Abstract: In order to study the impact on offshore platforms caused by nonlinear freak waves, a numerical wave tank is built, in which the incompressible Navier-Stokes equations are solved, with the free surfaces reconstructed using a VOF method. Nonlinear freak waves based on the Peregrine breather solution are generated. A fluid-structure interaction (FSI) algorithm is used to calculate the structural response of the bottom deck, which is discretized with the finite element method (FEM). The impact loads underneath a platform model are calculated and compared with the ones caused by the 2nd-order regular waves with the same wave lengths and heights to reveal the unique features of the impact caused by nonlinear freak waves. The structural response of the bottom deck is calculated using the FSI algorithm and processed with an FFT method to analyze the wetted vibration. Additionally, the influence of the vertical initial velocity is considered during the structural response simulations.

Key words: nonlinear freak wave, Peregrine breather, fluid-structure interaction (FSI), structural response, impact underneath the bottom deck