The dynamics of an offshore wind turbine in parked conditions: a comparison between simulations and measurements

Abstract

Offshore wind turbines are complex structures, and their dynamics can vary significantly because of changes in operating conditions, e.g., rotor-speed, pitch angle or changes in the ambient conditions, e.g., wind speed, wave height or wave period. Especially in parked conditions, with reduced aerodynamic damping forces, the response due to wave actions with wave frequencies close to the first structural resonance frequencies can be high. Therefore, this paper will present numerical simulations using the HAWC2 code to study an offshore wind turbine in parked conditions. The model has been created according to best practice and current standards based on the design of an existing Vestas V90 offshore wind turbine on a monopile foundation in the Belgian North Sea. The damping value of the model's first fore-aft mode has been tuned on the basis of measurements obtained from a long-term ambient monitoring campaign on the same wind turbine. Using the updated model of the offshore wind turbine, the paper will present some of the effects of the different design parameters and the different ambient conditions on the dynamics of an offshore wind turbine. The results from the simulations will be compared with the processed data obtained from the real measurements. The accuracy of the model will be discussed in terms of resonance frequencies, mode shapes, damping value and acceleration levels, and the limitations of the simulations in modeling of an offshore wind turbine will be addressed.

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

“…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.…”

“…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.…”

“…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.…”