Upgrading Subsea Foundations of Mooring Systems

Abstract

A mooring system has a pre-existing subsea foundation at a first seabed location and a catenary mooring line connected to the pre-existing foundation and extending to a moored floating unit. A supplementary subsea foundation is installed at a second seabed location to upgrade the capacity of the mooring system. The mooring line is connected to the supplementary subsea foundation while the connection between the mooring line and the pre-existing subsea foundation is maintained.

Claims

1. A method of upgrading the capacity of a mooring system that comprises a pre-existing subsea foundation at a first seabed location and a mooring line connected to the pre-existing subsea foundation and extending to a moored floating unit, the method comprising: installing a supplementary subsea foundation at a second seabed location; and connecting the mooring line to the supplementary subsea foundation while maintaining the connection between the mooring line and the pre-existing subsea foundation.

2. The method of claim 1, comprising displacing a portion of the mooring line at the second seabed location to make space for installing at least part of the supplementary subsea foundation.

3. The method of claim 2, comprising lifting the displaced portion of the mooring line away from the seabed.

4. The method of 3, comprising moving at least part of the supplementary subsea foundation to a position beneath the displaced portion of the mooring line.

5. The method of claim 4, preceded by lowering at least part of the supplementary subsea foundation to a seabed location beside the mooring line.

6. The method of claim 4, comprising effecting translational movement of at least part of the supplementary subsea foundation across the seabed.

7. The method of claim 4, comprising lifting at least part of the supplementary subsea foundation across the seabed.

8. The method of claim 2, comprising diverting the displaced portion of the mooring line around the second seabed location and laying that displaced portion of the mooring line on the seabed.

9. The method of claim 2, comprising moving the displaced portion of the mooring line into connection with at least part of the supplementary subsea foundation.

10. The method of claim 1, comprising placing a frame of the supplementary subsea foundation on the seabed, embedding one or more foundation elements in the seabed, coupling the or each foundation element to the frame and connecting the mooring line to the frame.

11. The method of claim 10, comprising coupling the or each foundation element to the frame by installing the or each foundation element into or through a guide structure provided in or attached to the frame.

12. The method of claim 10, comprising placing a link between the frame and the or each foundation element, which foundation element is embedded in the seabed and spaced apart from the frame.

13. The method of claim 10, comprising placing the mooring line between first and second laterally-spaced foundation elements.

14. The method of claim 1, comprising coupling a frame of the supplementary subsea foundation with a foundation element of the supplementary subsea foundation pre-installed at the second seabed location.

15. The method of claim 14, comprising lowering the frame onto the foundation element.

16. The method of claim 1, wherein the supplementary subsea foundation is a pile, the method comprising connecting the mooring line to the pile via a connection mechanism integrated with the pile.

17. The method of claim 1, comprising continuing to moor the surface installation using the existing subsea foundation and the supplementary subsea foundation in combination to resist tension in the mooring line without disconnecting the line from either foundation.

18. The method of claim 1, wherein the second seabed location is spaced across the seabed from the first seabed location.

19. The method of claim 1, wherein a chain section of the mooring line is connected to the supplementary subsea foundation by clamping the chain section or by mechanically engaging links of the chain section.

20. The method of claim 1, wherein the mooring line is connected to the supplementary subsea foundation by embracing the mooring line.

21. A mooring system, comprising: a first subsea foundation at a first seabed location; a mooring line connected to the first subsea foundation and extending to a moored floating unit; and a supplementary subsea foundation at a second seabed location, connected to the mooring line by a remotely-operable mechanical connector.

22. The mooring system of claim 21, wherein the mechanical connector connects the supplementary subsea foundation to a chain section of the mooring line by clamping the chain section or by engaging links of the chain section.

23. The mooring system of claim 21, wherein the mechanical connector embraces the mooring line.

24. The mooring system of claim 21, wherein the mechanical connector defines an upwardly-opening receptacle to receive the mooring line.

25. The mooring system of claim 21, wherein the supplementary subsea foundation comprises: a frame supporting the connector; and one or more foundation elements embedded in the seabed and coupled to the frame.

26. The mooring system of claim 25, wherein the or each foundation element is coupled to the frame via a guide structure provided in or attached to the frame.

27. The mooring system of claim 25, comprising a link between the frame and the or each foundation element, which foundation element is embedded in the seabed and spaced apart from the frame.

28. The mooring system of claim 25, wherein the mooring line extends between first and second laterally-spaced foundation elements.

29. The mooring system of claim 21, comprising an interface on top of at least one foundation element and/or beneath the frame for coupling the frame to the foundation element.

30. The mooring system of claim 21, wherein the supplementary subsea foundation is a pile and the connector is integrated with the pile.

31. A supplementary subsea foundation, comprising: a frame; a remotely-operable mechanical connector supported by the frame, which connector is arranged to connect with a mooring line; and at least one foundation element attachable to or integrated with the frame, arranged to be embedded in seabed soil.

32. The foundation of claim 31, wherein the mechanical connector is arranged to connect the supplementary subsea foundation to a chain section of the mooring line by clamping the chain section or by engaging links of the chain section.

33. The foundation of claim 31, wherein the mechanical connector is arranged to embrace the mooring line.

34. The foundation of claim 31, wherein the mechanical connector defines an upwardly-opening receptacle to receive the mooring line.

35. The foundation of claim 31, comprising a guide structure provided in or attached to the frame for attachment of the or each foundation element to the frame.

36. The foundation of claim 31, comprising a tensile link between the frame and the or each foundation element, which foundation element is spaced apart from the frame.

37. The foundation of claim 31, wherein the frame is attached to the foundation element by an interface above the foundation element and beneath the frame.

Description

[0058] To illustrate the prior art background, reference has already been made to FIGS. 1 and 2 of the accompanying drawings, in which:

[0059] FIG. 1 is a side view of a mooring line extending from a moored surface vessel to a suction pile; and

[0060] FIG. 2 is an enlarged detailed view of the ground chain end of the mooring line and the pile, corresponding to Detail II of FIG. 1.

[0061] In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the drawings in which:

[0062] FIG. 3 is a side view of a mooring line corresponding to FIG. 1 but with the addition of a supplementary foundation interposed between the FPSO and the suction pile in accordance with the invention;

[0063] FIG. 4 is a schematic perspective view of a ground chain of a mooring line on the seabed before the addition of a supplementary foundation in accordance with the invention;

[0064] FIG. 5 corresponds to FIG. 4 but shows the ground chain lifted above the seabed and a support frame placed on the seabed beside the ground chain;

[0065] FIG. 6 corresponds to FIG. 5 but shows the support frame moved laterally across the seabed to lie under the still-lifted ground chain;

[0066] FIG. 7 is an enlarged schematic perspective view of the support frame of FIG. 6 with the ground chain now lowered into engagement with a chain stopper mechanism on the support frame;

[0067] FIG. 8 corresponds to FIG. 7 but shows the addition of piles that pin the frame to the seabed to complete the supplementary foundation of the invention;

[0068] FIG. 9 is a schematic perspective view of a second embodiment of the invention in which the supplementary foundation of the invention is completed by anchoring a support frame carrying a chain stopper mechanism to piles that are remote from the support frame;

[0069] FIG. 10 is a schematic perspective view of a third embodiment of the invention in which a support frame carrying a chain stopper mechanism is integrated with a pile to form a supplementary foundation, the ground chain being diverted laterally from its normal route to make space for installing the pile;

[0070] FIG. 11 corresponds to FIG. 10 but shows the ground chain moved into engagement with the chain stopper mechanism on the pile; and

[0071] FIG. 12 is a schematic perspective view of a fourth embodiment of the invention in which a support frame carrying a chain stopper mechanism is being connected to a pre-installed pile.

[0072] Referring firstly to FIG. 3, this largely corresponds to FIG. 1 and therefore like numerals are used for like parts. Here, a supplementary foundation 32 that supports the mooring line 10 in accordance with the invention is shown interposed between the FPSO 12 and the suction pile 16. The supplementary foundation 32 engages the ground chain 20 of the mooring line, where the ground chain 20 crosses or lies on the seabed 18.

[0073] Thus, the pile 16 and the supplementary foundation 32 cooperate with each other to resist tension in the mooring line 10. In other words, the supplementary foundation 32 adds to the capacity of the pile 16 to upgrade the overall capacity of the mooring system. Importantly, this upgrade is achieved without disconnecting the mooring line 10 from the pile 16. Movement of the FPSO 12 is restrained continuously by the mooring line 10 throughout.

[0074] Advantageously, the lowermost portion of the mooring line 10 between the pile 16 and the supplementary foundation 32, including the portion that is buried in the soil of the seabed 18, is restrained against movement by the supplementary foundation 32. The supplementary foundation 32 also isolates that portion of the mooring line 10 from movement of the remainder of the mooring line 10. Thus, the capacity of the pile 16 will no longer degrade significantly due to trenching effects. The upper portion of the mooring line 10 between the supplementary foundation 32 and the FPSO 12 can, of course, continue to move but its amplitude of lateral motion is restricted. In any event, movement of the upper portion of the mooring line 10 takes place largely above the seabed 18, where it cannot give rise to a trenching effect.

[0075] FIGS. 4 to 8 illustrate one way in which the supplementary foundation 32 of FIG. 3 may be installed in accordance with the invention.

[0076] FIG. 4 shows a portion of the ground chain 20 of the mooring line 10 lying on the seabed 18. Part of the catenary curvature of the mooring line 10 between the FPSO 12 and the seabed 18 is evident on the right side of FIG. 4, where the ground chain 20 rises above the seabed 18. The pile 16 to which the ground chain 20 is attached is not shown but will lie to the left of the portion of the ground chain 20 shown in FIG. 4, at the end of a further portion of the ground chain 20 buried in the seabed 18.

[0077] FIG. 5 shows a mid-point of the ground chain 20 lifted from and held above the seabed 18 by a wire 34 that hangs from a surface support vessel (not shown). The wire 34 suitably hangs from a winch on the vessel, such as an abandonment and recovery (A&R) winch, but could instead hang from a crane on the vessel.

[0078] In FIG. 5, the surface support vessel has also lowered a frame 36 onto the seabed 18 in a holding location beside the raised portion of the ground chain 20. The frame 36 carries a connection mechanism 38 that is arranged to grip or engage the ground chain 20. The connection mechanism 38 may, for example, employ the principle of a clamp or a chain stopper as disclosed in CN 104802934. In this example, the frame 36 also carries one or more guide tubes 40 to receive pin piles, as will be described more fully with reference to FIG. 8 of the drawings.

[0079] The frame 36 is suitably lowered from a crane on the surface support vessel but could instead hang from a winch on the vessel. An ROV 42 is shown in attendance to monitor the operation and to disconnect slings 44 that support the frame 36.

[0080] In FIG. 6, the ground chain 20 is still held above the seabed 18 by the wire 34 but the frame 36 has now been moved across the seabed 18 to lie under the raised portion of the ground chain 20. The direction of movement of the frame 36 is transverse to, preferably substantially orthogonal to, the general plane of the mooring line 10 so as to intersect that plane.

[0081] This translational, generally horizontal movement of the frame 36 could be achieved by using the crane on the surface support vessel to lift the frame 36 from the holding location on the seabed 18. In that case, the ROV 42 shown in FIG. 5 can, if needs be, disconnect and reconnect slings 44 to avoid the slings 44 clashing with the raised portion of the ground chain 20. Another approach is to slide the frame 36 across the seabed 18 like a sled into its final position under the ground chain 20.

[0082] The enlarged view of FIG. 7 shows the ground chain 20 now lowered into, and engaged with, with the connection mechanism 38 on the frame 36. The connection mechanism 38 defines an upwardly-opening receptacle for this purpose. The connection mechanism 38 embraces and receives the ground chain 20.

[0083] Finally, as shown in FIG. 8, pin piles 46 are inserted into the guide tubes 40 on the frame 36 to secure the frame 36 to the seabed 18. This completes the supplementary foundation 32 as also shown in FIG. 3.

[0084] The connection mechanism 38 is operable remotely to connect the ground chain 20 to the supplementary foundation 32. The connection mechanism 38 clamps the ground chain 20 or engages links of the ground chain 20 in the manner of a chain stopper.

[0085] The FPSO 12 remains moored thereafter using the pile foundation 16 and the supplementary foundation 32 in combination to resist tension in the mooring line 10 without disconnecting the line 10 from either foundation 16, 32. The line 10 remains connected to both foundations 16, 32 thereafter and may remain so connected at least until the whole mooring system is eventually decommissioned. This may therefore be for longer than a moored FPSO 12 or other surface installation remains on station, which could be for more than twenty years.

[0086] Turning next to FIG. 9, this shows a supplementary foundation 48 in a second embodiment of the invention, being a variant of the first embodiment shown in FIGS. 3 to 8. In this variant, a frame 36 supports a connection mechanism 38 as before but the guide tubes 40 are omitted. Instead, the frame 36 is anchored by remote foundation elements that are exemplified here as piles 50 but could take other forms such as drag anchors. The frame 36 is connected to the piles 50 by respective tensile links 52, which could be of chain or wire.

[0087] Preferably, as shown in FIG. 9, the ground chain 20 is placed between laterally-spaced foundation elements such as the piles 50. This balances forces on the system while leaving space for installing the piles 50 without having to divert the ground chain 20 laterally as shown in the next embodiments.

[0088] FIGS. 10 and 11 show a supplementary foundation 54 in a third embodiment of the invention, in which a connection mechanism 38 is integrated with a foundation member. The foundation member is exemplified here as a suction pile 56. In effect, the top of the pile 56 is equivalent to the frame 36 of the preceding embodiments.

[0089] FIG. 10 shows the ground chain 20 pulled laterally across the seabed 18 to allow space for installation of the pile 56 along the normal route of the ground chain 20. Once the pile 56 has been installed in the seabed 18, the ground chain 20 is lifted into engagement with the connection mechanism 38 as shown in FIG. 11.

[0090] Finally, FIG. 12 shows a supplementary foundation 58 in a fourth embodiment of the invention, in which a frame 36 that supports a connection mechanism 38 can be mounted to a pre-installed pile 60 via an interface structure 62. Like FIG. 10, the ground chain 20 extending across the seabed 18 is shown here diverted to allow space for installation of the pile 60 along the normal route of the ground chain 20.

[0091] As in the first embodiment, the frame 36 is suitably lowered from a crane on a surface support vessel but could instead hang from a winch on the vessel. An ROV like that shown in FIG. 5 can monitor the operation and then disconnect slings 44 that support the frame 36.

[0092] Once the frame 36 has been mounted onto the pile 60 via the interface structure 62, the ground chain 20 is lifted into engagement with the connection mechanism 38 in a similar way to the arrangement shown in FIG. 11.

[0093] Many variations are possible without departing from the inventive concept. For example, the principle of the invention could be used when mooring a unit that floats at a subsea location rather than at the surface, such as a subsea buoy.