Ultimate Longitudinal Strength-Based Safety and Reliability Assessment of Ship's Hull Girder

An attempt is made to investigate into a practical assessment procedure to evaluate the safety and reliability of ships on the basis of the ultimate longitudinal strength of ship's hull girder.The external bending moment acting on the hull girder is assumed by using an existing rule of classification society. The ultimate longitudinal strength is analyzed by idealized structural unit method, in which a structural model for plate element subjected to biaxial load is formulated by idealizing the actual nonlinear behaviour of the element taking account of the influence of initial imperfections as well as the interaction effect between the local and overall buckling of the structure. The central safety factor of the hull girder is evaluated by dividing the external bending moment into the ultimate bending moment. The reliability assessment is made by using Cornel's second-moment method.The proposed safety and reliability assessment procedure is then applied as an example to evaluate the central safety factor and the safety index of a double skin tanker. The influence of initial deflection, welding residual stress and the yield stress on the safety and reliability of the hull girder under sagging and hogging moment is investigated. The stiffening effect of the longitudinal girder and side stringer is also discussed.