A Study On The Prediction Method Of Ship Motions

In marine hydrodynamics field, potential flow theories are widely used to predict the ship motions and wave loads in waves. Among various potential theories, strip theory is the most popular, because it can give the predicted ship motions and wave loads with sufficient engineering accuracy that are useful for design. For the calculation of wave diffraction forces, there are several different methods that still need to be investigated. In addition, lots of experiments of ship models and ships show that the motion responses and wave loads of ships oscillating by a large amplitude are evidently nonlinear, and many problems in this field haven't been solved by now owing to their complexity. This paper tries to probe into the following problems in frequency and time domain.First, based on the strip theory, a model is established in frequency domain to describe the motion of ships at low speed, in which irregular frequency effects are removed with the extended boundary integral equation method being used. Diffraction forces and moments are obtained using different methods. Then their effects to the motion responses are discussed. Large amount of computations are carried out for ships, and the comparisons between calculation and model test are conducted.Second, the methods to predict the nonlinear motions of ships in head waves are discussed. With the nonlinear factors due to the instantaneous position variation of body surface being considered, the nonlinear motion equations are established and solved in time domain, in which the incident wave forces and hydrostatic restoring forces are calculated according to the instantaneous body surface condition and the radiation forces are obtained by radiation potential with memory effect and the diffraction forces are obtained in frequency domain., Theprogram has been developed with the dominant nonlinear factors of incident wave forces and hydrostatic restoring forces being considered. Then the nonlinear fluid forces, nonlinear motions of S-175 container ship are computed, and the comparison among the results of the linear frequency domain method and the linear time domain and the nonlinear time domain methods are offered.At last, the radiation forces are calculated using radiation potential (considering memory effect). Different solutions for a simplified model of the heave and pitch motions of a ship hull due to a moving load and a comparison with some experimental results are presented.