Change in Safety Level against Collapse of Ship's Hull Girder in Longitudinal Bending According to Change in Design Criteria

Abstract

Choosing four Panamax-size bulk carriers and four Aframax-size oil tankers designed by different criteria in different times, reliability analysis has been performed to compare their safety levels from the viewpoint of ultimate hull girder strength in longitudinal bending. For evaluation of the ultimate hull girder strength and its sensitivities with respect to design parameters, a series of progressive collapse analysis is performed applying the Smith's Method. On the other hand, applying the nonlinear Strip Method, time-dependent nonlinear ship motion analysis is performed to estimate wave-induced bending moment. On the basis of the obtained time history of wave bending moment, statistic characteristics of the extreme bending moment is estimated. Utilising the calculated results, reliability index and failure probability are calculated applying the First Order Reliability Method (FORM). Investigating into the obtained results, it has been found that in general the safety level becomes higher with the times depending on the design criteria.