Vibration based structural health monitoring of the substructures of five offshore wind turbines

In 2011 a first vibration monitoring system was installed on a single Belgian offshore wind turbine to research the possibility to monitor the structural integrity of the wind turbines substructure using accelerometers. From 2011 to 2017 four more wind turbines have been equipped with a similar setup. A combined total of 15 years of vibration measurements on all five turbines has been collected. In this contribution we will focus how vibration measurements using accelerometers can be used to support operators in decisions on the structural health of their assets. In the first part of this contribution the vibration behavior of a (offshore) wind turbine will be discussed using measurements obtained from the five monitored turbines. It will be shown how wind conditions, such as wind speed and turbulence, have an effect on the vibration levels of the turbine. In addition the interaction between loads and the tower dynamics will be investigated. In the second part the focus will be put on the automated operational modal analysis (OMA) that is applied to the measured accelerations. From this automated OMA a large dataset of resonance frequencies and damping values was obtained. The paper will discuss how results from monitoring the resonance frequencies can be used to detect bottom-erosion (i.e. scour) and potentially can be used to monitor the condition of the rotor