Method On Stability And Voyage Safety Of Large Container Ship In Wind And Waves

With the increasing ship dimensions and the improved status in maritime shipping,more and more attention are paid to container ships,and the navigation safety has been one of the most concerned problems by ship designers and ship operators.It is likely to occur parametric rolling in head seas and synchronous rolling in beam seas in container ships' navigation,which usually results in excessive motion,excessive acceleration and ensuing damage and missing of loading cargo.In some severe situations,the disastrous ship capsizing could happen,which causes huge loss of people and property.Meanwhile,the second generation intact stability criteria requires that the latest research results of modern ship mechanics be adopted for further study on mechanism of stability failure.For this reason,the nonlinear dynamics is adopted to study the nonlinear rolling stability and navigation safety of large container ships in wind and wave.The main contents can be summarized as follows:The large-amplitude ship rolling motion is in fact a nonlinear dynamical system.In order to analyze the characteristics of rolling motion,the nonlinear factors should be firstly determined.The nonlinear factors come mainly from the nonlinear restoring terms and nonlinear damping terms,and the two kinds of nonlinear terms are respectively determined by least squares fitting and carrying out roll decay experiment in calm water.Multiscale method is used to analytically solve the approximate rolling response in regular beam seas and the analytic solutions are verified by Runge-Kutta simulation,meanwhile,the effects of the two kinds of nonlinear terms on stable solutions are qualitatively analyzed by virtue of amplitude-frequency response.Taking the actual seaway conditions in shipping routes into account,research methods of stability in two kinds of dynamical systems are discussed,namely,deterministic system and stochastic system.For deterministic system,the general Melnikov method is widely used.For stochastic system,the random Melnikov mean square criterion and the rate of phase space flux are deduced in this paper.For large container ships which have huge above-water area subjected to wind load,the effects of wind load could not be omitted.Three methods of CFD numerical simulation,NK classification rule and Isherwood empirical formula are adopted,analyzed and compared for computation of wind force coefficients and wind heeling moments,and the wind force model is embedded into the nonlinear dynamical system which transforms the system form symmetric system to asymmetric system.For this case,the simple zero point of Melnikov function is computed to determine the chaos threshold which is further verified by the maximum Lyapunov exponent.When the value of wave excitation is beyond the threshold,the gradual erosion phenomenon of safe basin is observed,which indicates that the rolling system has been unstable and capsizing is about to happen.By stability analysis of the system under is quantitatively evaluated.From combined action of wind and waves,the effects of wind force on asymmetry degree and stability the view of actual seaway condition,ship mostly sails in irregular waves.Studying the asymmetric stability of ship rolling motion random seaway condition is imperative.According to the random Melnikov mean square criterion,the calculation methods for phase space flux are presented and the rate of phase space flux is taken as a measure of loss of ship stability.Taking into account the difficulties of solving orbital parametric equations in asymmetric system,the discrete time solutions are obtained instead,and the threshold value curve for wave excitation under combined action of wind and wave are determined by discrete time Fourier transformation.For random condition,the capsizing diagrams are given out as verification from the perspective of probability,and the contour results are applied to forecast the capsizing probability of container ship which sails in one specific route.Tracing the recent developments of the second generation intact stability criteria,some basic research on safety voyage is carried out in the last part of this paper.Safe factors,criteria and standards are respectively discussed from both crew and deck cargo,and the excessive motion and excessive acceleration are analyzed and evaluated.In the end,some operational guidance for container ship is summarized by referring to the up-to-date studies.