Compliant Offshore Platform

The present invention generally concerns offshore structures adapted to have a compliant response to waves, wind and ocean currents. More specifically, the present invention concerns a compliant offshore drilling and production platform in which a vertical restoring couple is used to counter platform sway.

Most offshore oil and gas production is conducted from platforms secured to the ocean bottom. A key design constraint for such platforms is that there be no substantial dynamic amplification of the platform's response to waves. This is accomplished by designing the platform to have natural vibrational periods which do not fall within that portion of the range of wave periods representing waves of significant energy. The several modes of platform vibration which are generally of greatest concern in platform design are pivotinf of the structure about the base (commonly termed "sway"), flexure ("bending") in the vertical plane, and torsion about the vertical axis. For most offshore locations the range of natural vibrational periods to be avoided is from 7 to 25 seconds, this representing the range of wave periods occurring with the greatest frequency.

For water depths up to about 300 meters, the technology for avoiding dynamic amplification of an offshore structure's wave response is quite well developed. Nearly all existing offshore structures designed for use in such water depths are fixedly secured to the ocean bottom and stiffened to cause each of the natural vibrational periods to be less than about 7 seconds. Such offshore structures are referred to as "rigid structures". However, as water depths exceed 300 meters, the tonnage of structural steel required to maintain sufficient platform rigidity to ensure that all natural vibrational periods remains below 7 seconds increases rapidly with depth. It has been suggested that for even the richest offshore oil fields the use of a rigid structure could not be economically justified in water depths exceeding about 420 meters due to the limitations imposed by the natural vibrational periods.

For deepwater applications, it has been proposed to depart from conventional rigid structure design and develop platforms having a sway period greater than the range of periods of ocean waves containing significant energy. Consequently, much of the environmental load imposed on the platform is resisted by its own inertia. Such platforms are termed "compliant structures." The use of a compliant platform effectively removes the upper bound on the sway period. This greatly reduces the increase in the structural steel, and hence cost, required for a given increase in water depth.

In one type of compliant structure, the guyed tower, the platform deck is supported on a slender space-frame structure extending from the ocean bottom to the ocean surface. A radially arranged set of guylines extend outward from an upper portion of the space-frame structure to the ocean bottom. These guylines provide a restoring force to counter platform sway induced by environmental forces. Guyed towers are disadvantageous in that the guyline system is expensive to fabricate and deploy. In certain applications the guylines may also present an obstacle to navigation and fishing in the vicinity of the platform.

A second type of compliant structure, the tension leg platform, uses buoyancy to provide a restoring force to resist the platform's lateral displacement. The deck of the tension leg platform is situated on a large buoyant hull which is secured to a foundation at the ocean bottom by a set of vertical tethers. The tethers are tensioned and hence maintain the hull at a deeper draft than it would assume if floating free. When the hull is displaced laterally by environmental forces, the net vertical buoyant force acting on the tethers produces a righting moment tending to restore the hull to its original vertical position.

A significant drawback of the tension leg platform is that its buoyancy requirements are great. This necessitates use of a large and expensive hull structure. This is undesirable in that it increases the cross sectional area of the structure exposed to wind, waves, and current. Additionally, the production wells system for a tension leg platform is substantially more complex than that required for a traditional rigid structure. Further, for use in water depths greater than about 600 meters it is highly desirable to provide the tethers with inherent buoyancy to minimize the loading the tethers impose on the hull. This presents numerous technical problems.

It would be desirable to develop a compliant tower which does not rely primarily on guylines or positive buoyancy to counter lateral displacement caused by environmental forces.

The present invention is directed to a compliant offshore platform in which platform sway is resisted by a vertical couple established by a set of flex piles. In the preferred embodiment, the platform includes a rigid space-frame structure having a base resting on the ocean bottom and extending upward to an upper portion positioned 15-30 meters above the ocean surface. A drilling and production deck is situated atop the space-frame structure. A set of shear piles prevents lateral displacement of the space-frame structure base, while permitting the space-frame structure to pivot about the ocean bottom in response to waves and other environmental forces. A plurality of flex piles are driven into the ocean bottom at preselected locations around the periphery of the platform. Each of these flex piles extends upward along a corresponding leg of the space-frame structure to a preselected elevation below the wave zone, where it is secured to the platform. In the preferred embodiment, the flex pile attachment location is at or near one-half the total height of the space-frame structure.

The flex piles provide substantially all of the platform's resistance to sway induced by environmental forces. Guylines are not required. As the platform sways, the flex piles attached to that side of the platform away from the direction of platform tilt are placed in tension, while the flex piles on the opposite side of the platform are placed in compression. Thus, the flex piles establish a restoring couple at the point of attachment to the space-frame structure which limits the magnitude of platform sway resulting from environmental forces. The stiffness, number and location of the flex piles are selected to yield a sway period of greater than 25 seconds. This is sufficiently great to ensure that there is no substantial dynamic amplification of the platform's response to waves.

For a better understanding of the present invention, reference may be made to the accompanying drawings, in which:

FIG. 1 is an elevational view of an offshore platform incorporating the present invention;

FIG. 2 is an enlarged view of portions of the platform shown in FIG. 1;

FIG. 3 is a graph illustrating the bending moment and environmental loading for the compliant offshore platform of the present invention as a function of the location of the attachment of the piles to the structure;

FIG. 4 is a comparison of the bending moment diagrams for a traditional fixed base jacket, a compliant offshore platform having its flex piles secured at the ocean surface, and a compliant offshore platform having its flex piles secured at one-half the platform height;

FIGS. 5-7 show resilient connectors useful in alternate embodiments of the present invention;

FIG. 8 shows a telescoped pile adapted for use with the present invention;

FIG. 9 is an elevational view of an alternate embodiment of the present invention in which a fixed base is used;

FIG. 10 is an elevational view of a second alternate embodiment of the present invention in which a fixed base is used;

FIG. 11 is an elevational view of an embodiment of the present invention in which tensioned cables rather than piles provide the restoring vertical couple against lateral deflection.

These drawings are not intended as a definition of the invention, but are provided solely for the purpose of illustrating certain preferred embodiments of the invention, as described below.

FIGS. 1 and 2 show an elevational view of a preferred embodiment of the compliant offshore platform 10 of the present invention. As will become apparent in view of the following discussion, the preferred embodiment of the compliant offshore platform 10 is adapted for use as an oil and gas drilling and production platform. However, the present invention can also be used for a variety of other purposes. To the extent that the following discussion is specific to drilling and production platforms, this is by way of illustration rather than limitation.

In the preferred embodiment, the compliant offshore platform 10 includes a drilling and production deck 12 situated atop a slender space-frame structure 14. The space-frame structure 14 is constructed of tubular steel in a manner well known to those skilled in the art. The space-frame structure 14 should be substantially rigid, having a natural bending period (flexure period) less than about 7 seconds. Drilling and production are performed through conductors 16 extending from the deck 12 to the ocean bottom 18. The conductors 16 are preferably situated proximate the central longitudinal axis of the platform 10. In certain embodiments it is desirable to rigidly secure the conductors 16 to the deck 12, permitting the conductors 16 to flex in response to platform sway. Placing the conductors 16 near the platform's longitudinal axis minimizes this flexing.

The legs 28 and other tubular components of the space-frame structure 14 are sealed to avoid being flooded with seawater upon platform installation. This decreases the in-water weight of the platform 10. In the preferred embodiments of this invention, the space-frame structure 14 and deck 12 together have a net negative buoyancy. As will be described below, the requisite degree of platform compliancy is obtained without the need for special buoyancy chambers.

Platform sway (substantially rigid rotation about the platform base) is resisted by tubular steel flex piles 20 driven into the soil surrounding the base 22 of the platform. The flex piles 20 extend upward to a flex pile connector 25 situated at preselected pile attachment location 24 on the periphery of the space-frame structure 14. At the pile attachment location 24, the flex piles 20 are welded, grouted or otherwise rigidly connected to the space-frame structure 14. In addition to resisting platform sway, the flex piles 20 also support a portion of the weight of the platform 10 and transmit lateral forces to the soil. In some embodiments shear piles 26 may be driven through the platform base 22 to provide additional resistance against lateral deflection of the platform base 22. The shear piles 26 are not grouted to the platform base 22 and accordingly do not restrain vertical motion of any portion of the platform base 22. Because the space-frame structure 14 is substantially rigid, lateral deflection of the upper portion of the platform 10 in response to waves and other environmental forces causes the space-frame structure 14 to pivot about the ocean bottom 18. This pivoting occurs about a horizontal pivot axis at or near the ocean bottom 18. This axis passes approximately through the geometric center of the platform base 22 and is orthogonal to the direction of platform motion.

As the platform 10 pivots, that portion of the platform base 22 away from the direction of platform deflection moves upward from the ocean bottom 18 an amount proportional to the magnitude of the deflection. The opposite portion of the platform base 22 moves downward into the ocean bottom 18 an equal amount. Accordingly, the flex piles 20 on that side of the platform 10 away from the direction of deflection are placed in tension while the flex piles 20 on the opposite side the platform 10 are placed in compression. This establishes a vertical couple acting at the pile attachment location 24 tending to resist further lateral deflection and restore the platform 10 to its initial vertical position. Buckling of the flex piles 20 as they are placed in compression is prevented by pile guides 27 secured to the space-frame structure 14.

The magnitude of the vertical couple for a given degree of platform deflection is a function of the length, cross sectional area, number, and composition of the flex piles 20 and the lateral distance from the pivot axis to the point at which each pile enters the ocean bottom. The magnitude of the vertical restoring couple as a function of platform sway should be established to cause the platform 10 to have a sway period exceeding 25 seconds.

The embodiment of the compliant offshore platform 10 shown in FIGS. 1 and 2 is designed for use in a water depth of 790 meters under Gulf of Mexico environmental conditions. The platform 10 has nine main legs 28 arranged in a 3.times.3 square array, 73 meters on a side. Each of the legs 28 is 1.83 meters in diameter and has a maximum wall thickness of 7.0 cm. Four flex piles 20 surround each of the corner legs 28. The flex piles 20 are 1.37 meters in diameter, have a thickness of 5.7 cm. and are grouted to the legs 28 at a location 440 meters above the ocean bottom 18. File guides 27 are provided every 36 meters along the length of each flex pile 20. Two shear piles 26 are driven adjacent each of the middle legs 28 along the periphery of the platform base 22. The weights of the space-frame structure 14, pile system and topsides are, respectively, 39,000 metric tons, 18,120 metric tons and 13,600 metric tons. During the design one-hundred year storm, the maximum deck offset from the vertical is 9.1 meters, the maximum platform tilt is 0.7.degree. and the maximum platform twist is 0.1.degree.. The platform 10 has a sway period of 37 seconds and natural periods of bending and torsion of 6.8 and 5.8 seconds, respectively.

We have discovered that it is highly desirable to avoid placing the pile attachment location 24 at or near the platform deck 12. The position of the pile attachment location 24 should be selected on the basis of minimizing the internal moment of the platform 10 in response to anticipated environmental loading. By minimizing the internal moment which the platform 10 must resist, it is possible to use a lighter and less expensive space-frame structure 14 than would otherwise be necessary. FIG. 3 is a graph showing the maximum bending moment and total environmental load on a 300 meter compliant offshore structure as a function of the elevation of the pile connection location. The maximum bending moment reaches its minimum value when the pile connection location is established at approximately one-half the total elevation of the space-frame structure 14. This result is substantially independent of the specific configuration of the space-frame structure 14 and is also substantially independent of water depth.

FIG. 4 compares the moment diagram for a compliant offshore platform with the flex piles secured at one-half the platform height to corresponding moment diagrams for a traditional fixed base jacket and a compliant offshore platform with the flex piles tied to the space-frame structure 14 at the ocean surface. The fixed base jacket must have a bending resistance sufficient to support a linearly increasing unidirectional moment. As can readily be seen in FIG. 4, the restoring moment of the compliant offshore platform 10 with the flex piles tied at one-half the total height of the platform results in a reduction in the absolute magnitude of the bending moment by dividing the moment into positive and negative components. Accordingly, the maximum single amplitude value of the bending moment which must be resisted by the space-frame structure is greatly reduced, allowing use of a structure which requires less structural steel than alternate platform configurations. The reduction in the internal moment which must be carried by the space-frame structure is made possible by transferring a portion of the moment to the flex piles 20. This is desirable since piles are far less susceptible to fatigue damage than the tubular connections of a space-frame structure. Additionally, on a per unit weight basis, the complexity and expense of fabricating piles is much less than that for space-frame structures.

A further advantage of placing the pile attachment location at one-half the total elevation of the platform is that this causes the location of the maximum bending moment to coincide with that portion of the platform at which additional stiffness is most effective in reducing the bending period. Accordingly, in the preferred embodiment shown in FIG. 1, the additional cross bracing at the pile attachment location 24 provides resistance to the greatest bending moment experienced by the space-frame structure 14 and is also placed to cause the greatest possible reduction in the bending period.

Another advantage of placing the pile attachment location at or near the midpoint of the platform height is that the total environmental load on the platform 10 is significantly decreased. As best shown in FIG. 2 the flex piles 20 represent a significant fraction of the total vertical cross-section of the platform 10. By placing the flex piles 20 below the wave zone the effective cross section of the platform 10 to environmental loading is significantly reduced, resulting in a significant decrease in total platform loading. This is illustrated in FIG. 3. Additionally, it is desirable to decrease the total length of the piles used in the platform 10 to minimize the fabrication and installation expense.

In the preferred embodiment of the present invention, the space-frame structure 14 is fabricated in separate upper and lower sections 29,31 of approximately equal length. This significantly decreases the complexity and cost of platform installation and fabrication. In platform installation the two sections 29,31 of the space-frame structure 14 are launched from separate barges. The legs 28 of each section 29,31 are capped and filled with air to have a net positive buoyancy. While floating, the sections 29,31 are aligned and temporarily locked together with mechanical connectors. The space-frame structure 14 is then positioned over the installation site, upended and set on the ocean bottom. As best shown in FIG. 2, the flex piles 20 supporting the platform 10 are each driven through corresponding upper and lower pile connection sleeves 21,23. These sleeves are secured, respectively, to the platform legs 26 at the lowermost portion of the upper section 29 and the uppermost portion of the lower section 31. The flex piles 20 are then grouted or otherwise permanently secured to both sleeves 21,23. This arrangement serves both to permanently join the upper and lower sections 29,31 of the space-frame structure 14 and to provide the necessary pile-platform connection.

It is critical to ensure that the stress imposed on the flex piles 20 under maximum lateral deflection of the platform 10 does not cause plastic deformation of the piles or failure of the ocean bottom soil in which the piles 20 are set. In the embodiment shown in FIGS. 1 and 2, the greatest design stress imposed on the flex piles 20 occurs when platform deflection occurs along a diagonal of the platform cross section during the design one-hundred year storm. This yields a maximum platform deflection of 0.7.degree., which causes the piles in the direction of platform tilt to be compressed a total of 59 cm., while the piles away from the direction of platform tilt elongate a like amount. The set of piles receiving the greatest design stress are those surrounding the leg which is in the direction of platform tilt. The total stress is 1.83.times.10.sup.5 kPa (26.6.times.10.sup.3 psi), of which 1.49.times.10.sup.5 kPa (21.6.times.10.sup.3 psi) results from tilt induced pile compression and 3.4.times.10.sup.4 kPa (5.0.times.10.sup.3 psi) is due to the portion of the platform weight supported by the piles. This total stress is 76% of the maximum buckling stress of 2.4.times.10.sup.5 kPa. The tensioned set of flex piles surrounding the leg away from the direction of tilt is under a smaller load due to the initial compressive loading resulting from the weight of the platform. In many applications, the limiting pile stress occurs in driving the pile. This imposes a minimum pile wall thickness dependent on the nature of the ocean bottom soil through which the pile is driven. This minimum wall thickness may be greater than that necessary to accommodate the maximum degree of pile compression/extension in the course of platform sway. To overcome this limitation it may be desirable to employ piles having a relatively thick-wall section which is driven into the ocean bottom 18 and a relatively thin-wall portion extending upward from the ocean bottom 18 to the pile attachment location 24.

Clearly, there is a minimum pile length which will provide the necessary platform compliancy for a given set of design conditions without imposing an unsafe pile stress or causing soil failure. The minimum pile length cannot be reduced simply by increasing the number of piles or increasing the cross section of each pile because this would decrease the platform compliancy. For a platform having the relative proportions and pile-leg configuration shown in FIGS. 1 and 2, the minimum pile length necessary to maintain an acceptable degree of compliancy is about 440 meters for Gulf of Mexico conditions and 760 meters for North Sea conditions.

One solution to this problem is to shift the location at which the flex piles 20 enter the ocean bottom 18 to a position nearer the centerpoint of the platform base 22. This results in a decrease in pile elongation/compression, and hence pile stress, for a given degree of platform deflection. Of course, it would be necessary to increase the number or cross-section of the piles in proportion to the decrease in pile stress to maintain the necessary magnitude of the vertical restoring couple.

Another manner of reducing the minimum pile length is to place a resilient connector between the platform 10 and the pile 20. This resilient connector 30 preferably takes the form of an elastomeric spring as shown in FIGS. 5-7. In the embodiment shown in FIG. 5, the resilient connector 30 is contained within a housing 32 rigidly secured to the platform 10 at the desired pile attachment location 24. Concentric with and interior to the housing 32 is a sleeve 34 through which the flex pile 20 is driven. The pile 20 is welded, grouted or otherwise rigidly connected to the sleeve 34. The sleeve 34 and housing 32 define an annular spring containment space 35 bounded at its upper and lower ends by reaction members 36 fixed to the housing 32. An annular piston 38 secured to the pile connection sleeve 34 extends into the spring containment space 35 intermediate the upper and lower reaction members 36. A stack of thin annular elastomeric spring elements 40 occupy the spring containment space 35. The spring elements 40 are separated one from the other by steel plates 42 to control the deformation of the spring elements 40 as they are placed in compression.

Operation of the resilient connection 30 occurs as follows. When the platform 10 tilts away from the pile 20 the housing 32 moves upward relative to the pile 20, placing the elastomeric elements 40 intermediate the annular piston 38 and lower reaction member 36 in compression. When the platform 10 tilts toward the pile 20, the upper set of elastomeric elements 40 are placed in compression. The resilient connection 30 should be configured so that in conjuction with the pile 20 it provides load-deflection characteristics appropriate to provide the desired maximum lateral platform deflection and natural sway period in response to the environmental conditions of the platform installation site. Stiffness of the resilient connection 30 is controlled both by the modulus of elasticity of the material from which the spring elements 40 are composed and the radial cross-sectional area of the individual spring elements 40. The maximum allowable deflection is controlled by the total thickness of the spring elements 40. For most elastomeric materials total spring compressive deformation should be limited to 10% of the unstressed thickness of the material in compression to avoid plastic deformation or other undesirable load-deformation behavior. In the ideal configuration, the combination of the resilient connector 30 and the flex pile 20 provides load-deflection characteristics equivalent to those yielded by use of a longer pile.

The resilient connector 30 could assume many other embodiments. FIG. 6 shows an elastomeric spring having a threaded preload mechanism. This preload mechanism 44 permits adjustment for material relaxation and creep and also prevents the piston 38 from separating from the elastromeric elements 40 when they are unloaded. Separation of the unloaded elastomeric elements 40 could also be prevented by bonding all of the elastomeric elements 40 and steel plates 42 together so that the elastomeric elements 40 could also act in tension. FIG. 7 shows an elastomeric spring in which the individual elastomeric elements 40 and steel plates 42 are bonded together with the spring being adapted to act in shear rather than compression. Those skilled in the art will recognize that the resilient connector 30 need not include an elastromeric spring. Metallic and hydraulic springs could be used instead.

Another alternative for platforms situated in water depths too shallow to avoid overstressing a standard tubular pile is to use a telescoped pile 46, as shown in FIG. 8. A complete description of telescoped piles is provided in U.S. Pat. No. 4,378,179, issued Mar. 29, 1983. As used in conjunction with the compliant platform 10 of the present invention, the telescoped piles 46 include a standard tubular pile element 47 driven into the ocean bottom 18 and extending upward to a position above the pile attachment location 24. A tubular pile sleeve 48 is concentric with and fixedly secured to the upper end of the tubular pile element 47. The pile sleeve 48 extends downward through pile guides 27 to the pile attachment location 24 where it is fixedly secured to the space-frame structure 14. The use of the telescoped pile 46 yields a pile having an effective length equal to the length of the pile element 47 plus the length of the pile sleeve 48. Thus, for a platform 10 in a 300 meter water depth with a desired pile attachment location of 150 meters, the use of telescoped pile 46 extending to the ocean surface yields an effective pile length of 450 meters.

Shown in FIGS. 9 and 10 are alternate embodiments of the present invention adapted for use in relatively deep water applications. In these embodiments the space-frame structure 14 is situated atop a fixed base segment 50. The space-frame structure 14 is secured to the fixed base segment 50 by a structural pivot joint 52 which is adapted to resist shear loads and torsional moments. A suitable pivot joint 52 is detailed in copending U.S. patent application Ser. No. 756,405, filed July 17, 1985. The base segment 50 is adapted to remain substantially free from tilting and bending, and hence serves as a fixed foundation about which the space-frame structure 14 pivots. FIG. 9 illustrates the base segment 50 as a battered space-frame structure fixed securely to the ocean bottom 18 by skirt piles 51 which are rigidly secured to the base segment 50. Alternately, the base segment 50 could be a conventional gravity structure or, as shown in FIG. 10, a space-frame structure with the flex piles 20 grouted or otherwise mechanically connected to sleeves 53 in its base to resist tilting. As in the previous embodiments, the flex piles 20 extend upward through pile guides 27 and are secured to the space-frame structure 14 by pile connectors 23.

In another alternate embodiment, shown in FIG. 11, cables at 54 are used in place of piles 20, Each cable 54 extends along the outer surface of the space-frame structure 14 from an anchor pile 56 to a cable connector 58 secured to the space-frame structure at the elevation at which the vertical restoring couple is to be applied. Alternately, the cables 54 could be run through the legs 28 of the platform 10. To reduce the possibility of snap loading the cables 54, it is important to prevent the cables 54 from going slack under extreme lateral displacement. This is accomplished by pretensioning the cables 50. In certain applications it may be desirable to have the cables extend from the ocean bottom 18 to a cable connection elevation at the ocean surface or deck. Buoyancy modules 57 are provided to offset the compressive loading imposed on the space-frame structure 14 by the tensioned cables 50.

The preferred embodiment of the present invention and the preferred methods of using it have been detailed above. It should be understood that the foregoing description is illustrative, and that other embodiments of the invention can be employed without departing from the full scope of the invention as set forth in the appended claims.