With the offshore wind industry rapidly expanding worldwide, geotechnical research is being devoted to foundation optimisation - most intensively for large-diameter monopiles. The analysis and design of monopiles still suffers from significant uncertainties in relation to cyclic/dynamic loading conditions. The aim of this work is to shed new light on dynamic soil-monopile interaction, based on the results of unique full-scale experiments performed at the Westermeerwind wind park (Netherlands). The response of a 24 m long, 5 m diameter monopile to harmonic lateral loading of varying amplitude and frequency is inspected. The analysis of original field measurements (soil accelerations and pore pressures) enables the lateral stiffness observed at the monopile head to be linked to dynamic effects occurring in the surrounding soil. The interpretation of measured data is supported by three-dimensional finite-element studies, also looking at the influence of drainage conditions and monopile size. The set of results presented supports the need for dynamics-based monopile design, as higher frequencies gain relevance in the most recent offshore wind developments.

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3D non-linear finite element analyses are proving increasingly beneficial to analyse the foundations of offshore wind turbines (OWTs) in combination with advanced soil modelling. For this purpose, the well-known SANISAND04 bounding surface plasticity model (Dafalias & Manzari 2004) is adopted in this work to incorporate key aspects of critical state soil mechanics into the analysis of monopile foundations in sand. The final 3D soil-foundation-OWT model is exploited to simulate the response of an 8 MW OWT to a long loading history of approximately 2 hours duration. The scope is to investigate/explain the drops in natural frequency observed in the field during storms, as well as its subsequent recovery. The numerical results point out a strong connection between transient frequency drops and pore pressure accumulation, whereas the original OWT natural frequency seems to be restored as a consequence of post-storm re-consolidation.@en

Soil stiffness and damping parameters are important to correctly model the structural response and energy dissipation, especially for embedded and lightly damped structures. This research aims to demonstrate that multichannel analysis of surface wave (MASW) techniques can be used to generate a reliable in-situ measurement of the soil damping in marine environments. The goal of the project is to refine and apply a reliable method which is able to estimate the soil stiffness and damping profile in the depth range of 0-30m. Love waves, which are rarely used for surface wave applications and to the author’s knowledge never used for a damping inversion from real surface waves measurements, are included in this analysis. The frequency content of the present dataset is lower than previous investigation, which is important for extrapolating the damping parameters to the even lower frequency range of relevance for structural models. Finally, it is proposed a novel technique for retrieving the frequency dependence of the material damping by using on-site collected data.@en