Patent Description:
Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly ("directly driven" or "gearless") or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.

The wind turbine hub may be rotatably coupled to a front of the nacelle. The wind turbine hub may be connected to a rotor shaft, and the rotor shaft may then be rotatably mounted in the nacelle using one or more rotor shaft bearings arranged in a frame inside the nacelle. The nacelle is a housing arranged on top of a wind turbine tower that may contain and protect the gearbox (if present) and the generator (if not placed outside the nacelle) and, depending on the wind turbine, further components such as a power converter, and auxiliary systems.

There is a trend to make wind turbine blades increasingly longer to capture more wind and convert the energy of the wind into electricity. That makes blades more flexible and more prone to vibrations of the blades. Wind turbine blades vibrating excessively may get damaged. Vibrations of the rotor blades may also result in the whole wind turbine structure oscillating e.g. fore-aft oscillations, or sideways oscillations. Vibrations in the wind turbine blade may also damage other components of the wind turbine due to excessive stress.

When the wind turbine is in operation (i.e. producing energy and connected to an electrical grid), a wind turbine controller may operate auxiliary drive systems such as a pitch system or a yaw system to reduce or change loads on the blades. This way, vibrations of the blades may be counteracted. However, the problem of vibrations can be serious as well in circumstances when the wind turbine is parked and disconnected from the grid.

When a wind turbine is parked, the wind may blow against the wind turbine from unusual directions, i.e. different from when in normal operation. The airflow around the wind turbine may cause the wind turbine to vibrate. Vibrations may stress and even damage one or more wind turbine components, which may compromise the performance of the wind turbine, can increase the need of reparations and reduce the lifespan of the wind turbine. As an orientation of a wind turbine blade cannot be adapted to the direction of the incoming wind, e.g. through yawing and/or pitching as opposed to when the wind turbine is operating, the effects of vibrations may be greater or different when the wind turbine is parked than when the wind turbine is operating normally and producing energy.

In particular, this may apply when the wind turbine is being installed or commissioned. For example, it may happen that an incomplete rotor is installed (e.g. a rotor having a single blade or two blades out of the total of three blades). The remaining blades may not be installed until a few days or a week later. In the meantime, the partially installed (or "incomplete") rotor may be in standstill. The rotor may or may not be locked, and the wind turbine can be exposed to varying wind conditions. This may likewise apply if the wind turbine is stopped during several hours, days or weeks, e.g. for maintenance reasons. A wind turbine blade can start to vibrate in any of these conditions depending particularly on the direction of the wind.

<CIT> and <CIT> provide examples where blade vibrations are mitigated during standstill.

In an aspect of the present disclosure, a device according to claim <NUM> configured to be removably mounted to a wind turbine blade having a root, a tip and exterior surfaces defining a pressure side, a suction side, a leading edge and a trailing edge, each surface extending in a generally spanwise direction from the root to the tip, is provided. The device is configured for mitigating vibrations when a rotor of a wind turbine is in standstill. The device comprises a portion configured to protrude substantially in a local chordwise direction beyond the leading edge of the wind turbine blade. The device is configured to be attached around the blade substantially along a local chordwise direction.

According to this aspect, the portion configured to protrude from the leading edge may change the air flowing around the wind turbine blade once attached to it, and avoid, or at least reduce, vortex and/or stall induced vibrations.

Throughout the present disclosure, the terms "standstill" and "parked" are used interchangeably, and may be understood as a situation in which the wind turbine is not producing electricity, and the rotor is substantially standing still. The rotor may or may not be locked in standstill. For instance, a wind turbine may be parked or in standstill during installation and/or commissioning. A wind turbine may also be parked for e.g. maintenance reasons after operating normally, i.e. producing energy, or in case of a prolonged grid loss.

Throughout the present disclosure, protruding beyond the leading edge in a local chordwise direction may mean that a portion configured to protrude beyond the leading edge is arranged in front of the leading edge. In some examples, the protruding portion may extend along an axis having an angle α between -20º to +20º with a local chord, more in particular between -10º and +10º, and more in particular between -5º and +5º. Such angle may be measured in a plane including a local chordwise direction and substantially perpendicular to a local leading edge direction. A local chordwise direction may therefore represent 0º.

In a further aspect of the disclosure, a method according to claim <NUM> for mitigating vibrations of a parked wind turbine comprising one or more wind turbine blades is provided. A wind turbine blade comprises a root, a tip and exterior surfaces defining a pressure side, a suction side, a leading edge and a trailing edge, each surface extending in a generally spanwise direction from the root to the tip. The method comprises releasably attaching a device comprising a portion configured to protrude substantially in a local chordwise direction beyond the leading edge of the wind turbine blade around a wind turbine blade along a local chordwise direction.

Still in a further aspect of the disclosure, a method for detaching a device from a leading edge of a wind turbine, the device comprising a suction side piece and a pressure side piece, each side piece including a portion configured to protrude substantially in a chordwise direction beyond the leading edge, is provided. The method comprises pulling a release rope for separating the pressure side and suction side pieces when the leading edge is pointing downwards or upwards before starting to operate the wind turbine.

Each example is provided by way of explanation of the invention, not as a limitation of the invention.

The rotor blades <NUM> are mated to the hub <NUM> by coupling a blade root region <NUM> to the hub <NUM> at a plurality of load transfer regions <NUM>.

In examples, the rotor blades <NUM> may have a length ranging from about <NUM> meters (m) to about <NUM> or more. Rotor blades <NUM> may have any suitable length that enables the wind turbine <NUM> to function as described herein. For example, non-limiting examples of blade lengths include <NUM> or less, <NUM>, <NUM>, <NUM>, <NUM> or a length that is greater than <NUM>. As wind strikes the rotor blades <NUM> from a wind direction <NUM>, the rotor <NUM> is rotated about a rotor axis <NUM>. As the rotor blades <NUM> are rotated and subjected to centrifugal forces, the rotor blades <NUM> are also subjected to various forces and moments. As such, the rotor blades <NUM> may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position.

In the example, the wind turbine controller <NUM> is shown as being centralized within the nacelle <NUM>, however, the wind turbine controller <NUM> may be a distributed system throughout the wind turbine <NUM>, on the support system <NUM>, within a wind farm, and/or at a remote-control center. The wind turbine controller <NUM> includes a processor <NUM> configured to perform the methods and/or steps described herein. Further, many of the other components described herein include a processor.

As used herein, the term "processor" is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific, integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. It should be understood that a processor and/or a control system can also include memory, input channels, and/or output channels.

<FIG> is an enlarged sectional view of a portion of the wind turbine <NUM>. In the example, the wind turbine <NUM> includes the nacelle <NUM> and the rotor <NUM> that is rotatably coupled to the nacelle <NUM>. More specifically, the hub <NUM> of the rotor <NUM> is rotatably coupled to an electric generator <NUM> positioned within the nacelle <NUM> by the main shaft <NUM>, a gearbox <NUM>, a high-speed shaft <NUM>, and a coupling <NUM>. In the example, the main shaft <NUM> is disposed at least partially coaxial to a longitudinal axis (not shown) of the nacelle <NUM>. A rotation of the main shaft <NUM> drives the gearbox <NUM> that subsequently drives the high-speed shaft <NUM> by translating the relatively slow rotational movement of the rotor <NUM> and of the main shaft <NUM> into a relatively fast rotational movement of the high-speed shaft <NUM>. The latter is connected to the generator <NUM> for generating electrical energy with the help of a coupling <NUM>. Furthermore, a transformer <NUM> and/or suitable electronics, switches, and/or inverters may be arranged in the nacelle <NUM> in order to transform electrical energy generated by the generator <NUM> having a voltage between 400V to <NUM> V into electrical energy having medium voltage (<NUM> - <NUM> KV). Said electrical energy is conducted via power cables from the nacelle <NUM> into the tower <NUM>.

The gearbox <NUM>, generator <NUM> and transformer <NUM> may be supported by a main support structure frame of the nacelle <NUM>, optionally embodied as a main frame <NUM>. The gearbox <NUM> may include a gearbox housing that is connected to the main frame <NUM> by one or more torque arms <NUM>. In the example, the nacelle <NUM> also includes a main forward support bearing <NUM> and a main aft support bearing <NUM>. Furthermore, the generator <NUM> can be mounted to the main frame <NUM> by decoupling support means <NUM>, in particular in order to prevent vibrations of the generator <NUM> to be introduced into the main frame <NUM> and thereby causing a noise emission source.

In some examples, the wind turbine may be a direct drive wind turbine without gearbox <NUM>. Generator <NUM> operate at the same rotational speed as the rotor <NUM> in direct drive wind turbines. They therefore generally have a much larger diameter than generators used in wind turbines having a gearbox <NUM> for providing a similar amount of power than a wind turbine with a gearbox.

For positioning the nacelle <NUM> appropriately with respect to the wind direction <NUM>, the nacelle <NUM> may also include at least one meteorological measurement system which may include a wind vane and anemometer. The meteorological measurement system <NUM> can provide information to the wind turbine controller <NUM> that may include wind direction <NUM> and/or wind speed. In the example, the pitch system <NUM> is at least partially arranged as a pitch assembly <NUM> in the hub <NUM>. The pitch assembly <NUM> includes one or more pitch drive systems <NUM> and at least one sensor <NUM>. Each pitch drive system <NUM> is coupled to a respective rotor blade <NUM> (shown in <FIG>) for modulating the pitch angel of a rotor blade <NUM> along the pitch axis <NUM>. Only one of three pitch drive systems <NUM> is shown in <FIG>.

In the example, the pitch assembly <NUM> includes at least one pitch bearing <NUM> coupled to hub <NUM> and to a respective rotor blade <NUM> (shown in <FIG>) for rotating the respective rotor blade <NUM> about the pitch axis <NUM>. The pitch drive system <NUM> includes a pitch drive motor <NUM>, a pitch drive gearbox <NUM>, and a pitch drive pinion <NUM>. The pitch drive motor <NUM> is coupled to the pitch drive gearbox <NUM> such that the pitch drive motor <NUM> imparts mechanical force to the pitch drive gearbox <NUM>. The pitch drive gearbox <NUM> is coupled to the pitch drive pinion <NUM> such that the pitch drive pinion <NUM> is rotated by the pitch drive gearbox <NUM>. The pitch bearing <NUM> is coupled to pitch drive pinion <NUM> such that the rotation of the pitch drive pinion <NUM> causes a rotation of the pitch bearing <NUM>.

Pitch drive system <NUM> is coupled to the wind turbine controller <NUM> for adjusting the pitch angle of a rotor blade <NUM> upon receipt of one or more signals from the wind turbine controller <NUM>. In the example, the pitch drive motor <NUM> is any suitable motor driven by electrical power and/or a hydraulic system that enables pitch assembly <NUM> to function as described herein. Alternatively, the pitch assembly <NUM> may include any suitable structure, configuration, arrangement, and/or components such as, but not limited to, hydraulic cylinders, springs, and/or servomechanisms. In certain embodiments, the pitch drive motor <NUM> is driven by energy extracted from a rotational inertia of hub <NUM> and/or a stored energy source (not shown) that supplies energy to components of the wind turbine <NUM>.

The pitch assembly <NUM> may also include one or more pitch control systems <NUM> for controlling the pitch drive system <NUM> according to control signals from the wind turbine controller <NUM>, in case of specific prioritized situations and/or during rotor <NUM> overspeed. In the example, the pitch assembly <NUM> includes at least one pitch control system <NUM> communicatively coupled to a respective pitch drive system <NUM> for controlling pitch drive system <NUM> independently from the wind turbine controller <NUM>. In the example, the pitch control system <NUM> is coupled to the pitch drive system <NUM> and to a sensor <NUM>. During normal operation of the wind turbine <NUM>, the wind turbine controller <NUM> may control the pitch drive system <NUM> to adjust a pitch angle of rotor blades <NUM>.

According to an embodiment, a power generator <NUM>, for example comprising a battery and electric capacitors, is arranged at or within the hub <NUM> and is coupled to the sensor <NUM>, the pitch control system <NUM>, and to the pitch drive system <NUM> to provide a source of power to these components. In the example, the power generator <NUM> provides a continuing source of power to the pitch assembly <NUM> during operation of the wind turbine <NUM>. In an alternative embodiment, power generator <NUM> provides power to the pitch assembly <NUM> only during an electrical power loss event of the wind turbine <NUM>. The electrical power loss event may include power grid loss or dip, malfunctioning of an electrical system of the wind turbine <NUM>, and/or failure of the wind turbine controller <NUM>. During the electrical power loss event, the power generator <NUM> operates to provide electrical power to the pitch assembly <NUM> such that pitch assembly <NUM> can operate during the electrical power loss event.

In the example, the pitch drive system <NUM>, the sensor <NUM>, the pitch control system <NUM>, cables, and the power generator <NUM> are each positioned in a cavity <NUM> defined by an inner surface <NUM> of hub <NUM>. In an alternative embodiment, said components are positioned with respect to an outer surface of hub <NUM> and may be coupled, directly or indirectly, to the outer surface.

A schematic perspective view of a wind turbine blade <NUM>, e.g. one of the rotor blades <NUM> shown in <FIG>, is illustrated as an example in <FIG>. The rotor blade <NUM> includes a blade root <NUM>, a blade tip <NUM>, a leading edge <NUM> and a trailing edge <NUM>. The blade root <NUM> is configured for mounting the rotor blade <NUM> to the hub <NUM> of a wind turbine <NUM>. The wind turbine blade <NUM> extends lengthwise between the blade root <NUM> and the blade tip <NUM>. A span <NUM> defines a length of the rotor blade <NUM> between said blade root <NUM> and blade tip <NUM>. A chord <NUM> at a given position of the blade is an imaginary straight line joining the leading edge <NUM> and the trailing edge <NUM>, the cross-section generally having airfoil shaped cross-section. As is generally understood, a chordwise direction is substantially perpendicular to a spanwise direction. Also, the chord <NUM> may vary in length <NUM> as the rotor blade <NUM> extends from the blade root <NUM> to the blade tip <NUM>. The wind turbine blade <NUM> also includes a pressure side <NUM> and a suction side <NUM> extending between the leading edge <NUM> and the trailing edge <NUM>. A tip region <NUM> may be understood as a portion of a wind turbine blade <NUM> that includes the tip <NUM>. A tip region may have a length of <NUM>%, <NUM>%, or <NUM>% of the span or less. A root region <NUM> may be understood as a portion of the blade that includes root <NUM>. A root region may have a length of e.g. <NUM>%, <NUM>% of the span or less.

The rotor blade <NUM>, at different spanwise positions, has different aerodynamic profiles and thus can have airfoil shaped cross-sections <NUM>, such as a symmetrical or cambered airfoil-shaped cross-section. Close to a root of the blade, the cross-section of the blade may be rounded, even circular or almost circular. Closer to a tip of the blade, the cross-section of the blade may be thinner and may have an airfoil shape.

When a wind turbine is parked or stopped, vibrations caused by the air flowing around the wind turbine, in particular around the wind turbine blades, may stress and damage the wind blades and the wind turbine. The wind turbine rotor may or may not be locked in these situations.

At least two types of oscillations or vibrations may happen particularly when the turbine is parked. The first ones are so-called vortex induced vibrations (VIVs), and may arise when an angle of attack for a blade or airfoil portion is around <NUM> degrees. Vortex shedding may contribute to enhance the wind turbine blade oscillation. The second type of oscillations are stall induced vibrations (SIVs), and may arise when the angle of attack is close to stall angles (e.g. -<NUM> degrees to +<NUM> degrees). The angle of attack may be understood as a geometrical angle between a flow direction of the wind and the chord of a rotor blade or a local chord of a rotor blade section.

<FIG> schematically show different examples of devices for reducing vibrations in a wind turbine. Any of these devices <NUM> is suitable for mitigating vibrations of a wind turbine <NUM> during standstill of the rotor. A device <NUM> comprises a portion <NUM> configured to protrude substantially in a local chordwise direction beyond a leading edge <NUM> of the wind turbine blade <NUM>. A device <NUM> is configured to be attached around the blade substantially along a local chordwise direction.

Devices <NUM> as described herein may reduce vibrations when the wind turbine is parked. The performance of the wind turbine is not negatively affected as the device is normally removed before the wind turbine starts normal operation. Devices <NUM> may be particularly useful during installation and/or commissioning of a wind turbine. They may also be useful if the wind turbine is stopped, e.g. for maintenance.

<FIG> schematically illustrates a cross-sectional view of a device <NUM> arranged around a wind turbine blade <NUM>. <FIG> schematically illustrates a top view of a blade <NUM> with three devices <NUM> similar to the one illustrated in <FIG> attached to the blade <NUM>.

A device <NUM> may comprise a suction side piece <NUM> and a pressure side piece <NUM>. The suction side piece is configured to be arranged on the suction side <NUM> of the wind turbine blade and the pressure side piece is configured to be arranged on the pressure side <NUM> of the wind turbine blade <NUM>, as illustrated in <FIG>. In this example, both of the suction side and pressure side pieces comprise a portion <NUM> configured to protrude beyond the leading edge <NUM>.

The suction and pressure side pieces may create a contour or profile of the leading edge <NUM> which varies along the leading edge. , the contour of the leading edge becomes irregular with one or more of these devices <NUM> arranged on the blade <NUM>. Therefore, the frequency at which vortices are shed may vary between blade sections, i.e. portions of the blade at different spanwise locations, having a device <NUM> attached and sections which do not. Thus, vibration of contiguous blade sections may happen at different times, and the vibrations of some sections may be compensated with the vibrations of other sections. For example, as illustrated in <FIG>, sections S1 without a device <NUM> may shed vortices at a certain frequency which is different from the frequency at which vortices may be shed from sections S2 having a device <NUM> attached to blade having a portion <NUM> protruding from the leading edge <NUM>.

Each of the suction side piece <NUM> and pressure side piece <NUM> may have a front projection <NUM>, <NUM>. A front projection may be understood as a portion <NUM> configured to protrude beyond a leading edge <NUM> of the wind turbine blade <NUM>. A front projection may be sharp, e.g. it may have sharp, pointed or spiky edges in cross-section. , there may be a certain angle between the surfaces of a front projection, e.g. of less than <NUM>º (degrees). The angle could be about <NUM>º or between <NUM>º and less than <NUM>º in other examples. In some examples, a piece may have a front projection with a triangular or beveled cross section. A front projection may be single beveled or double beveled. If double beveled, the bevel angles of each bevel may be different. In <FIG>, the portions configured to protrude beyond the leading edge are sharp, e.g. have sharp edges in cross-section.

A gap <NUM> may be provided between the front projections <NUM>, <NUM> of the suction side <NUM> and pressure side <NUM> pieces when attached to a blade <NUM>. , a gap <NUM> extending in a direction substantially perpendicular to a chord and a length of a wind turbine may be provided after connecting the suction side piece <NUM> and the pressure side piece <NUM> along the leading edge <NUM> of the wind turbine blade. The gap may also extend along the leading edge. The pressure <NUM> and suction <NUM> side pieces may be configured to this end, e.g. a certain shape of the pieces, and in particular of front projections <NUM>, <NUM>, may be chosen.

Front projections <NUM>, <NUM> separated by a gap <NUM> may create a non-aerodynamic contour of a leading edge <NUM>, or at least a less aerodynamic contour with respect to a leading edge without the device <NUM>. A non-aerodynamic contour may be understood as a non-streamlined or bluff body, in particular a body that, as a result of its shape, has separated flow over a substantial part of its surface. Such a contour may increase drag and avoid or reduce lift creation. SIVs may therefore be avoided or at least reduced by device <NUM>. Sharp front projections may increase this effect.

The portion of a piece <NUM>, <NUM> which is not the front projection <NUM>, <NUM> may be the main body <NUM>, <NUM> of the piece. In some examples, the main body may be configured to extend in a chordwise direction, e.g. up to at least <NUM>% of the chord as in <FIG>. The main body of the piece may be longer than the front projection of the piece along the chordwise direction. In these examples, the main body may be configured to disrupt crossflow, i.e. air flowing from the root <NUM> to the tip <NUM> or from the tip to the root. The main body may have a suitable height (measured perpendicularly to a to a spanwise direction ant to a chordwise direction) to this end. Perturbating crossflow may decorrelate vortex shedding along a length of the blade and may reduce VIVs.

The main body and the front projection of a piece may be about the same length along a chordwise direction in other examples. A piece (both the main body and the front projection) may also extend along a length of the blade, as illustrated in <FIG>.

Pieces <NUM>, <NUM> may be lightweight. They may float on water. A piece <NUM>, <NUM> may be rigid, i.e. configured to keep its shape. Thus, it may not bend under the action of the incoming wind when placed on a leading edge. Either of the suction and pressure side pieces may be made of foam, rubber or plastic. A piece may have a hollow inside, as a life buoy or rescue can.

Removable attachment <NUM> between pieces <NUM>, <NUM>, and in particular between front projections <NUM>, <NUM>, may be a zipper or one or more hook-and-loop fasteners, e.g. Velcro™. Other removable attachments are possible. Removable attachment enables using the device <NUM> more than once.

Device <NUM> may further comprise a trailing edge piece <NUM>. The trailing edge piece <NUM> may be configured to be attached to a trailing edge <NUM>. For example, a trailing edge piece may be clamped to the trailing edge, as illustrated in <FIG>. The trailing edge piece may be configured to extend along a trailing edge, as shown in <FIG>. Trailing edge piece <NUM> may provide protection against serrations <NUM> of the trailing edge. It may additionally or alternatively avoid or reduce damages to the serrated edges by the device <NUM>.

A leading edge piece <NUM>, <NUM> and a trailing edge piece <NUM> may have a similar or substantially equal length. Herein the length refers to the dimension of the piece configured to extend along the leading edge or the trailing edge.

A trailing edge piece <NUM> may be rounded, for example as shown in <FIG>. A rounded trailing edge piece <NUM> may change the profile of the trailing edge <NUM>, similarly as the protruding portions <NUM>, <NUM> do on the leading edge. With regard to the trailing edge, transitions between a relatively sharp, non-round, contour in the absence of a trailing edge piece <NUM> and a rounded or more rounded contour in the presence of the trailing edge piece <NUM> occur. This may enhance the effect of varying the frequency of the vortex shedding caused by the protruding portions <NUM>, <NUM>.

A trailing edge piece <NUM> may be made of foam, or in general of any material suitable for avoiding or reducing damage caused to or by serrations <NUM>.

Pressure and suction side pieces, as well as trailing edge pieces, may also protect a wind turbine blade <NUM> during transport if the devices <NUM> are attached to the blade before transporting them to an installation site.

A device <NUM> may further comprise one or more ropes or straps <NUM> connecting the suction side piece and the pressure side piece, the one or more ropes or straps being configured to secure the suction <NUM> and pressure <NUM> side pieces to a wind turbine blade <NUM>. The straps <NUM> may enable attaching a device <NUM> around a blade <NUM> along a chordwise direction. A strap may be configured to adapt to a certain extent to a contour of the blade <NUM> in a chordwise direction. A strap <NUM> may include any suitable tightener mechanism for the securing. For example, ratchet straps may be used.

In an example where a trailing edge piece <NUM> is absent, two straps <NUM> may be provided: one connecting a first longitudinal end <NUM> of a suction side piece <NUM> to a first (same) longitudinal end of the pressure side piece <NUM>; and another one connecting a second, opposite to the first, longitudinal end <NUM> of the suction side piece <NUM> to a second, opposite to the first, longitudinal end of the pressure side piece <NUM>. A strap <NUM> may be tightened for securing the device <NUM> to the blade <NUM>. The straps <NUM> may be substantially aligned along a chordwise direction of the blade once tightened. In some examples, attaching the removable attachment <NUM> between piece <NUM> and <NUM>, e.g. along a leading edge <NUM>, may be sufficient for tightening the straps <NUM>.

Similarly, one or more straps <NUM> may be provided when a trailing edge piece <NUM> is present. In some examples, a single strap may be used to connect the suction <NUM> and pressure <NUM> side pieces, the strap also being in contact with the trailing edge piece <NUM>. In some of these examples, one strap may connect first longitudinal ends <NUM> of pieces <NUM> and <NUM>, and another strap may connect second (opposite) longitudinal ends <NUM> of pieces <NUM> and <NUM>. In some other examples, a first strap may be provided between the suction side piece <NUM> and the trailing edge piece <NUM>, and a second strap may be provided between the pressure side piece <NUM> and the trailing edge piece <NUM>. In some of these examples, two straps may connect first longitudinal ends <NUM> of pieces <NUM> and <NUM>, and another two straps may connect second (opposite) longitudinal ends <NUM> of pieces <NUM> and <NUM>. This may likewise apply to a device <NUM> where a trailing edge piece is absent. For example, a strap joined to a longitudinal end <NUM>, <NUM> of a suction side piece <NUM> may be configured to be releasably attached to a strap joined to a (same) longitudinal end <NUM>, <NUM> of a pressure side piece <NUM>. A buckle fastener may be used to join and tighten the two straps around the blade <NUM>.

A strap <NUM> may be fixedly joined to a suction side <NUM> and/or pressure side <NUM> piece in some examples. In other examples, a strap may be releasably joined to a piece <NUM>, <NUM>. , a strap <NUM> and a piece <NUM>, <NUM> may be provided as separate elements, and joined to one another when mounting a device <NUM> to a wind turbine blade <NUM>.

Straps <NUM> may enable a versatile device <NUM> which may be secured at any desired position along a blade length. The surface of pieces <NUM>, <NUM> configured to face a blade surface may not need to completely touch the blade. Partial contact may be sufficient. Accordingly, a device <NUM> may be used with blade portions with different chord length and curvatures as well as with different wind turbine blades.

A suction or pressure side piece may have an anchor point <NUM> (see figures 11A-11E). An anchor point may keep a release rope <NUM> joined to the device <NUM> at least during the release of the device <NUM> from a blade <NUM>. An anchor point may be additionally or alternatively provided in a trailing edge piece <NUM> if present. In such a case, trailing edge piece may have two portions, e.g. suction and pressure side pieces, which may releasably attached; and the suction and pressure side pieces may be attached to one another in a fixed or in a releasable manner.

A device <NUM> may be configured to extend between a <NUM>% and a <NUM>% of a blade length, and more in particular between <NUM>% and <NUM>%, in some examples. One or more devices <NUM> may be attached to a wind turbine blade.

<FIG> schematically show another example of devices for reducing vibrations in a wind turbine.

In this example, the device comprises a tip fastener <NUM> attached or attachable to a first longitudinal end <NUM> of the projection <NUM> configured to protrude beyond the leading edge <NUM>. The projection <NUM> is configured to extend from the tip fastener <NUM> along the leading edge <NUM>. Projection <NUM> can change the rather rounded contour of the leading edge to a more sharp or angular one. The shape of the projection <NUM> can be adapted to do so. For example, the projection <NUM> may be sharp, e.g. it may have sharp edges in cross section. In some examples, the projection <NUM> may have a squared shape in cross-section. In other examples, the projection <NUM> may have other non-rounded, and therefore sharp, shapes.

The tip fastener <NUM> is configured to fit around a tip region <NUM>. The tip fastener <NUM> may be configured to adapt to a certain extent to a contour of the blade <NUM> in a chordwise direction. The tip fastener <NUM> may be provided in an open or a closed state. In an open state, the tip fastener <NUM> may have two ends which may be joined, e.g. around a blade in a chordwise direction. In a closed state, the tip fastener may be slid around the blade in a longitudinal direction of the blade. The tip fastener <NUM> may have circular or annular shape when not fastened to the blade. The tip fastener <NUM> may be a tip ring. The tip fastener <NUM> may be made of a bendable and robust material. In some examples, the tip fastener <NUM> may comprise one or more straps.

In some examples, the tip fastener <NUM> may be provided separately from the projection <NUM> and they may be joined to the projection <NUM> when mounting the device <NUM> to a blade <NUM>.

<FIG> shows a cross-sectional view of a wind turbine blade <NUM> with a substantially squared projection <NUM> arranged on the leading edge <NUM>. As may be seen in this figure, the projection <NUM> may have sharp or angular edges in cross section, e.g. about <NUM>º. <FIG> shows a top view of a wind turbine blade <NUM> with a device <NUM> attached to it. In other examples the projection <NUM> may be triangular or have any other sharp, non-round, contour.

Vortex shedding may be described by a dimensionless number called the Strouhal number St. The Strouhal number is usually defined as St = f·C/V, where f is the frequency of vortex shedding, C is the characteristic length (for an airfoil it is the projected width perpendicular to the flow direction, for example blade thickness) and V is the air flow speed.

When the frequency of vortex shedding reaches a so-called natural frequency of the wind turbine blade <NUM>, resonance may increase the amplitude of vibrations. Following the formula above, the frequency of vortex shedding may be expressed as a function of the air flow speed as f = St/C · V.

A leading edge, which is substantially rounded, has a Strouhal number St higher than a non-round leading edge, e.g. a leading edge with a projection <NUM>. According to the latest formula, and assuming that the blade with a rounded leading edge and a blade with a non-rounded leading edge have a same characteristic length C, the air flow speed V would need to be higher in the blade having a non-rounded leading edge in order to obtain a frequency of vortex shedding f which is equal for both blades.

Therefore, a blade with a non-rounded edge may require a higher air flow speed V in order to reach a frequency of vortex shedding f equal to a natural frequency of the blade than a blade with a rounded edge. Therefore, a natural frequency for a blade with a non-rounded edge may occur less frequently, thus reducing vortex induced vibrations. Risk of increased amplitude vibration due to resonance may accordingly be reduced.

A projection <NUM> may also act as a spoiler, increasing drag and therefore delaying stall. Accordingly, SIV risk may also be reduced.

At least the side of the projection <NUM> which is to contact the leading edge <NUM> may be made of a soft material, for example of foam. The projection <NUM> may include rubber or plastic. The fitting of the projection to the blade may be better in this way, and also damage to the blade may be avoided. A soft material, e.g. foam, may enable using device <NUM> with different blades.

The device <NUM> may further comprise a handling rope <NUM> attached or attachable to a second longitudinal end <NUM> of the projection <NUM>. The second longitudinal end <NUM> of the projection <NUM> is opposite to the first longitudinal end <NUM> of the projection <NUM>. When joined to the leading edge, the second longitudinal end of the projection <NUM> is closer to the root of the blade and the first longitudinal end of the projection is closer to the tip of the blade.

The handling rope <NUM> may help to keep the projection <NUM> fixed to, in contact with or clinging to the leading edge <NUM>. When a handling rope is present <NUM>, a first end <NUM> of the rope <NUM> may be attached to the second longitudinal end <NUM> of the projection <NUM>, and a second opposite end <NUM> of the rope may be attached to the blade <NUM>, the nacelle <NUM> or the hub <NUM>. Attachment may be direct, e.g. to an anchor point, or may be indirect, e.g. a fastener connecting the handling rope <NUM> to the blade, hub or nacelle may be provided.

In some examples, a root fastener <NUM> may be provided as a fastener to a blade. If the rope <NUM> is to be attached to the hub or the nacelle, the rope may also be secured to the blade for having the projection <NUM> fixed or clinging to the blade. A rope retaining element may be provided to this end. The rope retaining element may be provided as part of the device <NUM> or separately. In some examples, a rope retaining element may be attached to a blade surface and then the retaining element and the rope may be removably attached.

The device may further comprise a root fastener <NUM> attached or attachable to a second longitudinal end <NUM> of the projection <NUM>. The root fastener <NUM> may also be attached or attachable to the handling rope <NUM> if the handling rope is present. In some examples, the root fastener <NUM> may be attached or attachable to the second end <NUM> of the rope, i.e. the end which is not joined or attachable to the projection <NUM>.

A root fastener <NUM> may be configured to fit around a more central region of the blade or to a root region <NUM> of the blade. Like the tip fastener <NUM>, the root fastener <NUM> may have circular or annular shape when not fastened to the blade. The root fastener <NUM> may be configured to adapt to a contour of the blade <NUM> in a chordwise direction. The root fastener <NUM> may be provided in an open state in some examples, but it may be provided in a closed state in other examples. The explanations with regard to the tip fastener are in general applicable to the root fastener.

A projection <NUM> may be elongated, see <FIG>. A projection <NUM> may be configured to extend along <NUM> and <NUM>% of the span along the leading edge. A device <NUM> may be configured to extend between <NUM>% and <NUM>%, in particular between <NUM> and <NUM>%, along the leading edge. This may particularly apply if a root fastener <NUM> and a handling rope <NUM> are used. In some of these examples, the handling rope <NUM> may be shorter than the projection <NUM>.

In other examples, for instance if the handling rope <NUM> is to be attached to the hub or the nacelle, the handling rope may be longer than the projection <NUM>. In these examples, a root fastener <NUM> may be present or may be absent.

Once mounted to a blade <NUM>, projection <NUM> may be in part loose. The surface of projection <NUM> configured to face a blade surface may not need to completely touch the blade, in particular during use. As projection <NUM> may be secured by its ends, it may move with respect to the blade, e.g. separate from the leading edge <NUM>, when subjected to an air flow. Such movement may add drag and may therefore help in damping vibrations, both VIVs and SIVs.

A wind turbine blade (<NUM>) comprising one or more devices (<NUM>) according to the examples of <FIG> or one or more devices according to the examples of <FIG> may be provided. A wind turbine (<NUM>) comprising one or more of such wind turbine blades (<NUM>) may also be provided.

In another aspect, a method <NUM> for mitigating vibrations of a parked wind turbine comprising one or more wind turbine blades, a blade having a root, a tip and exterior surfaces defining a pressure side, a suction side, a leading edge and a trailing edge, each surface extending in a generally spanwise direction from the root the tip, is provided. A wind turbine may be parked during installation, commissioning and maintenance of the wind turbine.

As illustrated in <FIG>, the method comprises, at block <NUM>, releasably attaching a device <NUM> comprising a projection <NUM> configured to protrude substantially in a local chordwise direction beyond the leading edge <NUM> of a wind turbine blade <NUM> around a wind turbine blade <NUM> along a chordwise direction.

Method <NUM> may further include, at block <NUM>, removing the device from the blade before starting to operate the wind turbine. As disclosed herein, several methods for releasably attaching one or more devices to the leading edge, and for remove them later on, are possible. As this step is optional, it has been indicated in a dashed box in <FIG>.

Any of the devices described with respect to <FIG> may be releasably attached to the blade. When mounted to a wind turbine blade, the VIVs and/or SIVs acting on the wind turbine blade <NUM>, and in general on the wind turbine <NUM>, may be mitigated.

In some examples, releasably attaching may comprise at least one of surrounding the wind turbine blade <NUM> with the device and sliding the device <NUM> around the wind turbine blade.

An example of attaching a device <NUM> by surrounding the wind turbine blade <NUM> with it is provided in <FIG> schematically show cross-sectional views of a blade <NUM> and a device <NUM> according to the description related to <FIG> and <FIG> being mounted to the blade <NUM>.

As illustrated in <FIG>, surrounding the blade may comprise joining the suction <NUM> and pressure <NUM> side pieces via removable attachment <NUM> along the leading edge <NUM>. A zipper or a hook-and-loop fastener like Velcro ™ may be used. Then, if necessary, one or more straps or ropes <NUM> may be tightened in order to secure the device <NUM> to the blade <NUM>, see e.g. <FIG>. The one or more straps <NUM> may connect the suction and pressure side pieces over the trailing edge <NUM> along a substantially chordwise direction.

In <FIG>, an open device <NUM> may be arranged on top of a trailing edge <NUM> which points upwards, e.g. on top of a horizontal wind turbine blade <NUM> with the trailing edge <NUM> pointing upwards, such that a suction side piece <NUM> is placed on (or near) the suction side of the blade, and a pressure side piece <NUM> is placed on (or near) the pressure side of the blade. An open device may be herein understood as a device having the suction and pressure side pieces not joined to one another.

Upwards may for example be vertically upwards. This may facilitate mounting the device <NUM> to the blade <NUM>, but it is not necessary that the trailing edge points exactly vertically upwards. The blade position may be such that a chordwise direction may have a certain angle with a vertical direction in some examples. For example, an angle between <NUM> and less than <NUM>º or <NUM>º (or between -<NUM>º or -<NUM>º and <NUM>º) with an axis substantially parallel to a chordwise direction may be possible.

Likewise, the blade may not need to be positioned horizontally, but doing so may facilitate installation of the device <NUM>. In some examples, a spanwise direction of the blade may have a certain angle with a horizontal direction, e.g. an angle between <NUM> and <NUM> or <NUM>º.

Horizontally may refer to a three or nine o'clock position of the blade if the blade is already mounted to the hub <NUM>, for example if the device <NUM> is mounted to a rotor installed atop a wind turbine tower.

One or more devices <NUM> may be releasably attached to a wind turbine blade before the blade is joined to the hub <NUM> or lifted to be joined to a hub already mounted on top of a tower <NUM>. One or more devices <NUM> may be releasably attached to a wind turbine blade when the blade <NUM> is already joined to a hub <NUM> on top of a tower <NUM>.

If the device <NUM> comprises a trailing edge piece <NUM>, the trailing edge piece <NUM> may be placed over the trailing edge and the suction <NUM> and pressure <NUM> side pieces may be placed in the corresponding side of the blade.

A release rope <NUM> may be passed through an anchor point <NUM> (see figures 11A-11E) and between the suction <NUM> and pressure <NUM> side pieces. If the device is mounted on a support surface, e.g. the ground <NUM>, this may be performed before installing the blade <NUM> or the rotor <NUM> on top of the tower <NUM>. Passing the rope <NUM> between the suction side <NUM> and pressure <NUM> side pieces may enable separating the pieces <NUM>, <NUM> when pulling the rope <NUM>. The anchor point <NUM> (see <FIG>) may keep the release rope <NUM> and a piece <NUM>, <NUM> connected once the pieces have been separated, which may help to control the lowering of the device <NUM>. The anchor point <NUM> may be in a (longitudinal) end of a suction side or pressure side piece, e.g. in a front projection <NUM>, <NUM>.

More than one device <NUM> may be mounted to a wind turbine blade <NUM> in this way. For example, between <NUM> and <NUM> devices <NUM> may be placed on a blade <NUM> for mitigating vibrations of a parked wind turbine.

In some examples, the removable attachment <NUM> may be located in the trailing edge piece <NUM>. In these examples, the pressure and suction side pieces may be non-detachably connected or detachably connected. The pressure and suction side pieces may be placed around the leading edge and the device <NUM> may be joined to the blade <NUM> by closing a releasable attachment in the trailing edge piece. One or more straps, e.g. ratchet straps, may be used to adjust, e.g. tighten, the device <NUM> to the blade.

In some examples, a device <NUM> may be installed on a blade before transporting the blade <NUM> to an installation site. At least a portion of the leading edge of the blade, and optionally of the trailing edge, may be protected from damage during transportation.

An example of a method <NUM> for attaching a device <NUM> by surrounding the wind turbine blade <NUM> with it and/or sliding it around the wind turbine is provided in <FIG>. The steps indicated below may particularly apply to the device described with respect to <FIG>.

The method comprises, at block <NUM>, arranging (attaching) a tip fastener <NUM> around a tip region <NUM> of the blade <NUM> along a substantially chordwise direction for attaching a first longitudinal end <NUM> of the projection <NUM> configured to protrude beyond the leading edge. The tip fastener <NUM> may be already connected to the projection <NUM> before arranging it around the blade in some examples, but in other examples the tip fastener and the portion may be joined when the tip fastener is arranged, e.g. fastened, around the blade.

The tip fastener <NUM> may be fastened by joining two portions of the tip fastener <NUM> in some examples. For example, this may be easily performed when the device <NUM> is attached to a blade <NUM> on the ground. The blade may be in a substantially horizontal position for fastening the tip fastener <NUM> around it.

In some other examples, the tip fastener <NUM> may be slid around the tip region <NUM>. The tip fastener may therefore be in a closed position. The tip fastener may be slid up to a point in which it grips the blade and beyond which it cannot continue sliding. For example, a radius of the tip fastener may become substantially equal to a chord of the blade. Sliding the tip fastener <NUM> may be performed before lifting the blade on a wind turbine tower.

The blade may be in a substantially vertical position for sliding the tip fastener <NUM> around it in some examples. For example, a lifting device such as a crane may lift the wind turbine blade by its root or a root region <NUM> such that the tip is pointing vertically downwards. The tip fastener may be then slid around the blade upwardly.

The method may further comprise, at block <NUM>, attaching, e.g. fastening, a second longitudinal end of the projection <NUM> to the blade <NUM>, e.g. around a central or a root region <NUM> of the blade <NUM>. A root fastener <NUM> may be used in some examples.

The projection <NUM> may first be extended along the leading edge. Then, a tip fastener and a root fastener may be fastened around the blade for securing the projection <NUM> to the blade. Alternatively, a fastener, e.g. the tip fastener, may be first attached around the blade, then the projection may be extended along the leading edge, and then the other fastener, e.g. the root fastener, may be attached around the blade. This may for example be performed with the blade in a substantially horizontal position, but the blade could also be in an inclined or vertical position.

If a handling rope <NUM> is present, the rope <NUM> attached to a second longitudinal end <NUM> of the projection <NUM> may be pulled to cling the projection <NUM> to the leading edge <NUM>. If provided separately from the projection <NUM>, the handling rope, in particular a first end of the rope <NUM>, may be attached first to a second longitudinal end <NUM>, opposite to the first longitudinal end <NUM>, of the projection <NUM>. This option is represented at block <NUM>.

In some examples, if provided separately from a root fastener <NUM>, the handling rope <NUM>, e.g. a second end of the rope <NUM>, may be attached to the root fastener <NUM>. This may be performed before or after pulling the rope.

Therefore, once the tip fastener <NUM> secures the first longitudinal end <NUM> of the projection <NUM> to a tip region <NUM>, the handling rope <NUM> may be pulled and the root fastener <NUM> may be placed around the blade.

In some examples the handling rope <NUM> may be lowered from a hub <NUM> or a nacelle <NUM> at block <NUM>. The rope <NUM> may be then joined to the projection <NUM>. This may be performed before or after attaching the first longitudinal end <NUM> of the projection <NUM> to the blade. A lifting rope may be attached to the tip fastener <NUM> and the blade may be lifted by pulling, at block <NUM>, both ropes or only the handling rope <NUM>.

It may also be possible to lift the blade by pulling from at least the handling rope but without lowering the rope <NUM> from the hub or the nacelle. For example, the rope <NUM> may be attached to the elongated projection <NUM> on the ground and then the rope <NUM> may be pulled upwardly.

The handling rope <NUM> may be directly or indirectly attached to a portion of the blade <NUM>. Direct attachment may include attaching the rope <NUM> to an anchor point of the blade. Indirect attachment may include attaching the rope to the blade by a fastener, e.g. a rope retaining element. A root fastener <NUM> may be a rope retaining element in some examples. Attaching the handling rope to the blade may be considered as an implementation of attaching a second longitudinal end of the projection <NUM> to the blade in some examples. Direct connection between the second longitudinal end of the projection and the blade may additionally be provided if a handling rope is used.

The handling rope <NUM> may be attached to the hub or the nacelle. In this case, the rope <NUM> may be kept close to the blade, e.g. by passing the rope through an eyelet on the blade or by using a rope retaining element (an eyelet may be a rope retaining element), and it may then be routed to the hub or nacelle. In this way the projection <NUM> may be cling to the blade.

In some examples, the device <NUM> may be mounted to a wind turbine blade in a rotor already placed on top of the tower. This may be the case for performing maintenance of the wind turbine, but also for installation or commissioning. In these examples, the handling rope <NUM> may be attached to the projection <NUM> and a lifting rope may be attached to the tip fastener <NUM>. The tip fastener may be in a closed state. By pulling the handling and lifting ropes, the tip fastener may be slid around a tip region <NUM> and the handling rope may be attached to the blade, hub or nacelle as explained above. The blade may be positioned vertically with the tip pointing down.

In some examples, a device <NUM> may also be installed on a blade before transporting the blade <NUM> to an installation site. The leading edge of the blade may be protected from damage during transportation.

Optional steps of method <NUM> have been indicated by dashed lines in <FIG>.

Any device <NUM> attached to a wind turbine blade <NUM> for avoiding or reducing vibrations when the wind turbine is parked may be detached from the blade before starting or resuming operation of the wind turbine.

The devices <NUM> disclosed herein may be detached in several ways from a blade before the wind turbine's operation is started or resumed.

<FIG> schematically illustrates a method <NUM> for releasing a device <NUM> from a wind turbine blade <NUM>. This method may be performed after method <NUM>. The combination of methods <NUM> and <NUM> provide a further method. The device may be in particular the one described with respect to <FIG> and <FIG>. An example of method <NUM> is further illustrated in <FIG> show two views: a cross-sectional view of a blade <NUM> and a device <NUM> being unmounted from the blade <NUM> on the left side of the figures, and a corresponding side view on the right side of the figures. A blade may have one or more devices <NUM> attached to it.

Herein detaching comprises pulling <NUM> a release rope <NUM> for separating the pressure side <NUM> and suction side <NUM> pieces when the leading edge is pointing downwards or upwards. The blade may be in a substantially horizontal position (<NUM> o'clock or <NUM> o'clock position). An example of this action is illustrated in <FIG>, where the leading edge <NUM> is pointing downwards. Before the pulling, the rope may for example extend along and between the two pieces, and it may hang from both longitudinal ends of the pieces, as shown in <FIG>. One of the pieces <NUM>, <NUM> may include an anchor point <NUM>, which may enable the release rope <NUM> to remain connected to the piece once the pressure and suction side pieces are separated, see <FIG>. Pulling may be performed from the ground, the hub, the nacelle or any suitable support. The pressure and suction side pieces may be separated from a tip towards a root direction of the blade in some examples.

The release rope <NUM> may in some examples be pulled when the leading edge <NUM> of the blade <NUM> is pointing downwards (or the trailing edge <NUM> pointing upwards), e.g. as illustrated in <FIG>. The method may comprise, see <FIG>, placing a blade <NUM> with the trailing edge <NUM> pointing upwards, e.g. vertically upwards. Pulling the release rope <NUM> may also be performed with the blade in a three or nine o'clock position. This or a close to a horizontal position of the blade may help to stabilize the device on the blade and control better its release. If not in a substantially horizontal and/or with the trailing edge <NUM> pointing upwards, the blade may be moved to one or both of them.

For example, the blade may be first positioned in a three or nine o'clock position at block <NUM> and then pitched at block <NUM> to make the trailing edge to point upwards. Then, the pressure and suction side pieces may be separated by pulling the release rope <NUM> at block <NUM>. The optional steps have been marked by dashed lines in <FIG>.

Once the suction and pressure side pieces <NUM>, <NUM> are separated, the trailing edge <NUM> of the wind turbine blade <NUM> may be made to point downwards at block <NUM>, e.g. vertically downwards. The blade may be pitched to this end. The device <NUM> may fall by the action of gravity and the release rope <NUM> may help to control the descent of the device <NUM>, see <FIG>. If gravity is not enough to make the device <NUM> to fall, pulling the rope <NUM> may help to release the device <NUM> from the blade <NUM>. Problems of the device getting stuck with serrations may also be prevented in this way.

In some other examples, the method may start by pulling the release rope <NUM> when the trailing edge of the blade <NUM> is pointing downwards, e.g. vertically downwards. The blade may also be in a horizontal (three or nine o'clock) position. For example, the blade may be placed in a three or nine o'clock position and then pitched for making the trailing edge <NUM> to point vertically downwards, similarly to <FIG>. The release rope <NUM> may be pulled and the suction <NUM> and pressure <NUM> side pieces separated. Gravity, or gravity and pulling the release rope <NUM>, may cause the device <NUM> to separate from the blade <NUM> and fall.

In some other examples where a trailing edge piece <NUM> is present, the removable attachment <NUM> may be located in the trailing edge piece <NUM>. In these examples, the pressure and suction side pieces may be non-detachably connected or detachably connected. The release rope <NUM>, when pulled, may then separate the trailing edge piece <NUM> in two portions. Gravity may help the device <NUM> to fall from the blade <NUM>.

<FIG> schematically illustrates a method <NUM> for releasing a device <NUM> from a wind turbine blade <NUM>. Method <NUM> may be applied after method <NUM>. The combination of methods <NUM> and <NUM> provide a further method. The device may be in particular the one described with respect to <FIG>.

Herein detaching comprises, at block <NUM>, separating a longitudinal end <NUM> of the portion configured to protrude beyond the leading edge <NUM> from the blade <NUM>. The end <NUM> may be called upper end in some examples and may be understood as the end which is at a highest altitude, e.g. with respect to the ground.

Separating may be performed when the blade <NUM> is pointing downwards, e.g. vertically downwards (six o'clock position). The blade may be put in this position before separating the projection <NUM>, e.g. its upper end, from the blade <NUM>. With the blade in this position, the upper end would be the end closest to the root of the blade.

Separating may comprise unfastening a root fastener <NUM> or unfastening a handling rope <NUM>. At block <NUM>, the method comprises letting the device <NUM> fall by the action of gravity. The tip fastener <NUM> may slid downwards when the root fastener or the handling rope have been released. The handling rope may be unfastened from a nacelle or a hub in some examples.

A rope may be attached to a tip fastener <NUM> before detaching the upper part of the device <NUM> and letting the device fall. A rope may be additionally or alternatively attached to a root fastener <NUM> before releasing the upper end of the device. The rope(s) may help to control the lowering and collection of the device.

In some other examples, the tip fastener <NUM> may be unfastened for separating the portion <NUM> from the blade. For example, the root and the tip fasteners may be unfastened for unmounting device <NUM>.

When reference is made to placing a wind turbine in a certain position (e.g. in a three, six or nine o'clock position), it is understood that such step is to be omitted if the blade is by whatever reason already in that position.

Claim 1:
A device (<NUM>) configured to be removably mounted to a wind turbine blade (<NUM>) having a root (<NUM>), a tip (<NUM>) and exterior surfaces extending in a generally spanwise direction from the root (<NUM>) to the tip (<NUM>) and defining a pressure side (<NUM>), a suction side (<NUM>), a leading edge (<NUM>) and a trailing edge (<NUM>);
the device (<NUM>) being configured for mitigating vibrations of a wind turbine (<NUM>) when a rotor (<NUM>) of the wind turbine (<NUM>) is in standstill; and
the device (<NUM>) being configured to be attached around the blade (<NUM>) substantially along a local chordwise direction;
the device (<NUM>) comprising a portion (<NUM>) configured to protrude beyond the leading edge (<NUM>) in a local chordwise direction which is characterized by having an angular contour comprising adjacent edges with an angle between them in cross-section