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

• Published in: Journal of physics. Conference series Vol. 524; no. 1; pp. 12088 - 10
• Main Authors: Versteijlen, W G, van Dalen, K N, Metrikine, A V, Hamre, L
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.01.2014
• Subjects: Building codes; Design parameters; Finite element method; Foundations; Half spaces; Interaction models; Mathematical models; Offshore; Physics; Resonant frequencies; Resonant frequency; Shear modulus; Soil (material); Soil structure interactions; Soil-structure interaction; Springs (elastic); Stiffness; Three dimensional models; Wind turbines
• ISSN: 1742-6596; 1742-6588; 1742-6596
• DOI: 10.1088/1742-6596/524/1/012088

The fundamental natural frequency as measured on installed offshore wind turbines 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. The small-strain soil stiffness is an important design parameter, as it has a defining influence on the first natural frequency of these structures. In this contribution, in situ seismic measurements are used to derive the small-strain shear modulus of soil as input for 3D soil-structure interaction models to assess the initial soil stiffness at small strains for offshore wind turbine foundations. A linear elastic finite element model of a half-space of solids attached to a pile is used to derive an equivalent first mode shape of the foundation. The second model extends the first one by introducing contact elements between pile and soil, to take possible slip and gap-forming into account. The deflections derived with the 3D models are smaller than those derived with the p- y curve design code. This higher stiffness is in line with the higher measured natural frequencies. Finally a method is suggested to translate the response of 3D models into 1D engineering models of a beam laterally supported by uncoupled distributed springs.