Abstract
Abstract. The design of foundations for offshore wind turbines (OWTs) requires the assessment of long-term performance of the soil–structure interaction (SSI), which is subjected to many cyclic loadings. In terms of serviceability limit state (SLS), it has to be ensured that the load on the foundation does not exceed the operational tolerance prescribed by the wind turbine manufacturer throughout its lifetime. This work aims at developing a probabilistic approach along with a reliability framework with emphasis on verifying the SLS criterion in terms of maximum allowable rotation during an extreme cyclic loading event. This reliability framework allows the quantification of uncertainties in soil properties and the constitutive soil model for cyclic loadings and extreme environmental conditions and verifies that the foundation design meets a specific target reliability level. A 3D finite-element (FE) model is used to predict the long-term response of the SSI, accounting for the accumulation of permanent cyclic strain experienced by the soil. The proposed framework was employed for the design of a large-diameter monopile supporting a 10 MW offshore wind turbine.
Highlights
Offshore wind turbines are slender and flexible structures which have to withstand diverse sources of irregular cyclic loads
The aim of the contour diagrams is to provide a 3D variation in the accumulated permanent strain in the average stress ratio (ASR), which is the ratio of the average shear stress to the initial vertical pressure or confining pressure; the cyclic stress ratio (CSR), which is the ratio of the cyclic shear stress to the initial vertical pressure or confining pressure; and the number of cycles (N )
During the lifetime of wind turbines, storms, typhoons or seismic action are likely to cause permanent deformation of the structure owing to the accumulation of plastic strain in the soil surrounding the foundation
Summary
Offshore wind turbines are slender and flexible structures which have to withstand diverse sources of irregular cyclic loads (e.g. winds, waves and typhoons). The design of an offshore structure mostly starts with the static load design step in which a loop between the geotechnical and structural engineers is required to converge to a set of optimal design dimensions (pile diameter, pile length and can thickness) This phase is governed by the ULS in which it must be ensured that the soil’s bearing capacity withstands the lateral loading of the pile within the allowable deformations (i.e. pile deflection and pile rotation at the mud-line). The cyclic stresses transferred to the soil can reduce the lateral resistance by means of liquefaction (ULS); can change the soil stiffness which can cause resonance problems (FLS); and can progressively accumulate deformation into the soil, leading to an inclination of the structure (SLS) If one of these limit states is not fulfilled, cyclic loads are driving the design and the foundation dimensions should be updated. An application of the proposed RBD framework is presented for a large-diameter monopile supporting a 10 MW offshore wind turbine
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