Subsea foundations

Abstract

A subsea foundation 16 for anchoring a mooring line 14 comprises a mooring base 20, such as a chain that is simply laid upon the seabed in a straight, curved or looped configuration, hence extending across the seabed 18 substantially parallel to the seabed. The mooring base is then anchored by one or more deadman anchors 22 that are embedded in the seabed soil. For this purpose, one or more links 24 extend through the soil to couple the mooring base to the or each deadman anchor. One or more mooring lines can then be coupled to the mooring base.

Description

This invention relates to subsea foundations that are suitable to anchor mooring lines of floating installations. Such installations may include offshore energy installations such as floating wind turbines (FWTs) or surface facilities for production, processing or storage of hydrocarbons, such as floating platforms or FPSOs, Common subsea foundations include suction piles, torpedo piles and driven piles, also known as pin piles. A typical suction pile is described in KR 102456098 and anchors multiple downwardly-convergent mooring chains. Advantageously, such piles centralise or unify all foundation functions at a single location. However, they suffer from drawbacks of size and difficulty of installation, noting that the greater the load on the mooring chain(s), the larger the foundation must be. Increasing the size of a subsea foundation adds to its cost, expressed as both capital cost and installation cost, which is multiplied by the number of foundations required. High cost becomes a significant constraint in, for example, offshore windfarms where numerous floating wind turbines could require a total of hundreds of such foundations. Foundations comprising deadman anchors (DMAs) are known in onshore and offshore applications, for example for anchoring a containment wall, but may not be sufficiently robust for mooring large floating installations. For example, DMAs or clump weights that are laid directly on the seabed without being embedded significantly into the seabed soil could slide across the seabed under tension in an attached mooring line. In KR 200492432, an embedded DMA is connected to a pile foundation to improve resistance of the pile to lateral traction. US 2009 / 123235 also discloses a pile foundation with supplementary supports. However, in each case, the pile remains the primary foundation. Thus, the DMA or other supplementary support is merely a secondary or auxiliary reinforcement that complicates installation and adds to the already high cost of the pile. WO 2023 / 117966 proposes an alternative solution for a rocky seabed in which an elongate foundation is embedded in a concrete block. The foundation is laid in a shallow trench, substantially in alignment with the axial direction of a mooring chain that is connected directly to the foundation. The foundation of WO 2023 / 117966 is therefore unlike a DMA, which would instead extend perpendicularly to the chain axis, and is bulky, costly and complex to install. There is a need to simplify subsea foundations to reduce their capital cost but without undermining their effectiveness or overly complicating their installation. In this respect, the invention takes the approach of improving cheap and simple anchors and has advantages of simplicity, flexibility and repeatability, hence reducing cost and easing installation in comparison with foundations previously known in the art. Against this background, the invention resides in a subsea foundation that comprises: a mooring base laid upon seabed soil; at least one deadman anchor embedded in the seabed soil; and at least one link extending through the seabed soil to couple the mooring base to the at least one deadman anchor. At least one mooring line can be coupled to the mooring base. The or each deadman anchor is suitably elongate in a direction that is substantially orthogonal to the at least one link. The mooring base lying upon the seabed can extend across the seabed substantially parallel to the seabed, mudline or seafloor. The mooring base can therefore be horizontally elongate, either substantially straight or curved in plan view. The mooring base can also be circumferentially elongate in the sense of describing a partial or complete loop in plan view. The loop may, for example, have an elliptical, circular or polygonal shape in plan view. For example, at least one link may be coupled to a face or a vertex of the polygonal shape. The mooring base may be rigid or pliant, for example comprising a chain or other flexible or articulated element Similarly, the or each link may be pliant or rigid. Also, the or each link may include a tensioner. The or each mooring line can be coupled to an end, an intermediate point, a face or a vertex of the mooring base. The or each mooring line may be opposed about the mooring base with respect to, and may be substantially aligned with, a respective link. A plurality of links may be spaced apart along or around the mooring base, each of the links extending to a respective one of the deadman anchors. The mooring base and the or each link may lie in a common upright plane or along a common path in plan view, but at least one link could extend laterally from an upright plane or path containing the mooring base. For exampie, a plurality of links could alternate along the mooring base in mutually opposed lateral directions from that upright plane or otherwise from a line following the plan shape of, and directly beneath, the mooring base. There could be a plurality of mooring bases, each of those mooring bases being coupled to at least one of the deadman anchors by at least one of said links. For example, the foundation could be arranged such that, in plan view, the deadman anchors are grouped inwardly and the links radiate outwardly from the deadman anchors to the mooring bases. Correspondingiy, the inventive concept embraces a method of installing a subsea foundation. The method comprises: placing a mooring base onto a seabed; and anchoring the mooring base with at least one deadman anchor that is coupled to the mooring base by at least one link extending through the soil of the seabed. The mooring base can be laid, preferably progressively along its length, in a straight or curved configuration. The or each deadman anchor can be driven into the seabed soil. Cement can be injected into the seabed soil to embed or to constitute the deadman anchor. At least one mooring line can then be coupled to the mooring base. Optionally, the or each link can then be tensioned. In embodiments of the invention, an anchoring system comprises: at least one deadman anchor embedded in the seabed; a mooring chain arranged on the seabed; and at least one tension member extending from the or each deadman anchor to the mooring chain. The mooring chain may be substantially straight, or curved, or looped or otherwise ring-shaped. The tension member may be tensioned by a mechanism that comprises a hydraulic cylinder, a turnbuckle or other tensioner. Successive deadman anchors may be embedded in the seabed in locations substantially beneath the chain or in a pattern alternating between each side of the chain. The or each deadman anchor may be, or may comprise, any generally horizontal elongate element, for example a cylindrical or tubular element such as a log or a pipe, or a plate-like element The or each deadman anchor may be driven into the seabed and may be sunk or embedded into the seabed by vibration, for example using a vibrating actuator such as a vibrating plate. The or each deadman anchor may be, or may comprise, a body of concrete embedded in the seabed. For this purpose, concrete may be injected into the seabed, either in isolation or around or beside a structural element of the deadman anchor. Embodiments of the invention also implement a method to install a subsea foundation, the method comprising: embedding at least one deadman anchor in the seabed, while extending a tension member between the deadman anchor and the seabed; arranging a mooring chain on the seabed; connecting the or each tension member to the mooring chain; and tensioning the or each tension member. In summary, the invention provides a low-cost but efficient subsea foundation for anchoring a mooring line. The foundation comprises a mooring base such as a chain that is simply laid upon the seabed in a straight, curved or looped configuration, hence extending across the seabed and lying substantially parallel to the seabed. The mooring base is then anchored by one or more deadman anchors that are embedded in the seabed soil. For this purpose, one or more links extend through the soil to couple the mooring base to the deadman anchor(s). One or more mooring lines can then be coupled to the mooring base. In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic side view of a floating wind turbine moored to a subsea foundation of the invention; Figure 2 is a schematic side perspective view of the foundation shown in Figure 1; Figure 3 is a schematic plan view of the foundation shown in Figure 2; Figure 4 is a schematic side view of a variant of the foundation shown in Figures 1 to 3; Figure 5 is a schematic plan view of the foundation variant shown in Figure 4; Figures 6a and 6b are schematic plan views of another subsea foundation of the invention, showing a sequence of installation; Figures 7, 8 and 9 are schematic plan views of further subsea foundations of the invention; and Figures 10, 11 and 12 are schematic side views showing various deadman anchor arrangements for subsea foundations of the invention. Referring firstly to Figure 1 of the drawings, an offshore installation or floater exemplified here by a floating wind turbine 10 is shown supported buoyantly at the surface 12 of the sea. Catenary mooring lines 14, acting under tension, connect the wind turbine 10 to subsea foundations 16 of the invention, positioned at the seabed 18. The resulting spread mooring pattern could comprise mooring lines 14 and foundations 16 additional to the two that are shown here in mutual opposition about the wind turbine 10. Conversely, small floating items such as buoys could be moored by a single mooring line 14 and foundation 16. In this example, each mooring line 14 has its own foundation 16. in other examples, at least one of the foundations 16 could serve as an anchorage for more than one mooring line 14, which could extend from one foundation 16 to more than one floater. Figures 2 and 3 show details of one of the foundations 16 of Figure 1. The foundation 16 comprises an elongate element, exemplified here by a pliant flexible or articulated element in the form of a chain lying on the seabed 18. That element defines a mooring base 20 of the foundation 16. Whilst the mooring base 20 is laid on and continues to lie substantially on top of the seabed 18, also called the seafloor or mudline, it could become embedded in the soil of the seabed 18 to some extent if that soil is sufficiently soft or if the soil drifts and accumulates on top of the mooring base 20 over time. A mooring line 14 extending toward the surface 12 is attached to the mooring base 20, in this example at one end of the mooring base 20. In another example, at least one mooring line 14 could be attached at an intermediate position between the ends of the mooring base 20, hence along the length of the mooring base 20. Alternatively, separate mooring lines 14 could be attached to both ends of the mooring base 20, for example to extend from the mooring base 20 in mutually-opposed directions. The mooring base 20 is anchored by at least one deadman anchor (DMA) 22 that is embedded in the soil of the seabed 18 and is coupled to the mooring base 20 by a link 24 extending through that soil. In this example, there is a series of DMAs 22 coupled to the mooring base 20 by respective links 24 that are spaced apart along the length of the mooring base 20. The links 24 of the series are shown here as being of substantially equal length but in other examples, at least one of the links 24 could differ in length from at least one of the other links 24. Each DMA 22 extends transversely with respect to the associated link 24, which connects to a longitudinal mid-point of the DMA 22. in this example, each DMA 22 is an elongate member extending along an axis that is substantially orthogonal to a longitudinal axis of the link 24 and to the longitudinal axis of the mooring base 20. When the DMAs 22 are embedded in the seabed 18, each link 24 is inclined to extend downwardly from the mooring base 20 of the foundation 16 into the soil of the seabed 18 in a direction generally opposed to the mooring line 14. Thus, the links 24 converge toward the mooring base 20 in the direction of the mooring line 14. The links 24 shown in Figures 2 and 3 all lie substantially in a common vertical plane that also contains the mooring base 20. The DMAs 22 therefore extend substantially orthogonally with respect to that vertical plane and substantially parallel to each other. Figures 4 and 5 show a variant of the foundation 10 in which like numerals are used for like features. Here, the links 24 diverge at acute angles from the vertical plane that contains the mooring base 20, such that the associated DMAs 22 no longer extend orthogonally with respect to the longitudinal axis of the mooring base 20. Moreover, in this example, the links 24 and the associated DMAs 22 alternate from one side of the mooring base 20 to the other along the length of the mooring base 20. Thus, the links 24 are in a longitudinally staggered herringbone arrangement when the foundation 10 is viewed from above as in Figure 5. Again, the links 24 converge toward each other and toward the mooring base 20 in the direction of the mooring line 14. In the plan views of the foundations 10 shown in Figures 3 and 5, and in plan views of other foundations 10 of the invention shown in Figures 6a, 6b and 7 to 9, features that are embedded into the seabed 18 are shown in dotted lines. Conversely, features that lie on seabed 18 are shown in solid lines whereas mooring lines 14 extending from the foundations 10 above the seabed 18 are shown in dashed lines. In each embodiment to be described with reference to Figures 6a, 6b, 7 and 8, a DMA 22 may be defined by a cylindrical or tubular rigid body like those shown in Figures 2 to 5, which could be made of concrete, metal, polymer, composite and / or wood. However, as exemplified in other embodiments to be described with reference to Figures 9 to 12, a DMA 22 could instead be defined by a plate that extends substantially horizontally and has an upright orientation to present a major face toward the associated link 24. Each link 24 acts in tension and so couid be pliant, flexible or articulated, for example being a wire or a chain as shown in Figures 2 to 5, 6a, 6b and 8. In other arrangements like those shown in Figures 8 to 12, each link 24 could be a rigid rod or strut that is capable of acting in tension or in compression. Each DMA 22 can be embedded into the seabed 18 before or after laying the mooring base 20 on the seabed 18, and can be coupled to the mooring base 20 subsequently via the associated link 24. For example, each link 24 can be attached initially to the mooring base 20 and then to the DMA 22, or can be attached initially to the DMA 22 and then to the mooring base 20. More generally, each link 24 can be installed while attached initially to the mooring base 20 and / or to the DMA 22. As the mooring base 20 shown in Figures 2 to 5 comprises a chain or other pliant elongate element, it can be installed easily from a vessel with minimal lifting capacity and can be laid easily onto the seabed 18 in any desired shape in plan view. Thus, the mooring base 20 need not be straight but could instead be curved in plan view if the mooring requirements of an offshore installation so dictate. However, the mooring base 20 could instead be defined by a rigid elongate element, which could be pre-formed with a straight or curved shape. It is also possible for the ends of the mooring base 20 to be conjoined before or after laying to form a loop, which may for example be circular, elliptical or polygonal in plan view. An example of a looped mooring base 20 formed by a pliant element is shown in Figures 6a and 6b. Examples of looped bases 20 formed by rigid elements are shown in Figures 7 and 8. The mooring base 20 can therefore be an upright tube of circular or non-circular crosssection. In other examples, the mooring base 20 could be a solid structure rather than a hollow or looped structure. In the examples shown in Figures 6a, 6b, 7 and 8, three DMAs 22 and associated links 24 are spaced equiangulariy around the looped mooring base 20, hence in an equilateral triangular configuration. The links 24 extend radially from the mooring base 20 and the DMAs 22 are orthogonal to the respective links 24 and lie in a common horizontal plane. Thus, each DMA 22 is oriented tangentially with respect to a circle that is concentric with the mooring base 20. In other examples, there could be more or fewer DMAs 22 and links 24, and there need not necessarily be equal angular spacing between them. The examples of Figures 6a, 6b, 7 and 8 also show, in dashed lines, three mooring lines 14 spaced equiangularly around the looped mooring base 20. The mooring lines 14 are disposed between, and hence are in circumferential alternation with, the links 24. Again, there could be more or fewer mooring lines 14 and there need not necessarily be equal angular spacing between them. Advantageously, however, each mooring line 14 may be aligned with, and in mutual opposition about the mooring base 20 to, a respective link 24 extending to a respective DMA 22. Referring now specifically to Figures 6a and 6b, these drawings show a foundation 16 whose mooring base 20 laid on the seabed 18 comprises a looped flexible or articulated element, again exemplified here by a chain. In this example, the embedded links 24 are also represented by chains acting in tension between the mooring base 20 and respective DMAs 22 also embedded in the seabed 18. Each link 24 incorporates a tensioner 26 that employs a mechanism such as a turnbuckle or a hydraulic cylinder to shorten and hence to apply tension to the link 24. In this embodiment, the tensioned links 24 pull the initially circular chain of the mooring base 20 as shown in Figure 6a into a triangular shape as shown in Figure 6b. Similar tensioners 26 could be added to the links 24 of any embodiment of the invention, such as the foundation 16 shown in Figure 7 in which the embedded links 24, shown in dotted lines, instead comprise wires. Figure 7 also exemplifies the mooring base 20 as a rigid triangular frame. In this example, the links 24 are coupled to respective faces of the triangular frame and mooring lines 14 extend from respective apices, corners or vertices of the triangular frame. However, the links 24 could instead be coupled to the vertices and the mooring lines 14 could instead extend from the sides or faces of the triangular frame. In the foundation 16 of Figure 8, the mooring base 20 is instead represented as a rigid circular frame and the links 24 are exemplified by rigid rods or struts, capable of acting in tension or in compression between the mooring base 20 and the respective DMAs 22. Figure 9 shows a foundation 16 in which embedded DMAs 22 are grouped together inwardly in plan view and are optionally joined together before or after installation, for example by brackets 28 as shown. The DMAs 22 could be plates with upright orientation or could be cylindrical or tubular as previously. Embedded links 24 radiate outwardly from the grouped DMAs 22 to individual bases 20 that lie on the seabed 18. Here, each mooring base 20 is exemplified by a flat panel that lies parallel to the seabed 18 and may incorporate a peripheral skirt to improve engagement with the seabed 18. The links 24 are represented by rigid rods or struts but could instead comprise pliant elements such as chains or wires. Mooring lines 14 radiate outwardly from the respective bases 20, in this case in mutual alignment with the associated links 24. In the example shown in Figure 9, three mooring lines 14, bases 20, DMAs 22 and links 24 are spaced equiangularly in an equilateral triangular configuration. In other examples, however, there could be more or fewer mooring lines 14, bases 20, DMAs 22 and links 24, and there need not necessarily be equal angular spacing between them. Turning finally to Figures 10 to 12, these drawings show one of the DMAs 22 of the foundation 16 of Figure 9 in conjunction with the associated link 24 and mooring base 20, together with the associated mooring line 14 extending from the mooring base 20 above the seabed 18. Figure 10 shows these features in exploded form and also shows an installation tool 30 that vibrates or hammers the DMA 22 to drive it into the seabed 18, optionally after a trench 32 is dug to receive the DMA 22. Typically, the DMA 22 will be embedded in the seabed 18 before coupling the link 24 to the DMA 22. The mooring base 20 is then coupled to the link 24 before the mooring line 14 is coupled to the mooring base 20. Figure 11 shows the DMA 22 embedded in a body of concrete 34 formed by injecting cement into a trench 32 in the seabed 18. Functionally, the concrete 34 therefore forms part of the DMA 22. Figure 12 shows a similarly-installed body of concrete that serves as the DMA 22 and may conveniently embed an end of the link 24 remote from the mooring base 20.

Claims

1. A subsea foundation, comprising:a mooring base laid upon seabed soil;at least one deadman anchor embedded in the seabed soil; andat least one link extending through the seabed soil to couple the mooring base to the at least one deadman anchor.

2. The foundation of Claim 1, wherein the mooring base extends across the seabed substantially parailel to the seabed.

3. The foundation of Claim 1 or Claim 2, wherein the mooring base is horizontally elongate.

4. The foundation of Claim 3, wherein the mooring base is substantially straight in plan view.

5. The foundation of Claim 3, wherein the mooring base is curved in plan view.

6. The foundation of any preceding claim, wherein the mooring base describes a loop in plan view.

7. The foundation of any preceding claim, wherein the mooring base has a polygonal shape in plan view.

8. The foundation of Claim 7, wherein the at least one link is coupled to a face or a vertex of the polygonal shape.

9. The foundation of any preceding claim, wherein the mooring base is rigid.

10. The foundation of any of Claims 1 to 8, wherein the mooring base is pliant.

11. The foundation of Claim 10, wherein the mooring base is formed by a chain.

12. The foundation of any preceding claim, wherein the at least one link is pliant.

13. The foundation of any of Ciaims 1 to 11, wherein the at least one link is rigid.

14. The foundation of any preceding claim, wherein the at least one link comprises a tensioner.

15. The foundation of any preceding claim, further comprising at least one mooring line coupled to the mooring base.

16. The foundation of Claim 15, wherein the at least one mooring line is coupled to an end, an intermediate point, a face or a vertex of the mooring base.

17. The foundation of Claim 15 or Ciaim 16, wherein the at least one mooring line is opposed about the mooring base with respect to the at least one link.

18. The foundation of any of Claims 15 to 17, wherein the at least one mooring line is substantially aligned with the at least one link.

19. The foundation of any preceding claim, comprising a plurality of links spaced apart along or around the mooring base, each of the links extending to a respective one of the deadman anchors.

20. The foundation of any preceding claim, wherein the mooring base and the at least one link lie in a common upright plane.

21. The foundation of any of Claims 1 to 19, wherein the at least one link extends laterally from an upright plane containing the mooring base.

22. The foundation of Claim 21, comprising a plurality of links that alternate along the mooring base in mutually opposed lateral directions from said upright plane.

23. The foundation of any preceding claim, wherein the at least one deadman anchor is elongate in a direction that is substantially orthogonal to the at least one link.

24. The foundation of any preceding claim and comprising a plurality of mooring bases, each of those mooring bases being coupled to at least one of said deadman anchors by at least one of said links.

25. The foundation of Claim 24, arranged such that, in plan view, the deadman anchors are grouped inwardly and the links radiate outwardly from the deadman anchors to the mooring bases.

26. A method of installing a subsea foundation, comprising: placing a mooring base onto a seabed; and anchoring the mooring base with at least one deadman anchor that is coupled to the mooring base by at least one link extending through the soil of the seabed.

27. The method of Claim 26, comprising laying the mooring base in a straight configuration.

28. The method of Claim 26, comprising laying the mooring base in a curved configuration.

29. The method of any of Claims 26 to 28, comprising driving the at least one deadman anchor into the seabed soil.

30. The method of any of Claims 26 to 29, comprising injecting cement into the seabed soil to embed or to constitute the deadman anchor.

31. The method of any of Claims 26 to 30, further comprising tensioning the at least one link.

32. The method of any of Claims 26 to 31, further comprising coupling at least one mooring line to the mooring base.13

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