ASYMMETRIC FLOATING WIND TURBINE INSTALLATION

Abstract

A floating wind turbine installation including an asymmetric floating wind turbine structure that is tethered to the floor of a body of water by a mooring system. The floating wind turbine structure includes a wind turbine mounted on a semi-submersible floating platform, and is oriented such that the wind turbine is positioned on an upwind side of the centre of mass of the floating wind turbine structure when the wind approaches the wind turbine structure in the direction of the prevailing wind at the location of the wind turbine installation.

Claims

1. A floating wind turbine installation, comprising an asymmetric floating wind turbine structure tethered to the floor of a body of water by a mooring system, wherein: the floating wind turbine structure comprises a wind turbine mounted on a semi-submersible floating platform, and the floating wind turbine structure is held in position by the mooring system such that the wind turbine is positioned on an upwind side of the centre of mass of the floating wind turbine structure in the direction of the prevailing wind at the location of the wind turbine installation.

2. A floating wind turbine installation as claimed in claim 1, wherein an angle between the prevailing wind direction and a straight line passing through the position of the wind turbine and the centre of mass of the floating wind turbine installation is 60 or less.

3. A floating wind turbine installation as claimed in claim 1, wherein the wind turbine is positioned substantially directly upwind of the centre of mass of the floating wind turbine structure in the direction of the prevailing wind.

4. A floating wind turbine installation as claimed in claim 1, wherein the semi-submersible floating platform comprises three columns connected by connecting members in a ring configuration.

5. A floating wind turbine installation as claimed in claim 4, wherein the wind turbine is supported on one of the columns of the semi-submersible floating platform.

6. A floating wind turbine installation as claimed in claim 5, wherein the mooring system comprises four mooring lines connected to the floating wind turbine structure, wherein two mooring lines of the four mooring lines are connected to the column supporting the wind turbine, and wherein the other two mooring lines of the four mooring lines are respectively connected to a different one of the other two of the three columns.

7. A floating wind turbine installation as claimed in claim 6, wherein the mooring lines are connected directly to the floating wind turbine structure.

8. A floating wind turbine installation as claimed in claim 4, wherein the mooring system comprises three mooring lines connected to the floating wind turbine structure, wherein each mooring line of the three mooring lines is connected to the floating wind turbine structure by a respective bridle comprising two bridle lines, with each of the two bridle lines being connected to a different one of the columns of the semi-submersible platform such that the respective mooring line is connected to two columns of the semi-submersible platform.

9. A floating wind turbine installation as claimed in claim 8, wherein each column of the three columns is connected to two of the three mooring lines.

10. A floating wind turbine installation as claimed in claim 5, wherein the mooring system comprises three mooring lines connected to the floating wind turbine structure, wherein two mooring lines of the three mooring lines are connected to the column supporting the wind turbine, and wherein the other mooring line of the three mooring lines is connected to the other two of the three columns via a bridle.

11. A floating wind turbine installation as claimed in claim 10, wherein the two mooring lines connected to the column supporting the wind turbine are directly connected to the column.

12. A floating wind turbine installation as claimed in claim 1, wherein the mooring system comprises a plurality of mooring lines connected, directly or indirectly, to the floating wind turbine structure.

13. A floating wind turbine installation as claimed in claim 12, wherein at least one mooring line is connected to the floating wind turbine structure by a bridle.

14. A floating wind turbine installation as claimed in claim 6, wherein the mooring lines are catenary mooring lines.

15. A floating wind turbine installation as claimed in claim 1, wherein the mooring system is an asymmetric mooring system.

16. A floating wind turbine installation as claimed in claim 15, wherein the mooring system comprises two mooring lines that are connected to the floating wind turbine structure by respective bridles having a first length, and a third mooring line that is connected to the floating wind turbine structure by a bridle having a second length that is shorter than the first length.

17. A floating wind turbine installation as claimed in claim 1, wherein the wind turbine comprises a tower and a rotor mounted at an upper end of the tower, wherein the rotor comprises a rotor hub and a plurality of blades mounted to the hub.

18. A method of mooring an asymmetric floating wind turbine structure in a body of water, wherein the floating wind turbine structure comprises a wind turbine mounted on a semi-submersible floating platform, the method comprising tethering the floating wind turbine structure to the floor of the body of water using a mooring system such that the floating wind turbine structure is held in position by the mooring system with the wind turbine positioned on an upwind side of the centre of mass of the floating wind turbine structure in the direction of the prevailing wind at the location of the floating wind turbine structure.

19. A method as claimed in claim 18, wherein the floating wind turbine structure and the mooring system form a floating wind turbine installation as claimed in claim 1.

Description

[0061] Certain preferred embodiments of the present invention will now be described in greater detail by way of example only and with reference to the accompanying drawings, in which:

[0062] FIG. 1 is a schematic plan view of a floating wind turbine installation in a known orientation;

[0063] FIG. 2 is a schematic plan view of an alternative floating wind turbine installation;

[0064] FIG. 3 is schematic plan view of another floating wind turbine installation;

[0065] FIG. 4 is a schematic plan view of another floating wind turbine installation;

[0066] FIG. 5 is a schematic plan view of yet another floating wind turbine installation;

[0067] FIG. 6 is a schematic plan view of the wind turbine installation of FIG. 3 arranged at an angle with respect to the direction of the prevailing wind;

[0068] FIG. 7 is a graph showing the average height above the water of the nacelle of a wind turbine installation as a function of wind speed, for wind approaching from different directions;

[0069] FIG. 8 is a graph showing the average wind speed at the actual height of the nacelle of a wind turbine installation as a function of the average wind speed at the height of the nacelle in non-wind conditions, for wind approaching from different directions;

[0070] FIG. 9 is a graph indicating the available wind energy at the actual height of the nacelle of a wind turbine installation as a function of the average wind speed at the height of the nacelle in non-wind conditions, for wind approaching from different directions;

[0071] FIGS. 10A-10C are graphs showing the electrical power production for a wind turbine installation as a function of average wind speed, for wind approaching from different directions;

[0072] FIGS. 11A-11C are graphs showing the roll motion response for a wind turbine installation as a function of average wind speed, for wind approaching from different directions;

[0073] FIGS. 12A-12C are graphs showing the yaw motion response for a wind turbine installation as a function of average wind speed, for wind approaching from different directions;

[0074] FIGS. 13A-C are graphs showing the pitch motion response for a wind turbine installation as a function of average wind speed, for wind approaching from different directions; and

[0075] FIGS. 14A-C are graphs showing the tower bottom bending moment for a wind turbine installation as a function of average wind speed, for wind approaching from different directions.

[0076] The known wind turbine installation of FIG. 1 has been discussed above. FIG. 2 is a schematic plan of a proposed floating wind turbine installation 10 comprising an asymmetric floating wind turbine structure 11 that is held in position by three mooring lines 12a, 12b, 12c anchored to the sea floor. The floating wind turbine structure 11 includes a semi-submersible floating platform formed of three columns 13, 14 joined in a triangular ring configuration by three connecting members 15. Two of the columns 13 are empty (i.e. they do not support a wind turbine) whilst the third column 14 supports a wind turbine 16. The wind turbine 16 is a conventional horizontal-axis wind turbine and includes tower which supports a nacelle. The nacelle houses a generator and supports a rotor comprising a plurality, e.g. three, rotor blades. The tower is supported in a substantially upright orientation by the semi-submersible platform.

[0077] Each mooring line 12a, 12b, 12c is connected directly to the floating wind turbine structure 11 (specifically to a column 13, 14 of the floating wind turbine structure 11).

[0078] The columns 13, 14 comprise ballast tanks for containing air and ballast, such as water. Ballast may be added to and/or removed from the columns 13, 14 in order to achieve a near 0 angle of heel in non-wind conditions. To achieve this, column 14 supporting the wind turbine 16 may be predominantly filled with air. The floating wind turbine structure 11 may include one or more pumps to add and/or remove liquid ballast (e.g. sea water) to and from the ballast tanks.

[0079] As in FIG. 1, the prevailing wind at the location of the floating wind turbine installation 10 propagates along the positive x-axis, as shown in FIG. 2. The floating wind turbine structure 11 is oriented such that the column 14 on which the wind turbine 16 is mounted is on the upwind side of the floating wind turbine structure 11 in the direction of the prevailing wind. Specifically, the column 14 supporting the wind turbine 16 is positioned such that it is on an upwind side of the centre of mass C.sub.m of the floating wind turbine structure 11 when the wind approaches the wind turbine installation 10 in the direction of the prevailing wind (i.e. along the positive x-axis shown in FIG. 2). The mooring lines 12a, 12b, 12c provide resistance against yawing of the floating wind turbine structure 11 so as to maintain the wind turbine 16 on the upwind side of the wind turbine structure 11 in the direction of the prevailing wind. Whilst the floating wind turbine structure 11 may experience (relatively small) yawing motions (e.g. due to the effect of wind forces, current and/or waves on the wind turbine structure 11), the mooring lines 12a, 12b, 12c act to maintain the wind turbine structure 11 (substantially) in the desired orientation.

[0080] As discussed above, a wind thrust force acting on the floating wind turbine structure 11 will cause the floating wind turbine structure 11 to pitch about a transverse axis (i.e. side to side axis) passing through its centre of mass C.sub.m. With the orientation shown in FIG. 2, the pitching motion of the floating wind turbine structure 11 due to a wind thrust force acting in the direction of the prevailing wind will cause the columns 13 situated on the downwind side of the centre of mass C.sub.m to sink lower into the water and will cause the column 14 (supporting the wind turbine 16) on the upwind side of the centre of mass C.sub.m to float higher in the water. As a result, the average height of the wind turbine 16 (and the rotor of the wind turbine 16) above the water will be increased, causing the rotor of the wind turbine 16 to interact with wind at a higher altitude. This increase in average height can lead to an increase in the power output by the wind turbine 16. This is because the speed of the wind (and its kinetic energy) is typically greater at higher altitudes, meaning more energy can be extracted from the wind by the wind turbine 16 and converted into electrical power.

[0081] Whilst the floating wind turbine installation 10 shown in FIG. 2 provides benefits in terms of increased power production, it may suffer from undesirable motion characteristics and increased loading on the mooring lines 12a-c and other components of the floating wind turbine installation 10.

[0082] When the wind approaches the floating wind turbine structure 11 along the prevailing wind direction (i.e. the positive x-axis of FIG. 2) then no yawing moment will be generated by the wind forces acting on the structure 11. Thus, the arrangement shown in FIG. 2 is oriented at an equilibrium position when the wind approaches along the prevailing wind direction. However, this is an unstable equilibrium position. Due to the weathervaning effect discussed above, when the wind approaches the floating wind turbine structure 11 away from the direction of the prevailing wind, a yawing moment will be generated that forces the structure 11 to yaw towards an orientation in which the column 14 supporting the wind turbine 16 is positioned on the downwind side of the installation. Even small changes in the wind direction away from the prevailing wind direction will result in a relatively large yawing moment (e.g. compared to the typical yawing moments generated in the floating wind turbine installation shown in FIG. 1) because the moment arm between the wind thrust force and the centre of mass C.sub.m of the floating structure 11 is large.

[0083] Moreover, since the wind will most commonly approach along or close to the prevailing wind direction, the wind will typically approach the floating wind turbine structure 11 above single mooring line 12c. In this case, the restoring yaw stiffness of the mooring system will be at its lowest and the mooring system will be less able to counter the weathervaning yaw motion.

[0084] Also, when the wind approaches the wind turbine structure 11 over a single mooring line (e.g. mooring line 12c) a substantial fraction of the loading will be applied to that mooring line, whereas when the wind approaches between two adjacent mooring lines the loading will be distributed between the two mooring lines. Hence, the loading on each mooring line will be less when the wind approaches between two adjacent mooring lines. As a result, when the wind approaches over a single mooring line, that mooring line will experience greater loading and increased fatigue. This may shorten the lifetime of the mooring line.

[0085] An alternative wind turbine installation 20 designed at least in part to reduce these issues is shown in FIG. 3.

[0086] The floating wind turbine installation 20 of FIG. 3 includes a floating wind turbine structure 11 that is predominantly the same as the one described above in respect of FIG. 2. It includes a semi-submersible floating platform formed of three columns 13, 14 joined in a triangular ring configuration by three connecting members 15, with a wind turbine 16 being supported by one of the columns 14. Similar to the floating wind turbine installation 10 of FIG. 2, the floating wind turbine installation 20 is oriented such that the column 14 supporting the wind turbine 16 is positioned at an upwind side of the centre of mass C.sub.m of the floating wind turbine structure 11 when the wind approaches the wind turbine installation 20 in the direction of the prevailing wind (i.e. along the positive x-axis shown in FIG. 3).

[0087] In FIG. 3, the floating wind turbine structure 11 is held in position with a mooring system comprising three mooring lines 21a, 21b, 21c and three bridles 22. Each of the mooring lines 21a, 21b, 21c is connected to the floating wind turbine structure 11 (specifically the columns 13, 14 of the floating wind turbine structure 11) via a respective bridle 22. Hence, the mooring system comprises three mooring lines 21a-c that are each connected to the floating wind turbine structure via a respective bridle 22.

[0088] As with the arrangement discussed above, the mooring lines 21a, 21b, 21c provide resistance against yawing of the floating wind turbine structure 11 so as to maintain the wind turbine 16 on the upwind side of the wind turbine structure 11 in the direction of the prevailing wind.

[0089] Each bridle 22 comprises two bridle lines 22a. In each bridle 22, one bridle line 22a is connected to one of the columns 13, 14, and the other bridle 22a is connected to another one of the columns 13, 14 (i.e. a different column 13, 14). Hence, each mooring line 21a-c is connected to two different columns 13, 14 via a bridle 22.

[0090] Mooring line 21a is connected to the column 14 supporting the wind turbine 16 via one bridle line 22a and is connected to an empty column 13 via another bridle line 22a. Mooring line 21b is connected to the column 14 supporting the wind turbine 16 via one bridle line 22a and is connected to a (different) empty column 13 via another bridle line 22a. Mooring line 21c is connected to an empty column 13 via one bridle line 22a and is connected to a (different) empty column 13 via another bridle line 22a. Hence, each column 13, 14 is connected to two mooring lines 21a-c via respective bridle lines 22a.

[0091] The presence of the bridles provides the wind turbine installation 20 with more favourable motion characteristics compared to the mooring system shown in FIG. 2.

[0092] By connecting the mooring lines 21a-c to the floating wind turbine structure 11 via the bridles 22, for a given pre-tension the yaw restoring stiffness of the mooring system is significantly increased compared to the mooring systems shown in FIGS. 1 and 2. Moreover, due to the orientation of the mooring lines 21a-c the restoring yaw stiffness of the mooring system is greatest when the wind approaches the floating wind turbine structure 11 along the prevailing wind direction (i.e. along the positive x-axis). As discussed above, the restoring yaw stiffness of the mooring system is greatest when wind approaches between two adjacent mooring lines, which will occur for the installation 20 illustrated in FIG. 3 when the wind approaches along the prevailing wind direction.

[0093] Since the floating wind turbine installation 20 shown in FIG. 3 is oriented with the column 14 that supports the wind turbine 16 at an upwind side of the floating platform, the floating wind turbine installation 20 benefits from increased power production much like the wind turbine installation 10 shown in FIG. 2. The thrust force exerted on the floating wind turbine structure 11 by wind blowing in the prevailing wind direction (i.e. along the positive x-axis as shown in FIG. 3) will cause the floating wind turbine structure 11 to pitch about a transverse axis passing through its centre of mass C.sub.m. This will cause the columns 13 located on the downwind side of the installation 20 to sink lower into the water, and cause the column 14 supporting the wind turbine 16 to rise higher in the water. As a result, the rotor of the wind turbine 16 will be caused to interact with wind at a higher altitude which typically has higher mean speeds, leading to an increase in the power output by the wind turbine 16.

[0094] Another wind turbine installation 30 is shown in FIG. 4. The floating wind turbine installation 30 of FIG. 4 includes a floating wind turbine structure 11 that is predominantly the same as the one described above in respect of FIGS. 2 and 3, so is not described in detail here to avoid repetition. The floating wind turbine installation 30 is oriented such that the column 14 supporting the wind turbine 16 is positioned at an upwind side of the centre of mass C.sub.m of the floating wind turbine structure 11 in the direction of the prevailing wind (i.e. along the positive x-axis shown in FIG. 4).

[0095] In the installation 30 of FIG. 4, the floating wind turbine structure 11 is held in position with a mooring system comprising four mooring lines 31a, 31b, 31c, 31d. The mooring lines 31a, 31b, 31c, 31d provide resistance against yawing of the floating wind turbine structure 11 so as to maintain the wind turbine 16 on the upwind side of the wind turbine structure 11 in the direction of the prevailing wind.

[0096] Each of the mooring lines 31a, 31b, 31c, 31d is connected directly to the floating wind turbine structure 11 (specifically to a column 13, 14 of the floating wind turbine structure 11), i.e. without bridles.

[0097] Two mooring lines 31a, 31b are directly connected to the column 14 supporting the wind turbine 16. Hence, column 14 is directly connected to two mooring lines 31a, 31b.

[0098] The two other mooring lines 31c, 31d are each respectively connected directly to an empty column 13 so that each empty column 13 is directly connected to one of the mooring lines 31c, 31d. That is, one mooring line 31c is connected to an empty column 13, and another mooring line 31d is connected to a different empty column 13.

[0099] The arrangement illustrated in FIG. 4 may provide an increased yaw restoring stiffness from the mooring system compared to the arrangement shown in FIG. 1.

[0100] Yet another wind turbine installation 40 is shown in FIG. 5. The floating wind turbine installation 40 of FIG. 5 includes a floating wind turbine structure 11 that is predominantly the same as the one described above in respect of FIGS. 2-4, so is not described in detail here to avoid repetition. The floating wind turbine installation 40 is oriented such that the column 14 supporting the wind turbine 16 is positioned at an upwind side of the centre of mass C.sub.m of the floating wind turbine structure 11 in the direction of the prevailing wind (i.e. along the positive x-axis shown in FIG. 5).

[0101] In the installation 40 of FIG. 5, the floating wind turbine structure 11 is held in position with a mooring system comprising three mooring lines 41a, 41b, 41c and one bridle 42 which is connected to the mooring line 41a. The mooring lines 41a, 41b, 41c and the bridle 42 provide resistance against yawing of the floating wind turbine structure 11 so as to maintain the wind turbine 16 on the upwind side of the wind turbine structure 11 in the direction of the prevailing wind.

[0102] The mooring line 41a is connected to the wind turbine structure 11 via the bridle 42. The bridle comprises two bridle lines 42a. Each of the bridle lines 42a is connected to one of the two empty columns 13 such that one bridle line 42a is connected to each empty column 13.

[0103] The two further mooring lines 41b, 41c are connected directly to the column 14 supporting the wind turbine 16, such that column 14 is connected to two mooring lines 41b, 41c.

[0104] The mooring system of FIG. 5 provides improved yaw stiffness compared to the arrangement shown in FIG. 1.

[0105] The wind turbine installations 10, 20 shown in FIGS. 2-5 are oriented so that the column 14 supporting the wind turbine 16 is directly upwind of the centre of mass C.sub.m of the floating wind turbine structure 11 in the direction of the prevailing wind (i.e. the positive x-axis shown in the Figures). That is, there is an angle of 0 between the prevailing wind direction and a straight line that passes through the centre of the wind turbine 16 and the centre of mass C.sub.m. However, it has been found that an appreciable, although lesser, increase in the power output of the wind turbine 16 may still be achieved when the wind turbine installation 20 is oriented at an angle of up to +/60 from the direction of the prevailing wind, i.e. when there is an angle of up to +/60 between the prevailing wind direction and a straight line passing through the centre of the wind turbine 16 and the centre of mass C.sub.m. FIG. 6 shows the wind turbine installation 20 of FIG. 3 oriented at an angle from the prevailing wind direction, which propagates along the positive x-axis shown in the Figure.

[0106] Although, with the orientation shown in FIG. 6, the increase in power output by the wind turbine may be reduced compared to when the column 14 supporting the wind turbine 16 is located directly upwind of the centre of mass C.sub.m of the floating wind turbine structure 11 (as shown e.g. in FIG. 3), wind approaching the floating wind turbine installation 10 at an angle of up to +/60 may still provide an adequate thrust force to cause the column 14 supporting the wind turbine 16 to rise higher in the water as a result of a pitching motion. This can lead to an increase in the power output by the wind turbine, as discussed above. Typically, a greater increase in the height of the wind turbine 16, and hence a greater increase in power output, will be achieved at smaller angles , for instance 45 or 30.

[0107] The bridle lines 22a, 42a and the mooring lines 12a-c, 21a-c, 31a-d and 41a-c described above may be made of various materials including mooring chain, wire rope, polyester rope, etc. The bridle lines 22a, 42a and the mooring lines 12a-c, 21a-c, 31a-d and 41a-c may be made of the same materials or different materials. In some floating wind turbine installations 10, 20, 30, 40 the mooring lines 12a-c, 21a-c, 31a-d and 41a-c may be formed of a plurality of segments, which may comprise different materials.

[0108] The bridle lines 22a, 42a and the mooring lines 12a-c, 21a-c, 31a-d and 41a-c may have the same or different thicknesses.

[0109] The bridle lines 22a, 42a may be connected to the mooring lines 21a-c, 41a with a joint such as a vacuum-explosion welded transition joint, e.g. Triplate.

[0110] The bridle lines 22a, 42a and/or the mooring lines 12a-c, 21a-c, 31a-d and 41a-c may be connected to the floating wind turbine structure 11 (e.g. the columns 13, 14 of the floating wind turbine structure 11) with a connector such as a fairlead.

[0111] Simulations have been carried out to compare the response of the floating wind turbine installation 20 of FIG. 3 when wind approaches at an angle of 0 to the response of the floating wind turbine installation 20 when the wind approaches at an angle of 180. It will be appreciated that this provides a comparison between the response of the floating wind turbine installation 20 when the column 14 supporting the wind turbine 16 is positioned at the upwind side of the floating wind turbine structure 11 (e.g. as shown in FIG. 3) and the response of the floating wind turbine installation 20 when the column 14 supporting the wind turbine 16 is positioned at the downwind side of the floating wind turbine structure 11 (similar to the known orientation shown in FIG. 1).

[0112] The simulation data was obtained by modelling the motion characteristics of a floating wind turbine installation 20 having three columns 13, 14 that extend 18 m above the waterline, and a 23 MW wind turbine 16 having a 138 m tall tower and a rotor situated at the top of the tower and comprising three blades of 136 m in length. In this example, the connecting members 15 connecting the columns 13, 14 are each 77.45 m in length. The modelled system has bridle lines 22a that are each 100 m long and made of steel wire, and mooring lines 21a-c that are each 855 m in length and made of polyester rope. An anchor chain of 30 m length connects the end of each mooring line to the sea floor.

[0113] In the simulations, waves were assumed to approach the floating wind turbine installation 20 from the same direction as the wind.

[0114] A base case simulation study was performed using a wind shear profile exponent of 0.14, as recommended by IEC standard 61400-1:2019. The study found that the annual power production of the wind turbine 16 was increased by 1.5% for the case where the wind approaches the wind turbine installation 20 at an angle of 0 compared to when the wind approaches the wind turbine installation 20 at an angle of 180.

[0115] Another simulation study was performed using a wind shear profile exponent of 0.10. This study also showed an increase in the annual power production of the wind turbine 16 for the case where the wind approaches the wind turbine installation 20 at an angle of 0 compared to when the wind approaches the wind turbine installation 20 at an angle of 180. In this case, the increase in annular power production was 1.1%.

[0116] Selected results from the simulation study will now be described with reference to FIGS. 7-14.

[0117] FIG. 7 shows the simulated average height of the nacelle of the wind turbine 16 above the water compared to the wind speed at the height of the nacelle in non-wind conditions. The graph shows how the average height of the nacelle changes with increasing wind speed for the case where =0 and the case where =180. It can be seen that when the wind approaches the floating wind turbine installation 20 at an angle of 0 the average height of the nacelle tends to increase as the wind speed increases. On the other hand, when the wind approaches the floating wind turbine at an angle of of 180 the height of the nacelle tends to decrease with increased wind speed. FIG. 7 shows that a difference in height of more than 16 m is seen at a wind speed of around 11 ms.sup.1 (+/1 ms.sup.1), which is a typical rated wind speed for a wind turbine located in the North Sea.

[0118] Wind tends to propagate at different speeds at different altitudes, and is typically faster at higher altitudes. It will therefore be appreciated that the nacelle, and the rotor of the wind turbine 16, will interact with wind of differing speeds depending on its height above the water. FIG. 8 shows the simulated average wind speed at the (actual) height of the nacelle (i.e. the height at which the nacelle sits at the average pitch angle of the floating wind turbine structure 11 due to wind thrust effects) compared to the average wind speed at the (nominal) height of the nacelle in non-wind conditions. As can be seen, the average wind speed at the actual nacelle height when the wind approaches the floating wind turbine installation 20 at an angle of 0 is typically faster than the average wind speed at the actual nacelle height when the wind approaches the floating wind turbine installation 20 at an angle of 180.

[0119] The amount of energy that can be extracted from the wind by a wind turbine is proportional to the cube of the wind speed. FIG. 9 shows how the cube of the simulated average wind speed (which is proportional to the available wind energy) at the actual nacelle height changes with the average wind speed at the nominal nacelle height. As can be seen, the cube of the average wind speed at the actual nacelle height, and thus the available wind energy, is higher when the wind approaches the floating wind turbine installation 20 at an angle of 0 compared to when the wind approaches the floating wind turbine installation 20 at an angle of 180. For example, at an average wind speed of 10.5 ms.sup.1 at the nominal nacelle height the available wind energy was found to be approximately 6% higher when the wind approaches the floating wind turbine installation 20 at an angle of 0 compared to when the wind approaches the floating wind turbine installation 20 at an angle of 180.

[0120] The simulated effect that the direction of the wind has on the power production of a wind turbine having a rated wind speed of 11 ms.sup.1 is shown in FIGS. 10A-C. The simulated maximum power production is shown in FIG. 10A, the simulated mean power production is shown in FIG. 10B, and the simulated minimum power production is shown in FIG. 10C. From FIG. 10B it can be seen that the mean power production is consistently greater at around rated wind speed when the wind approaches the floating wind turbine installation 20 at an angle of 0 compared to an angle of 180.

[0121] The simulated roll motion of the floating wind turbine structure 11 having a mooring system as shown in FIG. 3 is shown in FIGS. 11A-C. FIG. 11A shows the simulated maximum roll motion, FIG. 11B shows the simulated mean roll motion, and FIG. 11C shows the simulated minimum roll motion.

[0122] The simulated yaw motion of the same floating wind turbine structure 11 is shown in FIGS. 12A-C. FIG. 12A shows the simulated maximum yaw motion, FIG. 12B shows the simulated mean yaw motion, and FIG. 12C shows the simulated minimum yaw motion.

[0123] The simulated pitch motion of the same floating wind turbine structure 11 is shown in FIGS. 13A-C. FIG. 13A shows the simulated maximum pitch motion, FIG. 13B shows the simulated mean pitch motion, and FIG. 13C shows the simulated minimum pitch motion.

[0124] The tower bottom bending moments for the simulated floating wind turbine installation 20 are shown in FIGS. 14A-12C. FIG. 14A shows the maximum bottom bending moments, FIG. 14B shows the mean bottom bending moments, and FIG. 14C shows the minimum bottom bending moments.

[0125] FIGS. 11-14 show that the roll, yaw and pitch motions of the floating wind turbine structure 11 and the loads on the floating wind turbine structure 11 are of the same order of magnitude when the wind approaches the floating wind turbine installation 20 from an angle of 0 and an angle of 180.