FLOATING FOUNDATION FOR AN OFFSHORE WIND TURBINE AND METHOD OF CONSTRUCTION

Abstract

A floating foundation for an offshore wind turbine having a tower defining a vertical direction, the floating foundation comprising at least three vertical sections and at least two horizontal sections, wherein the vertical sections and the horizontal sections are tubular members, arranged in an alternating manner, and connected together by interpenetrating tube joints and wherein one of the vertical sections is arranged to receive the tower.

Claims

1. A floating foundation for an offshore wind turbine having a tower defining a vertical direction, the floating foundation comprising at least a first vertical section, a final vertical section, and an intermediate vertical section and at least two horizontal sections, wherein the vertical sections and the horizontal sections are tubular members, arranged in an alternating manner, and connected together by interpenetrating tube joints and wherein one of the vertical sections is arranged to receive the tower.

2. The floating foundation according to claim 1, wherein the intermediate vertical section is arranged to receive the tower.

3. The floating foundation according to claim 1, wherein the horizontal sections comprise at least one underwater section and at least one above-water section.

4. The floating foundation according to claim 1, wherein the horizontal sections are of a constant first diameter and the vertical sections are all of a constant second diameter and the first diameter of the horizontal sections is preferably-smaller than the second diameter of the vertical sections.

5. The floating foundation according to claim 1, wherein the vertical sections are equally spaced and form nodes of an equilateral triangle in a horizontal plane.

6. The floating foundation according to claim 4, wherein for each interpenetrating tube joint an end of the horizontal section passes completely through the vertical section and is welded both at an entry side and at an exit side.

7. The floating foundation according to claim 1, comprising exactly three vertical members and exactly two horizontal members.

8. The floating foundation according to claim 1, wherein each section of the floating foundation further comprises a plurality of ring frames attached to an inner surface and extending radially inwardly.

9. The floating foundation according to claim 1, wherein each section of the floating foundation has a circular external cross-section with a diameter in a range between 7.5 m and 15 m.

10. The floating foundation according to claim 1, wherein the final vertical section is arranged to terminate underwater and is provided with a heave plate.

11. The floating foundation according to claim 1, wherein an interior of the floating foundation is hollow and comprises a plurality of separated ballast tanks.

12. The floating foundation according to claim 1, wherein the floating foundation is made of steel and configured to support a wind turbine with a total weight of more than 1000 t.

13. The floating foundation according to claim 1, wherein the tubular members are arranged in an alternating manner to define a meandering path extending from a proximal end to a distal end with the first vertical section located at the proximal end, the intermediate vertical section located in between a horizontal underwater section and a horizontal above-water section and the final vertical section located at the distal end of the floating foundation.

14. A method for constructing a floating foundation for an offshore wind turbine, wherein the method comprises: a) providing first and intermediate annular feet each having an upper side with a recess of a first diameter, the first and second feet having a second diameter; b) providing a first horizontal section having a first end and an intermediate end, the first horizontal section being tubular and having a diameter corresponding to the first diameter, c) placing the first horizontal section onto the first and second annular feet such that it is aligned with the respective recesses with the first and intermediate ends extending beyond the respective feet, d) providing first and intermediate vertical sections, being tubular and having the second diameter and being provided with recesses of the first diameter at their respective lower sides, e) placing the first and intermediate vertical sections onto the first horizontal section, aligned respectively with the first and intermediate annular feet and welding all elements together to form first and lower intermediate, interpenetrating tube joints, f) providing further interpenetrating tube joints to at least a second horizontal section and a final vertical section.

15. The method according to claim 14, wherein at least a) to c) take place on shore and at least f) takes place with the first horizontal section floating.

16. The method according to claim 14, further comprising providing a heave plate to a lower end of the final vertical section.

17. The method according to claim 14, further comprising preparing the intermediate vertical section for receipt of a wind turbine tower and optionally connecting a wind turbine tower to the intermediate vertical section.

18. A floating wind turbine having a tower and a floating foundation, the floating foundation comprising at least a first vertical section, a final vertical section, an intermediate vertical section and at least two horizontal sections, wherein the vertical sections and the horizontal sections are tubular members, arranged in an alternating manner, and connected together by interpenetrating tube joints and wherein the tower extends from one of the vertical sections.

19. The floating wind turbine of claim 18, wherein the tubular members are arranged in an alternating manner to define a meandering path extending from a proximal end to a distal end with the first vertical section located at the proximal end, the intermediate vertical section located in between a horizontal underwater section and a horizontal above-water section and the final vertical section located at the distal end of the floating foundation.

20. The floating wind turbine of claim 18, wherein the tower extends from the intermediate vertical section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

[0061] FIG. 1 depicts a floating wind turbine installation in perspective view;

[0062] FIG. 2 depicts the installation of FIG. 1 in side elevation;

[0063] FIG. 3 depicts the installation of FIG. 2 in plan view;

[0064] FIG. 4 depicts in an exploded view, sections of the floating foundation of FIGS. 1 to 3;

[0065] FIG. 5 depicts an enlarged portion of an interpenetrating tube joint marked V in FIG. 2; and

[0066] FIG. 6 depicts an assembly process of the floating foundation.

DESCRIPTION OF EMBODIMENTS

[0067] The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are shown. The drawings are intended exclusively for illustrative purposes and not as a restriction of the inventive concept which is to cover all modifications, equivalents, and alternatives falling within the scope of the present invention. The scope of the invention is only limited by the definitions presented in the appended claims.

[0068] FIG. 1 illustrates in perspective view a first embodiment of a floating wind turbine installation 1 according to the present invention. The installation 1 comprises a wind turbine 2 having a tower 4, a nacelle 6 and blades 8. The wind turbine is mounted on a floating foundation 10 comprising an elongate hollow body 12 having three vertical sections 24 and two horizontal sections 11. The floating foundation 10 is unmanned although access for service personnel may be provided. The elongate hollow body 12 has a proximal end 26 which in use is above the water surface S and a distal end 28 provided with a heave plate 30. The installation 1 is kept in place by mooring lines 15. The illustrated wind turbine 2 has a rated capacity of 15 MW and a weight of 2400 t. The height of the nacelle 6 above the waterline is around 150 m and the blade length is around 110 m. The following description of the floating foundation 10 is based on a turbine of this scale. The skilled person will nevertheless, understand that dimensions of the floating foundation 10 will vary according to the size of the turbine.

[0069] FIG. 2 shows the installation 1 of FIG. 1 in side elevation in the installed condition, floating on a body of water having a surface S. The horizontal sections include an underwater section 14 and an above-water section 18. The horizontal sections 11 are tubularly shaped with closed ends. The vertical sections 24 A,B,C pass through the splash zone at the water surface S. FIG. 2 further shows compartmentation of the interior of the elongate hollow body 12 into eight inner chambers 31 A,B,C,D,E,F,G,H. The inner chambers 31 B,C,D,E,H are ballasted with sea water. The inner chambers are separated by water-tight bulkheads.

[0070] FIG. 3 shows the installation 1 of FIG. 2 in plan view, illustrating the positions of the vertical sections 24 A,B,C in the horizontal plain. A first vertical section 24 A is located at the proximal end 26, a final vertical section 24 C is located at the distal end 28 and the wind turbine 2 is connected to an intermediate vertical section 24 B, located in between the above-water section 18 and the underwater section 14. As can be seen, in the illustrated embodiment, these vertical sections are located at the corners of an equilateral triangle. The underwater section 14 and the above-water section 18 form two sides of the triangle. In this case, the length of the respective sides is 85 m calculated at a centreline CL of the elongate hollow body 12. A deck 23 extends along the above-water section 18 and circumferentially around the vertical sections 24 A,B,C at the same level with the above-water section 18. An interior of the above-water section 18 can be accessed from the deck 23 via manholes (not shown) to perform inspections and maintenance of the floating foundation 10. The inspection and the maintenance of the tower 4 may be carried out via a watertight door (not shown) located on the intermediate vertical section 24 B.

[0071] FIG. 4 shows in an exploded view the sections of the floating foundation 10 of FIGS. 1 to 3. The elongate hollow body 12 can be considered as five sections. The underwater section 14 has a first end portion 16 and a second end portion 13. A diameter of the first end portion 16 of the underwater section 14 is smaller than a diameter of the first vertical section 24 A and configured to pass through a first pair of holes 33 A,B on a surface of the first vertical section 24 A defining a first through-hole 33. Similarly, the intermediate vertical section 24 B has a second pair of holes 35 A,B defining a second through-hole 35 configured to receive the second end portion 13 of the underwater section 14. The intermediate vertical section 24 B has a third pair of holes 37 A,B defining a third through-hole 37 configured to receive a first end portion 17 of the above-water section 18 and the final vertical section 24 C has a fourth pair of holes 39 A,B defining a fourth through-hole 39 for receiving a second end portion 19 of the above-water section 18. The intermediate vertical section 24 B has an opening 41 for receiving the tower 4 of the wind turbine 2. Each hole is formed by connecting a recessed annular foot 43 A,B,C,D to a recessed end portion 42 A,B,C, D of the vertical section 24 A,B,C using a weld line 45 A,B,C,D. Recessed areas of the recessed end portions 42 A,B,C,D and the recessed annular feet 43 A,B,C,D each define a half circle arranged to form the first 33, the second 35, the third 37, and the fourth 39 through-hole upon connecting the elements by the weld line 45 A,B,C,D.

[0072] FIG. 5 illustrates a detail of the interpenetrating tube joint marked V in FIG. 2, showing an inner space 58 of the floating foundation 10. The intermediate vertical section 24 B and the underwater section 14, as well as other sections of the floating foundation, are formed from a steel wall 56 having a thickness d and defining the inner space 58. On an inner surface 62 of the wall 56 ring frames 54 are welded to reinforce the section. The sections further include bulkheads 52 which divide the inner space 58 of the floating foundation 10 into different compartments. The second end portion 13 of the underwater section 14 extends through the second through-hole 35 and beyond the intermediate section 24 B to form the interpenetrating tube joint. A diameter of the second end portion Dt and a diameter of the second through-hole Dh are designed to form a tight-fit and the interpenetrating tube joint is fixed by welding the two sections together around the second pair of holes 35 A,B.

[0073] FIG. 6 illustrates assembly steps of the floating foundation 10. In Step 1, a first recessed annular foot 43 A and a second recessed annular foot 43 B are placed at a distance from each other. In Step 2, the underwater section 14 is positioned on top of them such that the first end portion 16 and the second end portion 13 of the underwater section 14 are placed over recessed areas of the foot 43 A and the foot 43 B, respectively, and connected by a first weld line 49A and a second weld line 49B to form a first structure 102. In Step 3, the first vertical section 24 A and the second vertical section 24 B are placed over the first structure 102. The first vertical section 24 A has a first recessed end portion 42 A which is placed over the first annular foot 43 A and the first end portion 16 of the underwater section 14 to completely enclose the first end portion 16 of the underwater section 14. The same process is repeated with the intermediate vertical section 24 B at the second end portion 13 of the underwater section 14. The first 24 A and the second 24 B vertical sections are connected to the first structure 102 by a third 45 A and a forth 45 B weld line to form a second structure 103. The first three process steps are performed at an onshore location and then the second structure 103 is launched to a floating position together with a final vertical section 24 C which is placed apart from it. The final vertical section 24 C has a heave plate 30 connected to it to ballast the floating foundation upon assembly and in use. The respective second structure 103 and the final vertical section 24 C can be stably floated at the correct height by suitable ballasting and the use of lifting bags. In step 5, the above-water section 18 is positioned over the intermediate vertical section 24 B and the final vertical section 24 C. The intermediate vertical section 24 B and the final vertical section 24 C have a third recessed end portion 42 C and a fourth recessed end portion 42 D, and first and second end portions 17, 19 of the above-water section 18 are placed thereover. The above-water section 18 is connected to the rest of the structure by a fifth and a sixth weld line 49 C, D to form a third structure 105. Finally, in Step 6, third and fourth recessed caps 43 C, D are placed over the end portions 17,19 of the above water section 18 which is thus enclosed by the third and fourth recessed end portions 42 C, D. The elements are connected by seventh and eighth weld line 45 C, D to form the floating foundation 10.

[0074] Due to its simple concept, the floating foundation according to the invention is fully scalable and suitable for wind turbines of different sizes. The floating foundation can be provided with an active ballast system or a passive ballast system depending on the needs of the user and the conditions of the environment at the project location. Exemplary characteristics of different embodiments of the floating foundation according to the invention are shown in Table 1.

TABLE-US-00001 TABLE 1 Waves Centre spacing, Foundation Turbine (Hs) Outfitting typical diameter weight 9.5 MW 6.5 m Active 65 m, 9.3 m 2500 t 9.5 MW 6.5 m Passive 84 m, 12 m 3900 t 9.5 MW 11 m Active 65 m, 9.3 m in development 9.5 MW 11 m Passive 84 m, 12 m 4900 t 15 MW 6.5 m Active 84 m, 12 m 4700 t 15 MW 6.5 m Passive 105 m, 15 m 7600 t 15 MW 11 m Active 84 m, 12 m 5200 t 15 MW 11 m Passive 105 m, 15 m 9000 t

[0075] The floating foundation according to the invention can be efficiently produced in series since many elements are identical. Further, the sections can be easily stored and moved for a final assembly. The final assembly can be performed inshore and/or afloat with a minimal number of welds or other types of connection. The minimal draught for the floating foundation may be just 8 m, while the operational draught of the floating foundation may be around 24 m.

[0076] The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive to the inventive concept. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. It will be apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice. In addition, many modifications may be made to adapt a particular configuration or material to the teachings of the invention without departing from the essential scope thereof.

[0077] All modifications which come within the meaning and range of equivalency of the claims are to be embraced within their scope.