Floating wind turbine foundations

Abstract

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, a lower support structure 12 fixed to the tubular central column 10, three outer column members 20,21,22 and three horizontally extending pontoon members 30,31,32, each fixed to and extending between the lower support structure 12 and an outer column member 20,21,22; and three horizontally extending beam members 40,41,42, each fixed to and extending between the upper support structure 11 and an outer column member 20,21,22. The upper support structure 11 may comprise a plurality of vertically arranged interconnected flat panels (11 a-f, Fig.6) and corresponding support plates (11g-l, Fig.6) and the pontoons and upper beam members may be box beams. There is also provided a method of constructing a floatable foundation 100 for a wind turbine generator.

Description

The present disclosure relates to floating wind turbine foundations. 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 floatable foundation for a wind turbine generator, the floatable foundation comprising: a tubular central column; an upper support structure arranged about and fixed to the tubular central column, the tubular central column extending through the upper support structure; a lower support structure fixed to the tubular central column; three outer column members disposed about the tubular central column; three horizontally extending pontoon members, each horizontally extending pontoon member fixed to and extending between the lower support structure and a respective one of the three outer column members; and 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 three outer column members. In one example, there is provided a method of constructing a floatable foundation for a wind turbine generator, the method comprising: manufacturing a first tower support part, the first tower support part comprising a first central column part and an upper support structure arranged about and fixed to the first central column part; providing a second tower support part comprising a second central column part; manufacturing a third tower support part, the third tower support part comprising a lower support structure; fixing the first central column part to the second central column part and the second central column part to the third tower support part; providing three outer column members disposed about the tubular central column; fixing three horizontally extending pontoon members between the lower support structure and a respective one of the three outer column members; and fixing three horizontally extending beam members between the upper support structure and a respective one of the three outer column members. 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. 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, 11d, 11f of the upper support structure 11. The flat panels 11a,11 c, 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 that h1=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 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 11 a-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 (11 a, 11 c, 11 e) 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 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). 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. The top of the beam members 40-42 and / or the top surface of the column members 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 5 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. 10

Claims

1. 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); andthree 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).

2. The floatable foundation (100) according to claim 1, wherein the upper support structure (11) comprises a plurality of vertically arranged, interconnected flat panels (11 a-f) arranged about the tubular central column (10), wherein the plurality of vertically arranged, interconnected flat panels (11a-f) forms a polygonal collar about the tubular central column (10).

3. The floatable foundation (100) according to claim 2, wherein the upper support structure (11) comprises a plurality of vertically arranged support plates (11 g-l) extending radially outwardly between the tubular central column (10) and the interconnected flat panels (11a-f).

4. The floatable foundation (100) according to claim 2 or 3, wherein each beam member (40,41,42) is fixed at one respective flat panel of the upper support structure (11) and 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).

5. The floatable foundation (100) according to any of claims 2-4, wherein each of the three outer column members (20,21,22) is a right prism and comprises one rectangular side wall panel onto which one pontoon member (30,31,32) and one beam member (40,41,42) are fixed.

6. The floatable foundation (100) according to claim 5, 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).

7. The floatable foundation (100) according to claim 2 or 3, whereina width (b1) of each beam member (40,41,42) is equal to a width (b2) of each pontoon member (30,31,32),each beam member (40,41,42) is fixed at one respective flat panel of the upper support structure (11) and a width (b4) of the one respective flat panel of the upper support structure (11) is equal to the width (b1) of each beam member (40,41,42),each pontoon member (30,31,32) is fixed at one respective flat panel of the lower support structure (12) and a width (b5) of the one respective flat panel of the lower support structure (12) is equal to the width (b2) of each pontoon member (30,31,32), andeach of the three outer column members (20,21,22) is a right prism and comprises one rectangular side wall panel onto which one pontoon member (30,31,32) and one beam member (40,41,42) are fixed, and 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 the width (b2) of the respective pontoon member (30,31,32) and the width (b1) of the respective beam member (40,41,42).

8. The floatable foundation (100) according to any preceding claim, 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, andeach 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).

9. The floatable foundation (100) according to claim 8, wherein the column members (20,21,22), the upper support structure (11) and the lower support structure (12) have an identical cross-sectional outline profile in the horizontal plane, the profile being hexagonal.

10. The floatable foundation (100) according to claim 7 or 9, wherein a height of the upper support structure (11) is equal to a height (hi) of the beam members (40,41,42) and a height of the lower support structure (12) is equal to a height (h2) of the pontoon members (30,31,32).

11. The floatable foundation (100) according to claim 7 or 9, wherein the tubular central column (10) comprises a flange (15) above the upper support structure (11), the flange (15) configured for connection to a wind turbine generator tower (16).

12. The floatable foundation (100) according to claim 7 or 9, 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).

13. 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); andfixing 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).

14. The method of claim 13, 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).

15. The method of claim 13, 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).

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