Floating wind turbine or equipment foundations

Abstract

A floatable foundation 100 for a wind turbine generator and / or electrical equipment 61, the foundation comprising: a central column structure 10,11,12; at least three outer column members 20,21,22 dis

Description

The present disclosure relates to floating wind turbine foundations, including but not limited to ballast arrangements and structural design of floating wind turbine foundations, 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 155 538 A1; 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 (100) for a wind turbine generator and / or electrical equipment 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 a floatable foundation according to another example. Fig. 11 illustrates a floatable foundation according to another example. Figs 12-13 illustrate a floatable foundation according to another example. Figs 14-22 illustrate floatable foundations according to other examples, comprising ballast arrangements. Figs 23-26 illustrate floatable foundations according to other examples, comprising inner corner supports. Figs. 27-30 illustrate floatable foundations according to other examples, comprising watertight decks in foundation column members. Figs. 31-33 illustrate floatable foundations according to other examples, comprising ballast arrangements. Figs. 34 -58 illustrate floatable foundations according to other examples, comprising ballast arrangements. DETAILED DESCRIPTION Fig. 1 illustrates an example of 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 column members 20,21,22 and beam member 40,41,42 may advantageously be offset by 120 degrees. The tubular central column 10 in this example 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 in this example 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 11 g-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 Fig. 3) 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 Fig. 4) 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,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. 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 Fig. 4) 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. 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 b1,b2 (see Fig. 3), and only differ in their height h1,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. 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, for example at a tower flange 15 (see Fig. 1) 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. (Indicated as rectangular side wall panel 22x in Fig. 3.) 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. (Fig. 3 illustrates the width b3 at the adjacent rectangular side wall panel, but it will be understood that the rectangular side wall panel onto which the beam member 42 and the pontoon member 32 are fixed has the same width b3 when the horizontal cross-sectional shape is a regular polygon.) 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 when the horizontal cross-sectional shape is a regular polygon.) 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 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. Figs 9 and 10 illustrate, in top and perspective views, another example foundation 100 in which the column members 20,21,22 have a quadrilateral horizontal crosssection. The cross-section may, for example, be rectangular or square. The foundation 100 may otherwise comprise any or all of the features described in relation to Figs 1-8 above. Fig. 11 illustrates an example of a foundation 100 having 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. 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 slanting upwardly towards the upper support structure 11. Applying non-horizontal beam members 40,41,42 may provide structural advantages in relation to vertical forces, e.g. heave-related forces acting on the foundation 100. Illustrated in Figs 12 and 13, the foundation 100 may optionally be a foundation 100 for holding equipment 61. The foundation 100 may, for example, be a foundation 100 used as an offshore substation at an offshore wind park, where the equipment 61 is electrical equipment such as transformers, switchgears, etc. As illustrated, the equipment 61 can be arranged at a deck 60, which is supported at, or formed by, the beam members 40,41,42, the central column 10 / upper support structure 12, and / or the column members 20,21,22. As illustrated in Fig. 12, equipment 61 may also be provided in equipment compartments 62 in the column members 20,21,22. As illustrated in Fig. 13, the deck 60 may comprise a deck extension 63 which extends beyond the beam members 40,41,42, the central column 10 / upper support structure 12, and / or the column members 20,21,22. In any of the examples or embodiments described or claimed herein, the term fixed to can mean fixed to by welding. In any of the examples or embodiments described or claimed herein, the width b2 (see Fig. 3) of the pontoon members 30,31,32 may be constant along the entire length of the respective pontoon member 30,31,32. The width b2 may be equal to the width b3 of the one rectangular side wall panel 22x onto which the pontoon member 30,31,32 is fixed and equal to the width b5 of the one respective flat panel of the lower support structure 12 onto which the pontoon member 30,31,32 is fixed. Similarly, in any of the examples or embodiments described or claimed herein, the width b1 of the beam members 40,41,42 may be constant along the entire length of the respective beam member 40,41,42. The width b1 may be equal to the width b3 of the one rectangular side wall panel 22x onto which the beam member 40,41,42 is fixed and 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. Reference is now made to Figs 14-22 , which describe various ballast arrangements for a floatable foundation 100. As the skilled reader will understand, the ballast arrangements are here indicated schematically in a side view of the floatable foundation 100. The floatable foundation 100 may be a floatable foundation 100 according to any of the examples described above, or variations thereof. The floatable foundation 100 may, for example, alternatively have round columns, truss beams interconnecting the central column structure 10,11,12 with the column members 20,21,22, etc. As illustrated in Fig. 14, the a ballast tank 80 may be arranged in the central column structure 10,11,12. The ballast tank 80 may, as illustrated, be arranged in the central column 10 and above the lower support structure 12. Alternatively, the ballast tank 80 may be arranged partly or fully in the lower support structure 12. Each column member 20,21,22 may comprise a ballast tank 81. The ballast tanks 81 may be configured for holding water ballast, and the ballast tank 81 may vertically span a design waterline 91 of the respective column member 20,21,22. (i.e. an upper ballast water level in the ballast tank 81 may thus be vertically higher than the seawater surface 90.) The ballast tanks 81 may be formed as compartments in the respective column member 20,21,22. The column members 20,21,22 may have vertical divisions, such as steel plates, defining the ballast tanks 81, and separating the ballast tanks 81 to for example air-filled interior volumes inside the column members 20,21,22 above the ballast tanks 81. Also illustrated in Fig. 14, each column member 20,21,22 may comprise a buoyancy compartment 89 arranged vertically below the ballast tank 81. In this example, the buoyancy compartment 89 have a vertical extension which is equal to a height h2 (see Fig. 5) of the pontoon members 30,31,32. The buoyancy compartments 89, the pontoon members 30,31,32 and the lower support structure 12 may make up a pontoon layer of the floatable foundation 100, where the pontoon layer has no water ballast tanks arranged therein. Fig. 15 illustrates a ballast arrangement similar to that shown in Fig. 14, but wherein no ballast is arranged in the central column structure 10,11,12. Fig. 16 illustrates another arrangement wherein each column member 20,21,22 comprises a second ballast tank 82. The second ballast tank 82 may be arranged below the first ballast tank 81, for example entirely below a design waterline 91 of the respective column member 20,21,22. The first ballast tank 81 may, as described above, vertically span the design waterline 91. In this example, the foundation 100 further comprises buoyancy compartments 89 (as described above) and the second ballast tank 82 is arranged between the first ballast tank 81 and the buoyancy compartment 89. The second ballast tank 82 may be open to the sea 92 via a fluid opening 93 (a so-called “soft-tank”). This can allow automatic flooding of the second ballast tank 82 when the foundation 100 is in use, indicated by curved arrows in Fig. 16, illustrating seawater flowing into the second ballast tanks 82. Fig. 17 similarly illustrates a foundation having second ballast tanks 82, but where a closure member 83, such as a valve, is arranged in the fluid opening 93. The closure member 83 is operable to selectively open the second ballast tank 82 to the sea 92. The example in Fig. 17 further comprises an access pipe 87 extending through the first ballast tank 81 and into the second ballast tank 82. The access pipe 87 is arranged to allow a ballast water pipe and / or ballast water pump (not shown) to be lowered into the second ballast tank 82 in order to drain the second ballast tank 82, if desired. The access pipe 87 may be a vertical pipe extending from the second ballast tank 82 to an upper part 20a (such as above the design waterline 91), or to (or above) a top 20b of the respective column member 20,21,22. Optionally, a bottom wall of the second ballast tank 82 may comprise a sump 86 located vertically below the access pipe 87. The sump 86 can be arranged to receive a drainage pump such as to allow complete or near-complete emptying of the second ballast tank 82, if desired. Fig. 18 illustrates an example in which the first ballast tanks 81 comprise solid ballast 88, as fixed ballast arranged in the first ballast tanks 81. The solid ballast 88 may be provided in addition to water ballast in the first ballast tanks 81. Fig. 19 illustrates an example where the ballast tank 81 is arranged about a buoyancy tank or air-filled interior 85 in the respective column member 20,21,22. The ballast tank 81 may, for example, be arranged as a “double skin” of the respective column member 20,21,22. The “double skin” may be arranged with a first, outermost vertical plate structure 84a (indicated at the right hand side of Fig. 19) and a second, internal vertical plate structure 84b, where the first and second plate structures 84a,b define the ballast tank 81 therebetween. The first, outermost vertical plate structure 84a can make up an outward-facing side of the respective column member 20,21,22, whereby the plate structure 84a interfaces the sea 92 at its outward-facing side and the ballast tank 81 at its other, inward-facing side. The internal vertical plate structure 84b can, for example, be a circular or polygonal structure arranged inside the column member 20,21,22. Corresponding top and bottom plates may be provided such as to define a closed volume inside the ballast tank 81. The ballast tank 81 can be arranged as (or in the case of polygonal structures, similar to) an annulus volume (forming an annular tank) in the column member 20,21,22. As described above, the ballast tank 81 may vertically span the design waterline 91, as illustrated. Fig. 20 illustrates a similar example as that shown in Fig. 19, but where the air-filled interiors 85 of the column members 20,21,22 comprise solid ballast 88. Illustrated in Fig. 21, the ballast tank 81 may extend from a bottom 20c of the respective column member 20,21,22 and to a position above the design waterline 91. Alternatively, as shown in Figs 19 and 20, the ballast tank 81 can be spaced from the bottom 20c of the column member 20,21,22, for example to only be arranged at or near a typical collision zone around the design waterline 91. Fig. 22 illustrates a similar arrangement as that shown in Fig. 21, but with solid ballast 88, similarly as described above in relation to Fig. 20. Figs 23-25 illustrate parts of a floatable foundation 100 having an inner corner support 70 between the pontoon members 30,31,32 and the respective column member 20,21,22, and / or an inner corner support 70 between the beam members 40,41,42 and the respective column member 20,21,22. Fig. 26 illustrates a schematic side view of the floatable foundation 100 having such an inner corner support 70. The inner corner supports 70 are here illustrated in relation to beam member 40, pontoon member 30 and column member 20, but may be arranged equivalently for the other beam members 41,42, pontoon members 31,21 and column members 21,22. The floatable foundation 100 may have inner corner supports 70 between pontoons 30,31,32 and columns 20,21,22 only, between beam members 40,41,42 and columns 20,21,22 only, or both. The floatable foundation 100 may otherwise be configured as described in any of the examples above. Each of the inner corner supports 70 comprises a rectangular plate 71 fixed to the same side wall panel of the column member 20 onto which the pontoon member 30 and beam member 40 are fixed. The rectangular plate 71 is further fixed to the pontoon member 30 at a top wall thereof or the beam member 40 at a bottom wall 42b (see Fig. 7) thereof. The rectangular plate 71 can, for example, be a steel plate which is welded to the column member 20 and to the pontoon member 30 or beam member 40. The rectangular plate 71 advantageously extends from the column side wall panel towards the pontoon member 30 or beam member 40 with an angle between 30 and 60 degrees, such as 45 degrees. In this manner, an effective load transfer can be provided. The rectangular plate 71 may extend across the full width, or substantially the full width (such as at least 95%) of the column side wall panel and the pontoon member 30 or beam member 40. The inner corner supports 70 may further comprise a pair of side plates 72, and the rectangular plate 71 can be arranged between the side plates 72 and welded to respective inward-facing faces of the side plates 72. The rectangular plate 71 may thus be welded at all its four edges: At two opposite edges to the side plates 72, at one edge to the column member 20 and at one edge to the pontoon member 30 or beam member 40. Each side plate 72 has a first edge welded to the column member 20 and a second edge welded to the pontoon member 30 or beam member 40. The first edge may, for example, be welded to the same rectangular side wall panel onto which the pontoon member 30 or beam member 40 is fixed. The second edge may, for example, be welded to a bottom wall 42b (see Fig. 7) of the beam member 40 or to a top wall of the pontoon member 30. Each side plate 72 may further have a third edge which is curved. Each side plate 72 may have exactly three edges. Referring now to Figs 27-29, the foundation 100 may, in each of the three outer column members 20,21,22, comprise a first watertight deck 78 and a second watertight deck 79 which define the ballast tank 81 therebetween. The watertight decks 78,79 may each be a horizontal deck, for example a horizontal wall or bulkhead arranged in the respective outer column member 20,21,22. The first and second watertight decks 78,79 may be arranged vertically lower than the beam members 40,41,42 and vertically higher than the pontoon members 30,31,32. By means of the watertight decks 78,79, a compartment 76 in the column members 20,21,22 above the first watertight deck 78 may be arranged to be open to an interior 75 of the respective beam member 40,41,42. Similarly, a compartment 77 in the column members 20,21,22 below the second watertight deck 79 may be open to an interior 74 of the respective pontoon member 30,31,32. In the illustrated example, the foundation 100 further comprises inner corner supports 70 arranged between each pontoon member 30,31,32 and the respective column member 20,21,22, and inner corner supports 70 arranged between each beam member 40,41,42 and the respective column members 20,21,22. (See also the description in relation to Figs 23-26 above.) Each of the inner corner supports 70 comprises a rectangular plate 71 being fixed to the one rectangular side wall panel 20x (see Fig. 28; see also item 22x in Fig. 3) of the respective column member 20,21,22 onto which the pontoon member 30,31,32 or beam member 40,41,42 is fixed, and to the pontoon member 30,31,32 or beam member 40,41,42. Side plates 72 may further be used, similarly as described above. The rectangular plate(s) 71 may advantageously be fixed to the one rectangular side wall panel 20x,22x of the respective column member 20,21,22 at a height which corresponds to a vertical position of the respective watertight deck 78,79. For example, the rectangular plate(s) 71 and a respective watertight deck 78,79 can be fixed to the rectangular side wall panel 20x,22x at the same vertical height and at opposite sides thereof. Each pontoon member 30,31,32 may further comprise a bulkhead 73a, and each beam member 40,41,42 may comprise a bulkhead 73b. The rectangular plates 71 may be fixed to the pontoon member 30,31,32 at a location along the longitudinal length of the pontoon member 30,31,32 which corresponds to a position of the bulkhead 73a. For example, the rectangular plates 71 and the bulkhead 73a may be fixed to the pontoon member 30,31,32 at opposite sides of an outer flat panel defining a top wall of the pontoon member 30,31,32. Similarly, the rectangular plates 71 may be fixed to the beam member 40,41,42 at a location along the longitudinal length of the beam member 40,41,42 which corresponds to a position of the bulkhead 73b. For example, the rectangular plates 71 and the bulkhead 73b may be fixed to the beam member 40,41,42 at opposite sides of an outer flat panel defining a bottom wall 42b (see Fig. 7) of the beam member 40,41,42. This arrangement of the watertight decks 78,79 and rectangular plates 71 may be structurally beneficial, and / or may reduce fatigue / stress loading (or correspondingly ease design or construction requirements) at the interface region 99 between a beam member 40,41,42 and column member 20,21,22, and similarly at the interface region 98 between a pontoon member 30,31,32 and column member 20,21,22. (See Fig. 29.) Advantageously, the bulkhead 73a, the rectangular plate 71, side plates 72, a lower part 20d of the column member 20,21,22, the second watertight deck 79, and an outer part 30a of the pontoon member 30,31,32 may define a compartment 74,77. This is illustrated schematically in Fig. 30. The compartment 74,77 can be formed partly in the pontoon member 30,31,32 (outside the bulkhead 73a) and partly in the column member 20,21,22 below the ballast tank 81. Similarly, the bulkhead 73b, the rectangular plate 71, side plates 72, an upper part 20a of the column member 20,21,22, the first watertight deck 78, and an outer part 40a of the pontoon member 30,31,32 may define a compartment 75,76. The compartment 75,66 can be formed partly in the beam member 40,41,42 (outside the bulkhead 73b) and partly in the column member 20,21,22 above the ballast tank 81. These configurations can be beneficial structurally, as well as in terms of the ballast arrangements. For example, if having the ballast tank 81 arranged to span the design waterline 91, which generally corresponds to the “collision zone” where damage due to external impacts is most likely to occur, the risk of damage to the compartments 74,77 and 75,76 is reduced, and it may be permissible to arrange these as compartments extending from the column member 20,21,22 and into the beam member 40,41,42 or the pontoon member 30,31,32. (I.e. make these into “common” compartments, where the respective interior volume 76,77 of the column member 20,21,22 is not structurally separated from, but open to, an interior volume 74,75 of the beam member 40,41,42 or pontoon member 30,31,32.) Additionally, this may allow material savings (e.g. save steel) in the interface regions 98,99, in that there can be openings between the column member 20,21,22 and the beam member 40,41,42 or pontoon member 30,31,32. Figs 31-41 illustrate various ballast arrangements for the foundation 100 as illustrated in Fig. 30. In Fig. 31, the central column 10 and the lower support structure 12 holds water ballast, and the ballast tanks 81 also holds water ballast. In Fig. 32, the pontoon members 30,31,32 comprise water ballast in a ballast compartment 30e,32e which is separated from the compartment 74,77 by the bulkhead 73a. In this example, the ballast tanks 81 are not filled, but they may alternatively also comprise water ballast similarly as in Fig. 31. The central column 10 and lower support structure 12 also holds water ballast in this example, but one or both of these may alternatively be empty. In Fig. 33, the compartment 74,77 comprises solid ballast 88 and the pontoon members 30,31,32 comprise water ballast in a ballast compartment 30e,32e which is separated from the compartment 74,77 by the bulkhead 73a. The central column 10 and lower support structure 12 also holds water ballast in this example, but one or both of these may alternatively be empty. Figs. 35 to 42 illustrate various ballast arrangements for the foundation 100 as illustrated in Fig. 34 which is an illustration of the foundation 100 as described previously, but without any ballast. The “basic” features of Fig. 34 are replicated in the variants of Figs. 35 to 41 and the same reference numerals are used to identify corresponding features. The views of Figs. 35 to 42 are slightly different from the views of Figs. 26, 27 and 30 to 33. The views of Figs. 26, 27 and 30 to 33 are schematic cutaway cross-sectional views looking in a continuous plane extending vertically through an extension of the longitudinal axis A-A (see Fig. 9) of one of the pontoon members 31, whereas the views of Figs. 35 to 42 are schematic cutaway cross-sectional views looking in a non-continuous plane extending vertically through the longitudinal axis A-A of a first pontoon member 30, 31, 32 to the vertical longitudinal axis X of the central column 10 and from there through a plane extending vertically through the longitudinal axis A-B of a second one of the of one of the pontoon members 30, 31, 32 As described previously, the foundation 100 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 column members 20,21,22 and beam member 40,41,42 may advantageously be offset by 120 degrees. The tubular central column 10 in this example extends through the upper support structure 11. The foundation is illustrated as having the inner corner supports 70 between the pontoon members 30,31,32 and the respective column member 20,21,22, and the inner corner supports 70 between the beam members 40,41,42 and the respective column member 20,21,22. As described previously, the corner supports 70 comprise a rectangular plate 71 fixed to the same side wall panel of the column member 20 onto which the pontoon member 30 and beam member 40 are fixed. The rectangular plate 71 is further fixed to the pontoon member 30 at a top wall thereof and to the beam member 40 at a bottom wall thereof. As described previously, each pontoon member 30,31,32 further comprises a bulkhead 73a and each beam member 40,41,42 comprises a bulkhead 73b. The longitudinal positions of the bulkheads 73a, 73b may be arranged to coincide with the position where the rectangular plates 71 are secured to the pontoon members 30,31,32 and the beam members 40,41,42. In addition, each column member 20,21,22 is provided with a horizontal bulkhead 79 positioned at a height where the rectangular plates 71 are secured to the 30,31,32 and which define a compartment or hopper 77. In the variant of Fig. 35, water ballast 110 is provided in the lower compartment / hopper 77 of each of the column members 20, 21, 22. In this variant, the lower end of the tubular central column 10 extends down to the level of the base of the lower support structure 12 and additional water ballast 112 is provided in a ballast compartment 115 formed at the lowermost portion of the tubular central column 10. In the variant of Fig. 36, the same water ballast 110 is provided in the lower compartment / hopper 77 of each of the column members 20, 21, 22. In this variant, the lower end of the tubular central column 10 terminates at the upper face of the lower support structure 12 and additional water ballast 114 is provided in a ballast compartment 115a formed at a lowermost portion of the central tubular column 10. In the variant of Fig. 37, the same water ballast 110 is provided in the lower compartment / hopper 77 of each of the column members 20, 21, 22. In this variant, no ballast is provided in the tubular central column, but instead additional water ballast 116 is provided in a ballast compartment 117 formed by the interior each of the pontoon members 30, 31, 32 between the bulkhead 73a with the associated lower compartment / hopper 77 and the lower support structure 12, and extending for substantially the whole length of the pontoon members 30, 31, 32. In the variant of Fig. 38, the same water ballast 110 is provided in the lower compartment / hopper 77 of each of the column members 20, 21,22 and no ballast is provided in the tubular central column. In addition, each of the pontoon members 30, 31, 32 is provided with a further bulkhead 118 radially inwardly of the bulkhead 73a, and which can be positioned anywhere along the length of the pontoon members. This forms a ballast compartment 117a which extends only along part of the length of the pontoon members 30, 31, 32 in order to allow adjustment of the quantity and location of water ballast 120 in the pontoon members 30, 31, 32. The variant of Fig. 38 may be further adjusted, if desired, to remove one or more of the bulkheads 73a between the pontoon members 30, 31, 32 and the respective column member 20, 21,22 (as indicated by a dotted line to the bulkheads 73a in the figure) to form continuous ballast tanks formed by the associated lower compartment / hopper 77 and the portion of the pontoon members 30, 31, 32 radially outwardly of the inner bulkhead 118. The variant of Fig. 39 is the same as the variant of Fig. 38 above, but additional water ballast 114 is also provided in the ballast compartment 115a of the lowermost portion of the central tubular column 10, in the same way as for the variant of Fig. 36. If the bulkheads 118 are omitted, then the variant of Fig. 39 would be the same as the variant of Fig. 37, with the additional water ballast 114 in the ballast compartment 115a of the central tubular column 10. The variant of Fig. 40 is very similar to the variant of Fig. 37, except that the lower support structure 12 is of a reduced radius and the ballast compartments 117a of the pontoon members 30, 31, 32 are slightly longer. This arrangement avoids having ballast within the central tubular column 10 and the lower support structure 12, but increases the amount of ballast 116 within the pontoon members 30, 31, 32 nearer to the centre of the foundation 100 (i.e. nearer the longitudinal axis of the central tubular column 10). The variant of Fig. 41 is very similar to the variant of Fig. 38, with the bulkheads 73a located between the pontoon members 30, 31, 32 and the respective column member 20, 21, 22 and the further, inner bulkheads 118 located in the pontoon members. However, instead of introducing the ballast 120 of Fig. 38 into the ballast compartments 117a of the pontoon members 30, 31, 32 radially outwardly of the further bulkheads 118, ballast 122 is instead located in ballast compartments 117b located radially inwardly of the inner bulkheads, 18, between the bulkheads 118 and the lower support structure 12. Fig. 42 illustrates a variant of the foundation 100 as illustrated in Fig. 34, with individual compartments isolated from each other (e.g. compartments 77, the interiors of the pontoon members 30, 31, 32, the region of the column members 20, 21,22 above the horizontal bulkhead 79). Isolation of the compartments within the foundation 100 allows ballast to be positioned where required. In the variant of Fig. 42, solid ballast 124, 126 is located at desired positions within the column members 20, 21, 22, and as shown, the size and / or shape and / or weight and / or height and / or radial position and / or lateral position and / or orientation of the solid ballast 124, 126 may differ between compartments. Also, although not shown in Fig. 42, the solid ballast 124, 126 need not be positioned symmetrically with respect to the longitudinal axes of the pontoon members 30, 31, 32 and / or may be positioned to one side (or to both sides) of the longitudinal axes of the pontoon members 30, 31, 32. Solid ballast of the desired shape, type and weight may be positioned at the desired location in one, a plurality or all of the compartments of the foundation 100, to achieve the desired balance and / or stability of the foundation 100. Alternatively, or in addition, in any of the examples illustrated in Figs 31-42, the central column structure 10,11,12 may comprise solid ballast 88. In the variant of Fig. 43, a ballast tank 130 formed by the internal volume of the lower support structure 12 communicates with the interior of each of the pontoon members 30, 31, 32, which allows water ballast to pass between the lower support structure 12 and the pontoon members 30, 31, 32, as indicated by the arrows in the figure. In the variant of Fig. 44, the internal volume of the lower support structure 12 is isolated from the interior of each of the pontoon members 30, 31, 32 by means of a bulkhead 132. In this variant, the ballast tank 130 formed by the internal volume of the lower support structure 12 is filled with ballast water 134, but no ballast water is present within the pontoon members 30, 31, 32. In the variant of Fig. 45, which is a modification of the variant of Fig. 44 (and for which the same reference numerals identify the same features) the ballast tank 130 formed by the internal volume of the lower support structure 12 is filled with ballast water 134 and no ballast water is present within the pontoon members 30, 31, 32. However, the ballast tank 130 is a so-called “soft tank” which is open to the sea water within which the foundation 100 is positioned in use, as indicated schematically by one or more apertures 136 in one or more of the side walls 140, 142, 444 of the lower support structure 12 and / or one or more apertures 138 in the upper wall 12a and / or the lower wall 12b of the lower support structure 12. This allows sea water to circulate within the ballast tank 130, as indicated schematically by the arrow in the figure. The variant of Figs. 46 to 49 is a further modification of the variant of Fig. 44. Fig. 46 is a horizontal cross-section through the lower support structure 12, Fig. 47 is a horizontal cross-section through one of the pontoons 31 and its associated column member (but the pontoon members 30, 32 and their associated column members 20, 22 are identical), Fig. 48 is a vertical cross-section through the pontoon 31 and the column member 21 and Fig. 49 is a vertical cross-section through the support structure looking in the direction of arrows A - B indicated in Fig. 9. In this modification, the bulkheads 132 of Fig. 44 have been removed, so that the internal volume of each of the pontoon members 30, 31, 32 communicates with the ballast tank 130’ formed by the internal volume of the lower support structure 12. However, three identical, equally angularly spaced vertical walls 150 extend radially from the exterior surface of the central tubular column 10 to the junction of a respective side wall 152 of the lower support structure 12 with a side wall 154 of an adjacent pontoon member 30, 31, 32. The vertical walls 150 extend between the upper and lower walls of the support structure 12 and divide the ballast tank 130’ of the structure 12 into three separate, identical ballast tanks 130a, 130b, 130c, each in communication with the interior of a respective one of the three pontoon members 30, 31, 32. As shown in Figs. 47 to 49, the pontoons 30, 31, 32 are open all the way out to the respective column member 20, 21,22. In particular, the bulkheads 73a between the outer ends of the pontoons 30, 31, 32 and the respective column member 20, 21, 22, shown e.g. in Fig. 37, are omitted and the ballast tanks 77 at the base of the column members 20, 21,22 are isolated from the interior volumes of the respective pontoon 30, 31, 32, but may still be filled with ballast as indicated at 110 in Figs. 47 to 49. The arrangement illustrated in Fig. 50 is a variant of the arrangement of Fig. 21 in which a ballast tank 81 is arranged about a buoyancy tank or air-filled interior 85 in the respective column member 20,21,22 and is arranged as a “double skin” of the respective column member 20,21,22. In this variant, the ballasting is arranged towards the centre of the floatable foundation 100. Some possible arrangements are illustrated in Figs. 51 to 53. In Fig. 51, two vertical walls 160 extend between a respective one of the apexes of the inner wall 84b of the column members 21, 21,22 adjacent to the respective pontoon 30, 31, 32 and the junction of the outer wall 84a and the associated pontoon 30, 31, 32, defining a radially inner ballast sub-chamber 81a. In Fig. 51, each sub-chamber 81a is in communication with a ballast chamber 162 at the radially outer end of the associated pontoon 30, 31, 32 defined by an internal bulkhead 164. Reference numerals have only been included in Fig. 51 for one of the column members 20 and its associated pontoon 30, but the construction of the three column members 20, 21, 22 and pontoon members 30, 31, 32 is identical. The variant of Fig. 52 is almost identical to that of Fig. 51, except that the subchambers 81a do not communicate with a ballast chamber in the associated pontoon 30, 31, 32. In the variant of Fig. 53, additional internal walls 166 extend between the middle apexes (in the radial direction of the pontoons 30, 31, 32) of the outer and inner walls 84a, 84b of the column members 21, 21,22 and define two additional ballast sub-chambers 81 b, 81 c adjacent to, and on either side of, the ballast sub-chamber 81a. Alternatively, the double-skinned construction of the of the column members 21,21, 22 makes it possible to position the ballast in the of the column members 21, 21,22 outwardly, as illustrated in Figs. 54 to 57. The arrangement illustrated in Fig. 54 is a further variant of the arrangement of Fig. 21 in which a ballast tank 81 is arranged about a buoyancy tank or air-filled interior 85 in the respective column member 20,21,22 and is arranged as a “double skin” of the respective column member 20,21,22. In this variant, the ballasting is arranged towards the periphery of the floatable foundation 100. Some possible arrangements are illustrated in Figs. 55 to 57. The variant of Fig. 55 is constructed in the same way as the arrangement of Fig. 53, but the additional internal walls 166 define a further ballast sub-chamber 81 d within each column member 20,21,22, radially outwardly of the internal walls 166. In the variants illustrated in Figs. 56 and 57, which are further variants of the arrangement of Fig. 21, the base of each column member 20,21,22 is also doubleskinned, by the provision of a horizontal bulkhead 170 above the base of each column member 20,21,22, defining a further ballast chamber 172 at the base of each column member, which may contain ballast 172 (as shown in Fig. 56) or may be empty (as shown in Fig. 57). Figs. 56 and 57 also show radially inner ballast sub-chambers 81a (i.e. the variants of Figs. 50 to 53), but the variation of Figs. 56 and 57 is equally applicable to arrangements having radially outer ballast sub-chambers in the column members 20,21,22, e.g. as shown at 81 b in the variants of Figs. 54 and 55, and indeed to further variants in which the column members 20,21,22 have both radially inner ballast sub-chambers 81a (e.g. as shown at 81a in the variants of Figs. 50 to 53) and radially outer ballast sub-chambers (e.g. as shown at 81b in the variants of Figs. 54 and 55). Advantageously, according to examples and embodiments described herein, a structurally beneficial design with enhanced safety can be obtained. For example, in the event of damage to one of the column members 20,21,22, for example due to collision, the stability of the floatable foundation 100 may still be maintained at satisfactory levels (“damage stability”). If another vessel or item hits and punctures one of the column members 20,21,22, the ballast water which is located vertically higher than a sea surface 90 may drain out of the ballast tank 81, thereby reducing weight. Buoyancy can, in such an event, still be ensured via for example the buoyancy compartments 89, the air-filled interior 85, compartments 74,77, via buoyancy in the pontoon members 30,31,32, and / or via buoyancy in the central column structure 10,11,12. Alternatively, or additionally, the second ballast tanks 82 may be drained to provide additional buoyancy in such an event. Buoyancy compartments such as compartments 89, compartments 74,77 and / or the second ballast tanks 82, 110, 112, 114, 116, 120, 122, 130 may advantageously be positioned below a typical collision zone for this purpose, so as to reduce the risk of damage to these in the event of a damage incident. The pontoon members 30,31,32 may, equivalently, be positioned below a typical collision zone. In the variant of Fig. 58, the corner supports 70 of Figs. 23 - 42, 48 and 49 are modified. The rectangular plates 71 between the column member 20 and each of 22 the pontoon member 30 and beam member 40 are replaced with a V-shaped plate 171 formed from first and second rectangular plates 171a, 171b inclined to each other at an obtuse angle a and welded together along one transversely extending edge, the other transversely extending edge of each plate 171a being welded to the column member 20 and the other transversely extending edge of each plate 171b being welded to the pontoon member 30 or beam member 40 as appropriate. The side plates 72 described previously are replaced with generally arrowheadshaped side plates 172 comprising first and second mutually perpendicular edges 172a, 172b welded to the column member 20 and to the pontoon member 30 or beam member 40, and first and second inner edges 174, 176 inclined to each other at the same angle a as the first and second rectangular plates 171a, 171b and welded to a respective lateral edge of the first and second rectangular plates 171a, 171b. Further inventive aspects and embodiments are outlined in the following numbered clauses. A1 .A floatable foundation (100) for a wind turbine generator and / or electrical equipment (61), the foundation (100) comprising: a central column structure (10,11,12); at least three outer column members (20,21,22) disposed about the central column structure (10,11,12); at least three horizontally extending lower connection members, such as pontoon members (30,31,32), each horizontally extending lower connection member fixed to and extending between the central column structure (10,11,12) and a respective one of the at least three outer column members (20,21,22); and at least three horizontally extending upper connection members, such as beam members (40,41,42), each horizontally extending upper connection member fixed to and extending between the central column structure (10,11,12) and a respective one of the at least three outer column members (20,21,22). A2.The floatable foundation (100) according to any preceding clause, comprising a ballast tank (80) arranged in the central column structure (10,11,12). A3. The floatable foundation (100) according to any preceding clause, wherein each column member (20,21,22) comprises a ballast tank (81). A4.The floatable foundation (100) according to any preceding clause, wherein the ballast tank (81) is configured for holding water ballast and the ballast tank (81) vertically spans a design waterline (91) of the respective column member (20,21,22). A5.The floatable foundation (100) according to any preceding clause, wherein each column member (20,21,22) comprises a buoyancy compartment (89) arranged vertically below the ballast tank (81). A6.The floatable foundation (100) according to any preceding clause, wherein the horizontally extending lower connection members are pontoon members (30,31,32) and the buoyancy compartment (89) has a vertical extension which is equal to a height (h2) of the pontoon members (30,31,32). A7.The floatable foundation (100) according to any preceding clause, wherein the ballast tank (81) is a first ballast tank (81) and each column member (20,21,22) comprises a second ballast tank (82). A8.The floatable foundation (100) according to any preceding clause, wherein the second ballast tank (82) is arranged below the first ballast tank (81). A9.The floatable foundation (100) according to any preceding clause, wherein the second ballast tank (82) is arranged entirely below a design waterline (91) of the respective column member (20,21,22). A10. The floatable foundation (100) according to any preceding clause, wherein the second ballast tank (82) is arranged between the first ballast tank (81) and the buoyancy compartment (89). A11. The floatable foundation (100) according to any preceding clause, wherein the second ballast tank (82) is open to the sea (92) via a fluid opening (93). A12. The floatable foundation (100) according to any preceding clause, wherein the second ballast tank (82) is comprises a fluid opening (93) with a closure member (83) operable to selectively open the second ballast tank (82) to the sea (92). A13. The floatable foundation (100) according to any preceding clause, wherein the first ballast tank (81) comprises solid ballast (88). A14. The floatable foundation (100) according to any preceding clause, wherein each column member (20,21,22) comprises an access pipe (87) extending through the first ballast tank (81) to the second ballast tank (82). A15. The floatable foundation (100) according to any preceding clause, wherein the access pipe (87) is a vertical pipe extending from the second ballast tank (82) to an upper part (20a) or to a top (20b) of the respective column member (20,21,22). A16. The floatable foundation (100) according to any preceding clause, wherein a bottom wall of the second ballast tank (82) comprises a sump (86), the sump (86) located vertically below the access pipe (87). A17. The floatable foundation (100) according to any preceding clause, wherein the ballast tank (81) is arranged about a buoyancy tank or air-filled interior (85) in the respective column member (20,21,22). A18. The floatable foundation (100) according to any preceding clause, wherein the air-filled interior (85) comprises solid ballast (88). A19. The floatable foundation (100) according to any preceding clause, wherein the ballast tank (81) extends from a bottom (20c) of the respective column member (20,21,22), or wherein the ballast tank (81) is spaced from a bottom (20c) of the respective column member (20,21,22). A20. The floatable foundation (100) according to any preceding clause, comprising a first, outer vertical plate structure (84a) and a second, internal vertical plate structure (84b), the first and second plate structures (84a,b) defining the ballast tank (81) therebetween. A21. The floatable foundation (100) according to clause A20, comprising a plurality of walls extending between the outer vertical plate structure (84a) and a second, internal vertical plate structure (84b). A22. The floatable foundation (100) according to clause A21, wherein the plurality of walls extending between the outer vertical plate structure (84a) and a second, internal vertical plate structure (84b) define a radially inner ballast chamber. A23. The floatable foundation (100) according to clause A20 or A21, wherein the plurality of walls extending between the outer vertical plate structure (84a) and a second, internal vertical plate structure (84b) define a radially inner ballast chamber. A24. The floatable foundation (100) according to any of clauses A20 to A23, wherein the plurality of walls extending between the outer vertical plate structure (84a) and a second, internal vertical plate structure (84b) define a plurality of ballast chambers. A25. The floatable foundation (100) according to any preceding clause, wherein the first, outer vertical plate structure (84a) makes up an outwardfacing side of the respective column member (20,21,22). A26. The floatable foundation (100) according to any preceding clause, wherein the buoyancy compartments (89), the pontoon members (30,31,32) and the lower support structure (12) make up a pontoon layer, the pontoon layer having no water ballast tanks arranged therein. A27. The floatable foundation (100) according to any preceding clause, wherein each of the three outer column members (20,21,22) comprises a first watertight deck (78) and a second watertight deck (79), the first and second watertight decks (78,79) defining the ballast tank (81) therebetween. A28. The floatable foundation (100) according to any preceding clause, wherein the first watertight deck (78) is a horizontal deck, for example a horizontal wall or bulkhead arranged in the respective outer column member (20,21,22). A29. The floatable foundation (100) according to any preceding clause, wherein the second watertight deck (79) is a horizontal deck, for example a horizontal wall or bulkhead arranged in the respective outer column member (20,21,22). A30. The floatable foundation (100) according to any preceding clause, wherein the first and second watertight decks (78,79) are arranged vertically lower than the beam members (40,41,42) and vertically higher than the pontoon members (30,31,32). A31. The floatable foundation (100) according to any preceding clause, wherein: a compartment (76) in the column members (20,21,22) above the first watertight deck (78) is open to an interior (75) of the respective beam member (40,41,42), and / or a compartment (77) in the column members (20,21,22) below the second watertight deck (79) is open to an interior (74) of the respective pontoon member (30,31,32). A32. The floatable foundation (100) according to any preceding clause, comprising: inner corner supports (70) arranged between each pontoon member (30,31,32) and the respective one of the three outer column members (20,21,22), and / or inner corner supports (70) arranged between each beam member (40,41,42) and the respective one of the three outer column members (20,21,22). A33. The floatable foundation (100) according to any preceding clause, wherein each of the inner corner supports (70) comprises a rectangular plate (71) being fixed to the one rectangular side wall panel (20x,22x) of the respective column member (20,21,22) onto which the pontoon member (30,31,32) and / or beam member (40,41,42) is fixed, and to the pontoon member (30,31,32) or beam member (40,41,42). A34. The floatable foundation (100) according to any preceding clause, wherein the rectangular plate (71) extends from the one rectangular side wall panel with between 30 and 60 degrees, such as about 45 degrees or 45 degrees, towards the pontoon member (30,31,32) or beam member (40,41,42). A35. The floatable foundation (100) according to any preceding clause, wherein the rectangular plate (71) has a width which is at least 95% of a width (b3) of the one rectangular side wall panel onto which the pontoon member (30,31,32) or beam member (40,41,42) is fixed. A36. The floatable foundation (100) according to any preceding clause, wherein each of the inner corner supports (70) comprises a pair of side plates (72), and the rectangular plate (71) is arranged between the side plates (72), for example arranged, such as welded, between faces of the side plates (72). A37. The floatable foundation (100) according to any preceding clause, wherein each side plate (72) has a first edge welded to the respective column member (20,21,22), for example to the one rectangular side wall panel onto which the pontoon member (30,31,32) or beam member (40,41,42) is fixed or to an adjacent rectangular side wall panel thereof, and a second edge welded to the pontoon member (30,31,32) or beam member (40,41,42), for example to a bottom wall (42b) of the beam member (40,41,42) or to a top wall of the pontoon member (30,31,32). A38. The floatable foundation (100) according to any preceding clause, wherein each side plate (72) has a third edge which is curved. A39. The floatable foundation (100) according to any of clauses A32 to A38, wherein each of the inner corner supports (70) comprises first and second rectangular plates (171a, 171b) secured to each other at an obtuse angle (a) along an edge, the first rectangular plate (171a) being fixed to the one rectangular side wall panel (20x,22x) of the respective column member (20,21,22) onto which the pontoon member (30,31,32) and / or beam member (40,41,42) is fixed, and the second rectangular plate (171b) being fixed to the pontoon member (30,31,32) or beam member (40,41,42). A40. The floatable foundation (100) according to clause A39, wherein the side plates (172) between which the first and second rectangular plates are arranged. A41. The floatable foundation (100) according to clause A40, wherein each side plate (72) has a first edge (172a) welded to the respective column member (20,21,22), for example to the one rectangular side wall panel onto which the pontoon member (30,31,32) or beam member (40,41,42) is fixed or to an adjacent rectangular side wall panel thereof, a second edge welded to the pontoon member (30,31,32) or beam member (40,41,42), for example to a bottom wall (42b) of the beam member (40,41,42) or to a top wall of the pontoon member (30,31,32), a third edge (174) welded to an edge of the first rectangular plate (171a) and a fourth edge (176) welded to an edge of the second rectangular plate (171b). A42. The floatable foundation (100) according to any preceding clause, wherein the rectangular plate(s) (71) is(are) fixed to the one rectangular side wall panel (20x,22x) of the respective column member (20,21,22) at a height which corresponds to a vertical position of the respective watertight deck (78,79), for example wherein the rectangular plate(s) (71) and the watertight deck (78,79) are fixed to the rectangular side wall panel (20x,22x) of the respective column member (20,21,22) at the same vertical height and at opposite sides thereof. A43. The floatable foundation (100) according to any preceding clause, wherein each pontoon member (30,31,32) comprises a bulkhead (73a), and wherein the bulkhead (73a), the rectangular plate(s) (71), the side plates (72), a lower part (20d) of the column member (20,21,22), the second watertight deck (79), and an outer part (30a) of the pontoon member (30,31,32) define a compartment (74,77). A44. The floatable foundation (100) according to clause A43, wherein the compartment (74,77) is formed partly in the pontoon member (30,31,32) and partly in the column member (20,21,22) below the ballast tank (81). A45. The floatable foundation (100) according to any preceding clause, wherein the rectangular plate(s) (71) is(are) fixed to the pontoon member (30,31,32) at a location which corresponds to a position of the bulkhead (73a), for example wherein the rectangular plate(s) (71) and the bulkhead (73a) are fixed to the pontoon member (30,31,32) at opposite sides of an outer flat panel defining a top wall of the pontoon member (30,31,32). A46. The floatable foundation (100) according to any preceding clause, wherein each beam member (40,41,42) comprises a bulkhead (73b), and wherein the bulkhead (73b), the rectangular plate(s) (71), the side plates (72), an upper part (20a) of the column member (20,21,22), the first watertight deck (78), and an outer part (40a) of the pontoon member (30,31,32) define a compartment (75,76). A47. The floatable foundation (100) according to any preceding clause, wherein the compartment (75,66) is formed partly in the beam member (40,41,42) and partly in the column member (20,21,22) above the ballast tank (81). A48. The floatable foundation (100) according to any preceding clause, wherein the rectangular plate(s) (71) is(are) fixed to the beam member (40,41,42) at a location which corresponds to a position of the bulkhead (73b), for example wherein the rectangular plate(s) (71) and the bulkhead (73b) are fixed to the beam member (40,41,42) at opposite sides of an outer flat panel defining a bottom wall (42b) of the beam member (40,41,42). A49. The floatable foundation (100) according to any preceding clause, wherein the compartment (77) or the compartment (74,77) comprises solid ballast (88). A50. The floatable foundation (100) according to any preceding clause, wherein the central column structure (10,11,12) comprises solid ballast (88) or water ballast. A51. The floatable foundation (100) according to any preceding clause, wherein the pontoon members (30,31,32) comprise water ballast. A52. The floatable foundation (100) according to any preceding clause, wherein the pontoon members (30,31,32) comprise water ballast in a ballast compartment (30e,32e) which is separated from the compartment (74,77) by the bulkhead (73a). A53. The floatable foundation (100) according to any preceding clause, wherein the central column structure (10,11,12) comprises a tubular central column (10), a lower support structure (12) fixed to the tubular central column (10), and an upper support structure (12) fixed to the tubular central column (10). A54. 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), for example wherein the plurality of vertically arranged, interconnected flat panels (11 a-f) forms a polygonal, such as hexagonal, collar about the tubular central column (10). A55. The floatable foundation (100) according to clause A54, wherein the collar comprises a ballast compartment (130). A56. The floatable foundation (100) according to clause A55, wherein the ballast compartment (130) communicates with the interior of the horizontally extending lower connection members (30,31,32). A57. The floatable foundation (100) according to clause A56, wherein the collar comprises a plurality of ballast compartments (130a, 130b, 130c) which are isolated from each other, and wherein each of the ballast compartments communicates with the interior of a respective horizontally extending lower connection member (30,31,32). A58. 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. A59. 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). A60. 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). A61. 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). A62. 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). A63. 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. A64. 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). A65. 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. A66. 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. A67. The floatable foundation (100) according to any preceding clause, wherein each outer column member (20,21,22) comprises one rectangular side wall panel (20x,22x) onto which one pontoon member (30,31,32) and one beam member (40,41,42) are fixed. A68. The floatable foundation (100) according to any preceding clause, wherein a width (b3) of the one rectangular side wall panel (20x,22x) 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). A69. The floatable foundation (100) according to any preceding clause, wherein b1=b2=b3=b4=b5. A70. 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). A71. The floatable foundation (100) according to any preceding clause, wherein the central column structure (10,11,12), for example the tubular central column (10), comprises a flange (15), such as a bolt flange (15), the flange (15) configured for connection to a wind turbine generator tower (16). 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 and / or electrical equipment 61 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 and / or electrical equipment 61. 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 100 having watertight decks 78,79 and inner corner supports 70 as described above. Alternatively, or additionally, the foundation 100 may be a foundation 100 having compartments 75,76 and / or 74,77 arranged partly in a pontoon member 30,31,32 and partly in a column member 20,21,22, as described above. The method may include adjusting the three-dimensional (3D) computer model by adjusting a parameter associated with the watertight decks 78,79, the inner corner supports 70, the compartments 75,76 and / or the compartments 74,77 while keeping dimensions (height / width / depth) of the column members 20,21,22, pontoon members 30,31,32 and beam members 40,41,42 unchanged. The parameter associated with the watertight decks 78,79, the inner corner supports 70, the compartments 75,76 and / or the compartments 74,77 may, for example, be a size, structural strength (e.g. thickness or stiffener arrangements), size / location of openings in steel plates, orientation of parts or arrangement (e.g. localisation of interfaces) of e.g. the watertight decks 78,79, the rectangular side wall panel 20x,22x and / or the rectangular plates 71. In this manner, a basic design can form a 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. A72. A method of designing a floatable foundation (100) for a wind turbine generator and / or electrical equipment (61), the method comprising: (a) generating a three-dimensional (3D) computer model of a foundation (100) according to any of clauses A1-A61; (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. A73. 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). A74. The method according to any of the two preceding clauses, wherein the foundation (100) is a foundation having watertight decks (78,79), inner corner supports (70), compartments (75,76) and / or compartments (74,77), and wherein the step of adjusting the three-dimensional (3D) computer model comprises adjusting a parameter associated with the watertight decks (78,79), the inner corner supports (70), the compartments (75,76) and / or the compartments (74,77), while keeping dimensions of the column members (20,21,22), pontoon members (30,31,32) and beam members (40,41,42) unchanged. A75. The method according to any of the 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. A76. 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 design and manufacturing of a floatable foundation 100 for a wind turbine generator and / or electrical equipment 61 can be achieved. The foundation 100 can advantageously be realized with beneficial structural properties and ballast arrangements. Further inventive aspects and embodiments are outlined in the following numbered clauses. A77. A floatable foundation (100) for a wind turbine generator and / or electrical equipment (61), the foundation (100) comprising: a central column (10) having upper and lower support structures (11,12); at least three outer column members (20,21,22) disposed about the central column structure (10,11,12); at least three horizontally extending lower connection members, such as pontoon members (30,31,32), each horizontally extending lower connection member fixed to and extending between the lower support structure (12) and a respective one of the at least three outer column members (20,21,22); at least three horizontally extending upper connection members, such as beam members (40,41,42), each horizontally extending upper connection member fixed to and extending between the upper support structure (11) and a respective one of the at least three outer column members (20,21,22), wherein each of the at least three outer column members (20,21,22) comprises a base wall (20b, 21b, 22b) and a watertight deck (78) above the base wall, and wherein a ballast compartment (77) is defined between the base wall (20b, 21b, 22b) and a watertight deck (78). A78. The floatable foundation (100) according to clause A77, wherein at least one of the at least three horizontally extending lower connection members (30, 31, 32) comprises a ballast compartment (117). A79. The floatable foundation (100) according to clause A77 or clause A78, wherein the ballast compartment (117) of the horizontally extending lower connection member (30, 31, 32) extends along substantially the whole length of the lower connection member. A80. The floatable foundation (100) according to any of clauses A77 to A79, wherein the ballast compartment (117) of the horizontally extending lower connection member (30, 31, 32) extends partly along the lower connection member. A81. The floatable foundation (100) according to clause A80, wherein the ballast compartment (117) of the horizontally extending lower connection member (30, 31, 32) is adjacent to the ballast compartment (77) in the outer column member (20, 21,22) to which is fixed. A82. The floatable foundation (100) according to clause A81, comprising a bulkhead (73a) which isolates the ballast compartment (117) of the horizontally extending lower connection member (30, 31, 32) from the ballast compartment (77) in the outer column member (20, 21, 22) to which is fixed. A83. The floatable foundation (100) according to clause A77 to A82, wherein the ballast compartment (117) of the horizontally extending lower connection member (30, 31, 32) is adjacent to the lower support structure (12). A84. The floatable foundation (100) according to clause A83, wherein the lower support structure (12) comprises a ballast compartment (130) which communicates with the ballast compartment (117) of the horizontally extending lower connection member (30, 31, 32). A85. The floatable foundation (100) according to any of clauses A77 to A84, wherein the lower support structure (12) comprises a ballast compartment (130). A86. The floatable foundation (100) according to clause A84 or clause A85, wherein the ballast compartment (130) of the lower support structure (12) comprises a soft tank. A87. The floatable foundation (100) according to clause A86, wherein the lower support structure (12) comprises one or more apertures (136, 138) which connects the ballast compartment (130) to the exterior of the lower support structure (12). A88. The floatable foundation (100) according to any of clauses A77 to A87, wherein the central column (10) comprises a hollow tubular column with a ballast tank (112, 114) at its base. A89. The floatable foundation (100) according to clause A88, wherein the base of the hollow tubular column (10) is at substantially the same level as the base of the lower support structure (12). A90. The floatable foundation (100) according to clause A88, wherein the base of the hollow tubular column (10) is above the level of the base of the lower support structure (12). A91. The floatable foundation (100) according to clause A88, wherein the base of the hollow tubular column (10) is at substantially the same level as the top of the lower support structure (12). A92. The floatable foundation (100) according to any of clauses A77 to A91, comprising a plurality of buoyancy compartments (77, 112, 114, 117,. 130) containing solid buoyancy material. A93. The floatable foundation (100) according to clause A92, comprising at least two buoyancy compartments in which the solid buoyancy material differs in size and / or shape and / or weight and / or height and / or radial position and / or lateral position and / or orientation. The invention is not limited by the embodiments described above; reference should be had to the appended claims.

Claims

1. A floatable foundation (100) for a wind turbine generator and / or electrical equipment (61), the foundation (100) comprising:a central column structure (10,11,12);at least three outer column members (20,21,22) disposed about the central column structure (10,11,12);at least three horizontally extending lower connection members, such as pontoon members (30,31,32), each horizontally extending lower connection member fixed to and extending between the central column structure (10,11,12) and a respective one of the at least three outer column members (20,21,22); andat least three horizontally extending upper connection members, such as beam members (40,41,42), each horizontally extending upper connection member fixed to and extending between the central column structure (10,11,12) and a respective one of the at least three outer column members (20,21,22),wherein each column member (20,21,22) comprises a ballast tank (81) configured for holding water ballast and the ballast tank (81) vertically spans a design waterline (91) of the respective column member (20,21,22).

2. The floatable foundation (100) according to claim 1, wherein the ballast tank (81) is arranged about a buoyancy tank or air-filled interior (85) in the respective column member (20,21,22).

3. The floatable foundation (100) according to claim 2, wherein the ballast tank (81) extend from a bottom (20c) of the respective column member (20,21,22).

4. The floatable foundation (100) according to claim 2, wherein the ballast tank (81) is spaced from a bottom (20c) of the respective column member (20,21,22).

5. The floatable foundation (100) according to any of claims 2-4, comprising a first, outer vertical plate structure (84a) and a second, internal vertical platestructure (84b), the first and second plate structures (84a,b) defining the ballast tank (81) therebetween.

6. The floatable foundation (100) according to claim 5, wherein the first, outer vertical plate structure (84a) makes up an outward-facing side of the respective column member (20,21,22).

7. The floatable foundation (100) according to claim 1, wherein each of the three outer column members (20,21,22) comprises a first watertight deck (78) and a second watertight deck (79), the first and second watertight decks (78,79) defining the ballast tank (81) therebetween.

8. The floatable foundation (100) according to claim 7, wherein the first and second watertight decks (78,79) are horizontal decks arranged in the respective outer column member (20,21,22).

9. The floatable foundation (100) according to claim 7 or 8, wherein the first and second watertight decks (78,79) are arranged vertically lower than the beam members (40,41,42) and vertically higher than the pontoon members (30,31,32).

10. The floatable foundation (100) according to any of claims 7-9, wherein: a first compartment (76) in the column members (20,21,22) above the first watertight deck (78) is open to an interior (75) of the respective beam member (40,41,42), anda second compartment (77) in the column members (20,21,22) below the second watertight deck (79) is open to an interior (74) of the respective pontoon member (30,31,32).

11. The floatable foundation (100) according to any of claims 7-10, comprising: inner corner supports (70) arranged between each pontoon member (30,31,32) and the respective outer column member (20,21,22), each of the inner corner supports (70) comprising a rectangular plate (71) fixed to the one rectangular side wall panel (20x,22x) of the respective column member (20,21,22) onto which the pontoon member (30,31,32) is fixed and to thepontoon member (30,31,32), and a pair of side plates (72) between which the rectangular plate (71) is arranged,wherein each pontoon member (30,31,32) comprises a bulkhead (73a), andwherein the bulkhead (73a), the rectangular plate (71), the side plates (72), a lower part (20d) of the column member (20,21,22), the second watertight deck (79), and an outer part (30a) of the pontoon member (30,31,32) define a compartment (74,77).

12. The floatable foundation (100) according to claim 11, wherein the rectangular plate (71) has a width which is at least 95% of a width (b3) of the one rectangular side wall panel onto which the pontoon member (30,31,32) is fixed.

13. The floatable foundation (100) according to claim 11 or 12, wherein each side plate (72) hasa first edge welded to the respective column member (20,21,22), and a second edge welded to the pontoon member (30,31,32).

14. The floatable foundation (100) according to any of claims 11-13, wherein the rectangular plate (71) is fixed to the one rectangular side wall panel (20x,22x) of the respective column member (20,21,22) at a height which corresponds to a vertical position of the second watertight deck (79).

15. The floatable foundation (100) according to any of claims 11-14, wherein the first compartment (74,77) is formed partly in the pontoon member (30,31,32) and partly in the column member (20,21,22) below the ballast tank (81).

16. The floatable foundation (100) according to any of claims 11-15, wherein the rectangular plate (71) is fixed to the pontoon member (30,31,32) at a location which corresponds to a position of the bulkhead (73a).

17. The floatable foundation (100) according to any of claims 7-10, comprising: inner corner supports (70) arranged between each beam member (40,41,42) and the respective outer column member (20,21,22), each of theinner corner supports (70) comprising a rectangular plate (71) fixed to the one rectangular side wall panel (20x,22x) of the respective column member (20,21,22) onto which the beam member (40,41,42) is fixed and to the beam member (40,41,42), and a pair of side plates (72) between which the rectangular plate (71) is arranged,wherein each beam member (40,41,42) comprises a bulkhead (73b), andwherein the bulkhead (73b), the rectangular plate (71), the side plates (72), an upper part (20a) of the column member (20,21,22), the first watertight deck (78), and an outer part (40a) of the beam member (40,41,42) define a compartment (75,76).

18. The floatable foundation (100) according to claim 17, wherein the rectangular plate (71) has a width which is at least 95% of a width (b3) of the one rectangular side wall panel onto which the beam member (40,41,42) is fixed.

19. The floatable foundation (100) according to claim 17 or 18, wherein each side plate (72) hasa first edge welded to the respective column member (20,21,22), and a second edge welded to the beam member (40,41,42).

20. The floatable foundation (100) according to any of claims 17-19, wherein the rectangular plate (71) is fixed to the one rectangular side wall panel (20x,22x) of the respective column member (20,21,22) at a height which corresponds to a vertical position of the first watertight deck (78).

21. The floatable foundation (100) according to any of claims 17-20, wherein the second compartment (75,76) is formed partly in the beam member (40,41,42) and partly in the column member (20,21,22) above the ballast tank (81).

22. The floatable foundation (100) according to any of claims 17-21, wherein the rectangular plate (71) is fixed to the beam member (40,41,42) at a location which corresponds to a position of the bulkhead (73b).

23. The floatable foundation (100) according to any of claims 17-21, comprising a central column (10) having upper and lower support structures (11,12);and wherein:the at least three horizontally extending lower connection members are fixed to and extend between the lower support structure (12) and a respective one of the at least three outer column members (20,21,22);the at least three horizontally extending upper connection members are fixed to and extend between the upper support structure (11) and a respective one of the at least three outer column members (20,21,22); andeach of the at least three outer column members (20,21,22) comprises a base wall (20b, 21b, 22b) and a watertight deck (78) above the base wall, and wherein a ballast compartment (77) is defined between the base wall (20b, 21b, 22b) and a watertight deck (78).

24. The floatable foundation (100) according to any of the preceding claims, wherein the column member comprises a central column (10) in the form of a hollow tubular column with a ballast tank (112, 114) at its base.

25. The floatable foundation (100) according to any of the preceding claims, wherein the central column structure (10,11,12) comprises a central column (10) having upper and lower support structures (11,12) and wherein the lower support structure (12) comprises a buoyancy compartment (130).

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