Method of constructing precast modular marine structures

Details Method of constructing precast modular marine structures Method of constructing precast modular marine structures

2003-03-15

2004-01-06

Pezzuto, Robert E. (Department: 3671)

Hydraulic and earth engineering

Marine structure or fabrication thereof

C405S203000

Reexamination Certificate

active

06672803

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method of constructing precast modular marine structures.
BACKGROUND OF THE INVENTION
Much of the World's production of oil and gas is derived from offshore wells. While the early offshore oil and gas fields were located in relatively shallow water, the need to develop oil fields in deep water has become more important as the shallow water oil and gas fields become depleted. As a result, many deep-water basins throughout the world have been opened to oil and gas exploration and drilling.
During the exploration for, and production of sub-sea resources like oil and gas, an array of marine vessels, structures and appurtenances are employed. Prior proposed vessels used for exploration, drilling, production and storage of oil and gas at sea included: ships, boats, mobile offshore drilling units, semi-submersible units, submersible units, jack-up rigs, platforms, spars, deep draft caisson vessels, tension leg platforms and various combination of these and other components often in conjunction with a riser or sub-sea system.
Platforms, spars, deep draft caisson vessels, and tension leg platforms typically include a long vertical cylindrical hull that supports a platform above the water line. The platform provides space for drilling and maintaining oil or gas wells where the production wells may be positioned along an outside edge of the platform. Alternatively, the production wells may be located in the center of the platform within a moon bay or pool. Likewise, the above water platform of such a marine structure can be configured for use such as a launch pad for aeronautical and space vehicles, housing, hotels, resorts, and manufacturing and processing facilities.
Generally, traditional construction methods and materials for marine structures, including platforms, spars, deep draft caisson vessels, tension leg platforms, jack-up rigs, semi-submersible units, mobile offshore drilling units, ships and boats require the erection of frames about which plates, planks or sheets of material such as metal, wood or resin impregnated cloth are faired by and attached (permanently or otherwise) to the frames by skilled labor to form a complete or at least a significant portion of the marine structure's hull. Thereafter, the marine structure is launched or introduced into the water for further outfitting or operation.
Traditional materials of metal and/or wood require fairing, fixing and supporting the material(s) between frames. However, due to limitations in the structural and strength characteristics of traditional construction materials and the lack of economical labor with the proper skills, alternative construction methods have been developed. For example, the world's first metal oil/gas production spar hull was constructed as two separate sections in Finland. The two separate sections were shipped across the Atlantic Ocean aboard heavy lift vessels until reaching the Gulf of Mexico. There, the two separate sections of the spar hull were brought back to shore and welded together. The entire welded hull was then towed horizontally to the project site and upended to the vertical position by filling its lower ballast tanks with water.
Marine structures, such as the Troll A Platform, have been constructed from concrete materials using the slip form construction technique. This technique typically calls for the pouring of concrete in a vertically movable form. The form is connected to jack rods with hydraulic jacks, which move the form vertically in minute increments as the concrete is being poured. Once pouring begins, it continues until the top of the structure is reached, allowing for a monolithic poured concrete structure. Utilizing the slip form construction technique for marine structures requires a transportation path of sufficient clearances (in terms of water depth and overhead clearances) to accommodate the vertical monolithic poured structure. Furthermore, the scantlings of the lower regions of the pour must be of sufficient strength to accommodate the weight of the upper regions of the structure while being poured.
The structural sections may include either plated hull tank sections, or a combination of tank and truss-type section. An example of such spar platforms is depicted in U.S. Pat. No. 5,558,467 issued on Sep. 24, 1996 to Horton (hereinafter Horton '467). The Horton '467 patent describes a hull having a passage longitudinally extending through the hull in which risers run down to the sea floor. However, the Horton '467 patent fails to provide for a precast modular marine structure or incorporation of an equalized pressure system that adjusts internal pressure of the structure in relation to external pressure, namely hydrostatic pressure, exerted thereupon.
An alternative design of an existing spar platform is depicted in U.S. Pat. No. 5,875,728 issued on Mar. 2, 1999 to Ayers, et al. (hereinafter Ayers '728). The Ayers '728 patent provides for a spar platform incorporating an essentially vertical cylindrical buoyant vessel and a shroud surrounding the vessel. The shroud includes two intersecting sets of foam-filled fiberglass elements that are secured to the vessel using standoffs. Nevertheless, the Ayers '728 patent neither describes nor claims a precast modular marine structure or incorporation of an equalized pressure system, which gives the structure the ability to withstand an increasing hydrostatic force as the water depth increases.
Without an equalized pressure system, a spar system and any other marine structure requires additional reinforcement to withstand the significant hydrostatic forces. Such structures, including spars, risers, tension legs, and buoyancy cans must include greater wall thickness; stronger, lightweight materials; pressure resistant shapes; pre-pressurization of the structure and combinations of these techniques, especially when operating water depth increases. Utilizing the greatest wall thickness to withstand the maximum hydrostatic pressure over the complete depth of operation of the marine structure results in a simplified construction, but with a significant increase in weight and limit upon the ultimate water depth at which the marine structure can operate. A significant weight reduction can be achieved by varying the wall thickness in relation to the depth of water. Such a solution, however, significantly increases the complexity and cost to construct the marine structure, yielding only a modest increase in the limit of the ultimate operating water depth. The same result is true with the use of stronger lightweight materials, different shapes or combinations of the same. Each of these approaches use the strength of the construction material to withstand the hydrostatic pressure exerted on the external surface or wall of a typically hollow, closed marine structure.
Another known solution requires an increase in the internal pressure of the marine structure to a pressure that approximates the hydrostatic pressure that will be experienced at the depth at which the structure is planned to be operated. The obvious goal is to significantly reduce or eliminate the pressure differential experienced at the marine structure's wall. One approach is to pre-pressurize the marine structure, or compartments thereof, in order to eliminate or significantly reduce the pressure differential that will be experienced once the marine structure is located in its operational position. As can be appreciated, pre-pressurization calls for designing the marine structure to be, in effect, a pressure vessel with a positive pressure contained inside until finally positioned at the prescribed depth. This pre-pressurization requires increased wall thickness and presents a potential safety hazard because of the often-high pressures that must be contained within the vessel during handling prior to, and during installation. One method of delaying pre-pressurization is contemplated in U.S. Pat. No. 5,636,943 issued on Jun. 10, 1997 to Haney (hereinafter Haney '943). Acc

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