AU664448B2

AU664448B2 - Offshore structures

Info

Publication number: AU664448B2
Application number: AU39948/93A
Authority: AU
Inventors: Murray Charles Ferguson
Original assignee: VOLKER STEVIN OFFSHORE BV; Kvaerner Earl and Wright
Current assignee: VOLKER STEVIN OFFSHORE BV; Kvaerner Earl and Wright
Priority date: 1992-06-02
Filing date: 1993-06-01
Publication date: 1995-11-16
Anticipated expiration: 2013-06-01
Also published as: CN1035782C; GB2267525B; DK61293A; GB9311365D0; DE4318321A1; DK61293D0; MY131366A; NL9300899A; CN1079521A; GB2267525A; AU3994893A

Description

664448

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT

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5609 9 0*5* Applicants: 1) KVAERNER EARL AND WRIGHT 2) VOLKER. STEVIE~ OFFSHORE by Invention Title: OFFSHORE STRUCTURES The following statement is a full description of this invention, including the best method of performing it known to me/us: OFFSHORE STRUCTURES The invention relates to offshore structures.

The development of offshore oil and gas fields has stimulated the design of structures to support drilling, production and loading facilities.

Fluctuations in energy prices have led to a requirement for optimised structures.

For large topsides loads, gravity platforms or multi-legged piled steel jackets have been found to provide economical substructures. However, for relatively light topsides loads, a substructure formed with a single column piercing the sea surface has proved to be attractive. Such single column substructures have been proposed for developments in the Southern North Sea and offshore Australia.

With single column substructures, a major part of the weight and cost lies in the column itself, because this column has to react all forces applied to the topsides. To reduce bending stresses in the column, there is a requirement to minimise the free length of the column.

To provide on-bottom stability for the structure when first installed (and not yet secured to the seabed) it may be appropriate to place a triangular base on the seabed, and to arrange a tripod support on this base.

Heretofore tripod'tower structures have been proposed, in which the column is supported by three legs spaced apart by bracing at seabed level.

Such structures are illustrated in UK Patent Specification 2,116,237.

However, this configuration has the disadvantages of requiring the column to extend the full height from seabed to topsides,.and of needing bracing members on the seabed.

Subsea oil and gas fields have conventionally been exploited using fixed drilling/production platforms feeding subsea pipelines leading back to onshore terminals. The platform may support facilities for partially 30 processing the oil or gas so that it can be pumped through the subsea pipeline.

To exploit relatively small subsea oil reservoirs at remote locations,

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it may be uneconomic to provide a pipeline all the way from the location of the reservoir to an onshore terminal. In these cases floating vessels (eg.

shuttle tankers) may be used to transport the oil to the terminal.

-2- In pumping crude oil from subsea wells (and/or subsea storage) to floating vessels for transport to the onshore terminal, there is a problem in passing oil through the wave effected zone. The problem arises because any equipment used to transfer the oil from the subsea well(s) through that zone to the vessel will be subject to forces arising from winds, waves and current and tidal effects.

The equipment must be designed to resist the ;extreme events' of these environmental forces. Such events may include a 100 year wave, and freak hurricane conditions. To design a transfer system against such extreme events is a complex task, since the floating vessel to transport the oil to shore will normally 'weathervane' or lie head to wind. Thus from a fixed point on the seabed, oil must be transferred through equipment sufficiently robust to withstand extreme events, and also capable of allowing the vessel to weathervane.

An installation including this equipment must additionally protect the 'riser' or risers against damage from ship impact. (The word 'riser' has a particular meaning in the offshore industry and designates a pipeline bringing oil from the seabed through the wave effected zone to above the sea surface.) 20 Heretofore it has been known for vessels to load oil offshore from large

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loading buoys or articulated columns having flexible risers or jointed oo• pipework. (An arrangement for loading from a buoy is shown in our U.K.

Patent Specification 2,250,253). In the past both buoys and columns have 25broken free from their assigned locations, with a resulting loss of equipment and consequential loss of production.

It has also been proposed to make a fixed tower structure with a swivel mounted loading arm at its top. The fixed tower structure would necessarily be a larger fabrication than a loading buoy or articulated column. To form o" an economical installation it has been found advantageous to simplify the oamm ,30 design of the fixed tower structure so that it comprises only a minimum number of essential members.

One known proposal comprises a tower structure having a central column, S three support legs and a base structure anchored to the seabed by piles.

The legs are attached at their upper ends to a sleeve, and the central column passes through and is rigidly connected to this sleeve. The central column extends from the seabed to a position above the height of the highest extreme event wave expected at its particular location. (Such a tower structure is illustrated in U.K. Patent Specification 2,136,860).

3 This known proposal suffers from the disadvantages that a riser either must be attached to the outside of the central column, or must turn through a sharp corner at the foot of the central column so that it can pass up through the wave effected zone within that column.

In the first case, the riser will be exposed to ship impact if it is attached to the outside of the column.

Irs the second case, turning the pipeline through a sharp corner at the foot of the column introduces additional complexity. Thus a central column extending all the way down to the seabed may be unsuited to accommodate a riser (or J-tube).

The invention provides an offshore structure having at least three feet adapted to rest on a seabed and at least three legs extending upwardly and inwardly from respective feet to an apex which is below the wave effect zone when the structure is installed offshore, bracing in the plane of a face defined by a pair of adjacent legs, and a central column arranged to extend vertically upwards when the structure is installed offshore, in which the apex of the legs supports an intermediate point on the height of the column, and a lower point on the height of the column is stabilised by horizontal plan bracing extending from positions on the at least three legs between the feet and 25 the apex, such that the lower end of the column is supported a significant distance above the seabed.

It is preferred that the column extends to a o* location which is above the wave effected zone when the structure is installed offshore.

30 It is preferred that the peripheral and radial horizontal members are connected to the legs approximately 'g midway along the length of the legs.

It is still further preferred that the feet are adapted to be secured to the seabed by piling. *e r -4- The invention also provides an offshore structure forming part of a loading tower; and having a swivel assembly to be mounted at the top of the central column, a loading arm to be mounted on the swivel assembly, and provision for at least one riser or J-tube to lead from a pipeline on the seabed thruugh the central column to the swivel assembly and loading arm, such that a floating vessel may receive oil from the pipeline.

In this form it is pre erred that the lower end of the central column is spaced above the seabed by a distance slightly greater than the radius of curvature of the riser or J-tube which is to connect the seabed pipeline to the swivel assembly and loading arm through the interior of the column.

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 view of an offshore loading tower; Figure 2 is a more detailed side view of a base and central column forming a lower part of that tower; Figure 3 to 6 are plan views at levels III to VI shown in Figure 2; and Figure 7 is a side view of the base and central column showing the arrangement of a riser and J-tube in relation to the base and column.

Figure 1 shows an offshore loading tower comprising a triangular base 0* portion having three legs supporting a central column which is surmounted s. by a loading assembly.

The offshore structure has three feet 10 adapted to rest on a seabed, and three steel tubular legs 11 extending upwardly and inwardly from those *0 S feet to an apex 12. When the structure is installed offshore, eg. by piles 14 driven through pile sleeves 15, the apex 12 is below the wave effected zone.

S Referring now to Figures 2 to 6, which show details of the support arrangement for the central column, Figure 2 is a detailed side view of the structure. Lines Roman III to VI on Figure 2 designate the elevational 30 heights above the seabed at which Figures 3 to 6 provide plan views.

X bracing (indicated generally as 16) is arranged in the plane of each face defined by adjacent legs 11. Thus there are three inclined X braced panels directed towards the apex 12. Each panel has two lower arms 17 connected to adjacent feet 10, and two upper arms 18 connected to points 19 on the legs about midway between the feet 10 and the apex 12. The points 19 are tied together by peripheral horizontal members 21 extending between adjacent legs (as best seen in Figure 5) and by radial horizontal members 22 extending from respective legs to a central point (or node) 23." The members 21 and 22 form a level of horizontal plan bracing.

All the inclined arms (17 and 18) and horizontal members (21 and 22) are made from relatively small diameter tubular steel, and are welded to the legs 11 and to each other as applicable.

A relatively large diameter hollow steel column 24 extends upwardly from the node 23, through the apex 12', through the sea surface 25, to a deck support or swivel mount 26 (which carries a swivel assembly 27 details of which form no part of this invention). A mid portion of the column 24 pierces the sea surface, so that only the single column impedes the passage of waves past the tower. It this way, wave loads on the tower are.

minimised. The upper ends of the legs 11 support the column 24 at the apex 12 which is at an intermediate height between the node 23 and the sea surface. The radial members 22 meeting at the node 23 locate the lower end of the column 24 at a significant distance above the seabed.

The swivel assembly 27 is located on top of the swivel mount 26, and supports a loading arm 28 with an oil pipeline. In use a shuttle tanker (not shown) can receive oil from the pipeline for transport to an onshore terminal.

The arrangement illustrated has particular advantages in that the inclined X bracing 16 between the legs, and the horizontal bracing 21/22, .9 S 20 'forms a structurally efficient support for the lower end of the column 24.

The arrangement shown does not require a full length column, because bending in the column can be reacted at the apex 12 and in the plane of the members 21/22. Moreover, the structure does not require plan bracing at seabed level, so that the feet 10 can stand alone on the seabed.

Figure 7 is a view generally corresponding to Figure 2, showing a riser 29 and J-tube 31 running in from a level close to the seabed, and turning through a suitable radius to pass into the lower end of the central column 24 at the level of the radial members 22.

By arranging the lower end of the column 24 well above the seabed, the 30 riser and J-tube can adopt a suitable radius of curvature to pass directly up within the central column to the swivel mount 26, the swivel assembly 27 and the loading arm 28.

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Claims

1. An offshore structure having at least three feet adapted to rest on a seabed and at least three legs extending upwardly and inwardly from respective feet to an apex which is below the wave effected zone when the structure is installed offshore, bracing in the plane of a face defined by a pair of adjacent legs, and a central column arranged to extend vertically upwards when the structure is installed offshore, in which the apex of the legs supports an intermediate point on the height of the column, and a lower point on the height of the column is stabilised by horizontal plan bracing extending from positions on the at least three legs between the feet and the apex, such that the lower end of the column is supported a significant distance above the seabed.

2. A structure as claimed in Claim 1 in which there is diagonal bracing in the planes of all the faces defined by pairs of adjacent legs.

3. An offshore structure as claimed in Claim 1 or Claim 2 comprising a tripod having three feet adapted to rest on a seabed and three legs extending upwardly apd inwardly from their respective feet to an apex which is below the wave effected zone when the structure is installed offshore, X-bracing between pairs of adjacent legs, each X having two longer lower arms connected to adjacent feet and two shorter upper arms connected to points on the legs between the feet and the apex, in which the upper ends of the X-bracing are tied together by the horiz6ontal plan bracing comprising peripheral horizontal members extending between adjacent legs and radial horizontal members extending from respective legs to a central point; and in which the central column extends upwardly from that central point through the apex.

4. A structure as claimed in Claim 3 in which the column extends to a 'location which is above the wave effected zone when the structure is installed offshore.

5. A structure as claimed in Claim 3 or Claim 4 in which the peripheral and radial horizontal members are connected to the legs approximately midway along the length of the legs. 7

6. A structure as claimed in any one of Claims 3 to 5 in which the feet are adapted to be secured to the seabed by piling. .A structure as claimed in any one of the preceding claims and forming part of a loading tower, having a swivel assembly to be mounted at the top of the central column, a loading arm to be mounted on the swivel assembly, and provision for at least one riser to lead from a pipeline on the seabed through the central column to the loading arm, such that a fioating vessel may receive oil from the pipeline.

8. A structure as claimed in Claim 7 in which the lower end of the central column is spaced above the seabed by a distance slightly greater than the radius of curvature of a riser or J-tube which is to connect the seabed pipeline to the swivel assembl'y and loading arm through the interior of the column.

9. An offshore structure substantially as hereinbefore described with reference to and as shown in the accompany'4g drawings. Dated this 7th day of February 1995 1) KVAERNER EARL AND WRIGHT and 2) VOLKER STEVIN OFFSHORE BV By Their Patent Attorneys: C. GRIFFITH HACK CO. Fellows Institute of Patent Attorneys of Australia. C C C CC go* *o *e 0 ABSTRACT 1. An offshore structure for an offshore loading tower) having a central column (24) arranged to extend vertically upwards when the structure is installed offshore, and in which there are at least three feet (10) adapted to rest on a seabed and at least three legs (11) extending upwardly and inwardly from respective feet to an apex (12) which is below the wave effected zone when the structure is installed offshore, the apex of the legs supports an intermediate point on the height Gf the column, and a lower point (23) on the height of the column is stabilised by bracing extending from positions (19) on the at least three legs between the fet ancd the apex, such that the lower end of the column is supported a significant distance above the seabed. oe t 0 ee 0* 0 •3 0 0 0 0 0 U *0 5* 1