Progressive collapse of damaged ship structures

Abstract

This research investigates the progressive collapse of stiffened panels in a ship’s structure under several damaged conditions. The main focus is on the behaviour of stiffened panels under three conditions: intact condition; damage represented by a circular, clear-cut-out hole; and damage represented by penetration simulations. The same damage conditions have also been applied to double bottom box girders. The results of these analyses are used to better understand the behaviour of damaged ship structures and develop a novel modification to a simplified method for predicting a ship’s ultimate strength. The non-linear, finite element method is used in order to simulate the damaged condition and to estimate ultimate strength behaviour in both undamaged and damaged stiffened panels. The damaged conditions are divided into two categories: damage represented by a circular, clear-cut hole and damage represented by penetration with an indenter. The damaged scenario assumes the damage to be located in the middle of the stiffened panel. The diameter of the damaged area and diameter of indenter are controlled by a ratio between the diameter of damaged area (D) or diameter of indenter (Din) and the width of the stiffened panels (W) respectively. Pre-existing characteristics of the structure are considered as an average level in terms of both residual stress and geometric imperfection. An in-plane compression load is applied to the stiffened panel in order to generate the ultimate strength, which is affected by the damaged condition. The results are used to extend an existing hull girder progressive collapse method, using a novel approach to adapt the load shortening curves. A knockdown factor is generated by using regression formulae from the finite element models and is applied to modify a load shortening curve for damaged ship structures. The modification curves are combined with moment curvature to find the ultimate strength of the damaged hull girder. The method is verified with case study analyses of double bottom box girders. The same damaged conditions applied for the stiffened panels are used with the hull girder. The damaged area is located in the middle of the bottom part of the structure. The hogging condition is applied for the verification model. The validation results show excellent agreement between the finite element method and modified hull girder progressive collapse method, which can be used to predict the ultimate strength of a damaged ship structure.