Lessons Learnt From Back-Analysis of Offshore Monopile Lateral Response

Monopiles are a widely used foundation type for offshore wind turbine structures. They support the tall and slender superstructure and have an impact on the dynamic behaviour of the entire structure. The thrust loads generated by the wind on the wind turbine blades and tower and the loads due to wind, waves and current on the submerged part of the structure lead to cyclic loading, with oscillating bending moments in the monopile. These bending moments are absorbed by the soil surrounding the monopile and the lateral pile-soil interaction governs the magnitude of the bending moments below the ground level. The dynamic behaviour of the wind turbine structure and the resulting fatigue stress cycles govern the design of offshore wind turbine structures. Hence, it is important to accurately predict the natural frequency of the wind turbine structure and the influence of the pile-soil interaction on the dynamics. To validate existing theoretical models, the WINDSOIL research project was set up to back-analyze pile-soil interaction stiffness and wind turbine structure natural frequency across all the structures in an offshore wind farm. The project made use of in-situ monitoring of natural frequencies for benchmarking the natural frequency estimates. The approach followed in the WINDSOIL project can be applied to other types of piled foundations or retaining structures where accurate predictions of displacement or dynamics is required. This paper describes the more accurate soil stiffness modelling which was carried out based on the results of in-situ tests and shows how the small-strain stiffness of the soil influences leads to more accurate estimation of the pile dynamics. The importance of modelling surcharges around the pile (scour protection in this case) is also highlighted.&nbsp