Structural health monitoring of offshore wind turbines using automated operational modal analysis

Abstract

This article will present and discuss the approach and the first results of a long-term dynamic monitoring campaign on an offshore wind turbine in the Belgian North Sea. It focuses on the vibration levels and modal parameters of the fundamental modes of the support structure. These parameters are crucial to minimize the operation and maintenance costs and to extend the lifetime of offshore wind turbine structure and mechanical systems. In order to perform a proper continuous monitoring during operation, a fast and reliable solution, applicable on an industrial scale, has been developed. It will be shown that the use of appropriate vibration measurement equipment together with state-of-the art operational modal analysis techniques can provide accurate estimates of natural frequencies, damping ratios, and mode shapes of offshore wind turbines. The identification methods have been automated and their reliability has been improved, so that the system can track small changes in the dynamic behavior of offshore wind turbines. The advanced modal analysis tools used in this application include the poly-reference least squares complex frequency-domain estimator, commercially known as PolyMAX, and the covariance-driven stochastic subspace identification method. The implemented processing strategy will be demonstrated on data continuously collected during 2 weeks, while the wind turbine was idling or parked.

Structural health monitoring of offshore wind turbines: A review through the Statistical Pattern Recognition Paradigm

“…This can be achieved through making the turbines more structurally flexible, which directly affects their modal parameters, i.e. the resonance frequency [47]. Another important aspect is that OWTs' inspection and maintenance is considerably more expensive than onshore turbines'.…”

“…These analyses are particularly relevant when they are carried out in service conditions [74]. However, performing these analyses accurately to a full scale OWT during operation is extremely difficult due to the high number of uncertainties which the offshore environment presents [47] and, therefore, special effort has been given to solve this issue in the past years [75]. One reason that makes this analysis difficult is the fact that wind and wave loading applied to the structure cannot be measured accurately in a continuous manner.…”

“…This phenomenon represents a threat for the turbine as the natural frequency gets closer to the rotor's frequency of rotation [47]. Therefore, continuous monitoring of WTs' dynamics variations due to scour is recommended as it is expected to be a useful tool for developing maintenance plans regarding scour protection [82].…”

A modal decomposition and expansion approach for prediction of dynamic responses on a monopile offshore wind turbine using a limited number of vibration sensors

“…In the present work, a state of the art operational modal analysis technique [29][30][31] called pLSCF, that has been automated [32][33][34][35][36][37]39] was used to identify the modal parameters (natural frequencies, modeshapes and damping ratios) from 2 weeks of operational data while the wind turbine was in parked conditions. During these two weeks, the frequencies and mode shapes can vary due to changing environmental conditions [32,[34][35][36][37].…”

One Year of Continuous Dynamic Monitoring of a Wind Turbine. Structural Integrity and Fatigue Assessment

“…Some examples of implementation of monitoring systems based on OMA in wind turbines are presented in [5,6].…”