METHOD OF MONITORING THE STRUCTURAL INTEGRITY OF THE SUPPORTING STRUCTURE OF A WIND TURBINE

Abstract

Provided is a method of monitoring the structural integrity of a supporting structure of a wind turbine, which method includes the steps of 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; comparing the working structural indicator value to a reference working structural indicator value; and issuing an alarm if the difference between the working structural indicator value and the reference structural indicator value exceeds a predefined threshold. Also provided is a system for monitoring the structural integrity of a supporting structure of a wind turbine, a wind turbine, and a computer program product for carrying out the steps of the inventive method.

Claims

1. A method of monitoring the structural integrity of a supporting structure of a wind turbine, which method comprises the steps of: 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; comparing the working structural indicator value to a reference working structural indicator value; and reporting a fault signal if the difference between the working structural indicator value and the reference structural indicator value exceeds a predefined threshold.

2. A method according to claim 1, wherein a structural indicator value is a ratio of fore-aft tower oscillation frequency to side-to-side tower oscillation frequency.

3. A 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.

4. A 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.

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

6. A 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.

7. A 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.

8. A 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.

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

10. A method according to claim 9, comprising a step of adjusting wind turbine operation parameters as a cautionary measure until the structural integrity of the supporting structure can be inspected.

11. A system for monitoring the structural integrity of a supporting structure of a wind turbine tower, which system comprises: 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; and a comparator module configured to compare the structural indicator value with a reference structural indicator value and to issue a fault report signal if the difference between the structural indicator value and the reference structural indicator value exceeds a predefined threshold.

12. 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 using the method according to claim 1.

13. A wind turbine according to claim 12, wherein a fore-aft axis of the nacelle is parallel to the axis of rotation of the aerodynamic rotor.

14. A wind turbine according to claim 12, 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.

15. 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 a method for carrying out the steps of the method according to claim 1 when the computer program product is loaded into a memory of a programmable device.

Description

BRIEF DESCRIPTION

[0026] Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein:

[0027] FIG. 1 is a simplified diagram of a wind turbine seen from above;

[0028] FIG. 2 is a block diagram showing the main steps of the inventive method;

[0029] FIG. 3 shows a graph of fore-aft and side-to-side frequency frequencies over time;

[0030] FIG. 4 shows the ratio of fore-aft frequency to side-to-side frequency over time;

[0031] FIG. 5 shows a graph of fore-aft and side-to-side frequency frequencies;

[0032] FIG. 6 shows the ratio of fore-aft frequency to side-to-side frequency over time.

DETAILED DESCRIPTION

[0033] FIG. 1 is a simplified diagram of an offshore wind turbine 1 seen from above, indicating the aerodynamic rotor 10 (rotor blades mounted to a hub), the nacelle 11, and the position of the nacelle 11 over the tower 13, to which the nacelle 11 is mounted by means of a circular yaw ring 12. The tower 13 is mounted to a foundation 14, which in turn is embedded to a large extent under the seabed. The tower 13 and the foundation 14 collectively act as the supporting structure of the wind turbine 1. To measure tower frequency, a number of accelerometers A.sub.FA, A.sub.S2S are mounted about the yaw ring. Here, a fore-aft accelerometer A.sub.FA is mounted on the fore-aft axis FA of the nacelle 11 and is used to measure tower movement in the corresponding fore-aft direction D.sub.FA. A side-to-side accelerometer A.sub.S2S is mounted on the side-to-side axis S2S of the nacelle 11 and is used to measure tower movement in that side-to-side direction D.sub.S2S. The positions of the accelerometers A.sub.FA, A.sub.S2S are fixed relative to the nacelle 11, so that when the wind turbine 1 is yawed to track the wind, the fore-aft direction D.sub.FA and side-to-side direction D.sub.S2S change with respect to the wind turbine tower 13. Data collected by an accelerometer A.sub.FA, A.sub.S2S is analyzed to estimate the frequency of the tower oscillation in that direction. For example, the frequency of oscillation can be computed using a suitable Fourier transform technique, as will be known to the skilled person.

[0034] FIG. 2 is a block diagram showing the main steps of the inventive method. After commissioning a wind turbine of the type described in FIG. 1, data from the accelerometers are collected for a length of time, for example at least over several hours, or even over several days. Collecting data over a relatively long duration can ensure a favorable degree of accuracy. In this time, the fore-aft acceleration is measured using the fore-aft accelerometer A.sub.FA and processed in analysis block 20.sub.FA to obtain an estimation of the healthy fore-aft frequency f.sub.FA. Similarly, the side-to-side acceleration is measured using the side-to-side accelerometer A.sub.S2S and processed in analysis block 20.sub.S2S to obtain an estimation of the healthy side-to-side frequency f.sub.S2S.

[0035] The tower frequency f.sub.FA in the fore-aft direction D.sub.FA is generally slightly different from the tower frequency f.sub.S2S in the side-to-side direction D.sub.S2S. The difference arises from various factors, while one main contributing factor is that the rotor blades have different stiffness in their flap-wise and edge-wise directions.

[0036] The estimated tower frequencies can be compensated based on operational parameters such as rotor speed, turbine output power, rotor thrust and pitch angle as these may slightly affect the frequency. Compensation can be done individually for the fore-aft frequency f.sub.FA and the side-to-side frequency f.sub.S2S since these may be affected to different extents by the different parameters.

[0037] The inventors have recognized that the difference between the fore-aft frequency f.sub.FA and the side-to-side frequency f.sub.S2S can be exploited to more reliably detect tower damage. To this end, instead of using only one control frequencyi.e. either the fore-aft frequency f.sub.FA or the side-to-side frequency f.sub.S2Sas known from the conventional art, the inventive method includes a step of determining a working structural indicator value R using both the fore-aft frequency f.sub.FA and the side-to-side frequency f.sub.S2S. In this exemplary embodiment, the working structural indicator value R computed in block 21 is a ratio of fore-aft frequency f.sub.FA to side-to-side frequency f.sub.S2S. The structural indicator value R determined during the tower frequency measurements following installation and commissioning of the wind turbine is stored in a memory 22 as a reference structural indicator value R.sub.ref. As long as the tower remains structurally sound, the working structural indicator value R should remain essentially identical to this reference structural indicator value R.sub.ref. During the lifetime of the wind turbine, the working structural indicator value R is constantly computed and compared to the reference structural indicator value R.sub.ref. If the working structural indicator value R differs from the reference structural indicator value R.sub.ref by more than a predefined threshold .sub.OK, a fault report signal 230 is issued by a watchdog module 23 and passed to a suitable control module 24, for example to a control module 24 of the wind turbine controller or to a module 24 in a remote monitoring environment. Appropriate steps can then be taken, for example steps to shut down the wind turbine until damage to the tower structure can be repaired. A suitable value for the threshold .sub.OK can be based on data collected in the past for similar towers, and/or can be derived from simulations carried out for that type of tower structure.

[0038] FIG. 3 shows a graph of fore-aft and side-to-side frequencies f.sub.FA, f.sub.S2S (Y-axis) over time (X-axis), for example in a time frame extending over several weeks, months or even years. The time scale extends from an initial time 0, representing the commissioning of the wind turbine. In a brief interval up to time t.sub.0 following commissioning, a reference structural indicator value R.sub.ref is computed and stored in a memory as explained in FIG. 2 above.

[0039] In this exemplary embodiment, the curves indicate a possible alteration in frequency following a gradual change to the ground underneath the wind turbine. For example, the seabed may subside slightly around time t.sub.s in the region of an offshore wind turbine, so that the tower height is effectively slightly larger, and a tower oscillation frequency is therefore slightly lowered. However, since the altered tower height affects tower oscillation in all directions, the working structural indicator value R of fore-aft frequency f.sub.FA to side-to-side frequency f.sub.S2S will remain essentially constant, as indicated in FIG. 4.

[0040] Of course, a change to the underlying ground may result in an increase in tower oscillation frequency, but since the tower oscillation is affected equally in all directions, the working structural indicator value R, i.e. the ratio of fore-aft frequency f.sub.FA to side-to-side frequency f.sub.S2S, will remain essentially unchanged.

[0041] FIG. 5 shows a graph of fore-aft and side-to-side frequencies f.sub.FA, f.sub.S2S (Y-axis) over time (X-axis), again over a time frame that might extend over several weeks or months. The curves indicate a possible alteration in frequency following the development of a crack at time t.sub.c in the tower structure. A crack will have a more noticeable effect in some oscillation directions and will be less noticeable in other oscillation directions. FIG. 5 shows a situation in which the fore-aft frequency f.sub.FA is unaffected by the crack, but the side-to-side frequency f.sub.S2S clearly deviates from the expected value. In this case, the working structural indicator value R, i.e. the ratio of fore-aft frequency f.sub.FA to side-to-side frequency f.sub.S2S will exhibit a noticeable step, as indicated in FIG. 6, clearly deviating from the reference structural indicator value R.sub.ref (computed up to time t.sub.0 during an interval following commissioning, as explained in FIG. 3 above). If this difference .sub.R exceeds an acceptable threshold .sub.OK, a fault report signal is issued as explained in FIG. 2 above.

[0042] Although embodiments of the present invention has been disclosed in the form of embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of embodiments of the invention. For example, acceleration and frequency could be evaluated in directions other than the fore-aft and side-side directions mentioned above. It may also be advantageous to interrupt measurement of the fore-aft and side-to-side frequencies during certain conditions that would deliver erroneous results, for example at certain rotational speeds of the aerodynamic rotor, during certain extreme pitch angles, etc. Furthermore, the reference structural indicator value R.sub.ref can be recalculated in the event of major repairs or alterations to the wind turbine such as rotor blade replacement, or following any permanent change to the seabed conditions such as a level change.

[0043] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0044] For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements.