A Study On Nonlinear Phenomena In Ship Wave-making And Motions Of Floating Structures

The marine industry of China has a great demand on the numerical methods and software for the efficient and effective design of ship and floating structures with respect to hydrodynamics. Rankine panel methods are very effective tools for investigating the ship nonlinear wave making and motions of floating structures in time domain. The numerical methods based on Rankine panel method for the analysis of ship nonlinear wave making, nonlinear ship motions and the motions of moored ocean structures are established in this paper. The main contents can be summerised as follows:Firstly, the pre-process system for the efficient generation of hull panels is developed by using the non-uniform B spline method. For the sake of engineering utility of the pre-process system, the interface between pre-process system and ship design software is developed.Based on the numerical calculation method for the linear wave-making, the numerical method for the nonlinear wave-making is developed in this paper. In order to satisfy the nonlinear free surface conditions, it is necessary to apply iterative approach and to take into account the actually wetted surface during iteration. To raise the free surface panels above the calm water is to imporve the numerical robustness, while the collocation points shift forward to meet the radiation condition. Numerical simulations and experimental validations of two commercial ships show that the present method is suitable for the lines optimzaiton.Furthermore, adopting the results of wave-making numerical simulation as the initial values of RANS solver, a new numerical method is developed. Computations for two ships are carried out to investigate the effect on parallel computing speed of RANS solver by this initialization method. In comparison with the traditional method, the computational time of the method presented in the paper can be reduced by about 40%~50%.By adopting the Rankine panel method, the linear and nonlinear time domain numerical methods for ship motions in waves are established. The linear boundary conditions on free and mean wetted body surface are still used in present method, while numerical damping is adopted to satisfy the radiation conditions. The numerical simulations of several vessles in head, oblique regular waves and long, short crested irregular waves are carried out by the linear and nonlinear time domain methods separately, and the numerical results of nonlinear method agree well with the results of model tests. It's shown the nonlinear time domain numerical method can be applied to predict the nonlinear ship motions in rough seas.At last, the method presented in the paper is extended to predict the motions of moored SPAR platform in waves. A direct time domain coupled analysis is developed by adding the restoring forces, which is computed by centenary theory, to the 6DOF equations of motions. Three levels of simulations are carried out in this paper, the computed results show that: the Rankine panel method can capature the nonlinear effects of wave amplitudes of regular waves; the computed transfer functions of a spring-moored SPAR agree well with the experimental results; the low frequency effects of the coupling motions of SPAR and mooring systems can figured out by nonlinear simulations, as the partical second-order forces can be included in the nonlinear time domain numerical method.