GB2615652A

GB2615652A - Foundations for offshore wind turbines

Info

Publication number: GB2615652A
Application number: GB2300871.7A
Authority: GB
Inventors: John Macleay Alan
Original assignee: Subsea 7 Ltd
Current assignee: Subsea 7 Ltd
Priority date: 2022-01-21
Filing date: 2023-01-20
Publication date: 2023-08-16
Also published as: GB202200812D0; NL2034000B1; GB202300871D0; DE202022101137U1; GB2614902B; NO20230047A1; NL2034000A; GB2614902A; CN219012775U

Abstract

A foundation for an offshore wind turbine installation. The foundation is made of at least three suction caissons 16 and a connector body 22, the connector body having radially extending arms 50 that connect the caissons to a central hub. Each of the caissons has an upright tube consisting of: a lower tube 42 defining an open-bottomed suction chamber; an upper tube 44 defining a ballast receptacle 46; and an internal partition 38 that divides the lower tube portion from the upper tube portion. Each arm terminates by the upper tube portion of the associated suction caisson. Each arm may be a box-section structure which may be closed by the upper tube portion of the associated suction caisson and may have top and bottom walls. The partition of each caisson may be coplanar with the bottom wall of each arm. The top wall may be level with or bellow a top edge of each upper tube portion. The foundation may also have a curtain system (72, fig. 9) suspended from each arm.

Description

Foundations for offshore wind turbines This invention relates to foundation structures for offshore wind turbines. With the introduction of increasingly large offshore wind turbines that require correspondingly large foundation structures, the invention aims to simplify such structures and the processes required to manufacture them.

Offshore wind turbines can float at the surface or be fixed to the seabed. For example, FR 3079204 discloses a semi-submersible base for a floating wind turbine. The base floats at the surface and is anchored to the seabed. The base comprises a partially submerged central column that supports the turbine, surrounded by outer columns that are also partially submerged. Submerged pontoon-shaped branches join the outer columns to the central column. The various columns and the branches are hollow, thus imparting positive buoyancy by virtue of their submerged volume.

In contrast to FR 3079204, the invention is particularly concerned with wind turbine foundation structures that are fixed to the seabed and that, therefore, do not have positive buoyancy once installed. Many such structures are of a monopile design comprising a single leg atop a single foundation. Other such structures comprise multiple legs, for example jackets or tripods, where each leg is atop its own foundation.

The optimal foundation structure design for a particular application depends on the size of the wind turbine, the characteristics of the seabed and the loads expected in service, especially bending or tilting due to horizontal forces and moments transmitted from the wind turbine due to water motion and wind. This invention particularly relates to foundation structures comprising multiple legs and hence multiple foundations, as required to resist loads imparted by the larger offshore wind turbines that are now coming into service.

Typically, offshore wind turbine foundation structures are embedded into the seabed soil or into underlying rock. For example, a foundation structure may comprise one or more suction piles, also known as suction buckets or suction caissons. Each suction caisson comprises a tubular skirt with a closed top and an open bottom. The bottom of the skirt embeds initially into seabed soil under self-weight, hence defining a suction chamber within the skirt above the soil. The skirt then embeds further, after an initial gravity penetration phase, by underpressure generated as water is pumped out of the suction chamber. Gravity foundations are also known, in which the weight of the foundation structure, supplemented by additional ballast material, is sufficient to fix the structure to the seabed.

Hybrid foundations combine attributes of embedded foundations and gravity foundations by providing for ballast material to be placed atop a pile structure. For example, KR 20130128929 discloses foundations for offshore wind turbines in which an open-topped ballasting compartment added to the top of a suction caisson provides positive buoyancy for transportation by towing at the surface and is then ballasted to lower the caisson to the seabed during installation. Ballasting involves initially flooding the compartment with water and then placing heavier ballasting material such as concrete or rock into the compartment.

Some of the foundations in KR 20130128929 are monopiles comprising a single suction caisson; others, for larger wind turbines like those of relevance to the present invention, are tripod arrangements comprising three suction caissons. The caissons are joined by a connecting piece that is surmounted by a common open-topped ballasting compartment. Such assemblies are complex to manufacture. Also, a ballasting compartment with a large enough displacement to provide buoyancy for towing requires a correspondingly large amount of material. This may be considerably more material than is needed by the foundation in service.

WO 2021/221506 and WO 2019/074363 to SPT also disclose tripod foundation structures for an offshore wind turbine. In those examples, the structure comprises three suction caissons surmounted by a hollow connector body, which may be made of reinforced concrete or steel. The connector body is star-shaped in plan view, comprising box-section arms that extend radially with mutual angular spacing from a central upright axis. Ballasting is effected by filling the hollow connector body with water or another ballasting material.

In the primary embodiments of WO 2021/221506 and WO 2019/074363, the footprint of the connector body overlaps the caissons in side view and in plan view. Thus, in those embodiments, the connecting structure lies at a level above the caissons, such that the outwardly-tapering arms can extend outwardly beyond the radially inner sides of the caissons to join the tops of the caissons.

In a secondary embodiment of WO 2021/221506, the arms of the connector body do not overlap the caissons; instead, the arms join the inner sides of the caissons and the tops of the arms are at the same level as the tops of the caissons. However, the tops of the caissons, like the tops of the arms, are closed.

The -Fri-Suction Pile Caisson' foundation structure commercialised by SPT and disclosed at https://www.sptoffshoracomiwind-turbine-generator-wiri-foundations: shares key features with the primary embodiments of WO 2021/221506 and WO 2019/074363. In particular, the structure comprises a star-shaped connector body whose radially-extending arms embrace the tops of the respective caissons and that therefore overlap the caissons in plan view. Again, each arm tapers outwardly but in this instance, tapering is mostly evident in side view by virtue of outward convergence between a steeply-inclined top face and a horizontal bottom face of the arm.

Each arm terminates in a ring that encircles and embraces the top of the associated caisson, hence being of greater external diameter than the caisson. An outward part of the ring has a horizontal top surface, level with the lowermost extent of the inclined top of the arm. An inward part of the ring is integral with, and hence steeply inclined to match, the inclined top face of the arm.

The resulting structure is bulky and is complex in its structure and manufacturing requirements. Also, whilst the ring stands slightly proud of the closed top of the caisson and therefore defines a shallow upwardly-concave recess on top of the caisson, the recess is too shallow, and is inappropriately shaped, to retain a useful volume of ballast material necessary to stabilise a wind turbine. This means that the caissons must be enlarged to increase their supporting capacity, and in turn means that the connector body must be enlarged to suit the larger caissons. The additional material increases cost and greater bulk and weight increase the challenges of installation.

CN 214401870 discloses a composite foundation structure for an offshore wind turbine, in which the foundation includes a ballast support pan arranged concentrically with a pile of the supported turbine. The foundation also includes suction buckets and ribs under the support pan that join the suction buckets with the innermost wall of the support pan. The ribs are solid flanges that stiffen the support pan rather than independent hollow arm-like structures.

CN 106638662 discloses a combined foundation structure comprising three bucket foundations whose central portions are connected in a triangular arrangement. Each bucket foundation has a tubular base, with a ballast tank structure on top. Concrete structures join a central concrete pillar structure to the bucket foundations. The foundation structure can be used to support wind turbine installations.

Against this background, the invention resides in a foundation for an offshore installation, the foundation comprising at least three suction caissons joined by a connector body. The connector body comprises arms that radiate outwardly from a central hub to join respective associated ones of the suction caissons. Each suction caisson comprises an upright tube which, beneficially, may be of substantially constant horizontal cross section throughout its height or vertical length. The tube has a lower tube portion being an open-bottomed skirt for defining a suction chamber, an upper tube portion defining an open-topped ballast receptacle, and an internal partition that divides the lower tube portion from the upper tube portion.

Each arm is terminated by the upper tube portion of the associated suction caisson. For example, each arm may have a box-section structure, in which case an outer end of the box-section structure may elegantly be closed or terminated by the upper tube portion of the associated suction caisson.

The box-section structure of each arm suitably comprises top and bottom walls joined by side walls. The top and/or bottom walls may be substantially horizontal and the side walls may be substantially vertical. The partition of the associated suction caisson may be substantially level with, and is preferably co-planar with, the bottom wall.

Conversely, the top wall is preferably level with, or at a level beneath, a top edge of the upper tube portion. The full circumference of that top edge is, preferably, substantially horizontal to maximise the capacity of the ballast receptacle.

Each arm may comprise an array of stiffening members or beams that extends along the arm to the upper tube portion of the associated suction caisson. Where the arm has a double-skinned structure, at least some members of the array may be disposed between inner and outer walls or skins of that structure. At least some of those members can extend into the upper tube portion, for example by extending through a wall of the tube. The stiffening members may form part of a frame within the tube that reinforces the partition. Such a frame may conveniently lie on top of the partition at a base of the ballast receptacle Each of the arms may comprise a plurality of mutually-spaced structural members and each of these structural members may be hollow. The arrangement of the structural members in each arm may be symmetrical about a central vertical plane of that arm. The structural members of each arm may comprise at least one upper member and at least one lower member, each extending from the central hub to the respective suction caisson. At least one upper member of an arm may converge downwardly with the at least one lower member towards the respective suction caisson. The at least one upper member may conjoin the at least one lower member at a position radially outboard of the upright tube of the suction caisson.

The inventive concept embraces an offshore installation comprising the foundation of the invention. In that case, the lower tube portions of the suction caissons are embedded in soil of the seabed, and ballast material may be deposited in their ballast receptacles. A column may extend upwardly from the hub to an above-surface level to support an above-surface structure such as a wind turbine.

Where there is a gap between the seabed and an underside of each arm, that gap may be bridged by an anti-scour curtain system that is suspended from, and extends to a level beneath, each arm. The curtain system suitably comprises an array of compliant elongate barrier elements, which elements may be arranged in successive rows parallel to the radial orientation of the arm and may be weighted for negative buoyancy.

Such a gap may also be bridged by a berm of material deposited on the seabed.

However, an underside of each arm could instead lie on the seabed, hence leaving no significant gaps beneath the arms.

Thus, the invention provides a hybrid gravity suction foundation for offshore wind turbines, allowing for installation of wind turbines in offshore locations where soil conditions make conventional foundation solutions difficult.

In embodiments of the invention, a foundation for an offshore wind turbine comprises at least three suction caissons and a star-shaped support piece that connects the caissons together and supports a mast of the wind turbine. The support piece may, for example, comprise branches each with a rectangular cross-section. Lateral walls of the suction caissons extend above the seabed to a height or level that is at least flush with a top wall of the support piece, so that each branch of the support piece is terminated by a lateral wall of the associated suction caisson. Nevertheless, the top plate of each suction caisson may be lower than the upward extension of the lateral wall to define an open-topped volume, recess or compartment suitable for ballasting by dumping rocks.

A bottom wall or floor of each branch may be flush with, or embedded slightly into, the seabed after installation. Alternatively, there could be clearance between the bottom wall of each branch and the seabed beneath. For example, the bottom walls of the branches could be at a distance of between 1m and 5m above the seabed after installation Where there is clearance between the branches and the seabed beneath them, a curtain system may be suspended from the branches as an anti-scour measure. The top of the curtain system would be above the seabed but may be less than 10m, and preferably less than 2m, above the seabed. The curtain system may, for example, hang freely in the water between the bottom walls of the branches and the seabed. The curtain system may be positioned in line with the sides of the branches, may be positioned inwardly of the sides of the branches or may extend inwardly from those sides.

A curtain system could comprise compliant elongate elements that may be mounted side-by-side and/or in inwardly-successive rows, which could be in staggered relation. The elements of the curtain system may comprise one or more of fabrics, webs, fronds or strips that may be of artificial or natural seaweed, plastics, elastomers, polymers or rubber. The elements could also take the form of ropes, wires, filaments or small-diameter chains. The elements could be of negatively-buoyant materials and/or may comprise additional weights at their lower ends, thus enabling positively-buoyant materials to be used for the elements.

In another approach where there is clearance between the branches and the seabed beneath them, scour protection material may be placed or arranged on the seabed outside and/or within the footprint of the support piece, along the sides of each branch. Scour protection material may, for example, comprise rocks or concrete blocks, which could be loose or held inside flexible bags of mesh or fabrics.

In summary, a hybrid foundation of the invention combines attributes of embedded foundations and gravity foundations by providing for ballast material to be placed atop suction caissons that are joined by a connector body. Arms of the connector body radiate outwardly from a central hub to join the respective suction caissons, which each comprise an upright tube. An internal partition divides each tube into a lower portion and an upper portion. The lower portion is an open-bottomed skirt to define a suction chamber. The upper portion terminates the associated arm of the connector body and defines an open-topped ballast receptacle. Where the arm has a box-section structure, the outer end of the arm may be closed by the upper portion of the tube.

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 side view of an offshore wind turbine installation comprising a subsea foundation of the invention; Figure 2 is a perspective view of the foundation shown in Figure 1; Figure 3 is a simplified side view of the foundation shown in Figure 1, showing rock ballast; Figure 4 sectional plan view on line IV-IV of Figure 3; Figure 5 corresponds to Figure 4 but with the rock ballast removed, showing internal structural members of the foundation; Figure 6 is a sectional view on line VI-VI of Figure 5; Figure 7 is a sectional view on line VII-VII of Figure 5; Figure 8 is a schematic sectional view on line VI-VI of Figure 5 showing anti-scour berms deposited on the seabed adjoining an arm of the foundation; Figure 9 is a schematic sectional view on line VI-VI of Figure 5 showing anti-scour curtains extending from an arm of the foundation to the seabed; Figure 10 is a schematic sectional view on line VI-VI of Figure 5 showing an arm of the foundation lying on the seabed; Figure 11 is a top perspective view of an alternative embodiment in which each arm comprises a plurality of structural elements or members; and Figure 12 is a bottom perspective view of the embodiment shown in Figure 11.

Referring firstly to Figure 1 of the drawings, an offshore wind turbine installation 10 comprises a conventional wind turbine 12 atop a subsea foundation structure 14 of the invention. The foundation structure 14 comprises suction caissons 16 that are embedded in the seabed 18. In this example, three suction caissons 16 are in an equilateral triangular arrangement about a central vertical axis 20. In other examples, there could be four or more suction caissons 16 angularly spaced around the vertical axis 20.

The suction caissons 16 are joined by a star-shaped connector body 22 from which a column 24 centred on the vertical axis 20 extends upwardly to protrude above the surface 26. A mast 28 of the wind turbine 12 surmounts the column 24 in coaxial relation, extending to a nacelle 30 atop the mast 28. The nacelle 30 supports a rotor 32.

A work platform 34 is shown in Figure 1 cantilevered from the top of the column 24, beneath the mast 28 of the wind turbine 12. Figure 2 shows the work platform 34 in conjunction with ladders 36 that give access from sea level. Figure 2 also shows details of the suction caissons 16 and the connector body 22.

Each suction caisson 16 comprises a tube that is rotationally symmetrical about a vertical axis and so is of circular cross-section in this example. The tube is of substantially constant diameter along its length and is open at its top and bottom ends, which lie in respective substantially horizontal planes. The internal volume of the tube is divided by a partition 38 at an intermediate level. The partition 38 also lies in a substantially horizontal plane. Reinforcing beams 40 surmount the partition 38 and will be described in more detail with reference to later drawings.

On installation, a lower portion 42 of the tube is embedded in the seabed 18. The level of the seabed 18 is shown in Figure 2 in dashed lines. The lower portion 42 of the tube therefore serves as a skirt surrounding a suction chamber of the suction caisson 16. The partition 38 closes the top of the suction chamber.

An upper portion of the tube or each suction caisson 16 extends continuously above the partition 38 as a circular wall 44. Thus, the partition 38 is recessed beneath the top of the wall 44. Together, the partition 38 and the wall 44 define an open-topped ballast receptacle 46.

The wall 44 and the lower portion 42 of the tube can be attached or joined to each other or, conveniently, can be formed integrally or from the same component. In this example, the wall 44 is an upward extension of the lower portion 42 of the tube that defines the skirt of the suction caisson. This simplifies the construction of the foundation structure 14. In this respect, it will be noted that the wall 44 and the lower portion 42 of the tube have the same outer diameter. The wall 44 and the lower portion 42 of the tube may also have the same inner diameter and hence the same wall thickness.

Conveniently, during installation, ballast material 48 may be lowered or dumped into the ballast receptacles 46 of the foundation structure 14 as shown in Figures 3 and 4 without obstruction from above. The ballast material 48 is shown in those drawings as a mass of loose rock but other forms of ballast material are possible, such as poured or precast concrete or a granular material such as gravel or sand, which may be loose or bagged. The horizontal top edge of the wall 44 maximises the capacity of each ballast receptacle 46 and helps to capture and retain the ballast material 48 within.

The connector body 22 shown in Figure 2 comprises hollow box-section arms 50 that extend radially outwardly with respect to the central axis 20 of the column 24 shown in Figure 1. The arms 50 extend outwardly from a common root or hub, where they are conjoined, to respective ones of the suction caissons 16. Figure 2 also shows optional features of the connector body 22, namely a cable outlet 52 at the base of the column 24 and arrays of sacrificial anodes 54 extending along the arms 50 for corrosion protection of the subsea foundation structure 14.

As best appreciated in Figures 4, 5 and 6, each arm 50 of the connector body 22 is of rectangular cross-section and is of constant section, width and height along its length. Specifically, as shown in Figure 6, each arm 50 comprises parallel planar upright side walls 56 joined by a horizontal top wall 58 and a horizontal bottom wall 60, each of which is also planar. The side walls 56 are orthogonal to the top wall 58 and the bottom wall 60. More specifically, Figure 6 shows that each arm 50 is of double-skinned construction. Thus: each side wall 56 comprises an inner side plate 56A and an outer side plate 56B; the top wall 58 comprises an inner top plate 58A and an outer top plate 58B; and the bottom wall 60 comprises an inner bottom plate 60A and an outer bottom plate 60B.

It will be apparent from Figures 2, 3 and 6 that each arm 50 joins the wall 44 of the associated suction caisson 16, being the upper portion of the tube whose lower portion 42 defines the skirt of the suction caisson 16. In this example, the bottom wall 60 of each arm 50 is at a level above the seabed 18, hence leaving a gap between the underside of the arm 50 and the seabed 18.

The side walls 56 and the top wall 58 of each arm 50 are also above the level of the partition 38 within the suction caisson 16. However, as in this example, the bottom wall 60 of each arm 50, and therefore the bottom edges of the side walls 56, may be at the level of the partition 38. Specifically, as best appreciated in Figure 3, the partition 38 is a horizontal extension of the bottom wall 60 of the arm 50. Again, this simplifies the construction of the foundation structure 14. More specifically, as best appreciated in Figure 7, the partition 38 is a horizontal extension of the outer bottom plate 60B of the bottom wall 60. Conversely, the top wall 58 of each arm 50, and therefore the top edges of the side walls 56, are substantially level with the top of the wall 44 of the associated suction caisson 16. This also simplifies the construction of the foundation structure 14 and allows unimpeded access from above to fill the ballast receptacles 46 of the caissons 16 with ballast.

Figures 5 and 6 show arrays of stiffening members in the form of the aforementioned beams 40 that extend longitudinally along each arm 50 of the connector body 22. Figure 6 shows that those longitudinal beams 40 are sandwiched between the inner plates 56A, 58A, 60A and the corresponding outer plates 56B, 58B, 60B of the double-skinned structure of an arm 50.

Figure 5 shows that the lowermost array of parallel longitudinal beams 40 of each arm 50, disposed between the inner plate 60A and the outer plate 60B of the bottom wall 60, extends beyond the outer end of the arm 50. Thus, the outer end portions of those longitudinal beams 40 extend through the wall 44 that encircles the ballast receptacle 46 and into the base of the ballast receptacle 46, above the partition 38. Here, those longitudinal beams 40 join intersecting beams 40 to form a protective frame that extends across the base of the ballast receptacle 46 and that reinforces the partition 38, as also shown in Figure 7.

Turning next to Figures 8 and 9, these drawings show anti-scour provisions to prevent erosion of the seabed 18 due to flows of water that would otherwise traverse a gap 62 between an arm 50 and the seabed 18. In Figure 8, parallel elongate berms 64 of rock or granular material are placed on the seabed 18 beneath the arm 50 to close the gap 62. Each berm 64 is trapezoidal, comprising a horizontal top face 66 between inner and outer faces 68, 70 that are oppositely inclined toward the seabed 18. The inner face 68 is substantially inboard of the side wall 56, within the gap 62, and the outer face 70 and most or all of the top face 66 are outboard of the side wall 56. In this example, the inner face 68 is more steeply inclined than the outer face 70.

Figure 9 shows a curtain system 72 that is suspended from the underside of the arm 50 to bridge the gap 62 between the bottom wall 60 and the seabed 18. The curtain system 72 comprises one or more arrays of compliant elongate elements 74 such as fronds or strips that, in this example, are mounted side-by-side in inwardly successive rows extending into the gap 62 from beneath the side walls 56. To block water flow through the gap 62 more effectively, the elements 74 of a row may be staggered in relation to elements 74 of neighbouring rows. At least some of the elements 74 may have weighted lower ends to keep them hanging in an upright orientation across the gap 62. The suspended portion of each element 74 is longer than the height of the gap 62 so that the elements 74 hang in contact with and drape against the seabed 18.

Many variations are possible within the inventive concept. For example, Figure 10 shows that the suction caissons 16 could be embedded in the seabed 18 to such an extent that no gaps 62 remain between the arms 50 and the seabed 18. Consequently, the bottom wall 60 of an arm 50 is shown here lying on the seabed 18. In this case, the flat bottom wall 60 of the arm 50 spreads the weight load and further stabilises the wind turbine installation 10. The arm 50 could of course be embedded slightly in the soil of the seabed 18 where that soil is sufficiently soft.

Finally, Figures 11 and 12 show a variant of the connector body 22 in which each arm 50 comprises a plurality of tubular or hollow structural members 76U, 76L spaced apart from each other. In plan view, the structural members 76U, 76L of each arm 50 extend parallel to each other and to a radius that intersects the central axis 20 of the column 24 serving as a common root or hub.

In this example, each arm 50 comprises four structural members 76U, 76L, namely a pair of upper members 76U and a pair of lower members 76L. The members 76U, 76L of each pair are mutually parallel, being positioned symmetrically about a vertical plane that contains the vertical axis 20 and the central vertical axis 78 of the respective suction caisson 16.

The lower members 76L extend substantially horizontally from the column 24 to each suction caisson 16 whereas the upper members 76U each have a central portion 80 that is inclined to the horizontal, extending downwardly from the column 24 to the suction caisson 16 in the manner of a flying buttress. Thus, the upper and lower members 76U, 76L converge outwardly in a triangulated brace arrangement.

At their outward ends, the upper and lower members 76U, 76L each include horizontally-oriented outer box structures 82 that join the upper and lower members 76U, 76L to the suction caisson 16. The outer box structure 82 of each lower member 76L is joined to the outer box structure 82 of the upper member 761.1 directly above, thus coupling those upper and lower members 76U, 76L to each other radially outwardly of the caisson 16 to define the outward end of the arm 50 that adjoins the caisson 16. Each outer box structure 82 comprises a concave-curved interface that fits the convex outer curvature of the circular wall 44 of the caisson 16 so that the wall 44 again terminates the hollow members 76U, 76L that together form the arm 50.

Correspondingly, each upper member 76U includes a horizontally-oriented inner box structure 84 at its inward end that joins the upper member 76U to the column 24. The inner box structure 84 comprises a concave-curved interface that fits the convex outer curvature of the column 24.

In this example, each of the structural members 76U, 76L is rectangular in cross section, comprising parallel planar upright side walls 56 that are joined by parallel top and bottom walls 58, 60 oriented orthogonally with respect to the side walls 56. Thus, the top and bottom walls 58, 60 of the upper members 76U each comprise an inclined central portion between horizontal end portions.

In this example, the conjoined outer box structures 82 of the upper and lower members 76U, 76L extend from the top of the wall 44 to the level of the partition 38 within each suction caisson 16. Thus, the horizontal bottom walls 60 of the lower members 76L are substantially coplanar with the partitions 38 within the caissons 16.

The top and bottom walls 58, 60 of each member 76U, 76L extend outwardly beyond the side walls 56 to form flanges 86 that extend along the length of the members 76U, 76L to enhance their rigidity. The flanges 86 are joined at longitudinal intervals by upright gussets 88 that project orthogonally from the side walls 56. Conveniently, the gussets 88 align with the interfaces between the outer and inner box structures 82, 84 and the remaining portions of the respective structural members 76U, 76L.

Each of the structural members 76U, 76L forming the arms 50 in Figures 10 and 11 could be of single-skinned construction or of double-skinned construction like the arrangement shown in Figure 6.

Claims

Claims 1. A foundation for an offshore installation, the foundation comprising at least three suction caissons joined by a connector body, wherein: the connector body comprises arms that radiate outwardly from a central hub to join respective associated ones of the suction caissons; each suction caisson comprises an upright tube, the tube having a lower tube portion being an open-bottomed skirt for defining a suction chamber, an upper tube portion defining an open-topped ballast receptacle, and an internal partition that divides the lower tube portion from the upper tube portion; and each arm is terminated by the upper tube portion of the associated suction caisson.
2. The foundation of Claim 1, wherein each arm has a box-section structure.
3. The foundation of Claim 2, wherein an outer end of the box-section structure is closed by the upper tube portion of the associated suction caisson.
4. The foundation of Claim 2 or Claim 3, wherein the box-section structure comprises top and bottom walls joined by side walls.
5. The foundation of Claim 4, wherein the top and/or bottom walls are substantially horizontal.
6. The foundation of Claim 4 or Claim 5, wherein the partition of the associated suction caisson is substantially level with the bottom wall.
7. The foundation of Claim 6, wherein the partition and the bottom wall are coplanar.
8. The foundation of any of Claims 4 to 7, wherein the top wall terminates upwardly at a level aligned with or beneath a top edge of the upper tube portion.
9. The foundation of Claim 8, wherein a full circumference of the top edge is substantially horizontal.
10. The foundation of any preceding claim, wherein each tube is of substantially constant horizontal cross section throughout its height.
11. The foundation of any preceding claim, wherein each arm comprises an array of stiffening members that extends along the arm to the upper tube portion of the associated suction caisson.
12. The foundation of Claim 11, wherein at least some members of the array extend into the upper tube portion.
13. The foundation of Claim 12, wherein said members extend through a wall of the 15 tube.
14. The foundation of Claim 12 or Claim 13, wherein said members form part of a frame within the tube that reinforces the partition.
15. The foundation of Claim 14, wherein the frame lies on top of the partition at a base of the ballast receptacle.
16. The foundation of any of Claims 11 to 15, wherein at least some members of the array are disposed between inner and outer walls of a double-skinned arm structure.
17. The foundation of any preceding claim, wherein each of the arms comprises a plurality of mutually-spaced structural members.
18. The foundation of Claim 17, wherein each of the structural members is hollow.
19. The foundation of Claim 17 or Claim 18, wherein the structural members of each arm are arranged symmetrically about a central vertical plane of that arm.
20. The foundation of any of Claims 17 to 19, wherein the structural members of each arm comprise at least one upper member and at least one lower member, each extending from the central hub to the respective suction caisson.
21. The foundation of Claim 20, wherein the at least one upper member converges downwardly with the at least one lower member toward the suction caisson.
22. The foundation of Claim 20 or Claim 21, wherein the at least one upper member conjoins the at least one lower member at a position radially outboard of the upright tube of the suction caisson.
23. The foundation of any preceding claim, wherein each arm further comprises a curtain system that is suspended from, and extends to a level beneath, the arm.
24. The foundation of Claim 23, wherein the curtain system comprises an array of compliant elongate barrier elements.
25. The foundation of Claim 24, wherein the elements of the curtain system are arranged in successive rows parallel to orientation of the arm.
26. The foundation of Claim 24 or Claim 25, wherein the elements of the curtain system are weighted for negative buoyancy.
27. An offshore installation comprising the foundation of any preceding claim, wherein a supporting column extends upwardly from the hub to an above-surface level and the lower tube portions of the suction caissons are embedded in soil of the seabed.
28. The installation of Claim 27, wherein there is a gap between the seabed and an underside of each arm
29. The installation of Claim 28 when dependent on any of Claims 23 to 26, wherein the gap is bridged by the curtain system.
30. The installation of Claim 28, wherein the gap is bridged by a berm of material deposited on the seabed.
31. The installation of Claim 27, wherein an underside of each arm lies on the seabed.
32. The installation of any of Claims 27 to 31, further comprising ballast material deposited in the ballast receptacles of the suction caissons.