Study On Survival Probability Of Nonlinear Roll Of Ship In The Random Seas

The stability criteria on the hydrostatics theories are used to estimate whether aship in the heavy seas capsize or not,this method ignores the time domaincharacteristics, frequency characteristics and the randomicity of ship motions. Thecapsizing accidents occur now and then although ships are designed according tostability criterion. So, it is also the important problem for human beings to investigatethe nonlinear characteristics of ship motions and evaluate the survival ability of ships.The dynamic characteristics of ship's rolling and the survival probability of ships areinvestigated in this thesis. The main contents and results are in the following:The six-degree-freedom differential equation of ships' motion was establishedconsidering the hydrodynamics and the propulsion of screw propeller. Taking intoaccount the large amplitude of rolling motion, the nonlinear differential equation ofrolling motion was obtained. Considering the randomicity of waves, the nonlinearityof damping and restoring moment and the instantaneous states, the method ofpredicting the survival probability was investigated by the harmonic accelerationmethod and the safe basin method in the hydrodynamics. The rolling response and thesafe basin of a 30m long fishing vessel were calculated for different initial conditions,significant heights and wave parameters. The safe basin and survival probability ofdifferent random phase angles were obtained. The influence on rolling motion andsurvival probability of wave parameters and initial conditions was investigated.Simultaneously considering the ship speed and the heading angle, the ship's safe basinand the survival probability were obtained under different significant wave heights inthe bow seas and the quartering seas respectively.The results show that the instantaneous states of ship motions and waveparameters influence ships' capsizing very much. The safe basins are stochastic in therandom waves. The distribution and the shape of the safe basin are different as therandom phase angles are changed. The ship in the bow seas is safer than in the beamseas. The more speed of ship increased, the more ship is safe for bow seas. But it ismore dangerous in the quartering seas than in the bow seas. As the ship speedincreases, the survival probability decreases in the quartering seas.