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EP-4543744-B1 - FLOATING FOUNDATION FOR AN OFFSHORE WIND TURBINE AND METHOD OF CONSTRUCTION

EP4543744B1EP 4543744 B1EP4543744 B1EP 4543744B1EP-4543744-B1

Inventors

  • LUSTHOF, JEROEN
  • SCHUILING, Christiaan
  • GROOTE WOORTMANN, Paul

Dates

Publication Date
20260506
Application Date
20230619

Claims (15)

  1. A floating foundation (1) for an offshore wind turbine (2) having a tower (4) defining a vertical direction, the floating foundation comprising at least a first vertical section (24A), a final vertical section (24C), and an intermediate vertical section (24B) and at least two horizontal sections (11), wherein the vertical sections and the horizontal sections are tubular members, arranged in an alternating manner to define a meandering path extending from a proximal end (26) to a distal end (28) with the first vertical section located at the proximal end, the intermediate vertical section located in between a horizontal underwater section (14) and a horizontal above-water section (18) and the final vertical section located at the distal end of the floating foundation, the tubular members being 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 any one of the preceding claims, 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.
  4. The floating foundation according to any one of the preceding claims, wherein the vertical sections are equally spaced and optionally, wherein the vertical sections form nodes of an equilateral triangle in a horizontal plane.
  5. The floating foundation according to any one of the preceding claims, 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.
  6. The floating foundation according to any one of the preceding claims, consisting of three vertical members and two horizontal members.
  7. The floating foundation according to any one of the preceding claims, wherein each section of the floating foundation further comprises a plurality of ring frames (54) attached to an inner surface and extending radially inwardly.
  8. The floating foundation according to any one of the preceding claims, 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, and more preferably between 9 m and 12 m.
  9. The floating foundation according to any one of the preceding claims, wherein the final vertical section is arranged to terminate underwater and is preferably provided with a heave plate (30).
  10. The floating foundation according to any one of the preceding claims, wherein an interior of the floating foundation is substantially hollow and comprises a plurality of separated ballast tanks (31B,C,D,E,H).
  11. The floating foundation according to any one of the preceding claims, wherein the floating foundation is made of steel and configured to support a wind turbine with a total weight of more than 1000 t.
  12. A floating wind turbine having a tower and a floating foundation according to any one of the preceding claims.
  13. A method for constructing a floating foundation for an offshore wind turbine, wherein the method comprises steps of: 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 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. The method according to claim 13, wherein at least steps a. to c. take place on shore and at least step f. takes place with the first horizontal section floating.
  15. The method according to claim 13 or claim 14, further comprising providing a heave plate to a lower end of the final vertical section and/or preparing the intermediate vertical section for receipt of a wind turbine tower and optionally connecting a wind turbine tower to the intermediate vertical section.

Description

Field of the invention The present invention relates to floating foundations, in particular for supporting offshore wind turbines and to methods of constructing and installing such foundations and turbines. Background art Floating wind turbines are an excellent source of renewable energy as they can be situated offshore where the winds are stronger and more consistent. They show a great promise for wind farming as the floating wind turbines can be installed in deeper waters which significantly increases available sea area contrary to fixed wind turbines which are restricted to shallow waters close to the shore. Floating wind turbines generally have a wind turbine comprising a tower, a nacelle and blades attached to a foundation that is afloat and anchored to the seabed by a catenary or a taut mooring system. Existing floating foundations can be spar-buoy type, wherein the foundation comprises a steel and/or concrete cylinder filled with a ballast of water and gravels to keep it floating up-right. However, these types of floating foundations are not globally suitable for large wind turbines as the draught of the foundation needed for keeping the foundation and the wind turbine afloat is directly related to the size and weight of the wind turbine. This results in extreme draughts for large turbines, making construction, transport and installation complicated. Other systems use platform constructions, similar to oil and gas type floating platforms. US8471396B2 discloses one such floating wind turbine platform including a floatation frame that includes at least three columns that are coupled to each other with horizontal main beams. A wind turbine tower is mounted in the centre of the three columns or above a tower support column to simplify the system construction and improve the structural strength. Another such platform is shown in US 2015/329180. EP2387528A2 discloses a tension-leg offshore submerged platform, in hybrid concrete-steel solution, having a pre-stressed concrete central body, a steel peripheral structure, which is connected to the central body through steel stiffeners, and further to a basement for such a platform. However, floating foundations in deep waters are exposed to harsh environments of strong winds and large waves. In these environments high forces are induced on constituting elements of the floating foundation. The coupling and/or the connecting elements such as joints and/or struts are particularly vulnerable as the forces are highly concentrated in these regions. Hence, such floating foundations require regular inspections and maintenance. A floating foundation has been proposed in unpublished application WO2023/098994 that uses a continuous tubular structure of meandering shape. This had the particular advantage of a relatively simplified assembly process, since the continuous structure had no closed rings in its structure requiring precise positioning and could thus even be assembled afloat. That floating foundation avoided the use of joints and struts by the use of smooth curved transitions. Such curved sections are however more complex to engineer and construct and less adapted to high volume production processes. It would be desirable to provide a floating foundation suitable for large wind turbines that would be resilient to the harsh conditions of the deep-water environment yet relatively cost-effective and simple to construct, transport and install. Summary of the invention Therefore, according to a first aspect of the invention there is provided a floating foundation having a tower as defined in claim 1. The floating foundation comprises 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 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, the tubular members connected together by interpenetrating tube joints and wherein one of the vertical sections is arranged to receive the tower. The vertical sections comprise a first, a final, and an intermediate vertical section spaced from each other in a horizontal plane to ensure stability of the floating foundation. In the present context, the term interpenetrating tube joint is intended to denote a joint where a first tube passes through a front wall of a second tube and is provided with a connection to the rear wall thereof. The skilled structural engineer will understand that this provides a significantly stronger joint than one that is merely connected at the line of intersection on the front wall. The term is intended to encompass joints where the first tube terminates within the second tube and fully interpenet