Not applicable
The present invention generally relates to floating offshore mineral exploration and production platforms and, more particularly, is concerned with a compliant guide for protecting the buoyancy cans and components of the floating offshore platform from damage from impacts that may occur as a result of hydrodynamic loads (e.g. Froude-Krylov impact forces) on the buoyancy cans.
The spacing between the buoyancy can outer wall and the contact point of the guide structure in the centerwell of a Spar type floating offshore mineral exploration and production platform has been found to be very important in determining loads on the buoyancy can. The buoyancy can will have contact points (most typically four to six), in the form of built-up wear strips. These contact points on the buoyancy can will face corresponding contact points on the guide structure. See U.S. Pat. No. 4,702,321 to Edward Horton for “Drilling, Production, and Oil Storage Caisson for Deep Water” and U.S. Pat. No. 4,740,109 to Edward Horton for “Multiple Tendon Compliant Tower Construction”, both incorporated herein by reference.
Although sensitivity to gap size had previously been noticed in both model tests and in some calculations, efforts to determine the optimum gap size had assumed that once a small enough gap had been achieved, the nature and magnitude of the loads, including impact loads, would converge to those of a zero gap. Efforts were aimed at finding the point of diminishing returns on an exponential-type either load or bending moment response curve, where forces were determined without consideration for impact loads.
Previous attempts to minimize the gap have been dependent on the tolerances that are achievable in fabricating buoyancy cans, guides, and supporting structures. Recent analytical and model test work has indicated that the conclusions made previously did not fully account for impact loads, and that the nature of the signal is quite different if there is a gap that is large enough for these fabrication tolerances. Loads on the buoyancy can and guide have been found to be large and numerous enough to make practical design for both strength and fatigue difficult. Therefore, there is a need to reduce loads, particularly impact loads, on buoyancy cans.
It has been found that the solution to the above-described problem involves the insertion of an additional flexible element between the guide, the guide support structure, and the buoyancy can. One result of such an insertion is reduction of the effective gap size. In some embodiments of the invention, therefore, the gap will be, effectively, zero, (potentially with some preload). Thus, the insert provides for practical fabrication tolerances. Since the gap size is small, the relative velocity at impact is also small. If the gap is effectively zero, the loads are roughly equivalent to the loads calculated using the closed gap assumption. Additionally, if there were to be an impact load, the stiffness of the connection is reduced, in some embodiments, by designing the compliant guide stiffness to meet load requirements.
Using a computer simulation program, loads on the guides were computed for a given random excitation for a number of gap sizes both with and without the compliant guide. Results for maximum load from these simulations are shown in
According to one exemplary embodiment of the invention, a guide for a buoyancy can on a floating offshore platform is provided. The platform includes at least one support structure adjacent the buoyancy can. The guide comprises at least one compliant guide member supported by the support structure adjacent the exterior surface of the buoyancy can. Lateral movement of the buoyancy can toward the support structure compresses the compliant member so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A wear pad disposed between each guide structure and the buoyancy can protects the guide and buoyancy can from friction wear.
According to another exemplary embodiment of the invention, the support structure has at least one projection attached thereto. A buoyancy can guide is provided that comprises at least one elastomeric compression pad supported by the support structure and adjacent the exterior surface of the buoyancy can. Lateral movement of the buoyancy can toward the support structure compresses the elastomeric compression pad so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A wear pad disposed between each elastomeric compression pad and the buoyancy can protects the compression pad from friction wear against the buoyancy can. At least one carriage is attached to the guide. The carriage has a channel therein that slidingly engages the projection on the support structure.
According to still another exemplary embodiment of the invention, the support structure has upper and lower projections attached thereto. A buoyancy can guide is provided that comprises a plurality of elastomeric compression pads supported by the support structure and adjacent the exterior surface of the buoyancy can. Each compression pad has first and second opposite sides. Lateral movement of the buoyancy can toward the support structure compresses the elastomeric compression pads so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A first rigid plate is associated with the first side of the compression pad. A second rigid plate is disposed between and affixed to the support structure and the second side of the compression pad for affixing the compression pad to the support structure. A wear pad support is attached to the first rigid plate. The wear pad support has upper and lower ends and comprises a base plate, a pair of spaced side plates attached to and extending from the base plate, and a top plate extending between the side plates. A wear pad is secured to the wear pad support. The wear pad is disposed between the compression pad and the buoyancy can for protecting the compression pad and buoyancy can from friction wear. Upper and lower carriages extend from the upper and lower ends, respectively, of the wear pad support. Each carriage has a channel therein that slidingly engages a respective projection on the support structure.
According to yet another exemplary embodiment of the invention, the support structure has upper and lower projections attached thereto. A buoyancy can guide is provided that comprises a plurality of elastomeric compression pads supported by the support structure and adjacent the exterior surface of the buoyancy can. Each compression pad has first and second opposite sides. Lateral movement of the buoyancy can toward the support structure compresses the elastomeric compression pads so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage. A bearing plate is affixed to the first side of the compression pad. A first rigid plate is affixed to the bearing plate. A second rigid plate is disposed between and affixed to the support structure and the second side of the compression pad for affixing the compression pad to the support structure. A wear pad support is attached to the first rigid plate. The wear pad support has upper and lower ends. The wear pad support comprises a base plate, a pair of spaced side plates attached to and extending from the base plate, and a top plate extending between the side plates. A wear pad is secured to the wear pad support. It is disposed between the compression pad and the buoyancy can for protecting the compression pad and buoyancy can from friction wear. Upper and lower carriages extend from the upper and lower ends, respectively, of the wear pad support. Each carriage has a channel therein that slidingly engages a respective projection on the support structure.
In one particular preferred embodiment, the compliant buoyancy can guide comprises a pair of nested elastomeric compression pads, that is, a first compression pad circumferentially surrounding a second compression pad. The first elastomeric compression pad is more compliant (i.e., less stiff) than the second pad, so that a non-linear load deflection response is achieved in a compact and cost-effective structure. For relatively small deflections, the compliant can guide of this design provides small loads, a result that is essential during the installation process, and that reduces stick-slip fatigue on the risers supported by the buoyancy cans. For relatively large deflections, this embodiment provides higher loads that reduce the fatigue loads on the buoyancy can and the platform.
According to still another exemplary embodiment of the invention, apparatus for compliantly guiding a buoyancy can on a floating offshore platform is provided. The apparatus comprises a plurality of spaced support structures attached to the platform and arranged radially around the exterior circumferential surface of the buoyancy can. At least one elastomeric compression pad is attached to each support structure and disposed adjacent the exterior surface of the buoyancy can. Lateral movement of the buoyancy can toward one of the support structures compresses the elastomeric compression pad attached thereto so as to absorb the force generated by the buoyancy can movement, and so as to protect the buoyancy can and components of the floating offshore platform from damage.
According to a further exemplary embodiment of the invention, for a floating offshore platform having at least one buoyancy can and a support structure adjacent the buoyancy can, a method is provided for protecting the buoyancy can and the support structure from damage caused by impact of the buoyancy can with the support structure. The method comprises supporting at least one compliant member between the buoyancy can and the support structure. The method further comprises absorbing the force generated by lateral movement of the buoyancy can by compressing the compliant member between the buoyancy can and the support structure.
According to still another exemplary embodiment of the invention, for a floating offshore platform having at least one buoyancy can, a support structure for supporting a compliant guide for the buoyancy can is provided. The support structure comprises a T-girder and means for supporting the guide from the support structure.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following Detailed Description of the Invention taken in conjunction with the accompanying drawings, in which:
In
Referring now to
Other combinations of different elastomeric stiffnesses, or the use of spring components with different spring constants, will occur to those of skill in the art. For example, a spring or other compliant member is used in alternate embodiments instead of the elastomeric compression pads 18, 20, and 22 to absorb the force generated by movement of the buoyancy can 12.
Referring next to
In some embodiments, a bearing plate and pad retainer 30 is affixed to the first side of the compression pads 18, 20, and 22. A first rigid plate 32 is affixed to the side of the bearing plate 30 opposite each of the compression pads 18, 20, and 22. The wear pad support 28 is attached to the sides of the first rigid plates 32 opposite the bearing plates 30. For the upper and lower compression pads 18 and 22, junction plates 34 are affixed to the bearing plates 30 near their outer edges. The wear pad support 28 is removably attached to the first rigid plate 32, the bearing plate 30, and the junction plate 34 by means such as bolts 36 or other suitable mechanical fasteners, or by welding. A second rigid plate 38 is disposed between, and affixed to, the support structure 16 and the second side of the upper and lower compression pads 18 and 22, respectively, for affixing the upper and lower compression pads 18 and 22, respectively, to the support structure 16, as shown in
For each compression pad 18, 20, and 22, a retainer basket 40 extends out from the bearing plate 30 adjacent to the sides of the compression pad for capturing and retaining the compression pad in the unlikely event that it becomes detached from its bearing plate 30. The retainer basket 40 also helps to distribute the bolting force equally around the bearing plate 30. Equal force distribution helps to avoid damaging the elastomeric pad.
In some embodiments, the wear pad support 28 comprises a base plate 42, a pair of spaced side plates 44 attached to and extending from the base plate 42, and a top plate 46 extending between the side plates 44. In some exemplary embodiments, the top plate 46 and the outer edges of side plates 44 form a receptacle for securing the wear pad 26 therein. Other suitable wear pad supports and structural components that may be used will occur to those of skill in the art. Referring to
Referring to
Referring to
Referring to
In some embodiments, the compression pads 18, 20, and 22 are made of an elastomeric compound. In other embodiments, the compression pads 18, 20, and 22 are replaced by helical or leaf springs, air or liquid filled bumpers, or other passive or active compliant elements that provide increased force with increased displacement. The bearing plates 30, the first and second rigid plates 32 and 38, respectively, the junction plates 34, the base plates 42, the side plates 44, the top plates 46, the side plates 58, the bottom plates 60, and the end plates 64 preferably made of rigid steel plate.
As seen in
Referring to
Each guide 114 is a guide assembly that comprises a metal (e.g. steel) base plate 122 that is attached to the support structure 116, preferably by means such as bolts 120, as mentioned above. The base plate 122 is formed with a central aperture 124, and a first elastomeric compression pad 126 has a first or inner end that is fixed to the base plate 122 around the aperture 124. The first compression pad 126 has a circular, or preferably (as shown) an elliptical cross-sectional shape, defining a hollow core area in which a second elastomeric compression pad 128 is located. Preferably, the compression pads 126, 128 are arranged coaxially. The second compression pad 128 is seated in the central aperture 124 of the base plate 122 so that its inner end surface seats against the support structure 116. The outer diameter of the second compression pad 128 is measurably less than the inner diameter of the first compression pad 126, so that there is a peripheral channel 130 formed between the two pads. Also, the second compression pad 128 has a central axial bore 132.
The first compression pad 126 is formed of an elastomeric material that is less stiff (more compliant) than the material from which the second compression pad 128 is made, for reasons that will be explained below. Both compression pads may be made of a vulcanized natural rubber (latex) or any suitable synthetic elastomeric polymer that will readily suggest itself to those skilled in the pertinent arts.
Secured to the outer end surface of the first compression pad 126, preferably by bonding, is a contact plate 134, preferably of steel. The respective axial dimensions of the first and second compression pads are such that there is a gap 136 between the outer end surface of the second compression pad 128 and the inner surface of the contact plate 134. As best shown in
The outer surface of the contact plate 134 is advantageously covered with a wear pad 138 that is preferably made of a durable, low-friction polymer, such as UHMW polyethylene. The wear pad 138 is advantageously provided with a longitudinal channel 140 that substantially prevents, or at least minimizes, the rotation of the buoyancy can 112 relative to the guide 114 when the wear pad 138 seats against the buoyancy can 112.
An end plate 142 may advantageously be secured to one or both of the upper and lower ends of the contact plate 134. Each end plate 142 extends inwardly toward the base plate 122. One or both of the end plates 142 may be secured to the base plate 122 by a bracket 144, as shown in
Also as shown in
As also shown in
The nesting of the stiffer second contact pad 128 within the softer first contact pad 126 provides a non-linear load deflection response in a compact and cost-effective structure. For relatively small deflections, the compliant can guide 114 of this second embodiment provides small loads, a result that is essential during the installation process, and that reduces stick-slip fatigue on the risers supported by the buoyancy cans. For relatively large deflections, this embodiment provides higher loads that reduce the fatigue loads on the buoyancy can and the platform. This non-linear response is illustrated in
In
It will, of course, be appreciated that any desired load-versus-deflection characteristic can be obtained by changing the geometry, dimensions, and/or material characteristics of the compression pads 126, 128. Furthermore, the geometry of the compression pads, particularly the softer first compression pad, is designed to provide the above-described non-linear load-versus-deflection characteristic without significant buckling.
The compliant buoyancy can guide, in accordance with the above-described embodiments of the present invention, will be understood from the foregoing description of exemplary embodiments, and it will be apparent that, although specific examples of the invention and their advantages have been described in detail, various modifications and alterations will occur to those of skill in the art. Such modifications and alterations should be considered equivalents to the structure specifically disclosed herein, and should therefore be considered to be within the spirit and scope of the invention, as defined by the appended claims.
This application is a continuation-in-part of co-pending application Ser. No. 09/850,599; filed May 7, 2001, the disclosure of which is incorporated herein by reference, which, in turn, claims the benefit, under 35 U.S.C. Section 119(e), of provisional application No. 60/283,240; filed Apr. 11, 2001, the disclosure of which is likewise incorporated herein by reference.
Number | Date | Country | |
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60283240 | Apr 2001 | US |
Number | Date | Country | |
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Parent | 09850599 | May 2001 | US |
Child | 10760807 | Jan 2004 | US |