Stabilising cables around subsea foundations

Abstract

A scour protection structure comprises a cable stabilisation system that has elongate guides extending in side-by-side relation and mutually spaced to define an elongate valley between them. The valley has an inner end facing toward an opening for receiving a subsea structure and an outer end facing away from the opening, and flares in width toward the inner end. The guides have oppositely inclined faces that are mutually opposed across the valley, the valley thereby widening upwardly. The faces are convex-curved lengthwise along the valley and in cross-section transverse to the lengthwise direction. A base of the valley is defined by a mudmat of the scour protection structure or by a base panel mounted on the mudmat. The guides may be movable relative to the mudmat to accommodate misalignment of the subsea structure.

Description

[0001] Stabilising cables around subsea foundations

[0002] This invention relates to the challenges of stabilising cables around subsea foundations. The invention is exemplified in this specification in the context of a cable extending from a monopile foundation of a bottom-fixed wind turbine.

[0003] Aspects of the invention relate to the problem of scour due to erosion of submerged soil, and the consequential need to protect structures that rely upon submerged soil for support. Scour can occur where water flows around a structure that stands above the bed of a body of water in marine, riverine and lacustrine environments. Examples of such structures include upright elongate structural elements such as bridge piles, suction piles, legs of offshore oil platforms and pile foundations of fixed offshore wind turbines. A lower end of such a structure is typically embedded in the soil, in which case the stability of the structure often relies upon the depth of its embedment into the soil and the uniformity of embedment around the periphery of the structure.

[0004] On encountering and dividing around a submerged stationary structure, the divided water flow tends to generate eddies or vortices around and downstream of the object. Such vortices can entrain, transport and erode the soil in a process of excavation that, over time, decreases the embedded depth of the object and so could undermine its bearing capacity and stability.

[0005] The problem of scour is especially acute where fixed offshore wind turbines are mounted on a monopile that is driven into seabed soil. Monopiles have a generally smooth cylindrical surface of circular cross-section. As is well known, a cylindrical body presents challenges of vortex formation when exposed to fluid flowing in a direction transverse to the central longitudinal axis of the cylinder. Additionally, where the seabed intersects the cylindrical surface, a ‘horseshoe vortex’ can develop close to the interface between the monopile and the seabed. A horseshoe vortex follows an arc that wraps around the upstream side and approaches the downstream side of the monopile. In that vortex, water tumbles about a curved horizontal axis in a manner that strongly promotes scour.

[0006] EP 2767637, WO 2013 / 167121, WO 2018 / 004340 and WO 2024 / 189168 disclose scour prevention structures to surround a monopile base. Similarly, US 2011 / 0016644 discloses scour prevention structures for surrounding bridge pier foundations, US4279545 discloses scour prevention structures for surrounding underwater columns, and US 3529427 discloses scour prevention structures for the legs of oil rigs.

[0007] GB 2267107 discloses flexible foundation supports for offshore structures, comprising cable nets and a geotextile membrane cover.

[0008] The flow dynamics that give rise to scour can also affect elements, such as cables, that are disposed in proximity to a structure exposed to water flow. For example, in the context of a bottom-fixed offshore wind turbine as will be described below, a cable extends from the base of the foundation to convey electrical energy generated by the wind turbine. The cable is exposed to flow dynamics, at least where it extends between the foundation and a trench in which the cable is buried under the seabed.

[0009] Consequently, the length of the cable between its exit from the foundation and the burial point, including any protection system around the cable, can experience significant oscillatory movement. Over time, that movement can result in cable failure.

[0010] If measures are adopted to stabilise a cable and its protection system against oscillatory motion, another challenge is to cope with any misalignment between the exit of the cable from the foundation and the burial point. Misalignment between the foundation and the cable stabilisation system is also a challenge, particularly if it is convenient to install the cable stabilisation system before the foundation.

[0011] WO 2024 / 186220 aims to stabilise a subsea cable by anchoring the cable against movement under dynamic conditions. Clamping devices include a pile anchoring system that locks the cable to the seabed around a monopile or on top of a scour prevention structure placed around the monopile. The clamping devices are apt to be installed after rather than before the monopile, are complex to install and to operate, are themselves exposed to dynamic conditions, and can add to stress in the cable.

[0012] CN211735479 discloses an umbrella-type anti-scour structure that is lowered onto a pile foundation. The structure includes a central ring that surrounds the pile and rests on brackets that are welded to the side of the pile. An array of angularly-spaced ribs extend radially outwardly from the ring to support an umbrella surface of fabric material. A series of mutually spaced cable supports are arranged along one of the ribs, each cable support comprising a pair of struts arranged in a V-formation to receive a cable between them.Against this background, the invention resides in a scour protection structure for a subsea structure. The scour protection structure comprises an opening for receiving the subsea structure and may further comprise a sacrificial web that extends at least partially across the opening. The scour protection structure also comprises a cable stabilisation system that can be sheltered in an upwardly open recess disposed between upright walls of the scour protection structure.

[0013] The cable stabilisation system comprises elongate guides that extend in side-by-side relation and are mutually spaced to define an elongate valley between them. The valley widens upwardly and has an inner end facing toward the opening and an outer end facing away from the opening. The inner end may be wider than the outer end.

[0014] The guides have oppositely inclined faces that are mutually opposed across the valley. For example, the faces of the guides can be convex curved in cross-section transverse to a lengthwise direction of the valley. The guides can also have upper surfaces that are inclined downwardly with mutually opposed inclination toward the valley, in which case the face and the upper surface of each guide can be joined by a continuous convex curve. Whilst the valley is preferably open-topped, at least one restraint can be positioned on at least one of guides to bridge across the valley.

[0015] The faces of the guides can also be convex-curved lengthwise along the valley. The valley can flare in width toward the inner end and can also flare in width toward the outer end from a throat disposed along the valley, the throat being narrower than either of the inner or outer ends. The throat may, for example, be offset toward the outer end of the valley.

[0016] The guides may be mounted on a base panel that defines a base of the valley. The base panel can be a mudmat of the scour protection structure or can be mounted on such a mudmat. In the latter case, the base panel may be movable relative to the mudmat, for example pivotable relative to the mudmat about an axis orthogonal to a plane of the mudmat. Similarly, at least one of the guides may be movable relative to the base panel, for example being pivotable relative to the base panel about an axis orthogonal to a plane of the base panel or being resiliently deflectable relative to the base panel.The inventive concept embraces a subsea installation that comprises: the scour protection structure of the invention, lying on a seabed; a subsea structure, such as a monopile foundation of an offshore wind turbine, extending into the seabed through the opening; and a cable extending from an exit point of the subsea structure to the seabed, a portion of the cable extending along the valley of the cable stabilisation system. The cable may extend into a trench under the seabed.

[0017] The exit point can lie substantially on an upright plane that bisects the valley, in which case a portion of the cable extending from the exit point along the valley can lie substantially on that plane. Alternatively, the exit point can be offset from an upright plane that bisects the valley, in which case a portion of the cable extending from the exit point along the valley can curve out of that plane.

[0018] Conveniently, the scour protection structure can be lowered to the seabed as an assembly that includes a cable stabilisation system. For example, the cable stabilisation system can be pre-attached to the scour protection structure aboard an installation vessel that then lowers the assembly to the seabed. Afterwards, a subsea structure can be lowered into the scour protection structure and embedded in the seabed. A cable can then be installed, the cable extending from the subsea structure to the seabed via the cable stabilisation system.

[0019] Thus, the invention addresses the challenge of cable failure in the offshore wind industry by providing an integrated cable stabilisation and scour protection system. In this way, two functions are integrated into one simple system, namely cable stabilisation as a primary function and scour protection as a secondary function.

[0020] A pair of walls serve as guides to embrace and stabilise the cable throughout its operational lifetime, protecting the cable from excessive motion to mitigate failure due to fatigue. The walls are located in a sheltered area or recess of a scour protection structure, substantially aligned with a cable entry or exit hole or aperture of a monopile. The walls curve vertically to guide the cable during installation or pull-in of the cable into the aperture. The walls also curve horizontally to guide the cable routing in case of misalignment between the aperture and the cable path extending away from the scour protection structure beneath the seabed. Whilst stabilising the cable, the system avoids excessive constraints on the cable.When landed on the seabed, a doughnut-shaped or annular mudmat of the scour protection structure protects the soil from erosion or scouring before and after installation of a monopile foundation, ensuring structural integrity of the foundation. The annular shape of the mudmat leaves a free central area through which a monopile can extend into the seabed. The integrated system can be installed before the monopile to leave a small gap between the tubular wall of the monopile and the periphery of the central area. The mudmat can include a sacrificial material to protect this peripheral gap area from potential scour.

[0021] The cable stabilisation system is preferably integrated with, and sheltered by, an entire scour protection structure that extends around a full circumference of the monopile. However, the cable stabilisation system could be integrated with, and sheltered by, only a portion of the structure extending around, for example, a half or quarter of the circumference of the monopile.

[0022] The mudmat can include conventional provisions to facilitate lifting, such as beams or other structural reinforcement, and to facilitate installation, such as an array of perforations across the mudmat. Provisions may also be made to ensure reliable stabilisation on the seabed, such as skirts underneath the mudmat.

[0023] For operational optimisation, the cable stabilisation system and the scour protection system can be transported and loaded-out separately on the same vessel. The cable stabilisation system and the scour protection system can then be assembled offshore on deck of the vessel, for example via a smart manual locking system, before being installed together offshore.

[0024] In summary, the invention provides a scour protection structure that comprises a cable stabilisation system with elongate guides extending in side-by-side relation and mutually spaced to define an elongate valley between them. The valley has an inner end facing toward an opening for receiving a subsea structure and an outer end facing away from the opening, and may flare in width toward the inner end.

[0025] The guides may have oppositely inclined faces that are mutually opposed across the valley, the valley thereby widening upwardly. The faces can be convex-curved lengthwise along the valley and / or in cross-section transverse to the lengthwise direction.A base of the valley may be defined by a mudmat of the scour protection structure or by a base panel mounted on the mudmat. The guides may be movable relative to the mudmat or the base panel to accommodate any misalignment of the subsea structure.

[0026] 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:

[0027] Figure 1 is a perspective view of a monopile foundation whose base is surrounded by a scour protection structure fitted with a cable stabilisation system of the invention;

[0028] Figure 2 corresponds to Figure 1 but shows a variant of the scour protection structure;

[0029] Figure 3 is an enlarged detail perspective view of a cable stabilisation system of the invention;

[0030] Figure 4 is a perspective view of a mudmat for scour protection structures similar to those shown in Figures 1 and 2 and incorporating a cable stabilisation system of Figure 3;

[0031] Figure 5 is a perspective view of a monopile in conjunction with the mudmat shown in Figure 4;

[0032] Figure 6 is a perspective view of a variant of the cable stabilisation system of Figure 3;

[0033] Figure 7 is a schematic top plan view of a monopile and a cable stabilisation system of the invention;

[0034] Figures 8 and 9 correspond to Figure 7 but show variants of the cable stabilisation system; andFigures 10a to 10c are a sequence of schematic perspective views showing a scour protection structure and a cable stabilisation system being assembled aboard an installation vessel and then installed on the seabed.

[0035] Referring firstly to Figure 1 of the drawings, a scour protection structure 10 of the invention is shown encircling a tubular monopile 12 of a wind turbine. The structure 10 lies on a seabed 14 into which the monopile 12 is embedded. By way of example, the structure 10 can be made of steel, with an expected weight of less that forty metric tons, or can be made of mixed materials as appropriate. The monopile could have a diameter of ten metres.

[0036] The structure 10 comprises a circumferentially continuous upright inner wall 16 of circular cross section. The wall 16 is a shallow tube or sleeve that is substantially concentric with the monopile 12 about a common vertical central longitudinal axis. The wall 16 defines a central opening or through-passage that extends vertically from top to bottom through the structure 10 and is spaced radially from the monopile 12 to define an annular clearance or gap between them, consistent with installation tolerances.

[0037] In this respect, the structure 10 can be pre-installed on the seabed 14 before the monopile 12 is lowered to the seabed 14. In that case, the lower end of the monopile 12 is captured and guided into the central through-passage surrounded by the wall 16. Conversely, the structure 10 could be suspended and lowered toward the seabed 14 encircling the pre-installed monopile 12. Thus, the monopile 12 can serve as a guide for installation of the structure 10 as an alternative to the structure 10 serving as a guide for installation of the monopile 12.

[0038] A circumferential array of triangular vanes 18 adjoins, surrounds and tapers radially outwardly from the wall 16. The vanes 18 lie in respective upright radial planes that together converge on, intersect at, and contain the central longitudinal axis of the monopile 12. In this example, the vanes 18 are equi-angularly spaced around the monopile 12 in plan view. Together, the vanes 18 lie within an upwardly-tapering frusto-conical volume that is rotationally symmetrical about the central longitudinal axis of the monopile 12. More specifically, the structure 10 has a faceted, frusto-pyramidal shape with a polygonal outline in plan view, the shape of that outline having a number of facets or sides that corresponds to the number of vanes 18.The spaces between neighbouring vanes 18 are closed at the bottom by a floor or base that may be defined by a mudmat 20 extending across the full width of the structure 10. The mudmat 20 extends between and adjoins the neighbouring vanes 18 and also adjoins the wall 16. The mudmat 20 may be perforated, as shown, to ease its downward passage through the water column during installation.

[0039] Figure 2 shows a variant of the structure 10 shown in Figure 1 in which the vanes 18 are replaced by inclined struts 22 that correspond to upper edges of the vanes 18. In this variant, the spaces between neighbouring struts 22 are closed across the top of the structure 10, in this instance by inclined cover panels 24. The cover panels 24 may be continuous, as shown here, or perforated; they may be rigid plates, for example of steel or reinforced plastics, or of tensioned fabrics.

[0040] Figures 1 and 2 show a cable 26 emerging from an exit point being a submerged aperture 28 in a side wall of the monopile 12 above the level of the seabed 14. In this example, the cable 26 conveys electrical power generated by the wind turbine. The cable 26 emerges radially from the monopile 12 through the aperture 28 and follows a cable path that crosses the radial width of the scour protection structure 10 and then extends beneath the seabed 14 into a trench 30. In this example, the cable 26 enters the trench 30 via an articulated protective sleeve 32 as shown.

[0041] In service, the cable 26 may be subjected to lateral loads, uplift loads and pull-out loads. Lateral loads, typically caused by environmental influences such as waves or currents, are generally parallel to the plane of the seabed 14 and transverse to the plane of the cable 26, in either horizontal direction. Uplift loads, also typically caused by environmental influences, are generally orthogonal to the plane of the seabed 14. Pullout loads, arising in consequence of lateral loads and uplift loads, are generally aligned with the longitudinal direction of the cable 26 and act toward the point of origin of the inner end of the cable 26, being within the monopile 12 in this example.

[0042] For stability against the abovementioned loads and for protection from additional threats such as storms, dropped equipment, anchor dragging or overtrawling, most of the length of the cable 26 is buried or embedded in the soil or rock of the seabed 14, wholly beneath the level of the seabed 14. For this purpose, the cable 26 is positioned in the trench 30, which can be excavated in the seabed 16 before or after laying the cable 26. Then, the trench 30 can be covered over or filled in with soil, rock and / or otherprotective measures such as mats. Conventionally, the cable 26 will emerge from the trench 30 to terminate at its outer end at an offshore substation at which power generated by multiple wind turbines of a windfarm is gathered for export to a terrestrial grid.

[0043] The portion of the cable 26 extending between the monopile 12 and the seabed 14 is particularly subject to lateral loads, uplift loads and consequent pull-out loads. In accordance with the invention, that exposed portion of the cable 26 is stabilised by a stabilisation system 34. The stabilisation system 34 is located in a sheltered space above the mudmat 20 between neighbouring vanes 18 of the structure 10 of Figure 1 or between neighbouring struts 22 of the structure 10 of Figure 2. Advantageously, therefore, the structures 10 of Figures 1 and 2 provide scour protection while also serving as a preferred support base for the stabilisation system 34.

[0044] As shown in more detail in Figure 3, the stabilisation system 34 comprises a pair of elongate cable guides 36 in mutual side-by-side opposition. The guides 36 are mutually spaced to define an elongate, upwardly open groove or valley 38 between them that can accommodate a length of the cable 26 extending along the valley 38. The valley 38 is closed downwardly by a base 40 that extends beneath and between the guides 36. In the examples of Figures 1 and 2, the base 40 is a plinth that lies atop the mudmat 20.

[0045] The valley 38 extends generally radially with respect to the structure 10 to be bisected longitudinally by an upright central longitudinal plane that contains the central longitudinal axis of the tubular wall 16 and hence of the monopile 12 disposed concentrically within that wall 16. The cable 26 extending from the monopile 12 is received in, and extends along, the valley 38, embraced between the guides 36 and lying on the base 40. The cable 26 can simply be lowered into the valley 38 through the open top of the valley 38, conveniently as part of the process of installing the cable 26 on the seabed 14.

[0046] The cable 26 may bear against either or both of the guides 36 or there may be a clearance between the cable 26 and the guides 36. In the absence of such clearance, the cable 26 can slide relative to the guides 36 and the base 40. If the cable 26 is wide enough to bear against both of the guides 36, it is possible for the cable 26 to be supported only by the downwardly convergent guides 36 and therefore not to lie on the base 40. However, it is preferred that the cable 26 has limited freedom of movementrelative to the guides 36 and the base 40 of the stabilisation system 34 and so is not excessively constrained, with slight movement of the cable 26 being permitted to minimise stress on the cable 26.

[0047] Each guide 36 has an inclined wall or inner face 42 that is exposed for contact with the cable 26. Each inner face 42 is curved along its length in plan view, presenting convex curvature to the corresponding inner face 42 of the other guide 36 opposed across the valley 38. The inner faces 42 are symmetrically curved about the upright central longitudinal plane that bisects the valley 38.

[0048] In plan view, the valley 38 narrows toward its radially outer end 44. Thus, the radially inner end 46 of the valley 38 is wider than the outer end 44 of the valley 38. In this example, the valley 38 narrows in plan view to a throat 48 near its outer end 44 and flares in width from the throat 48 in opposed radial directions toward its outer and inner ends 44, 46. However, the sides of the valley 38 could instead be generally parallel from the outer end 44 until they flare apart toward the inner end 46.

[0049] The inner face 42 of each guide 36 is also convex-curved in cross-section orthogonal to the upright central longitudinal plane that bisects the valley 38. Thus, the inner face 42 of each guide 36 has double convex curvature. The valley 38 flares upwardly in width from the base 40. Specifically, in this example, the inner faces 42 meet the base 40 substantially orthogonally and curve smoothly upwardly and outwardly from there through nearly 90° of arc. The tops of the guides 36 have opposite inward and downward inclination leading into the valley 38, hence serving to guide the cable 36 into the valley 38 from above.

[0050] A mudmat 20 for a polygonal scour protection structure similar to the structure 10 of Figures 1 and 2 is shown in Figures 4 and 5 without the wall 16, the vanes 18, the struts 22 or the panels 24. In this case, the polygonal shape of the mudmat 20 has more sides than the structures 10 of Figures 1 and 2. Also, the stabilisation system 34 for the cable 26 is mounted directly to the mudmat 20 in this example. Thus, the guides 36 are mounted directly to the mudmat 20 and the mudmat 20 defines the base 40 of the valley between the guides 36.

[0051] It will be apparent from Figures 4 and 5 that the mudmat 20 has radially-extending skirts 50 on its underside. When the structure 10 is landed on the seabed 14, the skirts 50embed into the soil of the seabed 14 to resist sliding across the seabed 14. The structure 10 thereby provides a convenient template for receiving and guiding the bottom end of a monopile 12 to ensure that the monopile 12 is correctly positioned both on landing and when being driven into the seabed 14.

[0052] A central region of the mudmat 20 corresponding to the circular area within the wall 16 may comprise a sacrificial web 52 of thin material such as a steel plate or a geotextile. When inserted into the through-passage surrounded by the wall 16 during installation into the pre-installed structure 10, the bottom of the monopile 12 penetrates the web 52 as shown in Figure 5. Afterwards, an annular peripheral region of the web 52 may remain in place around the installed monopile 12, corresponding to the radial clearance between the monopile 12 and the wall 16. Thus, the web 52 protects the seabed 14 beneath from scour when the monopile 12 has been installed and also when the structure 10 has been pre-installed.

[0053] Figure 6 shows a variant of the stabilisation system 34 in which one or more restraints 54 bridge across the valley 38 between the guides 36. The restraints 54 serve as a locking system to hold a cable 36 within the valley 38. The restraints 54 can also bear against the cable 36 to limit upward movement of the cable 36 away from the mudmat 20, or other base 40, under uplift loads. However, an advantage of the invention is that this locking system is an optional feature rather than an essential feature.

[0054] Figure 7 represents an ideal installation outcome in which there is correct angular alignment of the aperture 28 of the monopile 12 relative to the trench 30 and the stabilisation system 34. Consequently, the aperture 28 and the trench 30 both lie on the aforementioned upright central longitudinal plane 56 that extends between the guides 36 of the stabilisation system 34 and bisects the valley 38. The exposed length of the cable 26 extending between the aperture 28 and the trench 30 also lies in that plane 56.

[0055] The disposition and shape of the guides 36 and the valley 38 allow the stabilisation system 34 to handle considerable angular misalignment of the monopile 12 relative to the scour prevention structure 10. However, Figures 8 and 9 show two ways in which the stabilisation system 34 can adapt to support the cable 26 if the monopile 12 is imperfectly positioned relative to the structure 10, especially where there is angular misalignment of the aperture 28 relative to the trench 30 and the stabilisation system34. In each case, the cable 26 departs from the plane 56 and curves around the guide 36 that lies on the intrados of that curve.

[0056] In Figure 8, the base 40 can be reoriented, in this case pivoted about a vertical axis relative to the mudmat 20, to alter the azimuth of the stabilisation system 34 that is mounted on the base 40. In this case, the base 40 may be a plinth like that shown in Figures 1 and 2, movable relative to the underlying mudmat 20 on a pivot or rail system. Conversely, Figure 9 shows one of the guides 36, in this case the guide 36 shown on the left, pivoted or deflected relative to the underlying base 40. For example, that guide 36 can bend resiliently under lateral pressure from the cable 26. As in Figures 4 and 5, the portion of the mudmat 20 that lies under the valley 38 between the guides 36 could serve as the base 40. However, guides 36 that are pivotable or deflectable relative to the underlying base 40, as in Figure 9, can be mounted on a base 40 that is itself pivotable relative to the mudmat 20, as in Figure 8.

[0057] The opposed inner walls 42 of the guides 36 and / or the base 40 extending along the valley 38 between the guides 36 may be coated or covered with a low-friction material such as PTFE to protect the cable 26 from the effects of friction if the cable 26 experiences slight motion relative to the stabilisation system 34.

[0058] Finally, Figures 10a to 10c are a sequence of views that represent a scour protection structure 10 and a cable stabilisation system 34 being assembled together aboard an installation vessel 58 and then installed together on the seabed 14.

[0059] In Figure 10a, the structure 10 is shown on a deck of the vessel 58 with the stabilisation system 34 being lowered into a recess 60 atop the mudmat 20 of the structure 10. The recess 60 opens upwardly and is disposed between upright walls of the structure 10, such as vanes 18 of the structure 10 shown in Figure 1. The stabilisation system 34 is thereby sheltered in the recess 60, at a level beneath the top of the structure 10.

[0060] Figure 10b shows the assembly 62 of the structure 10 and the stabilisation system 34 now complete while still on the deck of the vessel 58, ready to be lowered into the sea beneath the vessel 58.

[0061] Figure 10c shows the assembly 62 having been lowered from the vessel 58 onto the seabed 14. The assembly 62 is thereby pre-installed on the seabed 14, ready forsubsequent installation of a monopile 12 followed by installation of a cable 26, to be engaged with the stabilisation system 34 as previously described.

[0062] Efficiently, therefore, the stabilisation system 34 is installed in the same operation as the structure 10 into an appropriate position to align with the aperture 28 through which the cable 26 will exit the monopile 12. The stabilisation system 34 can handle a considerable degree of angular misalignment of the monopile 12 that may displace the aperture 28 from ideal alignment.

[0063] Many other variations are possible within the inventive concept. For example, provisions may be added to control or to modify the catenary curvature of, and stresses experienced by, the exposed portion of the cable 26 extending between the monopile 12 and the seabed 14.

[0064] In particular, a foundation interface device can extend through the aperture 28 to surround the cable 26 at the transition where the cable 26 exits the monopile 12 through the aperture 28. A bend restrictor may also surround the cable 26 outside the monopile 12, for example outboard of the foundation interface device. The cable 26 can also have a negatively-buoyant weighted section, for example comprising a series of weighted modules surrounding the cable 26, which may be disposed outboard of the bend restrictor. References to the cable 26 in this specification include such provisions, which may individually or collectively be regarded as a cable protection system.

Claims

Claims1. A scour protection structure for a subsea structure, the scour protection structure comprising:an opening for receiving the subsea structure; anda cable stabilisation system comprising elongate guides that extend in side-by- side relation and are mutually spaced to define an elongate valley between them, the valley having an inner end facing toward the opening and an outer end facing away from the opening;wherein the guides have oppositely inclined faces that are mutually opposed across the valley, the valley widening upwardly.

2. The scour protection structure of Claim 1, wherein the faces of the guides are convex-curved in cross-section transverse to a lengthwise direction of the valley.

3. The scour protection structure of Claim 1 or Claim 2, wherein the guides have upper surfaces that are inclined downwardly with mutually opposed inclination toward the valley.

4. The scour protection structure of Claim 3, wherein the face and the upper surface of each guide are joined continuously by a convex curve.

5. The scour protection structure of any preceding claim, wherein the faces are convex-curved lengthwise along the valley.

6. The scour protection structure of any preceding claim, wherein the valley flares in width toward the inner end.

7. The scour protection structure of any preceding claim, wherein the valley flares in width toward the outer end from a throat disposed along the valley, the throat being narrower than either of the inner or outer ends.

8. The scour protection structure of Claim 7, wherein the throat is offset toward the outer end of the valley.

9. The scour protection structure of any preceding claim, wherein the inner end of the valley is wider than the outer end of the valley.

10. The scour protection structure of any preceding claim, wherein the valley is opentopped.

11. The scour protection structure of any preceding claim, further comprising at least one restraint that is positionable on at least one of guides to bridge across the valley.

12. The scour protection structure of any preceding claim, wherein the guides are mounted on a base panel that defines a base of the valley.

13. The scour protection structure of Claim 12, wherein the base panel is a mudmat of the scour protection structure.

14. The scour protection structure of Claim 12, wherein the base panel is mounted on a mudmat of the scour protection structure.

15. The scour protection structure of Claim 14, wherein the base panel is movable relative to the mudmat.

16. The scour protection structure of Claim 15, wherein the base panel is pivotable relative to the mudmat about an axis orthogonal to a plane of the mudmat.

17. The scour protection structure of any of Claims 12 to 16, wherein at least one of the guides is movable relative to the base panel.

18. The scour protection structure of Claim 17, wherein the at least one guide is pivotable relative to the base panel about an axis orthogonal to a plane of the base panel.

19. The scour protection structure of Claim 17, wherein the at least one guide is resiliently deflectable relative to the base panel.

20. The scour protection structure of any preceding claim, wherein the cable stabilisation system is located in an upwardly open recess disposed between upright walls of the scour protection structure.

21. The scour protection structure of any preceding claim, further comprising a sacrificial web extending at least partially across the opening.

22. A subsea installation, comprising:the scour protection structure of any preceding claim, lying on a seabed;a subsea structure extending into the seabed through the opening; anda cable extending from an exit point of the subsea structure to the seabed, a portion of the cable extending along the valley of the cable stabilisation system.

23. The installation of Claim 22, wherein the cable extends into a trench under the seabed.

24. The installation of Claim 22 or Claim 23, wherein the subsea structure is a monopile foundation of an offshore wind turbine.

25. The installation of any of Claims 22 to 24, wherein the exit point lies substantially on an upright plane that bisects the valley and a portion of the cable extending from the exit point along the valley lies substantially on that plane.

26. The installation of any of Claims 22 to 24, wherein the exit point is offset from an upright plane that bisects the valley and a portion of the cable extending from the exit point along the valley is curved out of that plane.

27. A method of installing a scour protection structure, the method comprising lowering the scour protection structure to a seabed as an assembly that includes a cable stabilisation system, followed by lowering a subsea structure into the scour protection structure and embedding the subsea structure in the seabed.

28. The method of Claim 27, wherein the cable stabilisation system is pre-attached aboard an installation vessel that lowers the assembly to the seabed.

29. The method of Claim 27 or Claim 28, followed by installing a cable extending from the subsea structure to the seabed via the cable stabilisation system.

30. The method of Claim 29, comprising lowering the cable into a valley defined between elongate guides of the cable stabilisation system.

31. The method of any of Claims 27 to 30, wherein the scour protection structure is as defined in any of Claims 1 to 21.

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