Reliability Analysis Of Ship Structure Based On Random Process Theory

The environmental loads and the damage of ship structure included corrosion and fatigue fractures were the primary factors of the ship strength problem. In this case, the time variability of the loads was considered, so was the degradation of the ship strength. Namely, compared with the traditional method which adopted the extremum theory, the method presented in this paper considered the factor of time, so the corresponding reliability analysis was the time-dependant reliability analysis for ship structure based on random theory. Recently, many scholars have paid much attention to the study of time-dependant reliability of ship.Generally, the fatigue and corrosion would inevitably happen to the ship due to the repeat effect of environment and wave loads, which would be all over the ship life. Based on above instance, the ship strength would change with time, so it must be treated as a random process. The expanding of fatigue crack and the reducing of component thickness would induced the decreasing of effective section module, finally the vertical bending strength reduced, which would have impact on the time-dependant reliability.A method for time-dependent reliability assessment of the strength of a degraded ship hull girder in the presence of fatigue and corrosion was developed. According to the characteristics of loading environment for existing ship structures, the variation of strength with time due to fatigue and corrosion was studied by applying fracture mechanics and general corrosion theory, then the time-variant model of ship hull modulus was formulated using the random process theory. The time-dependent reliability of ship was analyzed with up-crossing analysis and the reliability method of parallel system. This analysis can not only exhibit continuous decrease of reliability of the ship under fatigue and corrosion but also avoid numerical integration, which was easy to compute. Furthermore, the time-dependent reliability and annual instantaneous reliability was compared through an example, which indicates the time-dependent reliability of ship structures could be predicted using the proposed method in a more reliable way, and it could provide some references to the decision of the reliability, maintenance and safeguard.When the atrocious sea happened in the sailing period of ship, the serious slamming would happen. And the slamming induced dynamic loads effect was equal to the effect of low wave loads, which would induced the whole flutter failure and local damage of ship. Based on the random process theory, the random process model of performance function of the ship hull girder subjected to slamming, wave-induced and stillwater loads was set up. According to the real condition, the correlation of the peak value of slamming and wave-induced load was considered. The reliability analysis was done by means of up-crossing analysis and parallel system reliability method. Finally, the influences of damping ratio, slamming rate and coefficient of correlation on the reliability were analyzed, and the reliability of all failure modes were also calculated. It was indicated that the damping ratio dominated the influence of slamming load on the reliability. Namely, when the damping ratio was little, the influence was great. The results also show the buckling failure was the primary one.The buckling of ship deck structure was induced by the combined of slamming and wave loads, once the middle section was destroy, then the ship hull girder would be failure when it subjected to the loads. So the reliability analysis of the ship deck subjected to slamming load was significant to the design of ship. Considering the three primary buckling failure modes of ship deck, the reliability was studied by the random process theory and parallel system reliability method. The result showed that the stiffener tripping failure was the most dangerous, although it was partial destroy, which would reduced the ship strength and happened the whole failure at the next serious sea. Thereby, reinforcing the stiffeners of middle ship or strengthening the buckled stiffeners properly would be enhancing the reliability of whole ship deck, which can provide some references to the ship design.The unstable of the whole ship, i.e., the capsizing of ship was the serious incident which made greatly losing of personnel and wealth, so more attention was paid on by shipbuilding circle and sailing circle. The ship capsizing study included the random of the loads and the nonlinear of great range rolling, which was complicated. Because of the random of the wave and wind load, the capsizing was a random event. The determination of this event probability could provide references to the engineering.The path integration method, based on Gauss-Legendre integration scheme, was applied to the motion of ship subjected to random wind and beam seas. Considering the nonlinear damping moment, restoring moment and random wind and seas, the nonlinear differential equation for ship's rolling motion was established. The rolling angle probability density was analyzed using the path integration method, when the ship was subjected to white noise disturb and a stationary wind moment, then capsizing probability was also calculated according to the presented rules. The presented method was capable of producing accurate results, which could be proved through the example. At the same time, the influence of each parameter on the probability density was analyzed. Furthermore, the capsizing probability at different velocity of wind and sailing was calculated, and the probability with time was also computed. It could be seen that this method was easy to achieve. Thus the rolling capsizing probability of warship in random wind and beam seas could be predicted quantitatively by the proposed method.