Blade pitch control in a wind turbine installation

1. A blade pitch controller for a floating wind turbine structure comprising a support structure supporting a rotor having a plurality of blades, the controller comprising: standard blade pitch control means; and active damping means; wherein the standard blade pitch control means is arranged to control a blade pitch using a transfer function between a rotor speed error and the blade pitch; and the active damping means is arranged to further control the blade pitch on the basis of a speed of a point on the wind turbine structure by converting the speed of a point on the wind turbine structure into a rotor speed error and using the same transfer function that is used in the standard blade pitch control means to convert the rotor speed error into a correction to the blade pitch.

2. A blade pitch controller as claimed in claim 1 , wherein controller parameters can be changed by remote operation.

3. A blade pitch controller as claimed in claim 1 , wherein the active damping means comprises a low pass filter.

4. A blade pitch controller as claimed in claim 3 , wherein the low pass filter is arranged to filter out changes in the speed of a point on the wind turbine structure with frequencies above the natural frequency of the rigid body oscillations of the wind turbine structure in pitch.

5. A blade pitch controller as claimed in claim 3 , wherein the low pass filter is arranged to filter out changes in the speed of a point on the wind turbine structure with frequencies above 0.05 Hz.

6. A blade pitch controller as claimed in claim 3 , wherein the low pass filter is arranged to filter out changes in the speed of a point on the wind turbine structure with frequencies above 0.04 Hz.

7. A blade pitch controller as claimed in claim 3 , wherein the low pass filter is a sharp filter.

8. A blade pitch controller as claimed in claim 3 , wherein the low pass filter is a second or third order Butterworth low pass filter.

9. A blade pitch controller as claimed in claim 1 , wherein the active damping means comprises active damping gain means which converts the speed of a point on the wind turbine structure into a rotor speed error.

10. A blade pitch controller as claimed in claim 9 , wherein the active damping gain means is arranged to reduce or prevent negative damping of rigid body oscillations of the wind turbine structure in pitch.

11. A blade pitch controller as claimed in claim 9 , wherein the active damping gain means is arranged to provide net positive damping of rigid body oscillations of the wind turbine structure in pitch.

12. A blade pitch controller as claimed in claim 1 , wherein the transfer function that is used in both the active damping means and the standard blade pitch control means is implemented in the form of a proportional integral controller.

13. A blade pitch controller as claimed in claim 11 , wherein the controller comprises two proportional integral controllers, one in the active damping means and one in the standard blade pitch control means.

14. A blade pitch controller as claimed in claim 12 , wherein the controller comprises a proportional integral controller that is arranged to be used by both the active damping means and the standard blade pitch control means.

15. A blade pitch controller as claimed in claim 1 , wherein the blade pitch can be adjusted separately for each rotor blade.

16. A method of controlling the blade pitch of a floating wind turbine structure comprising a support structure supporting a rotor having a plurality of blades, the method comprising: adjusting a blade pitch on the basis of the output of a transfer function between a rotor speed error and the blade pitch; and further adjusting the blade pitch on the basis of a horizontal speed of a point on the wind turbine structure; wherein the speed of a point on the wind turbine structure is converted into a rotor speed error which is then converted into a blade pitch using the same transfer function.

17. A method as claimed in claim 16 , wherein the blade pitch is only further adjusted for changes in the speed of a point on the wind turbine structure with frequencies above a certain value.

18. A method as claimed in claim 17 , wherein the certain value is above the natural frequency of the rigid body oscillations of the structure in pitch.

19. A method as claimed in claim 17 , wherein the certain value is 0.05 Hz.

20. A method as claimed in claim 17 , wherein the certain value is 0.04 Hz.

21. A method as claimed in claim 17 , wherein the changes in the speed of a point on the wind turbine structure are filtered using a low pass filter.

22. A method as claimed claim 21 , wherein the low pass filter is a second or third order Butterworth low pass filter.

23. A method as claimed in claim 16 , wherein the speed of a point on the structure is converted into a rotor speed error using an active damping gain means.

24. A method as claimed in claim 23 , wherein the active damping gain means is arranged to reduce or prevent negative damping of rigid body oscillations of the wind turbine structure in pitch.

25. A method as claimed in claim 23 , wherein the active damping gain means is arranged to net provide positive damping of rigid body oscillations of the wind turbine structure in pitch.

26. A method as claimed in claim 16 , wherein the transfer function that is used in both the active damping means and the standard blade pitch control means is implemented in the form of a proportional integral controller.

27. A floating wind turbine installation comprising a blade pitch controller as claimed in claim 1 or controlled according to claim 16 .