Advanced simulations of monitored wind turbine monopiles located in the Belgian North Sea under operational quasi-static loading

Design codes and recommended practices for geotechnical structures generally include some conservatism to take into account any possible source of uncertainty, with conservative design being generally accepted as safe. Such approaches are not appropriate for offshore wind turbines where both over-and-under designs would result in a sub-optimal performance in terms of structural dynamics. The soil surrounding wind turbines is a main contributor to their stiffness, it is also one of the major sources of uncertainty, which makes an accurate prediction of the structural dynamics a rather difficult task. Monitoring data on wind turbines can provide useful insight into their real in-situ behaviour, allowing designers to examine the validity of design assumptions further. In this paper, the bending moments of five monitored wind turbine monopiles are back analysed, using 3D FEM and 1D beam–column models in which the soil is modelled according to API RP2 GEO recommendations, and the rule-based PISA reaction curves. It is found that despite the efforts to get an as accurate soil response as possible, the monitored bending moments are overestimated by the 3D FEM models at some locations, reflecting a mismatch between the simulated and monitored wind turbine monopiles. The possible effect of scour protection is highlighted. The API-based 1D beam–column models overestimate bending moments, rotations, and displacements and underestimate the wind turbine stiffness, while the PISA models are able to give satisfactory results.