Floating foundations for offshore wind turbines

Abstract

A floating foundation for an offshore wind turbine having a tower is disclosed. The floating foundation comprises an elongated hollow member defining a serpentine path and having a submerged section, an above-water section, and a transition section therebetween. The transition section passes through the water surface, and the floating foundation comprises at least three transition sections spaced apart from one another to form a stable base.

Description

[Technical Field]

[0001]

[0001] The present invention relates to floating foundations, particularly floating foundations for supporting offshore wind turbines, and to methods of constructing and installing such foundations and turbines. [Background technology]

[0002]

[0002] Floating wind turbines are an excellent source of renewable energy because they can be installed offshore where winds are stronger and more stable. Because floating wind turbines can be installed in deeper waters, they significantly expand the available ocean area, in contrast to fixed wind turbines which are limited to shallow waters closer to shore, and therefore offer great potential for wind power generation.

[0003]

[0003] Floating wind turbines typically mount the wind turbine, including its tower, nacelle, and blades, on a floating foundation secured to the seabed by a catenary or mooring system. Existing floating foundations are known as spar buoy-type foundations, which consist of a steel and / or concrete cylinder filled with a water and gravel ballast to keep it upright and floating. However, these types of floating foundations are generally unsuitable for large wind turbines because the foundation draft required to keep the foundation and wind turbine afloat is directly related to the size and weight of the wind turbine. As a result, large turbines have extreme drafts, complicating construction, transportation, and installation.

[0004]

[0004] Other systems employ platform construction similar to oil and gas floating platforms. U.S. Patent No. 8,471,396 discloses one such floating wind turbine platform, which includes a floating frame including at least three columns connected to each other by horizontal main beams. The wind turbine tower is attached to the center of the three columns or above the tower support column to simplify construction of the system and improve structural strength.

[0005]

[0005] EP 2387528 discloses an offshore underwater platform with telescopic legs in a hybrid concrete-steel solution having a prestressed concrete central body and a steel perimeter structure connected to the central body via steel stiffeners and further connected to the foundations for such a platform.

[0006]

[0006] However, deep-sea floating foundations are exposed to harsh environments, such as strong winds and large waves. In such environments, large forces are applied to the components of the floating foundation. Linking and / or connecting elements, such as connecting struts, are particularly vulnerable, as forces are highly concentrated in these areas. Therefore, such floating foundations require regular inspection and maintenance.

[0007] It would be desirable to provide a floating foundation suitable for large wind turbines that can withstand the harsh conditions of a deep sea environment, yet is relatively cost-effective and easy to construct, transport and install. Summary of the Invention

[0008]

[0008] Thus, according to a first aspect of the present invention, there is provided a floating foundation as defined in claim 1. The floating foundation comprises an elongated hollow member defining a serpentine path and having a submerged section, an above water section and a transition section therebetween which, in use, passes through the water surface, the floating foundation comprising at least three transition sections spaced apart to form a stable base. The elongated hollow member may be in the form of a large tube or pipe having walls forming inner and outer surfaces and defining one or more internal chambers.

[0009]

[0009] "Meandering" in the context of this application should be understood in the broadest sense of the word, meaning "changing direction rather than extending in a straight line." Thus, meandering can be interpreted as winding, zigzagging, curved, twisting, turning, etc., to name a few.

[0010] For further explanation, the following directions will be used throughout this application: The direction parallel to the tower of the wind turbine is the vertical direction. The plane perpendicular to the vertical direction is the horizontal plane. When the floating foundation is floating, the horizontal plane is defined by the water surface. The above-water section remains clear of the water surface when in use, and the underwater section remains submerged below the water surface when in use. Only the transition section passes through the water surface when the floating foundation is floating, connecting the underwater and above-water sections. It will be understood that the exact depth to which the foundation floats will depend on the weight of the turbine and the presence or absence of ballast in the foundation, and that under certain conditions, part of the underwater section may extend above the water line, or vice versa.

[0011] A floating foundation according to the present invention comprises at least three transition sections spaced apart to form a stable base. A base is stable if the system comprising the floating foundation and wind turbine can return to its equilibrium position when displaced from the equilibrium position by external forces such as waves and / or wind. In other words, the system must be sufficiently hydrodynamically stable to resist tilting moments due to strong winds, currents, and waves. Generally, this requires that the transition sections be sufficiently spaced apart so that the center of buoyancy is far from each of the transition sections. Additionally, the above-water section must be sufficiently sized to avoid being submerged by the maximum force or moment likely to act on the wind turbine. In embodiments, the transition sections may be spaced apart from each other by at least 40 m, or at least 60 m, or even more than 100 m, measured center-to-center.

[0012] According to one embodiment, the proximal end of the floating foundation comprises a first transition section configured to fixedly receive the tower of the wind turbine. The tower may be connected to the first transition section by one or more connection methods, such as welding, bolts, flanges, couplers, sleeves, and / or the like. In one embodiment, a portion of the tower and a portion of the first transition area may overlap. For the avoidance of doubt, the tower in this context is considered to begin above the waterline, with the third transition section of the floating foundation passing through the waterline. However, this is not intended to exclude a tower that passes the waterline and connects to the floating foundation underwater. In such cases, the base of the tower may form the third transition section.

[0013]

[0013] According to a further embodiment, the distance between successive transition sections in the horizontal plane is equal. The transition regions are equally spaced apart. The transition sections preferably form equilateral triangular nodes in the horizontal plane. This symmetrical configuration provides optimized stability to the base of the floating foundation. It is particularly suitable for foundations that are directional tethered in place. It will be understood that other configurations and different numbers of transition sections are possible, with three being a minimum for stability. In particular, bases tethered to rotate with wind direction may benefit from a different distribution of transition sections.

[0014]

[0014] According to another embodiment, the serpentine path includes alternating straight and curved sections. The underwater and / or abovewater sections may be substantially straight, with curved elbows at each end leading to vertical transition sections. Straight sections are advantageous because they are easiest to manufacture and assemble. The length of the sections must be selected to provide adequate spacing for the transition sections and ensure the necessary stability when the wind turbine is attached to the floating foundation. This length depends not only on the parameters of the wind turbine, but also on the parameters of the floating foundation, such as the material, the diameter of the sections, the presence or absence of heave plates, etc.

[0015]

[0015] Additionally, the use of one or more straight sections in the underwater section can be advantageous in limiting the draft of the floating foundation. In preferred embodiments, the draft of the loaded foundation may be less than 20 m, or even less than 10 m. It may then be ballasted to a deeper operating position. In an unloaded state, before the tower is installed, the draft may be less than 10 m or even less than 8 m, and for foundations with a capacity sufficient for wind turbines of over 10 MW, it may be as little as 6 m. Such a shallow draft facilitates relocation and / or movement in coastal waters. The one or more straight sections can also provide a stable base when the foundation is on land or grounded in shallow waters.

[0016] According to one embodiment, the radius of curvature of the elbow section may be in the range of 7.5 m to 30 m. Alternatively, no part of the serpentine path has a radius of curvature less than the diameter of the pipe, e.g., less than 7.5 m for a 7.5 m pipe. As a result, stresses induced in the elbow during use are reduced, allowing for the use of thinner walls. The floating foundation is also less fatigue-prone and requires fewer inspections. As noted above, the floating foundation may include an elbow section or elbows arranged to interconnect the transition section with the submerged section and / or above-water section. With respect to the manufacture of the hollow member, the elbow may be an integral part of the submerged section, above-water section, and / or transition section. The elbow section is a curved portion of the elongated hollow member, and the curvature of the elbow section defines the serpentine path of the elongated hollow member. Preferably, the elbow section has a large radius of curvature. Such elbows are desirable because they result in lower stresses than conventional submersible or jacket joints. As mentioned above, the elbow section preferably has a radius of curvature that is greater than the diameter of the pipe.

[0017]

[0017] The elongated hollow member may be manufactured from a plurality of short tubular elements. The tubular elements are interconnected to form various sections of the floating foundation, such as submerged sections, above-water sections, transition sections, and elbows. The tubular elements can be interconnected using one or more connection methods, such as welding, bolts, flanges, couplers, sleeves, and / or the like. Welding is the most preferred option. The tubular elements have the shape of a hollow cylinder or tube cross section. The walls of the tubular elements must be thick enough to withstand forces generated by waves and wind, as well as to support the weight of a wind turbine when the tubular elements are interconnected to form the floating foundation. Using tubular elements to form a floating foundation is advantageous because the tubular elements are small in size compared to the various sections of the floating foundation, making them easier to produce in series. Elements exposed to greater stresses may be provided with thicker walls, or the entire hollow member can be manufactured with a constant wall thickness. In a preferred embodiment, the walls of the hollow member have a thickness of 10 mm to 60 mm.

[0018]

[0018] In one embodiment, the wall thickness of different sections of the hollow member is constant. However, different sections of the floating foundation may have different thicknesses from each other. The wall thickness of the above-water section may be smaller than the wall thickness of the underwater section. For example, the wall thickness of the above-water section may be 75% or less of the wall thickness of the underwater section. In this way, the overall weight and material costs of the floating foundation may be optimized.

[0019] According to one embodiment, each tubular element has an inner surface to which at least one ring frame may be attached. The ring frame is connected, for example, by welding. The ring frame is preferably in the form of a flat flange that abuts the inner surface and is configured to reinforce the tubular element against buckling or collapse. However, other forms, such as T-bars or angle bars, are also foreseeable. This allows for a reduction in the thickness of the tubular element, resulting in a lighter floating foundation with improved buoyancy characteristics. Surprisingly, it has been found that by using such a ring frame every 3 to 10 meters, preferably every 4 meters, of the length of the hollow element, the overall wall thickness required can be reduced by a factor of three, thereby correspondingly reducing the weight of the floating foundation. Several ring frames can completely close the interior of the hollow element as a sealing bulkhead. The thickness of the ring frame may correspond to 25% to 100% of the wall thickness, preferably in the range of 10 to 50 mm. The radial extension of the ring frame may be 5% to 30% of the diameter of the tubular element, or 5 cm to 100 cm, preferably 20 cm to 50 cm.

[0020] The tubular elements can be manufactured and assembled using conventional monopile construction techniques or may be manufactured off-site in a dedicated manufacturing facility. Due to the large radius and / or segmented nature of the curved sections of the pathway, the tubular elements used in these sections can be formed from single-axis curved plates. As a result, the manufacturing process eliminates the need for double-curved plates, greatly simplifying the manufacture and assembly of the tubular elements. The tubular elements may each have a length of 3 to 10 meters, with the length defined by the outer radius for curved elements. The use of a ring frame can also ensure that the shape of the tubular elements is maintained during the manufacturing and assembly process. This allows for the use of thinner material, which would otherwise require thicker plates to maintain sufficient precision during manufacturing.

[0021]

[0021] The tubular elements may be joined together, for example by welding, to form the respective underwater and above-water sections. The different sections of the floating foundation may be produced at one location, such as a factory or yard close to a port or quay, partially assembled, and then transported to another location, such as a port or quay, or to an onshore or offshore location, for final assembly to form the floating foundation.

[0022] The various sections may be designed to minimize the number and location of connection points for final assembly. For example, the various sections may be designed so that connections are made only at vertically positioned transition sections. While flanges may be provided between the sections, the sections are most preferably welded, for example, using field welding on-site. In a preferred embodiment, by providing connections at the vertically positioned transition sections, only horizontal welds are performed during final assembly of the floating foundation. Horizontal welds are advantageous because they are easier to perform under heavy lifting conditions, such as in ports or offshore locations using cranes or jacking mechanisms. In one embodiment, the floating foundation may be fabricated from submerged and abovewater sections joined together at their respective transition zones by horizontal welds. The welds may be located at the waterline or splash zone. Alternatively, the welds may be located in locations that will remain submerged during normal use, or in locations that will remain above water during normal use.

[0023]

[0023] Wind turbine installation can be done onshore or even offshore. Onshore installation is preferred because it significantly reduces costs and installation time. Furthermore, installation in shallow waters offshore facilitates inspection and commissioning of the system. This differs from fixed foundations and spars, which must be installed offshore, i.e., in deep water at the project site. Alternatively, the wind turbine can be installed offshore, on the proximal end of a floating foundation, allowing good access for (single lift) construction vessels.

[0024]

[0024] Preferably, the hollow member has a circular cross section. However, it should be noted that the cross section of the hollow member may also be triangular, rectangular, octagonal, or any other suitable shape, preferably with rounded corners.

[0025]

[0025] The interior of the hollow member may define a single interior chamber, which may also be in open communication with the interior of the tower. However, for various reasons, it may be desirable to divide this space into multiple separate interior chambers that are hermetically closed from one another. Additionally or alternatively, some ring frames may extend completely across the inner diameter and form partitions used to define the separate interior chambers.

[0026] According to one embodiment, at least one internal chamber is configured to receive a volume of water to act as a ballast tank, allowing the foundation to be ballasted with water during commissioning to achieve an optimal draft for stable operation. The floating foundation may be ballasted with more seawater than the combined weight of the foundation and turbine, resulting in a draft that is more than doubled. It will be appreciated that ballast should be distributed among the internal chambers and / or different sections of the floating foundation to maintain the stability of the floating foundation in use.

[0027]

[0027] The ballast system may be passive, meaning that the amount and location of ballast water remains the same during operation of the turbine. The inflow of water to the various sections of the foundation may be controlled by sea valves.

[0028] Alternatively, the internal chamber may be provided in combination with an active ballast system that can be used to compensate for static tilt caused by turbine thrust, and may include pumps, valves, and controls. In this way, it is possible to adjust the buoyancy of the floating foundation in response to wind loads, for example. This is particularly relevant for smaller systems where wave heights and maximum load conditions may exceed the limits of "static" ballast.

[0029]

[0029] An active system may be based on ballast transfer between internal chambers in a closed system, which is the preferred solution, or on methods that involve inflow and outflow into the sea.

[0030] According to one embodiment, the hollow member may have at least four internal chambers or ballast tanks. The number and configuration of the internal chambers need not depend on the type of ballast system. Preferably, the hollow member comprises at least seven internal chambers, and the underwater section comprises at least two internal chambers. Internal chambers belonging to different sections are closed by watertight bulkheads. However, internal chambers belonging to the same section may be interconnected, for example, by valves, pipes, and / or similar flow control methods.

[0031] According to one embodiment, the floating foundation further comprises a distal end arranged to receive a heave plate. The heave plate may be in the form of a massive disk or body made of steel or other suitable material. The weight of the heave plate is arranged to control the center of gravity of the floating foundation and improve stability while limiting its overall draft, particularly during erection.

[0032] The floating foundation may be made predominantly of steel, however, one or more other materials may be present, such as iron, concrete, fiberglass, resin, plastic, copper, aluminum, and / or the like.

[0033]

[0033] The floating foundation is preferably configured to support a wind turbine with a capacity ranging from 2 MW to over 15 MW and a weight including tower, nacelle and blades of over 1000 t, even over 9000 t. The length of the submerged and above water sections in the horizontal plane may be in the range of 60 m to 120 m. The diameter of the tubular elements may be in the range of 9 m to 15 m. The weight of the floating foundation when the ballast tanks are empty may be in the range of 2500 t to 9000 t.

[0034] According to one embodiment, the floating foundation is secured to the seabed by a catenary or a teaching or semi-teaching system. The exact form of the mooring system depends on various factors, including the depth and nature of the seabed. A semi-teaching mooring system may be preferred for water depths below 200 m. A catenary system may be preferred for water depths above 200 m.

[0035]

[0035] The present invention further contemplates a floating foundation comprising an elongated hollow member in the form of a single bent pipe. The bent pipe may have no branches or forks above the waterline, or may be completely devoid of branches or forks, and may have only proximal and distal ends for receiving the wind turbine tower. In one embodiment, the distal end is a submerged section. The floating foundation may also be devoid of columns, spars, braces, and joints. In this context, it is understood that there are no additional external structures that form part of the structural integrity and load-bearing capacity of the floating foundation. This is not intended to exclude the presence of auxiliary structures such as walkways, ladders, decks, lifting points, and anchor points.

[0036]

[0036] The present invention further relates to a floating wind turbine having a tower and a floating foundation as described above or below. The tower may be connected to the proximal end of the hollow member, and in particular the tower may be arranged in a straight line with the hollow member and therefore preferably vertical at this location. The cross section of the tower may coincide with the cross section of the hollow member at the location of the connection, so that the tower can be considered as an extension of the elongated hollow member.

[0037]

[0037] The tower may be connected to the floating foundation by any suitable method as described above, including flange connections or welded connections. Welded connections using field joining techniques may be preferred as they require less manufacturing precision at the joining interface. Another option is a sleeve connection.

[0038] According to another aspect of the present invention, there is provided a method for constructing a floating foundation for a wind turbine, the method comprising: providing a plurality of tubular elements; interconnecting the tubular elements to form at least one nonlinear underwater section and at least one nonlinear abovewater section; launching the underwater section to a floating position; assembling an above-water section onto a floating underwater section to form an elongated hollow member; A method comprising:

[0039]

[0039] In a preferred embodiment, the tubular elements comprise a ring frame. Alternatively or additionally, each tubular element has a length and a diameter greater than the length. The ring frames may be connected by welding. In addition, the underwater section and the abovewater section may also be assembled by welding.

[0040] Construction can be done on a single unit at a time, or multiple units can be produced in series in the form of a production line.

[0041] In a further embodiment, the tubular elements are interconnected at a first location on land and the above-water and underwater sections are assembled at a second location, such as a quay, a wharf or offshore. In this context, a quay is understood to be a location where the various sections of the floating foundation can be lifted into the water using one or more lifting means, such as heavy lifting cranes, floating crane vessels, jacking systems and / or launching barges.

[0042]

[0042] A distinction is thus made between the joining of tubular elements, which takes place in a production environment, and the assembly of sections, which takes place in a sheltered assembly location such as a harbour, bay or fjord. Preferably, these sections can be transported to a quay where they can be assembled floating in shallow water or even ashore.

[0043]

[0043] The quay may be arranged to have a storage area and an assembly area. The storage area may be used to stack different sections of the floating foundation. The sections can be moved from the storage area to an assembly area adjacent to the storage area by self-propelled modular transport (on land) or by barge (offshore). The assembly area may be equipped with lifting means for lowering and / or lifting the sections of the floating foundation into the water. The sections are assembled one by one while floating. To keep the sections floating during assembly, lifting bags may be attached to the sides of the sections and / or the internal chambers may be ballasted with water.

[0044]

[0044] Unlike assembly in a dry dock, assembly at sea does not require a large assembly area during assembly because sections can be easily moved / rotated by winches, barges, carriers, ships, etc. during the assembly process. After assembly, the floating foundation can be towed by a towing means such as a ship or barge and leave the assembly area for the next assembly. This significantly reduces assembly time, ensures flexibility in serial production, and enables mass assembly of floating foundations in a relatively small assembly area.

[0045] In a further embodiment, the hollow member has a proximal end, and the method includes connecting a wind turbine tower to the proximal end of the hollow member. This step may also occur at or adjacent to a second location. The wind turbine tower is preferably installed above the water surface. If this occurs offshore, the completed wind turbine may be towed to an offshore location, and the foundation may be ballasted and secured. Alternatively, a floating foundation may be towed to an offshore location, and the tower and wind turbine installed offshore. Unballasted floating foundations have a shallow draft, making them easier to navigate offshore. The wind turbine's electrical cable preferably passes through the tower and exits internally near the level of the boarding deck of the first abovewater section. The cable may exit the foundation via a J-shaped tube. This configuration facilitates installation of the delivery cable and avoids permanently submerged hull penetrations.

[0046]

[0046] Furthermore, the floating foundation has a distal end, and the method further includes connecting a heave plate to the distal end. The heave plate improves the motion behavior of the foundation by decoupling the heave and pitch motions of the foundation when floating. Furthermore, the heave plate limits the draft of the foundation by compensating for the underwater volume of the underwater section, allowing the foundation to be assembled and the wind turbine to be installed in shallower drafts.

[0047] Further advantages of the disclosed invention will become apparent below.

[0048]

[0048] The invention will now be described in more detail with reference to the accompanying drawings. [Brief explanation of the drawings]

[0049] [Figure 1] FIG. 1 is a perspective view of a floating wind turbine installation. [Figure 2] FIG. 2 is a side view showing the facility of FIG. 1. [Figure 3] FIG. 3 is a plan view showing the facility of FIG. 2. [Figure 4]FIG. 4 shows a section of the floating foundation of FIGS. 1 to 3 before assembly. [Figure 5] FIG. 5 shows tubular elements for forming the floating foundation of FIG. 4. [Figure 6] FIG. 2 shows an enlarged area of ​​FIG. 1 around the curved portion of the above-water section. [Figure 7] FIG. 1 illustrates an alternative embodiment of the present invention. [Figure 8] FIG. 10 illustrates another alternative embodiment of the present invention. [Figure 9] FIG. 10 illustrates yet another alternative embodiment of the present invention. [Figure 10] FIG. 10 illustrates another alternative embodiment of the present invention. [Figure 11] FIG. 10 illustrates yet another alternative embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0050]

[0056] The present invention will now be described in more detail with reference to the drawings in which exemplary embodiments thereof are shown. The drawings are for illustrative purposes only and are not intended to limit the scope of the present invention, which encompasses all modifications, equivalents, and alternatives falling within the scope of the invention. The scope of the present invention is limited only by the definition set forth in the appended claims.

[0051]

[0057] FIG. 1 shows a perspective view of 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 elongated hollow member 12 having a centerline CL that defines a serpentine path, as described further below. The installation 1 is held 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 approximately 150 m, and the blades are approximately 110 m long. The following description of the floating foundation 10 is based on a turbine of this size. However, those skilled in the art will understand that the dimensions of the foundation 10 will vary depending on the size of the turbine.

[0052]

[0058] FIG. 2 shows the installation 1 of FIG. 1 in a side view in an installed state, floating on a body of water having a surface S. The elongated hollow member 12 has submerged sections 14, 16 and abovewater sections 18, 20. Transition sections 24A, 24B, 24C pass through the splash zone of the water surface S. The elongated hollow member 12 has a first proximal end 26 connected to the base of the tower 4 and a second distal end 28 provided with a heave plate 30. FIG. 2 further shows that the interior of the elongated hollow member 12 is divided into seven internal chambers 31A, 31B, 31C, 31D, 31E, 31F, and 31G. The internal chambers 31B, 31C, 31D, 31E, and 31G are ballasted with seawater. The internal chambers are separated by watertight bulkheads.

[0053]

[0059] 3 shows the installation 1 of FIG. 2 in plan view, illustrating the location of the transition sections 24A, 24B, and 24C in a horizontal plane. As can be seen, in the illustrated embodiment, these transition sections are located at the corners of an equilateral triangle. The first underwater section 14 and the second abovewater section 20 form two sides of the triangle. In this case, the length of each side is 105 m, calculated from the centerline CL of the hollow member.

[0054]

[0060] Figure 4 shows the floating foundation 10 of Figures 1-3 in a perspective view prior to assembly. The elongated hollow body 12 is formed from four sections. The first above-water section 18 extends from a proximal end 26 to a first transition section 24A. The first underwater section 14 extends from the first transition section 24A to a second transition section 24B. The second above-water section 20 extends from the second transition section 24B to a third transition section 24C. The second underwater section 16 extends from the third transition section 24C to a heave plate 30 at a distal end 28.

[0055]

[0061] As can be seen, the first submerged section 14 and the second above-water section 20 have a straight portion 34 and two curved portions 36A, 36B. As can be seen, each of the sections 14, 16, 18, 20 is also formed from a plurality of tubular elements 40 that are abuttingly welded together.

[0056]

[0062] FIG. 5 shows a perspective view of a single tubular element 40 from a portion of the straight section 34 of the hollow member 12 of FIG. 4. The element 40 is a circular tubular section of diameter D, with a wall 42 of thickness t and an inner surface 44. The tubular element 40 has a length L along a centerline CL. In the illustrated embodiment, the diameter D is 15 m and the length L is 5 m. The thickness t is 35 mm. Attached to the inner surface 44 is a ring frame 46 that extends radially inward a distance of 50 cm. The ring frame 46 provides additional stiffening to the tubular element 40, keeping the thickness t small and allowing for material and weight savings.

[0057]

[0063] In the illustrated embodiment, the straight portions 34 of the submerged section 14 and abovewater section 20 are generally parallel to the water surface S, and the curved portions 36A, 36B form split elbows having a large average radius of about 15 m, while the transition sections 24A, 24B, 24C are generally perpendicular to the water surface S. However, one skilled in the art will understand that the floating foundation 10 may be formed without any straight sections, and the elongated hollow member 12 may be continuously curved.

[0058]

[0064] Figure 6 shows an enlarged view of a portion of the segmented curved section 36A of the second abovewater section 20. As can be seen, the curved section 36A is constructed from a series of straight tubular elements with beveled ends. This allows for the use of a single curved plate, which is significantly simpler than using a double curved plate. The abovewater section 20 has a walkway 50, a ladder 52, and anchor points 54 from which the mooring chain 15 extends. The walkway 50 also provides access through a door or manhole 56 leading to the interior of the elongated member.

[0059]

[0065] Due to its simple concept, the floating foundation of the present invention is fully scalable and suitable for wind turbines of various sizes. The floating foundation can be equipped with an active or passive ballast system depending on the needs of the user and the environmental conditions of the project site. Exemplary features of different embodiments of the floating foundation of the present invention are shown in Table 1.

[0060] [Table 1]

[0061]

[0066] Floating foundations of the present invention can be efficiently produced in series because many elements are identical. Furthermore, sections can be easily stored and transported for final assembly. Final assembly can be performed onshore or offshore with as few as three welds or other types of connections. The minimum draft of the floating foundation can be as little as 6m. Because all welding of the assembled sections is in a horizontal plane, the welding process is simplified by using a crane or float-over with a barge.

[0062]

[0067] A further alternative embodiment is shown in Figures 7 through 11, which illustrate that the cross section of the tubular member need not be constant and may have more than three transition sections. According to Figure 11, the tubular member forms a closed loop and has no distal end. Note that in this embodiment, there is a single branch of the tubular member, but this is below the waterline, and there are still three transition sections, forming an isosceles triangle in this embodiment.

[0063]

[0068] 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 of the inventive concept. Accordingly, the scope of the invention is indicated by the appended claims rather than by the foregoing description. Those skilled in the art will readily appreciate that they may conceive and implement alternative and equivalent embodiments of the invention. In addition, many modifications may be made to adapt a particular structure or material to the teachings of the present invention without departing from the essential scope thereof.

[0064]

[0069] All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

1. 1. A floating foundation for an offshore wind turbine having a tower, comprising an elongated hollow member defining a serpentine path and having a submerged section, an above-water section and a transition section therebetween that passes above the water surface in use, the floating foundation comprising at least three transition sections spaced apart from one another to form a stable base.

2. The floating foundation of claim 1 , wherein a proximal end of the hollow member comprises a first transition section configured to fixedly receive the tower of the wind turbine.

3. 3. A floating foundation according to claim 1 or 2, wherein the distances between successive transition sections are equal, and optionally said transition sections form the nodes of an equilateral triangle in a horizontal plane.

4. The floating foundation according to any one of claims 1 to 3, wherein the serpentine path includes alternating straight and curved portions.

5. A floating foundation according to any one of claims 1 to 4, wherein the transition section is substantially vertical.

6. 6. A floating foundation according to any one of claims 1 to 5, wherein the elongated hollow member has an inner surface to which a plurality of ring frames extending radially inward are attached.

7. A floating foundation according to any one of claims 1 to 6, wherein the hollow member has a circular outer cross section with a diameter of at least 7.5m.

8. A floating foundation according to any one of claims 1 to 7, wherein no part of the serpentine path has a radius of curvature less than the diameter of the pipe.

9. A floating foundation according to any one of claims 1 to 8, wherein the interior of the hollow member is provided with a plurality of separated ballast tanks.

10. A floating foundation according to any one of claims 1 to 9, wherein the hollow member further comprises a distal end provided with a heave plate.

11. A floating foundation according to any one of claims 1 to 10, wherein a majority of the floating foundation is made of steel.

12. A floating foundation according to any one of claims 1 to 11, wherein the floating foundation is configured to support a wind turbine having a total weight of more than 1000 t.

13. The floating foundation according to any one of claims 1 to 12, wherein the hollow member is a single bent pipe without any branches or branches.

14. A floating wind turbine comprising a tower and a floating foundation according to any one of claims 1 to 13.

15. 15. The floating wind turbine of claim 14, wherein the tower is connected to a proximal end of the hollow member, the cross section of the tower matching the cross section of the hollow member at the location of the connection.

16. 16. A floating wind turbine according to claim 14 or 15, wherein the floating foundation is fixed to the seabed.

17. 1. A method of constructing a floating foundation for an offshore wind turbine, comprising: providing a plurality of tubular elements; interconnecting the tubular elements to form at least one nonlinear underwater section and at least one nonlinear abovewater section; launching the submersible section into a floating position; assembling the above-water section onto the floating underwater section to form an elongated hollow member; A method comprising:

18. The method of claim 17 , wherein each of the tubular elements comprises one or more ring frames.

19. 19. The method of claim 17 or 18, wherein each of the tubular elements has a length and a diameter greater than the length.

20. A method according to any one of claims 17 to 19, wherein the underwater section and the abovewater section are assembled by welding, preferably at a levelled joint.

21. 21. The method of any one of claims 17 to 20, comprising at least two submersible sections, the two submersible sections both being launched into a floating position, and the above-water section being assembled to join the submersible sections together.

22. 22. The method of any one of claims 17 to 21, wherein the tubular elements are interconnected at a first location on land and the above-water section and the underwater section are assembled at a remote second floating location or quay.

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