Probability Study Of Pure Loss Of Stability And Surf-riding/Broaching In Irregular Waves

It is possible that a ship will have a large nonlinear motion or even capsize in dangerous sea conditions.It seriously endangers people’s lives and property.Traditional static stability criterion is difficult to ensure the safe ship navigation.In recent years,IMO has devoted itself to formulating the Second Generation Intact Stability Criteria.Under the background,this paper takes the pure loss of stability and riding/broaching as research objects,and studies the failure probability of stability by split-time method,Monte-Carlo method,Melnikov method and Gaussian fitting,etc.In this paper,various models are introduced to simulate the ship motion in irregular waves,include the classic/improved 1.5-DOF rolling motion equation,1-DOF surge motion equation and a nonlinear 6-DOF united model.The improved 1.5-DOF rolling motion equation is used to simulate the ship’s stability change in irregular waves.The 6-DOF united model combines the ship maneuvering and seakeeping motion.It can accurately simulate the ship’s motion in longitudinal waves and oblique waves.The 4-order Runge-Kutta method is used to approximately solve the above equation of motion.The single degree of freedom models have more advantages in computational efficiency,while the united model has higher accuracy.At the same time,this paper is devoted to improving and verifying the process of split-time method,which is used to estimating capsizing probability in pure loss of stability and surf-riding probability.Split-time method separates the complex problem into two less complex problems: a non-rare problem and a rare problem.It obtains the probability of ship instability by fitting distribution and statistical extrapolation,and solves nonlinearity,randomness and rarity of ship instability.In this paper,the influence of the variation of roll threshold and critical roll angular velocity on the estimation results is analyzed,and a process of capsizing probability in pure loss of stability is proposed.The method is also extended to the calculation of surf-riding probability in irregular longitudinal waves.The above processes are applied to ITTC ship A2.The probabilities estimated by split-time method and Monte-Carlo method are compared,and the results are similar.It proves the superiority and accuracy of split-time method in estimating capsizing probability in pure loss of stability and surf-riding probability.In this paper,the probability of surf-riding is also studied by other methods,which includes Melnikov method and fitting probability density distribution.The ship broaching phenomenon is also simulated by the united model of maneuvering and seakeeping equation.And the probability of broaching is counted.It is found that with the increase of the speed,both the probability of surf-riding and broaching increases.So maintaining a reasonable speed is very important to ensure the normal navigation of ships.