FIELD OF THE INVENTION

The present invention relates to a tower vibration damper for a wind turbine, more specifically a pendulum damper for mounting in a tower that forms part of an offshore or onshore wind turbine.

BACKGROUND

A typical wind turbine includes a tower, mounted upon which is a nacelle housing a generator connected by a shaft to a drive hub, and rotor blades attached to the drive hub. To install the wind turbine onshore, the base of the tower is fixed to the ground, which provides a foundation. For an offshore wind turbine installation the foundation will usually be provided by the seabed or a floating foundation moored to the seabed. Once the tower has been assembled, the nacelle is then lifted by crane for mounting to the top of the tower. Once the nacelle has been secured to the tower, the rotor blades are manoeuvred into place by the crane for attachment to the drive hub, and hence the nacelle. Alternatively, the nacelle and the blades may be assembled prior to installation of the nacelle.

The assembled wind turbine is a tall, heavy structure that needs to have dynamic stability in strong winds. In the case of an offshore wind turbine, the wind turbine is also subject to forces from water waves, in addition to wind forces that are typically stronger than those experienced onshore. These external forces can cause the tower to oscillate (e.g. vibrate at low frequency) such as to produce considerable lateral motion (or swaying) of the wind turbine, which increases towards the upper part of the tower. For example, the tower may be caused to bend under the forces to such an extent that the nacelle is displaced laterally from its resting position above the base of the tower by as much as one metre.

Rather than use conventional mechanical dampers to minimise these low frequency vibrations, wind turbines instead often employ “tuned” mass dampers in which a mass, which is free to oscillate relative to the structure of the wind turbine (i.e. the tower and/or nacelle), is attached to that structure. The properties (or characteristics) of such a mass damper are typically tuned to enable the mass to oscillate out of phase with the motion of the structure, the effect of which is to damp the vibrations in the wind turbine. A form of tuned mass damper considered to be particularly suitable for controlling the naturally low frequency vibrations of wind turbine towers is a pendulum damper.

The general construction and operation of an exemplary pendulum damper in a wind turbine can be found in WO2018/059638 and WO2018/153416, for example. As will therefore be understood, a pendulum damper in a wind turbine tower can be considered to comprise a pendulum structure having a pendulum body that is immersed (or submerged) in a chamber containing a damping liquid (e.g. an oil bath). A plurality of damping elements, such as springs, may be arranged to help damp movements of the suspended pendulum body. Motion of the wind turbine tower is transferred to the pendulum structure as kinetic energy, which can be dissipated through the damping elements.

However, the kinetic energy is typically dissipated in the form of heat generated via movement of the pendulum body within the damping liquid held in the chamber (e.g. which may be referred to as internal damping). By submerging the “free” pendulum body in a chamber of damping liquid, the motion of the pendulum is damped as it moves as a result of drag forces acting on the pendulum body within the chamber.

A pendulum damper typically has damping and tuning characteristics configured depending on a ratio between the mass of the pendulum body, the mass of the complete wind turbine and the operation characteristics of the wind turbine. During installation/construction of the wind turbine, however, the mass of the wind turbine changes as it is assembled, in particular once the nacelle is mounted and the blades are attached thereto. Thus, as the mass of the pendulum body is chosen based on the mass of the completed wind turbine, it does not provide optimum damping during assembly of the wind turbine. This can cause the wind turbine to experience excessive motion during its assembly, due to wind or waves, for example, which inhibits assembly of the wind turbine, thereby delaying its installation. The present invention aims to address this problem.

SUMMARY OF THE INVENTION Described herein is a tower vibration damper for mounting in a tower of a wind turbine, the damper comprising: a pendulum structure configured to be suspended in the tower, said pendulum structure comprising a pendulum body; a suspension arrangement for suspending the pendulum structure in the tower such that the pendulum body is allowed to displace from a neutral position for the pendulum body; a chamber holding a damping liquid into which the pendulum body is at least partly immersed such that movement of the pendulum body within the chamber is inhibited due to a drag force asserted by the damping liquid on the pendulum body; and means for changing the drag force asserted on the pendulum body by the damping liquid held in the chamber, so as to adjust a damping characteristic of the damper.

The present invention recognises that, as movement of the pendulum body through the damping liquid held in the chamber is inhibited by drag force asserted on it by the damping liquid held in the chamber, a damping characteristic of the damper can be adjusted by changing said drag force. As used herein the term “drag” preferably connotes the force on an object that resists its motion through a fluid. It is therefore a force opposing the relative motion of any object moving with respect to a fluid (e.g., with the present invention, a specific type of “damping” liquid) that surrounds it. When the drag takes place in water, it is known as hydrodynamic drag. In fluid dynamics, the following equation can conveniently be used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid:

1

PD = 2 2 P ^A C _D V where:

F _D is drag force, p is the density of the fluid, A is the (cross-sectional) area of the object facing the fluid in the direction of movement, C _D is the drag coefficient of the object in the fluid, and v is the speed of the object relative to the fluid. While other factors may affect the drag force on an object, the general principles of the above equation nonetheless hold true. Thus, for the pendulum body, the magnitude of the drag force, F ₀ , will be proportional to the square of the speed of the moving object and the area, A, of the object facing the fluid in the direction of movement. In addition, it is well-understood that the viscosity, h, of a fluid, which is proportional to drag force, decreases as its temperature increases, and vice versa.

The means for changing the drag force is preferably operable to change, e.g. increase and/or decrease, the drag force asserted on the pendulum body by the damping liquid held in the chamber while the pendulum structure is suspended in the tower. In this way, the effectiveness of the damper may preferably be optimised throughout installation of a wind turbine as the overall mass of the wind turbine structure changes.

Indeed, with the present invention, it is possible to adjust the amount of pendulum damping, in situ, so that the wind turbine may be optimally damped at all times. In addition to preventing damage to the wind turbine during use in adverse conditions, maintaining optimal damping of the wind turbine prior to its completion will make the final stages of the installation process easier and safer. This will also increase the window of, e.g. weather, conditions where installation of the tower, nacelle, and blades can be conducted due to better damping of tower vibrations.

As will become apparent, there are more than one means for changing the drag force asserted on the pendulum body by the damping liquid held in the chamber.

In one aspect, the means for changing the drag force may be configured to change a submersion distance that the pendulum body is immersed in the damping liquid. Optionally, said means for changing the drag force may comprise means for raising or lowering the pendulum body relative to the chamber. Said means for changing the drag force may comprise one or more winch(es) mounted in the tower. Each winch may be operably coupled to one or more wire attached to the pendulum body. If multiple wires are attached they are preferably spaced evenly about the pendulum body such that the suspended pendulum body is evenly balanced.

Alternatively, or additionally, a liquid tank may be arranged to hold damping liquid, said liquid tank being fluidly coupled to the chamber, and arranged such that damping liquid can flow from the liquid tank to the chamber under the force of gravity. The liquid tank may be positioned higher than the chamber, and may further comprise a valve to control the flow of damping liquid from the liquid tank to the chamber. A second liquid tank may be arranged to hold damping liquid and a second valve, wherein said liquid tank is fluidly coupled to the chamber via the second valve and arranged lower than the chamber such that, when the second valve is open, damping liquid can flow from the chamber to the second liquid tank under the force of gravity.

Alternatively, or additionally, a pump may be provided that is operable to pump damping liquid between the liquid tank and the chamber. In another aspect, the means for changing the drag force on the pendulum body may include means for changing temperature of the damping liquid so as to change its viscosity. Preferably, a pump is provided that is operable to pump damping liquid from the chamber to the temperature changing means and back to the chamber, wherein the temperature changing means is external to the chamber, optionally wherein it is external to the tower.

The temperature changing means may comprise a heating arrangement and/or a cooling arrangement.

The heating arrangement may comprise an immersion heater disposed within the chamber. The immersion heater can be arranged such that a heating element is submerged in the damping liquid, in use. Alternatively, or additionally, the temperature changing means may comprise a heat exchanger.

As used, herein, the term “means for changing a temperature [of the damping liquid]” (equivalently referred to as “temperature changing means” or “temperature changing arrangement”) preferably means a device, apparatus or system that can be used in cooling and/or heating processes. It therefore preferably connotes an arrangement for heating and/or cooling the damping liquid, which arrangements for convenience may be described herein as a “heater” and/or a “cooler”, respectively without limitation to the form, construction or operation of that arrangement. Such arrangements for heating and cooling liquids are well known and therefore need not be described in detail. The temperature changing means may comprise a liquid “heater”, a liquid “cooler”, or a combination of both, either individually or as part of a single device, apparatus or system. Advantageously providing both a heater and a cooler increases the range of temperatures that can be reached by the damping liquid and allows for faster adjustment of the temperature. Furthermore, it will allow the temperature changing means to compensate for changes in ambient temperature of the tower.

The pendulum body is preferably annular, but other shapes, such as block shaped or stick shaped, for example, may be utilised.

According to further aspect, there is provided a wind turbine comprising the tower vibration damper described above and herein.

According to yet a further aspect, there is provided a method of adjusting a damping characteristic of the tower vibration damper in the wind turbine described above and herein during its installation, comprising the steps of: at a (e.g. first) stage of installation, changing the drag force asserted on the pendulum body by the damping liquid held in the chamber such that the damper is configured for damping movement of the wind turbine at said stage of installation; and at a subsequent (e.g. second) stage of installation, changing the drag force asserted on the pendulum body by the damping liquid held in the chamber such that the damper is configured for damping movement of the wind turbine at said subsequent stage of installation.

According to yet a further aspect, there is provided a method of adjusting a damping characteristic of the tower vibration damper in the wind turbine described above and herein during its decommissioning, comprising the steps of: at a (e.g. first) stage of decommissioning, changing the drag force asserted on the pendulum body by the damping liquid held in the chamber such that the damper is configured for damping movement of the wind turbine at said stage of decommissioning; and at a subsequent (e.g. second) stage of decommissioning, changing the drag force asserted on the pendulum body by the damping liquid held in the chamber such that the damper is configured for damping movement of the wind turbine at said subsequent stage of decommissioning.

The pendulum structure may be made of iron and its mass may be in the range 3-30 tons depending on the specific requirement. The chamber may comprise an outer wall, an inner wall and a bottom part extending between the outer wall and the inner wall. The chamber may thus form a container structure suitable for holding the damping liquid into which the pendulum body is at least partly immersed. The chamber may be a separate element, or it may somehow be integrated in the wind turbine tower structure. For example, the outer wall of the chamber may form part of the wind turbine tower wall.

The suspension arrangement may comprise a plurality of wires suspending the pendulum structure. Tuning means configured for adjusting the natural frequency of the tower damper may be provided as well. The natural frequency may be adjusted by adjusting the length of the plurality of wires. The tuning means may comprise, for each of said plurality of wires, a clamp secured to the tower at one end and to the wire at the other end. In order to adjust the length of the wires, and thereby adjust the natural frequency of the tower damper, the securing of the clamp to the tower is configured such that the clamp may be movable along the longitudinal direction of the wire. In the present context the term “length of the wires” should be taken to mean the length of the wires that are free to swing, i.e. the distance between the tuning means, where the wires are attached to the tower structure, and the pendulum structure. The wires may move angularly below the tuning means thereby allowing the pendulum structure to swing. The damping liquid may comprise a suitable damping oil, includes products such as Texaco Way Lubricant x320, Exxon Mobilgear 600 XP 320 or Uno Vibration Absorber 320, for example. Other damping liquids such as glycol and silicone oils may also be applicable.

While the present invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of examples in the drawings and will be described in details herein. It should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

An example of the present invention will now be described with reference to the accompanying figures, in which:

Figure 1 shows a schematic side view of a structure of a typical wind turbine;

Figure 2A shows a schematic plan view of a part of a pendulum structure immersed within a chamber inside a wind turbine to provide a tower vibration damper;

Figure 2B shows a schematic sectional side view of the pendulum structure of Figure 2A;

Figure 3 shows a schematic sectional side view of a pendulum structure according to a first embodiment;

Figure 4A shows a schematic sectional side view of a pendulum structure according to a second embodiment; Figure 4B shows a schematic sectional side view of a pendulum structure illustrating a variant of the second embodiment of Figure 4A; and

Figure 5 shows a schematic sectional side view of a pendulum structure according to a third embodiment. DETAILED DESCRIPTION

In the following description and accompanying drawings, corresponding features may preferably be identified using corresponding reference numerals to avoid the need to describe said common features in detail for each and every embodiment. Furthermore, as will be appreciated, the accompanying drawings are in the form of simplified schematic views for the purpose of illustrating the general relationship between components and features of the disclosed vibration damper with a view simply to understanding what is disclosed herein. Thus, no limitation as to the form or construction of any of the embodiments of the disclosed vibration damper should be inferred from the drawings; rather, any such limitations to the vibration damper are instead defined by the following description.

Figure 1 shows a typical wind turbine 100. It comprises a tower 120, a nacelle 140, a rotor hub 160, and a plurality of rotor blades 180. The tower 120 comprises a tubular structure 122 having a longitudinal or vertical axis Z. A lower end 124 of the tower 120 is fixed to the ground 125, or in the case of an offshore wind turbine 100, the seabed or a floating foundation moored to the seabed. The nacelle 140 is mounted to an upper end 126 of the tower 120. The nacelle 140 contains a generator (not shown). The rotor hub 160 extends from the nacelle 140 and is connected (directly or indirectly) to the generator by a shaft (not shown) having an axis X. The rotor blades 180 are attached to the rotor hub 160. In use, a wind force acting on the rotor blades 180 causes the rotor blades 180 to rotate about the axis X, thereby to drive the generator via the shaft to produce electrical energy. Figure 2A is a sectional schematic plan view illustrating a conventional tower vibration damper 200, for a wind turbine 100, in the form of a pendulum damper. A pendulum body 210 is immersed within a liquid chamber 220.

The shown pendulum body 210 is annular, and the vibration damper 200 will therefore be described herein accordingly, by way of example. However, it will be appreciated that other shapes, such as block shaped or stick shaped, are also useful and may be utilised. An annular pendulum is for example advantageous as it can allow service personnel safe passage through the damper. The mass of the pendulum body 210 may be in the range of 3-10 tons depending on the required damping performance and the size of the tower 120 of the wind turbine 100. Typically the mass of the pendulum body 210 is around 3-20 tons.

To accommodate the annular pendulum body 210, the chamber 220 has a base 222, an inner wall (or boundary) 224 with diameter c/,, and an outer wall (or boundary) 226 with diameter d ₀ . The annular pendulum body 210 has a diameter d _p . The outer wall 226 of the chamber 220 may be shared with the tubular structure 122 of the tower 120 of the wind turbine 100. The inner wall 224 of the chamber 220 can provide an opening 228 for safe passage of service personnel through the damper 200 when required, for example in case upper parts of the wind turbine 100 need to be serviced or repaired. In use, the chamber 220 is at least partly filled with a suitable damping liquid 240 within which the pendulum body 210 is immersed.

A pendulum structure comprising the pendulum body 210 is secured to, and suspended internally within, the tower 120. The pendulum structure is suspended via a suspension arrangement that includes a plurality of wires (not shown) that are secured to the pendulum body 210 via attachment points 214, 215, 216, which may be evenly spaced around the pendulum body 210. The suspension arrangement is configured to suspend the pendulum structure within the tower 120 such that the pendulum body 210 is allowed to displace horizontally from a neutral position for the pendulum structure. Referring now to Figure 2B a sectional side view of the damper 200 is depicted. As discussed above, the annular pendulum body 210 is suspended in a chamber 220 by three wires 211, 212, (213 not shown) which are attached to the pendulum body 210 at spaced-apart attachment points 214, 215, (216 not shown), thereby forming part of the suspension arrangement. The length of the schematic wires 211, 212, 213 determines the natural frequency of the pendulum damper 200. Thus, by varying the length of the wires 211 , 212, 213, the natural frequency of the damper 200 may be adjusted and thereby “tuned” to specific demands.

As previously noted, the term “length of the wires” connotes the length of the wires that are free to swing. This may be from the attachment point of the wires 211 , 212, 213 to the tower 120. Alternatively, or additionally, the length of the wires can be set by clamps (or “runners”) 217, 218, (219 - not shown), which are movable vertically along the wires 211 , 212, 213 and thereby control the length (e.g. the “effective length”) of each wire 211, 212, 213 that is free to swing.

In use, the chamber 220 holds a damping liquid 240, which fills the chamber 220 to a height (h), measured from the base 222 of the chamber 220. The pendulum body 210 is then submerged in the damping liquid 240. Movement of the pendulum body 210 is then damped as a result of drag forces asserted by the damping liquid on the pendulum body 210 as it moves within the chamber 220. To allow it to move within the damping liquid 440, the pendulum body 210 is submerged in the damping liquid so the bottom 221 of the pendulum body 210 is at a submersion distance (s) below the surface of the damping liquid 440 that leaves a gap (g) between the base 222 of the chamber 220 and the pendulum body 210. The submersion distance (s) is also referred to as the distance that the pendulum body 210 is submerged in the damping liquid 240.

When unwanted motion (e.g. sway) is induced in the wind turbine 100 by factors such as wind or waves, the pendulum body 210 will also move, relative to the tower 120, through the damping liquid 240 held in the chamber 220. The viscosity of the chosen damping liquid 240 will cause the motion of the pendulum body 210 to be damped due to the effect of viscous drag through the damping liquid 240, which acts to dissipate unwanted kinetic energy causing the unwanted motion, typically in the form of heat. The effectiveness of the overall damping (“turbine damping”) on the wind turbine 100 depends on the level to which the pendulum body 210 is damped by the damping liquid 240 (“pendulum damping”).

The pendulum damper 200 is, ideally, configured to be installed at a position as high as possible inside a tower 120. Typically, an installation of the pendulum damper 200 within the upper half, and preferably in the upper third, of a tower 120 is considered to provide the most effective damping of vibrations in the tower 120.

As the wind turbine 100 is installed and its mass increases, it is desirable to change damper characteristics to be optimised for the increasing mass of the wind turbine 100. The simplest way to change damper characteristics is to change (i.e. increase) the mass of the pendulum body. However, since the pendulum body is very heavy (typically in the range of 3-20 tons) it cannot therefore be easily replaced or changed significantly in mass, so this is simply not feasible.

As will now be described, maintaining optimal damping of the wind turbine 100 at all stages of its installation can be achieved by changing the drag force experienced by the pendulum body 210 as it moves through the damping liquid 240 held in the chamber 220, i.e. the drag force asserted on the pendulum body 210 by the damping liquid 240 held in the chamber 220, so as to adjust a damping characteristic of the damper 200, and thereby provide an adjustable damper 200.

As noted above in relation to the drag equation, the magnitude of the drag force, F _d , experienced by the pendulum body 210 as it moves through the damping liquid 240 depends on multiple factors, one of which is the area, A, of the pendulum body 210 facing the damping liquid 240 in the direction of movement (i.e. causing the damping liquid 240 to move out of the way). Another parameter is the density of the damping liquid 240. In addition, the drag force asserted on the pendulum body 210 by the damping liquid 240 held in the chamber 220 is related to the viscosity of the damping liquid 240.

Figure 3 is a sectional schematic view of an embodiment of a tower vibration damper 300 (i.e. in the form of a pendulum damper) in which the drag force can be changed to adjust the level of damping in the damper 300, according to the present invention. The sectional view of Figure 3A is from the same perspective as shown in Figure 2B. The damper 300 is of similar construction to the damper 200 described in relation to Figure 2A, except that the wires 311, 312, 313 which attach to the pendulum body 310 are each coupled to a separate winch 351 , 352, 353 at the top of the tower 120.

By increasing the length of the wires 311 , 312, 313 with the winches 351, 352, 353, the pendulum body 310 can be lowered into the damping liquid 340 held in the chamber 320. As the submersion distance (s) increases, the size of the gap (g) decreases. Moreover, the area, A, of the pendulum body 310 that is submerged, and which therefore faces the damping liquid 340 in the direction of movement, increases. Thus, the drag force experienced by the pendulum body 310 when it moves in the damping liquid 340 increases, which in turn increases the level of pendulum damping. Conversely, by retracting the wires 311 , 312, 313 with the winches 351 , 352, 353, the pendulum body 310 can be raised so that it is less submerged in the damping liquid 340 held in the chamber 320. As the submersion distance (s) to which the pendulum body 310 is submerged in the damping liquid 340 decreases, the size of the gap (g) increases. Moreover, the area, A, of the pendulum body 310 that is submerged, and which therefore faces the damping liquid 340 in the direction of movement, decreases. The reduction in the area, A, of the pendulum body 310 that is submerged therefore results in decreased drag force experienced by the pendulum body 310 when it moves in the damping liquid 340, which in turn decreases the level of pendulum damping. Thus, a means for changing the drag force asserted on the pendulum body 310 by the damping liquid 340 held in the chamber 320 is thereby provided. In this way, the winches 351 , 352, 353 can initially raise the pendulum body 310 slightly out of the damping liquid 340 during installation of the wind turbine 100, to configure the drag force to be appropriate for the mass of, and hence pendulum damping required by, the partially completed wind turbine 100. Then as the wind turbine 100 is further installed, and its mass increases, the winches 351, 352, 353 can lower the pendulum body 310 further into the damping liquid 340 to increase the drag force asserted on the pendulum body 310, accordingly. If the wind turbine 100 later needs to be repaired or decommissioned, then the reverse process can occur. When raising or lowering the pendulum body 310, it is possible that the winches 351, 352, 353 may change the effective length of the wires 311, 312, 313 slightly, which may independently affect frequency tuning or damping characteristics of the damper. Even though the change in length of the wires 311 , 312, 313 will typically be small relative to their initial length, the clamps 317, 318, 319 may be adjusted to compensate for the change in length.

There may be fewer or more than one winch 351 , 352, 353 per wire 311 , 312, 313, for example one winch 351 may be configured to retract or let out more than one wire 311, 312, 313.

Table 1 , below, provides an exemplary indication of the changing weight of a wind turbine (WTG) 100 during installation, and the corresponding changes in optimum submersion of the pendulum body 310 into the damping liquid 340 (having constant viscosity) held in the chamber 320 for optimal damping, which is indicated by the gap (g) between the pendulum body 310 and the base of the chamber 320: Table 1: Figure 4A shows a sectional schematic view of a second embodiment of a tower vibration damper 400 (in the form of a pendulum damper) in which the drag force, FD, can be changed to adjust the level of damping in the damper 400, according to the present invention. The sectional view of Figure 4A is again from the same perspective as shown in Figure 2B, and the damper 400 is again similar in construction to the tower damper 200 described in relation to Figure 2A, except that in this embodiment there is also provided a tank 460 for holding damping liquid 440 fluidly coupled to the chamber 420 via a pump 470. The tank 460 may be cylindrical, or any other suitable shape. The tank 460 is preferably located internally within the tower 120, adjacent the tubular structure 122 of the tower 120. The pump 470 is configured to pump liquid 440 from the tank 460 into or out of the chamber 420.

By pumping damping liquid 440 from the tank 460 into the chamber 420, the height (h) of the damping liquid 440 in the chamber 420 can be increased, and the pendulum body 410 thereby becomes more submerged in the damping liquid 440. As discussed above in relation to Figure 3, as the area, A, of the pendulum body 410 that is submerged and therefore faces the damping liquid 440 in the direction of movement increases, the drag force on the pendulum body 410 when it moves in the damping liquid 440 also increases, which in turn increases the level of pendulum damping. Conversely, by pumping damping liquid 440 from the chamber 420 to the tank 460, the height (h) of the damping liquid 440 held in the chamber 420 can be decreased, and the pendulum body 410 thereby becomes less submerged in the damping liquid 440. Again, as noted above, as the area, A, of the pendulum body 410 that is submerged and therefore faces the damping liquid 440 in the direction of movement decreases, the drag force experienced by the pendulum body 410 when it moves in the damping liquid 440 decreases, which in turn decreases the level of pendulum damping. Thus, another means for changing the drag force asserted on the pendulum body 410 by the damping liquid 440 held in the chamber 420 is thereby provided.

In this way, during installation of the wind turbine 100, the chamber 420 can initially be filled with an amount of damping liquid 440 suitable to configure the drag force, and hence the pendulum damping, appropriate for the mass of the partially completed wind turbine 100. The tank 460 contains the remainder of the required damping liquid 440. Then as the wind turbine 100 is further installed and its mass increases, the pump 470 can transport damping liquid 440 from the tank 460 into the chamber 420 so that the height (h) of the damping liquid 440 in the chamber can be increased and such that pendulum body 410 becomes more submerged in the damping liquid 460. If the wind turbine 100 later needs to be repaired or decommissioned, then the reverse process can occur. A variation on this second embodiment of a damper 400 is shown in the schematic sectional view of Figure 4B, which is viewed from the same perspective as Figure 4A. Here, the tank 460 is shown installed in the tower 120 above the chamber 420 to which it is connected via an adjustable valve 461. There is also a second tank 465 for holding damping liquid 440 installed in the tower 120 below the chamber 420, to which it is fluidly coupled via a second adjustable valve 466. In this configuration, as the tower 120 is further installed, valve 461 can be opened to control flow of the damping liquid 440 from the first tank 460 to the chamber 420 in order to increase the drag force and increase pendulum damping. Then, if the wind turbine 100 is decommissioned, valve 466 can be opened to control flow of the damping liquid 440 from the chamber 420 to the second tank 465 in order to decrease the drag force and decrease pendulum damping. Advantageously, with this variant, the damping liquid 440 can be transported via gravity, which removes the requirement for a pump. Thus, another means for changing the drag force asserted on the pendulum body 410 by the damping liquid 440 held in the chamber 420 is thereby provided.

Table 2, below, provides an exemplary indication of the changing weight of a wind turbine (WTG) 100 during installation and corresponding changes in height (h) of the damping liquid 440 (having constant viscosity) held in the chamber 420. Table 2:

In each of the arrangements depicted in Figures 4A and 4B, essentially the level of damping is changed by altering the amount of damping liquid in the chamber, rather than raising or lowering the pendulum structure within the tower, as described in relation to Figure 3. However, each of these embodiments have in common the feature that drag force on the pendulum body is changed by changing the area, A, of the pendulum body that is submerged in the damping liquid held in the chamber Figure 5 shows a schematic view of a third embodiment of a tower vibration damper 500 (in the form of a pendulum damper) in which the drag force, F _D , can be changed to adjust the level of damping in the damper 500, according to the present invention. The sectional view of Figure 5 is again from the same perspective as shown in Figure 2B, and the damper 500 is similar to the tower damper 200 described in relation to Figure 2A, except that in this embodiment a heating arrangement (or “heater”) 590 is used to change the temperature of the damping liquid 540, which thereby changes its viscosity. A change in the viscosity of the damping liquid 540 changes the drag force asserted on (e.g. experienced by) the pendulum body 510 as it moves through the damping liquid 540.

To change the temperature of the damping liquid 540 to achieve this change in drag force, a pump 580 can receive damping liquid 540 via an outlet 582 in the chamber 520, transport it through the heater 590 and back into the chamber 520 via an inlet 584. By activating the pump 580 and increasing power supplied to the heater 590, the damping liquid 540 in the chamber 520 can be heated and maintained substantially at a specific (i.e. higher) temperature. Similarly, to reduce the temperature of the damping liquid 540, the power supplied to the heater 590 may be reduced or turned off entirely, such that the damping liquid 540 can cool to the temperature of its surroundings, or ambient temperature. The chamber 520 may be equipped with a temperature sensor (not shown) to help monitor the temperature of the damping liquid 540.

Alternatively, or additionally, a liquid cooler (not shown) may be installed. Indeed, the temperature changing arrangement 590 may comprise both a heater and a cooler. Utilising both a heater and a cooler can allow the temperature of the damping liquid 540 to be changed more quickly, e.g. the cooler can be operated when the heater 590 is switched off, and vice versa. Furthermore, the use of a heater and/or a cooler can allow the damping liquid 540 to be adjusted to a temperature either above or below the ambient temperature (e.g. of the surroundings) at the wind turbine 100, and further to maintain the damping liquid 540 at said temperature, if required.

As previously mentioned, there are numerous suitable damping liquids, e.g. oils, which exhibit suitable properties for use here. When the damping liquid 540 is maintained at a lower temperature, the liquid will be more viscous than when the damping liquid 540 is maintained at a higher temperature. The increased viscosity of the damping liquid 540 at the lower temperature increases the drag force experienced by the pendulum body 510 when it moves in the damping liquid 540, which increases the level of pendulum damping. If the damping liquid 540 is instead maintained at a higher temperature, it will be less viscous. This lower viscosity decreases the drag force experienced by the pendulum body 510 when it moves in the damping liquid 540, which decreases the level of pendulum damping.

Thus, another means for changing the drag force asserted on the pendulum body 510 by the damping liquid 540 held in the chamber 520 is thereby provided. In this way, during installation of the wind turbine 100, the pump 580 and temperature changing arrangement 590 can be activated to control the viscosity of the damping liquid 540 such that it is appropriate for the mass, and hence required damping, of the partially completed wind turbine 100. Then, as the wind turbine 100 is further installed and its mass increases, the temperature can be adjusted to make the liquid 540 become cooler, and hence more viscous. This increases the pendulum damping as the mass of the wind turbine 100 increases, which can allow the wind turbine 100 to remain optimally damped during the entire installation process. If the wind turbine 100 later needs to be repaired or decommissioned, then the reverse process can occur. The damping liquid may of course be heated in other ways, including by using an immersion heater (not shown) disposed in the chamber.

Table 3, below, provides an exemplary indication of the changing weight of a wind turbine (WTG) 100 during installation, and the optimum viscosity of the damping liquid 540 and corresponding temperature (and hence temperature change required from ambient temperature):

Table 3: It will be appreciated that the temperature changing arrangement 590 can be configured as one device capable of both heating and cooling, or could have separate heaters and coolers in different locations within the damper 500. The temperature changing arrangement 590 may have heating and cooling elements that are in direct thermal contact with the damping liquid 540 in use. Alternatively, the temperature changing arrangement 590 may be configured as a heat exchanger, where the heating and cooling elements change the temperature of the damping liquid 540 via an intermediate liquid, which is in thermal contact with both the damping liquid 540 and the heating and cooling elements of the temperature changing arrangement 590. The damping liquid 540 is, preferably, chosen such that it provides the required damping for a fully installed wind turbine 100 at a temperature that is approximately equal to the average ambient temperature at the wind turbine 100. As such, the damping liquid 540 should not require heating or cooling during general operation of the fully installed wind turbine 100.

The dampers described herein may be used throughout use of the wind turbine, rather than just through its installation and decommissioning. For example, during adverse weather, such as a storm, the temperature of the damping liquid may increase due to friction from increased motion of the pendulum body, which could result in decreased viscosity and a decrease in damping. This adverse effect may be to temporarily offset by increasing the pendulum damping according to the present invention, for example by way of one or more of the embodiments described herein.

Indeed, it will be appreciated that any of the embodiments described herein can be implemented alone, or together in any combination, for example to increase the maximum range of pendulum damping that can be achieved. Moreover, any feature in a particular embodiment described herein may be applied to another embodiment, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects described herein can be implemented and/or supplied and/or used independently. Any apparatus feature described herein may also be incorporated as a method feature, and vice versa.

While the foregoing is directed to exemplary embodiments of the present invention, it will be understood that the present invention is described herein purely by way of example, and modifications of detail can be made within the scope of the invention. Indeed, other and further embodiments of the invention will be apparent to those skilled in the art from consideration of the specification, and may be devised without departing from the basic scope thereof, which is determined by the claims that follow.