Dynamic response of real offshore wind turbines on monopiles in stratified seabed

Abstract

This paper studies the influence of the superstructure and foundation parameters of offshore wind turbine (OWT) systems, as well as of the soil profile, on the magnitude of the associated soil-structure interaction (SSI) phenomena. The analyses are carried out by assuming characteristic properties of real OWTs and of soil profiles based on North Sea boreholes. To do so, a simplified sub structuring model is proposed for the computation of the fundamental frequency and equivalent damping of OWTs founded on monopiles including the SSI effects. The whole superstructure is reduced to a three-degrees-of-freedom system through its modal mass and height, while the foundation stiffness is represented by impedance functions. The pile impedance functions are computed by a time-harmonic integral model that makes use of Green's Functions for the layered half space to represent the soil behaviour, while the pile is represented by finite elements as a Timoshenko beam and treated as a load-line within the soil. The obtained results confirm the necessity of considering the SSI effects for an accurate estimation of both the fundamental frequency and equivalent damping of the soil-structure system. Regarding the pile dimensions, the pile diameter plays a significant role on the magnitude of the SSI effects, while the pile length has almost no influence. On the other hand, the results highlight the importance of a good knowledge of the soil profile, as high differences are produced between the homogeneous and variable-with-depth profiles, even when both present the same mean shear velocity. The superficial soil layers are found to be the ones of crucial importance when evaluating the SSI effects on the dynamic properties of OWT systems.