Abstract

Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, rotor and blade shadowing. Under cyclic loading, most soils change their characteristics including stiffness, which may cause the system natural frequency to approach the loading frequency and lead to unplanned resonance and system damage or even collapse. To investigate such changes and the underlying micromechanics, a series of cyclic simple shear tests were performed on the RedHill 110 sand with different shear strain amplitudes, vertical stresses and initial relative densities of soil. The test results showed that: (a) Vertical accumulated strain is proportional to the shear strain amplitude but inversely proportional to relative density of soil; (b) Shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes and the shear modulus remains below an asymptote; (c) Shear modulus increases with increasing vertical stress and relative density, but decreasing with increasing strain amplitude. Coupled DEM simulations were performed using PFC2D to analyse the micromechanics underlying the cyclic behaviour of soils. Micromechanical parameters (e.g. fabric tensor, coordination number) were examined to explore the reasons for the various cyclic responses to different shear strain amplitudes or vertical stresses. Both coordination number and magnitude of fabric anisotropy contribute to the increasing shear modulus.

Highlights

  • Designing foundations for Offshore wind turbine (OWTs) are challenging as these are dynamically sensitive structures in the sense that natural frequencies of these structures are very close to the forcing frequencies [1]

  • A designer apart from predicting the global natural frequency of the structure, must ensure that the overall natural frequency due to dynamic-soil-structureinteraction does not shift towards the forcing frequencies [2, 3]

  • The global natural frequency is dependent on the soil-structure stiffness, an investigation of soil stiffness under cyclic loading is desired

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Summary

Introduction

Designing foundations for Offshore wind turbine (OWTs) are challenging as these are dynamically sensitive structures in the sense that natural frequencies of these structures are very close to the forcing frequencies [1]. A designer apart from predicting the global natural frequency of the structure, must ensure that the overall natural frequency due to dynamic-soil-structureinteraction does not shift towards the forcing frequencies [2, 3]. The interactions between the monopile and the surrounding soil could be represented by cyclic simple shear tests. Cyclic responses of soil stiffness have been studied intensively [4, 5], but few of them were focused on the micromechanism underlying these responses. The aim of this paper is to study the soil stiffness responses under cyclic simple shear loading with a focus on the micromechanics. The micromechanical parameters are analysed to find the relationships between the micromechanical parameters and macroscopic responses

Experimental tests
Macro-scale responses
Findings
Fabric
Full Text
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