Monopile Lateral Response Calibration from in-situ Monitoring Data

Monopiles are currently the most installed foundation type for offshore wind turbines but there is still a discrepancy between the predicted response in design and the in-situ lateral response during operation. While advances have been made in the understanding of lateral pile-soil interaction using numerical modelling, laboratory tests and field tests, this study uses in-situ strain measurements above and below the mudline to back-analyze the lateral pile-soil interaction. Monitoring data collected from offshore wind turbine monopiles allows for the derivation of measured bending moment profiles. Using insights from the literature and site-specific bathymetric and geotechnical data, response curves are optimized to obtain a best fit to the measured bending moment for a wide range of loading conditions. The calibration exercise highlights the importance of the mudline level, the resistance provided by the scour protection and the initial stiffness of the response curves. The calibration shows that the calculated response with site-specific small-strain stiffness and the PISA guidance corresponds well to the field observations. Using the calibrated response curves, a baseline for verification of the lifetime response of the offshore wind turbine can be created. Changes to the bathymetry or soil properties (e.g. cyclic degradation after severe storms) can thus be identified directly from the monitoring data.