Offshore wind farms located in a shallow water environment and in areas prone to hurricanes may experience extreme breaking waves. Although little is known about their characteristics, breaking waves may represent a dominant load that a wind turbine structure is subjected to during its design lifetime. Existing analysis approaches may have limited applicability, especially for large structures.
The objectives of the conducted study were to:

• create a computational (CFD) environment of a wave tank (wave generation and propagation with minimum dissipation, wave breaking initiation and wave termination without reflection)

• perform CFD simulations of wave behaviour in an intermediate and shallow depth water environment and to validate the computational results against published experimental data

• perform CFD simulations of wave slamming on a cylindrical monopile for previously studied wave conditions

• statistically evaluate the simulation results to obtain the time variation of breaking wave forces and associated moments

The modelling analysis was performed with ANSYS CFX and Fluent. The heterogeneous multiphase model was used in CFX, and the homogeneous multiphase model with Volume-of-Fluid and Level Set method in Fluent. The simulation domain was 1500 m long with the water depth of 54 m. Waves were generated with a flap type wave generator [1]. Their initial height was 13.6 m and the wave length 248 m.

Time variation of the streamwise force was calculated for a monopile with the diameter of 6 m. The diagram shows individual records divided into 50 s time intervals.

When the wave breaks at the monopile, the force the monopile is subjected to is the highest. It is significantly reduced if the wave breaks before or after the monopile. Single peaks in the force diagram mark waves passing the monopile before breaking, whereas groups of multiple peaks identify breaking waves upstream of the monopile. The diagram shows that the peak wave forces range between 5 and 34 MN.
The height of breaking waves was compared with the experimental data collected by Battjes [2] and a good agreement was demonstrated. The calculated forces to the monopile were also compared to the empirical correlation used by IEC guidelines [3]. Although, the force amplitude is identical and determined by the wave height, the CFD simulation shows the force variation and duration that cannot be captured by the empirical correlation.