Full-field strain estimation on an offshore wind support structure for fatigue monitoring

The design of monopile foundations for offshore wind turbines is most often driven by fatigue. With the foundation price contributing to the total price of a turbine structure by more than 30% wind farm operators seek to gain knowledge about the amount of consumed fatigue. Monitoring concepts are developed to uncover structural reserves coming from conservative designs in order to prolong the lifetime of a turbine. Amongst promising concepts is a wide array of methods using in-situ measurement data and extrapolate these model-based to desired locations below water surface and even seabed. The modal decomposition algorithm is used to obtain modal amplitudes from acceleration and strain measurements. In the subsequent expansion step these amplitudes are expanded to virtual measurements at arbitrary locations. The algorithm uses a reduced order model which is identified by operational modal analysis. Furthermore the measurements which are used as input for the algorithms are constrained to measurements from dry part of the substructure. However, with subsoil measurement data available from a dedicated campaign even validation for locations below mud-line is possible. Time and frequency domain analysis confirms the method capable of reconstructing strains with high precision over the entire range of operational and environmental conditions above and below seabed.