Reduction of the ultimate strength due to crack propagation in damaged ship structure

Abstract

The aim of this conceptual study is to analyze a propagation of the crack for damaged tanker during the towing period because of a fluctuating wave loading and then to investigate possibility of ultimate hull girder collapse during salvage operation. The present study represent an extension of state-of-the-art in residual strength assessment of damaged ships, as ultimate longitudinal bending capacity is considered to decrease with the time. Crack growth assessment is performed by the fracture mechanics approach. It is assumed that the bottom crack propagates through outer bottom panels and the side crack through deck panels. The outcome of the analysis is the crack length as a function of number of wave-load cycles for damaged tanker. The main deck and ship bottom are simplistically represented as unstiffened plates, just to demonstrate the procedure and to find out approximate solutions. It is assumed that fluctuating stresses enforcing crack to propagate are caused by the vertical wave bending moments which are taken from IACS Rules. The ultimate strength calculation of damaged tanker hull subject to vertical bending moments is performed by modified Paik-Mansour formula taking into account degradating effect of crack propagation on ultimate bending capacity. The possibility that extreme vertical bending load, composed of still water bending moment of damaged ship and wave bending moment, exceeds ultimate bending moment capacity, is investigated.