Reliability Research On Ultimate Ship Hull Girder Strength

For a long time, traditionally deterministic method has been utilized in ship design and strength assessment. As a fact, the dominant factors in structural analysis are uncertain parameters. Therefore, it is very difficult to specify various influential factors by means of deterministic method. With the development of ship structural mechanics, the implication of reliability method has been widely acknowledged. Currently, structural reliability analysis approach has been applied in many foreign counterparts, and the probability-based reliability theory has been successfully used to treat such uncertain variables as loads, material and cargo distribution. By cpntrast, domestic ship research is still dominated by deterministic safety factor method, and far lag behind that in the field of aeronautic and civil engineering.In the paper, the ultimate hull girder strength, the most important and meanwhile basic strength of ship structures has been combined with reliability analysis, the world-wide research hotspot, in order to precisely calculate the real ship hull capacity and assess the true safety margin of ship structures. As a result, the unified safety standard will be obtained and the ship specification will be improved as well. The impact of essential influential factors, including corrosion, fatigue and maintenance, has been considered in reliability context. Meanwhile, the advantage of ship maintenance decision in economy is addressed.The content of the thesis is illustrated as followes(1) A comparison is made between different calculation methods, and then the most effective one named progressive collapse approach is used to calculate ultimate ship hull girder strength.(2) Two important influential factors, corrosion and fatigue are systematically studied. The research is focused on two aspects, one is coating life prediction and the other is close relationship between corrosion and fatigue. Up to now, the research on coating belongs to quanlitative analysis. In this thesis, however, a quantatitative analysis is given to predict coating life and decide its probabilistic characters.As to relationship between corrosion and fatigue, corrosion enchancement factor is traditionally applied to reflect their interaction, which sounds too simplified. In the paper, correlation theory is introduced between corrosion and fatigue, which is thought to approach reality further. Moreover, Nataf transformation is applied to achieve the new correlation coefficient after normalization.(3) Two influential external loads, still water load and vertical wave-induced load, are illustrated in detail. On the basis of comparison between various kinds of load combination methods, Ferry-Borges method is preferred to combine two loads and the sensitive analysis on load combination factor is carried out.(4) Ship maintenance technique is simply reviewed. Thereafter, (semi-) Markov decision process is applied to optimize maintenance approach to minimize the long cost rate.(5) Surface response approach combined with reflection transformation method is suggested to carry out ultimate hull girder strength-based reliability analysis. Since the limit state function is implicitly expressed, the surface response approach is taken to transform it into equivalent explicit-expressed function, and then reflection transformation method is used to analyze the structural reliability. Finally, two real ships are examplified.