Floating offshore support structures

Abstract

A support structure suitable for a floating offshore wind turbine comprises a floater and upright tension legs extending from respective leg anchors to the floater The legs correspond to side edges of a prism, with upright side faces of the prism being defined between neighbouring legs. Additionally, inclined tensile braces extend diagonally across the side faces from individual or shared brace anchors to the floater. Each tension leg lies in a plane that extends between a pair of the braces, the braces of the pair extending across respective adjoining side faces and converging downwardly toward that plane. Braces of different pairs cross each other with mutually opposed inclination as they extend across the side faces.

Description

[0001] Floating offshore support structures

[0002] This invention relates to floating offshore support structures, particularly to tension-leg structures in which floaters are moored by upright tensile tethers or tendons.

[0003] Floating offshore wind turbines have been proposed for, and more recently installed in, waters that are too deep for bottom-fixed foundations such as monopiles or jackets to be practical. Various proposals have been made for mooring a floater that supports an offshore wind turbine. Some recent proposals employ tension-leg solutions, inspired by the tension-leg platforms used for many years in offshore oil and gas production.

[0004] Typically, tension-leg arrangements have great axial strength and hence resistance to loads that have a substantial vertical component but they lack capacity to resist lateral loads arising from a substantial horizontal component. This characteristic is shared by the foundations or ground anchors that are preferentially used for tension-leg arrangements, namely vertically-extending elongate piles at the base of each tension leg, those piles being embedded in the seabed by suction, driving or drilling.

[0005] A floating offshore wind turbine places considerable demands on the stability of a supporting floater and hence on the moorings and foundations that are chosen for the floater. There is a need, especially, to resist an overturning moment arising from wind loads acting on the rotor and transferred to the nacelle, high above the floater atop the tower or mast of the wind turbine. Resisting that overturning moment places substantial lateral loads on tension legs, for which they are not ideally suited as noted above.

[0006] To cater for lateral loads, it is known to supplement tension legs with inclined supplementary moorings such as catenary lines that radiate outwardly from the floater in plan view. However, inclined moorings of that type take up a large area of the seabed and so can clash with other moorings or other subsea infrastructure such as cables and substations, hence reducing flexibility for designing installation layouts to suit a congested seabed. Inclined moorings also present challenges with foundations, as vertical piles are inefficient at bearing inclined loads that have a substantial horizontal component. Bulky and expensive foundations may be required as a result.

[0007] WO 2024 / 018190, WO 2024 / 018189 and EP 4396415 disclose anchor piles that are configured to be embedded in a borehole. GB 2533772 discloses a gravity base foundation for a floating structure, the foundation incorporating a drill to create a socket for an anchor in a rocky seabed. The socket is drilled into the seabed at an angle that approximates to, but does not match, the inclination of an inclined mooring. This foundation is complex and bulky and is accompanied by a mooring arrangement that lacks any tension legs.

[0008] EP 4389582 discloses a mooring arrangement for an offshore wind turbine in which a floating platform is moored by inclined tensioned tendons. US 3086368 and JP S5940987 also disclose inclined mooring lines.

[0009] In a mooring arrangement with inclined and crossed mooring lines, US 8657534 discloses a floating platform for an offshore wind turbine provided with anchors that are arranged vertically beneath connecting locations on a floating support structure. Vertical mooring cables extend from each anchor to corresponding connecting locations and inclined mooring cables extend to further connecting locations outboard of the centre of the support structure. EP 2789849 also discloses a mooring arrangement with inclined and crossed mooring lines.

[0010] Other examples of documents representing the state of the art are: GB 1542247; FR 2290346; US 8657532; US 3086368; KR 101332655; US 2019 / 0078556; CN 117341915 and “A modular TLP floating substructure to maximize the flexibility within the supply chain" - Author: Adam F., et al., paper presented at the 27th International Ocean and Polar Engineering Conference, San Francisco, June 2017.

[0011] Against this background, the invention resides in an offshore support structure that comprises a buoyant support and upright tension legs that extend from respective leg anchors to the buoyant support, the legs being arrayed as side edges of a prism defining upright side faces of the prism between neighbouring legs. Inclined tensile braces extend diagonally across the side faces from brace anchors to the buoyant support, and may extend across substantially a full width of those side faces. Each tension leg lies in a plane extending between a pair of the braces, the braces of the pair extending across respective adjoining side faces and converging downwardly toward that plane. Braces of different pairs can cross each other with mutually opposed inclination as they extend across the side faces. The braces can extend outwardly and downwardly from the side faces in plan view and / or in side view. More generally, the braces may lie predominantly outboard of the side faces, or the brace anchors may lie outside a polygonal footprint whose vertices correspond to the leg anchors.

[0012] The brace anchors may extend substantially coaxially from respective ones of the braces. For example, the braces of the pair can extend from respective brace anchors. Alternatively, the braces of the pair can be conjoined at their lower ends to extend from a common brace anchor, which may be substantially coplanar with the braces of the pair.

[0013] The brace anchors may be driven or drilled piles whereas the leg anchors may be driven, drilled or suction piles.

[0014] The buoyant support may have a polygonal plan form whose vertices correspond to the tension legs, and can further comprise a wind turbine mounted thereon. The wind turbine may be offset laterally on the buoyant support at a position above one of the tension legs.

[0015] In summary, the invention provides compact diagonal bracing arrangements to resist lateral forces when mooring the floaters of floating offshore wind turbines, particularly those that are moored by tension-leg arrangements. The invention enables simple, compact ground anchors that primarily resist tension loads to be used for floating wind. Uniquely, the invention allows ground anchors to be inclined at an appropriate angle in combination with a diagonal arrangement of mooring lines that serve as braces. Those braces can be disposed largely under the floater or can extend between and beside the tension legs to extend across the width of the floater, to the benefit of compactness while maintaining an optimal angle of inclination.

[0016] The mooring system of the invention allows inclined braces to be added to a tension-leg arrangement and to be combined with correspondingly-angled piles serving as ground anchors. This supports the tension-leg moorings to handle horizontal loads and enables the ground anchors to transfer those horizontal loads into the seabed effectively.

[0017] Thus, the invention provides a support structure that is apt to support a floating offshore wind turbine. The structure comprises a floater and upright tension legs that extend to the floater from respective leg anchors. The legs correspond to side edges of a prism, with upright side faces of the prism being defined between neighbouring legs.

[0018] Additional inclined tensile braces extend to the floater from brace anchors embedded in or placed on the seabed. The braces extend diagonally across the side faces of the prism in side view. The braces may have individual or shared brace anchors.

[0019] Each tension leg lies in an upright plane that extends between a pair of the braces. The braces of the pair extend across respective adjoining side faces and converge downwardly toward that plane. Braces of different pairs can cross each other with mutually opposed inclination as they extend across the side faces.

[0020] 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:

[0021] Figure 1 is a schematic perspective view of a support structure for a floating offshore wind turbine including a brace arrangement of the invention;

[0022] Figure 2 corresponds to Figure 1 but shows a variant of the brace arrangement;

[0023] Figures 3 and 4 are schematic top plan views of the support structures shown in Figures 1 and 2 respectively;

[0024] Figures 5 and 6 are schematic side views of the support structures shown in Figures 1 and 2 respectively;

[0025] Figures 7 and 8 are schematic perspective views of further variants of wind turbine support structures that include brace arrangements of the invention; and

[0026] Figures 9 and 10 are schematic top plan views of the support structures shown in Figures 7 and 8 respectively.

[0027] The drawings show variants of a support structure 10 of the invention for a floating offshore wind turbine 12. The structure 10 comprises a buoyant support in the form of a tethered floater 14. The buoyancy of the floater 14 maintains tension in substantially vertical tension legs 16 that extend downwardly from the floater 14 to respective piles that are embedded in the seabed 18 to serve as leg anchors 20. The leg anchors 20 extend substantially coaxially from the tension legs 16 and can be driven or drilled into the seabed 18, or could be suction anchors.

[0028] In these examples, the floater 14 is a conventional semi-submersible structure 10 that floats at the surface 22 but in other examples, the floater 14 could be a sub-surface buoy. The floater 14 exemplified here comprises vertically-extending buoyancy columns 24 that are spaced apart but connected by horizontally-extending pontoons 26 to form a polygonal arrangement in plan view. Commonly, the pontoons 26 will have a lattice structure 10 and will be supported by additional trusses but such details have been omitted from the drawings for simplicity.

[0029] The wind turbine 12 surmounts one of the buoyancy columns 24, with its mast shown here schematically in dashed lines. Thus, the wind turbine 12 is offset laterally with respect to the floater 14, to a position above one of the tension legs 16.

[0030] In these examples, the tension legs 16 are attached to the buoyancy columns 24 of the floater 14 to maximise the area of the collective polygonal footprint 28 of the tension legs 16 and their leg anchors 20 on the seabed 18. Widening the foundation area in this way increases the stability of the support structure 10. However, in principle, the tension legs 16 could instead be attached to the pontoons 26 or to other parts of the floater 14.

[0031] Figures 1 to 6 show ‘tripod’ variants of the support structure 10 that have three tension legs 16 extending to respective buoyancy columns 24 of the floater 14. The floater 14 and the corresponding footprint 28 therefore have a triangular shape in plan view. In these examples, as the pontoons 26 are all of equal length, the floater 14 and the footprint 28 each have an equilateral triangular shape in plan view. However, other triangular shapes are possible.

[0032] Conversely, Figures 7 to 10 show ‘quad’ variants of the support structure 10 that have four tension legs 16 extending to respective buoyancy columns 24 of the floater 14. In these examples, the floater 14 and the corresponding footprint 28 have a rectangular shape in plan view. More specifically, as the pontoons 26 shown here are all of equal length, the floater 14 and the footprint 28 each have a square shape in plan view. In accordance with the invention, supplementary moorings in the form of inclined tensile braces 30 extend from the floater 14 to the seabed 18, where they are anchored by respective embedded piles that serve as brace anchors 32. Again, the brace anchors 32 may be driven or drilled into the seabed 18. Whilst the braces 30 are represented as being straight in the drawings due to the large tensile forces they sustain, in practice the braces 30 may sag slightly under gravity acting on their inclined length. However, for practical purposes, the braces 30 can be regarded as nominally straight, like the tension legs 16.

[0033] To orient the braces 30 at a usefully shallow angle to the horizontal for lateral location while keeping the mooring arrangement as compact as possible, each tension leg 16 is disposed between lower end portions of a respective pair of the braces 30. The braces 30 of that pair diverge upwardly from the seabed 18 toward the floater 14, extending generally toward the longitudinal axes of respective neighbouring tension legs 16 as the braces 30 approach the floater 14. In these examples, the upper ends of the braces 30 do not intersect the longitudinal axes of the neighbouring tension legs 16 but are instead attached to the buoyancy columns 24 that surmount those tension legs 16. Again, in principle, the braces 30 could be attached to the pontoons 26 or to other parts of the floater 14.

[0034] Geometrically, the volume defined between the tension legs 16 approximates to a polyhedron, or more specifically as a prism whose polygonal end faces correspond to the shapes of the floater 14 and the footprint 28 in plan view. Side edges of the prism are defined by the tension legs 16 whereas vertical side faces 34 of the prism are defined between pairs of neighbouring tension legs 16. The side edges corresponding to the tension legs 16, and hence also the side faces 34, are substantially vertical. Furthermore, the side faces 34 are generally planar.

[0035] It will be apparent that the braces 30 extend diagonally across the side faces 34 defined between neighbouring tension legs 16. As each tension leg 16 is associated with a pair of braces 30 extending across neighbouring side faces 34 of the prism, it will also be apparent that each side face 34 is crossed by two oppositely-inclined braces 30 that intersect centrally in side and plan view in an X-formation.

[0036] In more general terms, as shown in Figures 3, 4, 9 and 10, each tension leg 16 lies in a substantially vertical plane 36 that extends between, and divides the convergent angle between, the associated pair of braces 30. That plane 36 may, for example, bisect the acute angle between the pair of braces 30 in plan view, hence being in equiangular relation with each of those braces 30. The plane 36 can also bisect the floater 14 in plan view as shown, hence intersecting the opposed pontoon 26 orthogonally in the triangular arrangements of Figures 1 to 6 or including the diagonally opposed tension leg 16 in the square arrangements of Figures 7 to 10.

[0037] In Figures 1 , 3, 5, 7 and 9, the braces 30 of each pair extend to respective brace anchors 32. The piles serving as brace anchors 32 are preferably aligned substantially coaxially with the respective braces 30 as shown here.

[0038] In Figures 2, 4, 6, 8 and 10, the braces 30 of each pair extend to, and are joined at or by, a common brace anchor 32 that is shared between those braces 30. In this case, the pile serving as a brace anchor 32 may be substantially coplanar with the associated pair of braces 30 as shown here. More generally, the brace anchor 32, and hence the point of convergence or intersection between the pair of braces 30, may lie on the plane 36 that includes the associated tension leg 16, whether inboard or outboard of the tension leg 16.

[0039] In the illustrated examples, when seen either from above in plan view or from the side in elevation view, each brace 30 diverges from the side face 34 that it crosses, moving downwardly from its point of attachment at the floater 14 to its brace anchor 32 embedded in the seabed 18. Thus, in these examples, the brace anchors 32 are located outside the footprint 28 of the tension legs 16 although the brace anchors 32 could instead be located at least partially within that footprint 28. Similarly, in plan view, the braces 30 are also largely outside the footprint 28 of the tension legs 16 as shown although they could instead be at least partially within that footprint 28.

[0040] More generally, the brace anchors 32 can be within, on the border of, or outside the footprint 28 of the tension legs 16 whose positions coincide with the leg anchors 20, corresponding to vertices of the polygonal area of the footprint 28.

[0041] Many other variations are possible within the inventive concept. For example, anchors other than driven or drilled piles, such as suction piles or gravity foundations, may be possible for the braces 30 at least, although inclined driven or drilled piles are preferred for the braces 30. Other four-sided shapes are possible for the floater 14 and hence the footprint 28 defined by the tension legs 16, such as a diamond shape. More generally, other regular or irregular polygonal shapes are possible for the floater 14 and the footprint 28, those shapes having three, four or more than four sides.

Claims

Claims1 . An offshore support structure comprising: a buoyant support; upright tension legs extending from respective leg anchors to the buoyant support, the legs being arrayed as side edges of a prism defining upright side faces of the prism between neighbouring legs; and inclined tensile braces extending diagonally across the side faces from brace anchors to the buoyant support; wherein each tension leg lies in a plane extending between a pair of the braces, the braces of the pair extending across respective adjoining side faces and converging downwardly toward that plane.

2. The structure of Claim 1 , wherein braces of different pairs cross each other with mutually opposed inclination as they extend across the side faces.

3. The structure of Claim 1 or Claim 2, wherein the braces extend outwardly and downwardly from the side faces in plan view.

4. The structure of any preceding claim, wherein the braces extend outwardly and downwardly from the side faces in side view.

5. The structure of any preceding claim, wherein the braces lie predominantly outboard of the side faces.

6. The structure of any preceding claim, wherein the brace anchors lie outside a polygonal footprint whose vertices correspond to the leg anchors.

7. The structure of any preceding claim, wherein the braces extend across substantially a full width of the side faces that they cross.

8. The structure of any preceding claim, wherein the brace anchors extend substantially coaxially from respective ones of the braces.

9. The structure of any preceding claim, wherein the braces of the pair extend from respective brace anchors.

10. The structure of any of Claims 1 to 8, wherein the braces of the pair extend from a common brace anchor.

11. The structure of Claim 10, wherein the braces of the pair are conjoined at their lower ends.

12. The structure of Claim 10 or Claim 11 , wherein the common brace anchor is substantially coplanar with the braces of the pair.

13. The structure of any preceding claim, wherein the brace anchors are driven or drilled piles.

14. The structure of any preceding claim, wherein the leg anchors are driven, drilled or suction piles.

15. The structure of any preceding claim, wherein the buoyant support has a polygonal plan form whose vertices correspond to the tension legs.

16. The structure of any preceding claim, further comprising a wind turbine mounted on the buoyant support.

17. The structure of Claim 16, wherein the wind turbine is offset laterally on the buoyant support at a position above one of the tension legs.

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