Numerical and experimental investigation of breaking wave forces on a monopile-type offshore wind turbine

Abstract

Monopiles are the most commonly used foundation types for offshore wind turbines. This study investigates numerically and experimentally the effects of breaking wave impact on a monopile-type offshore wind turbine installed at different locations at the edge of a 1:25 slope. The numerical model uses a two-phase flow model based on solving the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with using the volume of fluid method and the k-ω shear stress transport turbulence model. Typical environmental conditions from the East China Sea have been examined. The wave run-up around the monopile and the nonlinear total horizontal wave force on the monopile are examined and compared with relevant experimental data originally presented in the present paper in order to validate the accuracy of the developed numerical model. A good agreement between the experimental data and numerical predictions is observed. Moreover, the wave breaking evolution and impact kinematics are investigated numerically for the different monopile locations. The results show that there is a 23.5% interval between the maximum wave force in the breaking zone and the wave force at the beginning of breaking. The first-order energy proportion of the breaking wave force will decrease maximum 61.65%, while, the second-order and high-order energy proportion of the breaking wave force will increase maximum 25.92% and 17.57%, respectively, between the different examined monopile locations. The time of occurrence of the secondary load cycle has no great relation with the form of the breaking wave for all examined locations, and the duration time of secondary load cycle impact on the monopile lasts for approximately 16.2% of the wave period. The secondary load cycle is smaller compared to the total horizontal breaking force in a range between 16.37% and 8.5%. The pressure on the monopile increases along with the wave breaking gradually. Compared with the pressure impact on the monopile when the wave begins to break, the pressure exerted on the monopile after the wave is rolled increases by 149%.