GB2319004A - Offshore Platform Assembly - Google Patents

Abstract

In an offshore platform assembly including a platform 12, four inclined 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. Each footing 18 provides an amount of angular adjustment between it and its respective leg 10 (see Figs 7-12). 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

OFFSHORE PLATFORM ASSEMBLY The present invention relates to an offshore platform assembly known as a jack-up. These vessels are used for production, exploration drilling for oil or gas, or offshore maintenance.

The assembly uses a floatable hull with three or four tubular or latticed legs which may be circular, square or triangular.

Most jack-up designs use straight i.e. vertical 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 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 present invention 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. Elevation of the hull on slanted legs however, induces loading in the legs which is in addition to the loads from the operational or storm design condition. An example is shown in U.S. Patent 5,092,712.

The present invention aims to eliminate or reduce this additional loading.

Accordingly, the invention proposes 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 the other having a single degree of freedom in the plane of the platform. In a preferred form, 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 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.

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. There is thus no limitation placed on the working height (or air gap) which is therefore a major improvement over prior art.

An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: 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 2-30.

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 stages of engagement.

The preferred embodiment provides a jack-up platform (Figs. 1 to 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 Figures 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 therewithin and pivot along a single axis as a result of pivoting of the coupling members 24.

Figures 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 Figures 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.

In an alternative embodiment, no pivotable couplings are provided. Instead, the legs are located in bores which allow some rotation of the legs. This alternative structure will be readily apparent to the skilled person.

Figure 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.

Figures 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.

Referring 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 moments 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 therewithin and dispense with pivotable coupling members.

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.

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.

Claims (9)

1. An offshore platform assembly comprising: a plurality of slant legs; a platform supported by the legs and two vertically spaced bearings provided on the platform for each leg; the first of said bearings having a laterally fixed location and a single degree of rotational freedom in a direction of leg inclination, the other of said bearings having a single degree of translational freedom in the plane of the platform and a rotational degree of freedom in the direction of leg inclination.

2. An offshore platform assembly according to claim 1, including a footing for each of said legs, each footing providing an amount of angular adjustment between it and its respective leg.

3. An offshore platform assembly according to claim 2, including means to fix each of said legs in position in its respective footing.

4. An offshore platform assembly according to claim 1, wherein said one of said bearings is laterally adjustable but lockable during raising or lowering of the platform.

5. An offshore platform assembly according to claim 1, wherein each of said bearings includes a plurality of guide members disposed around a respective leg and within which a respective leg can slide.

6. An offshore platform assembly according to any preceding claim, wherein said bearings are pivotable.

7. An offshore platform assembly according to any one of claims 1 to 5, wherein said bearings are not pivotable but allow pivoting of the legs.

8. An offshore platform assembly according to claim 7, wherein said bearings are in the form of bores within which the legs can slide and pivot.

9. An offshore platform assembly substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.