Supporting subsea wellhead equipment

Abstract

A subsea installation comprises equipment such as a BOP mounted atop a subsea wellhead. A support arrangement for the equipment comprises an anchor structure that is fixed to the seabed around or beside the wellhead and an array of elongate damping members that connect the anchor structure to the equipment. The damping members each comprise a double-acting damper that damps motion of the equipment relative to the anchor structure and the seabed. Each damper comprises a piston that is reciprocable through a hydraulic fluid within a cylinder. Springs in mutual opposition about the piston bias the piston toward an equilibrium position within the cylinder. The damper is disposed between or attached to at least one elongate element of the damping member. The elements are fixed to the piston and / or to the cylinder and can include rigid rods to transmit compressive force along the damping member.

Description

[0001] Supporting subsea wellhead equipment

[0002] This invention relates to techniques for supporting subsea wellhead equipment used in the offshore oil and gas industry.

[0003] Examples of subsea wellhead equipment include blowout preventers (BOPs), lower marine riser packages (LMRPs), trees, and drilling riser adaptors, which may be installed and used together as a BOP stack atop a subsea wellhead. For brevity, all such equipment will be referred to hereinafter simply as a BOP.

[0004] A BOP may require support to minimise wellhead fatigue cycles that could otherwise be caused by excessive oscillatory movement of the BOP, for example as driven by movement of a drilling riser connected to a drilling rig at the surface. More generally, support may be required to resist lateral and axial loads and bending moments acting on the BOP.

[0005] Conventionally, as shown in Figures 1 and 2, a subsea BOP 10 stands above the seabed 12, mounted on a subsea wellhead 14. Standard practice in the offshore oil and gas industry is to support or brace a BOP 10 using an array of tensioned flexible ropes, wires or cables 16 that are angularly spaced around the BOP 10 in plan view to tether the BOP 10 against lateral and vertical movement. Commonly, four such cables 16 are used, equiangularly spaced around the upright flow axis of the BOP 10, hence with a mutual angular spacing of 90°.

[0006] Each cable 16 extends from the BOP 10 to a respective anchor on the seabed 12. For example, Figure 1 shows anchors in the form of gravity base anchors 18 that are each topped by a respective winch 20. Each winch 20 serves as a tensioner for the associated cable 16. Figure 2 shows another example, in this case with suction anchors 18 that are each topped by a respective linear tensioner 22 acting on the associated cable 16. In a further example, the anchors can be drilled or impact-driven piles.

[0007] US 2014 / 0374116 discloses a system for tethering a subsea wellhead using suction anchors, tensioning systems and tension members. The system includes pile top assemblies, locking rams, and winches to apply tensile preloads and to balance lateral loads. In US 10577768 and US 2020 / 0003025, flexible tension members apply lateral and vertical preloads to stabilise a BOP. US 11473387 also discloses an arrangement for tethering a subsea BOP, focusing in that case on a tensioning system that comprises a rope gripper and a tension cylinder.

[0008] The above prior art disclosures disclose tethering a BOP to individually-installed anchors. US 10724349 and GB 2527386 take a different approach of tethering a BOP to a template structure that is installed on the seabed around the BOP or the wellhead to serve as a common anchor for all of the tethers. Similarly, WO 2021 / 091397 proposes tethering to a template or other frame structure fixed to the seabed. Again, in each case, tethering relies upon tensioned flexible members such as cables extending between the BOP and the structure that is fixed to the seabed.

[0009] A problem suffered by all of the above prior art teachings is the need for ROV intervention to set and adjust tension in the tethers individually, noting that tension can drift or be lost to issues such as cable creep, sliding in soft soil and tensioner failure. Consequently, tensioning or checking the tension of the tethers is necessary not only upon installation of the BOP but also periodically during the operational life of the BOP, hence requiring the expense of ongoing, lengthy visits from ROV support vessels. The BOP tethering system of US 11525338 attempts to mitigate this problem with a hydraulic interface that provides centralised tension adjustment, enabling tension in the tethers to be adjusted either individually or as a group. However, ROV intervention is still required.

[0010] US 2024 / 0200631 eschews tethering in favour of damping lateral motions of a BOP instead. For this purpose, horizontally-extending mass dampers are fixed to the BOP structure to act on mutually orthogonal horizontal axes.

[0011] Each mass damper of US 2024 / 0200631 comprises a horizontally elongate hollow cylinder in fixed relation to the BOP. The cylinder contains springs acting in mutual opposition on a mass that is disposed centrally between the springs. The springs surround a bar along which the mass can slide within the cylinder, hence biasing the mass to a central rest position along the length of the cylinder.

[0012] The cylinder of US 2024 / 0200631 also contains a viscous fluid that surrounds the mass. A narrow clearance between the mass and the tubular wall of the cylinder serves as a restriction that hinders flow of the fluid around the mass as the cylinder fixed to the BOP moves relative to the mass, whose inertia resists movement. The restricted fluid flow resists relative motion between the cylinder and the mass, and so damps lateral motion of the BOP to which the cylinder is attached.

[0013] The mass of US 2024 / 0200631 is not a piston: there is no rigid connection extending out of the cylinder that is capable of transmitting a longitudinal force to, or from, the mass. The mass damper is instead akin to the damping arrangements that are used to protect tall buildings from damage due to the oscillations of earthquakes. Also, a horizontally-movable mass cannot damp vertical oscillations.

[0014] It is against this background that the invention has been devised. In one sense, the invention resides in a method of supporting equipment mounted atop a subsea wellhead, the method comprising damping motion of the equipment relative to a seabed around the wellhead by operation of an array of damping members that connect the equipment to an anchor structure fixed to the seabed.

[0015] The damping members of the array may be connected to a single or common anchor structure, which structure may comprise a plurality of seabed-engaging anchors.

[0016] Compressive forces from the equipment can be transmitted along the damping members. Motion of the equipment can be damped with double-acting dampers of the damping members, for example by reciprocating a piston through a fluid within a cylinder of each damper. A restricted flow of the fluid from a compression chamber to an expansion chamber within the cylinder can be permitted through or around the piston. The piston can be biased toward a central equilibrium position within the cylinder.

[0017] Conveniently, the equipment can be installed onto the wellhead with the damping members attached to the equipment, which may be followed by deploying the damping members from the equipment to connect the damping members to the anchor structure.

[0018] The anchor structure can be landed on the seabed around the equipment previously installed on the wellhead. Alternatively, the anchor structure can be landed on the seabed and the equipment can be installed subsequently on the wellhead within or beside the anchor structure.

[0019] Correspondingly, the inventive concept embraces a subsea installation comprising equipment mounted atop a subsea wellhead and a support arrangement for the equipment, wherein the support arrangement comprises: an anchor structure fixed to a seabed around or beside the wellhead; and an array of elongate damping members connecting the anchor structure to the equipment, the damping members being configured to damp motion of the equipment relative to the anchor structure. Side members of the anchor structure may surround an aperture that defines an upright through-passage extending through the anchor structure to accommodate the equipment.

[0020] The damping members can surround the equipment and extend upwardly and inwardly from the anchor structure to the equipment, with inclination to the horizontal. The anchor structure can further comprise outwardly-extending anti-trawl structures, each anti-trawl structure having an upwardly- and inwardly-inclined upper surface that may be substantially aligned with a respective one of the damping members.

[0021] The damping members can each include at least one double-acting damper that comprises a piston reciprocable through a damping fluid within a cylinder. Springs in mutual opposition about the piston can bias the piston toward the equilibrium position within the cylinder. Each damping member may further comprise elongate elements between which the damper is positioned, one of the elongate elements being fixed to the piston and another of the elongate elements being fixed to the cylinder. For example, each damping member may comprise at least one rigid rod that is capable of transmitting compressive force along the damping member.

[0022] Thus, the invention provides a subsea damping system and methods to minimise wellhead fatigue cycles due to excessive movement of a BOP. In embodiments to be described, a subsea anchoring structure and four subsea hydraulic actuators form a damping system to minimise excessive movement of a BOP stack and wellhead system, such movement mainly arising during a drilling operation.

[0023] The subsea damping system of the invention is a more efficient and reliable solution than a traditional tensioner system, which requires ROV intervention for re-tensioning of cables or wires to guarantee a minimum tensioner capacity. Additionally, the damping system of the invention offers more precise time response to BOP displacement. The system of the invention also offers a substantial reduction in capital expenditure compared with a traditional system that employs separately-installed anchors, that reduction being in the order of 25%. The anchoring system comprises a subsea anchoring structure or frame and plural subsea damping members that connect the frame to a BOP. The frame is also connected to anchors such as suction piles that provide lateral soil support for the system.

[0024] The subsea damping members minimise excessive movement of a BOP through a combination of an actuator and springs, for example helical springs. Subsea actuators of the invention are designed for deepwater applications, with hydraulic operating pressure of, for example, 3000psi (20.7mPa) or 5000psi (34.5mPa) and axial force resistance from 120kN to 500kN.

[0025] An optimised design of a subsea anchoring structure, weighing about thirty tons, allows for installation by a crane of an ROV support vessel in water depths up to 2500 metres. The design of the anchoring structure also reduces logistical complexity compared to traditional suction piles used as anchors.

[0026] The subsea anchoring structure can be deployed from an installation vessel using standard field-proven installation aids and techniques, ensuring efficient, precise and reliable placement of the structure onto the seabed. Installation of the structure on the seabed can be effected or assisted by ROVs to ensure precise positioning and placement of anchoring elements such as suction piles around a subsea wellhead. Next, a BOP can be deployed by a drill rig and connected to the wellhead.

[0027] The subsea damping members, which may be carried by the BOP, are attached to the subsea anchoring system by ROV operation. This ensures precise positioning and secure attachment of the damping members, effectively reducing the potential for unwanted movements and vibrations in the BOP stack. Buoyancy can be added to reduce the submerged apparent weight of the damping members, making them easier for an ROV to manipulate.

[0028] Damping members of the invention combine the resilient stiffness of a spring element with the damping effect of a viscous fluid such as oil. This offers a reliable solution to minimise BOP movement while reducing the response time of the system and eliminating the need for ROV intervention to maintain anchor loads which, in turn, reduces overall operational costs. A double-acting damping member of the invention can be employed in conjunction with the subsea anchorage structure. The damping member comprises an external cylinder that may, for example, be able to support operation in up to 2000 metres of water depth and with an internal pressure of 3000psi (20.7mPa). A double spring system that can counterbalance an external force of, for example, up to 150kN acts on a piston within the cylinder. A beam or rod structure is connected to the piston to transfer external loads to the damping member, for which purpose a similar structure can be connected to the cylinder.

[0029] The damping effect of the damping member is provided by a hydraulic fluid within the cylinder, through which the piston moves. The piston is penetrated by restrictive passageways that allow fluid communication between internal chambers of the cylinder, one chamber on each side or end of the piston.

[0030] In summary, a subsea installation of the invention comprises equipment such as a BOP mounted atop a subsea wellhead. A support arrangement for the equipment comprises an anchor structure that is fixed to the seabed around or beside the wellhead and an array of elongate damping members that connect the anchor structure to the equipment. The damping members can each comprise a double-acting damper that damps motion of the equipment relative to the anchor structure and the seabed.

[0031] Each damper may comprise a piston that is reciprocable through a hydraulic fluid within a cylinder. Springs in mutual opposition about the piston bias the piston toward an equilibrium position within the cylinder. The damper may be disposed between or attached to at least one elongate element of the damping member. An elongate element can be fixed to the piston and / or to the cylinder and can be a rigid rod to transmit compressive force along the damping member.

[0032] To put the invention into context, reference has already been made to Figures 1 and 2 of the accompanying drawings, which are perspective views of prior art solutions for supporting a BOP atop a subsea wellhead.

[0033] 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: Figure 3 is a perspective view of support arrangement of the invention supporting a BOP;

[0034] Figures 4, 5 and 6 are perspective views showing installation stages of the support arrangement shown in Figure 3;

[0035] Figure 7 is a perspective view of a damper of the invention that can be used in the support arrangement shown in Figure 3; and

[0036] Figures 8a and 8b are side views in longitudinal section of the damper shown in Figure 7, with a piston of the damper shown in rest and deflected positions respectively.

[0037] Figure 3 shows a support arrangement 24 of the invention in combination with a BOP 10 atop a subsea wellhead 14. The arrangement 24 comprises a lattice frame 26 that surrounds the wellhead 14 and that supports an integral array of anchors 16. In this example, the anchors 16 are suction anchors to be embedded in the seabed 12 when the frame is landed on the seabed 12 as shown in Figures 4, 5 and 6.

[0038] The frame 26 is generally square in plan view, surrounding a central aperture 28 that opens upwardly and downwardly as a through-passage to accommodate the BOP 10 and the wellhead 14. The frame 26 can be lowered from the surface and landed on the seabed 12 around a pre-installed BOP 10 or can be landed around the wellhead 14 before the BOP 10 is lowered into the aperture 28 and installed onto the wellhead 14.

[0039] The aperture 28 is defined between four side members 30 of the frame 26, those side members 30 being paired in mutually parallel opposition and the pairs being in mutually orthogonal relation. The side members 30 are horizontally elongate lattice structures that lie in respective upright planes.

[0040] Where they intersect at corners of the aperture 28, the side members 30 define lower corners where the frame 26 supports the anchors 16 and upper corners where the frame 26 supports lower mounts 32 at which damping members or damping struts 34 connect to the frame 26. The damping struts 34 converge inwardly and upwardly from the lower mounts 32, above and across the aperture 28, to connect to respective angularly-spaced upper mounts 36 on the BOP 10. Thus, the damping struts 34 are each inclined at an acute angle to the horizontal. A load cell may be incorporated into each damping strut 34 or into the lower or upper mounts 32, 36 to allow monitoring of tensile or compressive loads in the damping strut 34 during operation.

[0041] Outriggers 38 extend outwardly from the corners of the aperture 28 with equiangular spacing in plan view. Each outrigger 38 comprises a horizontal beam 40 that extends outwardly from the lower corner and an inclined buttress 42 that extends downwardly from the upper corner to meet the outer end of the beam 40.

[0042] Each outrigger 38 is coplanar with a respective one of the damping struts 34. The inclination of the buttress 42 of each outrigger 38 matches or approximates to the inclination of the coplanar damping strut 34. The inclination of the buttress 42 provides overtrawling protection for the support arrangement 24, additional to overtrawling protection provided by the inclination of the damping strut 34 that adjoins the buttress 42 end-to-end.

[0043] Figure 4 shows the frame 26, complete with suction anchors 16, being lowered through the water column from the surface to the seabed. Lifting wires 44 are shown temporarily attached to the frame 26 for this purpose, to be released from the frame 26 by an ROV after the frame 26 has been landed in the seabed. The lifting wires 44 may, for example, hang from a spreader frame that is suspended, in turn, from a crane of an installation vessel.

[0044] Figure 5 shows the frame 26 landed on the seabed 12 around a BOP 10, with the suction anchors 16 beginning to embed into the seabed 12 due to self-weight and momentum. An ROV 46 is shown operating suction pumps atop the suction anchors 16 to embed the suction anchors 16 further.

[0045] Eventually, as shown in Figure 6, the suction anchors 16 are fully embedded in the soil of the seabed 12. The damping struts 34 are then connected to the BOP 10 and / or to the frame 26. In this respect, an ROV 46 is shown coupling one of the damping struts 34 to the frame 26, that damping strut 34 having already been attached to the BOP 10. As Figure 5 shows, it may be most convenient to pre-attach the damping struts 34 to the BOP 10 and to install them in an upright orientation with the BOP 10, and then to pivot them away from the BOP 10 about the upper mounts 36 to be connected to the frame 26 after the frame 26 has been landed around the BOP 10, or after the BOP 10 has been landed within the frame 26. This reduces the rig time required to install the BOP 10.

[0046] Referring now also to Figures 7, 8a and 8b, the damping strut 34 comprises an elongate double-acting damping cylinder 48 disposed coaxially between elongate elements being upper and lower rods 50, 52. An upper end of the upper rod 50 is attached to the BOP 10 at one of the upper mounts 36 whereas a lower end of the lower rod 52 is attached to the frame 26 at a corresponding one of the lower mounts 32.

[0047] One of the rods 50, 52, in this example the upper rod 50, is fixed to the cylinder 48. The other one of the rods 50, 52, hence in this example the lower rod 52, is fixed to a piston 54 that can slide within the cylinder 48 as shown in Figures 8a and 8b. The rods 50, 52 are substantially rigid in this example, thus being capable of bearing both tensile and compressive loads. This enables the rods 50, 52 to both pull and push on the cylinder 48 and the piston 54 in response to motion of the BOP 10 relative to the seabed 12 and hence relative to the frame 26 that is fixed to the seabed 12.

[0048] The piston 54 is slidable within the cylinder 48 through a viscous hydraulic fluid that fills the interior of the cylinder 48. The cylinder 48 has closed ends, save for an aperture in one end that accommodates and effects a sliding seal around the lower rod 52 protruding from the cylinder 48. The lower rod 52 is fixed to the piston 54 to move longitudinally with the piston 54. Conversely, the upper rod 50 is fixed to the opposite end of the cylinder 48.

[0049] A piston ring 56 encircles the piston 54 to seal against the tubular side wall of the cylinder 48. The piston 54 thereby divides the interior of the cylinder 48 into a compression chamber 58 on one side of the piston 54 and an expansion chamber 60 on the other side of the piston 54. The compression chamber 58 reduces in volume whereas the expansion chamber 60 increases in volume as the piston 54 moves longitudinally along its stroke within the cylinder 48.

[0050] Having regard to the direction of movement of the piston 54 along its stroke, which is from right to left as illustrated in Figure 8b, the compression chamber 58 is ahead of the piston 54 and the expansion chamber 60 is behind the piston 54. Consequently, the compression chamber 58 and the expansion chamber 60 swap positions as the piston 54 reciprocates within the cylinder 48 as the damping strut 34 cyclically shortens and lengthens with oscillatory motion of the BOP 10.

[0051] The chambers 58, 60 are sealed by the piston 54 and the surrounding walls of the cylinder 48, save for longitudinal ports 62 that penetrate the piston 54 to effect fluid communication between the compression chamber 58 and the expansion chamber 60. Thus, as the piston 54 moves from right to left relative to the cylinder 48 as shown in Figure 8b, fluid pressure in the compression chamber 58 increases transiently and fluid pressure in the expansion chamber 60 reduces transiently. Fluid pressure within the cylinder 48 is then equalised by a flow of the hydraulic fluid through the ports 62 from the compression chamber 58 to the expansion chamber 60. Restricted flow through the narrow ports 62 coupled with the viscosity of the hydraulic fluid resists and so damps relative longitudinal movements between the piston 54 and the cylinder 48.

[0052] Compression springs 64 act in mutual opposition between the piston 54 and the respective ends of the cylinder 48. Thus, one spring 64 is in the compression chamber 58 and the other spring 64 is in the expansion chamber 60. The springs 64 have substantially equal stiffness and so bias the piston 54 toward a central rest or equilibrium position within the cylinder 48. Resilient end-stops 66 cushion the piston 54 against impact with the end walls of the cylinder 48 at the ends of the stroke of the piston 54.

[0053] The springs 64 can simply bear against the piston 54 and the respective ends of the cylinder 48, hence acting solely in compression. Alternatively, the springs 64 could be attached to the piston 54 and to the respective ends of the cylinder 48, hence potentially acting in tension when extended by movement of the piston 54 relative to the cylinder 48.

[0054] In each case, opposing forces exerted by the springs 64 on the piston 54 are in balance when the piston 54 is in the equilibrium position shown in Figure 8a. Conversely, the spring 64 in the compression chamber 58 exerts an increasing restoring force on the piston 54 with increasing displacement of the piston 54 away from the equilibrium position as shown in Figure 8b. If acting in tension, the spring 64 in the expansion chamber 60 also exerts an increasing restoring force on the piston 54 with increasing displacement of the piston 54 away from the equilibrium position. Many other variations are possible within the inventive concept. For example, there could be more or fewer damping struts 34 surrounding the BOP 10, such as three or six damping struts 34. There could be a corresponding number of outriggers 38 extending outwardly from the frame 26. Similarly, the frame 26 could have a polygonal shape other than a square shape in plan view.

[0055] One or both of the upper and lower rods 50, 52 of the damping struts 34 could be replaced by flexible tendons that extend between the cylinder 48, the piston 54 and the respective upper and lower mounts 36, 32. Such tendons could be held in tension by bias of the springs 64 that act on the piston 54 and / or the cylinder 48.

Claims

Claims1 . A method of supporting equipment mounted atop a subsea wellhead, the method comprising damping motion of the equipment relative to a seabed around the wellhead by operation of an array of damping members that connect the equipment to an anchor structure fixed to the seabed.

2. The method of Claim 1 , wherein the damping members of the array are connected to a single anchor structure.

3. The method of Claim 1 or Claim 2, wherein the anchor structure comprises a plurality of seabed-engaging anchors.

4. The method of any preceding claim, comprising transmitting compressive forces from the equipment along the damping members.

5. The method of Claim 4, comprising damping the motion of the equipment with double-acting dampers of the damping members.

6. The method of Claim 5, comprising damping the motion of the equipment by reciprocating a piston through a fluid within a cylinder of each damper.

7. The method of Claim 6, comprising allowing a restricted flow of the fluid through or around the piston from a compression chamber to an expansion chamber within the cylinder.

8. The method of Claim 6 or Claim 7, comprising biasing the piston toward an equilibrium position within the cylinder.

9. The method of any preceding claim, preceded by installing the equipment onto the wellhead with the damping members attached to the equipment.

10. The method of Claim 9, comprising deploying the damping members from the equipment and connecting the damping members to the anchor structure.

11. The method of any preceding claim, comprising landing the anchor structure on the seabed around the equipment previously installed on the wellhead.

12. The method of any of Claims 1 to 10, comprising landing the anchor structure on the seabed and subsequently installing the equipment on the wellhead within or beside the anchor structure.

13. A subsea installation comprising equipment mounted atop a subsea wellhead and a support arrangement for the equipment, the support arrangement comprising: an anchor structure fixed to a seabed around or beside the wellhead; and an array of elongate damping members connecting the anchor structure to the equipment, the damping members being configured to damp motion of the equipment relative to the anchor structure.

14. The installation of Claim 13, wherein the damping members of the array are connected to a common anchor structure.

15. The installation of Claim 13 or Claim 14, wherein the anchor structure comprises a plurality of seabed-engaging anchors.

16. The installation of any of Claims 13 to 15, wherein the anchor structure comprises side members surrounding an aperture that defines an upright through-passage extending through the anchor structure and accommodating the equipment.

17. The installation of any of Claims 13 to 16, wherein the damping members surround the equipment and extend upwardly and inwardly from the anchor structure to the equipment, with inclination to the horizontal.

18. The installation of any of Claims 13 to 17, wherein the anchor structure comprises outwardly-extending anti-trawl structures, each anti-trawl structure having an upwardly- and inwardly-inclined upper surface.

19. The installation of Claim 18 when dependent on Claim 17, wherein the upper surface of each anti-trawl structure is substantially aligned with a respective one of the damping members.

20. The installation of any of Claims 13 to 19, wherein the damping members each comprise at least one double-acting damper.

21. The installation of Claim 20, wherein each damper comprises a piston that is reciprocable through a fluid within a cylinder.

22. The installation of Claim 21 , wherein the piston is penetrated by at least one passageway that effects fluid communication between a compression chamber and an expansion chamber within the cylinder.

23. The installation of Claim 21 or Claim 22, comprising springs in mutual opposition about the piston that bias the piston toward an equilibrium position within the cylinder.

24. The installation of any of Claims 21 to 24, wherein the damper is disposed between elongate elements, one of the elongate elements being fixed to the piston and another of the elongate elements being fixed to the cylinder.

25. The installation of any of Claims 13 to 24, wherein each damping member comprises at least one rigid rod that is capable of transmitting compressive force along the damping member.

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