Research On Time-domain Three-dimensional Nonlinear Wave-induced Loads Calculation Method

Wave-induced loads of ships with large flares and forward speed will present obvious nonlinear effect. Linear potential theory cannot reflect that the nonlinearity. It is necessary to investigate the nonlinear potential theory of a ship in waves.It has been proved by many references that the nonlinearity caused by position variation of body surface is rather important. A method which considers the nonlinearity caused by the hydrostatic store forces and incident forces has been adopted in this paper. In this method, the pressures caused by hydrostatic store forces and incident waves are integrated on the instantaneous wetted hull surfaces while the pressures from the radiation potential and diffraction potential are integrated on the mean hull surface. Main effect of nonlinearity of wave-induced loads can be taken into consideration in this theory, and meanwhile it avoids updating the meshes at each moment. The Green function only needs to be calculated once just at the beginning time of the calculation. It is very effective.To obtain the velocity potential on the hull surface, the mixed-source method has been applied. The Rankine source and the transient Green function need be used in this method. The calculation method of the Rankine source is based on the near field and far field methods. The numerical computation of the transient Green function is really complex, the method of series expansion or asymptotic expansion is adopted in this paper to deal with this problem. Examples about this area are provided.Time-domain-linear, time-domain-nonlinear methods and software WASIM have been utilized to predict motions and wave-induced loads of a container ship and a bulk carrier respectively. The results of nonlinear method coincide with those of linear method in small wave heights according to the comparison. To investigate the nonlinearity of vertical bending moments, the evaluations of the nonlinear loads have been conducted in large wave heights. It shows that the nonlinearity of vertical bending moments is noticeable for the container ship with non-wall sides. However, the nonlinearity for the bulk carrier with wall sides is not so evident. The investigation of this paper has proved that the nonlinear theory can be employed for the prediction of nonli near wave-induced loads f or ships with non-wall sides and hi gh forward speeds.