Dynamic soil-structure interaction in offshore wind turbines on monopiles in layered seabed based on real data

The present paper studies the soil-structure interaction (SSI) effects on the dynamic properties of offshore wind turbines (OWT) founded on monopiles. For that purpose, a three degrees-of-freedom substructuring model based on modal parameters is proposed. The whole superstructure is reduced to a punctual mass by its fundamental modal mass and height, while the pile-soil stiffness is represented by the corresponding impedance functions. The proposed model, together with characteristic relations between the fixed-base fundamental frequency and the modal parameters obtained from data of existent OWTs found in the literature, is used to analyse the influence of the superstructure and foundation dimensions and soil profile on the magnitude of the SSI phenomena. The obtained results confirm the relevance of including the foundation stiffness in the design stage of OWT systems, as variations in the fundamental frequency close to 15% can be produced. The homogeneous assumption, even if cs,30 mean values are assumed, yields to misleading results if the actual soil profile presents properties that vary with depth. The superficial layers of the soil profile are found to play a major role in the estimation of the OWT system fundamental frequency and damping when the SSI phenomena are included.