GB2356378A

GB2356378A - Offshore jack-up platform with inclined legs

Info

Publication number: GB2356378A
Application number: GB9927587A
Authority: GB
Inventors: Roy Malcolm Bennett
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-11-22
Filing date: 1999-11-22
Publication date: 2001-05-23
Also published as: GB9927587D0

Abstract

An offshore platform assembly comprises a plurality of inclined legs 102 supporting a platform 100, a first plurality of bearing surfaces (122, Fig 8) on each of the plurality of legs allowing no translational movement of the legs and a second plurality of bearing surfaces (124, Fig 8) on each of the plurality of legs allowing translational movement in at least one degree of freedom in a horizontal plane of the platform. The fixed bearing surfaces may be positioned above the moveable bearing surfaces. The legs may comprise three triangular shaped legs with a fixed and a moveable bearing positioned at each apex of the triangle of each leg.

Description

2356378 OFFSHORE PLATFORM ASSEMEBLY The present invention relates to an

offshore platform assembly known as a jack-up rig, used for production, exploration drilling for oil or gas, or offshore maintenance. 5 Most jack-up rig designs use vertical legs. The assembly uses a floatable hull with three or four tubular or latticed legs. 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 comer of the platform. These may be rack and pinion systems or hydraulic jacking systems which use friction clamps or pins which engage pinholes 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 improved offshore platform assembly is disclosed in the applicant's earlier British patent application 2,319,004.

The present invention seeks to provide an improved offshore platform assembly.

According to an aspect of the present invention there is provide an offshore platform assembly comprising:

a) a plurality of slant legs; b) a platform supported by the plurality of slant legs; c) a first plurality of bearing surfaces on each of the plurality of legs allowing no translational movement of the legs, as the platform is moved upward or downward; d) at least a second plurality of bearing surfaces on each of the plurality of legs, at least some of the plurality of the second surfaces allowing at least a translational degree of freedom in the plane of the platform and providing substantially no resistance to movement in said translation degree of freedom so that bonding stresses are negligible in the platform legs as the platform is moved upward or downward.

According to another aspect of the present invention there is provided an offshore platform assembly; comprising:

a) a plurality of slant legs; b) a platform supported by the slant legs, the platform being moveable upwardly and downward in relation to the slant legs; c) a first plurality of upper guide surfaces on each of the plurality of legs allowing no translational movement of the legs, as the platform is moved upwardly or downwardly; d) at least a second plurality of lower guide surfaces on each of the plurality of legs, at least some of the plurality of lower guide surfaces allowing at least a translational degree of freedom in the plane of the platform so that bending stresses are negligible in the platform legs as the platform is moved upwardly or downwardly while said guide surfaces provide substantially no resistance to movement in said translational degree of freedom during movement of the platform.

The preferred embodiment provides an offshore platform assembly with slant legs, each leg having two vertically spaced bearings in the platform, one bearing having a laterally fixed location and a single degree of rotational 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.

Another embodiment utilises a sliding lower leg guide installed in the four comers 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 these embodiments can provide a jack-up rig assembly that utilises a slant leg feature which is an improvement over the straight leg design due to the reduced 10ading in the legs from the wind and wave forces, the increased resistance to overturning, and the reduced lateral movement of the platform.

3 The jack-up rig assembly may have no limitation placed on the working height (or air gap) which is therefore a major improvement over prior art.

Advantageously, 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.

The described embodiments can 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.

An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows an elevation of a prior art platform in the transport condition with the legs fully elevated and the hull in a floating mode;

Figure 2 shows an elevation of the prior art platform of Figure I with the hull jacked up to its working height and the footings embedded in the ocean floor; Figure 3 shows a plan view of the prior art platform of Figure 1;

Figure 4 illustrates the change in inclination of the legs of the prior art platform of

Figure 1, which occurs when the hull is elevated to its working height, normally about 2-3 0; Figure 5 shows the location of the lower leg guides on the platform corners, and their direction of movement as the prior art platform of Figure 1 is raised or lowered;

4 Figure 6 shows a section cut through the prior art platform of Figure I illustrating the fixed upper and sliding lower guide, and the pivot connection at the footing;

Figure 7 illustrates an elevational view of an embodiment of deep-water platform using 5 multi-braced lattice legs; Figure 8 illustrates an isolated view of one of a plurality of chords that comprises part of each of the latticed legs of the platform of Figure 7; Figure 9 illustrates a representational view of the cross section of a three-legged platform, each of the legs triangular in configuration; Figures 10 and 11 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; Figure 12 shows a section cut through the leg footing; Figure 13 shows a plan view of the leg footing; and Figures 14, 15 and 16 show the footing split-collar guide at various states of engagement.

A prior art platform assembly as described in GB-A-2,319,004 is shown in Figures I to 6 and includes slanting legs 10 inclined at an 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 Figure 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 Figure 4, as the hull 12 climbs vertically, the angle of inclination of the legs 10 gradually reduces. This system 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 adopt 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 (Figures 1 to 11) 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.

The specific structure of this assembly is disclosed in detail in GB-A-2, 319,004.

The principal practice is that the leg coupling members are provided on a sliding mechanism, as shown by the arrow in Figures 5 and 6. The amount of slide would typically be in the region of 10 to 25 centimetres. 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.

Figure 6 depicts how the angle of inclination of the legs 10 can be changed as a result of adjustment of the coupling mechanism 16, 22.

6 Figures 7 to 11 illustrate a preferred embodiment of the system for use in a deep-water application, utilising, as illustrated, multi-braced lattice legs. In Figure 7, 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 Figure 7, 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 Figure 7. For purposes of discussion, the platform 10 will be a platform secured by three legs 102, of which a cross sectional representation is illustrated in Figures 9 to 11.

First, turning to Figure 8, each leg 102 of platform 100 would comprise three or more posts of the apices of the triangle, which are designated as chords 110 for example, in Figure 9. For the type of platform that is illustrated in Figure 7, as with the system discussed in GB-A-2,319,004, the platform is raised using a rack and pinion elevating system which is shown generally by numeral 112 in Figure 8. In the elevating system as illustrated, the rack 114 is located on each chord I 10 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 20 the platform is raised to its desired height above the water 106 as seen in Figure 7. In Figure 8, 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 25 Figure 8. There are four elevating pinions 116 which engage with the rock 114 during the elevating of the platform 100. Figure 9 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 30 serve to indicate the direction of leg inclination of the legs as the platform 100 is being raised into position.

7 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, and at the upper location, as seen in Figure 10, 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 Figure 11, 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 13 (Figure 9) when jacking down and in the opposite direction of the arrow 13 when jacking up. The third outboard chord 110, designated as chord 1 10B, 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 13 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 Figure 11, offering no significant restraint to movement of the legs in the direction as indicated by the arrow 132 in Figure 9.

Therefore, it is clear that during the elevational movement of the legs in relation to the platform, the upper guides 122 as illustrated in Figure 10, 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 Figure 11, the three guides allow translational movement of at least two of the chords, chords 10A, 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 Figure 7.

Figures 12 to 16 show schematically the structure of an embodiment of footing 18. As will be apparent, the legs 2 are a loose fit in their respective footings, to enable the legs to pivot once the footings 9 have been secured in to the sea-bed.

8 Figure 14 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 during the footing embedment operation. 5 Figure 15 shows the left-hand segment retracted allowing the legs to rotate unrestricted within the footing well during hull elevation.

Figure 16 shows both segments of the split collar 20 installed in the footing well. 10 The preferred embodiment provides for articulation or rotation of the legs 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.

With reference to Figure 16, the rotational fixing thereby achieved after jacking at the footing 18 assists in reducing the platform horizontal displacements and footing reactions 20 due to overturning moments from the wind and wave forces.

In the preferred embodiment, jack-up platforms that move frequently may have legs and footings 18 integrally welded together. The bottom surface may be conical or pointed thereby avoiding high restraining movement from the supporting soil which might cause 25 high upper guide forces whilst jacking.

In yet another embodiment, deeper water designs may employ legs 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 30 be added to each leg so as to locate the legs against lateral displacement.

9 In an alternative embodiment, the guides provide a loose fit of the legs there within and dispense with pivotable coupling members.

It will be apparent that the upper and lower guides may be revised such that the upper guides slide and the lower guides are fixed. However, it will be apparent that the improvement lies in the way the legs can pivot and move in translation and any mechanism could be provided to achieve this, as will be apparent to the skilled person.

Claims

1. An offshore platform assembly comprising:

2. An offshore platform assembly; comprising:

a) a plurality of slant legs; b) a platform supported by the slant legs, the platform being moveable upwardly and downwardly in relation to the slant legs; c) a first plurality of upper guide surfaces on each of the plurality of legs allowing no translational movement of the legs as the platform is moved upwardly or downwardly; d) at least a second plurality of lower guide surfaces on each of the plurality of legs, at least some of the plurality of lower guide surfaces allowing at least a translational degree of freedom in the plane of the platform so that bending stresses are negligible in the platform legs as the platform is moved upwardly or downwardly while said guide surfaces provide substantially no resistance to movement in said translational degree of freedom during movement of the platform.

11

3. An offshore platform assembly as in claim 1 or 2, wherein the plurality of slant legs comprises three triangular shaped legs.

4. An offshore platform assembly in claiin 1, 2 or 3, wherein the first plurality of bearing or guide surfaces are at a point above the second plurality of bearing or guide surfaces.

5. An offshore platform assembly in any preceding claim, wherein each of the plurality of slant legs is triangular shaped, with a vertically spaced bearing positioned at each apex of the triangle of each leg, and comprising an upper fixed bearing and lower bearings following translational movement of the leg in the direction of leg inclination.

6. An offshore platform assembly substantially as hereinbefore described with reference to and as illustrated in Figures 7 to 16 of the accompanying drawings.