Floating device for offshore wind turbine support and corresponding floating wind turbine assembly

Abstract

The invention relates to a floating device (2) intended to support a wind turbine (1) at sea and comprising a central floating column (21) arranged to fixedly receive a mast (12) of the wind turbine (1), at least three peripheral floats (22), an arm (23) per float, each arm (23) extending along a radial elongation direction relative to the central column (21) and each arm (23) having a proximal end fixed to the central column (21) and a distal end fixed to said float (22), the arms (23) comprising an external tubular element extending along the elongation direction of the arm (23) and having a curved section perpendicular to the elongation direction, and an internal tubular element extending along the elongation direction of the arm (23) and having a polygonal section perpendicular to the elongation direction, the polygonal section being inscribed in the curved section.The invention also relates to a floating wind turbine assembly comprising said device and a wind turbine.

Description

1. Field of the invention

[0001] The invention relates to the field of floating wind power. It relates more particularly to a floating device intended to support a wind turbine at sea and a floating wind turbine assembly comprising said device and a wind turbine. 2. Prior art

[0002] Floating wind turbine systems are known, comprising a floating device and an offshore wind turbine supported by the floating device. The wind turbine comprises a mast and a turbine. Examples of such wind turbine systems include HYWIND™, WINDFLOAT™, and IDEOL™.

[0003] One of the main difficulties encountered by floating devices intended to support an offshore wind turbine is the ability to withstand fatigue loads, due to the repetitive movements imposed on the floating wind turbine assembly, particularly by the turbine, wind and swell, and the extreme loads generated in the event of heavy seas. A disadvantage of current floating devices is that they are designed in materials that are not particularly resistant to fatigue loads and extreme loads. Indeed, they are conventionally designed in metal and in particular in steel. The design of such structures to withstand the fatigue loads and extreme loads previously mentioned therefore leads to bulky, heavy and expensive structures.

[0004] This results in a complex and costly manufacturing process for these structures. In particular, given the size of these structures, large dry docks are required for their manufacture.

[0005] These various disadvantages notably limit the power of the wind turbines that the floating devices of such sets can support.

[0006] Current floating assemblies are penalized not only by the use of materials that are not particularly resistant to fatigue, but also by the choice of a geometry that does not optimally limit the generation of new loads acting on the assembly. The loads generated are, for example, gravity loads linked to the inclination of the wind turbine mast relative to the vertical direction, inertial loads linked to the movements of the wind turbine mast around its resting position or loads resulting from the hydrostatic pressure acting on the walls of the floating device.

[0007] Furthermore, current floating wind turbine assemblies commonly use elements with solid volumes to withstand fatigue loads and extreme loads. However, these solid volumes do not contribute to the overall buoyancy of the system, which requires both increasing the dimensioning of the floating elements to increase their buoyancy and increasing the overall dimensioning of the system to withstand the bending forces that are transmitted between the floating elements and the non-floating elements. Another example in the prior art is disclosed by WO2004061302. 3. Objectives of the invention

[0008] The invention proposes a solution aimed at overcoming the aforementioned drawbacks.

[0009] Thus, the primary objective of the invention is to withstand the fatigue loads and extreme loads exerted on the floating device while limiting the generation of new loads, particularly on the turbine. This is ensured by the use of several materials, each used for their advantages from a mechanical point of view and placed at particular locations in the structure in order to absorb, under the best conditions, the forces transmitted both by the turbine, via the mast fixed to the floating device, and by the marine environment. This is also ensured by the geometry of the various elements so as to limit the generation of new loads and to confer neutral or positive buoyancy to the elements.

[0010] Another objective of the invention is to maintain similar behavior regardless of the orientation of environmental disturbances, such as wind, current or swell, thanks to shape symmetries opening up expanded possibilities with respect to installation sites. The invention provides greater flexibility with respect to installation sites as it makes it possible to obtain a floating wind turbine assembly with a relatively low draft compared to the existing one, which facilitates the transport and installation of such an assembly.

[0011] An objective of the invention is also to obtain a floating device having a small footprint, in particular compared to existing devices intended to support high-power turbines. As a corollary to this objective, the invention aims to propose a floating device that can be rapidly adapted to the use of turbines having a power greater than that of the turbines currently used in floating wind turbine assemblies, without modifying the architecture of the floating device. 4. Summary of the invention

[0012] The characteristics related to orientation in space are given throughout the document when the floating assembly floats on a body of water at rest, i.e. on a horizontal surface in the absence of wind, current and swell. The vertical direction is defined by the direction of gravity, the gravity field pointing downwards. Any direction orthogonal to the vertical direction is horizontal.

[0013] The invention relates to a floating device intended to support a wind turbine at sea and comprising: a floating central column arranged to fixedly receive a mast of the wind turbine, at least three peripheral floats, one arm per float, each arm extending in a radial elongation direction relative to the central column and each arm comprising a proximal end fixed to the central column and a distal end fixed to said float, the device being characterized in that the arms comprise: an external tubular element extending in the direction of elongation of the arm and of curved section perpendicular to the direction of elongation, and an internal tubular element extending in the direction of elongation of the arm and of polygonal section perpendicular to the direction of elongation, the polygonal section being inscribed in the curved section. The interlocking of the inner tubular element with a polygonal section into the outer tubular element with a curved section allows for good distribution of forces and therefore good mechanical strength. The inner tubular element, thanks to its polygonal section, allows for the in-plane bending caused by the wind thrust on the turbine, and the alternating in-plane bending induced by rolling, pitching and heave movements. The outer tubular element, thanks to its section delimited by a closed curve, allows for the transmission of external compressive pressure forces along the outer tubular element. In addition, the shell of the outer tubular element and the shell of the inner tubular element form a double shell, which reduces the risk of infiltration and therefore increases the safety of the floating device. The hollow and voluminous nature of the arms provides buoyancy.

[0014] According to a particular embodiment, the arms comprise compartments capable of being filled at least partially with ballast so as to adjust the buoyancy of the arms in order to avoid the appearance of undesirable additional bending forces due to excessively large buoyancy disparities.

[0015] According to a particular embodiment, the floating device further comprises at least two connecting pieces per pair of adjacent arms, each connecting piece comprising a first and a second end fixed respectively near the distal ends of the first and second arms of said pair of adjacent arms. The connecting pieces ensure mechanical continuity between the arms to limit out-of-plane moments generating deflected bending.

[0016] According to a particular embodiment, the arms extend horizontally. This embodiment is preferable in particular when the waterline crosses the arm. Indeed, in this case, additional forces are likely to appear if the arms are not horizontal.

[0017] According to a particular embodiment, radial planes being each defined by the direction of elongation of an arm and by a vertical direction, the device is arranged so that the radial planes are planes of symmetry of the device. This symmetrical configuration makes it possible to maintain similar behavior regardless of the orientations of environmental disturbances, such as wind, current or swell, so as to broaden the range of possible installation sites.

[0018] According to a particular embodiment, the floating device comprises four peripheral floats. This number of four floats allows a good compromise between the size and the hydrostatic performance of the floating wind turbine assembly, that is to say that it allows good buoyancy despite a relatively reduced arm length.

[0019] According to a particular embodiment, the peripheral floats comprise internal partitions. The partitioning ensures the stability of the entire float in the event of damage, for example with a compartment filled with water.

[0020] According to a particular embodiment, the central column comprises a fixing part comprising a hollow volume intended to receive the mast of the wind turbine, the hollow volume being cylindrical or truncated so as to match the shape of the mast. The shape of the mast of the wind turbine not being standard, the fixing part makes it possible to adapt the same floating device to several types of wind turbine.

[0021] According to a particular embodiment, the central column is hollow so as to have sufficient buoyancy to support the wind turbine. This makes it possible to avoid the appearance of additional undesirable bending forces due to excessively large buoyancy disparities.

[0022] According to a particular embodiment, the central column has, horizontally, a polygonal section whose number of sides is equal to twice the number of arms so that the arms are respectively fixed to sides of the polygonal section not adjacent to each other. This geometry makes it possible to achieve a weight / volume ratio promoting the buoyancy of the central column.

[0023] According to a particular embodiment, the floats comprise a truncated lower portion with a vertical axis and a cylindrical upper portion with a vertical axis. The shape of the floats provides a large buoyancy volume and good mechanical resistance to hydrostatic forces. In particular, the main objective sought is to limit movements, accelerations and forces on the turbine thanks to significant damping.

[0024] According to a particular embodiment, the floating device is made, at least partially, of reinforced concrete. Concrete has very good resistance to fatigue and extreme environments. It also makes it possible to avoid stress concentration phenomena at the welds, in particular for complex "wolf mouth" type connections, which are very expensive in steel. Concrete also makes it possible to produce elements with large dimensions, particularly with regard to floats. Large floats make it possible to reduce the overall trim of the assembly while maintaining good buoyancy because they make it possible to reduce the length of the arms. Concrete also has the advantage of being less expensive than steel in standard use. According to a particular embodiment, the floating device comprises steel portions, for example the reinforcement parts.Unlike concrete, steel resists tensile forces, making it particularly suitable for reinforcement parts that primarily work in tension. Furthermore, because steel is a relatively lightweight material, it helps limit the additional mass of reinforcement parts.

[0025] According to a particular embodiment, the floating device further comprises an anchoring system.

[0026] The invention also relates to a floating wind turbine assembly comprising: a device according to any one of the preceding claims, and a wind turbine. 5. List of figures

[0027] Other innovative features and advantages will emerge from the following description, provided for informational purposes and in no way limiting, with reference to the attached drawings, in which: There Figure 1 represents a front view of a floating wind turbine assembly according to the invention, The Figure 2represents a side perspective view of the floating wind turbine assembly of the Figure 1 , There Figure 3 represents a perspective view from below of the floating wind turbine assembly of the figures 1 And 2 , There Figure 4 represents a perspective view of a floating device according to the invention, The Figure 5 represents a bottom view of the floating device of the Figure 4 , There Figure 6 represents an exploded and partial view of the floating device of the Figures 4 and 5 , There Figure 7 represents a vertical sectional view of an arm of the device figures 4 to 6 , THE Figures 8a and 8b schematically represent the transmission of bending forces from a wind turbine to an arm in a floating wind turbine assembly according to the invention, The figure 9 schematically represents the transmission of compression forces in an arm of a floating device according to the invention. 6. Detailed description

[0028] To figures 1 to 3a floating wind turbine assembly according to the invention is shown. The floating wind turbine assembly comprises a wind turbine 1 and a floating device 2 intended to support the wind turbine 1.

[0029] The wind turbine 1 comprises a turbine 11 and a mast 12 supporting the turbine 11. The turbine 11 is suitable for offshore use and has a power of several megawatts. Preferably, it is a three-bladed horizontal-axis turbine. The mast 12 is made of cylindrical elements advantageously made of steel without longitudinal stiffening. The upper portion of the mast 12 is arranged to receive the turbine 11.

[0030] In reference to the Figure 4 , the floating device 2 comprises a central floating column 21 arranged to receive the mast 12, at least three peripheral floats 22, one arm 23 per float and advantageously at least two connecting pieces 24 per pair of adjacent arms 23.

[0031] In reference to the Figure 6, the central column 21 comprises a trunk 211 having the shape of a right prism with a vertical generator. The trunk 211 comprises a lower face 2111 and a lower face 2112 that are polygonal. The lower face 2111 and the lower face 2112 are horizontal. The lower face 2111 comprises a slab on which rests the base of the mast 12 of the wind turbine 1. In addition, the central column 21 comprises a fixing piece 212 comprising a hollow volume 2120 intended to receive the mast 12 of the wind turbine 1, the hollow volume 2120 being cylindrical or frustoconical so as to match the shape of the mast 12. The fixing piece 212 is preferably a steel piece fixed inside the central column 21. For example, the central column 21 being made of concrete, the fixing piece 212 is taken in the concrete of the column. The fixing part 212 is adapted to the wind turbine 1 that the floating device 2 is intended to receive.On the other hand, the rest of the floating device 2 is designed to be able to accommodate a wide variety of wind turbines.

[0032] The central column 21 is hollow to provide the buoyancy necessary to support the wind turbine 1. The central column 21 comprises compartments capable of being at least partially filled with ballast. The ballast is, for example, seawater or another solid, liquid or granular material, denser than seawater, allowing the mass of the entire system to be adjusted in order to adjust the draft for dockside operations, transit operations or on-site installation. In the case of liquid ballast, suitable adjustment means are, for example, liquid pumping means allowing ballast to be added to or removed from the compartments suitable for ballasting.

[0033] According to a preferred embodiment, the central column 21 is made at least partially of reinforced concrete in order to withstand the cyclic fatigue stresses generated in particular by the movements of the wind turbine 1.

[0034] According to a particular embodiment, the central column 21 is made at least partially of steel, although steel is less resistant to fatigue than concrete.

[0035] The peripheral floats 22 comprise a frustoconical lower portion 221 and a cylindrical upper portion 222. The frustoconical lower portion 221 and the cylindrical upper portion 222 extend along a vertical axis. Here, the vertical axis is an axis of symmetry of revolution of the floats. At the interface between the frustoconical lower portion 221 and the cylindrical upper portion 222, the diameter of the two portions is identical. The interface between the frustoconical lower portion 221 and the cylindrical upper portion 222 is materialized by a slab. The frustoconical portion flares downwards so as to provide better hydrodynamic damping to improve the dynamic behavior of the platform. The lower base of the frustoconical portion is materialized by a slab. Preferably, in the frustoconical portion, the diameter of the lower base is greater than the height.

[0036] The geometry of the floats 22 provides a significant buoyancy volume and mechanical resistance to hydrostatic forces. In a preferred embodiment, the waterline L is located at the level of the cylindrical part of the floats.

[0037] The floats are sized so that the natural periods of the floats in roll and pitch are beyond the periods of wave energy.

[0038] The cylindrical upper portion 222 comprises a platform forming a flat surface intended to allow human interventions or fixing of equipment necessary for towing, installation, or maintenance operations, for example.

[0039] The floats 22 further comprise a partitioning device inside the upper and lower portions to ensure the stability of the entire float in the event of damage, for example with a column filled with water.

[0040] On the same principle as that of the central column 21, the floats 22 comprise compartments capable of being at least partially filled with ballast. The ballast is for example sea water or another solid, liquid or granular material, denser than sea water, making it possible to adjust the mass of the entire system in order to adjust the draft for dockside operations, transit operations or on-site installation. In the case of liquid ballast, suitable adjustment means are for example liquid pumping means making it possible to add or remove ballast. The compartments intended for the ballast are located far from the axis of the float 22 and in the lower portion in order to contribute to the stability of the system by increasing its inertia in rolling and pitching.

[0041] In a preferred embodiment, the floats 22 are made at least partially of reinforced concrete in order to withstand the stresses from the wind turbine 1, the aquatic environment and the dynamics of the floating device 2.

[0042] According to a particular embodiment, the floats 22 are made at least partially of steel.

[0043] Preferably, the floating device 2 comprises between three and eight floats 22 and in particular four floats 22. The number of floats 22 is calculated to keep the size fairly small compared to the floating devices intended to support high-power turbines and to remain within a range of inclination in operation and in an extreme environment compatible with the use of existing turbines.

[0044] The arms 23 extend in a radial elongation direction relative to the central column 21 and each arm 23 has a proximal end fixed to the central column 21 and a distal end fixed to a float 22 associated with the arm 23. With reference to the Figure 7, the arms 23 further comprise an outer tubular element 231 and an inner tubular element 232. The inner and outer tubular elements extend in the direction of elongation of the arm 23. The outer tubular element 231 has a curved section, here circular perpendicular to the direction of elongation. The inner tubular element 232 has a polygonal section, here rectangular perpendicular to the direction of elongation. The rectangular section is inscribed in the circular section, the vertices of the rectangular section being fixed to an inner wall of the circular section. The geometry corresponding to a rectangular section inscribed in a circular section is designed to absorb the forces from the turbine and the marine environment.Furthermore, the shell of the outer tubular element 231 and the shell of the inner tubular element 232 form a double shell, which reduces the risks of infiltration and therefore increases the safety of the floating device 2. The inner tubular element 232, thanks to its rectangular section, makes it possible to take up the bending in the plane, generated by the thrust of the wind on the blades of the turbine, and the alternating bending in the plane, induced by the rolling, pitching and heaving movements. Indeed, the inner tubular element 232 of rectangular section comprises slabs located in particular in the extension of a high portion and in the extension of a low portion of the circular section of the arm 23 to ensure continuity in the transmission of forces, in particular bending moments transmitted from the wind turbine 1 to the arms 23. The . Figures 8a and 8bshow that the thrust P generated by the thrust P of the wind on the turbine blades induces a bending moment FC at the central column 21. The bending moment FC in the central column 21 is transmitted in the form of bending moments FB in the arms 23 having an elongation direction parallel to the thrust P. The bending moments FB in the arms are taken up in the form of CH stresses in the slab of the upper portion of the arm and in the form of CB stresses in the slab of the lower portion of the arm.

[0045] The external tubular element 231, thanks to its curved section, makes it possible to absorb external compression pressure forces generated by the aquatic environment. figure 9shows that the external compressive pressure forces C are transmitted to the outer tubular element 231 in the form of peripheral stresses CP . The peripheral stresses CP in the outer tubular element 231 are then transmitted to the inner tubular element 232 in the form of internal stresses CI .

[0046] Alternatively, the curved section is elliptical and / or the polygonal section is triangular, pentagonal or hexagonal.

[0047] In a preferred embodiment, the arms 23 have a cylindrical external shape of revolution.

[0048] In another embodiment, the arms 23 have a truncated external shape.

[0049] In a preferred embodiment, the arms 23 extend horizontally. The axes of elongation of the arms 23 are therefore coplanar.

[0050] In another embodiment, the axes of elongation of the arms 23 form generatrices of a cone of revolution whose apex is located on the axis of the mast 12.

[0051] The direction of elongation of an arm 23 and the vertical direction define a radial plane. According to a preferred embodiment shown in Figure 5, the device is arranged so that the radial planes are planes of symmetry of the device. In other words, the arms 23 are arranged in a star shape around the central column 21 in a regular manner, that is to say that the angle α formed by the elongation axes of two arms 23 of a pair of successive arms 23 is the same regardless of the pair of arms 23 and is equal to 360° divided by the total number of arms 23. This symmetrical configuration makes it possible to maintain similar behavior regardless of the orientations of the environmental disturbances, such as wind, current or swell, so as to broaden the range of possible installation sites. In this case, according to a particular embodiment, the turbine comprises a servo-controlled actuator which orients the turbine according to the direction of the wind.

[0052] The interfaces between the arms 23 and the central column 21 are located in vertical planes. At the interface between an arm 23 and the central column 21, the section of the arm 23 is tangent to one side of the polygon formed by the lower face 2111 of the trunk 211 of the central column 21; the diameter of the section of the arm 23 is less than or equal to the distance between the lower face 2112 and the lower face 2111 of the trunk 211. The trunk 211 has, horizontally, a polygonal section whose number of sides is equal to twice the number of arms 23 so that the arms 23 are respectively fixed to sides of the polygonal section that are not adjacent to each other. For example, when the floating device 2 comprises four floats 22 and therefore four arms 23, the lower face 2111 and the lower face 2112 of the trunk 211 are octagonal.

[0053] Each arm 23 is fixed to the lower truncated portion 221 of the float associated with it. The interface between the arm 23 and the lower truncated portion 221 is wolf's mouth, according to a structure conventionally used in the field of piping. At the interface between the arm 23 and the lower truncated portion 221, the section of the arm 23 is tangent in the lower part to the base of the lower truncated portion 221 and in the upper part to the interface between the lower truncated portion 221 and the upper cylindrical portion 222. Thus, the transmission of forces is ensured between the arm 23 and the slab constituting the base of the lower truncated portion 221 and between the arm 23 and the slab constituting the interface between the lower truncated portion 221 and the upper cylindrical portion 222.

[0054] The arms 23 are arranged to delimit a sufficient volume to have positive buoyancy and are designed to be hollow. On the same principle as that of the central column 21 and the floats 22, the arms 23 comprise compartments capable of being at least partially filled with ballast. The ballast is for example sea water or another solid, liquid or granular material, denser than sea water, making it possible to adjust the mass of the entire system in order to adjust the draft for dockside operations, transit operations or on-site installation. In the case of liquid ballast, suitable adjustment means are for example liquid pumping means making it possible to add or remove ballast from said compartments.

[0055] In a preferred embodiment, the arms 23 are completely submerged in operation so as to guarantee acceptable behavior from the point of view of movements and forces undergone in extreme conditions, that is to say when the wind turbine 1 is stopped and in the event of strong swell. According to another embodiment, the arms 23 are partially submerged.

[0056] In a preferred embodiment, the arms 23 are made at least partially of reinforced concrete to better withstand fatigue stresses and extreme loads from the turbine 11 and the aquatic environment.

[0057] According to a particular embodiment, the arms 23 are made at least partially of steel.

[0058] The connecting pieces 24 connect the arms 23 in pairs and extend horizontally. Each connecting piece 24 has a first and a second end fixed respectively near the distal ends of the first and second arms 23 of said pair of adjacent arms 23. The connecting pieces 24 connecting the same pair of arms 23 are located in the same vertical plane. The connecting pieces 24 ensure mechanical continuity between the arms 23 to limit the out-of-plane moments generating deflected bending. These connecting pieces 24 have a small section compared to the dimensions of the other elements.

[0059] In a preferred embodiment, the connecting pieces 24 are made of steel because they work mainly in traction, and steel resists tensile forces unlike concrete.

[0060] According to one embodiment, the floating device 2 comprises mooring means. The mooring means are a set of lines 30 connecting the platform to a fixed external element, for example, the bottom of the aquatic mass on which the platform floats. The lines 30 comprise a first end connected to the floating device 2 at one or more points and a second end connected to the fixed external element, for example via an anchor or a dead weight. The lines 30 may consist of cables, chains or other flexible elements suitable for maintaining in position a floating object subjected to drift forces under the effect of the environment. According to particular embodiments, the lines 30 are entirely or partially vertically or obliquely stretched.

[0061] In an alternative embodiment, the lines 30 are catenaries.

[0062] Depending on the mode of use, the draft of the floating wind turbine assembly is low, for example around ten meters unballasted and around fifteen meters ballasted, in order to facilitate towing and installation operations.

[0063] Alternatively, the draft is the same between the installation phase and the operation phase, which avoids ballasting / deballasting operations on the production site. The draft can nevertheless be adjusted by ballasting once on the production site.

Claims

1. Floating device (2) for supporting an offshore wind turbine (1) and comprising: - a central floating pillar (21) arranged for fixedly receiving a tower (12) of the wind turbine (1), - at least three peripheral floaters (22), - one leg (23) per floater, each leg (23) extending in a radial longitudinal direction in relation to the central pillar (21) and each leg (23) comprising a proximal end that is secured to the central pillar (21) and a distal end that is secured to said floater (22), an outer tubular element (231) which extends in the longitudinal direction of the leg (23) and has a curved cross-section perpendicularly to the longitudinal direction, the floating device (2) being characterised in that the legs (23) comprise: - an inner tubular element (232) which extends in the longitudinal direction of the leg (23) and has a polygonal cross-section perpendicularly to the longitudinal direction, the polygonal cross-section being inscribed in the curved cross-section.

2. Floating device (2) according to claim 1 characterised in that it further comprises at least two connection parts (24) per pair of adjacent legs (23), each connection part (24) comprising a first and a second ends secured respectively in the vicinity of the distal ends of the first and second legs (23) of said pair of adjacent legs (23).

3. Floating device (2) according to any preceding claim characterised in that the legs (23) extend horizontally.

4. Floating device (2) according to any preceding claim characterised in that, radial planes being each defined by the longitudinal direction of a leg (23) and by a vertical direction, the floating device (2) is arranged in such a way that the radial planes are planes of symmetry of the floating device (2).

5. Floating device (2) according to any preceding claim characterised in that the central pillar (21) comprises a securing part (212) comprising a hollow volume (2120) for receiving the tower (12) of the turbine (1), the hollow volume (2120) being cylindrical or truncated in such a way as to hug the shape of the tower (12).

6. Floating device (2) according to any preceding claim characterised in that the central pillar (21) has, horizontally, a polygonal cross-section of which the number of sides is equal to double the number of legs (23) in such a way that the legs (23) are respectively secured to sides of the polygonal cross-section that are not adjacent between them.

7. Floating device (2) according to any preceding claim characterised in that the floaters (22) comprise a truncated lower portion (221) with a vertical axis and a cylindrical upper portion (222) with a vertical axis.

8. Floating device (2) according to any preceding claim characterised in that it is made, at least partially, of reinforced concrete.

9. Floating device (2) according to any preceding claim characterised in that it comprises portions made of steel.

10. Floating wind turbine unit comprising: - a floating device (2) according to any preceding claim, and - a wind turbine (1).