Abstract
The response of bucket foundations for offshore wind turbines subjected to cyclic loading in saturated clay is explored through three-dimensional finite element numerical analyses. In the analyses, nonlinear cyclic hysteretic behavior of clay under undrained condition is modeled through a simple kinematic hardening constitutive model embedded in ABAQUS. The finite element model is validated against published in situ tests of bucket foundations under quasistatic cyclic loading in Bothkennar clay. The computed results agreed generally with those from in situ tests. The behavior of bucket foundations with different aspect ratios under displacement-controlled cyclic loading mode is investigated. Then, the evolution of foundation displacement with increasing number of cycles is studied subjected to wind and wave combined loading. The results show that, for the cycles of low-amplitude rotation, dimensionless moment-rotation curve is approximately elastic; however, the curve engenders obvious hysteresis loop, whose shape is influenced by soil-sidewall interface condition, during high-amplitude cycles. Under thousands of loading cycles, for bucket foundations of low aspect ratio, the oscillatory displacement component is smaller; however, the residual component will accumulate gradually until the serviceability rotation is exceeded. For foundations of high aspect ratio, the oscillatory component is relatively larger, but the accumulation rate of residual displacement decreases gradually.
Highlights
With the gradual consumption of nonrenewable resources and the increasing power shortage, international efforts are being made to develop offshore wind power
E forces exerted on the bucket foundation for offshore wind turbines vary greatly from those of offshore platform. e vertical load from the upper tower and turbine blade is relatively small, while the horizontal load and moment component is larger
It is noted that long-term cyclic response is typically important design situation in the wind industry, whereas bearing capacity is dominating the design in the oil and gas industry
Summary
With the gradual consumption of nonrenewable resources and the increasing power shortage, international efforts are being made to develop offshore wind power. E stability of bucket foundation for offshore wind turbines under combined loading was studied by Fan et al and Liu et al, and the characteristics of failure envelope of bucket foundations under different load components were analyzed [3, 4]. To study the mechanical response of bucket foundation for offshore wind turbines under cyclic loading in clay, in the paper, the kinematic hardening model is used to simulate the stress-strain relationship of saturated clay under undrained conditions. Where C is initial kinematic hardening modulus which can be taken as elastic modulus of soil E and η controls the rate of back stress that recovers with the accumulation of equivalent plastic strain. Erefore the following equation can be used to calculate the value of η: E√
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