Abstract:
An offshore platform assembly includes a platform 12, four legs 10 and four footings 18. Each leg 10 is coupled to the platform by upper and lower bearings which are pivotable in a direction of inclination of the legs, the upper bearing being fixed with respect to translational movements, whilst the lower bearing can slide in a plane common with the plane of the platform 12. In an alternative embodiment, the lower bearing may be fixed and the upper bearing sliding. This assembly enables the platform to be used in high waters and prevents bending of the legs, which can occur with prior art systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 08/893,658, filed Jul. 11, 1997, which has issued as U.S. Pat. No. 5,954,454 hereby incorporated by reference thereto. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     REFERENCE TO A &#34;MICROFICHE APPENDIX&#34; 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The apparatus of the present invention relates to an offshore platform assembly known as jack-up rigs used for production, exploration drilling for oil or gas, or offshore maintenance. More particularly, the present invention relates to an offshore platform assembly with slant legs, each leg having two vertically spaced bearings in the platform resulting in reduced loading in the legs from the wind and wave forces, the increased resistance to overturning, and the reduced lateral movement of the platform. 
     2. General Background of the Invention 
     Most jack-up rig designs use straight i.e. vertical legs. The assembly uses a floatable hull with three or four tubular or latticed legs which may be circular, square or triangular. The legs support the platform in the working condition, and are supported by the platform during transit. Once the legs are located on the sea bed, elevation of the hull to the platform working height is accomplished by elevating units installed at each corner of the platform. These may be rack and pinion systems or hydraulic jacking systems which use friction clamps or pins which engage pin holes spaced at regular intervals up the legs. The jacking system couples the hull to the legs and supports the weight of the hull when elevated. 
     An example is shown in U.S. Pat. No. 5,092,712. The design disclosed in the &#39;712 patent utilizes an offshore platform assembly which uses inclined legs. The legs pass through a vertical hull and the platform is elevated, flexible leg guides are adapted to move laterally, to some degree, absorbing much of the bending loads and shear forces imposed on the legs, by the use of a compressible member formed as a resilient vertical rectangular sleeve, a spring or other adjustable means which permits a limited lateral bending moment acting on the leg which passes through the guides in the platform hull. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention solves the problems in the art in a simple and straightforward manner. What is provided is an offshore platform assembly with slant legs, each leg having two vertically spaced bearings in the platform, one hearing having a laterally fixed location and a single degree of rotational freedom in the direction of the leg inclination, the other bearing having a single degree of translational freedom in the plane of the platform and a rotational degree of freedom in the direction of the leg inclination. In a preferred embodiment, the attachment of the bottom of each leg to its respective footing also allows an angular adjustment between the two. Even the fixed bearing may be laterally adjustable, but thereafter locked during the jacking process. 
     An additional embodiment utilizes a sliding lower leg guide installed in the four corners of the hull and a split collar guide installed in the footings which allow the hull to be jacked to its working height without bending the legs. 
     Therefore, it is a principal object of the present invention to provide a jack up rig assembly that utilizes a slant leg feature which is an improvement over the straight leg design due to the reduced loading in the legs from the wind and wave forces, the increased resistance to overturning, and the reduced lateral movement of the platform. 
     It is a further object of the present invention to provided a jack up rig assembly with no limitation placed on the working height (or air gap) which is therefore a major improvement over prior art. 
     It is a further object of the present invention to provided a jack up rig assembly wherein the sliding lower guide does not use springs or other resilient means to absorb loads from the leg during hull elevation and storm loading, while the rotational degree of freedom of the guides permits smooth jacking due to uniform bearing of the guides on the legs as the angle of leg inclination changes. 
     It is a further object of the present invention to provided a jack up rig assembly which aims to eliminate or reduce the additional loading incurred with elevation of the hull on slanted legs, with such loading, in the current state of the art, being in addition to the loads from the operational or storm design condition. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEW OF THE DRAWINGS 
     For a further understanding of the nature, objects, and advantages of the present invention, reference should be had to the following detailed description, read in conjunction with the following drawings, wherein like reference numerals denote like elements and wherein: 
     FIG. 1 shows an elevation of the platform in the transport condition with the legs fully elevated and the hull in a floating mode; 
     FIG. 2 shows an elevation of the platform with the hull jacked up to its working height and the footings embedded in the ocean floor; 
     FIG. 3 shows a plan view of the platform; 
     FIG. 4 illustrates the change in inclination of the legs which occur when the hull is elevated to its working height, normally about 2-3°; 
     FIG. 5 illustrates one of the platform upper guides which is fixed and unable to move horizontally, but which permits pivoting movement. The four segments of the guide are shown each with their own pivot pin; 
     FIG. 5A is a view of the upper guide in a direction parallel to the axis of the pivot pins; 
     FIG. 6 is a plan view of one of the lower guides which is adapted to slide horizontally in one direction but is able to react to loads from the leg in a direction orthogonal or perpendicular to the direction of sliding; 
     FIG. 6A is a view of one of the lower guides in a direction parallel to the axis of the pivot pins; 
     FIG. 6B is an end view of the lower guide showing the guide keyed into the hull supporting structure on each side of the guide; 
     FIG. 6C shows the location of the lower leg guides on the platform corners, and their direction of movement as the platform is raised or lowered; 
     FIG. 7 shows a section cut through the platform illustrating the fixed upper and sliding lower guide, and the pivot connection at the footing; 
     FIG. 8 shows a section cut through the leg footing; 
     FIG. 9 shows a plan view of the leg footing; 
     FIGS. 10, 11 and 12 show the footing split-collar guide at various states of engagement; 
     FIG. 13 illustrates an elevational view of a deep water platform using multi-braced lattice legs; 
     FIG. 14 illustrates an isolated view of one of a plurality of chords that comprises part of each of the latticed legs of the platform in FIG. 13; 
     FIG. 15 illustrates a representational view of the cross section of a three-legged platform, each of the legs triangular in configuration; 
     FIGS. 16 and 17 illustrate cross sectional views of each of the legs of a triangular leg platform illustrating the guide configuration in each of the legs of the platform. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention provides a jack-up platform (FIGS. 1-3) with slanting legs 10 inclined at a fixed angle of between 5 and 10 degrees which allows elevation of the hull 12 to a specified air gap above the surface of the sea without inducing bending moments in the legs. 
     Reference will now be made to FIG. 4 for discussion of the hull elevation. 
     The platform is towed to its location and the legs 10 are lowered to the sea bed 14. During the leg lowering phase, the sliding lower guides 16 are locked in position to ensure that the legs 10 contact the sea floor 14 at the correct angle of inclination. The locking mechanism may be mechanical or hydraulic. Penetration of the footings 18 is accomplished by extracting the water from inside the footings or by using hull ballast water. 
     With the legs 10 fully penetrated, the lower guide 16 locking mechanism is disengaged for the initiation of hull elevation. 
     Referring to FIG. 4, as the hull 12 climbs vertically, the angle of inclination of the legs 10 gradually reduces. 
     The present invention allows for unrestricted changes in inclination of the legs by allowing the hull lower guide to slide horizontally, and the base of the leg to pivot within a well formed in the footing. For some designs, it may be preferable to use a fixed lower guide and to adapt the upper guide to slide horizontally. 
     With normal air gap achieved, the lower guide 16 locking mechanism is engaged so that all legs 10 may resist loading equally due to the storm wind and wave loading. The split collar guides 20 (FIGS. 10 to 12) are installed at the top of the footing 18 well to fix the legs 10 at the sea-bed 14 which reduces the leg bending moments at the lower guide. 
     Referring to FIGS. 5 and 5A, the preferred structure for the upper leg guides 22 is shown. This includes four coupling members 24 pivotably connected to the platform and unable to move translationally relative thereto. The coupling members 24 hold one of the legs 10 so that it can slide there within and pivot along a single axis as a result of pivoting of the coupling members 24. 
     FIGS. 6, 6A, 6B and 6C show in detail the structure of the lower leg guides 16. This includes four coupling members 26 equivalent to the coupling members 24 of the upper guides 22. The principal difference is that the coupling members 26 are provided on a sliding mechanism, as shown by the arrow in FIGS. 6 and 6C. The amount of slide would typically be in the region of 5 to 10 inches. To assist in gliding, the sliding mechanism may be provided with friction reducing means, such as roller bearings; a friction reducing agent or with low friction surfaces. Movement of the sliding mechanism may be along a slight arc. 
     FIG. 7 depicts how the angle of inclination of the legs 10 can be changed as a result of adjustment of the coupling mechanisms 16, 22. 
     FIG. 7 to 9 show schematically the structure of the footing 18. As will be apparent, the legs 10 are a loose fit in their respective footings, to enable the legs to pivot once the footings 19 have been secured in to the sea-bed. 
     I refer now to FIGS. 10, 11 and 12. 
     FIG. 10 shows the left-hand segment of the split collar 20 installed in the footing well 18. The purpose of this arrangement is to ensure that the footing is correctly aligned with the leg 10 during the footing embedment operation. 
     FIG. 11 shows the left-hand segment retracted allowing the legs 10 to rotate unrestricted within the footing well during hull elevation. 
     FIG. 12 shows both segments of the split collar 20 installed in the footing well. 
     The present invention provides for articulation or rotation of the legs 10 as they pass through the hull 12, and also for relief from the leg rotational fixing at the leg footing connection during the jacking phase. 
     Jacking of the platform can be by any of the well known mechanisms. For example, there may be provided jacking pinions which co-operate with racks provided on the legs 10. 
     With reference to FIG. 12, the rotational fixing thereby achieved after jacking at the footing 18 assists in reducing the platform horizontal displacements and footing reactions due to overturning moments from the wind and wave forces. 
     In another embodiment, jack-up platforms that move frequently may have legs 10 and footings 18 integrally welded together. The bottom surface may be conical or pointed thereby avoiding high restraining movements from the supporting soil which might cause high upper guide forces whilst jacking. 
     In yet another embodiment, deeper water designs may employ legs 10 with pointed lower ends which simply dig into the sea bed. These are free to tilt, once engaged, as required for the jacking procedure. Once the assembly is jacked into position, anchor means may be added to each leg so as to locate the legs against lateral displacement. 
     In an alternative embodiment, the guides 16 and 22 provide a loose fit of the legs 10 there within and dispense with pivotable coupling members. 
     It is to be understood that various modifications and additions can be made to the above-described embodiments within the scope of the invention, which should only be interpreted in accordance with the claims. 
     It will be apparent that the upper and lower guides 22, 16 may be revised such that the upper guides slide and the lower guides are fixed. 
     FIGS. 13-17 illustrate an additional embodiment of the system of the present invention utilized in a deep water application, utilizing, as illustrated, multi-braced lattice legs. In FIG. 13, the two dimensional view of the platform 100 is supported by a plurality of multi-braced lattice legs 102 with the hull 100 elevated above the water surface 106. As illustrated initially in FIG. 13, legs 102 could be either set in a triangular configuration or a rectangular configuration. Legs 102 are supporting the platform at a certain height 104 above the level of the water 106, with each of the legs 102 of the platform mounted onto the sea bed 108 as illustrated in FIG. 13. For purposes of discussion, the platform 100 will be a platform secured by three legs 102, of which a cross sectional representation is illustrated in FIGS. 15-17. 
     First, turning to FIG. 14, each leg 102 of platform 100 would comprise three or more posts at the apices of the triangle, which are designated as chords 110 for example, in FIG. 15. For the type of platform that is illustrated in FIG. 13, as with the embodiments discussed in FIGS. 1 through 12, the platform 100 is raised using a rack and pinion elevating system which is shown generally by the numeral 112 in FIG. 14. In the elevating system as illustrated, the rack 114 is located on each chord 110 and the pinions 116 are positioned on each chord 110 with the pinions 116 engaging the rack 114 on each side of the chord 110. In this manner, each leg 102 is then guided on the tips 118 of the rack teeth 120 as the platform is raised to its desired height above the water 106 as seen in FIG. 13. In FIG. 14, the view through the elevating tower of one of the legs 102 illustrates one rack 114 of one chord 110. The bearing surfaces or guides 122, 124, are shown on each side of the rack 114 at the upper location 122 and at the lower location 124. The translational degree of freedom at the lower location 124 is indicated by the double arrow 130 in FIG. 14. There are four elevating pinions 116 which engage with the rack 114 during the elevating of the platform 100. 
     FIG. 15 illustrates a representational view of the three legs 102, having a chord 110 at each apex of the triangulated legs 102. There is further illustrated arrows 132 which serve to indicate the direction of leg inclination of the legs as the platform 100 is being raised into position. 
     In this particular embodiment, each of the three chords 110 located at the three apices of each triangulated leg 102 includes two bearing surfaces or guides 122 at the upper location and two bearing surfaces or guides 124 on each of the three chords 110 on each leg at the lower location. For purposes of construction and functioning, at the upper location, as seen in FIG. 16, all guides 122 are fixed and there is no lateral movement permitted between the guides 122 and the rack 114 during movement of the legs 102. 
     However, as seen in FIG. 17, at the lower location 124, all three chords, as members of the leg, have at least a translational degree of freedom in the plane of the platform 100 in the direction of the arrow 132 when jacking down and in the opposite direction of the arrow 132 when jacking up. The third outboard chord 110, designated as chord 110B, on each of the legs 102 is unable to move laterally normal or perpendicular to the direction of leg movement but is able to move in the direction of arrow 132 due to the clearances between the rack teeth 120 and the guides 122. The lower guides 124 in the lower location move freely translationally as indicated by the double arrow 134 in FIG. 17, offering no significant restraint to movement of the legs in the direction as indicated by the arrow 132 in FIG. 15. 
     Therefore, it is clear that during the elevational movement of the legs in relation to the platform, the upper guides 122 as illustrated in FIG. 16, are fixed and therefore allow no translational movement of the chords 110 at any corner of the three legs 102. However, in the lower location as illustrated in FIG. 17, the three guides allow translational movement of at least two of the chords, chords 110A, on each leg 102 so as to eliminate any significant bending stresses that may occur on the legs as the platform is moved upward and downward in relation to the legs 102 that are fixed as illustrated in FIG. 13. 
     The foregoing embodiments are presented by way of example only; the scope of the present invention is to be limited only by the following claims.