Geomechanics and Engineering

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Analytical framework for natural frequency shift of monopile-based wind turbines under two-way cyclic loads in sand

  • Yang Wang (School of Civil and Architectural Engineering, Shandong University of Technology) ;
  • Mingxing Zhu (School of Civil Engineering and Architecture, Jiangsu University of Science and Technology) ;
  • Guoliang Dai (School of Civil Engineering, Southeast University) ;
  • Jiang Xu (Institute of Geotechnical Engineering, Yangzhou University) ;
  • Jinbiao Wu (School of Resource and Safety Engineering, Central South University)
  • Received : 2023.09.12
  • Accepted : 2024.04.02
  • Published : 2024.04.25
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Abstract

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.

Keywords

Acknowledgement

The research described in this paper was financially supported by the National Natural Science Foundation of China (52078128) and Natural Science Foundation of Shandong Province (ZR2023QE172)

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