Strain estimation for offshore wind turbines with jacket substructures using dual-band modal expansion

Structural fatigue is a design driver for offshore wind turbines (OWT). In particular, the substructures, like jackets, are strongly affected by fatigue. Monitoring the fatigue progression in the welds is vital for the maintenance and a potential lifetime extension. However, inspections of critical locations are costly due to the limited accessibility of the mostly submerged jacket. Considering the high number of potentially critical welds, it is regarded as economically unfeasible to equip all fatigue hot spots with sensors. Thus, an indirect method to monitor the fatigue progress of the structure and point out critical locations is desirable. For a consistent support of ongoing maintenance, it has to yield reliable results for varying operational and environmental conditions. This paper applies a virtual sensing approach to jacket substructures. From a small set of sensors on the tower, fatigue at every desired location of the jacket is estimated using dual-band modal expansion. Simulations using the OC4 jacket design are performed to show potentials and limitations of the method. Namely fatigue progress on leg welds of K-joints is predicted with high accuracy over a wide range of load cases. However, some difficulties in fatigue prediction of X-joints due to the occurrence of local modes and limitations in the extrapolation of wave loading have to be resolved in future work.