GB2339228A

GB2339228A - Offshore platform

Info

Publication number: GB2339228A
Application number: GB9923832A
Authority: GB
Inventors: James William Bunce
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-10-09
Filing date: 1999-10-09
Publication date: 2000-01-19
Anticipated expiration: 2019-10-09
Also published as: GB9923832D0; GB2339228B

Description

2339228 OFFSHORE PLATFORM This invention relates to construction and

installation of an offihore platform.

According to UK Patent GB 2 300 211 B there was provided an offshore platform consisting of a superstructure supported on an elongate caisson founded on the sea floor, said caisson being supported against hydrodynamic loading by means of three or more vertically inclined flexible guy lines each attached at one end to said caisson near its top, above the water surface, with the other end anchored to the sea floor. Protection against boat impact was provided by three or more arrays of flexible fender lines attached to and running between said guy lines near the water surface, such that an approaching boat would be arrested on said fender lines before it could strike the platform.

Utilisation of such a platform in deep waters and a harsh environment is limited by a number of factors, but mainly excessive overall flexure and dynamic behaviour of said caisson over its unsupported length and risk of overall collapse if said guy lines were to fail, for example, by accidental loading from trawling gear. The present invention relates to a platform structure that has improved flexural and dynamic characteristics and robustness properties, suitable for application in deep water and harsh environments.

Thus, according to the present invention there is provided an offshore platform consisting of a superstructure supported on a tower founded on the sea floor, said tower being supported against hydrodynamic loading by means of two or more arrays of vertically inclined flexible guy lines disposed over the height of said tower, wherein each array of guy lines comprises three or more guy lines each attached at one end to said tower with the other end anchored to the sea floor, wherein the uppermost array of guy lines is attached to said tower above the water surface, wherein said guy lines 2 are proportioned such that any one array of guy lines can be lost or become ineffective without consequent loss of overall stability of the platform in storm conditions, wherein said uppermost array of guy lines is configured so as to protect lower guy line arrays from accidental loading and wherein protection against boat impact is provided by three or more arrays of flexible fender lines attached to and running between said uppermost array of guy lines near the water surface such that an approaching boat will be arrested on said fender lines before it can strike said tower.

Product conductors, risers and export lines are supported on said tower., some on its outside and some inside said tower.

Means of tensioning said guy lines are provided near the top of said tower, accessible from the superstructure and/or near the sea floor anchorages, operable by pulling on a line from a boat floating above. Means of slackening and lowering said uppermost array of guy lines are provided to allow close approach of a large vessel such as a deep-sea drilling rig. Means of tensioning, slackening, disconnecting and lowering said fender lines are also provided to allow controlled approach of a supply boat to the platform between said guy lines, for platform servicing purposes.

Offshore installation and retrieval of the platform may be by means of a jack-up drilling rig or by a floating crane vessel.

The invention will now be described by way of example only and with reference to Figures I to 4 of the accompanying drawings.

Figure I shows an elevation of the platform.

Figure 2 shows a part-elevation of the top of the platform to an enlarged scale.

3 Figure 3 shows a plan view on the uppermost array of guy lines.

Figure 4 shows a plan view on the lower array of guy lines.

Referring to the Figures I and 2, the platform comprises a superstructure I supported above the wave crest level on a tower 2 supported on a foundation 3 on the sea floor.

Two or more arrays 4,5 of vertically inclined flexible guy lines are attached to said tower 2, disposed over its height. Each guy line array comprises three or more guy lines one end of each being affixed to said tower 2 with the other end attached to an anchor system 6 on the sea floor. The uppermost array of guys 4 is affixed to said tower 2 near its top above the water surface.

The horizontal plan arrangement of said guy line arrays 4,5 and anchor systems 6 is symmetrical to resist the hydrodynamic loads, as shown in Figures 3 and 4.

Said uppermost array of guy lines 4 is configured directly above the lower guy line array 5 such that it will protect the lower guy lines from accidental loads such as snagging by trawling gear.

Said guy line arrays are proportioned such that the platform will remain stable under storm loads in the event that any one arTay of guy lines becomes lost or ineffective due to failure or damage.

Referring to Figures 2 and 3, three or more arrays of flexible fender lines 7 are attached to said uppermost array of guy lines 4, each array running between at least two guy lines near the water surface and configured such that a boat approaching from any direction will be arrested and its impact energy absorbed, mainly by strain energy in said fender lines 7 and guy lines 4, before it can strike the platform.

4 By way of example only, said tower 2 may be in the form of a cylindrical caisson, a lattice tower of tubular members or other construction of a form already known.

Said tower foundation 3 may comprise, for example, piles, spud-cans or shallow bases of forms already known. Said guy line anchor system 6 may comprise, for example, an array of ships anchors, piles, suction anchors or anchor blocks of forms already known.

Means of tensioning said guy line arrays 4,5 are provided at the top of said tower 2, for example, in the form of hydraulic jacks attachable to the end of each guy line, accessible from the superstructure 1, and/or near the sea floor anchorage, by means of anchor line tensioners of forms already known, operable by pulling on a line from a boat floating above.

By way of example only, each fender line array 7 may take the form of three main fender lines 8, 9 in an inverted 'Y' configuration with secondary horizontal lines 10 connected to said main fender lines and guy lines 4 as depicted in Figures 2 and 3. The lower legs 8 of said main fender lines are attached to the guy lines 4 below the water surface while the upper fender line leg 9, which forms the stem of the 'Y' is attached to the tower 2 near its top such that the fender line array can be tensioned by applying a force to said upper fender line leg 9. Said secondary horizontal fender lines 10 may be disconnected from said main fender lines 8,9, for example, by undoing shackles. Said main fender lines 8,9 may be slackened by reducing the tension in said upper fender line leg 9 and may be lowered below the water surface by paying out its length, thus allowing a supply boat under close control to approach the platform.

Connection of said guy lines 4,5 and main fender lines 9 to said tower 2 may be by means of padeyes and rope terminations, or cable stoppers or chain stoppers of forms already known.

Product conductors, risers and export lines 11 may be supported on said tower on its outside, by means of guides 12, and inside said tower 2.

Said tower 2 and foundation 3 are preferably made of steel for lightness.

Said guy line arrays 4,5 and fender lines arrays 7 may be steel wire rope, cable or spiral strand or chain or composite fibre construction, or a combination thereof.

6

Claims

I An offshore platform consisting of a superstructure supported on a tower founded on the sea floor, said tower being supported against hydrodynamic loading by means of two or more arrays of vertically inclined flexible guy lines disposed over the height of said tower, wherein each array of guy lines comprises three or more guy lines each attached at one end to said tower with the other end anchored to the sea floor, wherein the uppermost array of guy lines is attached to said tower above the water surface, wherein said guy lines are proportioned such that any one array of guy lines can be lost or become ineffective without consequent loss of overall stability of the platform in storm conditions, wherein said uppermost array of guy lines is configured so as to protect lower guy line arrays from accidental loading and wherein protection against boat impact is provided by three or more arrays of flexible fender lines attached to and running between said uppermost array of guy lines near the water surface such that an approaching boat will be arrested on said fender lines before it can strike said tower.
2 An offshore platform according to Claim I in which means are provided of tensioning or slackening said guy lines and fender lines, accessible from the superstructure.
3 An offshore platform according to Claims I or 2 in which means are provided near the sea floor anchorages of tensioning or slackening said guy lines, operable by pulling on a line from a boat floating above.
4 An offshore platform according to Claims 1, 2 or 3 in which means are provided of slackening and lowering said uppermost array of guy lines to allow close approach of a large vessel such as a deep-sea drilling rig.
An offshore platform according to Claim 1, 2, 3 or 4 in which said fender line arrays each take the form of three main fender lines in an inverted 'Y' configuration, in which the lower two legs of said main fender lines are each attached to a guy line below the water surface while the upper leg of said main fender lines, forming the stem of the inverted 'Y', is attached to said tower near its top and in which secondary 7 horizontal fender lines are attached to said main fender lines and said guy lines, such that a boat approaching from any direction will be arrested.
6 An offshore platform according to any of the preceding claims in which product conductors, risers and export lines are supported on said tower, some on its outside by means of guides and some inside said tower.
7 An offshore platform according to any of the preceding claims in which said foundation takes the form of piles, spud-cans or shallow bases of forms already known.
8 An offshore platform according to any of the preceding claims in which said tower takes the form of a cylindrical caisson, a lattice tower of tubular members or other construction of a form already known.
9 An offshore platform according to any of the preceding claims in which said tower and superstructure are made from metal and said guy lines and fender lines are made from metal wire rope, cable, or spiral strmd or chain or composite fibres, or a combination thereof.

An offshore platform according to any of the preceding claims in which the platform is used for offshore mineral extraction purposes, habitation, communications, control, power generation and transmission, vessel mooring and loading or unloading or observation purposes.

I I An offshore platforTn substantially as herein described and illustrated in the accompanying drawings.