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.

The work presented in this study describes a comparative study between different techniques aimed at identifying the damping values of an offshore wind turbine on a monopile foundation. It will be shown that damping ratios can directly be obtained from vibrations of the tower under ambient excitation from wave and wind loading. The results will be compared with the damping values obtained from a commonly used overspeed stop. Ambient vibration tests have the strong advantage of being more practical and less demanding for the wind turbine in comparison with the overspeed stop. Several identification algorithms, the standard exponential decay method, alternative procedures in the time domain as well as more advanced operational modal analysis techniques in the frequency domain will be applied to the experimental data. These data have been obtained during a short measurement campaign on an offshore wind turbine in the Belgian North Sea. The results of the used methods for estimating the modal damping of a wind turbine excited by ambient excitation will be discussed and compared. This study also presents some aspects related to the practical implementation of the measurements.

In this contribution, first, the results in the development of a structural health monitoring approach for the foundations of an offshore wind turbine based on its resonance frequencies will be presented. Key problems are the operational and environmental variability of the resonance frequencies of the turbine that potentially conceal any structural change. This article uses a (non-)linear regression model to compensate for the environmental variations. An operational case-by-case monitoring strategy is suggested to cope with the dynamic variability between different operational cases of the turbine. Real-life data obtained from an offshore turbine on a monopile foundation are used to validate the presented strategy and to demonstrate the performance of the presented approach. First, the results indicate an overall stiffening of the investigated structure.

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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.