Method for Assembling an Offshore Support Structure for a Wind Turbine - Patent application

Abstract

During the assembly of an offshore support structure (3) for a wind turbine (2), tubular braces (11, 12) are interconnected or connected to a tower support (8) with an injection connection. The end (12A) of the corresponding brace (12) is inserted into a sleeve (17) fixed to the interconnected brace (11) or tower support (8). The volume (20) between the sleeve (17) and the end (12A) of the inserted brace (12) is filled with an injection material, typically grout.

Description

[Technical field]

[0001] The present invention relates to a method, including the assembly and optionally also the setting up, of an offshore support structure for a wind turbine. In particular the invention relates to a method according to the preamble of the independent claim. [Background technology]

[0002] For offshore structures, for example to support wind turbines, the tetrahedral structure has the advantage of a high degree of stability and requires only moderate costs in terms of relative scale. There is a continuous effort to find improvements in manufacturing procedures in order to optimize the manufacturing methods and make offshore energy facilities more attractive, while at the same time reducing and minimizing the manufacturing costs.

[0003] Japanese patent application JP2000-087504A discloses a method of providing an offshore tower structure where the end of a connecting pipe is inserted through an opening in a larger brace and grout injection fills a portion of the larger brace and the end of the pipe which is provided with a shear key for additional stability.

[0004] The fact that grout is inserted throughout the entire pipe structure means that a large amount of grout is consumed. It is desirable to reduce the grout consumption while maintaining a high degree of stability and rigidity.

[0005] No. 4,245,928 discloses an offshore structure in which piles are driven into the seabed and braces are fixed to the piles to provide stability. The braces are connected to the piles at joints and secured in place using cement.

[0006] WO 2011 / 147472 discloses a segmented jacket structure, in particular for the foundation of a wind turbine installation, which comprises a lattice of segments interconnected by joints comprising tubular modules bonded by grout material.

[0007] When the lattice segments of the prior art are interconnected by grouting connections, the corresponding segments are pushed into the receiving cavities and grouted. This seems at first glance to be a simple procedure. However, this is not the case when the structures are very large, and thus the offshore structures that support modern large wind turbines. In particular, a simple opening in the tube is not stable until it is grouted, or at least until the opposite end is fixed. When the lattice segments are inserted into the cavities for grouting connections, it becomes desirable to achieve a better directional guidance. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2000-087504 A [Patent Document 2] U.S. Pat. No. 4,245,928 [Patent Document 3] International Publication No. 2011 / 147472 Summary of the Invention [Problem to be solved by the invention]

[0009] It is therefore an object of the present invention to improve upon the state of the art, in particular to provide an improved method for the construction of offshore platforms, in particular for wind turbines, of tetrahedral construction, in which grout injection connections are provided at the ends of the tubular segments, allowing a relatively simple assembly with directional guidance and a minimum of grout consumption. This object and other advantages are achieved by a method for assembling, and optionally setting, an offshore support structure for a wind turbine, as described below and claimed. [Means for solving the problem]

[0010] In summary, in the assembly of offshore support structures for wind turbines, tubular braces are interconnected or connected to tower supports with injection connections. The ends of the corresponding braces are inserted into sleeves fixed to the interconnected braces or tower supports. The volume between the sleeves and the inserted brace ends is filled with an injection material, typically grout.

[0011] Details are explained below.

[0012] For assembly, a first set of N first tubular braces and a second set of N second tubular braces are provided in addition to the tower support that will be used to support the tower of the wind turbine, where N=3, 4, 5, or 6. These components are then assembled into the support structure.

[0013] This assembly method is particularly useful for offshore support structures associated with offshore wind turbines, although the generality of the method does not preclude it being used as a support structure for other types of offshore platforms, such as floating platforms.

[0014] For each pair of one of the first braces and one of the second braces, the second end of the first brace is connected to the first portion of the tower support at a first connection, and the second end of the second tubular brace is connected to the second portion of the tower support at a second connection. Furthermore, the first end of the second brace is connected to the first brace at a third connection. When the support structure is oriented for operation and the wind turbine tower is in a vertical orientation, the second connection is above the first connection. Thus, the tower support, the first brace, and the second brace form a triangle in a vertical plane. Due to the triangular shape of the combination of the tower support, the radial brace, and the second brace, the second brace is also called a diagonal brace. The N pairs of braces are directed outward from the tower support in various directions around the vertical center axis of the tower support. Thus, the first brace is also called a radial brace.

[0015] In the following, various specific embodiments are presented in which an injection connection, for example grout injection, is used for the interconnection between the brace and the tower support.

[0016] In a first embodiment, the second end of the first brace is fixed to the lower part of the tower support by an injection connection, advantageously by grout injection connection. In particular, in this embodiment, the method comprises providing a wall opening in the first lower part of the tower support and a steel sleeve with a first sleeve end and a second sleeve end in the first connection. The first sleeve end is welded to the rim of the opening in the tower support, also made of steel. The sleeve extends only inwardly from the rim into the internal volume of the first part of the tower support, but does not extend outwardly from the rim beyond the opening. In other words, the sleeve is only inside the tower support and does not create any ends that would interfere with transportation and automated operations on the outer surface of the tower support, such as for example surface painting. The second end of the first brace is then inserted into the sleeve through the opening and an injection material, such as for example grout, is solidified to fill the volume between the sleeve and the first brace and to rigidly fix the first brace inside the sleeve. Optionally, the second end of the first brace is provided with a closed end, whereby injection material does not pass from the sleeve into the interior volume at the second end of the first brace.

[0017] In a second embodiment, the second end of the second brace is fixed to the upper part of the tower support by a connection by injection, advantageously by grout injection. In particular, in this embodiment, the method comprises providing a wall opening in the second part of the tower support and a steel sleeve with a first sleeve end and a second sleeve end in the second connection. The first sleeve end is welded to the rim of the opening in the tower support, also made of steel. The sleeve extends only inward from the rim into the interior volume of the second part of the tower support, but does not extend outward from the rim beyond the opening. Furthermore, in this embodiment, the sleeve is only inside the tower support and does not create any ends that would interfere with transportation and automated operations on the outer surface of the tower support, such as for example surface painting. The second end of the second brace is then inserted into the sleeve through the opening and an injection material, such as for example grout, is solidified to fill the volume between the sleeve and the first brace and to rigidly fix the first brace in the sleeve. Optionally, the second end of the second brace is provided with a closed end, whereby injection material does not pass from the sleeve into the interior volume at the second end of the second brace.

[0018] In a third embodiment, the first end of the second brace is fixed to the first brace by a connection by injection, advantageously by grout injection. In particular in this embodiment, the method comprises providing an opening in the wall of the first brace and a steel sleeve with a first sleeve end and a second sleeve end in the third connection. The first sleeve end is welded to a rim in the opening of the first brace, also made of steel, such that the sleeve extends from the rim in the opening of the first brace only inwardly into the internal volume of the first brace, but does not extend from the rim outwardly beyond the opening of the first brace. In this embodiment, the sleeve is only inside the first brace and does not create any ends that would interfere with transport and automatic operations on the outer surface of the tower support, for example surface painting. The first end of the second brace is then inserted into the sleeve through the opening and an injection material, e.g., grout, is allowed to solidify to fill the volume between the sleeve and the second brace and rigidly secure the second brace within the sleeve. Optionally, the first end of the second brace is provided with a closed end, whereby the injection material does not pass from the sleeve into the interior volume at the first end of the second brace.

[0019] As mentioned, the embodiment in which the sleeve extends only inwards is particularly advantageous for transport and working, as described above. However, in some alternative cases, it may be acceptable to have the sleeve extend outwards by a short distance. However, in a useful practical embodiment, it is not desirable for the sleeve to extend outwards from the rim more than 0.2 m, since this would make handling and working in transport more difficult. It is pointed out that typical diameters of diagonal braces are in the range of 2 m, so 0.2 m is around 10% of the diameter. Extending outwards from the rim by a distance corresponding to at most 10% of the diameter of the diagonal brace is another useful limit.

[0020] In some alternative embodiments, two or all three of the above three embodiments are combined.

[0021] In order to avoid using an excessive amount of injection material, in particular grout material, in the connection and to restrain the injection material in the sleeve, the sleeve is provided with a closed bottom at the second sleeve end, so that for fixing, the injection material, e.g. grout, is filled in the inner volume of the sleeve closed at one end, but not on the outside of the sleeve.

[0022] Advantageously, especially when the ends of the braces are provided with closed ends, the injection material, e.g. grout, is inserted into the volume between the outside of the end of the brace and the inside of the sleeve only after the corresponding end of the brace has been inserted into the sleeve, which makes it easier to position the end at the correct point inside the sleeve before the injection material is forced into the hollow volume inside the sleeve.

[0023] The injection material is a fluid or semi-fluid, for example a polymer, concrete, or grout. It is then cured to provide a solidified, rigid injection. Grout is the preferred material due to its high stiffness and longevity in salt water. In the following, grout is exemplified as the injection material, but other injection materials can be substituted if more suitable or useful.

[0024] The cavities formed by the sleeves are larger than the ends of the corresponding braces that they receive, which is advantageous not only to provide sufficient grout excess volume inside the cavity between the ends of the braces and the inner wall of the respective sleeve, but also to allow the braces to be angled slightly relative to the cavities during the assembly procedure to avoid jamming of the braces during insertion.

[0025] Generally, the sleeve is tubular with a central longitudinal axis that is parallel to the central longitudinal axis of the receiving tubular brace.

[0026] The sleeve is generally, but not necessarily, of circular cross-section perpendicular to its central longitudinal axis, in some cases the sleeve is polygonal, such as with a square or rectangular cross-section perpendicular to its central longitudinal axis.

[0027] For seabed fixed support structures, a rigid frame structure with tower support, N first braces and N second braces is generally sufficient for long term stability. For floating structures such as Tension Leg Platforms (TLP) for wind turbine towers or semi-submersible platforms, it is desirable to provide additional stability. For this reason, the following extended embodiment is useful as an option:

[0028] In this expanded embodiment, a third set of N third braces, typically tubular braces, are provided to interconnect the first braces with the third braces. For example, the third braces are connected to the first braces in a next step, optionally by grouting or alternatively by welding or by connecting to corresponding brackets. The braces are typically entirely made of steel, and welding is one of the available options. Bolting the third brace to the first brace, for example by first welding brackets to the braces and then using them for bolting, is another option.

[0029] For example, for N=4, the first brace forms an intersection with the tower support in the middle, and the third brace stabilizes the intersection in the plane formed by the intersection. Typically, the third set of N=4 braces form a square where the first braces together form an oblique angle. The first and third braces are optionally in a single plane. However, this is not required. For example, the third brace forms a square in one plane, and the first braces extend outward with their first ends at the tower support, e.g., in a plane below the plane of the square of the third brace. Additionally, although also not required, these braces may be of equal length, and one or two of the first braces may be longer than the remaining two, such that for an N=4 assembly, the third brace deviates from the square and forms a rectangle instead.

[0030] Another typical preferred example is N=3, where the third brace forms a triangle, and optionally the tower is in the center of the triangle. These third braces are also called side braces, since they form the sides of the triangle. The first braces are also generally called radial braces, since they extend radially from the tower support to each one of the corners of the triangle. In this case the first and third braces are optionally in a single horizontal plane. However, this is not necessary. For example, the third brace forms a triangle on one side, and the first brace extends outward, for example with a first end at the tower support, on a face below or above the face of the triangle of the third brace. Furthermore, the triangle does not necessarily have to be equilateral, so the braces do not necessarily have to be of equal length to form an equilateral triangle. Furthermore, the tower support does not have to be the center of the triangle.

[0031] Optionally, the interconnection of the first brace by the third brace includes interconnecting an end of the first brace by the third brace, however, this is not required as the connection can be offset a distance from the end.

[0032] For the case where N=3, the assembly can be a tetrahedral structure, optionally formed as a regular tetrahedron by the first, second, and third braces. In this case, the first brace is a radial brace, extending radially from the tower support. The third brace is a side brace, since it forms a side of the triangle. The second brace is a diagonal brace, since it extends diagonally from the first brace to the tower support. Each second brace forms a vertical triangle with the first brace and the tower support.

[0033] For example, the column support may be centered on a tetrahedral structure. Alternatively, it may be off-center, or the tower support may be at a corner of the support structure, or along a side of a triangle between two nodes.

[0034] Once the offshore support structure has been assembled, typically on shore or land, the wind turbine is attached to the top of this structure. This assembly is then moved to its final offshore point, typically by towing it with a vessel, and then anchored to the seabed, e.g., by floating the structure. As mentioned, an example is a TLP, which is a semi-submersible that typically floats in the sea. They float half-submerged above the sea surface.

[0035] To optimize the strength and service life of the grouted connections, shear keys are advantageously used at the brace inserts.

[0036] The first and second braces are tubular, and typically the third brace is also tubular. Optionally, these tubular braces have a volume with positive buoyancy. Optionally, this volume can be flooded to adjust the buoyancy. In the most common case, the braces are straight.

[0037] By way of example, the braces may optionally have diameters ranging from 1 to 6 meters, and may be as large as more than 50 meters in length. The brace ends are optionally inserted into their respective cavities at distances of 3 to 5 meters.

[0038] Optionally, the tower support itself is tubular, for example cylindrical, or conical, or combinations thereof, in adjacent sections of tubular support structure.

[0039] The invention will now be explained in more detail with reference to the drawings. [Brief description of the drawings]

[0040] [Figure 1] FIG. 1 illustrates a tetrahedral structure for offshore wind turbines. [Diagram 2] FIG. 1 illustrates a connection between two braces. [Diagram 3] FIG. 13 illustrates an alternative connection between two braces. [Figure 4] FIG. 13 is a perspective view of a sleeve in a section of a radial brace. [Diagram 5] FIG. 13 is a side perspective view of a sleeve in a radial brace. [Figure 6] FIG. 13 is a side perspective view of the sleeve at the tower support. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Figure 1 illustrates an offshore wind turbine installation 1. The installation 1 comprises a wind turbine 2 and an offshore support structure 3 on which the wind turbine 2 is mounted for operation and which supports it in offshore conditions. The wind turbine 2 comprises a rotor 5, a tower 7 and a nacelle 6 which connects the rotor 5 to the tower 7. The offshore support structure 3 comprises a tower support 8 on which the tower 7 of the wind turbine 2 is mounted. It is noted that the wind turbine 2 is not to scale with the support structure 3 and is shown at a smaller scale for ease of illustration.

[0042] The offshore support structure 3 is illustrated as a bottom support structure with a base 14 embedded in the seabed 13 below the sea surface 4. Such types of offshore support structures 3 are used in shallow waters. For deeper waters, floating structures are typically used, such as semi-submersible structures with mooring lines and buoyancy tanks that keep the structure 3 half submerged in water. In such cases, buoyancy tanks are attached to the joints 9 of the structure 3 instead of the base 14 if the tubular structure itself does not provide sufficient buoyancy. Alternatively, the structure 3 can be a tension leg platform (TLP), with a fully submerged floating support structure. The floating support structure 3 is held in place by mooring lines secured to the seabed 13.

[0043] The illustrated structure 3 has a tetrahedral shape with a central tower support 8. From a first, lower portion of the tower support, first braces 11 extend radially outward, so that these first braces 11 are also referred to as radial braces 11. From a second, upper portion of the tower support, second braces 12 extend to the first radial braces 11, so that the tower forms a flat triangle with each set of first braces 11 and second braces 12. Due to the triangular shape of the tower support 8, the radial braces 11, and the second braces 12, the second braces are also referred to as diagonal braces 12.

[0044] A triangular base for the tetrahedron is formed by the side braces 10 and the radial braces 11. The side braces 10 form a triangle by interconnecting via the radial braces 11.

[0045] The radial braces 11 connect at their second ends 11B to a first, lower portion of the tower structure 8, and the diagonal braces 12 connect at their second ends 12B to a second, upper portion of the tower structure 8. The first end 11A of each of the diagonal braces 11 typically connects to one of the radial braces 12 at or near the first end 11A of the corresponding radial brace 11.

[0046] The tower support 8 is illustrated as a support pillar, but may have other shapes than that shown. As shown, the tower support 8 extends to a position above the water surface 4, which is also typical for floating support structures.

[0047] As will be illustrated in more detail below, the connections between the braces 10, 11, 12 and the tower support 8 may be injection connections, e.g. grout injection connections. The ends 11A, 11B, 12B of the braces 11, 12 are received in cavities in another brace and / or in the tower support 8, which are then filled with an injection material, typically grout, which is then cured to provide a rigidly fixed connection.

[0048] An example of an injection-molded connection between the diagonal braces 12 and the radial braces 11 is explained in more detail with reference to the corresponding example below.

[0049] 2 shows a coaxial arrangement in which a first end 12A of the diagonal brace 12 is inserted into a tubular sleeve 17 provided in the radial brace 11. The sleeve 17 has a first sleeve end 17A and a second sleeve end 17B, the second sleeve end 17B being closed by an end wall 18. In particular, the radial brace 11 is provided with an opening 22 in a wall 23 of the radial brace 11, and the first sleeve end 17A, made of steel, is secured by a weld 16 to a rim 22A of the opening 22 in the radial brace 11, also made of steel. The weld 16 can be made, for example, by a robot.

[0050] In particular, the sleeve 17 extends from the rim 22A only inside the internal volume of the first brace 11, and does not extend from the rim 22A beyond the opening 22 of the first brace 11. This has certain advantages when large sizes of radial braces 12 are transported, since structural elements extending outward from the surface of the radial brace 12 would make it more difficult to handle. Furthermore, it is advantageous when performing operations on the radial brace 12, especially when painting it before assembly, that the brace 12 does not have any ends extending outward.

[0051] When the first end 12A of the diagonal brace 12 is inserted into the sleeve 17 through the opening 22, the sleeve 17 acts as a guide for the movement of the diagonal brace 12 and also defines the longitudinal direction of the diagonal brace 12. After the end 12A of the diagonal brace 12 is inserted into the sleeve 17, an injection material, typically grout, is inserted into the volume 20 between the inner wall of the sleeve 17 and the outer wall at the end 12A of the diagonal brace 12, after which the injection material is solidified to rigidly fix the diagonal brace 12 in the sleeve 17.

[0052] In the case where the sleeve 17 is open at the second end 17B, the injection material also fills a part of the internal volume of the radial brace 11. However, when the sleeve 17 has a closed bottom 18 as shown, the injection material, in particular grout, inserted in the sleeve 17 is constrained within a volume 20 between the sleeve 17 and the end 11A of the diagonal brace 11. This minimizes the volume of injection material required for the fixing of the diagonal brace 11, which is advantageous.

[0053] FIG. 3 shows an alternative embodiment in which the ends 12A of the diagonal braces 12 are provided with shear keys 21 for better longitudinal stiffness.

[0054] FIG. 4 is a perspective view in section of a radial brace 11 having a closed bottom 18 and inserted into a sleeve 17 in which the injection material, particularly grout, is constrained as described above.

[0055] 5 shows that the sleeve does not extend outward from the outer surface of the radial brace 11. In certain cases, there may be a slight elevation by the weld 16, but this does not create any obstruction to the transportation of the radial brace 11 or during working on the outer surface, such as when the outer surface is worked on automatically by a robot, and the tubular brace is rotated on a working support.

[0056] 6 shows that an opening 22 surrounded by a sleeve 17 is provided in the tower support 8 to receive the second end 11B of the diagonal brace 11 or the second end 12B of the radial brace 12. Moreover, in this case the sleeve 17 has a closed bottom. To prevent the injection material from flowing out of the volume 20 between the sleeve and the inserted brace 11, 12, a fastening gasket is provided around the brace 11, 12 along the rim 22A of the opening 22 while the sleeve 17 is filled with the injection material and until the injection material has hardened. In some cases this gasket remains along the rim, in other cases the gasket is removed again.

[0057] Although the system has been illustrated as a triangular, and in particular a tetrahedral, structure, it is also applicable to other polygonal structures, for example having four, five or six radial braces 11 and a corresponding number of diagonal braces 12. As a common option, side braces 10 are connected to the radial braces 11 to terminate the ends of the structure, improving stiffness, as shown in Figure 1.

Claims

1. A method for assembling an offshore support structure (3) for a wind turbine (2), comprising the steps of: Providing a tower support (8) for supporting a tower (7) of a wind turbine; providing N first braces (11) and N second braces (12), N being at least 3, each brace (11, 12) having a first end (11A, 12A) and a second end (11B, 12B), each said brace having a central longitudinal axis (31, 32); for each pair, one of the first braces (11) and one of the second braces (12), a first connection (29A) connects the second end (11B) of the first brace (11) to a first portion of the tower support (8), a second connection (29B) connects the second end (12B) of the second tubular brace (12) to a second portion of the tower support (8), and a third connection (29C) connects the first end (12A) of the second brace (12) to the first brace (11); when the support structure (3) is oriented for offshore operation, the second portion and the second connection portion (29B) of the tower support (8) is above the first portion and the first connection portion (29A) of the tower support (8), the tower support (8), the first brace (11), and the second brace (12) form a triangle in a vertical plane, and N pairs of braces (11, 12) are directed outward from the tower support (8) in different directions around a vertical central axis (30) of the tower support (8); Including, wherein the method comprises the steps of A, B, and C below: A: the method comprises providing the first connection (29A) with a wall opening (22) in the first part of the tower support (8) and a sleeve (17) with a first sleeve end (17A) and a second sleeve end (17B), the first sleeve end (17A) being welded to a rim (22A) of the opening (22), the sleeve (17) extending inwardly from the rim (22A) into an interior volume of the first part of the tower support (8), the sleeve (17) extending outwardly from the rim (22A) 0.2 m or the sleeve (17) does not extend outwardly from the rim (22A) beyond the opening (22), the method comprising inserting the second end (11B) of the first brace (11) through the opening (22) and into the sleeve (17), filling an injection material, e.g. grout, into a volume (20) between the sleeve (17) and the first brace (11), and allowing the injection material to solidify to rigidly fix the first brace (11) within the sleeve (17); B: the method comprises providing the second connection (29B) with a wall opening (22) in the second part of the tower support (8) and a sleeve (17) with a first sleeve end (17A) and a second sleeve end (17B), the first sleeve end (17A) being welded to a rim (22A) of the opening (22), the sleeve (17) extending inwardly from the rim (22A) into an interior volume of the second part of the tower support (8), the sleeve (17) extending outwardly from the rim (22A) by 0.2 m. or the sleeve (17) does not extend outwardly from the rim (22A) beyond the opening (22), the method including inserting the second end (12B) of the second brace (12) through the opening (22) and into the sleeve (17), filling an injection material, e.g. grout, into a volume (20) between the sleeve (17) and the second brace (12), and allowing the injection material to solidify to rigidly fix the second brace (12) within the sleeve (17); C: the method includes providing, at the third connection (29C), a wall opening (22) in a wall (23) of the first brace (11) and a sleeve (17) with a first sleeve end (17A) and a second sleeve end (17B), the first sleeve end (17A) being welded to a rim (22A) of the opening (22), the sleeve (17) extending inwardly from the rim (22A) into an internal volume of the first brace (11), the sleeve (17) extending outwardly from the rim (22A) not more than 0.2 m. or the sleeve (17) does not extend outwardly from the rim (22A) beyond the opening (22) in the first brace (11), the method comprising inserting the first end (12A) of the second brace (12) through the opening (22) and into the sleeve (17), filling an injection material, e.g. grout, into the volume (20) between the sleeve (17) and the second brace (12), and allowing the injection material to solidify to rigidly fix the second brace in the sleeve (17); The method according to claim 1, further comprising at least one of:

2. 2. The method of claim 1, wherein the sleeve (17) has a closed bottom (18) at the second sleeve end (17B), and the method includes filling an injection material, e.g. grout, into an inner volume (20) of the sleeve (17) but not outside the sleeve.

3. 3. The method of claim 1 or 2, wherein the sleeve (17) extends only inwardly from the rim (22A) into an interior volume of the first portion of the tower support (8), but the sleeve (17) does not extend outwardly from the rim (22A) beyond the opening (22).

4. The method comprises: A. providing a closed end (18) at the second end (11B) of the first brace (11); B, providing a closed end (18) at the second end (12B) of the second brace (11); C. providing a closed end (18) at the first end (12A) of the second brace (12); The method according to any one of claims 1 to 3, comprising:

5. The method according to any one of claims 1 to 4, comprising filling the volume (20) of the sleeve (16) with an injection material, optionally a grout, only after inserting the ends (11B, 12A, 12B) of the corresponding braces (11, 12) into the sleeve (17).

6. 6. The method according to any one of claims 1 to 5, comprising providing a third set of N third braces (10) and interconnecting the first braces (11) by the third braces (10) to increase stiffness between the first braces (11).

7. 7. The method of claim 6, wherein N is 3 and the third brace (10) forms a triangular structure.

8. 8. The method of claim 7, comprising forming a tetrahedral structure by the first brace (11), the second brace (12), and the third brace (10).

9. 9. The method of claim 8, comprising placing the tower support (8) in the center of the tetrahedral structure to form the tetrahedral structure as a regular tetrahedron.

10. The method according to any one of claims 1 to 9, comprising assembling the offshore support structure (3) on shore or on land, providing a wind turbine (2) on top of the support structure (3), hardening the injection material in the cavity (16) after assembly, transporting the support structure (33) to an offshore destination and anchoring the support structure (3) to the seabed (13).

11. 11. The method of claim 10, comprising setting the support structure (3) on a floating structure.

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