GB2296928A

GB2296928A - Offshore Tower Structure

Info

Publication number: GB2296928A
Application number: GB9600658A
Authority: GB
Inventors: Michael Timothy Smith
Original assignee: Kvaerner Earl and Wright
Current assignee: Kvaerner Earl and Wright
Priority date: 1995-01-12
Filing date: 1996-01-12
Publication date: 1996-07-17
Also published as: GB2296928A8; GB2296928A9; GB9500608D0; GB9600658D0

Abstract

An offshore tower structure having four corner legs 11 extending vertically from the seabed 14 to above the sea surface 15, the legs being disposed in plan to define a square or rectangle at or near the seabed, in which near to the seabed each corner leg is associated with at least one pile sleeve 17, and in which near to the seabed there are additional outrigger pile sleeves 18 located midway along each of two opposed sides of the square or opposed longer sides of the rectangle, so to provide an array of pile sleeves in the general form of a hexagon at seabed level to support the tower of square or rectangular cross section above sea level, and in which each outrigger pile sleeve is connected to the corner legs by horizontal members 21 and 23 in the plane of a lowest level of plan bracing, and by two inclined members 24 and 25 extending from the outrigger pile sleeve 18 to the legs 11 at the lowest level on the legs at which face bracing between the legs is connected to the legs.

Description

OFFSHORE TOWER STRUCTURE The invention relates to an offshore tower structure.

Development of offshore oil and gas fields has led to requirements for fixed drilling/production platforms to be placed in deeper and deeper waters, so calling for taller and more costly support structures.

Conventional support structures have been fabricated as three-dimensional lattices composed of tubular steel members, and known within the offshore industry as "jackets". Heretofore jackets have been built to their full height in fabrication yards, either upright, or - in the case of taller jackets - horizontally, lying on one side face.

These taller jackets have been skidded out from fabrication yards, transported to offshore sites in one piece on barges, and then either launched or lifted into the water for upending and piling on to the seabed.

While the height of a jacket for a specific offshore location is determined by the water depth in which the jacket is to stand, its plan dimensions above sea level can be arranged to accommodate various different deck payload requirements. Thus the dimensions between the upper ends of corner legs of a jacket will correspond to the spacing between support members beneath a cellar deck for the drilling and/or process facilities.

Many support structures have been designed to have sloping faces, so that the jacket tapers continuously from a large plan area at its base to a smaller plan area at its top. Tapering a jacket in this manner gives the structure a wide spread of foundation piling at its base to resist overturning moments. The taper also gives a reduced section in the wave effected zone near the top of the structure, which attracts relatively low wave induced loads. The taper can be designed to produce an optimum support spacing for the drilling and/or process facilities. Within the offshore industry, this feature of taper is known as "batter".

The requirement for constant batter all the way up a conventional jacket has led to the design of some unwieldy structures which have had oversized members near their bases. To alleviate these structural inefficiencies, it has been proposed to build jackets with a spread base, a tapered section, and a tower of uniform cross section. One example of this type of jacket is shown in our U.K. Patent Specification 2214548.

Conventional battered jackets, and spread base jackets of the kind shown in U.K.

Patent Specification 2214548, have to some extent achieved a rational array of piles at the base of the jacket. The piles have been arranged around the external periphery of the base planform. However, these arrangements are founded on the premise that for both fabrication convenience and geometric requirements of drilling and/or process facilities, conventional jackets have had square or rectangular base planforms, with legs at their four corners.

In many offshore locations, wind and wave loadings are generally similar from all directions. (Exceptions to this occur with respect to particular wind/wave directions when a jacket is set in the lee of a land mass.) With generally similar design loads to be applied from all directions, the most efficient disposition of foundation piles would be for those piles to be placed on the circumference of a circle. Clearly a very complicated framing arrangement would be necessary to transfer loads from a circle of piles set on the seabed to a square or rectangular support for the deck above sea level.

It has been proposed to build a jacket of square cross-section with outrigger piles arranged at the extremities of extended framing. In this case the pile sleeves have been connected to the legs of the jacket with cross braced vertical panels extending up the lowest bay of the jacket, and with angled brace members arranged to join these panels to the legs at the top of the second lowest bay of the jacket. This arrangement requires a multiplicity of members, and adds substantially to the weight (and cost) of the jacket.

Thus there is a requirement for an offshore tower structure to suit both an efficient arrangement of foundation piles on a seabed, and also a convenient deck support configuration above sea level. To meet this requirement it is desirable to have a minimum number of additional members as compared with a spread base jacket, or with a jacket having outrigger piles (of the kind described in the preceeding paragraph).

The invention provides a tower structure to stand upright at an offshore location, and having four corner legs extending vertically from the seabed to above the sea surface, the legs being disposed in plan to define a square or rectangle at or near the seabed, in which near to the seabed each corner leg is associated with at least one pile sleeve, and in which near to the seabed there are additional outrigger pile sleeves located midway along each of two opposed sides of the square or opposed longer sides of the rectangle, so to provide an array of pile sleeves in the general form of a hexagon at seabed level to support the tower of square or rectangular cross section above sea level, and in which each outrigger pile sleeve is connected to the corner legs by horizontal members in the plane of a lowest level of plan bracing, and by two inclined members extending from the outrigger pile sleeve to the legs at the lowest level on the legs at which face bracing between the legs is connected to the legs.

In one form the long sides of the rectangle are more than one third as long again as the short sides, and more specifically the long sides are more than half as long again as the short sides.

It is preferred that the outrigger pile sleeves are connected to the plan bracing between the corner legs by spacer members disposed in a horizontal plane.

It is further preferred that the spacer members join the plan bracing at positions where diamond plan bracing joins horizontal members connecting the legs directly.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a side elevation of an offshore tower structure; Figure 2 is an end elevation of that structure; Figure 3 is a plan of that structure on the line XX near its base; and, Figure 4 is a typical plan level showing restraint of the face bracing.

As shown in Figures 1 and 2, an offshore tower structure or jacket 10 has four vertical corner legs 1 , and face bracing members 1 2 in vertical planes between adjacent legs. The jacket 10 extends upwardly from sea bed 14, through the sea surface 15, to deck stab in points above the sea surface. All the four corner legs are vertically aligned, and all the legs and face bracing members are made of tubular steel components.

The jacket is designed (in this example) to stand in a water depth of approximately 90m, and to support drilling and/or process facilities on a deck (not shown) having stab in points set at spacings of 32m and 20m in the planes of Figures 1 and 2 respectively.

The 32m x 20m spacing between the legs is maintained from sea surface to sea bed because all the corner legs 11 are vertical. Thus the jacket has no "batter".

The leg spacing is convenient both for fabrication, loadout, transportation, and launch/lift; and also for deck support. However, the layout of the corner legs at the lowest level of plan bracing (16 as shown in Figure 3) is not well suited to the geometric requirements of a piled foundation. The piled foundation may be required to accept generally similar design loads from every direction. Ideally, all the piles would be placed on the circumference of a circle.

Corner pile sleeves 1 7 are attached to each of the corner legs 1 and additionally following the invention - outrigger pile sleeves 1 8 are positioned mid way along the longer sides of the rectangular planform. These outrigger pile sleeves are spaced outwardly from the longer sides by horizontal plan members 21, 22 and 23 and angled brace members 24 and 25. The angled brace members 24 and 25 extend upwardly and inwardly from the outrigger pile sleeves 1 8 into the corner legs 11 at nodes where the lowest level of face bracing joins the legs. The horizontal plan member 22 forms a spacing member to hold the outrigger pile sleeve 1 8 away from the face of the jacket.

In this way foundation piles can be driven through the pile sleeves 1 7 and 1 8 into the seabed strata in the shape of a slightly eiongated hexagon, so approximating to the idealised pitch circle that would join the most advantageous piling array.

The framing arrangement that connects the outrigger pile sleeves 1 8 to the main legs of the jacket shows several advantages. Use of the lowest level of plan bracing to react horizontal plan members 21, 22 and 23, coupled with angled brace numbers 24 and 25 to carry vertical loads directly into the corner legs, provides an economical framing arrangement. This arrangement is particularly advantageous in this case, in which the angled brace members frame directly into corner legs between restrained X bracing levels. The configuration optimises load paths while requiring the use of a minimum amount of additional structural steel.

The configuration of the jacket illustrated by way of example has additional advantages with respect to fabrication and loadout. By making all the corner legs parallel, the jacket can be fabricated as two flat panels at ground level. When substantially complete, these panels can be rotated upward and inward to define the 'long' sides of the jacket. The legs about which rotation was effected can then be used as skid rails for loadout.

Claims

1. A tower structure to stand upright at an offshore location, and having four corner legs extending vertically from the seabed to above the sea surface, the legs being disposed in plan to define a square or rectangle at or near the seabed, in which near to the seabed each corner leg is associated with at least one pile sleeve, and in which near to the seabed there are additional outrigger pile sleeves located midway along each of two opposed sides of the square or opposed longer sides of the rectangle, so to provide an array of pile sleeves in the general form of a hexagon at seabed level to support the tower of square or rectangular cross section above sea level, and in which each outrigger pile sleeve is connected to the corner legs by horizontal members in the plane of a lowest level of plan bracing, and by two inclined members extending from the outrigger pile sleeve to the legs at the lowest level on the legs at which face bracing between the legs is connected to the legs.

2. A tower structure as claimed in Claim 1 in which the long sides of the rectangle are more than one third as long again as the short sides.

3. A tower structure as claimed in Claim 2 in which the long sides of the rectangle are more than half as long again as the short sides.

4. A tower structure substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

4. A tower structure as claimed in any one of the preceding claims in which the outrigger pile sleeves are connected to the plan bracing between the corner legs by spacer members disposed in a horizontal plane.

5. A tower structure as claimed in Claim 4 in which the spacer members join the plan bracing at positions where diamond plan bracing joins horizontal members connecting the legs directly.

6. A tower structure substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS 1. A tower structure to stand upright at an offshore location, and having four corner legs extending vertically from the seabed to above the sea surface, the legs being disposed in plan to define a rectangle at or near the seabed, in which near to the seabed each corner leg is associated with at least one pile sleeve, and in which near to the seabed there are additional outrigger pile sleeves located midway along each of two opposed longer sides of the rectangle, so to provide an array of pile sleeves in the general form of a reguiar hexagon at seabed level to support the tower of rectangular cross section above sea level, and in which each outrigger pile sleeve is connected to the corner legs by horizontal members in the plane of a lowest level of plan bracing, and by two inclined members extending from the outrigger pile sleeve to the legs at the lowest level on the legs at which face bracing between the legs is connected to the legs, whereby all the pile sleeves are close to the circumference of an idealised pitch circle, so that the piled foundation can accept generally similar design loads from any direction.

2. A tower structure as claimed in Claim 1 in which the outrigger pile sleeves are connected to the plan bracing between the corner legs by spacer members disposed in a horizontal plane.

3. A tower structure as claimed in Claim 2 in which the spacer members join the plan bracing at positions where diamond plan bracing joins horizontal members connecting the legs directly.