A motion controller, a floating wind turbine, a method for controlling a floating wind turbine, and a computer program product.

Abstract

A motion controller 20 is provided for a floating wind turbine 1 comprising a plurality of rotor blades, the motion controller 20 configured to adjust the blade pitch of each rotor blade when the floating wind turbine 1 is operating in winds below a rated wind speed to generate a net force that dampens surge motion of the floating wind turbine. Also provided is a method of damping motion of a floating wind turbine 1, and a wind turbine 1 having such a motion controller 20.

Description

[Technical Field]

[0001] The present invention relates to a motion controller for a floating wind turbine, a method for damping the motion of a floating wind turbine, and a wind turbine equipped with such a motion controller, and more particularly to damping surge motion of a floating wind turbine when the wind turbine is operating at wind speeds below its rated wind speed. [Background technology]

[0002] A wind turbine installation typically comprises a support structure with an elongated tower, and a nacelle with a rotor mounted on top of the support structure, the rotor comprising a plurality of rotor blades, and a generator and associated electronics typically located within the nacelle.

[0003] A wind turbine installation may be a fixed-base wind turbine, fixed to land or the seabed, or a floating wind turbine. One exemplary floating wind turbine comprises a conventional wind turbine structure attached to a buoyant base, such as a platform or raft-like structure. Another example is a "spar buoy" type structure. Such structures are formed by a vertically elongated buoyant support structure with a rotor attached to the top. The support structure may be a monolithic structure or an elongated substructure with a standard tower attached.

[0004] The floating wind turbine installation may be moored to the seabed, for example, by one or several mooring lines having anchors or attached to the seabed, or by using one or more articulated (hinged) legs to hold it in a desired location.

[0005] A fixed-foundation wind turbine is firmly anchored to land at one end. When acted upon by a force, such as a force caused by a change in wind speed or direction, a fixed-foundation wind turbine acts as a cantilever, causing the tower to bend slightly and vibrate. These motions can be of low amplitude but high frequency, i.e., small, fast movements. In contrast, a floating wind turbine is not firmly anchored to land, and as a result, in addition to the same types of tower vibrations experienced by fixed-foundation turbines, the entire slender structure can move like a rigid body.

[0006] When a floating wind turbine is acted upon by forces such as those caused by changes in wind speed or direction, or by waves, the entire structure may move in the water. These motions may be of large amplitude but relatively low frequency, i.e., large and slow. These motions are low frequency in the sense that they are much lower than the rotational frequency of the turbine / rotor itself. They are rigid body motions (as opposed to bending motions). The resulting motions are "heave", which is a linear vertical (up / down) motion (e.g., in a vertical direction perpendicular to the rotor axis); "sway", which is a linear lateral (left / right) motion (e.g., in a horizontal direction perpendicular to the rotor axis); "surge", which is a linear longitudinal (forward / aft) motion (e.g., in a direction parallel to the rotor axis); "roll", which is a rotation of an object about a horizontal (forward / aft) axis (e.g., around the rotor axis); "pitch", which is a rotation of an object about a lateral (left / right) axis (e.g., around a horizontal axis perpendicular to the rotor axis); and "yaw", which is a rotation of an object about a vertical axis (e.g., around a vertical axis perpendicular to the rotor axis).

[0007] In certain circumstances, these movements can reduce the overall efficiency or power output of the turbine, and can also generate excessive structural stresses that can damage or weaken the wind turbine structure and / or associated moorings, or can lead to instability of the wind turbine. Therefore, it is desirable to control or damp these rigid body movements.

[0008] In conventional wind turbines, the pitch of the rotor blades is controlled to regulate power output. The power output generated by the turbine is maximized at a specific wind speed, known as the rated wind speed. When operating in winds below the rated wind speed, the blade pitch is held nearly constant at an angle that provides maximum power output. In contrast, when operating in winds above the rated wind speed, the blade pitch is adjusted to produce a constant power output and prevent excessively high power output that could damage the generator and / or its associated electronics. This constant power output is sometimes referred to as the rated power of the wind turbine. In this regime, the rotor may be controlled to rotate at a constant speed, which may be referred to as the desired rotor speed and / or target rotor speed.

[0009] Wind turbines may also have a cutout wind speed, which is the wind speed at which the turbine will stop to avoid damage.

[0010] To produce maximum power output when operating below rated wind speed, the blade pitch is set to produce an optimum tip speed ratio, λ, defined as the speed at which the outer tips of the rotor blades are traveling divided by the wind speed, and is given by the following equation:

number

number

[0011] When operating below rated wind speed, the blade pitch is kept nearly constant, so that the thrust acting on the rotor (in its axial direction) increases with wind speed. Thrust is approximately proportional to the square of the wind speed relative to the rotor. As a result, axial motions that increase the relative wind speed, such as surge motions, can be damped.

[0012] In contrast, when operating in winds higher than the rated wind speed, to generate a constant power output, the blade pitch is adjusted so that rotor thrust decreases with increasing wind speed. As wind speed increases, the blade pitch is increased, i.e., made more parallel to the wind direction, reducing the thrust acting on the blades and thereby maintaining constant power. However, as thrust is reduced, the damping force acting on the wind turbine vibrations is also reduced, which can have a negative effect. In other words, vibrations can become worse, increasing their amplitude. This then causes further changes in relative wind speed and further adjustments to blade pitch, further increasing the amplitude of vibrations. The opposite is true when the wind turbine is moving away from the wind, resulting in even worse vibrations. This is known as negative damping and can significantly destabilize floating wind turbines.

[0013] US Patent Nos. 5,999,029 and 5,999,033 describe wind turbine controllers designed to address the problem of negative damping that occurs at higher than rated wind speeds and reduce axial resonant low frequency motions by adjusting the blade pitch collectively to create axial damping and / or restoring forces. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] International Publication No. 2007 / 053031 Pamphlet [Patent Document 2] International Publication No. 2010 / 076557 Brochure Summary of the Invention [Means for solving the problem]

[0015] In a first aspect, the present invention provides a motion controller for a floating wind turbine having a plurality of rotor blades, the motion controller being configured to adjust a blade pitch of each rotor blade when the floating wind turbine is operating in winds below a rated wind speed to generate a net force that damps surge motion of the floating wind turbine.

[0016] The present inventors have recognized that when a wind turbine is operating below the rated wind speed, some axial motion, particularly surge motion, may still be significant despite the natural positive damping that occurs as described above. The inventors have discovered that for operation below the rated wind speed, the overall natural damping level may be relatively low, particularly at the natural frequency of the surge. Therefore, the present invention provides a controller configured to actively damp surge motion when the wind turbine is operating below the rated wind speed, which enhances the effectiveness of natural aerodynamic damping.

[0017] Previously known motion controllers for damping the axial motion of wind turbines utilize collective pitch adjustments of the turbine blades when the wind turbine is operating in winds above rated wind speed and negative damping is a concern. These controllers typically include a blade pitch control loop that attempts to maintain a desired rotor speed by adjusting blade pitch. This is done based on rotor speed error, which is the deviation from the desired (setpoint) rotor speed. However, negative damping is not an issue below rated wind speed because blade pitch adjustments (which cause negative damping) typically do not occur in that operating region. Furthermore, wind turbines operating below rated wind speed do not maintain the rotor speed at a constant desired value and therefore do not adjust blade pitch based on rotor speed error. Therefore, known controller algorithms used to avoid negative damping above rated wind speed are not suitable for use when the wind turbine is operating below rated wind speed.

[0018] The present invention enables effective damping of surge motion of a floating wind turbine while the turbine is operating below rated wind speed by adjusting the blade pitch of each rotor blade to generate a net force that damps the surge motion.

[0019] Thus, the controller of the present invention can be used to actively damp surge motions of floating wind turbines that could damage components of the structure such as mooring lines or other securing devices, even when the wind speed is below the rated wind speed and natural aerodynamic damping is expected to prevent large surge motions.

[0020] The controller may be particularly useful for floating wind turbines located in moderate wave climate locations, where wind induced loads may dominate the entire mooring load. Thus, the present invention may, for example, allow for the reduction of excess loads on the mooring system of a floating wind turbine, thereby extending the life of the mooring system in addition to reducing the load on the wind turbine structure itself.

[0021] The problems of mooring system fatigue and surge motions experienced by floating wind turbines are particularly evident in catenary chain mooring systems, but also occur to some extent in rope mooring systems (which typically use polyester ropes). The controller may therefore be useful in conjunction with a variety of mooring systems, including these two examples, particularly when the floating wind turbine is moored (e.g., to the water bottom) by a chain mooring system. Thus, a system may be provided that includes a floating wind turbine and a mooring system (e.g., a chain or rope mooring system) for mooring the floating wind turbine, the floating wind turbine comprising a motion controller as described herein.

[0022] Another particular application of the present invention is in shared mooring systems where multiple floating wind turbines share moorings in a grid or networked arrangement. In such shared mooring systems, fatigue and surge motion problems may become more pronounced, and the present invention finds particular application therein. Thus, a system can be provided that includes multiple floating wind turbines arranged in a network (or grid) arrangement via a shared mooring system, each floating wind turbine comprising a motion controller as described herein.

[0023] Damping of surge motion can be achieved by pitching the rotor blades individually or collectively to produce an axial (i.e., axial with respect to the rotor axis) force that counteracts the axial surge speed and / or displacement. This can be achieved by dynamically adjusting the blade pitch, thus providing additional blade angles based on the surge motion, e.g., based on the surge speed and / or displacement of the floating wind turbine in the direction of the surge motion. For example, the blade pitch can be continuously and gradually adjusted while the floating wind turbine is undergoing a surge motion, thus gradually increasing and then decreasing the blade pitch based on the oscillations of the surge motion. By varying the amplitude of the additional blade pitch angle, and therefore the force, at a frequency equal to the natural frequency of the surge motion, a net damping effect can be achieved.

[0024] The surge motion of a floating wind turbine has a long natural period compared to the period of rotor rotation, which means that the surge motion can be precisely controlled by appropriate amplitude, frequency, and phase of the blade pitching.

[0025] The surge motion may be within a particular frequency range. The frequency range may be approximately 0.004-0.017 Hz, or optionally, approximately 0.007-0.009 Hz. This frequency range may relate to any motion (or axial motion) of the floating wind turbine having a natural or driving frequency within this range. Motions of the floating wind turbine occurring within these ranges may be dominated by surge motions, but may also include other types of motion. The natural period of the surge motion may be within the range of approximately 60-250 seconds, or optionally, within the range of approximately 110-140 seconds, or optionally, approximately 125 seconds. These surge motions may be caused by changes in wind speed that excite the wind turbine's natural surge frequency and may be more likely to occur in calmer waters.

[0026] The controller is for damping and / or controlling surge motion of the floating wind turbine and may therefore be referred to as a motion controller and / or a floating wind turbine motion controller. The controller is also for controlling the blade pitch of one or more of the rotor blades and may therefore be referred to as a blade pitch controller.

[0027] The controller is preferably configured to adjust the blade pitch of each rotor blade in phase with respect to the floating wind turbine surge speed to provide the damping force.

[0028] Additionally or alternatively, the controller is configured to adjust the blade pitch of each rotor blade in phase with respect to the floating wind turbine surge motion to provide a restoring force.

[0029] Damping force refers to a force that opposes the velocity of the surge motion, as opposed to a restoring force that acts to oppose the deflection (displacement) of the surge motion.

[0030] The phase of the rotor blade pitch adjustment relative to the surge speed can be varied, and the force is in phase with the surge speed to create a damping force, or in phase with the surge deflection to create a restoring force. The controller can be tuned to obtain an optimum balance between damping and restoring forces.

[0031] Based on the surge motion input, the controller may calculate or determine a blade pitch adjustment to dampen the surge motion.

[0032] The surge motion input may be a surge speed. Thus, blade pitch adjustments to damp the surge motion may be based on the speed of the surge motion. The speed may be a measured or estimated surge speed. The surge speed estimate may be an estimate based on measurements of motion, speed, and / or acceleration.

[0033] The surge motion input can be measured and / or estimated using output from one or more sensors. The sensors can be configured to provide an output indicative of surge motion within a surge motion frequency range. The one or more sensors can comprise different types of sensors.

[0034] The surge motion input may be obtained from the output from a motion sensor (e.g., a speed sensor and / or an accelerometer), i.e., the sensor may be a motion sensor. The sensor may be a motion reference unit (MRU). The sensor may be for measuring rigid body surge motion within a relevant frequency range. The relevant frequency range may be a frequency range expected to include surge motion. The motion sensor for detecting wind turbine motion (e.g., surge motion) may be located at any point on the wind turbine. For example, the sensor may be located at the base of the wind turbine tower, in the nacelle of the wind turbine, or at any point along the wind turbine tower.

[0035] Additionally or alternatively, the surge motion input may be obtained from the output of a global positioning system (GPS), such as a differential global positioning system (DGPS), i.e. the sensor may be a GPS (or DGPS). A global positioning system can be used to measure the surge motion of a wind turbine, for example within a relevant frequency range.

[0036] Any other suitable sensing means may also be used to measure surge motion.

[0037] When a surge motion speed measurement or estimate is input, it is preferred that the controller use a low pass filter on the speed input. This ensures that the controller can act on the surge motion within a particular frequency range, for example at or near the resonant frequency of the surge motion. For surge motion, it is desirable to provide damping at the natural frequency of the surge motion and to consider higher frequencies, such as wave frequencies, as undesirable disturbances.

[0038] When a wind turbine is operating below rated wind speed, it is preferable to maximize power extraction and damp surge motion. Thus, the controller preferably controls blade pitch based on a blade angle that maximizes power extraction and an additional blade pitch adjustment to account for surge motion.

[0039] Thus, in a preferred embodiment, the controller is configured to calculate an original blade pitch for the plurality of rotor blades, calculate additional blade pitch adjustments for the plurality of rotor blades to counteract surge motion of the floating wind turbine, and combine the original blade pitch and additional blade pitch adjustments to obtain a total blade pitch adjustment to cause attenuation of the surge motion. The controller preferably adjusts the blade pitch of each rotor blade based on the total blade pitch adjustment.

[0040] In a preferred embodiment, the controller comprises a standard controller for determining the original blade pitch signal and an active damping controller for determining the additional blade pitch adjustment signal.

[0041] The original blade pitch may be an optimal blade pitch for maximum power extraction below rated wind speed, e.g., a pitch that produces an optimal tip speed ratio. It may be a constant or substantially constant value. The original blade pitch may not be determined based on surge motion of the floating wind turbine, i.e., the original blade pitch may be independent of surge motion. The additional blade pitch adjustment is preferably based on surge motion. Thus, the total blade pitch adjustment may include a component that is independent of surge motion (e.g., a constant or substantially constant component) and a component that varies based on surge motion.

[0042] The active damping controller may include a control loop that may be for calculating and / or determining an additional blade pitch adjustment signal to damp the surge motion.

[0043] The control loop may include filtering and / or parameter settings that are tuned and / or optimized for surge motion.

[0044] The control loop may receive inputs of surge motion, for example from motion sensors mounted on the floating wind turbine structure, and / or from or based on data from a Differential Global Positioning System.

[0045] The output of the control loop may be an additional blade pitch adjustment signal to dampen surge motion.

[0046] The controller may be configured to control the blade pitch of one or more of the plurality of rotor blades based on an additional blade pitch adjustment signal from the active damping controller. The controller may also be configured to control the blade pitch of one or more of the plurality of rotor blades based on the original blade pitch signal.

[0047] The active damping controller may also include a signal processing unit configured to obtain raw measurements from the sensors and apply one or more estimation techniques to estimate the speed of the surge motion of the wind turbine. The estimation techniques may include Kalman filtering. For example, measurements from a differential global positioning system may be combined with estimation techniques such as Kalman filtering to estimate the motion, e.g., surge speed.

[0048] The active damping controller may include a filter. The filter may be configured to filter out velocity changes at frequencies higher and / or lower than the natural surge frequency of the rigid body motion. For example, the filter may filter out frequencies above 0.017 Hz and / or below 0.004 Hz. The filter may be a low-pass filter. The low-pass filter may have a cutoff frequency in the range of about 0.004-0.017 Hz, preferably in the range of about 0.007-0.009 Hz. While any suitable filter may be employed, the filter is preferably a second- or third-order Butterworth low-pass filter. Such a filter may be configured to ensure that only vibrations having a desired frequency are actively damped.

[0049] As discussed above, the controller may comprise a standard controller in addition to the active damping controller. The standard controller may determine or receive an original blade pitch signal, which may be a signal corresponding to a desired blade pitch when surge motion of the floating wind turbine is not taken into account. The standard controller is preferably also for receiving an additional blade pitch adjustment signal from the active damping controller.

[0050] The controller may be configured to combine the original blade pitch signal and the additional blade pitch adjustment signal to generate a total blade pitch adjustment signal. The controller may be configured to adjust the blade pitch of one or more of the plurality of blades based on the total blade pitch adjustment signal. The total blade pitch adjustment may damp surge motion while maximizing power output when the wind turbine is operating at wind speeds below the rated wind speed.

[0051] The total blade pitch adjustment may be for controlling the pitch of the rotor blades collectively, and thus the controller may be for providing collectively blade pitch control.

[0052] Blade pitch adjustment can vary the rotor speed and can induce an optimal rotor speed while providing a force that damps surge motion.

[0053] The control loop (i.e., control law) of the active damping controller includes an additional blade pitch adjustment signal β to damp the surge motion. ref2 The additional blade pitch adjustment signal can be calculated based on the measured or estimated speed of the surge motion.

number

[0054] Additional blade pitch adjustments are preferably made based on the measured or estimated speed of the surge motion.

number

[0055] Therefore, the control law can be written in the following form:

number

number

number

[0056] T may be in the range of about 60 to 250 seconds, or optionally in the range of about 110 to 140 seconds, for example about 125 seconds.

[0057] The controller gain and / or low pass filter frequency may be adapted to the surge motion. The low pass filter frequency may be set according to the surge motion frequency range.

[0058]

number

[0059] β ref2 The original blade pitch signal β ref1 and the total blade pitch adjustment signal β ref In other words, β ref =β ref1 +β ref2 is.

[0060] β ref2 , the total blade pitch adjustment signal damps surge motion.

[0061] As discussed above, β ref2 may be proportional to the surge speed, and therefore the total blade pitch adjustment signal may also be proportional to the surge speed.

[0062] The controller is for controlling the floating wind turbine when the wind speed is below the rated wind speed. Thus, the controller is for controlling the floating wind turbine when the wind turbine is operating below its rated power. The controller may also be for controlling the floating wind turbine when the wind speed is higher than the rated wind speed, i.e., the controller may not be solely for use below the rated wind speed.

[0063] The floating wind turbine may be a spar buoy-type floating wind turbine. The floating wind turbine may be secured to the seabed by use of a mooring system, such as mooring lines and / or one or more articulated legs. Alternatively, the floating wind turbine may be a semi-submersible floating wind turbine or any other type of floating wind turbine.

[0064] A floating wind turbine may be equipped with one or more sensors.

[0065] In a second aspect, the present invention provides a floating wind turbine comprising a rotor having a plurality of rotor blades and a motion controller configured to adjust a blade pitch of each rotor blade when the floating wind turbine is operating in winds below a rated wind speed to generate a net force that damps surge motion of the floating wind turbine.

[0066] The floating wind turbine of the second aspect may comprise a controller according to the first aspect.

[0067] The invention also extends to corresponding control methods. Thus, in a third aspect, the invention provides a method of controlling a floating wind turbine comprising a plurality of rotor blades, the method comprising adjusting the blade pitch of each rotor blade when the wind turbine is operating at a wind speed below its rated wind speed to generate a net force that damps surge motion of the floating wind turbine.

[0068] The method preferably incorporates the optional and preferred features discussed above in relation to the first aspect of the invention.

[0069] The method of the third aspect may be carried out using the controller of the first aspect and / or the floating wind turbine of the second aspect.

[0070] The controller of the first aspect and / or the floating wind turbine of the second aspect may be configured to carry out the method of the third aspect.

[0071] As will be appreciated by those skilled in the art, the controller is typically provided in the form of software and therefore comprises a processor for executing this software, which may be, for example, a microprocessor.

[0072] The present invention also relates to a software product comprising instructions that, when executed by a processor, cause the processor to adjust the blade pitch of each rotor blade when the wind turbine is operating at a wind speed below a rated wind speed to generate a net force that damps surge motion of the floating wind turbine.

[0073] Preferably, the software product is a physical data carrier. Alternatively or additionally, the software product may be provided in the form of instructions transmitted over a network, for example downloaded over the internet.

[0074] The present invention may be an additional controller or additional software, which may be configured to perform the method or at least part of the method. The software may be stored on a physical medium, a cloud-based storage solution, or any other suitable medium.

[0075] The controller can be retrofitted to an existing floating wind turbine. This can be achieved by providing the existing floating wind turbine with additional inputs, additional sensors, and / or additional or updated code / software. For example, an existing floating wind turbine can be equipped with a standard controller, and an active damping controller can be added to the standard controller to obtain the motion controller of the present invention.

[0076] The active damping controller may be a code used to provide one or more additional blade pitch adjustments that can be used to damp surge motion.

[0077] Viewed from a fourth aspect, the present invention provides a computer program product comprising instructions which, when executed on a processing circuit for a floating wind turbine, configure the processing circuit to control blade pitch for one or more rotors of the floating wind turbine, the instructions comprising adjusting the blade pitch of each rotor blade when the wind turbine is operating at a wind speed below a rated wind speed to generate a net force that damps surge motion of the floating wind turbine.

[0078] A computer program product of the fourth aspect may be provided to the controller of the first aspect and / or the floating wind turbine of the second aspect. The computer program product of the fourth aspect may be used to perform the method of the third aspect. In other words, the computer program product may include instructions that, when executed on a processing circuit for a floating wind turbine, configure the processing circuit to perform the method of the third aspect.

[0079] Specific embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0080] [Figure 1] FIG. 1 illustrates a floating wind turbine. [Figure 2] FIG. 1 is a block diagram of a conventional controller for a floating wind turbine. [Figure 3] FIG. 1 is a block diagram of a controller for a floating wind turbine with active surge damping control according to one embodiment of the present invention. [Figure 4] 10 is a graph showing results from a simulation comparing a floating wind turbine with a controller without active surge damping control and a floating wind turbine with a controller with active surge damping control. [Figure 5] 10 is a graph showing results from a simulation comparing a floating wind turbine with a controller without active surge damping control and a floating wind turbine with a controller with active surge damping control. DETAILED DESCRIPTION OF THE INVENTION

[0081] Referring to FIG. 1 , a floating wind turbine assembly 1 is shown. The floating wind turbine assembly 1 comprises a turbine rotor 2 mounted to a nacelle 3. The nacelle is further mounted on top of a structure comprising a tower 4 fixed on top of a floating body 5, which in the illustrated example is a spar buoy-like structure. The principles of the present disclosure for controlling surge motion are applicable to all floating structures for floating wind turbines. The floating body is secured to the seabed by one or several anchor lines 7 (only one is shown), which may be taut or catenary mooring lines. The nacelle contains a generator connected to the turbine rotor by any known means, such as a reduction gearbox, which in turn is directly connected to a generator or a hydrostatic transmission (these elements are not shown). The nacelle also contains a control unit.

[0082] Floating wind turbines are designed to absorb incoming wind WThe floating wind turbine assembly 1 is subjected to a force of axial force and a force of waves 9 (waves 9 on the water surface are shown schematically). These forces cause the floating wind turbine assembly 1 to move in the water. The axial movement (i.e. axial with respect to the rotor axis A) is called surge motion.

[0083] The controller in the nacelle is configured to control the blade pitch of the rotor blades 2. In a conventional controller (e.g., the standard controller shown in FIG. 2), the blade pitch is approximately constant at an angle that produces maximum power when the wind turbine is operating in winds below the rated wind speed. In a controller according to the present invention, the blade pitch is adjusted to damp surge motion of the floating wind turbine when the wind turbine is operating in winds below the rated wind speed.

[0084] 2 shows a conventional blade pitch controller 10 for a floating wind turbine. The standard blade pitch controller 10 is specifically intended to control the pitch of the blades of a floating wind turbine when the wind speed is below the rated wind speed.

[0085] When the wind speed is below the rated wind speed, the standard blade pitch controller 10 reduces the original blade pitch signal β ref1 The blade pitch is kept at a constant angle according to the original blade pitch signal β ref1 The blade pitch corresponding to is typically selected to maximize power extraction at wind speeds below the rated wind speed and can be determined according to known methods. However, the standard blade pitch controller 10 does not take into account the motion of the wind turbine structure itself.

[0086] 3 illustrates a motion controller 20 for a floating wind turbine according to one embodiment of the present invention. The motion controller 20 can take into account surge motions that the floating wind turbine may experience. The motion controller 20 can provide an additional blade pitch adjustment signal β to dampen the surge motions. ref2The active damping controller 22 is coupled to a standard blade pitch controller, such as the standard blade pitch controller 10 of FIG.

[0087] The active damping controller 22 provides an additional blade pitch adjustment signal β to damp rigid body surge motion of the wind turbine. ref2 The damping control loop 24 includes a signal processing block 26, an active damping controller gain K s , and the controller transfer function h s (s)

[0088] The motion controller 20 operates as follows: A standard blade pitch controller 10 generates an original blade pitch signal β ref1 In this embodiment, the original blade pitch signal β ref1 corresponds to a substantially constant angle or pitch of the rotor blades while the wind speed is below the rated wind speed. In the modified embodiment, the original blade pitch signal β ref1 may not be constant and may include other adjustment signals.

[0089] In the active damping control loop 24, the measured or estimated surge speed v of the wind turbine is s (

number

[0090] Then, an additional blade pitch adjustment signal β ref2 is the original blade pitch signal β ref1 is added to produce a total blade pitch adjustment signal β ref is generated, and this signal βref is used to control the blades of the wind turbine to damp surge motion while maximizing power extraction, which reduces forces on the wind turbine structure and mooring system and maximizes power output for a given wind speed.

[0091] The signal processing block 26 uses a sharp low-pass filter with a filter frequency sufficiently lower than the wave frequency range (0.05-0.2 Hz) to avoid attenuation of wave-induced motions. Attenuation of wave-induced motions leads to poor performance with respect to critical wind turbine parameters. The filter frequency may depend on the natural surge frequency of the floating wind turbine. The filter frequency may be approximately 0.004-0.017 Hz.

[0092] Active damping controller gain K s The value of is adjusted depending on the frequency of the surge motion to be damped. The exact value to use for this parameter can be found by conventional controller tuning.

[0093] Figures 4 and 5 show simulation results to help illustrate the benefits of active wind turbine control below rated wind speeds that take surge motion into account. Figure 4 shows surge motion for a floating wind turbine with and without active surge damping control for wind speeds below rated wind speed. Figure 5 shows mooring line tension at the most heavily loaded mooring line from the same simulation. The simulation compares a scenario in which the floating wind turbine uses a standard blade pitch controller without active damping (e.g., the blade pitch controller of Figure 2) and two scenarios in which the floating wind turbine uses a motion controller with active damping for surge motion (e.g., the blade pitch controller of Figure 3) with different active damping control values. From Figure 4, it can be seen that surge motion response is reduced when active surge damping control is applied in accordance with the present invention. Furthermore, as shown in Figure 5, there is a corresponding reduction in tension fluctuations in the main mooring lines when active surge damping control is applied.

[0094] In this simulation, a 12MW floating wind turbine with an asymmetric mooring layout was modeled. Figures 4 and 5 show snapshots of the simulation from 0 to 600 seconds, with the total simulation length being 3700 seconds. Simulation parameters included a 8.5ms -1 The parameters included a mean wind speed of 1.3 m, the presence of turbulence class C, a significant wave height set to 1.3 m, and a characteristic peak duration of 6.3 seconds. The surge speed measurement used by the controller was modeled as the average speed between two DGPS measurements at a resolution of 1 Hz.

[0095] The fatigue damage of the main mooring line and tower bottom was also calculated for the two active surge damping controller settings, compared to the fatigue damage for the original control system for the case under consideration. For a range of 3700 seconds of simulation for this particular set of parameter values, the relative fatigue damage for the highest loaded mooring line (3) was reduced from 0.59 to 0.73, and the relative fatigue damage for the tower bottom was reduced from 0.82 to 0.84, depending on the active damping controller setting.

Claims

1. 1. A motion controller for a floating wind turbine having a plurality of rotor blades, the motion controller being configured to adjust a blade pitch of each rotor blade when the floating wind turbine is operating in winds below a rated wind speed to generate a net force that damps surge motion of the floating wind turbine.

2. The motion controller of claim 1 , configured to calculate a blade pitch adjustment for one or more of the rotor blades based on the surge motion input.

3. The motion controller of claim 2 , wherein the input is based on measurements and / or estimates of surge speed of the floating wind turbine.

4. The motion controller of claim 3 , wherein the blade pitch adjustment is proportional to the surge speed.

5. A motion controller as described in claim 3 or 4, configured to adjust the blade pitch of each rotor blade in phase with respect to the surge speed of the floating wind turbine to provide a damping force.

6. A motion controller according to any one of claims 2 to 5, wherein the surge motion input is measured and / or estimated using output from one or more sensors.

7. A motion controller according to any one of claims 2 to 6, configured to use a low pass filter on the input.

8. The motion controller of claim 7 , wherein the low pass filter comprises a transfer function.

9. 9. The motion controller of claim 7 or 8, wherein the low pass filter is configured to filter out frequencies above 0.017 Hz.

10. A floating wind turbine comprising a rotor having a plurality of rotor blades and a motion controller according to any one of claims 1 to 9.

11. 1. A method of controlling a floating wind turbine having a plurality of rotor blades, comprising: adjusting the blade pitch of each rotor blade when the floating wind turbine is operating at a wind speed less than a rated wind speed to generate a net force that damps surge motion of the floating wind turbine. A method comprising:

12. inputting the surge motion of the floating wind turbine into a controller; adjusting the blade pitch of each rotor blade based on the input of the surge motion; 12. The method of claim 11, comprising:

13. The method of claim 12 , wherein the input is based on measurements and / or estimates of surge speed of the floating wind turbine.

14. The method of claim 13 , wherein the blade pitch adjustment is proportional to the surge speed.

15. A method as described in claim 13 or 14, comprising a step of adjusting the blade pitch of each rotor blade in phase with respect to the surge speed of the floating wind turbine to provide a damping force.

16. A method according to any one of claims 12 to 15, wherein the input of the surge motion is measured and / or estimated using output from one or more sensors.

17. The method of claim 16 , wherein the one or more sensors include a motion sensor and / or a global positioning sensor.

18. A method according to any one of claims 12 to 17, comprising using a low pass filter on the input.

19. The method of claim 18 , wherein the low pass filter comprises a transfer function.

20. 20. The method of claim 18 or 19, wherein the low pass filter filters out frequencies above 0.017 Hz.

21. A method according to any one of claims 11 to 20, implemented using a controller according to any one of claims 1 to 9.

22. 22. A computer program product comprising instructions that, when executed on a processing circuit associated with a floating wind turbine, configure the processing circuit to perform the method of any one of claims 11 to 21.

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