GB2188356A - Offshore structures - Google Patents

Abstract

A steel lattice tower structure for use as a support for an offshore platform, and having peripheral legs spaced apart by 'X' braced panels; in which the intersections at the centres of the 'X's are deliberately displaced out of a plane defined by the ends of the four arms of the 'X', either outwardly (215), or inwardly (205), with respect to the periphery of the structure. <IMAGE>

Description

SPECIFICATION Improvements relating to offshore structures The invention relates to steel lattice tower structures for use as supports for offshore platforms.

Steel lattice tower structures are founded on the seabed at offshore locations to support, e.g. facilities forthe drilling forand/or production of subset oil and/or gas. Such towerstructures are designed to support the deadweight ofthe topsides loads, and to resist environmental loadings due to winds, waves and currents. In the region of the waterline, these structures are also designed to resist accidental imnpacts (e.g. from supply boats which may not be fully under control) and fatigue effects from repeated application of loads due to waves much smallerthan those of the limiting environmental condition.

The present invention is concerned with a reconfiguration of the lattice framework to resist more effectively damage which might be caused by ship impact, orfatigue damage.

The invention provides a steel lattice tower structure for use as a supportfor an offshore platform, and having peripheral legs spaced apart by diagonally braced panels; in which the bracing of at least one panel at or nearthewater level is in the configuration of an 'X', and the intersection at the centre ofthe 'X' is deliberately displaced out of a plane defined by the ends of the four arms of the 'X'.

It is preferred that said one panel is a face bracing panel.

It is further preferred that the ends of two adjacent arms ofthe 'X' are connected directly to one leg of the structure, and the ends of the other two arms of the 'X' are connected to an adjacent leg ofthe structure, said ends being connected to each ofthe adjacent legs at corresponding vertically spaced apart levels.

The out of plane displacement may be outward with respect to the periphery of the structure.

The out of plane displacement may be inward with respect to the periphery of the structure.

In these two last mentioned forms it is preferred that two opposed face panels, on opposite sides of the structure, are so configured.

It is further preferred that the intersections of X's are joined by a horizontal member extending across the interior ofthe structure.

An anotherform of the invention said one panel may be a plan bracing panel.

Specific embodiments of the invention will now be described by way of examples, with reference to the accompanying drawings, in which: Figure 1 is an isometric view of the top portion of a conventional steel lattice tower structure for use as a supportforan offshore plafform; Figure2 is a similarview, illustrating features of the invention; and Figure3is anothersimilarview, illustrating further features of the invention.

As shown in Figure 1, a conventional steel lattice tower structure (or jacket) has four peripheral legs, each battered inwardly towards the top of the tower.

Only the bays of the structure in the region ofthe waterline are illustrated in Figure 1, and it should be understood that the structure extends down to the seabed (notshown).Amodulesupportframeordeck (also not shown) would later be emplaced on top of the illustrated portion to complete the primary structure of the platform.

The legs are spaced apart by face bracing panels, each lying in a plane between and joining two battered legs, and by plan bracing panels lying in horizontal planes and joining all four legs. To maintain a consistentterminology'between Figure 1 (showing the conventional tower structure) and the other figures (illustrating features of the invention), the longerfaces of the tower will be designated 'A' and 'B', and the shorter faces '1' and '2'. Thus the legs become Al, B1, B2 and A2 in a clockwise sense around the structure. In a vertical sense, plan bracing is provided at level - 1 2m and at level +22m (both with respect to the waterline). Conductor guide bracing is in this case provided at level +6m. The legsAl to B2 are identical in all of Figures 1 through 3, and will be understood to be so designated in those figures.The plan bracings at levels - 1 2m and +22m are identical in Figures 1 and 2 and are similar in Figure 3, and will be understood to be designated by level in those figures.

In one aspect, the invention is concerned with the face bracing of the structures. As shown in Figure 1, the panel on face 'A' between levels -12m and +22m is diagonally braced between opposite corners with 'X' bracing comprising tubular members 101,102,103 and 104. These members lie in the plane of the face 'A', and intersect art a node 105, which also receives the ends of members 106, 107 and 108 of the conductor guide bracing. As may be seen, members 106 and 107 lie in the plane ofthe face 'A', and member 108 extends horizontally across the interior of the structure to a corresponding node on face 'B'.

On face '1', the panel between levels -1 2m and +22m is diagonally braced between opposite corners with 'X' bracing comprising tubular members 111,112,113 and 114. These members lie in the plane of the face '1', and intersect at a node 115, which also receives the ends of members 116 and ll7oftheconductorguidebracing.

The bracing pattern described above in respect of faces 'A' and '1' is repeated symmetrically on faces 'B' and '2' of the structure, and represents a typical sea-level bracing arrangement for a conventional jacket.

Figure 2 illustrates a generally similarjacket structure, but with the face bracing members arranged in accordance with features ofthe invention.

In this case, the 'X' bracing on face 'A' consists of members 201,202,203 and 204, which intersect at point 205. This point 205 is displaced inwardly from the plane of face 'A' (with respect to the periphery of the structure). The conductor guide bracing member 206/207 is still in the plane of face 'A'.

The 'X' bracing on face '1' consists of members 211,212,213 and 214, which intersect at point 215.

This point 215 is displaced outwardly from the plane of face '1' (with respect to the periphery of the structure). The conductor bracing 216/217 is still in the plane of face '1 '. A member 218 extends across the conductor guideframing from face '1' towards face '2', and members 221,222,223 and 224join member 218 at a point 225. This point 225 is displaced inward ly towards the point 215, so creating an assymetric arrangement when viewed looking onto face 'A'. The member 218 thus extends from point 215 (outside the periphery) on face '1' through the conductor guide framing members 216, 217 and the point 225 to the conductorguideframing members 226,227 on the far side of the structure.

In figure 2,the peripheral members ofthe conductorguideframing have been maintained in the planes ofthe faces of the structure.

Figure 3 illustrates a generally similarjacket structure to that shown in Figures 1 and 2 (and with the faces, legs and bracing levels identified with the same references) butwith both the face bracing members and the peripheral members of the conductor guide framing displaced from the planes ofthefaces in which they would conventionally lie.

In this embodiment, the jacket is symmetrical aboutthe centre line of both its long and its short faces 'A' and '1' respectively.

On face 'A', the 'X' bracing consists of members 301,302,303 and 304, which intersect at a ring 305.

The conductor guide bracing members 306 and 307 are displaced out of the plane of face 'A' to meet at this ring.

Similarly, on face '1', the 'X' bracing consists of members 311,312,313 and 314, which intersect at a ring 315. The conductor guide bracing members 316 and 317 are displaced out of the plane of face '1 'to meet at this ring.

Although not illustrated in the drawings, it will be understood that the plan bracings may also be displaced out of their pla nes.

Claims (9)

1. A steel lattice tower structure for use as a support for an offshore platform, and having peripheral legs spaced apart by diagonally braced panels; in which the bracing of at least one panel at or near the water level is in the configuration of an 'X', and the intersection at the centre of the 'X' is deliberately displaced out of a plane defined by the ends ofthefour arms ofthe 'X'.

2. A structure as claimed in Claim 1 in which said one panel is a face bracing panel.

3. Astructure as claimed in Claim 2 in which the ends of two adjacent arms of the 'X' are connected directly to one leg of the structure, and the ends of the other two arms ofthe 'X' are connected to an adjacent leg ofthe structure, said ends being connected to each of the adjacent legs at corresponding vertically spaced apart levels.

4. A structure as claimed in Claim 2 or Claim 3 in which the out of plane displacement is outward with respect to the periphery ofthe structure.

5. Astructure as claimed in Claim 2 or Claim 3 in which the out of plane displacement is inward with respect to the periphery ofthe structure.

6. A structure as claimed in Claim 4 or Claim 5 in which two opposed face panels, on opposite sides of the structure, are so configured.

7. A structure as claimed in Claim 6 in which the intersections of X's are joined buy a horizontal member extending across the interiorofthe structure.

8. A structure as claimed in Claim 1 in which said one panel is a plan bracing panel.

9. A steel lattice tower structure substantially as hereinbefore described with reference to and as shown in Figure 2 or Figure 3 of the accompanying drawings.