Method of monitoring the structural integrity of the supporting structure of a wind turbine

1. A method of monitoring the structural integrity of a supporting structure of a wind turbine, comprising:

determining a fore-aft tower oscillation frequency;

determining a side-to-side tower oscillation frequency;

computing a working structural indicator value from the fore-aft tower oscillation frequency and the side-to-side tower oscillation frequency, wherein the working structural indicator value is a ratio of the fore-aft tower oscillation frequency to the side-to-side tower oscillation frequency during monitoring of the wind turbine;

comparing the working structural indicator value to a reference working structural indicator value;

reporting a fault signal if the difference between the working structural indicator value and the reference structural indicator value exceeds a predefined threshold; and

adjusting wind turbine operation parameters as a cautionary measure until the structural integrity of the supporting structure can be inspected.

2. The method according to claim 1 , comprising a prior step of computing the reference structural indicator value over an interval following commissioning of the wind turbine.

3. The method according to claim 1 , wherein the predefined threshold value is established on the basis of simulations carried out for that type of wind turbine.

4. The method according to claim 1 , wherein the predefined threshold value is established on the basis of operation data collected for comparable wind turbines.

5. The method according to claim 1 , wherein the fore-aft tower oscillation frequency is determined on the basis of data collected by a number of accelerometers arranged on a fore-aft axis of the wind turbine nacelle.

6. The method according to claim 1 , wherein the side-to-side tower oscillation frequency is determined on the basis of data collected by a number of accelerometers arranged on a side-to-side axis of the wind turbine nacelle.

7. The method according to claim 1 , comprising a step of yawing the nacelle while computing structural indicator values to identify the location of a tower structure fault.

8. The method according to claim 1 , comprising a step of performing a fault verification procedure following the reporting of a fault signal.

9. A system for monitoring the structural integrity of a supporting structure of a wind turbine tower, comprising:

a frequency determination module configured to determine a fore-aft tower oscillation frequency;

a frequency determination module configured to determine a side-to-side tower oscillation frequency;

a structural indicator value computation module configured to compute a structural indicator value from the fore-aft tower oscillation frequency and the side-to-side tower oscillation frequency, wherein the computed structural indicator value is a ratio of the fore-aft tower oscillation frequency to the side-to-side tower oscillation frequency during monitoring of the wind turbine;

a comparator module configured to compare the computed structural indicator value with a reference structural indicator value and to issue a fault report signal if the difference between the computed structural indicator value and the reference structural indicator value exceeds a predefined threshold; and

a controller configured to adjust wind turbine operation parameters as a cautionary measure until the structural integrity of the supporting structure can be inspected.

10. A wind turbine comprising:

a supporting structure comprising a tower anchored to the ground by a foundation;

a nacelle mounted on top of the tower by a yaw assembly;

an aerodynamic rotor; and

a system for monitoring the structural integrity of the supporting structure by:

determining a fore-aft tower oscillation frequency;

determining a side-to-side tower oscillation frequency;

computing a working structural indicator value from the fore-aft tower oscillation frequency and the side-to-side tower oscillation frequency, wherein the computed working structural indicator value is a ratio of the fore-aft tower oscillation frequency to the side-to-side tower oscillation frequency during monitoring of the wind turbine;

comparing the computed working structural indicator value to a reference working structural indicator value; and

reporting a fault signal if the difference between the computed working structural indicator value and the reference structural indicator value exceeds a predefined threshold.

11. The wind turbine according to claim 10 , wherein a fore-aft axis of the nacelle is parallel to the axis of rotation of the aerodynamic rotor.

12. The wind turbine according to claim 10 , comprising a number of accelerometers arranged on a fore-aft axis of the nacelle, and a number of accelerometers arranged on a side-to-side axis of the nacelle.

13. A computer program product, comprising a computer readable hardware storage device having computer readable program code stored therein, said program code executable by a processor of a computer system to implement the method according to claim 1 .