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Geomechanics and Engineering
  Volume 37, Number 2, April25 2024 , pages 167-178

Analytical framework for natural frequency shift of monopile-based wind turbines under two-way cyclic loads in sand
Yang Wang, Mingxing Zhu, Guoliang Dai, Jiang Xu and Jinbiao Wu

    The natural frequency shift under cyclic environmental loads is a key issue in the design of monopile-based offshore wind power turbines because of their dynamic sensitivity. Existing evidence reveals that the natural frequency shift of the turbine system in sand is related to the varying foundation stiffness, which is caused by soil deformation around the monopile under cyclic loads. Therefore, it is an urgent need to investigate the effect of soil deformation on the system frequency. In the present paper, three generalized geometric models that can describe soil deformation under two-way cyclic loads are proposed. On this basis, the cycling-induced changes in soil parameters around the monopile are quantified. A theoretical approach considering three-spring foundation stiffness is employed to calculate the natural frequency during cycling. Further, a parametric study is conducted to describe and evaluate the frequency shift characteristics of the system under different conditions of sand relative density, pile slenderness ratio and pile-soil relative stiffness. The results indicate that the frequency shift trends are mainly affected by the pile-soil relative stiffness. Following the relevant conclusions, a design optimization is proposed to avoid resonance of the monopile-based wind turbines during their service life.
Key Words
    monopile; natural frequency; shift; soil deformation; two-way cyclic loads
Yang Wang: School of Civil and Architectural Engineering, Shandong University of Technology, Zibo, China
Mingxing Zhu: School of Civil Engineering and Architecture, Jiangsu University of Science and Technology, Zhenjiang, China
Guoliang Dai: School of Civil Engineering, Southeast University, Nanjing, China
Jiang Xu: Institute of Geotechnical Engineering, Yangzhou University, Yangzhou, China
Jinbiao Wu: School of Resource and Safety Engineering, Central South University, Changsha, China

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