GB1604357A

GB1604357A - Offshore structure and method

Info

Publication number: GB1604357A
Application number: GB2538778A
Authority: GB
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-12-09

Description

(54) OFFSHORE STRUCTURE AND METHOD (71) We, THE BRITISH PETROLEUM COMPANY LIMITED, of Britannic House, Moor Lane, London, EC2Y 9BU, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following state ment : - This invention relates to a tethered buoyant platform, to an anchoring system for the tethers thereof and to a method and apparatus for the installation and tensioning of the tethers.

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.

A tethered buoyant platform of improved construction has now been invented.

Thus, according to one aspect of the present invention a tethered buoyant platform has four vertical buoyancy chambers herein referred to as legs and four horizontal buoyancy chambers herein referred to as hulls, each leg having a plurality of conduits extending lengthwise therethrough to accommodate the tethers of the tethered buoyant platform two of the hulls being of relatively large cross section and two relatively small, the two relatively large hulls being disposed parallel to each other so as to be on opposite sides of a square or rectangle and the two relatively small hulls being disposed likewise.

The hulls may be subdivided into watertight compartments and the tanks may be suitable to be employed as ballast tanks.

The tethered buoyant platform can include an anchoring system comprising (a) a plurality of tethers each tether having at the end thereof a terminal (b) an anchor or anchors for installation on the sea bed the anchor or anchors having a plurality of members corresponding to the number of terminals and wherein the terminals and members co-operate in a releasable locking engagement.

The anchor can be a gravity anchor or drilled anchor. The system can further comprise a jacking means for applying tension to the tethers.

According to another aspect of the present invention a method of installing a tether of the tethered buoyant platform as hereinbefore defined in the previous but one paragraph comprises: (a) installing an anchor on the sea bed, (b) positioning the tethered buoyant platform over the anchor, (c) installing a guide wire between the anchor and the tethered buoyant platform and, (d) lowering one of said tethers and, by using a guide frame working on the guide wire, guiding the tether so that its terminal enters into releasable locking engagement with a respective one of the members on the anchor.

By the use of the word tethered we do not intend to be limited to platforms the tethers of which are moored to the sea bed but to include platforms in a condition suitable for moving to their location.

The invention is illustrated by reference to the accompanying drawings in which Fi & 1 is a vertical section of one leg of the tethered buoyant platform showing shafts for the tethers extending through the leg.

Fig. 2 is a perspective view showing an anchor located on the sea bed the anchor having six anchor members in the form of receptacles for the terminals of six tethers.

A guidance frame is also shown.

Fig. 3 is a perspective view on an enlarged scale of one leg of the tethered buoyant platform showing a movable jack for tensioning the tethers one at a time.

Fig. 4 is a vertical section of one embodiment of a terminal for the lower end of a tether.

Fig. 5 is a vertical section of a modification of the bottom terminal shown in Fig.

4 and is shown located in its receptacle.

Fig. 6 is a vertical section of the guid ance frame shown in Fig. 2.

Fig. 7 is a vertical section showing the guidance frame docked at the base of a leg of the tethered buoyant platform.

Fig. 8 is a perspective view of the gate on the guidance frame for a tether.

Fig. 9 is a perspective view of a receptacle for a lower terminal of a tether.

Fig. 10 is a vertical section on one embodiment of a lower terminal located on its receptacle.

Fig. 11 is a vertical section of an alternative embodiment of lower terminal located in its receptacle.

Fig. 12 is a vertical section of another alternative embodiment of lower terminal located in its receptacle.

Fig. 13 is a vertical section of another alternative embodiment of lower terminal located in its receptacle.

Fig. 14 is vertical section of an upper terminal and its anchoring assembly.

Figs. 15 to 19 show the various stages of installation of a tether.

Referring to Fig. 1 a tethered buoyant platform has four vertical buoyancy chambers (herein referred to as legs) only one of which is shown by reference numeral 2.

Between these legs four horizontal buoyancy chambers or hulls are disposed (only one of which 16 is shown), two being of relatively large cross section and two being of relatively small cross-section, the relatively large hulls being disposed parallel to each other so as to be on opposite sides of a square or rectangle, the two relatively small hulls being disposed likewise. The leg 2 has extending lengthwise therethrough six conduits in the form of tubular shafts symmetrically disposed with respect to the axis of leg 2 through which shafts the tethers pass, only two shafts 4 and 6 being shown. In the region of the water level 8 the leg 2 has a double skin, the two skins 10, 12 being held apart by stiffeners 14.

Located on the axis of the leg 2 is another tubular shaft 15 of larger diameter than the tether shafts for access and service.

Connected to leg 2 is a hull 16 which is divided into watertight compartments 18 (which is a permanent ballast tank) and controllably floodable tank 20 to provide a temporary ballast tank. A catwalk 22 extends through hull 16 to the access shaft 15.

Referring to Fig. 2 a gravity anchor 22 is located on sea bed 24 and has six re ceptacles 26, 28, 30. 32, 34 and 36 for the terminals 67 on six tethers and a central locking device (not shown) within sleeve 38 to engage the terminal on a guide wire 40. A guidance frame indicated generally by numeral 42 has six sleeves 44 for engag ing the tethers. Each sleeve 44 comprises two half sleeves 46 and 48 hinged together (see Fig. 8) at hinges 50, 50' and 50" and lockable by pin 52 and rings 54 and 54'.

Attached to half sleeve 48 are hydraulic rams 56 and 56' for moving the sleeve and tether radially with respect to the central guide wire 40 to engage the terminal with its corresponding receptacle. Guidance frame 42 is suppled with hydraulic power via umbilical 58 and suspended on chain 60.

With reference to Fig. 3 the leg 2 has decks 62 and 62' and 62". Deck 62 supports tether jack 64 which is rotatable about the service shaft 15 by means of crane 66 to work on each tether one by one.

With reference to Fig. 4 a lower terminal for a tether cable indicated generally by numeral 67 is attached to a tether cable 29 the strands of which are splayed out at its end and set in resin 68 enclosed in a housing 70 having an upper spherical bearing surface 72 to permit angular movement and immersed in an oil bath 71 within an outer casing 74 to prevent corrosion.

Where the tether 29 enters the outer casing 74 is provided an oil seal comprising a gaiter 75 held by a clamp 77. Around the gaiter 75 is provided a protection sleeve 79.

With reference to Fig. 5 a lower terminal for a tether cable indicated generally by reference numeral 67A is attached to a inch diameter tether cable 29A the strands of which are splayed out at its end and set in resin 68A enclosed in a housing 70A.

The housing is enclosed in an oil bath 71A (to prevent corrosion) within an outer casing in the form of mounting cone 76A.

The upper surfaces 72A of housing 70A act against a hard spherical seating or bearing 73A. The bearing 73A can be an elastomeric type bearing and may be made of layers of steel plate separated by layers of rubber, the layers being bonded to each other on both faces. At the top of the terminal is provided a flexible gaiter 77A made of polychloroprene e.g. Neoprene (RTM) or other material resistant to hydrocarbon oil and seawater to act as a seal and secured to the tether 29A by clamp 82A. Surrounding the gaiter 77A is a steel sleeve 78A having a flange 80A bolted to the mounting cone 76A. The base of the mounting cone 76A has a cover 84A secured thereto by bolts 86A and having a central plug 88A for oil emptying and filling. The mounting cone 76A is located in its open sided receptacle 90A (shown in more detail in Fig. 9) located on the sea bed. The terminal allows 8 of movement as indicated by the broken lines at the top of the figure. The terminal 67A is assembled into a unit above the surface and then located in the previously installed receptacle 90A as one unit.

Referring to Figs. 6 and 7 the guidance frame 42 has a floodlight and T.V. facility 41 and is shown in Fig. 7 in its docked position under the leg 2. The leg 2 has a cage 76 for protection of the frame 42 and a divers catwalk 78.

Referring to Fig. 9 the receptacle 34 for the lower terminal 67 of tether 29 has guide vanes 35. Terminal 67 is lowered to the position shown in Fig. 2, the hydraulic rams 56 and 56' actuated to move the terminal and tether radially with respect to the central guide wire 40 and tension then applied from the platform to lock the terminal in the position shown by 67'. The arrow shows the movement of the terminal 67.

In Fig. 10 the right hand side shows the locked position and the left hand side the unlocked position.

In Fig. 10 the terminal 67 forms part of an assembly having pawls 83 which lock against the lip 83A of upstanding pin 88.

Each pawl 83 has a lobe 86 which is engaged by sleeve 85 to effect unlocking.

Entry of terminal 67 is facilitated by guide cone 87.

The receptacle in Fig. 11 has locking pawls 100 (Shown locked on right hand side, unlocked on left hand side) pivoted at 101 and operated by hydraulic rams 102 and ram actuator 103 to which hydraulic fluid is supplied by lines 104 to provide remote control. The terminal 67 has a flange 105 which is engaged by the cams in the latched position. Release of the pawls is effected hydraulically by ram 102 acting on pawl 100 to cause same to pivot away from the locked position.

In Fig. 12 the terminal 67 forms part of an assembly having locking pawls 106 which can engage a lip 107 on the receptacle 109. Engagement is effected by lowering the assembly a small distance below the locking position, where pawls 106 are allowed to pivot outwardly into the locking position, and moving the assembly upwardly to engage the pawls 106 with lip 107. Check sleeve 108 can be used during the lowering to maintain pawls 106 upright and assist entry into the receptacle.

Disengagement is effected by downward movement of terminal 67 in the receptacle 109 to move pawls 106 into an upright position, and maintaining the pawls upright by means of check sleeve 108 and withdrawing assembly and check sleeve together.

In Fig. 13 the arrangement is generally similar to that shown in Fig. 10 except that the terminal assembly 81 incorporates a universal joint 110.

Referring to Fig. 14 tether cable 29 is splayed out at its end and set in resin 68' (as described with reference to Fig. 4) and enclosed within a housing 120' whose upper flange 114 is bolted to end plate 115.

End plate 115 has a plate 116 pivotally connected at pivot 119 to plate 117, the latter being attached to member 118 of a tensioning frame. Tensioning of cable 29 is effected by means of jacks working in uprights 120 the rams of the jacks being shown by the broken lines in uprights 120 and locked by means of shim plates 121 supported on the upper terminal seating 123.

Referring to Figs. 15 to 19 guidance frame 42 is unlocked from its docked position under the leg of the platform and the guide wire 40 unreeled and lowered and its terminal 41 locked onto upstanding pin 38 at the centre of the upper surface of anchor 22, manoeuvring the platform by its catenary moorings (not shown) and employing a diver or minisubmarine to assist the locking.

Tether 29 is then unwound from its drum (not shown), led over radius guide 130 and lowered through the opening in tether anchorage flat 131 and down through tether shaft 4 (Fig. 15). When the terminal 67 has passed below the guidance frame cage 76, the hydraulic arms of the frame 42 are extended and the sleeve 44 locked round the tether 29. Then the bottom terminal 67 is lowered and guided to its receptacle 34 using the guidance frame 42. The tether 29 is pulled taut using tail wire 132 and a cut point marked at a position above anchorage flat 131 and clamped at the top of the tether shaft 4 using slips 133. The end of the tether 29 is then cut and set in resin. The top terminal assembly 135 is then made up and, using balance weight 125 and pulley system 127, the slips 133 are removed to allow the tether to float independently of the platforms heave. The operation is then repeated for the other tethers 29A and when all the tethers at each leg are floating the upper terminal seatings 123 are positioned over the openings in anchorage flat 131 and the platform deballasted by pumping water out of the temporary ballast tanks until the upper terminal assemblies 135 are seated. The pulleys 127 and balance weights 125 are then removed.

Having regard to the provisions of Section 9 of the Patents Act 1949, attention is directed to our copending application 25388/78 (Serial No. 1604358).

WHAT WE CLAIM IS: 1. A tethered buoyant platform having four vertical buoyancy chambers herein referred to as legs and four horizontal buoyancy chambers herein referred to as hulls, each leg having a plurality of conduits extending lengthwise therethrough to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. receptacle 90A as one unit. Referring to Figs. 6 and 7 the guidance frame 42 has a floodlight and T.V. facility 41 and is shown in Fig. 7 in its docked position under the leg 2. The leg 2 has a cage 76 for protection of the frame 42 and a divers catwalk 78. Referring to Fig. 9 the receptacle 34 for the lower terminal 67 of tether 29 has guide vanes 35. Terminal 67 is lowered to the position shown in Fig. 2, the hydraulic rams 56 and 56' actuated to move the terminal and tether radially with respect to the central guide wire 40 and tension then applied from the platform to lock the terminal in the position shown by 67'. The arrow shows the movement of the terminal 67. In Fig. 10 the right hand side shows the locked position and the left hand side the unlocked position. In Fig. 10 the terminal 67 forms part of an assembly having pawls 83 which lock against the lip 83A of upstanding pin 88. Each pawl 83 has a lobe 86 which is engaged by sleeve 85 to effect unlocking. Entry of terminal 67 is facilitated by guide cone 87. The receptacle in Fig. 11 has locking pawls 100 (Shown locked on right hand side, unlocked on left hand side) pivoted at 101 and operated by hydraulic rams 102 and ram actuator 103 to which hydraulic fluid is supplied by lines 104 to provide remote control. The terminal 67 has a flange 105 which is engaged by the cams in the latched position. Release of the pawls is effected hydraulically by ram 102 acting on pawl 100 to cause same to pivot away from the locked position. In Fig. 12 the terminal 67 forms part of an assembly having locking pawls 106 which can engage a lip 107 on the receptacle 109. Engagement is effected by lowering the assembly a small distance below the locking position, where pawls 106 are allowed to pivot outwardly into the locking position, and moving the assembly upwardly to engage the pawls 106 with lip 107. Check sleeve 108 can be used during the lowering to maintain pawls 106 upright and assist entry into the receptacle. Disengagement is effected by downward movement of terminal 67 in the receptacle 109 to move pawls 106 into an upright position, and maintaining the pawls upright by means of check sleeve 108 and withdrawing assembly and check sleeve together. In Fig. 13 the arrangement is generally similar to that shown in Fig. 10 except that the terminal assembly 81 incorporates a universal joint 110. Referring to Fig. 14 tether cable 29 is splayed out at its end and set in resin 68' (as described with reference to Fig. 4) and enclosed within a housing 120' whose upper flange 114 is bolted to end plate 115. End plate 115 has a plate 116 pivotally connected at pivot 119 to plate 117, the latter being attached to member 118 of a tensioning frame. Tensioning of cable 29 is effected by means of jacks working in uprights 120 the rams of the jacks being shown by the broken lines in uprights 120 and locked by means of shim plates 121 supported on the upper terminal seating 123. Referring to Figs. 15 to 19 guidance frame 42 is unlocked from its docked position under the leg of the platform and the guide wire 40 unreeled and lowered and its terminal 41 locked onto upstanding pin 38 at the centre of the upper surface of anchor 22, manoeuvring the platform by its catenary moorings (not shown) and employing a diver or minisubmarine to assist the locking. Tether 29 is then unwound from its drum (not shown), led over radius guide 130 and lowered through the opening in tether anchorage flat 131 and down through tether shaft 4 (Fig. 15). When the terminal 67 has passed below the guidance frame cage 76, the hydraulic arms of the frame 42 are extended and the sleeve 44 locked round the tether 29. Then the bottom terminal 67 is lowered and guided to its receptacle 34 using the guidance frame 42. The tether 29 is pulled taut using tail wire 132 and a cut point marked at a position above anchorage flat 131 and clamped at the top of the tether shaft 4 using slips 133. The end of the tether 29 is then cut and set in resin. The top terminal assembly 135 is then made up and, using balance weight 125 and pulley system 127, the slips 133 are removed to allow the tether to float independently of the platforms heave.The operation is then repeated for the other tethers 29A and when all the tethers at each leg are floating the upper terminal seatings 123 are positioned over the openings in anchorage flat 131 and the platform deballasted by pumping water out of the temporary ballast tanks until the upper terminal assemblies 135 are seated. The pulleys 127 and balance weights 125 are then removed. Having regard to the provisions of Section 9 of the Patents Act 1949, attention is directed to our copending application 25388/78 (Serial No. 1604358). WHAT WE CLAIM IS:

1. A tethered buoyant platform having four vertical buoyancy chambers herein referred to as legs and four horizontal buoyancy chambers herein referred to as hulls, each leg having a plurality of conduits extending lengthwise therethrough to

accommodate the tethers of the tethered buoyant platform, and two of the hulls being of relatively large cross section and two being of relatively small cross section, the two relatively large hulls being disposed parallel to each other so as to be on opposite sides of a square or rectangle and the two relatively small hulls being disposed likewise.

2. A tethered buoyant platform as claimed in Claim 1 wherein each of the hulls is sub-divided into a plurality of watertight compartments which are suitable for employment as ballast tanks.

3. A tethered buoyant platform as claimed in claim 2 wherein one or more of the compartments are controllably floodable to serve both as ballast tanks and buoyancy tanks.

4. A tethered buoyant platform as claimed in any one of Claims 1 to 3 including an anchoring system said anchoring system comprising (a) a plurality of tethers, each tether having a terminal at the lower end thereof, (b) an anchor or anchors for installation on the sea bed, the anchor or anchors having a plurality of members corresponding to the number of tether terminals and wherein the terminals and members co-operate in a releasable locking engagement.

5. A tethered buoyant platform as claimed in claim 4 wherein each of the tethers of the anchoring system is a cable and the terminals of the tethers each comprise a housing containing a settable material into which the strands of the cable are splayed out and set.

6. A tethered buoyant platform as claimed in Claim 5 wherein the housing is enclosed in an oil bath within an outer casing.

7. A tethered buoyant platform as claimed in Claim 6 wherein a bearing is located between the upper surface of the housing and the interior surface of the outer casing.

8. A tethered buoyant platform as claimed in claim 7 wherein the bearing is made of steel plate reinforced rubber.

9. A tethered buoyant platform as claimed in any one of claims 4 to 8 wherein the upper end of each tether is provided with a terminal comprising a housing containing a settable material into which the strands of the cable are splayed out and set.

10. A tethered buoyant platform as claimed m any one of the preceding claims the platform having 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 member with which the terminal of the tether is to lock.

11. A tethered buoyant platform as claimed in Claim 10 wherein the second sleeve is movable in a radial direction with respect to the first sleeve and the guide tool includes means for moving same.

12. A method of installing a tether of a tethered buoyant platform as claimed in any one of claims 4 to 11 which method comprises: (a) installing an anchor on the sea bed, (b) positioning the tethered buoyant platform over the anchor (c) installing a guide wire between the anchor and the tethered buoyant platform and, (d) lowering one of said tethers and by using a guide wire frame working on the guide wire, guiding the tether so that its terminal enters into releasable locking engagement with a respective one of the members on the anchor.

13. A tethered buoyant platform substantially as hereinbefore described with reference to the accompanying drawings.