GB2451250A

GB2451250A - Offshore structure with braced legs

Info

Publication number: GB2451250A
Application number: GB0714425A
Authority: GB
Inventors: Rowan Patterson
Original assignee: UWG Ltd
Current assignee: UWG Ltd
Priority date: 2007-07-24
Filing date: 2007-07-24
Publication date: 2009-01-28
Also published as: GB0714425D0

Abstract

An offshore structure has a platform 10 supported on legs 12. The bottom ends of the legs are fixed in the seabed 14. At least one cross-bracing frame 18 is fitted between the legs at a position intermediate the seabed and the platform. Wires 26 run between the seabed, the frame and the platform, with each wire having one end fixed on one leg and the other end fixed on another leg, and the wires being put into tension to brace the structure. The frame may comprise tubular sockets 24 linked by rigid beams 20 and 22 which in use lie in the horizontal plane. Each leg may pass through one of the sockets, the sockets having a diameter slightly greater than that of the legs, so that the frame can slide up and down on the legs.

Description

Offshore Structure This inventon relates to an offshore structure where a platform is supported on legs fixed on or in the sea bed. The structure will typically be a drilling or

production platform in an oil and/or gas field.

The legs of the structure need to be sufficiently strong and rigid to support the platform above the sea level and to resist the stresses imposed on the structure by wind, waves and currents.

It is known, for example from US Patent 5,722,494 to link well tubulars S...

II' (specifically well conductors through which drilling and production flows will *:*::* pass), both below and above sea level by bracing structures which extend between the conductors, so that the conductors support one another. The well described in that patent is operated from a jackup drilling rig supported on the :: sea bed alongside the well, on legs, as is conventional for jackup rigs.

According to the invention, there is provided an offshore structure comprising a platform and a plurality of legs for supporting the platform, the bottom ends of the legs being adapted to be fixed at the sea bed, at least one frame slidable along the legs, the frame providing horizontal bracing between the legs, and tension wires which extend between the legs at or near the sea bed and the frame, and between the frame and the tops of the legs, and means for tensioning the wires.

The frame may be held in position by an equilibrium between the tension wires above and below the frame, before being clamped or otherwise fixed to the legs.

In this specification, the term wire" is not to be read narrowly. The term as used herein encompasses any elongate member which is relatively thin compared to the legs, and which may be flexible or rigid.

The use of tension wires in this way adds rigidity to the structure. Using wires which are relatively thin avoids adding other loads to the structure such as might appear if thicker bracing members were used which could induce vortexes when exposed to flowing water currents. The intermediate bracing structure provided by the wires is primarily required to prevent buckling of the legs when surface loads, environmental loads and water depth so dictate. The :.::s entire structure has a water depth and environment capability that is limited in its application by the environmental loads of wind, wave and currents. * S. * S *

The frame may comprise tubular sockets, and each leg can pass through one of the sockets, with the sockets having a diameter slightly greater than that of the *.*.e.

S * legs, so that the frame can slide up and down on the legs. The sockets are * ** preferably linked by rigid beams which, in use, will lie in a horizontal plane.

The legs will normally be tubular. There may be three, four or more the four legs.

Each tension wire is preferably linked to the frame at or close to the sockets, and extends diagonally between two of the legs.

The means for tensioning the wires may be located adjacent the top of the legs and may comprise another frame to which the wires are attached, and means for applying a tensioning force between the top frame and the platform mounted at the top of the legs.

According to a second aspect of the invention, there is provided an offshore well structure comprising a wellhead, a plurality of conductor legs extending through the welJhead and a surface structure supported above sea level with the conductor legs connected to the surface structure, wherein tension members are connected between the welihead, the conductor legs and the surface structure, each tension member being connected at one end to one of the conductors and at the other end to another of the conductors to provide cross-bracing between the conductors. Preferably the cross-bracing s applied over the whole height of the conductors.

: * The invention wHI now be further described, by way of example, with reference to the accompanying drawings, in which * ** * S S * Figure 1 is a perspective view of an offshore structure in accordance with the invention; and ** S... * S

Figure 2 is a side view of the structure shown in Figure 1.

The structure comprises a foundation structure (in this case a mudline template 16) for spacing three or more legs or conductors 12 that may be driven or cemented in predrilled holes in the seabed 14. Four such legs are shown in the Figures. A surface platform 10 is subsequently supported above sea level and attached to the legs.

This structure is most likely to be used in relatively shaflow water; ie water of a depth suitable for the use of jack-up rigs. Typically this will be up to 70 metres depth, but the principle can be used at depths of up to 100 metres and possibly greater.

The Figures show the structure once it has been erected and braced, A frame 18 is positioned between the top and bottom of the legs 12. This frame has horizontal beams 20 which brace between the legs in the horizontal plane. As can be seen in Figure 1 the frame may also have other horizontal strengthening and supporting members 22 which span between the beams 20.

The beams join tubular collars 24 which are able to slide up and down on the legs 12.

Tension wires 26 are tensioned between the lower ends of the legs 12 and the frame 18, and also between the frame 18 and the top of the legs. As shown in the Figures, the wires each extend from a point on one leg to a point at a different height on a different leg, so that the wires cross one another below and above the frame 18.

The tension in the wires will be determined by analysis in conjunction with the conductor analysis that is performed to design the legs or well conductors. * S * * .5

The wires may be flexible cables or metal rods, for example of the type used to support yacht masts.

The tension members can be attached by a suitable means to all sides of the foundation structure, cross frame or frames and surface structure and individually or collectively tensioned. All structures are attached to the legs to assist transmission of loads.

The wires may be tensioned in a number of different ways. The wires may be anchored to a bottom section 28 of the platform and may be tensioned from the top of the structure by using jacks to pull the bottom section up towards the top section 30 of the platform.

Instead of anchoring the wires to a section of the platform, a further frame 18 may be provided below the platform 10, with the wires being attached to that frame and with means for puffing that frame up towards the platform.

Alternatively, the wires could be tensioned from below. The bottom ends of the wires could be attached to a further, substantial frame just above the mudline which slides freely on the legs. If the top ends of the wires are anchored at or near the platform, the weight of the frame near the mudline can place sufficient tension in the wires.

Once the structure is complete and the wires have been tensioned, the frame or I...

frames may be fixed to the legs, eg by grouting or clamping, to lock the structure against relative movement between the components.

The legs may be structural components only, their function being only to support the platform 10. Alternatively, the legs can perform the function of conductors, so that drilling and/or production tasks can be undertaken through the tubular legs.

A further benefit of the tension wires are that they can be used to locate and hold the cross frames in place prior to clamping to the legs. The tension members could be installed with the foundation structure and cross frame structure from the surface, hauled into place and connected to the conductors, thus divers or ROVs may not be required.

Claims

Claims 1. An offshore structure comprising a platform and a plurality of legs for supporting the platform, the bottom ends of the legs being adapted to be fixed at the sea bed, at least one frame slidable along the legs, the frame providing horizontal bracing between the legs, and tension wires which extend between the legs at or near the sea bed and the frame, and between the frame and the tops of the legs, and means for tensioning the wires.
2. A structure as claimed in Claim 1, wherein the frame comprises tubular sockets with each leg passing through one of the sockets, with the sockets having a diameter slightly greater than that of the legs, so that the frame can slide up and down on the legs.
3. A structure as claimed in Claim 2, wherein the sockets are linked by rigid beams which, in use, will lie in a horizontal plane. *. . * S* * **
4. A structure as claimed in any preceding claim, wherein each tension wire is linked to the frame at or close to the sockets.
5. A structure as claimed in Claim 4, wherein each wire extends diagonally between two of the legs.
6. A structure as claimed in Claim 4 or Claim 5, wherein between each pair of legs, two wires cross one another in an X formation.
7. A structure as claimed in any preceding claim, wherein there are four legs.
8. A structure as claimed in any one of Claims 1 to 6, wherein there are three legs.
9. A structure as claimed in any preceding claim, wherein the legs are paraflel to one another.
10. A structure as claimed in any preceding claim, wherein the means for tensioning the wires is located adjacent the top of the legs.
11. A structure as claimed in Claim 10, wherein the means for tensioning the wires comprises an upper frame to which the wires are attached, and means for *::::* applying a tensioning force between the upper frame and the platform mounted *:*::* at the top of the legs
12. A structure as claimed in Claim 10, wherein the wires are anchored at their top ends and are attached at their lower ends to a lower frame and the *:*. lower frame is able to slide downwards on the legs such that the weight of the lower frame puts the wires in tension.
13. A structure as claimed in any preceding claim, wherein the frame may be held in position by an equilibrium between the tension wires above and below the frame, before being clamped or otherwise fixed to the legs.
14. An offshore structure substantially as herein described with reference to the accompanying drawings.