Analysis on nonlinear wave loads of a 20 000 TEU container ships based on 'IORM'

Objectives The aim of this paper is to study the influence of wave height on nonlinear wave load.

Methods First, for a 20 000 TEU container ship, numerical research on ship wave loads under different wave heights is carried out. Then, based on the rigid body theory, the three-dimensional boundary element method and IORM (inner and outer region method) are adopted to calculate the velocity potential and wave excitation forces, and the convergence analysis is conducted on the range and mesh of the free surface. Finally, considering two nonlinear factors: wave excitation forces caused by wetted surface and hydrostatic restoring force. The three-dimensional nonlinear method in time domain is used to investigate the influence of nonlinear factors with different wave heights on ship motions and wave loads.

Results The results show that compared to the motions of the ship's hull, the frequency doubling phenomenon of wave external loads on the hull profile is more pronounced, exhibiting stronger nonlinearity. And as the wave height increases, the frequency doubling response of the ship increases.

Conclusions The frequency doubling phenomenon of nonlinear wave load of ships is caused by the nonlinear wave excitation force and hydrostatic recovery force caused by the change of wet surface of the hull, and the frequency doubling phenomenon has a great influence on the peaks and cycle times of wave loads, as well as structural fatigue damages. In the actual engineering design stage, it is necessary to fully consider the harmonic response components of ship loads to improve the level of ship safety design.