GB2588124A

GB2588124A - Floating Wind Turbine

Info

Publication number: GB2588124A
Application number: GB1914509.3A
Authority: GB
Inventors: Knauer Andreas
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2019-10-08
Filing date: 2019-10-08
Publication date: 2021-04-21
Also published as: GB201914509D0

Abstract

Floating wind turbine 101 arranged so the cross-sectional area at the waterline 105 can be changed while the turbine is floating (in use). The turbine may; be arranged so the position if controllable while floating, comprise an adjustable ballast 120 capable of controlling the vertical position, comprise a support structure 102 with a varying cross-sectional area along its length, comprise a first and second portion where the first portion has a smaller cross-sectional area than the second and the second portion nearer the bottom of the floating wind turbine, the turbine is moveable between upper and lower positions with the upper position having the second portion at the waterline and the lower position having the first portion at the waterline, be moveable into a parked position which is vertically lower than the lower position, the parked position may be such that the lowermost point of the blades 108 do not contact the water when furthest from the bottom of the turbine (where the rotor may be locked) but do closest to the bottom. Also claimed is a method of doing such, and a turbine and method of locking the rotor as previously described.

Description

FLOATING WIND TURBINE

The invention relates to a floating wind turbine and a method of controlling a floating wind turbine.

A wind turbine structure is usually formed of a support structure comprising an elongate tower, with a nacelle and a rotor attached to the upper end of the tower. The generator and its associated electronics are usually located in the nacelle.

Fixed-base wind turbines that are fixed either to the land or the sea bed are well-established. However, recently there has been a desire to develop floating wind turbines and various structures have been proposed. One example is a wind turbine structure where a conventional wind turbine structure is mounted on a floating foundation such as a buoyant platform or raft-like structure. Another proposal is a "spar buoy" type structure. Such a structure is formed of an elongate buoyant support structure with a rotor mounted on the top. The support structure could be a unitary structure including the tower or the foundation part could be formed as an elongate sub-structure with a standard tower mounted thereon. Offshore floating wind turbines are exposed to wind and wave forces that exert loads on the floating wind turbine. Whilst at least some of these loads are essential for the wind turbine to be able to generate power, too much load on the wind turbine can have negative consequences.

For example under certain conditions these loads can damage the wind turbine and/or reduce its operating lifetime before it has to be serviced, repaired or taken out of service.

When a floating wind turbine structure is acted on by forces, such as those caused by changes in wind speed or waves, the whole structure moves about in the water. These motions may have a large amplitude but relatively low frequency, i.e. they are large slow motions -the motions are low frequency in the sense that they are much lower than the rotational frequency of the turbine itself.

The motions experienced by a wind turbine structure are described as "heave" which is linear vertical (up/down) motion, "sway" which is linear lateral (side-to-side) motion, "surge" which is linear longitudinal (front/back) motion, "roll" which is rotation of the body about its horizontal (front/back) axis, "pitch" which is rotation of the body about its transverse (side-to-side) axis and "yaW' which is rotation of the body about its vertical axis. -2 -

These motions can have a negative impact on the operation of the floating wind turbine and/or on components of or associated with the floating wind turbine, such as mooring.

Thus there is a desire to be able to control and/or reduce the loads acting on a floating wind turbine.

In a first aspect the present invention may provide a floating wind turbine for floating in a body of water, wherein the floating wind turbine is arranged such that the cross sectional area of the floating wind turbine at the surface of the body of water can be changed when the floating wind turbine is floating in the body of water.

In a second aspect the present invention may provide a method of controlling a floating wind turbine, wherein the method comprises: providing a floating wind turbine in a body of water; and changing the cross sectional area of the floating wind turbine at the surface of the body of water.

The floating wind turbine of the first aspect may be arranged to perform the method of the second aspect and/or one or more or all of the method steps described below.

It has been realised that by providing a floating wind turbine in which the cross sectional area of the floating wind turbine at the water line can be changed, the wave forces acting on the floating wind turbine may be controlled and/or reduced. For example, by reducing the cross sectional area of the floating wind turbine at the surface of the body of water may reduce the wave loads experienced by the floating wind turbine. This is because the wave forces acting on a floating wind turbine depend at least in part on the cross sectional area of the floating wind turbine at the surface of the body of water, e.g. at the waterline.

Wave loading on a floating wind turbine is greatest at the water line. Wave-induced velocities decrease with water depth exponentially. Thus by re-positioning of parts with larger cross sectional areas to deeper waters, the wave loads acting on the floating wind turbine may be reduced.

Thus the floating wind turbine may be arranged such that parts with larger cross sectional areas can be repositioned to deeper waters.

The method may comprise re-positioning of parts with larger cross sectional areas to deeper waters.

In a situation with rough weather, the cross sectional area of the floating wind turbine at the waterline may be reduced to reduce wave loads. The method -3 -may comprise reducing the cross sectional area of the floating wind turbine at the waterline to reduce wave loads.

For a given sea condition, the wave loads on the wind turbine structure may be less when the cross sectional area of the wind turbine structure at the water surface is reduced.

The surface of the body of water may be the mean waterline, i.e. the average water height over a time interval such as one hour. Thus the cross sectional area at the surface of the body of water may be the cross sectional area at the mean waterline.

The floating wind turbine may comprise a controller. The controller may be arranged to change the cross sectional area of the floating wind turbine at the surface of the body of water.

The floating wind turbine may comprise a support structure for floating in the body of water. The support structure may penetrate the surface of the body of water. As a result, it may have a portion that is below the water line, a point that is at the waterline (e.g. at the mean waterline) and a portion that is above the water line. The portion above the water line may have a smaller cross sectional area than the portion below the water line.

The floating wind turbine may be arranged so that the vertical position of the floating wind turbine, e.g. the vertical position of support structure, relative to the body of water is adjustable and/or controllable when the floating wind turbine is floating in the body of water. In other words, the point of the floating wind turbine that is at the water line may be adjusted and/or controlled when the floating wind turbine is floating in the body of water, e.g. during operation. The control may be performed by the controller. The method may comprise adjusting and/or controlling the vertical position of the floating wind turbine relative to the body of water.

The floating wind turbine may be arranged so that the vertical position of the support structure relative to the body of water is adjustable and/or controllable such that the cross sectional area of the floating wind turbine (i.e. the support structure) at and/or near the sea surface is adjustable and/or controllable.

The floating wind turbine may be arranged so that the vertical position of the support structure relative to the body of water is adjustable and/or controllable such that larger diameter portions of the support structure can be positioned to deeper waters so as to decrease wave induced loads. -4 -

The adjusting of the vertical position may be performed to control wave loading on the floating wind turbine. For example, in rough conditions, the floating wind turbine may be lowered in the water to reduce the cross sectional area of the floating wind turbine at the water surface and/or move larger diameter portions of the support structure further from the water surface.

The floating wind turbine may be arranged so that the vertical position of the support structure relative to the body of water is adjustable and/or controllable such that the wave forces acting on the floating wind turbine may be controllable.

Thus, the floating wind turbine and/or controller may be arranged so that the vertical position of the support structure relative to the surface of the body of water is adjustable and/or controllable.

The support structure may be shaped so that the cross sectional area of the support structure at the surface of the body of water can be changed by controlling the height of the support structure relative to the body of water.

The support structure may have a cross sectional area that varies along its length, i.e. that is not constant. This may for example be achieved by the support structure comprising one or more tapered portion(s) e.g. one or more frustoconical portion(s). The entire support structure may be tapered or only a portion of the support structure may be tapered.

The support structure may have a portion with a cross sectional area that is smaller than the cross sectional area of another portion. For example, the support structure may comprise a first portion with a cross sectional area that is smaller than the cross sectional area of a second portion.

The support structure may have a plurality of portions of different cross sections. Each of these portions may have a constant cross sectional area along its length, e.g. at least 1m. Between the portions of constant cross section the support structure may comprise transition sections with non-constant cross section. For example the transitions sections may be frustoconical sections between the portions of constant cross section.

The portion with a larger cross sectional area may be closer to the bottom of the floating wind turbine and/or support structure than the portion with a smaller cross sectional area.

The support structure may decrease in cross sectional area from the bottom of the floating wind turbine to the top. This may be achieved by a constant change -5 -in diameter, one or more step changes in diameter and/or one or more transition sections between sections of approximately constant cross section.

In use, i.e. when the floating wind turbine is floating in the body of water, the portion with the larger cross sectional area may be at a lower vertical height than the portion with the smaller cross sectional area.

The support structure may comprise a wider diameter portion that is connected to a smaller diameter portion. For example the wider diameter portion may be connected to the smaller diameter portion via a tapered connecting portion.

The height of the floating wind turbine relative to the waterline may be reduced to reduce the cross sectional area of the floating wind turbine at the waterline. This may reduce the wave forces acting on the floating wind turbine.

The method may comprise reducing the height of the floating wind turbine in the body of water. This may be when it is desired to reduce the wave loads acting on the floating wind turbine. The controller may be arranged to reduce the height of the floating wind turbine in the body of water. This may reduce the cross sectional area of the floating wind turbine at the surface of the body of water and thus reduce the wave forces acting on the floating wind turbine.

The method may comprise reducing the height of the floating wind turbine by up to 25m, for example the height may be reduced by between 1 and 20 meters or about 10 meters. By reducing the height of the floating wind turbine by about 10m it may be possible to reduce the wave loads on the floating wind turbine by up to 10%.

The vertical position of the floating wind turbine may be controlled based on information concerning the environmental conditions, e.g. wind speed and/or wave height, experienced and/or expected to be experienced by a floating wind turbine.

That the floating wind turbine, e.g. the controller, may be arranged to receive information concerning the environmental conditions and to control the floating wind turbine on the basis of this information. The information may be collected by sensors on the wind turbine itself and/or provided to the floating wind turbine from other sources.

The method may comprise controlling the vertical height of the floating wind turbine based on environmental conditions.

Beyond rated wind speed the wind turbine position might be lowered to reduce the wave forces acting on the floating wind turbine. Thus the method may comprise receiving information concerning environmental conditions, such as the -6 -wind speed (which may involve detecting the wind speed), and reducing the height of the wind turbine if the environmental conditions are above a threshold, e.g. if the wind speed is above rated wind speed. The controller may be arranged to adjust the height of the floating wind turbine depending on the wind speed. For example, the controller may be arranged to reduce the height of the floating wind turbine if the wind speed is above rated wind speed.

The wave forces may be linked to the wind speed. Thus controlling the wind turbine on the basis of the wind speed to control the wave forces is feasible. In the wind regime above rated wind speed reducing the height of the floating wind turbine may have no or limited impact on the power production of the wind turbine.

The method may comprise increasing the height of the floating wind turbine relative to the waterline. This may be performed when the conditions are such that the wave loads experienced when the floating wind turbine is in a raised position

are acceptable.

In use the floating wind turbine may be movable between an upper position and a lower position.

The upper position may be the position in which the relative height between the floating wind turbine and the water are greatest whilst still in a position in which the floating wind turbine can operate. For example, the upper position may be the position in which the length of support structure beneath the water line is smallest and/or the height at which the top of the floating wind turbine is furthest from the water line. The upper position may be a position in which the floating wind turbine is still stable enough to allow the floating wind turbine to operate.

The lower position may be the position in which the relative height between the floating wind turbine and the water are smallest whilst still in a position in which the floating wind turbine can operate. For example, the lower position may be the position in which the length of support structure beneath the water line is largest and/or the height at which the top of the floating wind turbine is nearest to the water line whilst the floating wind turbine is still operational.

The lower position may be a position in which the floating wind turbine is still high enough to allow the floating wind turbine to operate, e.g. the rotor blades can still rotate without contacting the water. -7 -

The cross sectional area of the floating wind turbine at the water surface may be larger when the floating wind turbine is in the upper position compared to the lower position.

For example, the diameter of the support structure at the water line when the floating wind turbine is in the upper position may be about 10 to 20m, e.g. around 15m, and the diameter of the support structure at the water line when the floating wind turbine is in the lower position may be about 5 to 10m, e.g. around 8m.

The floating wind turbine may be movable into any height position between the upper position and the lower position. Alternatively the floating wind turbine may be movable into certain positions between the upper position and lower position.

The cross sectional area of the floating wind turbine may decrease from the portion of the floating wind turbine at the water line when the floating wind turbine is in the upper position to the portion of the floating wind turbine at the water line when the floating wind turbine is in the lower position. This may be a continuous and/or gradual decrease or it may be a non-continuous decrease made up of decreasing diameter transition sections.

If the floating wind turbine support structure comprises constant diameter portions joined by changing diameter transition portions the constant diameter portions may be at the water line when the floating wind turbine is in the upper and/or lower position. Thus, the floating wind turbine may not be held at a position in which a changing diameter transition portion is at the water line.

The wind turbine may comprise a nacelle. The nacelle may be supported on the support structure. The support structure may comprise a tower. The nacelle may be mounted on the tower.

The nacelle may contain a generator and the associated electronics.

The floating wind turbine may comprise a rotor comprising rotor blades. The rotor may comprise and/or consist of three rotor blades. The nacelle may support the rotor comprising rotor blades.

The support structure may comprise a floating spar-buoy structure. The tower may be supported on the floating spar-buoy structure. The support structure, e.g. the floating spar-buoy structure, may be moored by a mooring system. The mooring system may comprise three anchor lines, e.g. anchor chains. -8 -

The floating wind turbine may comprise ballast. The ballast may be provided at the bottom of the support structure. The ballast may comprise adjustable ballast, e.g. water ballast. The ballast may comprise fixed ballast.

The adjustable ballast may be adjustable such that the vertical position of the floating wind turbine and hence support structure relative to the body of water is

adjustable.

The method may comprise adjusting the ballast to adjust the vertical position of the floating wind turbine and hence support structure relative to the body of water.

The controller may be arranged to control the ballast to adjust the vertical position of the floating wind turbine and hence support structure relative to the body of water.

The height of the floating wind turbine may be reduced by adding ballast to the adjustable ballast.

The adjustable ballast may be water ballast. Thus the height of the floating wind turbine may be reduced by adding water to the adjustable ballast. The height of the floating wind turbine may be increased by removing water and/or adding air to the adjustable ballast.

The floating wind turbine may be movable into a parked position. The parked position may be lower than the above described lower position. The parked position may be a position in which the floating wind turbine can no longer operate. When the floating wind turbine is in the parked position the rotor may be locked in a fixed position. In other words when the floating wind turbine is in the parked position the rotor may not rotate.

The fixed position of the rotor may be a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine. The parked position may be a position in which the rotor blades of the floating wind turbine when in the fixed position do not contact the water. However, the parked position may be a position in which if the rotor is rotated (i.e. the rotor blades are rotated about the turbine axis) at certain positions during rotation the rotor blades of the rotor would contact the water. This is because the lowermost point of the rotor changes as the rotor blades rotate. This change in height of the lowermost point of the rotor is greatest with the fewest number of rotor blades.

The parked position may be a position in which the floating wind turbine is at a vertical height such that when the rotor is in a position in which the lowermost -9 -point of the rotor blades is furthest from the bottom of the floating wind turbine the rotor blades do not contact the water but if the rotor is in a position in which the lowermost point of the rotor blades is nearest the bottom of the floating wind turbine the rotor blades do contact the water.

There are certain circumstances in which it is desirable to lock the rotor in a fixed position when the floating wind turbine is floating in the body of water, e.g. at very high wind speeds. It has been realised that when this is performed, it is beneficial for the fixed position to be a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine. This is so that the lowermost point of the rotor blades is furthest from the surface of the body of water.

This allows the rotor blades to be locked in a fixed position that minimises the risk of the rotor blades touching the surface of the water. Whilst this may be particularly beneficial when the height of the floating wind turbine is adjustable (as the lowermost acceptable position of the floating wind turbine may be lower), it has been realised that this may also be beneficial even if the height is not adjustable (and thus even if the cross sectional area of the floating wind turbine is not adjustable).

Thus, in a third aspect (independent or in addition to the first and second aspects), the present invention may provide a floating wind turbine comprising: a rotor comprising a plurality of rotor blades, wherein the rotor may be locked in a fixed position, and wherein the fixed position is a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine.

In a fourth aspect (independent or in addition to the first and second aspects), the present invention may provide a method of controlling a floating wind turbine, the method comprising: providing a floating wind turbine comprising a rotor comprising a plurality of rotor blades, locking the rotor in a fixed position, wherein the fixed position is a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine.

The following description is applicable to all four of the above disclosed and described aspects.

The fixed position may be a position in which the lowermost point of any of the rotor blades is furthest from the bottom of the floating wind turbine.

-10 -The fixed position may be a position in which the lowermost point of any of the rotor blades is at its highest position compared to the positions the lowermost point passes through during a full revolution of the rotor.

The fixed position may be a position in which the lowermost point of any of the rotor blades is closest in a vertical direction to the nacelle.

In the case of a floating wind turbine with three evenly circumferentially spaced rotor blades, the fixed position may be the position in which one rotor blade is directly vertical, i.e. at a 00 position (i.e. directed up and away from the support structure), a second rotor blade is at 1200 position and a third rotor blade is at 240° position.

The floating wind turbine may have its rotor locked in the fixed position and then the floating wind turbine moved into the above described parked position.

When in the parked position the lowermost point of the rotor may be above, but near (e.g. within lm of) the surface of the body of water.

The cross sectional area of the support structure at the waterline when the floating wind turbine is in the parked position may be less than the cross sectional area of the support structure at the waterline when the floating wind turbine is in the lower position.

The rotor may be fixed in the locked position when the wind speed is greater and/or expected to be greater than a threshold, e.g. 50m/s and/or. when it is 60 to 70m/s or more. This may for example be during an extreme event such as a hurricane.

The floating wind turbine may be moved into the parked position when the rotor is fixed in the locked position and/or when the wind speed is greater than 50m/s, e.g. when it is 60 to 70m/s or more. The floating wind turbine may be arranged so that it can only be moved into the parked position when the rotor is fixed in the locked position.

The method may comprise detecting environmental conditions such as the wind speed. If a wind speed above a threshold (e.g. above 60m/s) is detected and/or expected, the method may comprise locking the rotor in the fixed position and/or moving the floating wind turbine into the locked position. The floating wind turbine and/or controller may be arranged to perform this method.

In an aspect the invention may provide a floating wind turbine comprising a support structure for floating in a body of water, and a nacelle supported by the support structure, wherein the support structure has a cross sectional area that varies along its length, and wherein the floating wind turbine is arranged so that the vertical position of the support structure relative to the body of water is adjustable. In an aspect the invention may provide a method, the method comprising: providing a floating wind turbine that comprises a support structure for floating in a body of water, and a nacelle supported by the support structure, wherein the support structure has a cross sectional area that varies along its length, and wherein the method comprises adjusting the vertical position of the support structure relative to the body of water.

In an aspect the invention may provide a floating wind turbine comprising a support structure for floating in a body of water, wherein the floating wind turbine is arranged so that the vertical position of the support structure relative to the surface of the body of water is adjustable, and wherein the support structure is shaped so that the cross sectional area of the support structure at the surface of the body of water can be changed by adjusting the height of the support structure relative to the body of water.

In an aspect the invention may provide a method, the method comprising: providing a floating wind turbine comprising a support structure for floating in a body of water, wherein the support structure is shaped so that the cross sectional area of the support structure at the surface of the body of water can be changed by adjusting the height of the support structure relative to the body of water, and wherein the method comprises adjusting the vertical position of the support structure relative to the surface of the body of water.

The above described features, including the optional features, of the first to fourth aspects are also applicable to these four aspects.

The present invention may mean that for a given sea condition, the wave loads experienced by the floating wind turbine may be reduced. This may reduce the motions of the floating wind turbine. This may increase the operating efficiency of the floating wind turbine.

Reduced wave forces (i.e. for a given sea condition) on the floating wind turbine may allow for an increase the lifetime of the floating wind turbine and/or allow it to be made more economically, e.g. with less material, as less forces may be experienced by the floating wind turbine over its lifetime.

Reducing wave loads on the floating wind turbine may allow a reduction of the forces on the mooring. The invention may allow a reduction of extreme loads, -12 -e.g. by reducing wave forces in rough sea conditions. The reduction in wave loads may also allow for the static tension in the mooring system to be reduced.

The invention may allow for an increase in power output. This is because in calm conditions (e.g. when average wind speeds are less than about 20m/s) the height of the rotor may be increased to allow for a greater experienced wind speed (because the wind speed generally increases away from the surface of a body of water). Whilst this may increase wave loads experienced by the floating wind turbine, the wave loads in calm conditions may be acceptable. In rough conditions (e.g. when average wind speeds are greater than about 20m/s), the height of the rotor may be decreased to decrease wave loads experience by the floating wind turbine. However, this may have little to no effect on the power input as the wind turbine may already be operating at or near maximum power output and thus the reduction in wind speed by reducing the height of the rotor hub may have limited effect.

The present invention may allow optimisation of the levelised cost of electricity (LCOE) from the floating wind turbine. This is because it may allow optimised design and operation of the floating wind turbine that can reduce material, installation and maintenance costs and may increase the lifetime of the floating wind turbine.

The invention may alternatively or additionally allow the wind turbine to be locked in a sae position to minimise damage to the rotor.

Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a floating wind turbine in a first position, Figure 2 shows a floating wind turbine in a second position, Figure 3 shows a floating wind turbine in a third position, and Figure 4 shows an alternative floating wind turbine.

The floating wind turbine 1 shown in figures 1 to 3 comprises a support structure 2 which supports a nacelle 4. The support structure 2 is in the form of a floating spar-buoy structure. The floating wind turbine 1 is floating in a body of water 3 with a waterline 5.

The support structure may be moored by a mooring system, e.g. comprising three anchor lines, however this is not shown in the figures.

-13 -The nacelle 4 contains a generator and the associated electronics, and supports a rotor 6 comprising three blades 8.

The support structure 2 comprises On order from top to bottom of the floating wind turbine) a tower 10, upper cone 12, first portion 14, lower cone 16, second portion 18, adjustable ballast 20 (e.g. water ballast), and fixed ballast 22.

The section portion 18 has a greater diameter than the first portion 14 and the first portion 14 has a greater diameter than the tower 10.

The larger diameter second portion 18 is connected to smaller diameter first portion by lower cone 16. The lower cone 16 is a frustoconical portion that has a diameter at one end that is equal to the diameter of the second portion 18 and a diameter at the other (upper) end that is equal to the diameter of the first portion 14. The first portion 16 is connected to smaller diameter tower 10 by upper cone 12. The upper cone 16 is a frustoconical portion that has a diameter at one end that is equal to the diameter of the first portion 14 and a diameter at the other (upper) end that is equal to the diameter of the tower 10.

The first portion 14 and the second portion 18 each have a substantially constant diameter along their length. The tower 10 may also have a substantially constant diameter along its length. Although the tower 10 is illustrated as having a slight taper this may have a negligible effect on the cross sectional area along its length such that it can be regarded as having a substantially constant diameter.

The support structure 2 of the floating wind turbine is arranged such that the cross sectional area at the waterline 5 can be changed when the floating wind turbine 1 is floating in the body of water 3.

This is achieved by the floating wind turbine 1 being arranged so that vertical position of the support structure 2 relative to the body of water 3 is adjustable. The vertical position can be adjusted using the adjustable ballast 20. Water may be pumped into/air pumped out of the ballast 20 to lower the vertical position of the floating wind turbine 1 and water may be pumped out of/air pumped into the adjustable ballast 20 to raise the vertical position of the floating wind turbine 1. Owing to the fact that the support structure 2 has a cross sectional area that varies along its length, the cross sectional area of the floating wind turbine 1 at the water surface 5 can be changed by adjusting the vertical position of the floating wind turbine 1 in the body of water.

For example the floating wind turbine 1 may comprise a controller (not shown) that can be used to control the vertical position of the floating wind turbine -14 -by controlling the adjustable ballast 20. The controller may be housed in the nacelle 4.

The controller may receive information (e.g. from sensors) regarding environmental conditions (e.g. the wind speed and/or wave height) and adjust the height of the floating wind turbine 1, and hence adjust the cross sectional area of the support structure 2 at the water line 5, based on the information.

The floating wind turbine 1 may be moveable between an upper position as shown in figure 1 and a lower position shown in figure 2 whilst floating in the body of water 3. The floating wind turbine 1 is still capable of being operational, e.g. generating power, in both the upper and lower positions. In other words, in both the upper and lower positions the rotor blades 8 can still rotate about a rotor axis. In the upper position shown in figure 1 the floating wind turbine 1 is at a vertical position such that the greater diameter second portion 18 is at the water line 5. In the lower position shown in figure 2 the floating wind turbine 1 is at a vertical position such that the smaller diameter first portion 14 is at the water line 5. Given that the cross sectional area of the support structure 2 at the water line 5 is less in the lower position and the larger diameter portion 18 is lowered away from the water line 5 (where the wave forces are greatest) the waves forces acting on the floating wind turbine 1 for a given water condition are less when the floating wind turbine 1 is in the lower position compared to the upper position.

Thus, in calm conditions, e.g. wind speeds of less than 20m/s, the floating wind turbine 1 can be raised into the upper position shown in figure 1 by evacuating the adjustable ballast 20. This allows the rotor 6 to be moved into a higher position where the wind speed will be greater whilst acceptable level of wave loads are exerted on the floating wind turbine 1. Thus, the power output may be increased.

In rougher conditions, e.g. wind speeds greater than 20 m/s, the floating wind turbine can be lowered to the lower position shown in figure 2 by adding ballast to the adjustable ballast 20. This will reduce the wave loads experienced by the floating wind turbine 1 as explained above. Whilst this may result in the rotor 6 being moved to a position where the wind speed is less, this may be acceptable if the wind speed is already at or near rated wind speed such that a reduction in wind speed has little, if any, impact on the power output of the turbine.

In both the upper and lower positions the floating wind turbine 1 can still operate to generate power.

-15 -The wind turbine 1 may also be moved into a parked position as shown in figure 3.

In the parked position the floating wind turbine 1 may be lowered even further than in the lower position shown in figure 2. Given that the wide diameter portion 18 is moved yet further from the water line and optionally an even narrower portion of the wind turbine 1 is at the water line (e.g. the tower 10 as shown in figure 3) the wave forces may be reduced even further in the parked position compared to the lower position for a given water conditions.

Also, because the wind speeds are generally lower near the surface of the water, the wind loads on the wind turbine may be reduced.

Thus, in extreme conditions, such as during a hurricane, the wind turbine 1 may be lowered into the parked position. In such conditions the rotor 6 may be locked in a fixed position. The fixed position may be the position shown in figure 3 in which the lowermost point of all of the rotor blades 8 is furthest from the bottom of the floating wind turbine 1. This means for a given vertical position of the floating wind turbine the lowermost point of all of the rotor blades 8 is furthest from the surface 5 of the body of water 3. This means that the floating wind turbine 1 can be moved to a lower vertical position without the rotor blades 8 contacting the water.

The parked position may be a position in which the floating wind turbine 1 is at a vertical height such that when the rotor 6 is in a position in which the lowermost point of the rotor blades 8 is furthest from the bottom of the floating wind turbine 1 the rotor blades 8 do not contact the water but if the rotor 6 is in a position in which the lowermost point of the rotor blades 8 is nearest the bottom of the floating wind turbine 1 the rotor blades 8 do contact the water 3.

When the wind turbine 1 is in the parked position the rotor 6 may not rotate.

This is because, if it did, the blades may contact the water 3. As a result the wind turbine 1 may not generate power when in the parked position. This is acceptable when the environmental conditions are so severe that the primary aim is to minimise the risk of damage of the floating wind turbine.

When operating the wind turbine 1 information may be received regarding the environmental conditions. The floating wind turbine 1 may be controlled so that an optimum vertical position to minimise wave forces whilst maximising power output and/or avoiding damage to the wind turbine 1 may be achieved.

This may mean that in calm conditions the floating wind turbine 1 is in the upper position shown in figure 1, in rough conditions the floating wind turbine 1 is in -16 -the lower position shown in figure 2, and in extreme conditions the floating wind turbine is in the parked position shown in figure 3.

Figure 4 shows a floating wind turbine 101 with a different geometry of support structure 102 to the support structure 2 of the wind turbine 1 shown in figures 1 to 3.

The corresponding parts are labelled with the equivalent reference numeral 100 higher. For example, the tower 110 of the floating wind turbine 101 of figure 4 is labelled as 110 which is 100 higher than the tower 10 of the floating wind turbine 1 shown in figures 1 to 3.

The parts of floating wind turbine 101 are thus a support structure 102, nacelle 104, rotor 106, rotor blades 108, tower 110, upper cone 112, lower cone 116, support portion 118, adjustable ballast 120 (e.g. water ballast), and fixed ballast 122. The above description of figures 1 to 3 is equally applicable to the floating wind turbine 101 of figure 4 aside from the different support structure geometry which does not comprise first portion 14.

Figure 4 shows three waterlines 105, 105' and 105'. Line 105 represents the waterline when the floating wind turbine 101 is in the upper position, line 105' represents the waterline when the floating wind turbine 101 is in the lower position, and line 105" represents the waterline when the floating wind turbine 101 is in the parked position.

Claims

-17 -2. 3. 4. 5. 6. 7.CLAIMS: A floating wind turbine for floating in a body of water, wherein the floating wind turbine is arranged such that the cross sectional area of the floating wind turbine at the surface of the body of water can be changed when the floating wind turbine is floating in the body of water.
A floating wind turbine according to claim 1, wherein the floating wind turbine is arranged so that the vertical position of the floating wind turbine relative to the body of water is controllable when the floating wind turbine is floating in the body of water.
A floating wind turbine according to claim 2, wherein the floating wind turbine comprises adjustable ballast, wherein the adjustable ballast is adjustable so that the vertical position of the floating wind turbine relative to the body of water is controllable when the floating wind turbine is floating in the body of water.
A floating wind turbine according to claim 1, 2 or 3, wherein the floating wind turbine comprises a support structure, and wherein the support structure has a cross sectional area that varies along its length.
A floating wind turbine according to claim 4, wherein the support structure comprises a first portion with a cross sectional area that is smaller than the cross sectional area of a second portion, wherein the second portion is closer to the bottom of the floating wind turbine than the first portion.
A floating wind turbine according to claim 5, wherein, in use the floating wind turbine is movable between an upper position and a lower position, wherein at the upper position the second portion is at the water line and at the lower position the first portion is at the water line.
A floating wind turbine according to claim 6, wherein the floating wind turbine is movable into a parked position, wherein, when in the parked -18 -position, the floating wind turbine is at a lower vertical position relative to the body of water, than when in the lower position.
8. A floating wind turbine according to claim 7, wherein the floating wind turbine comprises a rotor comprising rotor blades, and wherein the parked position is a position in which the floating wind turbine is at a vertical height such that when the rotor is in a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine the rotor blades do not contact the body of water but if the rotor is in a position in which the lowermost point of the rotor blades is nearest the bottom of the floating wind turbine the rotor blades do contact the body of water.
9. A floating wind turbine according to any preceding claim, wherein the floating wind turbine comprises a rotor comprising rotor blades, and wherein the floating wind turbine is arranged so that the rotor can be locked in a fixed position.
10. A floating wind turbine according to claim 9, wherein the fixed position is a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine.
11. A method of controlling a floating wind turbine, wherein the method comprises: providing a floating wind turbine in a body of water; and changing the cross sectional area of the floating wind turbine at the surface of the body of water.
12. A method according to claim 11, wherein the method comprises adjusting the vertical position of the floating wind turbine relative to the body of water.
13. A method according to claim 12, wherein the method comprises adjusting an adjustable ballast to adjust the vertical position of the floating wind turbine relative to the body of water.
-19 - 14. A method according to claim 11, 12 or 13, wherein the floating wind turbine comprises a support structure, and wherein the support structure has a cross sectional area that varies along its length.
15. A method according to any of claims 11 to 14, wherein the method comprises reducing the vertical position of the floating wind turbine if the wind speed is above rated wind speed and/or increasing the vertical position of the floating wind turbine if the wind speed is less than rated wind speed.
16. A method according to any of claims 11 to 15, wherein the method comprises moving the floating wind turbine between an upper position and a lower position, and wherein the cross sectional area of the floating wind turbine at the water surface may be larger when the floating wind turbine is in the upper position compared to the lower position.
17. A method according to claim 16, wherein the method comprises moving the floating wind turbine into a parked position, wherein when in the parked position the floating wind turbine is at a lower vertical height than when in the lower position.
18. A method according to claim 17, wherein the parked position is a position in which the floating wind turbine is at a vertical height such that when the rotor is in a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine the rotor blades do not contact the body of water but if the rotor is in a position in which the lowermost point of the rotor blades is nearest the bottom of the floating wind turbine the rotor blades do contact the body of water.
19. A method according to any of claims 11 to 18, wherein the method comprises locking the rotor in a fixed position.
20. A method according to claim 19, wherein the fixed position is a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine.
-20 - 21. A method according to ay of claims 11 to 20, wherein the floating wind turbine provided is the floating wind of any of claims 1 to 10.
22. A floating wind turbine comprising: a rotor comprising a plurality of rotor blades, wherein the rotor may be locked in a fixed position, and wherein the fixed position is a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine.
23. A method of controlling a floating wind turbine, the method comprising: providing a floating wind turbine comprising a rotor comprising a plurality of rotor blades, locking the rotor in a fixed position, wherein the fixed position is a position in which the lowermost point of the rotor blades is furthest from the bottom of the floating wind turbine.