Floating wind turbine foundations

Abstract

A method of designing a floatable foundation 100 for a wind turbine generator, the method comprising generating a three-dimensional (3D) computer model of the foundation 100; providing a set of metocean data representative of environmental conditions at a proposed operational site for the foundation 100; simulating an operation of the foundation 100 at the proposed operational site; and determining a parameter indicative of stress and / or fatigue loads on the floatable foundation 100 under the environmental conditions at the proposed operational site. There is also provided a floatable foundation 100 for a wind turbine generator, the floatable foundation 100 comprising three outer column members 20,21,22 disposed about a tubular central column 10, three horizontally extending pontoon members 30,31,32, three horizontally extending beam members 40,41,42, and a plurality of reinforcement members (50, Fig.26) each extending between at least two pontoon members 30,31,32.

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 being studied and developed by various research and development (R&D) groups, both within academia and industry, 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 present disclosure include: 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; EP4 155 538A1; WO 2023 / 014230 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 method of designing a floatable foundation for a wind turbine generator as set out in appended claim 1. In one example, there is provided a floatable foundation for a wind turbine generator as set out in appended claim 6. 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 floatable foundation according to an example. Figs 2-8 illustrate details of a floatable foundation according to various examples. Figs 9 and 10 illustrate aspects of a floatable foundation and the construction thereof. Fig. 11 illustrates details of a floatable foundation according to an example. Figs 12-15 illustrate a floatable foundation according to an example, comprising cable tubes for an electric cable. Figs 16-19 illustrate floatable foundations according to other examples, comprising strengthening trusses. Figs 20-27 illustrate floatable foundations according to other examples, comprising reinforcement members. Fig. 28 illustrates a floatable foundation according to another example, comprising strengthening trusses and a central column support collar. Figs 29 and 30 illustrate a floatable foundation according to another example, comprising quadrilateral column members. Fig. 31 illustrates a floatable foundation according to an example, comprising a polygonal central column. Figs 32 and 33 illustrate floatable foundations according to other examples, comprising downwardly slanting beam members. DETAILED DESCRIPTION Fig. 1 illustrates 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. Figs 2-8 illustrate details of the foundation 100 according to various examples. The foundation comprises a tubular central column 10 having an upper support structure 11 arranged about and fixed to the tubular central column 10. A lower support structure 12 is further fixed to the tubular central column 10, and vertically spaced from the upper support structure 11. Three outer column members 20,21,22 are disposed about the tubular central column 10, and fixed to the tubular central column 10 by means of three horizontally extending pontoon members 30,31,32 and three horizontally extending beam members 40,41,42. 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, and each horizontally extending beam member 40,41,42 is fixed to and extend between the upper support structure 11 and a respective one of the three outer column members 20,21,22. The tubular central column 10 extends through the upper support structure 11. Fig. 6 illustrates details of the upper support structure 11, the interface between the central column 10 and the upper support structure 11, and further the interface to the beam members 40,41,42. The upper support structure 11 is made up of a plurality of vertically arranged, interconnected flat panels 11 a-f arranged about the tubular central column 10. The vertically arranged, interconnected flat panels 11 a-f form a polygonal, in this example a hexagonal, collar about the tubular central column 10. A plurality of vertically arranged support plates 11g-l extend radially outwardly between the tubular central column 10 and the interconnected flat panels 11 a-f. Advantageously, each of the support plates 11 g-l is fixed at an interface between two interconnected flat panels 11 a-f, i.e. at the vertices of the polygonal structure formed by the flat panels 11 a-f. Alternatively, or additionally, support plates 11 g-l can be arranged at other locations on the flat panels 11 a-f. The upper support structure 11 further comprises a top plate 11m (see Figs 3 and 9) which is arranged about and fixed to the tubular central column 10, and is also fixed to upper edges of the flat panels 11 a-f. Similarly, a bottom plate 11 n (see Figs 4 and 9) is arranged about and fixed to the tubular central column 10, and is fixed to lower edges of the flat panels 11 a-f. The support plates 11 g-l may also be fixed to the top and / or bottom plate(s) 11 m, 11 n. Each beam member 40,41,42 is fixed at one respective flat panel 11 b, 11 d, 11f of the upper support structure 11. The flat panels 11a,11c,11e form adjacent faces located between pairs of beam members 40,41,42. Illustrated in Fig. 7, each of the beam members 40,41,42 is a box beam made up of four outer flat panels, the four outer flat panels defining a top wall 42a, a bottom wall 42b, and a pair of side walls 42c,d. The beam members 40,41,42 may have internal strengthening features, for example features as illustrated in Figs 6-8. In one example, the beam members 40,41,42 comprise at least one internal corner plate 49 fixed between a side wall 42c,d and a top or bottom wall 42a,42b of the beam member 40,41,42. A corner plate 49 may be arranged at each internal corner of the beam members 40,41,42, such that each beam member 40,41,42 has four internal corner plates 49. The upper support structure 11 may further comprise at least one corner plate 48 arranged between the top and / or bottom plate 11n,11m of the upper support structure 11 and the corresponding flat panels 11 a, 11 c, 11 e which lie between the beam members 40,41,42, i.e. those flat panels 11a,11c.11e which are located adjacent to the flat panels 11 b, 11 d, 11f to which the beam members 40,41,42 are fixed. In this manner, the corner plates 48,49 can provide advantageous structural features at the upper support structure 11 and beam members 40,41,42, and in the interface between these. The lower support structure 12 may be arranged about and fixed to the tubular central column 10. It may be constructed similarly as the upper support structure 11, as illustrated in Fig. 6, with the exception that the tubular central column 10 does not necessarily need to extend through and below the lower support structure 12. The tubular central column 10 may stop inside the lower support structure 12, for example where a lower end of the tubular central column 10 is fixed to an upward-facing surface of a bottom plate 12b (see Figs 4 and 9) of the lower support structure 12. Alternatively, the tubular central column 10 may be positioned inside the lower support structure 12 adjacent the bottom plate 12b, and fixed inside the lower support structure 12 by other means, such as via a plurality of vertically arranged support plates extending radially outwardly between the tubular central column 10 and interconnected flat panels making up part of the lower support structure 12, similarly as shown in Fig. 6 for the upper support structure 11. As can be seen for example in Figs 2-4, each pontoon member 30,31,32 is fixed at one respective flat panel of the lower support structure 12 and extend outwardly to the column members 20,21,22. The configuration is equivalent as that of the beam members 40,41,42 and the upper support structure 11, described above. Similarly as for the beam members 40,41,42, the pontoon members 30,31,32 may be made up as a box beam of four outer flat panels defining a top wall, a bottom wall, and a pair of side walls. The pontoon members 30,31,32 and the lower support structure 12 may, similarly, comprise one or more corner plates 48,49 equivalently as described above in relation to beam members 40,41,42 and upper support structure 11. In some examples, the design of the beam members 40,41,42 and the pontoon members 30,31,32 may be identical, or may only differ in their dimensions. For example, the beam members 40,41,42 and the pontoon members 30,31,32 may have the same width b 1,b2 (see Fig. 3), and only differ in their height h 1,h2 (see Fig. 5), or they may have the same width (such that b1=b2) and the same height (such thath1=h2). Advantageously, the width b4 and b5 (see Fig. 3) of the interconnected flat panels 11 a-f forming part of the upper and lower support structures 11,12 is the same as the width b1,b2 of the beam members 40,41,42 and the pontoon members 30,31,32. Referring now to Figs 9 and 10, examples of a method of constructing a floatable foundation 100 are shown. The foundation 100 may be constructed by manufacturing a first tower support part 13a having a first central column part 10a and an upper support structure 11 as described above, which is arranged about and fixed to the first central column part 10a. The first central column part 10a may comprise a flange 15 above the upper support structure 11, the flange 15 configured for connection to a wind turbine generator tower 16 (see Fig. 1). A second tower support part 13b comprising a second tubular central column part 10b is provided. The second tower support part 13b may be a substantially tubular part. A third tower support part 13c is manufactured, wherein the third tower support part 13c comprises a lower support structure 12 as described above. After manufacturing the tower support parts 13a-c, the first central column part 10a is fixed to the second central column part 10b and the second central column part 10b is fixed to the third tower support part 13c to form the tower support parts of the foundation 100, i.e. the middle structure of the floater as illustrated in e.g. Figs 1 and 2. The first and second central column parts 10a, 10b, when fixed together, make up a tubular central column 10. Three outer column members 20,21,22 are then provided and arranged about the tubular central column 10, and three pontoon members 30,31,32 and three beam members 40,41,42 are fixed between the lower and upper support structures 11,12 and the respective column members 20,21,22. As illustrated in Fig. 9, the third tower support part 13c may be manufactured with a third central column part 10c and with the lower support structure 12 fixed to the third central column part 10c, for example in the manner described in relation to Fig. 6 above. The step of fixing the second central column part 10b to the third tower support part 13c may then comprise fixing the second central column part 10b to the third central column part 10c. The third central column part 10c may in this case make up part of the tubular central column 10. Optionally, as illustrated in Fig. 10, fixing the second central column part 10b to the third tower support part 13c may comprise fixing a lower end 10b’ of the second central column part 10b onto a top plate 12a of the lower support structure 12. The internal structure of the lower support structure 12 may in such a case be different from that described above. For example, the lower support structure 12 may be manufactured integrally with the pontoon members 30,31,32. The tubular central column 10 may be a closed cylindrical structure in the parts of the tubular central column 10 which are submerged, or the tubular central column 10 may otherwise be separated from surrounding sea water. The tubular central column 10 may for example only have a single opening located at its top end (at the flange 15) and otherwise be a closed cylindrical column. Alternatively, or additionally, a bottom plate 12b (see Fig. 4) of the lower support structure 12 may separate an interior of the tubular central column 10 from surrounding sea water. In some examples, the bottom plate 12b is a bottom plate of the lower support structure 12 and also a lower plate of the tubular central column 10, for example if the tubular central column 10 is fixed in a fluid-tight manner at an upward-facing surface of the bottom plate 12b. In one example, the bottom plate 12b of the lower support structure 12, bottom plates 30b,31b,32b (see Fig. 4) of the three horizontally extending pontoon members 30,31,32 and bottom plates 20b,22b of the three outer column members 20,21,22 define an uninterrupted, planar bottom face of the foundation 100. There may advantageously be no opening present in the uninterrupted, planar bottom face of the foundation 100. 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. In this manner, the cross-sectional shape (and consequently the width and distance from the tubular central column 10) is the same at the interface of the beam members 40,41,42 and at the interface of the pontoon members 30,31,32. Each outer column member 20,21,22 comprises one rectangular side wall panel onto which one pontoon member 30,31,32 and one beam member 40,41,42 are fixed, vertically spaced from each other. The one rectangular side wall panel onto which both the pontoon member 30,31,32 and the beam member 40,41,42 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, or other features enabling such fixation. Advantageously, the dimensions of the various parts can be designed to provide manufacturing advantages. For example, a width b3 of the one rectangular side wall panel onto which the pontoon member 30,31,32 and the beam member 40,41,42 are fixed can be made equal to the width b2 of the respective pontoon member 30,31,32 and the width b1 of the respective beam member 40,41,42. The width b2 of the pontoon members 30,31,32 can further be equal to a width b5 of the one respective flat panel of the lower support structure 12 onto which the pontoon member 30,31,32 is fixed, and the width b1 of each beam member 40,41,42 is equal to the width b4 of the one respective flat panel of the upper support structure 11 onto which the beam member 40,41,42 is fixed. (Fig. 3 illustrates the widths b4 and b5 at the adjacent flat panels of the upper and lower support structures 11,12, but it will be understood that the flat panels onto which the beam members 40,41,42 and the pontoon members 30,31,32 are fixed have the same widths b4 and b5.) Particularly advantageously, the widths may be arranged such that b1=b2=b3=b4=b5. In this manner, considerable manufacturing advantages (e.g. robotization advantages and automation advantages) can be achieved, in that the same plate dimensions can form the basis for many of the parts of the foundation 100. This facilitates efficient supply chains, manufacturing and construction. The height may, similarly, be adapted to correspond such that a height of the upper support structure 11, defined by the height of the plurality of vertically arranged, interconnected flat panels 11a-f, is equal to a height hi (see Fig. 5) of the beam members 40,41,42. Similarly, a height of the lower support structure 12, defined by the height of the plurality of vertically arranged, interconnected flat panels forming part of the lower support structure 12, may be equal to a height h2 of the pontoon members 30,31,32. The heights hi and h2 may be the same, or the height of the beam members 40,41,42 and upper support structure 11 may be smaller than that of the pontoon members 30,31,32 and lower support structure 12. In an advantageous example, the column members 20,21,22, the upper support structure 11 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. These parts may all have a horizontal outline in the form of a regular polygon, for example having a hexagonal cross-sectional profile. The tubular central column 10 may have a constant diameter from the lower support structure 12 (e.g. at the top plate 12a) 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. In one example, illustrated in Fig. 11, the tubular central column 10 may comprise stiffening members 10d,10e fixed at an inner surface of the tubular central column 10. With reference to Figs 12-15, the floatable foundation 100 may further comprise at least one cable tube 19 configured for receiving an electrical cable 18. As illustrated in Fig. 12, the cable tube 19 can be fixed to the upper support structure 11 at a flat panel 11a,11c,11e between the beam members 40,41,42, and to the lower support structure 12 at a flat panel between the pontoon members 30,31,32. Alternatively, as illustrated in Fig. 13, the cable tube 19 can be fixed inside one of the outer column members 20,21,22, and with an opening for the cable 18 at a bottom plate 20b,22b of the outer column member 20,21,22. Alternatively, as illustrated in Fig. 14, the cable tube 19 can be fixed inside the tubular central column 10 and with an opening at the bottom plate 12b for the cable 18. Alternatively, as illustrated in Fig. 15, the cable tube 19 can be fixed to a side wall 42c,d of one beam member 40,41,42 and to a side wall of one pontoon member 30,31,32. With reference to Figs 16-19, the floatable foundation 100 may further comprise a slanted strengthening truss beams 45 located between one of the three pontoon members 30,31,32 and a corresponding one of the three beam members 40,41,42 (i.e. the beam member 40,41,42 arranged vertically above the pontoon member 30,31,32). The truss beam 45 may be fixed at one end thereof to the pontoon member 30,31,32, the beam member 40,41,42, the tubular central column 10, the upper support structure 11 or the lower support structure 12, and at the other end thereof to the pontoon member 30,31,32, the one beam member 40,41,42, or one of the three outer column members 20,21,22. Fig. 16 illustrates a diagonal truss, and Fig. 17 illustrates a double diagonal truss. Fig. 18 illustrates a V-truss, and Fig. 19 illustrates an A-truss. In any of the examples or embodiments described or claimed herein, the term fixed to can mean fixed to by welding. Further inventive aspects and embodiments are outlined in the following numbered clauses. A1 .A floatable foundation (100) for a wind turbine generator comprising: a tubular central column (10) an upper support structure (11) arranged about and fixed to the tubular central column (10), a lower support structure (12) fixed to the tubular central column (10), three outer column members (20,21,22) disposed about the tubular central column (10), three horizontally extending pontoon members (30,31,32), each horizontally extending pontoon member (30,31,32) fixed to and extending between the lower support structure (12) and a respective one of the three outer column members (20,21,22); three horizontally extending beam members (40,41,42), each horizontally extending beam member (40,41,42) fixed to and extending between the upper support structure (11) and a respective one of the three outer column members (20,21,22). A2.The floatable foundation (100) according to any preceding clause, wherein the lower support structure (12) is arranged about and fixed to the tubular central column (10). A3. The floatable foundation (100) according to any preceding clause, wherein the tubular central column (10) extends through the upper support structure (11). A4.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 (11a-f) arranged about the tubular central column (10), for example wherein the plurality of vertically arranged, interconnected flat panels (11a-f) forms a polygonal, such as hexagonal, collar about the tubular central column (10). A5.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 (11g-l) extending radially outwardly between the tubular central column (10) and the interconnected flat panels (11a-f). A6.The floatable foundation (100) according to any preceding clause, wherein each of the support plates (11g-l) is fixed at an interface between two interconnected flat panels (11a-f). A7.The floatable foundation (100) according to any preceding clause, wherein the upper and / or lower support structure(s) (11,12) comprise(s) a top plate (11m,12a) arranged about and fixed to the tubular central column (10), and fixed to upper edges of the flat panels (11a-f). A8.The floatable foundation (100) according to any preceding clause, wherein the upper support structure (11) comprises a bottom plate (11 n, 12b) arranged about and fixed to the tubular central column (10), and fixed to lower edges of the flat panels (11a-f). A9.The floatable foundation (100) according to any preceding clause, wherein the top plate (11 m) and / or the bottom plate (11 n) is fixed to the plurality of vertically arranged support plates (11 g-l). A10. The floatable foundation (100) according to any preceding clause, wherein the lower support structure (12) comprises a top plate (12a), and wherein a lower end of the tubular central column (10) is landed onto the top plate (12a) and fixed thereto. A11. The floatable foundation (100) according to any preceding clause, wherein the lower support structure (12) comprises a bottom plate (12b), and wherein a lower end of the tubular central column (10) is landed onto or positioned adjacent an upward-facing surface of the bottom plate (12b) and fixed in the lower support structure (12), for example fixed to the upward-facing surface of the bottom plate (12b). A12. The floatable foundation (100) according to any preceding clause, wherein an interior of the tubular central column (10) is not open to surrounding sea water. A13. The floatable foundation (100) according to any preceding clause, wherein the tubular central column (10) comprises a single opening located at its top end and is otherwise a closed cylindrical column. A14. The floatable foundation (100) according to any preceding clause, wherein the lower support structure (12) comprises a bottom plate (12b) which separates an interior of the tubular central column (10) from surrounding sea water. A15. The floatable foundation (100) according to any preceding clause, wherein a bottom plate (12b) of the lower support structure (12), bottom plates (30b,31b,32b) of the three horizontally extending pontoon members (30,31,32) and bottom plates (20b,22b) of the three outer column members (20,21,22) define an uninterrupted, planar bottom face of the foundation (100), particularly wherein no opening is present in the uninterrupted, planar bottom face of the foundation (100). A16. The floatable foundation (100) according to any preceding clause, wherein each pontoon member (30,31,32) is fixed at one respective flat panel of the lower support structure (12). A17. The floatable foundation (100) according to any preceding clause, wherein a width (b2) of each pontoon member (30,31,32) is equal to a width (b5) of the one respective flat panel of the lower support structure (12). A18. The floatable foundation (100) according to any preceding clause, wherein each beam member (40,41,42) is fixed at one respective flat panel of the upper support structure (11). A19. The floatable foundation (100) according to any preceding clause, wherein a width (b1) of each beam member (40,41,42) is equal to a width (b4) of the one respective flat panel of the upper support structure (11). A20. The floatable foundation (100) according to any preceding clause, wherein each of the pontoon members (30,31,32) is a box beam made up of four outer flat panels, the four outer flat panels defining a top wall, a bottom wall, and a pair of side walls. A21. The floatable foundation (100) according to any preceding clause, wherein each of the beam members (40,41,42) is a box beam made up of four outer flat panels, the four outer flat panels defining a top wall (42a), a bottom wall (42b), and a pair of side walls (42c,d). A22. The floatable foundation (100) according to any preceding clause, wherein: each beam member (40,41,42) comprises at least one internal corner plate (49) fixed between a side wall (42c,d) and a top or bottom wall (42a,42b) of the beam member (40,41,42), and the upper support structure (11) comprises at least one corner plate (48) arranged between a top or bottom plate (11n,11m) of the upper support structure (11) and a flat panel (11a,11c,11e) which is adjacent to a flat panel (11 b,11d, 11 f) onto which the beam members (40,41,42) are fixed. A23. The floatable foundation (100) according to any preceding clause, wherein: each pontoon member (30,31,32) comprises at least one internal corner plate (49) fixed between a side wall and a top or bottom wall of the pontoon member (30,31,32), and the lower support structure (12) comprises at least one corner plate (48) arranged between a top or bottom plate (12a, 12b) of the lower support structure (12) and a flat panel which is adjacent to a flat panel onto which the pontoon members (30,31,32) are fixed. A24. The floatable foundation (100) according to any preceding clause, wherein each outer column member (20,21,22) is a polygonal prism comprising a plurality of rectangular side wall panels. A25. 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. A26. 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 pontoon member (30,31,32) and one beam member (40,41,42) are fixed. A27. The floatable foundation (100) according to any preceding clause, wherein a width (b3) of the one rectangular side wall panel onto which one pontoon member (30,31,32) and one beam member (40,41,42) are fixed is equal to a width (b2) of the respective pontoon member (30,31,32) and a width (b1) of the respective beam member (40,41,42). A28. The floatable foundation (100) according to any preceding clause, wherein b1=b2=b3=b4=b5. A29. The floatable foundation (100) according to any preceding clause, wherein hi <h2, or wherein h1=h2, or wherein h1>h2. A30. The floatable foundation (100) according to any preceding clause, wherein a height of the upper support structure (11), such as a height of the plurality of vertically arranged, interconnected flat panels (11a-f) forming part of the upper support structure (11), is equal to a height (hi) of the beam members (40,41,42). A31. 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 forming part of the lower support structure (12), is equal to a height (h2) of the pontoon members (30,31,32). A32. The floatable foundation (100) according to any preceding clause, wherein the column members (20,21,22) and the upper support structure (11), and optionally also the lower support structure (12), have an identical polygonal cross-sectional profile in the horizontal plane, for example a profile of a regular polygon. A33. 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. A34. The floatable foundation (100) according to any preceding clause, wherein the tubular 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). A35. The floatable foundation (100) according to any preceding clause, wherein the tubular central column (10) has a constant diameter from the lower support structure (12) to a position above the upper support structure (11), for example to a position at which the flange (15) is arranged on the tubular central column (10). A36. The floatable foundation (100) according to any preceding clause, wherein the tubular central column (10) comprises stiffening members (10d,10e) fixed at an inner surface of the tubular central column (10). A37. The floatable foundation (100) according to any preceding clause, comprising at least one cable tube (19) configured for receiving an electrical cable (18), wherein the cable tube (19) is fixed: to the upper support structure (11) at an outside surface of a flat panel (11a,11c,11e) which is adjacent to a flat panel (11 b, 11 d, 11f) onto which the beam members (40,41,42) are fixed, and to the lower support structure (12) at an outside surface of a flat panel which is adjacent to a flat panel onto which the pontoon members (30,31,32) are fixed; inside one of the outer column members (20,21,22) and in conjunction with an opening at a bottom plate (20b,22b) of the outer column member (20,21,22); inside the tubular central column (10) and in conjunction with an opening at a bottom plate (12b) of the lower support structure (12); or to a side wall (42c,d) of one beam member (40,41,42) and to a side wall of one pontoon member (30,31,32). A38. The floatable foundation (100) according to any preceding clause, comprising at least one slanted truss beam (45) located between one of the three pontoon members (30,31,32) and a corresponding one of the three beam members (40,41,42), the truss beam (45) being fixed at one end thereof to the one pontoon member (30,31,32), the one beam member (40,41,42), the tubular central column (10), the upper support structure (11) or the lower support structure (12), and at the other end thereof to the one pontoon member (30,31,32), the one beam member (40,41,42), or one of the three outer column members (20,21,22). A39. A method of constructing a floatable foundation (100) for a wind turbine generator, the method comprising: manufacturing a first tower support part (13a), the first tower support part (13a) comprising a first central column part (10a) and an upper support structure (11) arranged about and fixed to the first central column part (10a); providing a second tower support part (13b) comprising a second central column part (10b); manufacturing a third tower support part (13c), the third tower support part (13c) comprising a lower support structure (12); fixing the first central column part (10a) to the second central column part (10b) and the second central column part (10b) to the third tower support part (13c); providing three outer column members (20,21,22) disposed about the tubular central column (10) fixing three horizontally extending pontoon members (30,31,32) between the lower support structure (12) and a respective one of the three outer column members (20,21,22); and fixing three horizontally extending beam members (40,41,42) between the upper support structure (11) and a respective one of the three outer column members (20,21,22). A40. The method of any preceding clause, the method comprising manufacturing the third tower support part (13c) with a third central column part (10c) and with the lower support structure (12) fixed to the third central column part (10c), and wherein the step of fixing the second central column part (10b) to the third tower support part (13c) comprises fixing the second central column part (10b) to the third central column part (10c). A41. The method of any preceding clause, wherein the step of fixing the second central column part (10b) to the third tower support part (13c) comprises fixing a lower end (10b’) of the second central column part (10b) onto a top plate (12a) of the lower support structure (12). In other examples, illustrated in Figs 20-28, the pontoon members 30,31,32 and / or the column members 20,21,22 may be provided with reinforcement members 50 extending laterally outwardly from the pontoon members 30,31,32 and / or from the column members 20,21,22, or extending between pontoon members 30,31,32. Figs 20 and 21 illustrate, in top and perspective views, an example of a floatable foundation 100 having reinforcement members 50. The reinforcement members 50 are arranged in a common horizontal plane with the pontoon members 30,31,32 and the lower support structure 12, such that the reinforcement members 50 form a horizontal extension from the pontoon members 30,31,32 and / or the lower support structure 12. One reinforcement member 50 may be arranged in conjunction with each pair of neighbouring pontoon members 30,31,32. The reinforcement members 50 may be arranged symmetrically about the lower support structure 12. In the example illustrated in Figs 20 and 21, each of the reinforcement members 50 comprises a first reinforcement member part 51 fixed to and extending outwardly from the lower support structure 12. The first reinforcement member part 51 may be fixed to the lower support structure 12 adjacent or between two respective pontoon members 30,31,32. The first reinforcement member part 51 may be fixed to the lower support structure 12 for example at an outside surface of a flat panel 12c (see Fig. 24) which is adjacent to a flat panel 12b,12d onto which a pontoon member 30,31,32 is fixed, or at an outside surface of a flat panel 12c which is arranged between two flat panels 12b, 12d onto which pontoon members 30,31,32 are fixed. In the illustrated examples, the lower support structure 12 is hexagonal, with three flat panels having pontoon members 30,31,32 fixed thereto and three flat panels having first reinforcement member parts 51 fixed thereto. (See also the discussion below in relation to Figs 24 and 25.) In this example, each reinforcement member 50 further comprises two second reinforcement member parts 52,53, where each second reinforcement member part 52,53 is fixed to the first reinforcement member part 51 and to a respective distal end part of one of the pontoon members 30,31,32. The second reinforcement member parts 52,53 may comprises a beam, such as a box beam. As illustrated, the second reinforcement member parts 52,53 are in this example fixed to respective side walls of one of the pontoon members 30,31,32, e.g. welded to the outer flat panel making up the pontoon member 30,31,32. In the example shown in Figs 20 and 21, the reinforcement members 50 define openings 54 between the reinforcement members 50 and the pontoon members 30,31,32. In this example, the openings 54 are defined by the first reinforcement member part 51, one of the second reinforcement member parts 52,53, and the respective pontoon member 30,31,32. Arranging the reinforcement members 50 so as to define openings 54 can facilitate adjustments of masses and added mass and increase or reduce damping by means of adding or reducing drag. Alternatively, or additionally, arranging the reinforcement members 50 so as to define openings 54 can reduce material (e.g., steel) use. The amount and geometry of openings 54 can be varied and adjusted in order to tune damping or mass properties to desired levels. Alternatively, as illustrated in Figs 22 and 23, the first and second reinforcement member parts 51,52,53 may be arranged integrally and continuously fixed to the lower support structure 12 and respective pontoon members 30,31,32 such as not to form any opening 54. The example illustrated in Figs 22 and 23 is otherwise equivalent to that described in relation to Figs 20 and 21 above. Illustrated in Figs 20 and 22, the second reinforcement member parts 52,53 may advantageously be arranged with an obtuse angle a therebetween. The obtuse angle a may be larger than an angle between the two respective pontoon members 30,31,32 (i.e., the angle between longitudinal central axes of the pontoon members 30,31,32). The obtuse angle a may be larger than 120 degrees. The angle a may be measured as the angle between outward-facing sides 52a,53a (see Figs 21,23) of the second reinforcement member parts 52,53, or as the angle between longitudinal central axes of the second reinforcement member parts 52,53. Figs 24 and 25 illustrate further details of the example illustrated in Figs 20 and 21. Fig. 24 illustrate a sectional view and Fig. 25 a partial view of the lower support structure 12 and associated parts. As illustrated, the first reinforcement member part 51 may have a triangular horizontal cross-section and comprise two plates 51 a,b extending outwardly from the lower support structure 12. The lower support structure 12 is in this example hexagonal, with three flat panels 12b,d,f having pontoon members 30,31,32 fixed thereto and three flat panels 12a,c,e having first reinforcement member parts 51 fixed thereto. The arrangement with flat panels 12a-f may be equivalent to that described for the upper support structure 11 in Fig. 6 and associated description above. Vertically arranged support plates (equivalent to and arranged similarly as vertically arranged support plates 11g-l) may be arranged in conjunction with flat panels 12a-f. Each of the two plates 51 a,51b can be arranged coplanar with, such as in extension of, a respective adjacent flat panel 12b,12d onto which a pontoon member 30,31,32 is fixed. An edge (such as an outermost edge) of the plates 51a,51b may be fixed with the second reinforcement member parts 52,53 at or directly adjacent an interface 55 between the outward-facing sides 52a,53a (see Fig. 25). Advantageously, each of the two plates 51 a,51b has an extension in the horizontal plane which is equal to that of the respective flat panel 12b,12d onto which a pontoon member 30,31,32 is fixed. The extension of the respective flat panel 12b, 12d may be equal to the width b5 as indicated in Fig. 3 and described above. These examples may provide structural and construction advantages for example in that the arrangement of internal stiffeners in the lower support structure 12, the reinforcement members 50 and / or the pontoon members 30,31,32 can be simplified. In other examples, illustrated in Figs 26-28, the reinforcement members 50 may comprise elongate beams, such as box beams, extending between two pontoon members 30,31,32. Illustrated in Figs 26 and 27, the reinforcement member 50 may be fixed to outwardfacing side walls of two of the pontoon members 30,31,32, such as to side walls at distal end parts of the pontoon members 30,31,32, adjacent the column members 20,21,22, or at other locations of the side walls. Fig. 28 illustrates another example, wherein the foundation 100 has a support collar 46 fixed on the tubular central column 10 and slanted truss beams 45 arranged between the support collar 46 and each one of the three pontoon members 30,31,32. The foundation 100 illustrated in Fig. 28, as well as those illustrated in Figs 16-19, may have a combination of slanted truss beams 45 and reinforcement members 50. The reinforcement members 50 may, in such a case, for example be according to one or more of the examples illustrated in Figs 20-27. Applying one or more truss beams 45, for example as illustrated in Figs 16-19 or 28, may particularly provide structural advantages in relation to vertical forces, e.g. heave-related forces acting on the foundation 100. The examples illustrated in Figs 16-28 may advantageously allow for reduced overall steel weight of a 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 the interfaces between the pontoon members 30,31,31 and the support structure 12.) By providing strengthening truss beams 45 and / or reinforcement members 50, acceptable structural strength can be obtained without the need to enhance hull strength locally by e.g. use of thicker steel plates. This can provide more flexibility and more efficient manufacturing, in that the same basic foundation design can be used for various different locations / projects, but with strengthening truss beams 45 and / or reinforcement members 50 added on a case-by-case basis, according to local requirements at the intended operating site. Adding reinforcement members 50 such as illustrated in Figs 20-28 to the basic design (e.g. with a basic design as illustrated in Figs 2-4) can further facilitate adjustments of masses and added mass, and increase or reduce damping by means of adding or reducing drag. The specific design of reinforcement members 50 may thus be used at the design stage to influence both structural characteristics and hydrodynamic behaviour. This can provide design flexibility for use of the foundation 100 at different locations and under different metocean conditions, by providing a core basic design which can be adapted as required for different locations / projects. Reinforcement members 50 may particularly provide structural advantages in relation to horizontal forces acting on the foundation 100, for example to reduce stresses at the interface between the pontoon members 30,31,32 and the lower support structure 12, and / or to reduce stresses at the interface between the beam members 40,41,42 and the upper support structure 11. Figs 29 and 30 illustrate an example foundation 100 in which the column members 20,21,22 have a quadrilateral horizontal cross-section. The cross-section may, for example, be rectangular or square. Fig. 31 illustrates an example of a foundation 100 in which the tubular central column 10 is replaced by a polygonal central column 10’. Any of the aspects, examples or embodiments described or claimed herein may be realised with the tubular central column 10 replaced by such a polygonal central column 10’. Advantageously, the polygonal central column 10’ may have the same horizontal cross-section as the lower support structure 12 and the upper support structure 11. The polygonal central column 10’ may extend continuously between the lower support structure 12 and the upper support structure 11 and interconnect these to form a combined polygonal prism consisting of the polygonal central column 10’ and the lower and upper support structures 11,12. Additionally or alternatively, the polygonal central column 10’ may have the same horizontal cross-section as each of the column members 20,21,22. The polygonal central column 10’ 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 10’ can be done, by using the same plate dimensions and / or same construction equipment. Figs 32 and 33 illustrate examples of a foundation 100 in which the beam members 40,41,42 are arranged slanted downwardly as they extend towards the column members 20,21,22. In the example illustrated in Fig. 32, the outer end parts of the beam members 40,41,42 are fixed at or adjacent the interface between the column members 20,21,22 and the pontoon members 30,31,32. The beam members 40,41,42 may be fixed to both the column members 20,21,22 and to the pontoon members 30,31,32. Optionally, the beam members 40,41,42 may be fixed higher up on the column members 20,21,22, or may be fixed at a top face of the pontoon members 30,31,32. As illustrated in the example illustrated in Fig. 33, the upper support structure 11 may be arranged vertically higher than a top of the column members 20,21,22, and with the beam members 40,41,42 slanted downwardly and fixed at a top part of the column members 20,21,22. Any of the aspects, examples or embodiments described or claimed herein may be realised with non-horizontal beam members 40,41,42, for example with beam members 40,41,42 extending as illustrated in Figs 32 or 33. Applying non-horizontal beam members 40,41,42, for example as illustrated in Figs 32 or 33, may particularly provide structural advantages in relation to vertical forces, e.g. heave-related forces acting on the foundation 100. Further inventive aspects and embodiments are outlined in the following numbered clauses. A42. The floatable foundation (100) according to any preceding clause, comprising a plurality of reinforcement members (50), the reinforcement members (50): extending outwardly, such as laterally or horizontally outwardly, from the pontoon members (30,31,32), and / or extending between pontoon members (30,31,32). A43. The floatable foundation (100) according to any preceding clause, wherein the reinforcement members (50) are arranged in a common plane, such as a common horizontal plane, with the pontoon members (30,31,32) and the lower support structure (12). A44. The floatable foundation (100) according to any preceding clause, wherein each of the reinforcement members (50) comprises a first reinforcement member part (51) fixed to the lower support structure (12) and extending outwardly from the lower support structure (12), and two second reinforcement member parts (52,53), each second reinforcement member part (52,53) being fixed to the first reinforcement member part (51) and to a respective distal end part of one of the pontoon members (30,31,32). A45. The floatable foundation (100) according to any preceding clause, wherein each second reinforcement member part (52,53) is fixed to a side wall of one of the pontoon members (30,31,32). A46. The floatable foundation (100) according to any preceding clause, wherein the second reinforcement member parts (52,53) are arranged with an obtuse angle (a) therebetween, for example wherein the obtuse angle (a) is larger than an angle between the two respective pontoon members (30,31,32), for example wherein the angle (a) is larger than 120 degrees. A47. The floatable foundation (100) according to any preceding clause, wherein outward-facing sides (52a,53a) of the second reinforcement member parts (52,53) are arranged with an obtuse angle (a) therebetween, for example wherein the obtuse angle (a) is larger than an angle between the two respective pontoon members (30,31,32), for example wherein the angle (a) is larger than 120 degrees. A48. The floatable foundation (100) according to any preceding clause, wherein the second reinforcement member parts (52,53) each comprises a beam, such as a box beam. A49. The floatable foundation (100) according to any preceding clause, wherein the reinforcement members (50) define openings (54) between the reinforcement members (50) and the pontoon members (30,31,32). A50. The floatable foundation (100) according to any preceding clause, wherein the first reinforcement member part (51) is fixed to the lower support structure (12) at an outside surface of a flat panel (12c) which is adjacent to a flat panel (12b, 12d) onto which a pontoon member (30,31,32) is fixed, for example at an outside surface of a flat panel (12c) which is arranged between two flat panels (12b, 12d) onto which pontoon members (30,31,32) are fixed. A51. The floatable foundation (100) according to any preceding clause, wherein the first reinforcement member part (51) has a triangular horizontal cross-section and comprises two plates (51a,b) extending outwardly from the lower support structure (12). A52. The floatable foundation (100) according to any preceding clause, where each of the two plates (51 a,51b) is arranged coplanar with a respective adjacent flat panel (12b, 12d) onto which a pontoon member (30,31,32) is fixed. A53. The floatable foundation (100) according to any preceding clause, where each of the two plates (51a,51b) has an extension in the horizontal plane which is equal to that of the respective flat panel (12b, 12d) onto which a pontoon member (30,31,32) is fixed. A54. The floatable foundation (100) according to any preceding clause, wherein the reinforcement members (50) comprise elongate beams, such as box beams. A55. The floatable foundation (100) according to any preceding clause, wherein each reinforcement member (50) is fixed to side walls of two of the pontoon members (30,31,32), such as side walls at distal end parts of the pontoon members (30,31,32). A56. The floatable foundation (100) according to any preceding clause, comprising a support collar (46) fixed on the tubular central column (10) and at least one slanted truss beam (45) arranged between the support collar (46) and one of the three pontoon members (30,31,32), for example comprising slanted truss beams (45) arranged between the support collar (46) and each one of the three pontoon members (30,31,32) In another inventive example and aspect, there is provided 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. The method may comprise receiving the electric power at a land-based location for supply into a land-based electricity grid. Advantageously, the method can provide environmentally friendly generation of electric power for land-based consumers. In another inventive example and aspect, there is provided a method of designing a floatable foundation 100 for a wind turbine generator. The foundation 100 may be a foundation according to any of the examples, aspects or embodiments described above. 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, represent an operational life (fatigue life) of the foundation. 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 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. Advantageously, the foundation 100 may be a foundation having truss beams 45 and / or reinforcement members 50, as described above. The method may include adjusting the three-dimensional (3D) computer model by adjusting a parameter associated with the truss beams 45 and / or the reinforcement member(s) 50, while keeping dimensions of the column members 20,21,22, pontoon members 30,31,32 and beam members 40,41,42 unchanged. The parameter associated with the truss beams 45 may, for example, be a size, structural strength (e.g. thickness), orientation or arrangement (e.g. localisation of the interfaces where the truss beams 45 are fixed to the rest of the foundation structure) of the truss beams 45. See, for example, Figs 16-19 and 28. The parameter associated with the reinforcement member(s) 50 may, for example, be a size, structural strength (e.g. thickness), orientation or arrangement (e.g. localisation of the interfaces where the reinforcement member(s) 50 are fixed to the rest of the foundation structure) of the of the reinforcement member(s) 50. See, for example, Figs 20-28. 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. Further inventive aspects and embodiments are outlined in the following numbered clauses. A57. 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 clauses A1-A38 or A42-A56; (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. A58. 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 pre-determined 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). A59. The method according to any of the two preceding clauses, wherein the three-dimensional (3D) computer model is a three-dimensional (3D) computer model of a foundation (100) according to any of clauses A38 or A42-A56, and wherein the step of adjusting the three-dimensional (3D) computer model comprises adjusting a parameter associated with the truss beams (45) and / or the reinforcement member(s) (50), while keeping dimensions of the column members (20,21,22), pontoon members (30,31,32) and beam members (40,41,42) unchanged. A60. The method according to any of the two preceding clauses, 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. A61. The method according to the preceding clause, 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. According to examples and embodiments described here, efficient manufacturing of a floatable foundation 100 for a wind turbine generator can be achieved. The foundation 100 advantageously can be realized with a low steel weight compared to existing technology, due to its beneficial structural properties. For example, as illustrated schematically in Fig. 5, bending moments applied on the wind turbine generator tower 16 will be counteracted by a force couple acting on the tubular central column 10 from the upper and lower support structures 11,12. Advantageously, foundations 100 as disclosed herein can 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. 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, which again may equal the width of the beam members 40,41,42. The width of the interfaces at the upper and / or lower support structure(s) 11,12 may also correspond to the same. The top of the beam members 40-42 and / or the top surface of the column members 5 20-22 may be arranged to support equipment, such as electrical converters, substations, hydrogen (H2) production equipment, fluid storage, material storage, batteries, or other items. Improved technology as described herein may be employed to reduce cost and for more effective use of existing facilities, equipment and supply chains. In some io implementations, “serial production” can be achieved, with efficient repeated use of the same equipment, facilities and personnel to produce a series of foundations 100. The invention is not limited by the embodiments described above; reference should be had to the appended claims. 15

Claims

1. 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 the floatable foundation (100), the floatable foundation (100) comprising:a tubular central column (10);an upper support structure (11) arranged about and fixed to the tubular central column (10), the tubular central column (10) extending through the upper support structure (11);a lower support structure (12) fixed to the tubular central column (10);three outer column members (20,21,22) disposed about the tubular central column (10);three horizontally extending pontoon members (30,31,32), each horizontally extending pontoon member (30,31,32) fixed to and extending between the lower support structure (12) and a respective one of the three outer column members (20,21,22); andthree horizontally extending beam members (40,41,42), each horizontally extending beam member (40,41,42) fixed to andextending between the upper support structure (11) and a respective one of the three outer column members (20,21,22);(b) providing a set of metocean data, the set of metocean data being representative of environmental conditions at a proposed operational site for the floatable 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.

2. The method according to claim 1, 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 pre-determined 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).

3. The method according to claim 2, wherein the three-dimensional (3D) computer model is a three-dimensional (3D) computer model of a floatable foundation (100) comprising a plurality of reinforcement members (50), each reinforcement member (50) extending between at least two pontoon members (30,31,32),and wherein the step of adjusting the three-dimensional (3D) computer model comprises adjusting a parameter associated with the reinforcement members (50), while keeping dimensions of the column members (20,21,22), pontoon members (30,31,32) and beam members (40,41,42) unchanged.

4. The method according to claim 2 or 3, 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.

5. The method according to claim 4, 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.

6. A floatable foundation (100) for a wind turbine generator, the floatable foundation (100) comprising:a tubular central column (10);an upper support structure (11) arranged about and fixed to the tubular central column (10), the tubular central column (10) extending through the upper support structure (11);a lower support structure (12) fixed to the tubular central column (10);three outer column members (20,21,22) disposed about the tubular central column (10);three horizontally extending pontoon members (30,31,32), each horizontally extending pontoon member (30,31,32) fixed to and extending between the lower support structure (12) and a respective one of the three outer column members (20,21,22);three horizontally extending beam members (40,41,42), each horizontally extending beam member (40,41,42) fixed to and extending between the upper support structure (11) and a respective one of the three outer column members (20,21,22); anda plurality of reinforcement members (50), each reinforcement member (50) extending between at least two pontoon members (30,31,32).

7. The floatable foundation (100) according to claim 6, wherein the reinforcement members (50) are arranged in a common horizontal plane, with the pontoon members (30,31,32) and the lower support structure (12).

8. The floatable foundation (100) according to claim 6 or 7, wherein each of the reinforcement members (50) comprisesa first reinforcement member part (51) fixed to the lower support structure (12) and extending outwardly from the lower support structure (12), andtwo second reinforcement member parts (52,53), each second reinforcement member part (52,53) being fixed to the first reinforcement member part (51) and to a respective distal end part of one of the pontoon members (30,31,32).

9. The floatable foundation (100) according to claim 8, wherein each second reinforcement member part (52,53) is fixed to a side wall of one of the pontoon members (30,31,32).

10. The floatable foundation (100) according to claim 8 or 9, wherein the second reinforcement member parts (52,53) are arranged with an obtuse angle (a) therebetween, wherein the angle (a) is larger than 120 degrees.

11. The floatable foundation (100) according to any of claims 8-10, wherein outward-facing sides (52a,53a) of the second reinforcement member parts (52,53) are arranged with an obtuse angle (a) therebetween, wherein the angle (a) is larger than 120 degrees.

12. The floatable foundation (100) according to any of claims 8-11, wherein the second reinforcement member parts (52,53) each comprises a box beam.

13. The floatable foundation (100) according to any of claims 8-12, wherein the first reinforcement member part (51) is fixed to the lower support structure (12) at an outside surface of a flat panel (12c) which is arranged between two flat panels (12b, 12d) onto which pontoon members (30,31,32) are fixed.

14. The floatable foundation (100) according to any of claims 8-13, wherein the first reinforcement member part (51) has a triangular horizontal cross-section and comprises two vertical plates (51 a,b) extending outwardly from the lower support structure (12).

15. The floatable foundation (100) according to claim 14, where each of the two plates (51a,51b) is arranged coplanar with a respective adjacent flat panel (12b,12d) onto which a pontoon member (30,31,32) is fixed.

16. The floatable foundation (100) according to claim 14 or 15, where each of the two plates (51 a,51b) has an extension in the horizontal plane which is equal to that of the respective flat panel (12b,12d) onto which a pontoon member (30,31,32) is fixed.

17. The floatable foundation (100) according to claim 6, wherein each reinforcement member (50) is fixed to side walls of two of the pontoon members (30,31,32).

18. The floatable foundation (100) according to claim 17, wherein the reinforcement members (50) comprise elongate beams, such as box beams.

19. The floatable foundation (100) according to any of claims 6-18, wherein the reinforcement members (50) define openings (54) between the reinforcement members (50) and the pontoon members (30,31,32).34

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