Floating wind turbine foundations

Abstract

A floatable foundation (100, Fig 1) for a wind turbine generator comprises: a central column 10 having an upper support structure (11, Fig 1) and a lower support structure 12; at least three outer col

Description

The present disclosure relates to floating wind turbine foundations, including but not limited to floatable foundations for wind turbine generators, methods of constructing floatable foundations, methods of designing floatable foundations, and methods of producing electric power. BACKGROUND Floating wind turbine systems are under development by a number of industrial players, and provide a promising option for offshore electric power generation. Floating wind turbine systems rely on a moored, buoyant substructure base, onto which a wind turbine is mounted. Publications which may be useful to understand the field of technology include: US 2016 / 0341182 A1; US 2022 / 0348288 A1; FR 3 109 924 A1; WO 2009 / 131826 A2; WO 2021 / 148156 A1; WO 2013 / 110276 A1; WO 2020 / 167137 A1; EP 4 155 538 A1; WO 2023 / 014230 A1; WO 2024 / 172662 A1; WO 2024 / 170846 A1; and WO 2023 / 009010 A1. Such floating wind turbine systems are in many cases not presently cost-competitive compared to bottom-fixed systems, and not yet in widespread commercial use. However it is expected that further development of floating offshore wind technology can make such systems more competitive and a viable alternative at many locations in the near future. This applies particularly to offshore sites with large water depths (typically more than 50-70 m), which may be unsuitable for bottom-fixed installations. With a projected continued increase in the need for renewable electric power generation in the future, there is a need for further improved technology in this area. The present disclosure has the objective to provide such improvements, or at least useful alternatives, to the current state of the art. SUMMARY In an example, there is provided a floatable foundation for a wind turbine as set out in appended claim 1, a method of producing electric power as set out in appended claim 23 and a method of designing a floatable foundation for a wind turbine generator as set out in appended claim 24. In one aspect, there is provided a floatable foundation for a wind turbine generator, the foundation comprising: a central column having an upper support structure and a lower support structure; at least three outer column members disposed about the central column; at least three horizontally extending lower connection members, each horizontally extending lower connection member fixed to and extending between the lower support structure and a respective one of the at least three outer column members; at least three horizontally extending beam members, each horizontally extending beam member fixed to and extending between the upper support structure and a respective one of the at least three outer column members; wherein: each of the at least three horizontally extending lower connection members comprises a pair of lower connection members; and / or each of the at least three horizontally extending beam members comprises a pair of beam members. The present invention also provides a method of producing electric power, the method comprising operating a floatable foundation in accordance with the invention having a wind turbine generator arranged thereon at an offshore location to produce electric power and, optionally, comprising receiving the electric power at a land-based location for supply into a land-based electricity grid. The present invention also provides a method of designing a floatable foundation for a wind turbine generator, the method comprising: (a) generating a three-dimensional (3D) computer model of a foundation according to the present invention; (b) providing a set of metocean data, the set of metocean data being representative of environmental conditions at a proposed operational site for the foundation; (c) using the three-dimensional (3D) computer model and the set of metocean data, simulating an operation of the floatable foundation at the proposed operational site under the environmental conditions; and (d) based on the simulation, determining a parameter indicative of stress and / or fatigue loads on the floatable foundation under the environmental conditions at the proposed operational site. The sub-claims define further optional features of the invention. The detailed description below and appended claims outline further inventive aspects and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS The above and other characteristics will become clear from the following description of illustrative, non-restrictive examples, with reference to the attached drawings, in which: Fig. 1 is a perspective view of a first embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 2 is a perspective view, to an enlarged scale, of a beam member which forms part of the floatable foundation of Fig. 1; Fig. 3 is a further perspective view of the floatable foundation of Fig. 1; Fig. 4 is a perspective view from above, partly cut away and to an enlarged scale, of the lower central portion of the floatable foundation of Fig. 1; Fig. 5 is a perspective view from below showing the interior of a pontoon member which forms part of the floatable foundation of Fig. 1; Fig. 6 is a perspective view from above showing the interior of a pontoon member which forms part of the floatable foundation of Fig. 1; Fig. 7 is a perspective view of a second embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 8 is a perspective view, to an enlarged scale, of a beam member and a column member which forms part of the floatable foundation of Fig. 7; Fig. 9 is a perspective view of a third embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 10 is a perspective view, to an enlarged scale, of a beam member and a pontoon member which forms part of the floatable foundation of Fig. 9; Fig. 11 is a perspective view, partly cut away, of a first part of a fourth embodiment of floatable foundation for a wind turbine generator according to the present invention according to the present invention; Fig. 12 is a perspective view, partly cut away, of a second part of the floatable foundation of Fig. 11; Fig. 13 is a perspective view from below of a fifth embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 14 is a perspective view from above of the floatable foundation of Fig. 13; Fig. 15 is a perspective view from above of a sixth embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 16 is a perspective view from above of a portion of an seventh embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 17 is a perspective view from above of a portion of a eighth embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 18 is a perspective view from above of a portion of a ninth embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 19 is a perspective view, to an enlarged scale, of a connecting portion of the embodiment of Fig. 18; Fig. 20 is a an exploded perspective view of the connecting portion of Fig. 19; Fig. 21 is a perspective view from above of a portion of an tenth embodiment of floatable foundation for a wind turbine generator according to the present invention; Fig. 22 is a perspective view to an enlarged scale of a portion of the floatable foundation of Fig. 21, showing the connection between the end of a beam member and an associated buoyant column; Fig. 23 is a perspective view, to an enlarged scale, showing a modification of the a connecting portion of the embodiment of Fig. 21; and Fig. 24 is a perspective view from above of a portion of an eleventh embodiment of floatable foundation for a wind turbine generator according to the present invention. DETAILED DESCRIPTION Figs 1 - 6 illustrate a floatable (i.e. capable of floating or buoyant) foundation 100 for a wind turbine generator. The foundation 100 provides a base for a wind turbine generator tower 16, onto which a wind turbine generator (not shown) can be mounted. The foundation comprises a central column 10 having an upper support structure 11. The central column 10 is in this example a predominantly circular, tubular central column, however may optionally have a different shape, such as a polygonal crosssection. In this example, the upper support structure 11 is provided as a collar-like structure arranged about a tubular part of the central column 10. The central column 10 further has a lower support structure 12, which is vertically spaced from the upper support structure 11. Three outer column members 20,21,22 are disposed about the central column 10. Each outer column member 20,21,22 is fixed to the central column 10 by means of a horizontally extending lower connection member in the form of a pontoon member 30,31,32 and two horizontally extending beam members 40a-b,41a-b,42a-b. In this embodiment, and in the following embodiments, the lower connection members 30,31,32, are shown as pontoon members 30,31,32. The embodiments are described with the lower connection members being pontoon members 30,31,32, but it is to be understood that the lower connection members 30,31,32 may, alternatively, be beam members, pairs of beam members, or another appropriate structure. Each horizontally extending pontoon member 30,31,32 is fixed to and extend between the lower support structure 12 and a respective one of the three outer column members 20,21,22. In this embodiment, each of the column members 20, 21, 22 is a regular hexagon in cross-section and each of the pontoon members 30, 31, 32 is fixed to a radially inner panel 20x, 21x, 22x of the associated column member 20, 21, 22 which extends perpendicularly to the longitudinal axis of the pontoon member. Each pair of horizontally extending beam members 40a-b,41a-b,42a-b is fixed to and extends between the upper support structure 11 and a respective one of the three outer column members 20,21,22. In this embodiment, each of the column members 20, 21,22 is a regular hexagon in cross-section and each of the pair of beam members is fixed to a respective one of the two panels 20y, 20z; 21 y, 21z; 22y, 22z on either side of the radially inner panel 20x, 21x, 22x. A flange 15 is arranged on the central column 11 and configured for connection of the wind turbine generator tower 16. The central column 10 may have a tubular part which extends through the upper support structure 11, and on which the flange 15 is arranged. Fig. 4 illustrates details of the lower support structure 12, the interface between the tubular part of the central column 10 and the lower support structure 12, and further the interfaces to the pontoon members 30,31,32. The lower support structure 12 in this example is made up of a plurality of vertically arranged, interconnected flat panels 12a-f arranged about the tubular part of the central column 10. The vertically arranged, interconnected flat panels 12a-f form a polygonal, in this example a hexagonal, collar about the tubular part of the central column 10. A plurality of vertically arranged support plates 12g-l extend radially outwardly between the central column 10 and the interconnected flat panels 12a-f. Advantageously, each of the support plates 12g-l is fixed at an interface between two interconnected flat panels 12a-f, i.e. at the vertices of the polygonal structure formed by the flat panels 12a-f. Alternatively, or additionally, support plates 12g-l can be arranged at other locations on the flat panels 12a-f. The tubular part of the central column 10 may stop inside the lower support structure 12, for example where a lower end of the tubular part of the central column 10 is fixed to an upward-facing surface of a bottom plate of the lower support structure 12. Alternatively, the tubular part of the central column 10 may be positioned inside the lower support structure 12 and fixed inside the lower support structure 12 by other means, such as via the plurality of vertically arranged support plates 12g-l. In yet another alternative, the tubular part of the central column 10 may be fixed on a top plate of the lower support structure 12, i.e. not extending into the lower support structure 12. As can be seen in Figs 1 - 4, each pontoon member 30,31,32 is fixed at one respective flat panel 12b,d,f of the lower support structure 12 and extends outwardly to a respective one of the column members 20,21,22. The flat panels 12a,c,e form adjacent faces located between pairs of pontoon members 30,31,32. The upper support structure 11 may be constructed similarly as the lower support structure 12, having a similar or identical structural design, including flat panels similarly as flat panels 12a-f described in relation to lower support structure 12 above. Internal support plates, for example similar to support plates 12g-l, may also be used. The tubular part of the central column 10 may extend through the upper support structure 11 and be fixed to the upper support structure 11 via such support plates in this configuration, such that the upper support structure 11 forms a collar arranged about the tubular part of the central column 10. The flange 15 is in this example arranged on the tubular part of the central column 10, as illustrated in Figs 1-3. Alternatively, the tubular part of the central column 10 may be fixed at a downward-facing surface of the upper support structure 11. The flange 15 may in such a case be fixed at the upper support structure 11 or an extension thereof. Each beam member 40a-b,41a-b,42a-b is fixed at one or more flat panel(s) of the upper support structure 11. Each pair of beam members 40a-b,41a-b,42a-b may be arranged such that the two beam members 40a-b,41a-b,42a-b in the pair are arranged in parallel. Each pair of beam members 40a-b,41a-b,42a-b may be made up of two beam members 40a-b,41a-b,42a-b having identical design, such as having the same diameter and length. Illustrated in Figs 5 and 6, each of the pontoon members 30,31,32 is in this example a box beam made up of four outer flat panels, the four outer flat panels defining a top wall 32a, a bottom wall 32b, and a pair of side walls 32c,d. The pontoon members 30,31,32 may have internal strengthening features, for example plate-like annular strengthening ribs 34 interconnecting the top, bottom and side walls 32a-d and / or strengthening ribs 36 on one or more of the top, bottom and side walls 32a-d extending in the longitudinal direction of the pontoon members, as illustrated in Figs 4-6. As best seen in Figs. 4 - 6, the pontoon members 30,31,32 may optionally comprise at least one internal corner plate 49 fixed between a side wall 32c,d and a top or bottom wall 32a,32b of the pontoon member 30,31,32. A corner plate 49 may be arranged at each internal corner of the pontoon members 30,31,32, such that each pontoon member 30,31,32 has four internal corner plates 49. The lower support structure 12 may further comprise at least one corner plate 48 arranged between a top and / or bottom plate of the lower support structure 12 and the corresponding flat panels 12a,12c,12e which lie between the pontoon members 30,31,32, i.e. those flat panels 12a,12c,12e which are located adjacent to the flat panels 12b,12d,12f to which the pontoon members 30,31,32 are fixed. In this manner, the corner plates 48,49 can provide advantageous structural features at the lower support structure 12 and pontoon members 30,31,32, and in the interface between these. Advantageously, the width of the interconnected flat panels 12a-f forming part of the lower support structure 12 may be the same as the width b (see Fig. 5) of the pontoon members 30,31,32. Additionally, or alternatively, the height of the interconnected flat panels 12a-f forming part of the lower support structure 12 may be the same as the height h of the pontoon members 30,31,32. The outer column members 20,21,22 may have the form of a polygonal prism comprising a plurality of rectangular side wall panels. Particularly, each of the three outer column members 20,21,22 can be a right prism and / or have a constant crosssection in the horizontal plane. Each outer column member 20,21,22 comprises one or more rectangular side wall panel(s) onto which the respective pontoon member 30,31,32 and beam members 40a-b,41a-b,42a-b are fixed. The rectangular side wall panel(s) onto which the pontoon member 30,31,32 and the beam members 40a-b,41a-b,42a-b are fixed may, for each of the column members 20,21,22, be specifically prepared for this purpose, for example by means of additional internal strengthening members, a prepared connection profile at the side wall panel(s), or other features enabling such fixation. Advantageously, the dimensions of the various parts can be designed to provide manufacturing advantages. For example, a width of the one rectangular side wall panel onto which the pontoon member 30,31,32 is fixed can be made equal to the width b of the respective pontoon member 30,31,32. The width b of the pontoon member 30,31,32 can further be equal to a width of the one respective flat panel of the lower support structure 12 onto which the pontoon member 30,31,32 is fixed. This may facilitate efficient supply chains, manufacturing and construction. In an advantageous example, the column members 20,21,22 and the lower support structure 12 have an identical polygonal cross-sectional outline profile in the horizontal plane. This may include having the same size in the horizontal outline, i.e. the same polygonal side lengths. The column members 20,21,22 and the lower support structure 12 may all have a horizontal outline in the form of a regular polygon, for example having a hexagonal cross-sectional profile. The tubular part of the central column 10 may have a constant diameter between the lower support structure 12 to a position above the upper support structure 11 (for example up to the flange 15). Advantageously, this also facilitates efficient supply lines, manufacturing and construction. Alternatively, the central column 10 may be a polygonal central column. Any of the aspects, examples or embodiments described or claimed herein may be realised with the central column 10 of the illustrated examples replaced by a central column which is at least partly prismatic. Advantageously, the central column may have the same horizontal cross-section as the lower support structure 12 and the upper support structure 11 along its height up to and including the upper support structure 11. (A tubular or other part may nevertheless be fixed to the central column 10 above the upper support structure 11 for fixing the wind turbine generator tower 16 thereto.) The upper and lower support structures 11,12 may in such a case be formed integrally with the (part of the) polygonal central column having a form of a right prism. Additionally or alternatively, the central column (or a prismatic part thereof) may have the same horizontal cross-section as each of the column members 20,21,22. The central column, or a prismatic part thereof, may also have the same height as each of the column members 20,21,22. In this manner, efficient manufacturing of the column members 20,21,22 and the central column can be done, by using the same plate dimensions and / or same construction equipment. Illustrated in Fig. 3, in any of the examples or embodiments described or claimed herein, the floatable foundation 100 may have an inner corner support 70 between the pontoon members 30,31,32 and the respective column member 20,21,22. Each of the inner corner supports 70 may comprise a rectangular plate fixed to the same side wall panel of the column member 20,21,22 onto which the pontoon member 30,31,32 is fixed. The rectangular plate is further fixed to the pontoon member 30,31,32 at a top wall thereof. The rectangular plate can, for example, be a steel plate which is welded to the column member 20,21,22 and to the pontoon member 30,31,32. The rectangular plate advantageously extends from the column side wall panel towards the pontoon member 30,31,32 with an angle between 30 and 60 degrees, such as 45 degrees. In this manner, an effective load transfer can be provided. The rectangular plate may extend across the full width, or substantially the full width (such as at least 95%) of the column side wall panel and the pontoon member 30,31,32. The inner corner supports 70 may further comprise a pair of side plates, and the rectangular plate can be arranged between the side plates and welded to respective inward-facing faces of the side plates. The rectangular plate may thus be welded at all its four edges: At two opposite edges to the side plates, at one edge to the column member 20,21,22 and at one edge to the pontoon member 30,31,32. Each side plate has a first edge welded to the column member 20,21,22 and a second edge welded to the pontoon member 30,31,32. The first edge may, for example, be welded to the same rectangular side wall panel onto which the pontoon member 30,31,32 is fixed. The second edge may, for example, be welded to a top wall of the pontoon member 30,31,32. In the embodiment of Figs. 7 and 8, the tubular beam members 41a-b, 42a-b, 43a-b of the first embodiment are replaced with identical beam members 51a-b, 52a-b, 53a-b of constant square cross-section. As best seen in Fig. 8, the overall width w1 of the beam members 51a-b, 52a-b, 53a-b (i.e. the spacing between the parallel laterally outermost faces 54, 55 of the beam members) is equal to the spacing w2 between opposed vertices 56, 58 of the associated column member 20, 21, 22. As a consequence, the laterally outermost faces 54, 55 of the beam members are joined to the associated column member 20,21, 22 at opposed vertices 56, 57 of the column member. In addition, in this embodiment the spacing d between the laterally innermost faces 62, 64 of the beam members 51a-b, 52a-b, 53a-b is equal to the width of the innermost face 66 of the associated column member 20, 21, 22. As a consequence, the laterally innermost faces 62, 64 of the beam members are joined to the associated column member 20, 21, 22 at adjacent vertices 68, 70 of the column member. In addition, in this embodiment the spacing d between the laterally innermost faces 62, 64 of the beam members is equal to the width w4 of the pontoon members 30, 31, 32. However, the spacing d may be greater than the width w4 of the pontoon members. As a further consequence of the above, if the column 20, 21, 22 has the shape of a regular hexagon in cross-section, beam members 51a-b, 52a-b, 53a-b having a width w3 equal to Vi w2 will have their laterally outermost faces 54, 56 joined to the associated column member at opposed vertices 58, 60 of the column member and their laterally innermost faces 62, 64 joined to the associated column member at adjacent vertices 68, 70 of the column member. In this embodiment, it should also be noted that the width w3 of each of the beam members 51a-b, 52a-b, 53a-b is equal to half the width w4 of the pontoon members 30, 31, 32. Furthermore, the faces 72 of the support structure 11 which face the columns 20, 21, 22 are flat faces extending perpendicularly to the longitudinal axes of the beam members 51a-b, 52a-b, 53a-b and perpendicularly to the laterally innermost faces 62, 64 of the beam members. In the embodiment of Figs. 9 and 10, the one-piece pontoon members 30, 31, 32 of the first and second embodiments are replaced with identical split, two-piece pontoons 76, 78, 80, each formed from two elongate pontoon members 76a-b, 78a-b, 80a-b extending between the lower support structure 12 and the respective column member 20, 21, 22. Alternatively, the members 76a-b, 78a-b, 80a-b may be beam members (such as box beams or tubular beams) or another suitable connection structure. The embodiment of Figs. 11 and 12 is a variation of the first embodiment, and the same reference numerals are used to identify corresponding features. In addition to the features of the first embodiment, it will be noted that the interiors of the one-piece pontoon members 30, 31, 32 are provided with a plurality of plates 86, 88, 90 welded to, and extending perpendicularly to, the top wall 32a, bottom wall 32b and side walls 32c,d to form a plurality of watertight dividers and which divide the pontoon members into separate watertight sections. The first plate 86 is positioned at the junction of the pontoon members 30, 31, 32 with the lower support structure 12, the second plate 88 is positioned approximately halfway along the pontoon members 30, 31, 32 and the third plate 90 is located at a position where the inner corner support 70 meets the top wall 32a of the pontoon members 30, 31, 32. Similarly, the interiors of each of the column members 20, 21,22 are provided with a plurality of plates 92, 94 welded to, and extending perpendicularly to, the six planar walls forming the hexagonal column members 20, 21, 22 to form a plurality of watertight dividers and which divide the interior of the column members into separate watertight sections. The first plate 92 is positioned approximately one third of the way up the column members 20, 21, 22 and the second plate 94 is positioned approximately two thirds of the way up the column members 20, 21,22. The plates 86, 88, 90, 92, 94 help to retain the buoyancy of the pontoon members 30, 31, 32 and the column members 20, 21, 22 in the event of a leak. The embodiment of Figs. 13 and 14 is a further variation of the first embodiment, and the same reference numerals are used to identify corresponding features. In addition to the features of the first embodiment, two reinforcing wing plates 100 are welded to and project perpendicularly from each of the column members 20, 21, 22, the associated inner corner support 70 and the associated pontoon member 30, 31, 32. More specifically, each wing plate 100 comprises a straight base portion 102 having the same length as the inclined edges of the inner corner support 70, a shorter upper straight portion 104 inclined to the base portion 102 at the same angle as the column members 20, 21, 22 and a shorter lower straight portion 106 inclined to the base portion 102 at the same angle as the pontoon members 30, 31, 32. The base portion 102 of each wing plate 100 is welded to an inclined edge of an inner corner support 70, to the adjacent vertex / corner of the associated column member 20, 21,22 and to the adjacent corner of the associated pontoon member 30, 31, 32. It will also be observed that an enclosing plate lid 110 is welded to the inner end of each of the pontoon members 30, 31, 32 and to the outer cylindrical surface of the central column 10, and preferably resting on the upper edges of the support plates 12g-l, in order to exclude water from the region where the pontoon members 30, 31, 32 are joined to the central column 10, thereby reducing the likelihood of corrosion and the buildup of unwanted material in that region. The embodiment of Fig. 15 is a modification of the first embodiment, and the same reference numerals are used to identify corresponding features. In addition to the features of the first embodiment, reinforcing, strengthening cross-members 114 extend between, and are welded to, each pair of parallel beam members 40a-b,41a-b,42a-b of each beam 40, 41, 42. In this particular embodiment, three reinforcing, strengthening cross-members 114 extend between each pair of parallel beam members in a zigzag formation, with the inner and outer ends of the central crossmember 114 abutting the outer end of the innermost cross-member 114 and the inner end of the outermost cross-member 114 respectively. The inner end of the innermost cross-member 114 is secured to the junction of one of the beam members 40b, 41b, 42b and the upper support structure 11 and the outer end of the outermost crossmember 114 is secured to the junction of the other of the beam members 40a, 41a, 42a and the associated column member 20, 21, 22. The embodiment of Fig. 16 is another modification of the first embodiment, and the same reference numerals are used to identify corresponding features. In addition to the features of the first embodiment, the innermost ends of the beam members 40a-b,41a-b,42a-b of each beam 40, 41, 42 are provided with a flared or tapered connection 118 to the upper support structure 11. The connections 118 are generally frusto-conical in shape, to increase strength and stiffness of the assembly. The embodiment of Fig. 17 is similar to the embodiment of Fig. 16, and the same reference numerals are used to identify corresponding features. One variation is that the generally frusto-conical flared or tapered connections 118 of the embodiment of Fig. 16 are replaced with generally cylindrical connections 120. In addition, a deck 122 in the form of a metal grille is secured between the generally cylindrical connections 120 of the parallel beam members 40a-b,41a-b,42a-b of each beam 40, 41,42, to facilitate access and maintenance of the assembly. The embodiment of Figs. 18 to 20 is similar to the embodiment of Fig. 17, and the same reference numerals are used to identify corresponding features. A further variation is that the outermost ends of the parallel beam members 40a-b,41a-b,42a-b of each beam 40, 41, 42 are not secured directly to the associated column member 20, 21, 22 but instead each end is secured to a respective mounting interface 130 welded to the upper end of an inclined face 132, 134 of the associated column member 20, 21, 22 (i.e. not secured to the face 135 which faces radially inwardly and extends perpendicularly to the longitudinal axis of the respective beam members 40a-b,41a-b,42a-b. Each mounting interface 130 comprises upper and lower triangular plates 138, 140 shaped as a right angled triangle and whose hypotenuse is secured to the uppermost edge of the associated inclined face 132, 134 of the associated column member 20, 21, 22, a rectangular side plate 142 extending parallel to the longitudinal axis of the respective beam members 40a-b,41a-b,42a-b and whose outermost edge is secured to the vertex 57, 58 between the inclined faces 132, 134 and the adjacent inclined faces 144, 146 of the associated column member 20, 21, 22. A square plate 148 is welded to the innermost edges of the upper and lower triangular plates 138, 140 and to the upper end of the vertex between the faces 135 and 138 / 140 of the associated column member 20, 21,22, coplanar with the face 135. The square plate 144 carries an annular forged metal ring 150 which is shaped and dimensioned to receive the radially outermost end of a respective beam member 40a-b,41a-b,42a-b and to which it is sealed by means of an annular weld 152. A second variation of the arrangement of Figs. 18 to 20 as compared with the arrangement of Fig. 17 is that the upper support structure 11’ comprises three identical generally trapezoidally-shaped beam mounting interfaces 160 which are separated from each other by one of three identical generally trapezoidally-shaped spacers 162. Each beam mounting interface 160 receives, and is secured to, the innermost end of one beam member 40a-b,41a-b,42a-b of two adjacent beams 40, 41, 42. The beam mounting interface 160 comprises identical upper and lower generally trapezoidal plates 164 (only the upper plate is visible in Fig. 21) whose radially innermost edge is complementarily-shaped with the outer surface of the central column 10, to which it is welded. The spacers 162 comprise identical upper and lower generally trapezoidal plates 166 whose radially innermost edge is complementarily-shaped with the outer surface of the central column 10, to which it is welded. A rectangular plate 168 extends between the radially outermost edges of the upper and lower plates 166, and extends perpendicularly to the longitudinal direction of the associated beam member 40a-b,41a-b,42a-b, so that it faces the associated column 20, 21,22. The embodiment of Figs. 21 and 22 is similar to the embodiment of Figs. 18 to 20, and the same reference numerals are used to identify corresponding features. One variation is that in addition to the mounting interface 130, a further generally conical interface 170 is located at the end of each beam member 40a-b,41a-b,42a-b and is secured to the square plate 144 of the mounting interface 130. The outer end of the interface 170 has the same footprint as the square plate 144 and is welded to the periphery of the square plate 144 of the mounting interface 130. The generally conical interface 170 is shown in more detail in Fig. 22 and comprises four identical flat triangular plates 172 which alternate with four curved generally triangular plates 174 which are inverted with respect to the flat triangular plates 172. The curved innermost edge 176 of each of the curved triangular plates 174 is welded to the outer surface of the outermost end of an associated beam member 40a-b,41a-b,42a-b and one vertex of each of the flat triangular plates 172 intersects the ends of the curved edge 176 of two adjacent curved triangular plates 174, forming the interface 170 having an outer opening of the same size and shape as the square plate 144 and an inner opening of the same size and shape as the beam members 40a-b,41a-b,42a-b. Although the generally conical interface 170 is shown as being constructed from four identical flat triangular plates 172 and four curved generally triangular plates 174, there may be as few as three, or more than four, identical flat triangular plates and curved generally triangular plates 174. A variation of the embodiment of Figs. 21 and 22 is shown in Fig. 23 and differs from that embodiment in that the height of the flat triangular plates 172’ and of the generally triangular plates 174’ in the longitudinal direction of the beam members 40a-b,41a-b,42a-b is increased compared with the plates 172, 174, so that the conical interface 170’ is elongated in the longitudinal direction of the beam members 40a-b,41a-b,42a-b compared with the conical interface of Figs. 21 and 22. The embodiment of Fig. 24 is similar to the first embodiment, and the same reference numerals are used to identify corresponding features. A variation is that the height hi of the beam members 40a-b,41a-b,42a-b is equal to the height h2 of the upper support structure 11. If the beam members 40a-b,41a-b,42a-b have a circular crosssection, then their diameter would be equal to the height of the upper support structure 11. In any of the examples or embodiments described or claimed herein, the term fixed to can mean fixed to by welding. In any of the examples or embodiments described or claimed herein, the column members 20,21,22 and beam member 40a-b,41a-b,42a-b may advantageously be offset by 120 degrees about the central column 10. In any of the examples or embodiments described or claimed herein, the width b (see Fig. 5) of the pontoon members 30,31,32 may be constant along the entire length of the respective pontoon member 30,31,32. The width b may be equal to the width of the one rectangular side wall panel of the respective column member 20,21,22 onto which the pontoon member 30,31,32 is fixed, and equal to the width of the one respective flat panel of the lower support structure 12 onto which the pontoon member 30,31,32 is fixed. Further inventive aspects and embodiments are outlined in the following numbered clauses. 1. A floatable foundation (100) for a wind turbine generator, the foundation (100) comprising: a central column (10) having an upper support structure (11) and a lower support structure (12); at least three outer column members (20,21,22) disposed about the central column (10); at least three horizontally extending lower connection members (30,31,32), each horizontally extending lower connection member (30,31,32) fixed to and extending between the lower support structure (12) and a respective one of the at least three outer column members (20,21,22); at least three horizontally extending beam members (40a-b,41a-b,42a-b), each horizontally extending beam member (40a-b,41a-b,42a-b) fixed to and extending between the upper support structure (11) and a respective one of the at least three outer column members (20,21,22); wherein: each of the at least three horizontally extending lower connection members (30,31,32) comprises a pair of lower connection members; and / or each of the at least three horizontally extending beam members (40a-b,41a-b,42a-b) comprises a pair of beam members. 2. The floatable foundation (100) according to any preceding clause, comprising three outer column members (20,21,22), three horizontally extending lower connection members (30,31,32) and three horizontally extending beam members (40a-b,41a-b,42a-b). 3. The floatable foundation (100) according to any preceding clause, wherein the lower connection members (30,31,32) comprise pontoon members (30,31,32). 4. The floatable foundation (100) according to any preceding clause, wherein each horizontally extending beam member (40a-b,41a-b,42a-b) comprises a pair of horizontally extending beam members (40a-b,41a-b,42a-b) comprising a first beam member (40a,41 a,42a) and a second beam member (40b,41b,42b). 5. The floatable foundation (100) according to any preceding clause, wherein the first and second beam members (40a-b,41a-b,42a-b) are arranged in parallel. 6. The floatable foundation (100) according to any preceding clause, wherein the first and second beam members (40a-b,41a-b,42a-b) have the same size. 7. The floatable foundation (100) according to any preceding clause, wherein each horizontally extending lower connection member (40a-b,41a-b,42a-b) comprises a pair of lower connection members comprising a first lower connection member (40a,41 a,42a) and a second lower connection member (40b,41b,42b). 8. The floatable foundation (100) according to clause 7, wherein the first and second lower connection members (40a-b,41a-b,42a-b) are arranged in parallel. 9. The floatable foundation (100) according to clause 7 or clause 8, wherein the first and second lower connection members (40a-b,41a-b,42a-b) have the same size. 10. The floatable foundation (100) according to any preceding clause, wherein each outer column member (20,21,22) comprises a polygonal prism comprising a plurality of rectangular side wall panels. 11. The floatable foundation (100) according to any preceding clause, wherein, at each of the at least three outer column members (20,21,22), the respective lower connection member (30,31,32) is fixed at a first rectangular side wall panel (21x), the respective first beam member (40a,41 a,42a) is fixed at a second rectangular side wall panel (21y) and the respective second beam member (40b,41b,42b) is fixed at a third rectangular side wall panel (21z), the second and third rectangular side wall panels being arranged adjacent and at opposite sides of the first rectangular side wall panel. 12. The floatable foundation (100) according to any preceding clause, wherein the first and second beam members (40a-b,41a-b,42a-b) are or comprise tubular beams. 13. The floatable foundation (100) according to any preceding clause, wherein the first and second beam members (40a-b,41a-b,42a-b) are or comprise rectangular box beams. 14. The floatable foundation (100) according to any preceding clause, wherein the rectangular box beams each have a width which is equal to half of a width (b) of the lower connection members (30,31,32). 15. The floatable foundation (100) according to any preceding clause, wherein the rectangular box beams each have a width which is half of a largest width (w2) of the respective outer column member (20,21,22). 16. The floatable foundation (100) according to any preceding clause, comprising a plurality of first beam interfaces (130) positioned between, and secured to each of, a respective first and second beam member (40a-b,41a-b,42a-b) and a respective column member (20,21,22). 17. The floatable foundation (100) of clause 16, wherein at least one, and preferably all, of the column member / s (20, 21, 22) comprise / s a cross-section of a regular polygon of at least six sides forming a corresponding number of column wall portions, wherein a first wall portion (135) extends perpendicularly to the longitudinal axes of the first and second beam members (40a-b,41a-b,42a-b) and wherein each of the beam interfaces (130) are secured to a second and third respective wall portion (132, 134) adjacent to the first wall portion (135). 18. The floatable foundation (100) of clause 17 wherein the width of the beam interfaces (130) in a direction perpendicular to the longitudinal axes of the first and second beam members (40a-b,41a-b,42a-b) is the same as the lateral projection of the second and third wall portions (132, 134) beyond the first wall portion (135). 19. The floatable foundation (100) according to any of clauses 16 to 18, comprising mounting rings (150) to each of which a longitudinally outer end of a respective beam member (40a-b,41a-b,42a-b) is received, each ring member being secured in a respective beam interface (130). 20. The floatable foundation (100) according to any of clauses 16 to 19, further comprising a second beam interface (170) positioned between, and secured to, the first beam interface (130) and a longitudinally outer end of a respective beam member (40a-b,41a-b,42a-b). 21. The floatable foundation (100) according to clause 20, wherein the cross section of the second beam interface (170) tapers from the first beam interface (130) towards the respective beam member (40a-b,41a-b,42a-b). 22. The floatable foundation (100) according to clause 21, wherein the second beam interface (170) is generally conical and comprises at least three substantially flat triangular plates (172) which alternate with the same number of curved generally triangular plates (174) which are inverted with respect to the generally flat triangular plates (172). 23. The floatable foundation (100) according to clause 22, comprising four substantially flat triangular plates (172) and four curved generally triangular plates (174). 24. The floatable foundation (100) according to any preceding clause, comprising a plurality of beam interfaces (118, 130) positioned between, and secured to each of, a respective first and second beam member (40a-b,41a-b,42a-b) and the upper support structure (11). 25. The floatable foundation (100) according to clause 24 wherein the crosssection of the beam interfaces (118, 130) between the respective first and second beam member (40a-b,41a-b,42a-b) and the upper support structure (11) tapers from the upper support structure (11). 26. The floatable foundation (100) according to clause 25, wherein the beam interfaces (118, 130) between the respective first and second beam member (40a-b,41a-b,42a-b) and the upper support structure (11) comprise a frusto-conical portion (118). 27. The floatable foundation (100) according to any preceding clause, comprising a deck portion (122) extending between a pair of horizontally extending beam members (40a-b,41a-b,42a-b). 28. The floatable foundation (100) according to clause 23, comprising a deck portion (122) between each pair of horizontally extending beam members (40a-b,41a-b,42a-b). 29. The floatable foundation (100) according to clause 23 or clause 24, wherein the or each deck portion is located adjacent to the upper support structure (11). 30. The floatable foundation (100) according to any preceding clause, wherein the width (w1) of each pair of horizontally extending beam members (40a-b,41a-b,42a-b) is larger than a width (w4) of the respective lower connection member (30,31,32). 31. The floatable foundation (100) according to any preceding clause, wherein the upper support structure (11) comprises a planar face between the longitudinally inner ends of each pair of horizontally extending beam members (40a-b,41a-b,42a-b) extending perpendicularly to the longitudinal direction of the beam members. 32. The floatable foundation (100) according to any preceding clause, wherein a width (w1) of each pair of horizontally extending beam members (40a-b,41a-b,42a-b) is equal to or larger than a largest width (w2) of the respective outer column member (20,21,22). 33. The floatable foundation (100) according to any preceding clause, wherein, in each pair of horizontally extending beam members (40a-b,41a-b,42a-b), the beam members (40a-b,41a-b,42a-b) are spaced from each other by a distance (d) which is at least equal to a width (w4) of the respective lower connection member (30,31,32). 34. The floatable foundation (100) according to any preceding clause, wherein the lower support structure (12) is arranged about and fixed to a tubular part of the central column (10). 35. The floatable foundation (100) according to any preceding clause, wherein the central column (10) comprises a flange (15) above the upper support structure (11), such as a bolt flange (15), the flange (15) configured for connection to a wind turbine generator tower (16). 36. The floatable foundation (100) according to any preceding clause, wherein a tubular part of the central column (10) extends through the upper support structure (11). 37. The floatable foundation (100) according to any preceding clause, wherein the flange (15) is arranged on a tubular part of the central column (10). 38. The floatable foundation (100) according to any preceding clause, wherein the upper and / or lower support structure(s) (11,12) comprise(s) a plurality of vertically arranged, interconnected flat panels (12a-f) forming a polygonal, e.g. hexagonal, structure. 39. The floatable foundation (100) according to any preceding clause, wherein the plurality of vertically arranged, interconnected flat panels (12a-f) are arranged about a tubular part of the central column (10) such as to form a polygonal, e.g. hexagonal, collar about the tubular part of the central column (10). 40. The floatable foundation (100) according to any preceding clause, wherein the upper and / or lower support structure(s) (11,12) comprise(s) a plurality of vertically arranged support plates (12g-l) extending radially outwardly between the tubular part of the central column (10) and the interconnected flat panels (12a-f). 41. The floatable foundation (100) according to any preceding clause, wherein each of the support plates (12g-l) is fixed at an interface between two interconnected flat panels (12a-f). 42. The floatable foundation (100) according to any preceding clause, wherein the lower support structure (12) comprises a top plate (110), and wherein a lower end of a tubular part of the central column (10) is landed onto the top plate and fixed thereto. 43. The floatable foundation (100) according to any preceding clause, wherein the lower support structure (12) comprises a bottom plate, and wherein a lower end of a tubular part of the central column (10) is landed onto or positioned adjacent an upward-facing surface of the bottom plate and fixed in the lower support structure (12), for example fixed to the upward-facing surface of the bottom plate. 44. The floatable foundation (100) according to any preceding clause, wherein an interior of the central column (10) is not open to surrounding sea water. 45. The floatable foundation (100) according to any preceding clause, wherein each lower connection member (30,31,32) is fixed at one, such as exactly one, respective flat panel of the lower support structure (12). 46. The floatable foundation (100) according to any preceding clause, wherein a width (b) of each lower connection member (30,31,32) is equal to a width of the one respective flat panel of the lower support structure (12) to which the lower connection member (30,31,32) is fixed. 47. The floatable foundation (100) according to any preceding clause, wherein each of the lower connection members (30,31,32) comprises a box beam made up of four outer flat panels, the four outer flat panels defining a top wall (32a), a bottom wall (32b), and a pair of side walls (32c,d). 48. The floatable foundation (100) according to any preceding clause, wherein: each lower connection member (30,31,32) comprises at least one internal corner plate (49) fixed between a side wall (32c,d) and a top or bottom wall (32a,32b) of the lower connection member (30,31,32), and the lower support structure (12) comprises at least one corner plate (48) arranged between a top or bottom plate of the lower support structure (12) and a flat panel (12a,12c,12e) which is adjacent to a flat panel (12b, 12d, 12f) onto which the lower connection members (30,31,32) are fixed. 49. The floatable foundation (100) according to any preceding clause, wherein each of the three outer column members (20,21,22) is a right prism and / or wherein each of the three outer column members (20,21,22) has a constant cross-section in the horizontal plane. 50. The floatable foundation (100) according to any preceding clause, wherein each outer column member (20,21,22) comprises one rectangular side wall panel onto which one lower connection member (30,31,32) and no beam member (40a-b,41a-b,42a-b) is fixed. 51. The floatable foundation (100) according to any preceding clause, wherein a width of the one rectangular side wall panel onto which one lower connection member (30,31,32) is fixed is equal to a width (b) of the respective lower connection member (30,31,32). 52. The floatable foundation (100) according to any preceding clause, wherein a height of the lower support structure (12), such as a height of the plurality of vertically arranged, interconnected flat panels (12a-f) forming part of the lower support structure (12), is equal to a height (h) of the lower connection members (30,31,32). 53. The floatable foundation (100) according to any preceding clause, wherein the column members (20,21,22) and the lower support structure (12) have an identical polygonal cross-sectional profile in the horizontal plane, for example a profile of a regular polygon. 54. The floatable foundation (100) according to any preceding clause, wherein the polygonal cross-sectional profile in the horizontal plane is a hexagonal cross-sectional profile, for example a profile of a regular hexagon. 55. The floatable foundation (100) according to any preceding clause, wherein a tubular part of the central column (10) has a constant diameter between the lower support structure (12) and the upper support structure (11), optionally wherein the tubular part of the central column (10) has a constant diameter from the lower support structure (12) and through the upper support structure (11) to a position above the upper support structure (11) at which the flange (15) is arranged on the tubular part of the central column (10). 56. The floatable foundation (100) according to any preceding clause, further comprising reinforcing members (114) extending between a pair of horizontally extending beam members (40a-b,41a-b,42a-b). 57. The floatable foundation (100) according to any preceding clause, wherein the height (hi) of the beam members (40a-b,41a-b,42a-b) is substantially equal to the height (h2) of the upper support structure (11). 58. The floatable foundation (100) according to any preceding clause, wherein at least one, and optionally all, of the lower connection members (30,31,32), comprises a pair of pontoon members (78a, 78b) fixed to and extending between the lower support structure (12) and a respective one of the at least three outer column members (20,21,22). 59. The floatable foundation (100) according to any preceding clause, wherein the interior of the column members (20,21,22) and / or of the horizontally extending lower connection members (30, 31, 32) comprises one or more dividers (86, 88, 90, 92, 94) which separate / s the interior of the column members (20,21,22) and / or the horizontally extending lower connection members (30, 31, 32) into a plurality of watertight sections. 60. The floatable foundation (100) according to any preceding clause, wherein the upper support structure (11) comprises a plurality of beam mounting interfaces (160) to which one beam member (41b, 42a) of each of two adjacent pairs of beam members (41a, 41b; 42a, 42b) are attached. 61. The floatable foundation (100) according to clause 56, wherein the upper support structure (11) further comprises a plurality of spacers (162), each located between, and secured to, two beam mounting interfaces (160). 62. The floatable foundation (100) according to clause 56, wherein the spacers (162) comprise a planar wall (168) located between, and extending perpendicularly to, each pair of beam members (41a, 41b; 42a, 42b). 63. A method of producing electric power, the method comprising operating a floatable foundation (100) according to any of the preceding clauses having a wind turbine generator arranged thereon at an offshore location to produce electric power. 64. The method of clause 59, comprising receiving the electric power at a land-based location for supply into a land-based electricity grid. 65. A method of designing a floatable foundation (100) for a wind turbine generator, the method comprising: (a) generating a three-dimensional (3D) computer model of a foundation (100) according to any of the preceding clauses; (b) providing a set of metocean data, the set of metocean data being representative of environmental conditions at a proposed operational site for the foundation (100); (c) using the three-dimensional (3D) computer model and the set of metocean data, simulating an operation of the floatable foundation (100) at the proposed operational site under the environmental conditions; and (d) based on the simulation, determining a parameter indicative of stress and / or fatigue loads on the floatable foundation (100) under the environmental conditions at the proposed operational site. 66. The method according to the preceding clause, the method comprising: subsequent to step (d), adjusting the three-dimensional (3D) computer model in response to finding that the parameter does not meet a predetermined threshold; repeating steps (c) and (d) using the adjusted three-dimensional computer model (3D); and in response to finding that the parameter meets the pre-determined threshold, generating a final three-dimensional (3D) computer model of the foundation (100). 67. The method according to clause 62 or clause 63, wherein the step of generating the final three-dimensional (3D) computer model of the foundation (100) comprises producing a set of computer-aided design (CAD) drawings and / or a set of construction drawings of the foundation (100) according to the final three-dimensional (3D) computer model. 68. The method according to clause 63, further comprising producing one or more floatable foundations (100) based on the set of computer-aided design (CAD) drawings and / or the set of construction drawings. The method comprises generating a three-dimensional (3D) computer model of the foundation 100. The computer model may, for example, be created as a computer-aided design (CAD) model of a proposed design of the foundation 100. A set of metocean data representative of environmental conditions at a proposed operational site for the foundation 100 is provided. The metocean data may include measured data from the proposed operational site or data which is obtained in another manner and which is representative of the environmental conditions at the proposed operational site. Optionally, the data may be from a different operational site which is comparable to the proposed operational site, or from which information about the environmental conditions at the proposed operational site can be derived. The set of metocean data may, for example, include weather information relating to wind, wave and / or current at the proposed operational site. The set of metocean data may, for example, comprise data extending over one year. Alternatively, or additionally, the metocean data may comprise information about the likelihood and nature of specific weather conditions. Using the three-dimensional (3D) computer model and the set of metocean data, the operation of the floatable foundation 100 at the proposed operational site is simulated under the environmental conditions. Based on the simulation, a parameter indicative of stress and / or fatigue loads on the floatable foundation 100 under the environmental conditions at the proposed operational site is determined. The parameter may, for example, representan operational life (fatigue life) of the foundation when used at the operational site. The method may further comprise adjusting the three-dimensional (3D) computer model in response to finding that the parameter does not meet a pre-determined threshold, for example a pre-defined threshold for operational life. The adjustment may, for example, involve changing one or more dimensions of the three-dimensional (3D) computer model of the foundation 100, such as a length, width or height of parts, plate thicknesses, positioning / size / dimensions of stiffening arrangements, etc. The method may be used in an iterative process by repeating the above steps for the adjusted three-dimensional (3D) model to calculate updated parameter values and thus, for example, obtain updated values for the operational life of the foundation 100 under the environmental conditions at the proposed operational site. If (or when) the parameter meets the pre-determined threshold, a final three-dimensional (3D) computer model of the foundation 100 can be established. The final three-dimensional (3D) computer model of the foundation 100 may comprise a set of computer-aided design (CAD) drawings and / or a set of construction drawings of the foundation 100 according to the final three-dimensional (3D) computer model. The CAD drawings and / or the construction drawings may, for example, be such drawings used by a yard to produce (i.e., build) the foundation 100. The method may also comprise producing (i.e., building) one or more foundations 100 based on the set of computer-aided design (CAD) drawings and / or the set of construction drawings. For example, a plurality of foundations 100 for the proposed operational site may be produced base on the final three-dimensional (3D) computer model. In this manner, a basic design can form the basis for foundations 100 which can be used at various operational sites, and a construction yard may for example be set up for producing foundations 100 according to the basic design. (E.g. in relation to the plate sizes used, work processes associated therewith, etc.) Based on requirements for a specific operational site, e.g. fatigue loads on the foundation 100 due to the local metocean conditions, the foundation 100 may be designed and produced to comply with design requirements such as fatigue life. Such adjustments can thereby be undertaken without adversely affecting manufacturability in a significant manner, since the basic design remains unchanged. According to examples and embodiments described here, a foundation 100 can advantageously be realized with beneficial structural properties, e.g. allowing a low overall steel weight, and / or allowing for efficient manufacturing compared to existing technology. Foundations 100 as disclosed herein may provide enhanced structural characteristics in that the peak stresses imposed by bending moments from the wind turbine generator tower are better separated from the peak stresses imposed by waves acting on the floatable foundation 100. For example, certain metocean conditions, such as those found at some locations in the North Sea, may produce high fatigue loads on floating foundations like the ones disclosed here, which could in some cases dictate the use of thicker steel plates at local fatigue hotspots in the hull structure of the foundation 100. (For example, at or near the interfaces between the pontoon members 30,31,31 and the lower support structure 12.) Particularly, the use of pairs of beam members 40a-b,41a-b,42a-b which are spaced apart provide improved fatigue properties. Unexpectedly, the inventors have discovered that the arrangements and characteristics of the beam members 40a-b,41a-b,42a-b in examples described here can provide reduced fatigue loads also in the lower part of the foundation, such as at the pontoon members 30,31,32 and the lower support structure 12. In order to facilitate automated fabrication, geometrical standardization may be repeated throughout the foundation 100. The width of the pontoon members 30,31,32 may, for example, equal the width of the panels of the column members 20,21,22. The width of the interfaces at the lower support structure 12 may also correspond to the same. The invention is not limited by the embodiments described above; reference should be had to the appended claims.

Claims

1. A floatable foundation (100) for a wind turbine generator, the foundation (100) comprising:a central column (10) having an upper support structure (11) and a lower support structure (12);at least three outer column members (20,21,22) disposed about the central column (10);at least three horizontally extending lower connection members (30,31,32), each horizontally extending lower connection member (30,31,32) fixed to and extending between the lower support structure (12) and a respective one of the at least three outer column members (20,21,22);at least three horizontally extending beam members (40a-b,41a-b,42a-b), each horizontally extending beam member (40a-b,41a-b,42a-b) fixed to and extending between the upper support structure (11) and a respective one of the at least three outer column members (20,21,22);wherein:each of the at least three horizontally extending lower connection members (30,31,32) comprises a pair of lower connection members; and / oreach of the at least three horizontally extending beam members (40a-b,41a-b,42a-b) comprises a pair of beam members.

2. The floatable foundation (100) according to any preceding claim, comprising three outer column members (20,21,22), three horizontally extending lower connection members (30,31,32) and three horizontally extending beam members (40a-b,41a-b,42a-b).

3. The floatable foundation (100) according to any preceding claim, wherein the lower connection members (30,31,32) comprise pontoon members (30,31,32).

4. The floatable foundation (100) according to any preceding claim, wherein each horizontally extending beam member (40a-b,41a-b,42a-b) comprises a pair of horizontally extending beam members (40a-b,41a-b,42a-b) comprising a first beam member (40a,41 a,42a) and a second beam member (40b,41b,42b) and, optionally,wherein the first and second beam members (40a-b,41a-b,42a-b) are arranged in parallel.

5. The floatable foundation (100) according to any preceding claim, wherein each horizontally extending lower connection member (40a-b,41a-b,42a-b) comprises a pair of lower connection members comprising a first lower connection member (40a,41 a,42a) and a second lower connection member (40b,41b,42b) and, optionally,wherein the first and second lower connection members (40a-b,41a-b,42a-b) are arranged in parallel and, optionally,wherein the first and second lower connection members (40a-b,41a-b,42a-b) have the same size.

6. The floatable foundation (100) according to any preceding claim, wherein each outer column member (20,21,22) comprises a polygonal prism comprising a plurality of rectangular side wall panels.

7. The floatable foundation (100) according to any preceding claim, wherein, at each of the at least three outer column members (20,21,22), the respective lower connection member (30,31,32) is fixed at a first rectangular side wall panel (21x), the respective first beam member (40a,41 a,42a) is fixed at a second rectangular side wall panel (21y) and the respective second beam member (40b,41b,42b) is fixed at a third rectangular side wall panel (21z), the second and third rectangular side wall panels being arranged adjacent and at opposite sides of the first rectangular side wall panel.

8. The floatable foundation (100) according to any preceding claim, wherein the first and second beam members (40a-b,41a-b,42a-b) are or comprise tubular beams.

9. The floatable foundation (100) according to any preceding claim, wherein the first and second beam members (40a-b,41a-b,42a-b) are or comprise rectangular box beams and, optionally,wherein the rectangular box beams each have a width which is equal to half of a width (b) of the lower connection members (30,31,32), and / orwherein the rectangular box beams each have a width which is half of a largest width (w2) of the respective outer column member (20,21,22).

10. The floatable foundation (100) according to any preceding claim, comprising a plurality of first beam interfaces (130) positioned between, and secured to each of, a respective first and second beam member (40a-b,41a-b,42a-b) and a respective column member (20,21,22).

11. The floatable foundation (100) of claim 10, wherein at least one, and preferably all, of the column member / s (20, 21, 22) comprise / s a cross-section of a regular polygon of at least six sides forming a corresponding number of column wall portions, wherein a first wall portion (135) extends perpendicularly to the longitudinal axes of the first and second beam members (40a-b,41a-b,42a-b) and wherein each of the beam interfaces (130) are secured to a second and third respective wall portion (132, 134) adjacent to the first wall portion (135) and, optionally,.wherein the width of the beam interfaces (130) in a direction perpendicular to the longitudinal axes of the first and second beam members (40a-b,41a-b,42a-b) is the same as the lateral projection of the second and third wall portions (132, 134) beyond the first wall portion (135), and / orcomprising mounting rings (150) to each of which a longitudinally outer end of a respective beam member (40a-b,41a-b,42a-b) is received, each ring member being secured in a respective beam interface (130).

12. The floatable foundation (100) according to claim 10 or claim 11, further comprising a second beam interface (170) positioned between, and secured to, the first beam interface (130) and a longitudinally outer end of a respective beam member (40a-b,41a-b,42a-b).

13. The floatable foundation (100) according to claim 12, wherein the cross section of the second beam interface (170) tapers from the first beam interface (130) towards the respective beam member (40a-b,41a-b,42a-b) and, optionally,.wherein the second beam interface (170) is generally conical and comprises at least three substantially flat triangular plates (172) which alternate with the same number of curved generally triangular plates (174) which are inverted with respect to the generally flat triangular plates (172) and, optionally,comprising four substantially flat triangular plates (172) and four curved generally triangular plates (174).

14. The floatable foundation (100) according to any preceding claim, comprising a plurality of beam interfaces (118, 130) positioned between, and secured to each of, a respective first and second beam member (40a-b,41a-b,42a-b) and the upper support structure (11) and, optionally,wherein the cross-section of the beam interfaces (118, 130) between the respective first and second beam member (40a-b,41a-b,42a-b) and the upper support structure (11) tapers from the upper support structure (11) and, optionally,wherein the beam interfaces (118, 130) between the respective first and second beam member (40a-b,41a-b,42a-b) and the upper support structure (11) comprise a frusto-conical portion (118).

15. The floatable foundation (100) according to any preceding claim, comprising a deck portion (122) extending between a pair of horizontally extending beam members (40a-b,41a-b,42a-b), e.g.comprising a deck portion (122) between each pair of horizontally extending beam members (40a-b,41a-b,42a-b) and / orwherein the or each deck portion is located adjacent to the upper support structure (11).

16. The floatable foundation (100) according to any preceding claim, wherein the width (w1) of each pair of horizontally extending beam members (40a-b,41a-b,42a-b) is larger than a width (w4) of the respective lower connection member (30,31,32).

17. The floatable foundation (100) according to any preceding claim, wherein the upper support structure (11) comprises a planar face between the longitudinally inner ends of each pair of horizontally extending beam members (40a-b,41a-b,42a-b) extending perpendicularly to the longitudinal direction of the beam members.

18. The floatable foundation (100) according to any preceding claim, wherein a width (w1) of each pair of horizontally extending beam members (40a-b,41a-b,42a-b) is equal to or larger than a largest width (w2) of the respective outer column member (20,21,22).

19. The floatable foundation (100) according to any preceding claim, wherein, in each pair of horizontally extending beam members (40a-b,41a-b,42a-b), the beam members (40a-b,41a-b,42a-b) are spaced from each other by a distance (d) which is at least equal to a width (w4) of the respective lower connection member (30,31,32).

20. The floatable foundation (100) according to any preceding claim, wherein the height (hi) of the beam members (40a-b,41a-b,42a-b) is substantially equal to the height (h2) of the upper support structure (11).

21. The floatable foundation (100) according to any preceding claim, wherein the interior of the column members (20,21,22) and / or of the horizontally extending lower connection members (30, 31, 32) comprises one or more dividers (86, 88, 90, 92, 94) which separate / s the interior of the column members (20,21,22) and / or the horizontally extending lower connection members (30, 31, 32) into a plurality of watertight sections.

22. The floatable foundation (100) according to any preceding claim, wherein the upper support structure (11) comprises a plurality of beam mounting interfaces (160) to which one beam member (41b, 42a) of each of two adjacent pairs of beam members (41a, 41b; 42a, 42b) are attached and, optionally,wherein the upper support structure (11) further comprises a plurality of spacers (162), each located between, and secured to, two beam mounting interfaces (160) and / orwherein the spacers (162) comprise a planar wall (168) located between, and extending perpendicularly to, each pair of beam members (41a, 41b; 42a, 42b).

23. A method of producing electric power, the method comprising operating a floatable foundation (100) according to any of the preceding claims having a wind turbine generator arranged thereon at an offshore location to produce electric power and, optionally,comprising receiving the electric power at a land-based location for supply into a land-based electricity grid.

24. A method of designing a floatable foundation (100) for a wind turbine generator, the method comprising:(e) generating a three-dimensional (3D) computer model of a foundation (100) according to any of the preceding claims;(f) providing a set of metocean data, the set of metocean data being representative of environmental conditions at a proposed operational site for the foundation (100);(g) using the three-dimensional (3D) computer model and the set of metocean data, simulating an operation of the floatable foundation (100) at the proposed operational site under the environmental conditions; and(h) based on the simulation, determining a parameter indicative of stress and / or fatigue loads on the floatable foundation (100) under the environmental conditions at the proposed operational site.

25. The method according to the preceding claim, the method comprising: subsequent to step (d), adjusting the three-dimensional (3D) computer model in response to finding that the parameter does not meet a predetermined threshold;repeating steps (c) and (d) using the adjusted three-dimensional computer model (3D); andin response to finding that the parameter meets the pre-determined threshold, generating a final three-dimensional (3D) computer model of the foundation (100).5 26. The method according to claim 24 or claim 25, wherein the step of generating the final three-dimensional (3D) computer model of the foundation (100) comprises producing a set of computer-aided design (CAD) drawings and / or a set of construction drawings of the foundation (100) according to the final three-dimensional (3D) computer model and, optionally,10further comprising producing one or more floatable foundations (100) based on the set of computer-aided design (CAD) drawings and / or the set of construction drawings.

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