Structural monitoring for lifetime extension of offshore wind monopiles: verification of strain-based load extrapolation algorithm

Lifetime extension needs low-cost assessments that can identify the remaining useful life of offshore wind monopiles. A novel concept for load monitoring was developed that only needs strain gauges installed at one level of the support structure. Damage equivalent loads were calculated from strain measurements and extrapolated along a monopile using a regression algorithm. In this paper, the assumptions behind the load extrapolation algorithm were verified with two consecutive months of measurement data from an offshore wind park. The verification was performed separately for two offshore wind turbines. Both turbines had strain gauges installed at a distance of approximately 15 m and 25 m. Results show that monthly damage equivalent loads can be predicted with errors smaller than 4% based on measurement data only. Prediction using linear regression resulted in similar results for the total fatigue damage as a nonlinear k-nearest neighbor approach, but individual 10-min damage equivalent loads showed larger differences than for the more robust k-nearest neighbor algorithm, especially for small loads. These results are very promising and should motivate further research.