Monitoring resonant frequencies and damping values of an offshore wind turbine in parked conditions

Abstract

This study shows the first results of a long-term monitoring campaign on an offshore wind turbine in the Belgian North Sea. It focuses on the continuous monitoring of the resonant frequencies and damping values of the most dominant modes of the support structure. These parameters allow to better understand the dynamics of offshore wind turbines and are crucial in the fatigue assessment during the design phase. They can also help to minimise operation and maintenance (O&M) costs and to assess the lifetime of the offshore wind turbines structures during their operation. To do an accurate continuous monitoring of these parameters, a state-of-the-art operational modal analysis technique has been automated, so that no human-interaction is required and the system can track small changes in the dynamic behaviour of the offshore wind turbine. The study will analyse the resonant frequencies and damping values of the most dominant modes shapes while the wind turbine is in parked conditions.

Automatic Tracking of the Modal Parameters of an Offshore Wind Turbine Drivetrain System

“…From Tables 1 and 2, one can see that the mean value of the resonance frequencies of the global tower modes, the blade modes, and the first torsional drivetrain mode obtained from the signals [7].…”

“…To do so, a short-term tracking of the modal parameters of the vibration modes in a low frequency-band (i.e., 0-2 Hz) will be done over the different 12 datasets, and the results from this analysis will be compared to published ones [7,8] for the same turbine. The modes in this band are basically the dominant vibration modes of both the tower and the turbine's blades.…”

“…The modes in this band are basically the dominant vibration modes of both the tower and the turbine's blades. According to the results of some previous intensive studies [7][8][9][10][11][12] on the identification of the physical vibration modes of the tower, foundation system, and the blades of the instrumented OWT, • The mode shapes together with the resonance frequency values of those modes are shown in Figure 2. Based on the work done in [8], some other modes related to the blades and the drivetrain can be detected in the frequency band 0-2 Hz.…”

“…Those modes are the first drivetrain torsional mode (DTT1), the first blade asymmetric flapwise pitch (B1AFP) mode, the first blade asymmetric flapwise yaw (B1AFY) mode, the first blade collective flap (B1CF) mode, and the first blade asymmetric edgewise yaw (B1AEY) mode. In Tables 1 and 2, the mean value of the resonance frequencies of the tower, blades, and low-frequency drivetrain mode are summarized and compared to the reference values [7,8]. One can see from Table 1 that the dominant tower's modes are successfully detected except the third mode, i.e., FA2 at 1.20 Hz.…”

“…The selected 12 data records are consecutively processed in frequency domain for a frequency band goes up to 2 Hz, and the most dominant in-band modes are estimated and manually tracked based on the frequency values presented in [7,8]. The auto and cross power spectra are estimated using the Correlogram approach [13,14] as it was used in the reference article (i.e., [7]) to estimate the frequency-domain data.…”