Ships – Floating platform
Reexamination Certificate
1999-07-29
2001-08-14
Sotelo, Jesus D. (Department: 3617)
Ships
Floating platform
C114S293000
Reexamination Certificate
active
06273018
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to a buoyant substructure for an offshore platform, and to a method of forming that substructure.
More particularly the invention relates to a buoyant substructure which can be arranged to float upright over an offshore oil and/or gas field, in order to support drilling and/or production facilities.
BACKGROUND OF THE INVENTION
Oil and/or gas production in deep water may be carried out from facilities set on buoyant substructures. These substructures may comprise ship shaped Floating Production Storage and Offloading vessels (FPSO's), semisubmersibles, Tension Leg Platforms (TLP's) or spars. When using FPSO's or semisubmersibles for production, wellheads are usually located on the seabed. For both TLP's and spars, wellheads can be disposed above sea level.
The use of spars for drilling and production is a relatively recent development. (The Brent spar was merely for storage and offloading.) The Oryx Neptune spar (for production) was installed in August 1996. The design, fabrication and installation of that spar were described in OTC Papers 8384 and 8385. These papers were presented at the Offshore Technology Conference in Houston Tex. during May 1997. Subsequently, spars were designed for the Chevron Genesis (drilling and production) and Exxon Diana developments.
The spars referred to above were constructed primarily of steel. It has also been suggested that spars should be made of concrete. Spar configurations for construction in concrete were described on pages 29—33 of Offshore Engineer for April 1996.
In addition to the “spar” substructures described above, proposals have been made for other types of floating substructures designed generally on the spar principle.
One such proposal is set out in PCT Patent Specification No WO96/14473. This shows a single cylindrical hull, and a downward extension formed of four vertical legs of reduced diameter. The cylindrical hull is buoyant, and supports a deck. The vertical legs are connected together by diagonal truss members. The substructure is held in place by an array of semi taut mooring lines.
Our UK Patent Specification No 2,147,546A describes a multi column floating substructure. This has four corner columns which support a deck. The columns are buoyant, and are of substantial diameter. At the lower ends of the columns there are downwardly extending legs of reduced diameter. The downwardly extending legs are connected together by diagonal bracing members. The substructure is held in place by a conventional spread of (catenary) anchor chains.
In the substructures illustrated in these two patent specifications, the downwardly extending legs have diagonal truss or bracing members between them. Moreover, in both cases there are sudden transitions of cross section (from the hull or corner columns respectively, to the legs of reduced diameter).
The substructures in the patent specifications referred to above are designed to be built in a horizontal attitude. The truss or bracing members require significant fabrication activity. Fit up and welding of these members adds cost and takes up time in the fabrication schedule. When complete, the substructures can be floated out (still in a horizontal attitude) to their required locations. At those locations they have to be upended. The sudden transitions of cross section may create instability during floatout and upending.
Multi leg substructures have smaller diameter columns, and so need less reinforcement to resist hydrostatic crushing loads, than do the hulls of spars. This results in lighter substructures to support similar topside weights. Additionally, the spacing apart of the columns gives better stability characteristics.
The present invention is intended to take advantage of these characteristics of a multi leg configuration, while avoiding the need for complex truss or bracing members, extending between the columns. It is also intended to reduce the disadvantages of discontinuities in cross section in the columns.
DISCLOSURE OF THE INVENTION
The invention provides a buoyant substructure to float upright in water (e.g. for an offshore platform) and comprising at least three spaced apart columns joined together in generally aligned relationship, in which at least one of the columns is adapted to be ballasted at its end which is to float low down in the water, in which the columns are joined together by discrete cross members which are widely spaced apart along the length of the columns, and in which the discrete cross members have relatively small dimensions in the direction of alignment of the columns as compared with the total length of the columns.
It is preferred that at least the column adapted to be ballasted at its end which is to float low down in the water has at least two cylindrical portions, one of which portions is above and the other of which portions is below at least one of the discrete cross members.
It is also preferred that at least the column adapted to be ballasted at its end which is to float low down in the water has a surface piercing portion, a buoyancy portion, and a ballastable portion, and in which (when floating upright) the horizontal cross section of the buoyancy portion is greater than the horizontal cross section of the ballastable portion.
It is further preferred that all of the at least three columns have generally aligned surface piercing portions, buoyancy portions and ballastable portions.
It is further preferred that all of the columns are of identical external configuration.
It is yet still further preferred that two of the columns have identical respective horizontal cross sections along their aligned lengths.
Preferably the columns are aligned in parallel relationship.
Preferably the at least three columns are joined together in spaced apart relationship at three positions along their aligned lengths.
In a form in which there is a surface piercing portion, a buoyancy portion and a ballastable portion, it is preferred that the horizontal cross section of the buoyancy portion is greater than the horizontal cross section of the surface piercing portion.
In this last mentioned form it is further preferred that the at least three columns are joined together in spaced apart relationship between the surface piercing portions, and at or near the respective opposed ends of the ballastable portions.
The columns may be joined by cross members comprising discrete lattice truss frameworks in planes defined by adjacent elongate members.
Alternatively, the columns may be joined by cross members comprising box girder elements in planes defined by adjacent elongate members.
It is preferred that elements joining the columns together in spaced apart relationship constitute riser and/or conductor guides.
It is also preferred that elements joining the columns together in spaced apart relationship constitute heave suppression baffles.
According to a feature of the invention, a mooring line to keep the substructure on station is connected to the substructure by a bridle having upper and lower elements which can be controlled to adjust the line of action of the force in the mooring line.
The invention also provides a method of forming a buoyant substructure (e.g. for an offshore platform) which includes the steps of constructing at least three columns horizontally for joining in a spaced apart relationship, floating the substructure so formed horizontally on two of those columns, uprighting the substructure by selective ballasting with water, adding solid ballast to the lower end of at least one of the columns, and securing the substructure in position with mooring lines.
REFERENCES:
patent: 3347052 (1967-10-01), Steitle et al.
patent: 3859804 (1975-01-01), Koehler et al.
Christiansen Poul-Eric
Gottsche Graham Frederick
Piermattei Earl J.
Ostfeld David M.
Sotelo Jesus D.
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