Abstract
Offshore wind turbine foundations are subject to 107–108 cycles of loadings in their designed service life. Recent research shows that under cyclic loading, most soils change their properties. Discrete Element Modelling of cyclic simple shear tests was performed using PFC2D to analyse the micromechanics underlying the cyclic behaviours of soils. The DEM simulation were first compared with previous experimental results. Then asymmetric one-way and two-way cyclic loading pattern attained from real offshore wind farms were considered in the detailed parametric study. The simulation results show that the shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on the strain amplitude, initial relative density and vertical stress. It shows that the change of soil behaviour is strongly related to the variation of coordination number, rotation of principal stress direction and evolution of degree of fabric anisotropy. Loading asymmetry only affects soil behaviours in the first few hundred of cycles. In the long term, the magnitude of (γmax − γmin) rather than loading asymmetry dominates the soil responses. Cyclic loading history may change the stress–strain behaviour of a soil to an extent dependent on its initial relative density.
Highlights
1.1 Background and motivationOffshore wind turbine foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P and 2P/3P loads
Higher shear stress level and shear modulus are correlated to higher coordination number and higher magnitude of fabric anisotropy
In each symmetric loading cycle, majority of the principal direction rotation occurs between half of γmin and half of γmax in spite of the magnitude of γmax
Summary
Offshore wind turbine foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Intensive investigations have been conducted using laboratory element tests to examine the accumulated deformation [16, 17] and variation of stiffness [18, 19] of soils under cyclic loading in different load conditions These experimental studies have no access to the micro-mechanism underlying the soil responses. Discrete element modelling of cyclic tests were widely adopted to link the macroscale soil responses and underlying micro-mechanism during cyclic loading including [20,21,22] These studies were limited to symetric two-way loadings, which are different from the real loading scenarios in offshore wind farms as found by Jalbi et al [15]. This is a knowledge gap to fill in the current study
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