Controlling motions of floating wind turbines

1. A motion controller for a floating wind turbine with a plurality of rotor blades, wherein the controller is arranged to adjust a blade pitch of each of the rotor blades so as to create a net force to control a motion of the floating wind turbine in a yaw direction, wherein the controller includes a control action which is proportional to a yaw offset angle and/or a control action which is proportional to an integral of the yaw offset angle, wherein the controller is adapted to calculate a dynamic and collective pitch for all of the plurality of rotor blades to counteract an axial motion of the floating wind turbine to obtain a first adjustment, calculate a dynamic and individual blade pitch for each of the plurality of blades to counteract a yaw motion of the floating wind turbine to obtain a second adjustment calculate a dynamic and individual blade pitch for each of the plurality of blades and/or a variation in rotor torque to counteract an in-plane motion of the floating wind turbine to obtain a third adjustment; and combine the first, second and third adjustments to cause simultaneous control of the axial motions, yaw motions and in-plane motions of the floating wind turbine.

2. The motion controller as claimed in claim 1 , wherein the controller does not include a control action which is proportional to the derivative of the yaw off-set angle.

3. The motion controller as claimed in claim 1 , wherein the controller is arranged to operate only when the yaw off-set is above a certain threshold angle.

4. The motion controller as claimed in claim 1 , wherein the controller is also arranged to control a motion in the in-plane direction.

5. The motion controller as claimed in claim 1 , wherein the controller is also arranged to control a motion in the axial direction.

6. The motion controller as claimed in claim 1 , wherein the controller is arranged to control the motion of the floating wind turbine in a plurality of directions simultaneously.

7. The motion controller as claimed in claim 4 , wherein the controller is arranged to adjust the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion velocity so as to provide a damping force for at least one of the motions.

8. The motion controller as claimed in claim 1 , wherein the controller is arranged to adjust the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion displacement so as to provide a restoring force.

9. The motion controller as claimed in claim 1 , wherein the controller is further arranged to control the torque of the load presented to the rotor to control a motion in the in-plane direction.

10. The motion controller as claimed in claim 1 , wherein the input to the controller is based on a measurement of the velocity of the motions of the floating wind turbine.

11. The motion controller as claimed in claim 9 , wherein the controller uses a low pass filter on the velocity input.

12. A method of controlling a floating wind turbine structure comprising a plurality of rotor blades, the method comprising:

adjusting a blade pitch of each of the rotor blades so as to create a net force to control a yaw motion of the floating wind turbine, wherein a blade pitch adjustment is calculated using a controller which includes a control action which is proportional to a yaw offset angle and/or a control action which is proportional to an integral of the yaw offset angle,

wherein the controller is adapted to calculate a dynamic and collective pitch for all of the plurality of rotor blades to counteract an axial motion of the floating wind turbine to obtain a first adjustment, calculate a dynamic and individual blade pitch for each of the plurality of blades to counteract a yaw motion of the floating wind turbine to obtain a second adjustment; calculate a dynamic and individual blade pitch for each of the plurality of blades and/or a variation in rotor torque to counteract an in-plane motion of the floating wind turbine to obtain a third adjustment; and combine the first, second and third adjustments to cause simultaneous control of the axial motions, yaw motions and in-plane motions of the floating wind turbine.

13. The method as claimed in claim 12 , wherein the blade pitch adjustment is calculated using a controller which does not includes a control action which is proportional to the derivative of the yaw off-set angle.

14. The method as claimed in claim 12 , wherein the blade pitch is only adjusted when the yaw off-set is above a certain threshold angle.

15. The method as claimed in claim 12 , wherein the method comprises adjusting a blade pitch of each rotor blade so as to create a net force to also control an in-plane motion.

16. The method as claimed in claim 12 , wherein the method comprises adjusting the blade pitch of the rotor blades to also control a motion in the axial direction.

17. The method as claimed in claim 12 , the method comprising controlling the motion of the floating wind turbine in a plurality of directions simultaneously.

18. The method as claimed in claim 15 , the method comprising adjusting the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion velocity so as to provide a damping force for at least one of the motions.

19. The method as claimed in claim 12 , the method comprising adjusting the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion displacement so as to provide a restoring force.

20. The method as claimed in claim 12 , the method comprising controlling the torque of the load presented to the rotor to control a motion in the in-plane direction.

21. The method as claimed in claim 12 , the method comprising inputting to the controller a measurement of the velocity of the motions of the floating wind turbine.

22. The method as claimed in claim 21 , the method comprising using a low pass filter on the velocity input.

23. A motion controller for a floating wind turbine with a plurality of rotor blades, wherein the controller is arranged to adjust a blade pitch of each of the rotor blades so as to create a net force to control a motion of the floating wind turbine in a direction other than the axial direction, wherein the controller is arranged to only control a motion of the floating wind turbine when that motion exceeds a certain threshold.