Assessing the small-strain soil stiffness for offshore wind turbines based on in situ seismic measurements

In this contribution, in situ seismic measurements are used to derive the small-strain shear modulus of soil as input for two soil-structure interaction (SSI) models to assess the initial soil stiffness for offshore wind turbine foundations. This stiffness has a defining influence on the first natural frequency of offshore wind turbines (OWTs), which is one of the most important design parameters of these structures. The fundamental natural frequency as measured on installed OWTs is significantly higher than its designed value, and it is expected that the explanation for this can be found in the currently adopted modeling of soil-structure interaction. In this paper a method is suggested to improve the accuracy of estimating the small-strain soil stiffness. The field data used in this study is measured with a seismic cone penetration test. A method is suggested to identify the shear moduli, which together with the measured in situ mass densities and estimated Poisson's ratios are input for two static 3D SSI models. The first model is a linear elastic finite element model of a half-space of solids attached to a pile (shell). This is a straightforward and fast approach in which a static horizontal force and a bending moment are applied at the top of the pile. The second model extends the first one by introducing contact elements at the interface between pile and soil, where a contraction of the soil towards the pile is not allowed and maximum friction forces on the interface can be prescribed. Finally a method is suggested to translate the global response of a 3D model into an engineering model of a 1D beam laterally supported by uncoupled distributed springs. When comparing the deflections, those derived with the global 3D models, are smaller than deflections derived with the p-y curve design code. The deflections of the global models behave more rigidly, and especially the upper 10m deflections are smaller. The presented work is a small part of the research that still needs to be done before any conclusion can be drawn.