Method for fabricating a reduced-heave floating structure
A method for installing a heave plate (100) on a floating structure (10). The method allows for construction of the floating structure (10) and heave plate (100) on-shore, while providing for installation of the heave plate on the floating structure at sea.
This invention relates to a method for installing a heave plate on a floating structure, for example a spar, and to floating structures so constructed. The method includes providing a heave plate and a floating structure at an offshore joining site and methodology for connecting the heave plate to the floating structure.
BACKGROUNDAs the petroleum industry ventures to deep-water offshore locations to find sources of hydrocarbons, technology must be developed in order to retrieve these hydrocarbons. More and more, floating structures as opposed to bottom-founded structures are used in these deep-water applications. A particular floating structure used by the petroleum industry in deep water operations is the spar or deep-draft caisson vessel (DDCV), The spar or DDCV-type structure has a long vertically-disposed floating hull or caisson that, when in a floating operating position, has an upper portion above the water line that supports a drilling and/or production deck, and a lower portion below the ocean water line that provides buoyancy for the hull and deck. The spar or DDCV may be moored to the seafloor using mooring lines. Vertical pipes, known as risers, may be used to connect the subsea wells and the spar or DDCV to serve as conduits for the production of hydrocarbons and/or to provide access for drilling or workover activities.
The spar hull is typically cylindrical in shape, typically with a buoyant section at its upper end, a skirt-section at the middle, and a soft tank section at the lower end. The buoyant section is typically a hard tank, which provides the required buoyancy for the spar platform and can withstand hydrostatic pressure. The soft tank provides buoyancy for initial horizontal tow and subsequent upending. Once in place, both skirt section and soft tank can be filled with seawater. They are open to the sea at the bottom of the hull. In some cases the soft tank might be ballasted with heavy materials such as ore or concrete for added stability.
The spar's deep draft hull, which typically will extend below the wave zone of the body of water, offers excellent global motion responses such as heave or vertical motions caused by offshore wave forces. Reduction of these motions provides several operational benefits including the ability to use top-tensioned risers with surface well-control trees for the wells used to produce the field, which provides the operator with more options for efficiency and reduced costs.
Because of the severity of some offshore environments, additional reduction of the heave, pitch and roll motions of the DDCV structure may be desired. Many solutions have been proposed, including the attachment of a horizontally disposed heave plate to the DDCV hull. For example, see U.S. Pat. No. 5,558,467 “Deep water offshore apparatus” and U.S. Pat. No. 5,722,797 “Floating caisson for offshore production and drilling”. The heave plate provides added mass and damping to the floating structure by increasing the horizontal surface area at depths below the active wave actions that are located near the surface of the ocean.
However, there are logistical problems in implementing the heave plate concept. Because of convenience and cost efficiencies, offshore floating structures are almost always constructed on land and then transported to sea for installation. It is often cost-prohibitive and is logistically difficult to fabricate an offshore floating structure at sea. In regards to the heave plate configuration, constructing and combining the spar hull and the heave plate on land would provide similar efficiencies and costs savings. However, the reduced-heave configuration must still be transported to sea for installation. Because of the significant draft requirements caused by the heave plate configuration, transportation of the spar from an on-shore construction site to an offshore installation site is simply impractical.
Accordingly, what is needed is a method for providing a horizontally disposed heave plate on a floating structure, such as a DDCV or spar structure, which allows construction of the heave plate and the floating structure on land, while providing for the attachment of the heave plate to the floating structure at or near the offshore installation site. The current invention satisfies this need.
For additional information, see U.S. Pat. No. 3,101,798, Marine Drilling Apparatus; U.S. Pat. No. 3,572,041, Spar-Type Floating Production Facility; U.S. Pat. No. 3,921,557, Floating Storage Unit; U.S. Pat. No. 4,606,673, Spar Buoy Construction Having Production and Oil Storage facilities and Method of Operation; U.S. Pat. No. 4,702,321, Drilling, Production, and Oil Storage Caisson for Deep Water; Lowd, Judson D. and Hill, E. C., Use of a Spar Buoy Designed for Interim Production Processing; Offshore Technology Conference, OTC 1333, May 1971; U.S. Pat. No. 5,558,467, Deepwater Offshore Apparatus; U.S. Pat. No. 5,722,797, Floating Caisson for Offshore Production and Drilling; U.S. Pat. No. 6,206,614, Floating Offshore Drilling/Producing Structure; Tanaka, K., et al.; A Study on the Installation of a Submerged Caisson in Deep Waters, Offshore Technology Conference, OTC 5606, May 1987; Walker, S. et al.; STAPLA—The Design ofa Variable Draught Semi-Submersible Floating Production Vessel, Proceedings of the first European Offshore Mechanics Symposium, August 1990; U.S. Pat. No. 5,609,442, Offshore Apparatus and Method for Oil Operations; U.S. Pat. No. 5,924,822, Method for Deck Installation on an Offshore Substructure.
SUMMARY OF THE INVENTIONThe invention includes a method for installing a heave plate on an offshore floating structure. The method includes providing a heave plate and a floating structure at an offshore joining site. The heave plate may contain a template capable of receiving the horizontal cross section of the floating structure. The method includes vertically positioning the floating structure with the heave plate along a common vertical axis. The method includes contacting the floating structure with the template of the heave plate and connecting the heave plate to the floating structure.
The present invention will be described in connection with its preferred embodiments. However, to the extent that the following description is specific to a particular embodiment or a particular use of the invention, this is intended to be illustrative only and is not to be construed as limiting the scope of the invention. On the contrary, it is intended to cover all alternatives, modifications, and equivalents that are included within the spirit and scope of the invention, as defined by the appended claims.
As used herein and in the claims the phrase “floating structure” is meant to refer to any structure that will float in a body of water. Floating structure refers to a structure that floats when in its final installed position, however, such a structure may also be capable of sinking in a water body, for example, by ballasting. Examples of floating structures, include, but are not limited to, offshore floating structures, spars, classic spars, deep-draft caisson vessels, truss spars, cell spars, pipe spars and other structures used in the offshore petroleum industry.
As used herein and in the claims the term “spar” is meant to refer to any spar-type floating structure, including but not limited to, classic spars, deep-draft caisson vessels, truss spars, cell spars and pipe spars.
As used herein and in the claims the phrase “heave plate” is meant to refer to any device which is suitable for attachment to a floating structure and is useful for providing added mass and damping to the floating structure by increasing the horizontal surface area at depths below the active wave zones of a water body. A heave plate can be of any shape, for example, a relatively planar member in a rectangular, circular, polygonal, elliptical, or other regular or irregular shape. A heave plate may include a void area which may be used as a template and facilitate attachment to a floating structure.
As used herein and in the claims the term “template” is meant to refer to a cut out portion of a heave plate. The template may partially or completely pierce through the heave plate. In the alternative case where the template completely pierces through the heave plate a hole or void is created in the heave plate. In the alternative case where the template only partially pierces through the heave plate a well or depression is created in the heave plate. A template may also be designed to partially pierce some portion of the heave plate and completely pierce a portion of the heave plate to allow, for example, a moonpool opening in a heave plate that is attached to the bottom of a spar. The template may be designed to any shape and dimension to allow the heave plate to fit around a floating structure.
As used herein and in the claims the terms “hull” and “caisson” are used interchangeably to describe the underwater portion of a spar that provides buoyancy and stability.
In one alternative embodiment, the floating structure applicable for this invention is a DDCV or spar structure, for example as shown in
Referring now to
Referring to
After the heave plate 100 is structurally connected to the floating structure 10 the combined structure, i.e. the reduced-heave caisson vessel, can be towed, if necessary, to the field location and installed. Provision of the deck and installation of the facility at the field location can be accomplished using conventional techniques.
Referring now to
Referring now to
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To practice this embodiment of the invention, both the heave plate 100 and floating structure 10 are provided at the joining site 5. As shown in
Referring now to
After the heave plate 100D is structurally connected to the floating structure 10, the reduced-heave caisson vessel and deck can be towed to the field location, if necessary, and installed using conventional techniques.
Referring now to
If the floating structure 10 contains equipment or obstructions on the exterior of its hull that would impede the transfer of the heave plate/vessel 100D from the top to the lower end of the floating structure 10, for example fairleads or steel catenary riser porches, slots 120D in template 110D may be provided to allow an unimpeded transfer of the heave plate 100D, as shown in
Referring now to
Another potential benefit for providing a heave plate on a DDCV or spar structure is that the hull of the vessel may be designed and ultimately constructed smaller in size, thus providing cost savings in its construction. Generally, the hull of the vessel must be of a sufficient size to provide adequate buoyancy for the weight of the deck and hull, and be a sufficient length to minimize heave motions. The limiting factor for reducing the length of the hull may be heave motions. In other words, reducing the hull length may cause unacceptably large heave motions before it will cause buoyancy problems. By adding a horizontally disposed heave plate to decrease these motions, one may construct a DDCV or spar structure that is shorter in length, and obtain the corresponding cost savings while at the same time maintaining sufficient buoyancy.
The methodologies described herein may be used to construct offshore floating structures for use in exploring for and producing offshore hydrocarbon resources. The offshore floating structure may be for example a classic spar (e.g. a deep draft caisson vessel (“DDCV”) or a truss spar) that is equipped with a deck or a production or export riser. In the case of the spar, the deck can support offshore hydrocarbon resource (i.e. oil and gas) exploration, drilling and production operations. The deck may be use to conduct offshore seismic data collection. Alternatively, the deck can support offshore drilling equipment for oil and/or gas drilling operations. The deck may also support oil and/or gas production equipment for the production of oil and gas natural resources. Produced oil and/or gas may then be offloaded from the deck by, for example, pipeline to shore or a transport ship or barge and then moved to shore. The oil and gas may then be refined into usable petroleum products such as, for example, natural gas, liquefied petroleum gas, gasoline, jet fuel, diesel fuel, heating oil or other petroleum products.
The present invention has been described in connection with its preferred embodiments. However, to the extent that the foregoing description was specific to a particular embodiment or a particular use of the invention, this was intended to be illustrative only and is not to be construed as limiting the scope of the invention. On the contrary, it was intended to cover all alternatives, modifications, and equivalents that are included within the spirit and scope of the invention, as defined by the appended claims.
All documents cited herein are fully incorporated by reference for all jurisdictions in which such incorporation is permitted and to the extent they are not inconsistent with this specification. Although some dependent claims have single dependencies in accordance with U.S. practice, each of the features in any of the dependent claims can be combined with each of the features of one or more of the other dependent claims dependent upon the same independent claim or claims. Certain features of the present invention are described in terms of a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that ranges formed by any combination of these limits are within the scope of the invention unless otherwise indicated.
Claims
1. A method for installing a heave plate on an offshore floating structure, said method comprising:
- (a) providing said heave plate and said floating structure at an offshore joining site, said heave plate containing a template capable of receiving at least a portion of the horizontal cross section of said floating structure;
- (b) vertically positioning said floating structure with said heave plate along a common vertical axis;
- (c) contacting said floating structure with said template of said heave plate; and
- (d) structurally connecting said heave plate to said floating structure.
2. The method of claim 1, wherein said floating structure is a spar.
3. The method of claim 2, wherein said heave plate is connected to said floating structure so that said heave plate is horizontally disposed.
4. The method of claim 3, wherein said heave plate is a vessel capable of variable buoyancy.
5. The method of claim 4, wherein said offshore floating structure further comprises a deck, and wherein said vessel with variable buoyancy transports said deck to said offshore joining site.
6. The method of claim 3, wherein said heave plate is non-buoyant.
7. The method of claim 6, wherein said non-buoyant heave plate is transported to said offshore joining site on a vessel.
8. The method of claim 3, wherein said vertical positioning step comprises submerging said heave plate and positioning said heave plate beneath said floating structure.
9. The method of claim 8, wherein said template within said heave plate comprises an opening that extends through a portion of the height of said heave plate.
10. The method of claim 8, wherein said contacting step comprises contacting the lower end of said floating structure with the top of said template.
11. The method of claim 8, wherein said contacting step further comprises the use of guide lines.
12. The method of claim 3, wherein said vertical positioning step comprises submerging said floating structure and positioning said floating structure beneath said heave plate.
13. The method of claim 12, wherein said template within said heave plate comprises an opening that extends through the height of said heave plate.
14. The method of claim 12, wherein said contacting step further comprises the use of joining lines.
15. The method of claim 12, wherein said contacting step comprises contacting the upper end of said floating structure with the bottom of said template.
16. The method of claim 15, further comprising re-positioning said heave plate from the upper end of said floating structure to the lower end of said floating structure prior to said step of connecting said heave plate to said floating structure.
17. The method of claim 3, wherein said connecting step comprises latching said heave plate to said floating structure.
18. The method of claim 3, wherein said connecting step comprises grouting the annulus between said heave plate and said floating structure.
19. The method of claim 3, further comprising towing the combined floating structure and heave plate from said offshore joining site to an offshore production location.
20. The method of claim 5, wherein said vertical positioning step comprises submerging said floating structure and positioning said floating structure beneath said heave plate.
21. A method according to claim 3, further comprising:
- (e) producing offshore hydrocarbon resources using the floating structure.
22. The method of claim 21, further comprising:
- (f) transporting the hydrocarbon resources to shore.
23. An offshore floating structure with a heave plate installed according to any one of claims 1 through 20.
Type: Application
Filed: Jun 25, 2003
Publication Date: Jan 17, 2008
Inventors: Roald T. Lokken (Houston, TX), J. Ward Turner (Houston, TX)
Application Number: 10/554,631
International Classification: E02D 29/00 (20060101);