pontoons. The vertical heave of the tension leg platform (TLP) is reduced by tendons attached between the lower hull and the seabed. The tendons are always maintained in tension with the excess buoyancy designed into the lower hull. The application of a TLP is limited to water depths of say 5000 ft., over which, the tendons' vertical neutral period enters the energetic portion of the wave heave forces of the TLP. Secondly, the size of the lower hull increases the economics to always maintain tension in the tendons with respect to the increased water depth.
The SPAR is currently used in relatively large offshore oil production applications. The SPAR is a single large vertical column or caisson buoyant structure. Unlike the semi-submersibles, the stability of the vessel is not dependent on the water plane area and the moment of inertia of the water plane area. The stability of the SPAR is provided by lowering the center of gravity of the vessel below its center of buoyancy. Thus, the vertical buoyancy acts upwards above the center of gravity, and total weight acts at the center of gravity below the center of buoyancy. The SPAR has excellent heave performance like a TLP and dry-tree production systems like a fixed offshore platform are used. However, the SPAR has limitation in deck area and payload due to the size of the SPAR. The SPAR is fabricated and transported horizontally and up-ended vertically at the operating location. Then the fully equipped deck is installed on top of the SPAR vertical column. Thus the transportation and installation cost and risk are significantly increases with water depth and deck payload applications. The SPAR becomes uneconomical for larger production as a self-contained drilling unit in deepwater. Ultra deepwater posses problems to the riser tensioning system and limits the applicability of the SPAR. Mooring of the SPAR in deepwater over 5000 ft also posses a serious problem. The effectiveness of the mooring and handling are reduced in such water depth. The SPAR also poses vortex shedding vibration problems when the vertical single column hull becomes extremely slender.
Horton, U.S. Pat. No. 5,558,467 discloses a spar-type deep water offshore floating apparatus, for use in oil drilling and production in which an upper buoyant hull of prismatic shape is p

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