GB2035240A

GB2035240A - Offshore structures

Info

Publication number: GB2035240A
Application number: GB7939343A
Authority: GB
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1978-11-15
Filing date: 1979-11-14
Publication date: 1980-06-18

Abstract

A tether assembly for a tethered buoyant platform (101) suitable for installation in a shaft (122) in the leg (116) of the tethered buoyant platform comprises a tubular tether (100) having a terminal at one end thereof for connection to an anchor post (104) on the sea bed (110). The tether (100) has an upper flexible joint and a lower flexible joint and a tether length adjusting means comprising a tubular section having circumferential spaced apart grooves capable of engaging circular spigots on a releasable clamping means. Details are given of an arrangement for tensioning the tether and its mounting within the leg (116). <IMAGE>

Description

SPECIFICATION Offshore structure and method This invention relates to a tether assembly for a tethered buoyant platform and to a method of installing the tether assembly.

Tethered buoyant platforms, sometimes called tension leg platforms, have been previously proposed for the production of oil and/or gas at offshore locations. A tethered buoyant platform is a platform which, when in use, is moored vertically below its free floating position by means of tethers to which tension is applied.

By the word tethered in the present specification we do not intend to be limited to a platform the tethers of which are moored to the sea bed but to include a platform in a condition suitable for moving from one location to another.

Atethered buoyant platform having an improved anchoring system has now been invented.

Thus, according to the present invention a tether assembly for a tethered buoyant platform suitable for installation in a shaft of the leg of the tethered buoyant platform, comprises a tubulartether having a terminal at the end thereof for connecting to a corresponding member on the sea bed and having located at one or more positions along its length a flexible joint to reduce bending stresses in the tether.

Thetubulartethers can each comprise lengths of steel pipe joined endwise, conveniently by means of screw threads. The steel pipe can be the type used in drilling.

Conveniently one flexible joint is located in the region of the lower end of the tether and a second flexible joint is located in the region of the upper end.

The flexible joints can be universal joints.

By the term flexible joint is meant any joint which allows articulation between the sections of tether connected to the joint.

The tether can have located along its length a tether length adjusting means said means comprising a plurality of axially spaced apart members capable of engaging corresponding members associated with a releasable clamping means.

The tether length adjusting means can comprise a tubular member having a plurality of axially spaced apart circumferential grooves capable of engaging corresponding circular spigots on the releasable clamping means or vice versa.

The tether assembly can include a tool for engaging the walls of the shaft in which it is located for centralising the tether with respect to the shaft and to transmit horizontal components of loads from platform to the tether.

According to one embodiment of the invention a tether assembly as herein before described is installed in the shaft of a leg of a tethered buoyant platform and has located at a position along its length within the shaft and in the region of the lower end thereof a bearing to transmit the restraining forces from the tether to the shaft as the tethered buoyant platform surges. The bearing can have a tubular body and, in order to reduce loads, is relatively long in an axial direction and is located in a section of the shaft of reduced diameter.

The tether assembly can include a guidance apparatus for the installation of the tethers said guidance apparatus comprising: (a) a guide wire to be installed in a substantially vertical attitude, (b) a guide tool movable up and down the guide wire, the guide tool comprising a first sleeve for engaging the guide wire and a second sleeve for engaging the tether, the two sleeves being held substantially parallel in spaced apart relationship by a distance corresponding to the distance between the bottom end of the guide wire and the anchor terminal with which the terminal of the tether is to latch.

The distance between the first sleeve and the second sleeve can be adjustable and the guide tool can included means for effecting the adjustment.

According to another aspect of the present invention a method of installing a tubular tether on a tethered buoyant platform comprises: (a) installing a guide wire in a substantially vertical attitude between the platform and an anchor on the sea bed said guide wire being engaged by a guide tool movable up and down the guide wire, the guide tool having two sleeves one sleeve engaging the guide wire and the second sleeve engaging the tether, the two sleeves being held substantially parallel in spaced apart relationship by a distance corresponding to the distance between the bottom end of the guide wire and the anchor terminal with which the terminal of the tether is to latch, and (b) lowering thetubulartether employing the guide tool to guide the terminal on the tether into latching engagement with its anchorterminal.

The invention is illustrated by the accompanying drawings in which Figure 1 is a perspective view of the upper portion of one leg of the tethered buoyant platform showing tubular tethers located in shafts in the leg.

Figure 2 is a vertical section of a shaft centraliser and upper universal joint forming part of a tether.

Figure 3 is a view partly in section and partly in elevation of a portion of the tethered buoyant platform and shows a tether composed of sections of pipe running from an anchor terminal on the sea bed to a heave compensator on the platform.

Figure 4 is a vertical section of the arrangement for holding the upper end of a tether herein referred to as the top terminal assembly.

Figure 5 is a vertical section of the arrangement for holding the lower end of the tether herein referred to as the bottom terminal assembly (the tether is shown in latching engagement with an upstanding anchor pin).

Referring to Figure 1 a leg 2 of tethered buoyant platform 1 has tether shafts 4,6,8 and 10 and a central service and access shaft 11 extending vertically therethrough. A rig 12 of 100 ton capacity (shown installing a tether 14 in shaft 6) is mounted on deck 16 and is movable together with a slips table 18 to operate on each of the tether shafts in turn. The slips table 18 is for holding the tethers during installation. The leg 2 has decks 20 and 22 and on deck 20 is mounted a tether jack 24. The tethers located in shafts 4 and 8 are shown in the installed condition in which they are held by means of their terminal assemblies 5 and 7 respectively. Mounted on deck 16 is a pipe rack 26 holding pipe 28.

Referring to Figure 2 the shaft centraliser 30 and universal joint 32 are located in each tether near the bottom of the tether shaft (see Figure 3).The centraliser 30 comprises a body 34 connected to the pipe by a bolted flange 36 and having four hard rubber rollers to engage the walls of the shaft. Only two of the rollers 37 and 38 are shown each mounted on a horizonal axis. The centraliser 30 is connected by a bolted flange to a sub 40 which is connected also by a bolted flange to universal joint 32, comprising two parts 35 and 39 joined together at pivot 41. The upper part 35 can pivot about a horizontal axis relative to member 33 and about a horizontal axis at right angles to the first mentioned horizontal axis with respect to part 39. The part 39 is connected to the tubulartether 42 by bolted flange 44.

Referring to Figure 3 a tether 14 composed of sections of pipe joined endwise by means of screw threaded sleeve connectors 50, 52 and 54 extends downwardly from a 30 ton capacity heave compensator 60 through tether shaft 6 to an anchor 62 on the sea bed 64. The end of tether 14 is latched onto a terminal pin 66 attached to anchor 62. The tether 14 has a centraliser 30 and upper universal joint 32 and lower universal joint 68 the latter being adjacent bottom terminal assembly 70. A tether guidance frame 76 is partly shown and is used for guiding the tether during installation.

The upper end 72 of an anchored tether is shown supported by tether anchorage flat 74.

Referring to Figure 4 the uppermost length of pipe 77 comprising the tether 14 has connected thereto a relatively short make up piece 78 to provide water depth adjustment to which is connected a further make up piece 79 into which is screwed end piece 80 which is pivotally connected to a jacking system indicated generally by numeral 82 for tensioning the tether 14. Aftertensioning by the jacking system 82 tension is maintained by means of hard rubber and steel plate shims 84 supported by retractable seating plates 86.

The tubulartethers can be installed in a manner similar to that described in our co-pending UK Patent Application Nos. 25387/78 and 25388/78.

Referring to Figure 5 the lowermost section of pipe 90 comprising the tubular tether 14 is connected by means of bolted flanges 92 to lower universal joint 68 which is in turn connected by means of bolted flanges to the bottom terminal assembly 70 which has remotely hydraulically operated pawls or collets 94 and 96 (shown in the unlatched and latched positions respectively) which provide latching engagementwith anchor terminal in the form of pin 66.

Referring to Figures 6 and 7 the tubular tethers 100 and 102 of a tethered buoyant platform indicated generally by numeral 101 are connected at their lower ends to anchor posts 104 and 106 respectively upstanding from anchor base 108 located on the sea bed 110. The connections to the anchor posts 104 and 106 are made by connectors only one of which 112 is numbered adjacent to elastomeric joints 113.

Where the tethers 102 and 104 enter the lower ends of the leg 116 of the tethered buoyant platform, they are provided with a bearing 118 and a further elastomeric joint 120. The tethers pass through shafts 112and 124 at the upper ends of which are shaft sealing caps 123. The upper ends of the tethers are held in tether support clamp 121 and seat 125 at tether anchorage deck 126. Interposed between the tether seat 125 and clamp 121 is load cell 119.

Passing through clamp 121 is a tether length adjuster 117. Located above the tethers are electric chain hoist tongs 127 and holding tongs 128 and a tube lifting manipulator 129 above which is the main deck 130. On a cantilevered platform around the main deck 130 are haul down winches 131. On the main deck 130 are tensioning cylinders 132. Above is an upper deck 133 on which are tether winches 134 and on a platform around deck 133 are catenarywinches 135. Supported by the upper deck 133 is a gantry crane 136.

Each tether 100, 102 has an anchor connector 112 that latches automatically to the anchor post 104, 106 when it is pulled onto the post by the haul-down rope. Immediately above the connector 112 is the lower elastomeric joint 113 which reduces the bending moments on the anchor post and tether, and an adaptor piece 115 connects the elastomeric joint to the first tether tube. The high strength tether tubes are fastened together by screwed couplings (VAM joints) to form the tubular tether running to the underside of the tethered buoyant platform where another adaptor and elastomeric joint 120 are incorporated.

The elastomeric joint 120 is coupled to the tethered buoyant platform bearing 118 which is housed in the lower end of the tether shaft 122 to control the forces transmitted from the tether to the tether shaft as the tethered buoyant platform moves.

The bottom of the shaft 122 is flared to allow easy re-entry of the tether components during tether change-out. Further tether tubes cannot be tethered bouyant platform bearing 118 to the tether length adjustor 117 and tether clamp 121 which sits on shims and a load cell 119 on the tether seat 125. The tether seat transmits load into the tether anchorage deck 126 and through to the tethered buoyant platform structure.

Some of the platform buoyancy is used to induce a static pretension in the tethers in order to prevent the tethers from going slack.

The tether tubes are stored inside the tethered buoyant platform legs and moved by hydraulic manipulators 129. Each tube is screwed into the top of the tether string using slips, tongs 128 eievators and winches. The winch lowers the tether string after each joint has been made in a conventional manner.

Each tether 100 has a haul-down rope (not shown) attached to its anchor connector 112 and the rope passes through the anchor post 104 and back to a winch on the tethered buoyant platform to correctly position the anchor connector over the anchor post 104. The haul-down ropes are installed using a submersible.

Each tether 100 has atensioning cylinder 132 which lowers the tether 100 during the final installation stage. There are eight tensioning cylinders on each leg two of which act as heave compensators while the first tethers are being attached. Thereafter the tethered buoyant platform heave is controlled by these first tethers while the remaining tethers are connected to the anchor posts 104, 106.

The tethers 100 are tensioned by a combination of deballasting and use of the tensioning cylinders 132 to achieve the required excess buoyancy. If required, some of the installation equipment can be removed from the rig after all the tethers have been tensioned, leaving only sufficient for tether change-out.

The tether length adjustor 117 is designed to allow adjustment of tether tension and a small adjustment of tether length.

The tether length adjuster 117 is a thick walled steel tube 4.4 metres long, with concentric grooves 131 spaced evenly along part of its length in 150 mm increments. The tether clamp 121 bolts around the length adjuster and grips in grooves sized to take the full tether load. The corners of the grooves 131 are radiused to resist fatigue cracking. The lower end 133 has a female screw thread to allow the adjuster to be screwed onto the top tether tube. At the upper end is a transverse hole 135 to accommodate a pin 136 which fastens the adjuster 117 to the tensioning cylinder 132 make-up piece. The make-up piece is also connected to the tensioning cylinder 132 to allow the tether tension to be hydraulically adjusted.

The tether clamp 121 is positioned in the most suitable set of grooves and the tether is lowered onto the tether seat assembly which incorporates a split load cell 119 and shims 139 (if required). The tether load is recorded from the hydraulic pressure in the tensioning cylinder and can be checked by the load cell 119 after the tether has been lowered into its seat 125.

The tether clamp 121 is a steel ring split in two parts which are hinged to allow the clamp to be closed round the tether length adjuster 117. The two halves of the clamp are securely bolted to give a rigid unit that transfers the tether load to the tether seat assembly. The clamp 121 hs two circular spigots 137, 138, 150 mm apart, which locate in the circumferential grooves 131 in the tether length adjuster 117. The upper spigot takes the load and the lower spigot has a 2 mm clearance and acts as a secondary load path in the unlikely event of failure of the upper spigot (or the tether length adjuster). The loads are transferred through the top surface of the upper spigot 137.A ring of nitrile rubber protrudes from the underside of the spigots 137,138 to ensure that the clamp ring is concentric with the tether length adjuster 117 and to eliminate and axial backlash. All sharp corners are radiused to reduce the chance of fatigue cracking. The clamp 121 is lightly bolted through the shims and load cell 119 to the tether seat 125 such that it secures the tether but yet avoids significant loading of the load cell 119.

Tension adjusting shims 139 are located between the tether clamp 121 and the load cell 119. Each shim 139 is a split steel ring to allow assembly round the tether length adjuster 117. For 450m waterdepth the shims are 121/2, 25 and 50 mm thick giving a minimum load adjustment of about 7.5 tonnes. For 200m the shims are 6, 121/2,25 and 50 mm thick giving minimum load increments of about 7.8 tonnes.

The load cell 119 is an annular steel ring split in two parts to allow assembly round the tether length adjuster 117. There are top and bottom plates between which is sandwiched the load measuring element. The element has strain gauges bonded to its surface and is coated to minimise the risk of damage. Wheatsone bridge arrangements give an accurate measurement with temperature compensation provided by dummy gauges.

The load cell 119 sits on the tether seat 125 in the direct load path between the tether length adjuster 117 and the seat 125. There are vertical holes to allow the securing bolts to pass from the tether clamp 121 to the tether seat 125.

The gauges are linked to a central monitoring console where alarms may be set to trip when maximum and minimum loads are approached and if the rate of change of mean loads exceeds prescribed limits.

The tether seat 125 is a fabricated component which transmits load from the tether to the tether anchorage deck 126 during installation and operation. The seat has a radial slot 141 to allow itto be moved aside as the tethered buoyant platform bearing 118 is lowered through the anchorage deck 126, and each slot is closed by a loose section which is bolted into place to provide a continuous upper surface for the load cell 119. The seat 125 is secured to the tether anchorage deck by bolts 140 before the tether is tensioned. Holes in the upper surface of the seat 125 enable the tether clamp 121, shims 139, and load cell 119 to be fastened to the seat 125 with bolts.

The tethered buoyant platform bearing 118 trans- mits the restraining forces from the tether to the tethered buoyant platform as the tethered buoyant platform surges. The bearing is lubricated by water and allows differential movement to take place between the tether and the tether shaft as the tether stretches under the influence of fluctuating loads.

The tubular body of the bearing 118 is concentric with the tubular tethers 110 and is fastened to it by welded discs.

The bearing material is reinforced resin (e.g.

Railko) and in order to minimise loads the bearing is long and fits in a machined section of the tether shaft 122 with a reduced diameter to allow easy installation of the bearing.

The upper end of the bearing 118 is chamfered and the lower end of the tether shaft 12 is flared to enable the bearing to re-enter the shaft if it is inadvertently lowered too far. The upper end of the bearing 118 has a female butress thread to accept a tether tube and the lower end has a flange for connection to the upper elastomeric joint 120.

When the tethered buoyant platform is offset, local bending occurs in the tether where it emerges from the bottom of the tether shaft. The upper elastomeric joint 120 reduces the bending stresses in the tether 100 while at the same time allowing a clear bore for the passage of inspection equipment. The joint can flex + 20 and this is achieved by the shearing action of two rings of elastomer laminated with thin steel plates.

The tensile loads in the tether 100 are reversed to appear in the elastomeric rings as compression loads. The joint has upper and lower steel sections with flanges to connect to the tethered buoyant platform bearing and tether tu be adaptor respectively. An elastomeric ring is bonded to each section and a cylindrical steel shell connects to the two rings to complete the load path. A ball and socket arrangement aligns the two sections of the joint, and prevents the elastomer from experiencing tensile loads. Even in the event of severe failure of the elastomerthe steel shell provides a load path to allow removal of the tether.

The elastomeric joint to tether tube adaptor 115 consists of a tapered steel tube with a flange at one end to connect to the elastomeric joint 113 and a male butress thread to screw into a VAM coupling at the other end. The adaptor 115 provides a smooth transition between the elastomeric joint 113 and the tubular tether and acts to control bending stresses.

There are two in each tether, one below the upper elastomeric joint, and one above the lower elastomeric joint.

The lower elastomeric joint 113 is identical to the upper elastomeric joint 120 and is required to reduce bending stresses in the tether 100 and anchor post 104.

The anchor connector 112 is designed to latch automatically to the anchor post 104 when a tether is installed and then to transfer vertical and horizontal loads and bending moments from the tether to the anchor post. The anchor connector 112 is pulled into the anchor post 104 during installation and is therefore designed, as part of the pull-on system, with a flared mating skirt and a central alignment probe.

The automatic latching system is actuated by contact between the connector and the anchor post at which point a weight is released on the connector which drives a set of dogs into a groove on the anchor post. The weight is then locked in place. The mating faces on the connector and anchor post are tapered to allow for misalignment during installation or removal. In order to remove the connector the weight holding the dogs in place is unlocked and raised hydraulically and the connector 112 is pulled from the post 104.

A flange on the top of the connector 112 mates with a flange on the lower elastomeric joint 113. The long life wire of the tether haul-down system passes through the anchor connector flange where it is held in position by a split collet.

The anchor posts 104, 106 are thick walled hollow steel tubes 4 metres high and 610mm outside diameter. There are 10 posts evenly spaced on an 8 metre diameter pitch circle and the lower end of each post is built into the anchorage unit. The upper end is tapered to receive the anchor connector and there is a circular groove in the tapered section to accept the dogs of the anchor connector. The top of each post 104 is radiused both externally and internally to assist in tether connection and to minimise wear on the haul-down rope as it passes through the top of the post and out through a vertical slot in the side of the post.

The anchor post 104 will have a temporary debris lid fitted prior to anchorange installation, and removed prior to tether installation.

Claims

1. A tether assembly for a tethered buoyant platform suitable for installation in a shaft in the leg of the tethered buoyant platform, said tether assembly comprising a tubular tether having a terminal at the end thereof for connecting to a corresponding member on the sea bed and having located at one or more positions along its length a flexible joint to reduce bending stresses in the tether.

2. A tether assembly as claimed in claim 1 wherein one flexible joint is located in the region of the lower end of the tether and a second flexible joint is located in the region of the upper end.

3. A tether assembly as claimed in claim 1 or claim 2 wherein the tether has located along its length a tether length adjusting means said means comprising a plurality of axially spaced apart mem berscapable of engaging corresponding members associated with a releasable clamping means.

4. A tether assembly as claimed in claim 3 wherein the tether length adjusting means comprises a tubular member having a plurality of axially spaced apart circumferential grooves capable of engaging corresponding circular spigots on the releasable clamping means or vice versa.

5. A tether assembly as claimed in any one of the preceding claims wherein the tether is installed in the shaft of the leg of a tethered buoyant platform and has located at a position along its length within the shaft and in the region of the lower end thereof a bearing to transmit the restraining forces from the tether to the shaft as the tethered buoyant platform surges.

6. A tether assembly as claimed in claim 5 wherein the bearing has a tubular body and, in order to reduce loads, is relatively long in an axial direction and is located in a section of the shaft of reduced diameter.

7. A tethered buoyant platform installed at an offshore location and including a plurality of tether assemblies as claimed in any one of the preceding claims.

8 A tether assembly substantially as hereinbefore described with reference to any one of the drawings.