Objective The aim of this paper is to study the influence of wave height on nonlinear wave loads on ships.

Methods A 20000 TEU container ship is selected as the research object. First, numerical research on ship wave loads under different wave heights is carried out. Based on the rigid body theory, the three-dimensional boundary element method and inner and outer region matching method (IORM) are adopted to calculate the velocity potential and wave excitation forces, and convergence analysis of the free surface range and grid division is also performed. Furthermore, the nonlinear factors of wave excitation force caused by the wetted surface and the hydrostatic restoring force are considered. A three-dimensional nonlinear time-domain method is employed to analyze the effects of these nonlinear factors on ship motion and wave loads under different wave heights.

Results The results show that the nonlinearity of wave loads on the ship's sectional profile is more pronounced than that of ship motion, and the ship's second-harmonic response increases with wave height.

Conclusions The second-harmonic phenomenon of nonlinear wave loads is mainly caused by the interaction of the nonlinear wave excitation force due to the changing wetted surface and the hydrostatic restoring force. This phenomenon significantly affects the magnitude of wave loads, the number of load cycles, and structural fatigue damage. Therefore, it is necessary to consider the second-harmonic response components of ship loads in the engineering design stage to enhance ship safety.