Buoyant supports for floating offshore wind turbines

Abstract

A floater for a floating offshore wind turbine (FOWT) comprises a turbine support 14 and buoyancy structures 12 connected to the turbine support by respective arms 22, each joined to a brace 24 that i

Description

This invention relates to buoyant supports, also known as floaters, for supporting floating offshore wind turbines (FOWTs). Aspects of the invention relate to the challenges of simplifying and reducing the cost of manufacturing and installing such floaters. There have been many proposals for supporting offshore wind turbines but, to date, only a few of those proposals have made it to the market. In shallow waters, bottom-fixed wind turbines are typically mounted on tubular monopiles that are driven into the seabed although lattice-structure jacket foundations have been used where water is slightly deeper, for example around 50m. In significantly deeper waters, the only practical option is to adopt a FOWT solution in which the tower or mast of the wind turbine surmounts a raft, buoy or other floater that is moored to the seabed and floats at the surface. The need for installation in deeper waters has been driven by the requirement for ever-larger wind turbines that are not suitable for use on land or close to the shore. The larger the wind turbine, the larger the floater must be, not only for displacement to support the weight of the wind turbine but also for stability. In this respect, wind forces acting on the tower and rotor of the wind turbine will tend to tilt the tower away from the vertical. Extreme wind loading could capsize the floater and so collapse the wind turbine; but even moderate wind loading could tilt the tower to an extent that the rotor axis is no longer aligned efficiently relative to the incident wind direction. Consequently, it is desirable for the floater to have a wide footprint, for example at least 50m wide. Various FOWT solutions are feasible technically but are not necessarily feasible financially at the necessary scale. For this reason, very few FOWT solutions have even reached a prototype stage and fewer still have been the subject of a commercial project. To make exploitation of FOWTs sufficiently profitable, there is a need to optimise the design and manufacture of floaters to make them easier to assemble and to reduce their installation costs. One approach to the design of a floater involves pre-manufacturing modules that are subsequently assembled together. For example, WO 2024 / 062177 discloses the assembly of triangular frames into a central hub. WO 2020 / 167137 describes a floater structure made of welded steel panels arranged in a polyhedral cross-section and including stiffening elements. In WO 2023 / 284926 and WO 2023 / 110037, members of a floater are grouted together. WO 2020 / 169158 discloses another type of structural connection. In general, however, the prior art contemplates semi-submersible floaters made substantially entirely of a single material such as concrete or steel, constructed, fabricated or assembled using a single technology or construction philosophy. Against this background, the invention resides in a method of constructing a floater for a FOWT, the floater comprising a turbine support and a plurality of buoyancy structures connected to and spaced from the turbine support by respective elongate arms each joined to a respective elongate brace. The method comprises: joining an inboard end of each arm to the turbine support at a lower fixing point; joining an inboard end of each brace to the turbine support at an upper fixing point; and joining an outboard end of each arm to a respective one of the buoyancy structures at an outer fixing point. These various joining operations may be effected by clamping and grouting or by welding. The inboard end of each arm can be inserted into a respective lower receptacle provided on the turbine support at the lower fixing point. Similarly, the inboard end of each brace can be inserted into a respective upper receptacle provided on the turbine support at the upper fixing point. The inboard ends of the arm and / or the brace can be inserted into the lower and upper receptacles respectively in a substantially horizontal direction. Moreover, the inboard ends of the arm and / or the brace can be inserted into the respective receptacles simultaneously. In that case, the brace can be joined to the arm to form an assembly before that assembly is joined to the turbine support. Similarly, the outboard end of each arm can be inserted into an outer receptacle provided on the respective buoyancy structure at the outer fixing point. The outer receptacle could extend through a full width of the respective buoyancy structure. Such insertion may be effected in a substantially horizontal direction. In an alternative approach, the buoyancy structure can be inserted into an outer receptacle provided on the arm at the outer fixing point. For example, the outer receptacle could receive a lower spigot of the buoyancy structure in a substantially vertical direction or in a substantially horizontal direction. A closure can be positioned to capture the buoyancy structure in the outer receptacle, which could be an open-ended slot. An outboard end of each brace can be joined to an upper end of a respective one of the buoyancy structures. For this purpose, an upper spigot of the buoyancy structure can be inserted into an upper receptacle provided on the outboard end of the brace. Again, the upper receptacle could receive the upper spigot in a substantially vertical direction or in a substantially horizontal direction and a closure can be positioned to capture the upper spigot in the upper receptacle. The outboard end of the brace can be sandwiched between a platform and the upper end of the buoyancy structure. The platform can be coupled to the buoyancy structure, for example via the upper receptacle of the brace. A strut of the brace can be joined to an upper member of the brace, for example by clamping around the upper member. In this case, an outboard end of the upper member defines the outboard end of the brace and the strut also extends from the upper member to join the arm, for example by clamping around the arm. The inboard end of the arm and an inboard end of the upper member, defining the inboard end of the brace, can be joined simultaneously to the turbine support at the lower and upper fixing points respectively. However, the strut can be joined to the upper member and to the arm after joining the inboard end of the arm to the turbine support. Conveniently, the turbine support and / or each of the buoyancy structures can be manufactured by bundling a plurality of polymer or composite buoyancy tubes around a central core tube or steel or concrete. In that case, the central core tube may be longer than the buoyancy tubes to define the lower and / or upper spigot protruding longitudinally beyond the bundled buoyancy tubes. More generally, the turbine support may be of steel and / or concrete and each buoyancy structure may be of steel, optionally further including polymer or composite buoyancy tubes. Each arm and / or brace may comprise a steel tube, which can be double-skinned. For example, the steel tube may comprise inner and outer tubes defining an annular gap between them. One of the inner and outer tubes may be of polymer or composite material and the other of those tubes may be of steel. The inner tube may have a polygonal cross-section and the outer tube may have a circular or other elliptical crosssection. A hardened fluid such as grout or an epoxy resin can fill the gap between the inner and outer tubes. The method of the invention is apt to be performed at an assembly site adjoining a body of water, such as a shipyard. The turbine support can be constructed at the assembly site or transported to the assembly site across water. Conversely, the arms and the braces can be transported to the assembly site after constructing them at a facility remote from the assembly site. Similarly, the buoyancy structures, or at least polymer or composite buoyancy tubes of the buoyancy structures, can be transported to the assembly site after constructing them at a facility remote from the assembly site. Transportation from the remote facility to the assembly site can be effected across water or on land, in the latter case by road or rail. Correspondingly, the inventive concept embraces a floater for a FOWT, the floater comprising: a turbine support; a plurality of buoyancy structures; a plurality of elongate arms, each having an inboard end joined to the turbine support at a lower fixing point and an outboard end joined to a respective one of the buoyancy structures at an outer fixing point; and a plurality of elongate braces each joined to a respective one of the arms, each brace having an inboard end joined to the turbine support at an upper fixing point. The inboard end of each arm may be engaged with a respective lower interface such as a lower receptacle provided on the turbine support at the lower fixing point and the inboard end of each brace may be engaged with a respective upper interface such as an upper receptacle provided on the turbine support at the upper fixing point. At the outer fixing point, the outboard end of each arm may be engaged with a respective outer interface such as an outer receptacle provided on the respective buoyancy structure, or the buoyancy structure can be received in an outer receptacle provided on the arm, for example where a lower spigot of the buoyancy structure is inserted into the outer receptacle. A closure can be positioned to capture the buoyancy structure in the outer receptacle. An outboard end of each brace may be joined to an upper end of a respective one of the buoyancy structures, for example where an upper spigot of the buoyancy structure is inserted into an upper receptacle provided on the outboard end of the brace. The outboard end of the brace may be sandwiched between a platform and the upper end of the buoyancy structure. The brace may comprise a strut and an upper member joined to the strut, wherein an outboard end of the upper member defines the outboard end of the brace and the strut extends from the upper member to the arm. The strut may be clamped around the upper member and / or around the arm. Thus, the invention contemplates assembly of a multi-material hybrid floater. More specifically, the invention proposes an assembly sequence and associated fabrication techniques for a floating-wind floater of semi-submersible type, using multiple modules that can be made of different materials. The invention embodies the principle to adjust or to tailor materials to their function, using materials dedicated to the functions they serve. The sequence conceived for assembly and fabrication can be a primary design input to reduce final assembly constraints. In addition to optimising the cost and design of FOWT floaters, the invention proposes to industrialise production and to reduce costs by promoting access to multiple sources of materials and components. This reduces investment needs, diversifies sourcing and promotes sharing of subcontractor fabrication, thus reducing risks to supply. A floater assembly of the invention typically comprises a core element being a primary floater or support for supporting a wind turbine generator. To stabilise the floater assembly, this core element is connected to two or more secondary or outer floaters or buoyancy structures via at least two braces or arms. The core element can be centred with respect to the floater assembly, thus typically being disposed between three or more outer floaters. In this sense, the core element can be an inner element and the outer floaters can be outer elements. Alternatively, the core element can be off-centred with respect to the floater assembly, for example where there are two outer floaters. The core element may be, or may comprise, a steel can or a concrete reinforced tubular structure, or may be, or may comprise, an assembly comprising polymer tubes or composite buoyancy elements attached to a core structural member of steel or concrete. Similarly, each outer floater could be made of a core steel or concrete tube to which polymer tubes or composite buoyancy elements are attached. The arms that attach the outer floaters to the core element may be steel tubes or hybrid tubes. Hybrid tubes have a tube-in-tube subassembly defining an annulus that can be filled with concrete grout, or possibly a composite resin or foam-like epoxy resin. Main components made of different materials can be assembled by wedge junctions and / or grouted junctions or a combination of both wedging and grouting at one or more junctions, possibly with local clamping. A typical floater of the invention could have a central support column of concrete, braces or arms of steel tube, such as hybrid tubes, and external or secondary floaters. The outer floaters may be sub-assemblies with polymer or composite tubes grouped in bundles around a core or steel tube that provides for connection to the arms. The structural design of such a floater is driven by fabrication thinking. Specifically, the central support is apt to be fabricated locally or delivered to a port to minimise the need for transportation. The arms can be fabricated in steel by specialised yards or fabricators, being apt for high-density, high-volume work and being optimised for sea transport. Water pipes that can be used to construct outer floaters are apt to be transported by truck or in bulk. In an exemplary manufacturing sequence, core floaters or supports are fabricated in series at an assembly location such as a port situated close to the offshore installation site to avoid long-haul transport of heavy components. The assembly location could instead be a structural fabrication yard for delivering complex fabricated parts. The structural braces or arms may be prefabricated in a factory in series or in a dedicated station and delivered in batches to the assembly site. The outer floaters can also be delivered from a factory to the assembly site in batches. In some examples, the outer floaters can be preassembled by connecting polymer tubes to a core junction tube when preparing a stock of sub-assemblies. The support defining the core element can be placed in a launch-at-sea position, close to a quay for loadout inside or outside a dock. Then, the arms can be introduced and positioned in connection slots of the core support. For this purpose, the arms can be moved by multi-wheel vehicles or by cranes. The arms can then be grouted into the connection slots. Next, the outer floaters are landed at the external extremities of the arms and wedge-connected and / or grouted to the arms. Optionally, additionally bracing in the form of top junction arms may be attached to the core element and floated and landed on top of the outer floaters. Clamps and guiding shims can be incorporated in the arms. Then, all junctions can be grouted either progressively or sequentially or in parallel operations. Connections can be complemented by clamp bolting or wedging tubes. Embodiments of the invention contemplate making manufacturing modular by splitting the functions, materials, constructions and / or sourcing of the modules, those modules being: a main support column, which can be central or offset, that serves as a hub for the assembly and will carry the wind turbine tower; buoyancy columns; and connecting arms or braces such as lattice structures or beams, preferably extending radially from the main column to the buoyancy columns. The main column may be of steel and / or concrete and comprises interfaces required to connect to other parts of the assembly, particularly the arms. Each buoyancy column can be a steel or concrete tank or may comprise a steel or concrete core tube surrounded by plastic or composite buoyancy tubes. The arms or braces may be of steel, such as a hybrid steel structure. A main beam, which may be substantially horizontal, can be used in conjunction with bracing. Each such elongate member may comprise at least a tube, which can be reinforced by an internal structure. For example, a circular or elliptical tube may contain a polygonal tube of steel or polymer to define a double-skin tube, and a grout or other hardening fluid can be injected into the annulus or gap between those skins. Embodiments of the invention implement a method to assemble a FOWT, the method comprising: assembling radial structures comprising at least one radial truss and bracings; connecting a first end of the radial structures to a main column; and coupling buoyancy columns to a second end of the radial structures. Coupling or connection operations may involve welding and / or or clamping and grouting, for example by gluing or injecting epoxy resin into an inner volume of a clamp. Connecting the radial structures to the main column may comprise connecting an end of a radial truss to the main column, and coupling an end of any associated brace to the main column. Coupling a buoyancy column to a radial structure may comprise fitting a tubular end of the buoyancy column in a hole or a recess of the radial structure and either welding or grouting the assembly. The buoyancy columns could be interconnected by rods, beams, cables or tendons. Each radial structure may be pre-assembled by coupling the or each bracing to the radial truss. Extra bracing may connect the buoyancy column to the radial structure or to the main column. At least one of the radial trusses and bracings may be a doubleskin tube. Assembly of the radial structures may be completed after the buoyancy columns and the main column have been connected by the radial truss of the radial structure. Thus, a floater of the invention for a FOWT comprises a turbine support and buoyancy structures connected to the turbine support by respective arms, each joined to a brace that is also joined to the turbine support. The floater is constructed by joining an inboard end of each arm to the turbine support at a lower fixing point, joining an inboard end of each brace to the turbine support at an upper fixing point, and joining an outboard end of each arm to a respective one of the buoyancy structures at an outer fixing point. In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic plan view of a floater of the invention for an offshore wind turbine; Figure 2 is a schematic side view of the floater of Figure 1; Figure 3 is a sectional detail view taken on line III of Figure 1; Figure 4 is an exploded side view of components of the floater of Figures 1 to 3, showing some of those components being assembled together; Figure 5 is a schematic side view showing other components of Figure 4 being assembled together to build the floater; Figure 6 is a sectional detail view corresponding to Figure 3 but showing a variant of the floater; Figure 7 is a schematic plan view of an outer buoyancy structure of the floater shown in Figure 6; Figure 8 corresponds to Figure 7 but shows a variant of the outer buoyancy structure; Figure 9 is a schematic side view showing components being brought together to assemble the floater of Figure 6; Figure 10 is a sectional detail view of main column of a floater of the invention; Figure 11 is a cross-sectional view on line XI-XI of Figure 10; Figure 12 is a schematic side view of another floater of the invention; Figure 13 is a sectional detail view corresponding to Figure 3 but showing the variant of Figure 12; Figure 14 is an exploded side view of components of the floater of Figures 12 and 13, showing some of those components being assembled together; Figures 15 and 16 are schematic side views showing other components of Figure 14 being assembled together to build the floater; Figure 17 is a schematic side view of another floater of the invention; Figure 18 is a sectional detail view corresponding to Figure 3 but showing the variant of Figure 17; Figure 19 is an exploded side view of components of the floater of Figures 17 and 18, showing some of those components being assembled together; Figures 20 to 23 are schematic side views showing other components of Figure 19 being assembled together to build the floater; Figure 24 is a selection of cross-sectional views showing variants of structural members for floaters of the invention; and Figure 25 is a selection of perspective views showing some of the variants shown in Figure 24. In the various examples illustrated in the drawings, in which like numerals are used for like features, a floater 10 of the invention for a floating offshore wind turbine comprises outer buoyancy structures 12 that are angularly spaced, in plan view, around a turbine support 14 that serves as a core element or hub for the floater 10. The turbine support 14 is configured to support the mast or tower of the turbine (not shown) but in this example also contributes buoyancy to the floater 10, to the extent that the turbine support 14 is arranged to displace water in use. Moorings of the floater 10 can be conventional and so have also been omitted from the drawings for clarity. The buoyancy structures 12 and the turbine support 14 exemplified here are each of upright columnar configuration, being hollow elongate cylinders comprising tubular side walls that are closed at their ends to define respective sealed buoyancy chambers. The turbine support 14 extends along an upright central longitudinal axis 16 and the buoyancy structures 12 extend substantially parallel to that upright axis 16. When the floater 10 is floating level in a body of water 18, upper portions of the turbine support 14 and of the buoyancy structures 12 are designed to stand above the surface 20 of the water 18. This arrangement resists tilting pitch or roll movements of the floater 10 about horizontal axes. The floater 10 could be of all-steel construction but at least some components of the floater 10 could comprise other materials. For example, at least part of the turbine support 14 and / or the buoyancy structures 12 could comprise concrete and / or polymer tubes 42, as will be explained. Three buoyancy structures 12 are shown in the illustrated examples, equi-angularly spaced around the turbine support 14 in an equilateral triangular configuration. Each buoyancy structure 12 is connected to the turbine support 14 by an elongate tubular truss or arm 22 that extends radially from the turbine support 14 in plan view. In the illustrated examples, the arms 22 of each floater 10 are all of the same length and each arm 22 extends substantially horizontally from the turbine support 14. Each arm 22 is braced against bending loads by a tubular brace 24 that is positioned above the arm 22 in a common upright plane extending radially from, and containing, the central longitudinal axis 16 of the turbine support 14. Each brace 24 comprises, or consists of, a strut 26 that converges outwardly and downwardly with the associated arm 22 like a flying buttress before intersecting with the arm 22 inboard of the radially outer end of the arm 22. The strut 26 acts primarily in compression to transmit bending loads from the arm 22 into the turbine support 14. At a lower fixing point 28, the inboard end of each arm 22 is received by a respective lower receptacle 30 provided in a side wall of the turbine support 14. Similarly, the inboard end of each brace 24 is received by a respective upper receptacle 32 provided in a side wall of the turbine support 14 at an upper fixing point 34. When the floater 10 is moored offshore and in use, the lower fixing points 28 and hence the arms 22 may be submerged beneath the surface 20 whereas the upper fixing points 34 may be above the surface 20. In these examples, the receptacles 30, 32 each extend radially and orthogonally with respect to the central longitudinal axis 16 of the turbine support 14, hence extending substantially horizontally. Thus, the inboard ends of the arms 22 and the braces 24 are inserted into their respective receptacles 30, 32 in a radially inward direction, for example telescopically as shown. The arms 22 and the braces 24 can then be fixed in the receptacles 30, 32 mechanically, for example by clamping, and / or with grout or adhesive, such as an epoxy resin. Such a grout or adhesive is injected or otherwise applied into the annulus between an outer surface of the arm 22 or brace 24 and the surrounding wall of the receptacle 30, 32. Figures 1 to 5 of the drawings show a first embodiment of the invention, which may be of modular all-steel construction. Figure 1 includes exemplary dimensions to illustrate the massive scale of the floater 10, for example an overall width of more than 120m. By way of illustration, the turbine support 14 has an outer diameter of 12m, a height of 40m and a weight of around 1000t whereas each buoyancy structure 12 has an outer diameter of 8m, a height of 35m and a weight of around 400t. Each arm 22 is about 70m long and has an outer diameter of 4m, weighing about 470t, whereas each brace 24 is about 40m long and has an outer diameter of 2m, weighing about 80t. In this embodiment, each brace 24 is a strut 26 with a dogleg shape comprising a generally horizontal inboard portion, hence extending substantially parallel to the associated arm 22, and an inclined outboard portion converging with that arm 22. The inboard portion of each strut 26 is received in a respective upper receptacle 32 in the side wall of the turbine support 14. At an outer fixing point 36, an outboard end of each arm 22 is received in an outer receptacle 38 of the associated buoyancy structure 12. Again, the outer receptacle 38 is shown here extending horizontally so that the outboard end of the arm 22 is inserted horizontally. The arm 22 can then be fixed in the outer receptacle 38 mechanically and / or with grout or other adhesive, such as an epoxy resin, that is injected or otherwise applied into the annulus between an outer surface of the arm 22 and the surrounding wall of the outer receptacle 38. In this example, the outer receptacle 38 is a through-passage extending horizontally through the buoyancy structure 12 whereby the arm 22 can also extend through the full width of the buoyancy structure 12 in telescopic fashion. In this respect, it will be noted that the outboard end of the arm 22 is shown protruding from an outboard side of the buoyancy structure 12. However, the outer receptacle 38 could instead be a blind recess like the lower and upper receptacles 30, 32 of the turbine support 14. Thus, the outboard end of the arm 22 need not necessarily protrude from the outboard side of the buoyancy structure 12. Figures 4 and 5 show steps of a method for assembling the first embodiment at a coastal yard 40. The turbine support 14 and the buoyancy structures 12 can be fabricated at the yard 40 or fabricated at another yard and delivered, most conveniently by sea, to the yard 40 at which final assembly is performed. Conveniently, the turbine support 14 and the buoyancy structures 12 can be fabricated or transported in a horizontal orientation and then upended at the yard 40 into a vertical orientation, ready for assembly of the floater 10. Conversely, the arm 22 and the strut 26 are conveniently fabricated at a factory and delivered from there to the yard 40. Figure 4 shows the strut 26 being assembled with the arm 22, for example by welding, bonding and / or bolting at their junction where the outboard portion of the strut 26 adjoins the arm 22. Figure 5 then shows the assembly of the arm 22 and the strut 26 held in an upright plane and being joined to the lower and upper fixing points 28, 34 of the upright turbine support 14 by inserting and grouting the inboard ends of the arm 22 and the strut 26 into the lower and upper receptacles 30, 32. Similarly, Figure 5 shows the outboard end of the arm 22 being inserted and grouted into the outer receptacle 38 whereby the assembly of the arm 22 and the strut 26 is joined to the associated buoyancy structure 12 at the outer fixing point 36. For this purpose, Figure 5 shows the upright buoyancy structure 12 being moved horizontally toward the outboard end of the arm 22, such that the outer receptacle 38 receives the outboard end of the arm 22. As noted above, the floater 10 need not necessarily be of all-steel construction. As an example in this respect, Figures 6 to 9 show a variant of the first embodiment in which each buoyancy structure 12 is a bundled assembly of parallel polymer tubes 42 serving as auxiliary buoyancy elements fixed around a structural core tube 44 of steel or concrete. All of the tubes 42, 44 have closed ends to define respective sealed buoyancy chambers. Such a bundle structure may be light enough to be manufactured in a factory and delivered to the yard 40 at which final assembly is performed. Various bundle arrangements are possible. For example, there could be six polymer tubes 42 in a single-layer hexagonal bundle around the core tube 44 as shown in Figure 7, weighing around 80t, or eighteen polymer tubes 42 in a double-layer hexagonal bundle around the core tube 44 as shown in Figure 8. Non-hexagonal bundle arrangements are also possible; for example, the polymer tubes 42 need not be present in multiples of six. For optimal hexagonal bundling, the polymer tubes 42 are apt to have the same outer diameter as the core tube 44. In this example, the polymer tubes 42 and the core tube 44 each have a diameter of 2.7m. Conversely, the core tube 44 may be longer than the surrounding polymer tubes 42. For example, the core tube 44 may be 35m long whereas the polymer tubes 42 may be about 30m long. Thus, the core tube 44 projects longitudinally beyond the ends of the polymer tubes 42. Conveniently, the projecting end of the core tube 44 defines a lower spigot 46 that can be used to fix the buoyancy structure 12 to the associated arm 22 when the buoyancy structure 12 has been upended with the spigot 46 facing downwardly, as shown in Figure 9. Specifically, the arm 22 defines the outer receptacle 38 in this variant and receives the spigot 46 defined by the core tube 44. Thus, the load path of buoyant upthrust acting on the buoyancy structure 12 extends from the polymer tubes 42 to the arm 22 via the core tube 44. In this case, the outer receptacle 38 is a through-passage extending vertically through the full height or thickness of the arm 22 but could instead be a blind recess that only opens upwardly. The spigot 46 can then be fixed in the outer receptacle 38 mechanically and / or with grout or other adhesive, such as an epoxy resin, that is injected or otherwise applied into the annulus between an outer surface of the spigot 46 and the surrounding wall of the outer receptacle 38. Figures 10 and 11 show a variant of the turbine support 14 whose side wall is inset to define a circumferential recess 48 encircling the turbine support 14. The recess 48 accommodates a circumferential array of sealed polymer tubes 42 that extend parallel to the central longitudinal axis 16, in this example eight such tubes 42 serving as auxiliary buoyancy elements. The radially inner wall of the recess 48 may, for example, have an outer diameter of 6m, in which case the polymer tubes 42 may each have an outer diameter of 3m to maintain the overall diameter of the turbine support 14 at nominally 12m. Figures 12 to 16 show another embodiment of the invention in which the brace 24 of each arm 22 further comprises an upper member 50 extending from the turbine support 14 to the associated buoyancy structure 12. The upper member 50 extends generally parallel to the arm 22, hence substantially horizontally in this example, and the inclined strut 26 extends downwardly and outwardly from the upper member 50 to the arm 22. Here, therefore, the inboard end of the upper member 50 is received in the upper receptacle 32 at the upper fixing point 34 of the turbine support 14. In an assembly step shown in Figure 14, the strut 26 is welded or otherwise fixed at its upper, inboard end to the upper member 50 and at its lower, outboard end to the arm 22. Figure 14 also shows an assembly step of bundling polymer tubes 42 around a core tube 44 to form a buoyancy structure 12 that is shown completed and upended in Figure 15. These steps can be performed sequentially or in parallel. The bundled buoyancy structure 12 shown here is like that shown in Figures 6 to 9 in that the core tube 44 is again longer than the surrounding polymer tubes 42 and so projects longitudinally beyond the ends of the polymer tubes 42. In this case, however, the core tube 44 projects beyond both ends of the bundled polymer tubes 42. The projecting ends of the core tube 44 define a lower spigot 46 and an upper spigot 52 when the buoyancy structure 12 has been upended as shown in Figure 15. Figure 15 shows the assembly of the arm 22 and the brace 24 held in an upright plane and being joined to the lower and upper fixing points 28, 34 of the upright turbine support 14. This is done by inserting and grouting the inboard ends of the arm 22 and the upper member 50 of the brace 24 into the lower and upper receptacles 30, 32 in the side wall of the turbine support 14. Figure 16 shows how the lower spigot 46 defined by the core tube 44 of the buoyancy structure 12 can be used to fix the buoyancy structure 12 to the associated arm 22 whereas the upper spigot 52 can be used to fix the buoyancy structure 12 to the upper member 50 of the associated brace 24. Specifically, in this example, the lower spigot 46 is slid into a longitudinal slot 54 that serves as an outer receptacle 38 and that opens to the outboard end of the arm 22. The slot 54 is then closed by fitting an end closure 56 to the arm 22 to hold the lower spigot 46 captive in the slot 54. Conversely, the upper spigot 52 is received in a through-passage that extends vertically through the upper member 50 of the brace 24 to serve as an upper outer receptacle 38 but could instead be a blind recess that only opens downwardly. The lower and / or upper spigots 46, 52 can then be fixed in the outer receptacles 38 mechanically and / or with grout or other adhesive, such as an epoxy resin, that is injected or otherwise applied into the annulus between an outer surface of each spigot 46, 52 and the surrounding wall of the outer receptacle 38. A slot 54 and end closure 56 like those used for the lower spigot 46 could also, or instead, be used to capture the upper spigot 52 or indeed to capture a spigot 46 of a buoyancy structure 12 attached to an arm 22 or an upper member 50 of other embodiments. Figures 17 to 23 show a further embodiment of the invention in which, like the embodiment of Figures 12 to 16, the brace 24 of each arm 22 comprises an upper member 50 extending from the turbine support 14 to the associated buoyancy structure 12. Again, the inboard end of the upper member 50 is received in an upper receptacle 32 at the upper fixing point 34 of the turbine support 14. As also shown in the embodiment of Figures 12 to 16, there are bundled buoyancy structures 12 in which the core tube 44 projects longitudinally beyond the ends of the polymer tubes 42 to define lower and upper spigots 46, 52 engaged, respectively, with the arm 22 and the upper member 50. In this example, the lower and upper spigots 46, 52 engage in simple through-passages in the arm 22 and the upper member 50 but those outer receptacles 38 could instead be blind recesses or slots 54 closed with end closures 56, as in the preceding embodiment. The embodiment of Figures 17 to 23 differs from preceding embodiments in two main respects. One of those differences lies in how the strut 26 is attached to the upper member 50 of the brace 24 and to the arm 22. In this example, the strut 26 terminates at its inboard and outboard ends in tubular sleeves 58 that are oriented at matching angles offset from the longitudinal axis 16 of the strut 26, to align with and to encircle the upper member 50 and the arm 22 respectively. Each sleeve 58 is split longitudinally into two part-tubular sections, a first section 60 being fixed to the strut 26 and a second section 62 being cooperable with the first section 60 to complete the sleeve 58 around the upper member 50 or the arm 22 as appropriate. The sleeves 58 can then be fixed to the upper member 50 and the arm 22 mechanically, for example by clamping, and / or with grout or other adhesive, such as an epoxy resin, that is injected or otherwise applied into the annulus between an outer surface of the upper member 50 or the arm 22 and the surrounding wall of the sleeve 58. Another difference between the embodiment of Figures 17 to 23 and preceding embodiments is that the upper spigot 52 of the buoyancy structure 12 is engageable with a platform 64 that surmounts the buoyancy structure 12. For this purpose, the platform 64 and the upper spigot 52 have complementary engagement formations 66 in this example. Once the platform 64 is engaged with the upper spigot 52, the upper member 50 of the brace 24 is sandwiched between the platform 64 and the polymer tubes 42 of the buoyancy structure 12. The engagement formations 66 may be arranged to facilitate clamping the platform 64 and the buoyancy structure 12 together around the upper member 50. In an assembly step shown in Figure 19, polymer tubes 42 are bundled around a core tube 44 to form a buoyancy structure 12 shown completed and being upended in Figure 20. Figure 20 also shows the arm 22 being joined to the lower fixing point 28 of the upright turbine support 14 by inserting and grouting the inboard end of the arm 22 into the lower receptacle 30 in the side wall of the turbine support 14. Again, these steps can be performed sequentially or in parallel. Figure 21 shows the lower spigot 46 of the upright buoyancy structure 12 being inserted into an outer receptacle 38 of the arm 22. The lower spigot 46 can then be fixed in the outer receptacle 38 mechanically and / or with grout or other adhesive, such as an epoxy resin, that is injected or otherwise applied into the annulus between an outer surface of the lower spigot 46 and the surrounding wall of the outer receptacle 38. Figure 22 shows the upper member 50 of the brace 24 being joined to the upper fixing point 34 of the upright turbine support 14 by inserting and grouting the inboard end of the upper member 50 into the upper receptacle 32 in the side wall of the turbine support 14. The upper spigot 52 of the upright buoyancy structure 12 is inserted into an outer receptacle 38 of the upper member 50 and can then be fixed in the outer receptacle 38 mechanically and / or with grout or other adhesive, such as an epoxy resin, that is injected or otherwise applied into the annulus between an outer surface of the upper spigot 52 and the surrounding wall of the outer receptacle 38. Figure 22 also shows initial steps of installing the strut 26 of the brace 24, in which the strut 26 is lifted between the upper member 50 and the arm 22 and is then pivoted to bring the first sections 60 of the sleeves 58 at the ends of the strut 26 into embracing alignment with the upper member 50 and the arm 22. Figure 23 then shows a subsequent step of installing the strut 26, namely fixing the second sections 62 to the first sections 60 complete the sleeves 58. Figure 23 also shows the platform 64 being attached to the upper spigot 52 of the buoyancy structure 12, clamping the upper member 50 between the platform 64 and the polymer tubes 42 of the buoyancy structure 12. Turning finally to Figures 24 and 25, these drawings show possible configurations of the invention for tubular structural members 68 of the floater 10 such as the arms 22, the struts 26 and / or the upper members 50 of the brace 24. In each case, the structural member 68 has a double-walled hollow tubular structure in which an inner wall 70 of polygonal cross-section is disposed within an outer wall 72 of circular or other elliptical cross-section. Figure 24 exemplifies the polygonal cross-section of the inner wall 70 as triangular (variants a, e), square (variants b, f), hexagonal (variants c, g) or octagonal (variants d, h), although other polygonal sections with odd or even numbers of sides are possible instead. The vertices of the polygonal cross-section are joined to the interior of the outer wall 72 either directly as shown in the first group of variants (a, b, c, d) in Figure 24 or via short radial webs 74 as shown in the second group of variants (e, f, g, h) in Figure 24. Such webs 74 can be longitudinally continuous strips extending along the structural member or could instead be longitudinally interrupted. Thus, the inner wall 70 can have an outer diameter that substantially matches the inner diameter of the outer wall 72 or, by virtue of the webs 74, can have an outer diameter that is substantially less than the inner diameter of the outer wall 72. The inner and outer walls 70, 72 and any webs 74 may conveniently be of steel. The inner wall 70 and the webs 74 may, for example, be fabricated of steel plate. The disparity between the cross-sections of the inner and outer walls 70, 72 defines voids 76 of substantially segment-shaped cross-section between the inner and outer walls 70, 72, each void 76 corresponding to a respective side of the relevant polygon. Figure 25 shows variant (i) corresponding to variant (b) of Figure 24, variant (ii) corresponding to variant (c) of Figure 24 and variant (iii) corresponding to variant (h) of Figure 24. As shown in these examples, the voids 76 can be filled with a filler material that is initially in a flowable fluid state but cures, sets or solidifies in situ to form a rigid infill 78 in the segmented annulus defined by the voids 76 between the inner and outer walls 70, 72. The infill 78 thereby imparts additional rigidity to the structural member 68. Examples of such filler material include grouts, epoxy resins or cement mixtures that set to form a solid mass such as concrete within the voids. Thus, the resulting structural member 68 has a composite structure. Many other variations are possible within the inventive concept. For example, the buoyancy structures need not be equiangularly spaced, there could be four or more buoyancy structures, or there could be two buoyancy structures in addition to the 5 support structure in a triangular arrangement, whether equilateral or otherwise. Thus, the support structure need not be disposed centrally with respect to the buoyancy structures even though the illustrated examples show such central disposition. Similarly, the arms of each floater are all of the same length in the illustrated examples but could have different lengths in other examples or could be inclined to the horizontal. 10 The arms and the braces can be attached to the support structure and the buoyancy structures at their respective fixing points in other ways, without necessarily requiring them to be received in receptacles or other openings. 15 The braces can be joined to the arms via the buoyancy structures in addition to or instead of the direct connection between the braces and the associated arms shown in the exemplary embodiments.

Claims

1. A method of constructing a floater for a FOWT, the floater comprising a turbine support and a plurality of buoyancy structures connected to and spaced from the turbine support by respective elongate arms each joined to a respective elongate brace, the method comprising:joining an inboard end of each arm to the turbine support at a lower fixing point;joining an inboard end of each brace to the turbine support at an upper fixing point; andjoining an outboard end of each arm to a respective one of the buoyancy structures at an outer fixing point.

2. The method of Claim 1, comprising inserting the inboard end of each arm into a respective lower receptacle provided on the turbine support at the lower fixing point and inserting the inboard end of each brace into a respective upper receptacle provided on the turbine support at the upper fixing point.

3. The method of Claim 2, comprising inserting the inboard ends of the arm and the brace into the lower and upper receptacles respectively in a substantially horizontal direction.

4. The method of Claim 2 or Claim 3, comprising simultaneously inserting the inboard ends of the arm and the brace into the lower and upper receptacles respectively.

5. The method of Claim 4, comprising joining the brace to the arm to form an assembly before joining that assembly to the turbine support.

6. The method of any preceding claim, comprising inserting the outboard end of each arm into an outer receptacle provided on the respective buoyancy structure at the outer fixing point.

7. The method of Claim 6, comprising inserting the outboard end of the arm into the outer receptacle in a substantially horizontal direction.

8. The method of Claim 6 or Claim 7, comprising inserting the outboard end of the arm into the outer receptacle to extend through a full width of the respective buoyancy structure.

9. The method of any of Claims 1 to 5, comprising inserting the buoyancy structure into an outer receptacle provided on the arm at the outer fixing point.

10. The method of Claim 9, comprising inserting a lower spigot of the buoyancy structure into the outer receptacle.

11. The method of Claim 10, comprising inserting the lower spigot of the buoyancy structure into the outer receptacle in a substantially vertical direction.

12. The method of Claim 10, comprising inserting the lower spigot of the buoyancy structure into the outer receptacle in a substantially horizontal direction.

13. The method of any of Claims 9 to 12, comprising positioning a closure to capture the buoyancy structure in the outer receptacle.

14. The method of any preceding claim, comprising joining an outboard end of each brace to an upper end of a respective one of the buoyancy structures.

15. The method of Claim 14, comprising inserting an upper spigot of the buoyancy structure into an upper receptacle provided on the outboard end of the brace.

16. The method of Claim 14 or Claim 15, comprising sandwiching the outboard end of the brace between a platform and the upper end of the buoyancy structure.

17. The method of Claim 16, comprising coupling the platform to the buoyancy structure.

18. The method of Claim 17, comprising coupling the platform to the buoyancy structure via the upper receptacle of the brace.

19. The method of any of Claims 14 to 18, comprising joining a strut of the brace to an upper member of the brace, wherein an outboard end of the upper member defines the outboard end of the brace and the strut extends from the upper member to the arm.

20. The method of Claim 19, comprising clamping the strut to the upper member.

21. The method of Claim 19 or Claim 20, comprising simultaneously joining the inboard end of the arm and an inboard end of the upper member, defining the inboard end of the brace, to the turbine support at the lower and upper fixing points respectively.

22. The method of Claim 19 or Claim 20, comprising joining the strut to the upper member and the arm after joining the inboard end of the arm to the turbine support.

23. The method of any preceding claim, comprising clamping the brace to the arm.

24. The method of any preceding claim, comprising manufacturing the turbine support and / or each of the buoyancy structures by bundling a plurality of polymer buoyancy tubes around a central core tube or steel or concrete.

25. The method of Claim 24, wherein the central core tube is longer than the buoyancy tubes to define at least one spigot protruding longitudinally beyond the bundled buoyancy tubes.

26. The method of any preceding claim, comprising joining the inboard ends of the arm and / or the brace to the turbine support by clamping and grouting.

27. The method of any of Claims 1 to 25, comprising joining the inboard ends of the arm and / or the brace to the turbine support by welding.

28. The method of any preceding claim, wherein the turbine support is of steel and / or concrete and each buoyancy structure is of steel, optionally further including polymer buoyancy tubes.

29. The method of any preceding claim, wherein each arm and / or brace comprises a steel tube.

30. The method of any preceding claim, performed at an assembly site adjoining a body of water.

31. The method of Claim 30, comprising constructing the turbine support at the assembly site.

32. The method of Claim 30, comprising transporting the turbine support to the assembly site across water.

33. The method of any of Claims 30 to 32, comprising transporting the arms and braces to the assembly site after constructing them at a facility remote from the assembly site.

34. The method of any of Claims 30 to 33, comprising transporting the buoyancy structures to the assembly site after constructing them at a facility remote from the assembly site.

35. The method of any of Claims 30 to 33, comprising transporting buoyancy tubes of the buoyancy structures to the assembly site from a facility remote from the assembly site.

36. A floater for a FOWT, the floater comprising:a turbine support;a plurality of buoyancy structures;a plurality of elongate arms, each having an inboard end joined to the turbine support at a lower fixing point and an outboard end joined to a respective one of the buoyancy structures at an outer fixing point; anda plurality of elongate braces each joined to a respective one of the arms, each brace having an inboard end joined to the turbine support at an upper fixing point.

37. The floater of Claim 36, wherein the inboard end of each arm is inserted into a respective lower receptacle provided on the turbine support at the lower fixing point andthe inboard end of each brace is inserted into a respective upper receptacle provided on the turbine support at the upper fixing point.

38. The floater of Claim 36 or Claim 37, wherein the outboard end of each arm is inserted into an outer receptacle provided on the respective buoyancy structure at the outer fixing point.

39. The floater of Claim 38, wherein the outboard end of each arm extends through a full width of the respective buoyancy structure.

40. The floater of Claim 36 or Claim 37, wherein the buoyancy structure is inserted into an outer receptacle provided on the arm at the outer fixing point.

41. The floater of Claim 40, wherein a lower spigot of the buoyancy structure is inserted into the outer receptacle.

42. The floater of Claim 40 or Claim 41, further comprising a closure that can be positioned to capture the buoyancy structure in the outer receptacle.

43. The floater of any of Claims 36 to 42, wherein an outboard end of each brace is joined to an upper end of a respective one of the buoyancy structures.

44. The floater of Claim 43, wherein an upper spigot of the buoyancy structure is inserted into an upper receptacle provided on the outboard end of the brace.

45. The floater of Claim 43 or Claim 44, wherein the outboard end of the brace is sandwiched between a platform and the upper end of the buoyancy structure.

46. The floater of any of Claims 43 to 45, wherein the brace comprises a strut and an upper member joined to the strut, wherein an outboard end of the upper member defines the outboard end of the brace and the strut extends from the upper member to the arm.

47. The floater of Claim 46, wherein the strut is clamped to the upper member and / or to the arm.

48. The floater of any of Claims 36 to 47, wherein the turbine support and / or each of the buoyancy structures comprises a plurality of polymer or composite buoyancy tubes bundled around a central core tube or steel or concrete.

49. The floater of Claim 48, wherein the central core tube is longer than the buoyancy tubes to define at least one spigot protruding longitudinally beyond the bundled buoyancy tubes.

50. The floater of any of Claims 36 to 49, wherein the inboard ends of the arm and / or the brace are joined to the turbine support by grout.

51. The floater of any of Claims 36 to 50, wherein the turbine support is of steel and / or concrete and each buoyancy structure is of steel, optionally further including polymer or composite buoyancy tubes.

52. The floater of any of Claims 36 to 51, wherein each arm and / or brace comprises a steel tube.

53. The floater of Claim 52, wherein the steel tube comprises inner and outer tubes defining an annular gap between them.

54. The floater of Claim 53, wherein the inner tube has a polygonal cross-section and the outer tube has an elliptical cross-section.

55. The floater of Claim 53 or Claim 54, comprising a hardened fluid filling the gap between the inner and outer tubes.

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