The Ultimate Strength Of Ship Hulls And Its Reliability-based Research

The ultimate longitudinal strength of ship hull girder reflects the bending resistance ability under multi-load, including still water load, and wave load, etc. Therefore, the ultimate longitudinal strength is the basic index which measures the safety of ship hulls. It is meaningful to analyze and to assess the ultimate longitudinal strength of ship hull girder accurately. On the other hand, in the operational process of shipping, ships always undertake the risk of various kinds of accidents, among which collision and grounding do the maximum damage to the bearing capacity of ship hulls. Hence, it is also necessary to analyze and to assess the residual ultimate strength of damaged ships accurately.The traditional method to check the ultimate longitudinal strength of hull girder is deterministic method, which is simple but does not take account into the influence of uncertain factors. It is not accurate enough to assess the strength of complex ship hulls with the method. In contrast, the reliability-based method is more of a suitable method to check the ultimate longitudinal strength of hull girder to conform to the reality.According to the principles of reliability, the FOSM method is adopted to assess the structural reliability of an actual ship. And the result is compared with the result of the deterministic method. The ship is also assumed to have been damaged in collision and grounding, and the structural reliability of the damaged ship is also assessed.This thesis mainly includes the following five parts:(1) The probability distribution and the statistical characteristics of random variables such as still water load, the wave load, and the ultimate strength are studied.(2) The one-step method and the Non-linear FEM are studied and adopted to calculate and check the ultimate longitudinal strength of the hull girder of an actual ship, and their results are analyzed and compared.(3) The Rosenbluth method is adopted to calculate the statistical characteristics of the ultimate strength of the hull.(4) The relations between the residual strength index and the centroid of the broken area, and the size of the broken area are studied. Then, functions are fitted to correspond to the mutual relationship among the variables. Finally, the model of the damaged ship is built.(5) The model based on the principles of reliability is built and the FOSM method is adopted to calculate and analyze the ship.The results indicate that the methods are feasible.