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Abstract In this paper, the procedure is established to analyze the ultimate strength of damaged hull girder caused by the collision or grounding with consideration of the asymmetry of cross section and instantaneous shift of neutral axis of cross section. The advanced progressive collapse analysis method is applied. The relationship of the ship section element stress—strain is calculated with consideration of the interaction between the stiffer and attached plate, the influence of distorts of stiffener, torsional-bending buckling and local plating buckling. The effect of the damage location on the residual strength of hull girder is investigated in both sagging and hogging conditions. Through the study of varying parameter of damage extent, the changing rate of the residual strength index of damaged hull girder is studied and compared in different seagoing condition. An example, 34,000 ton bulk carrier with large deck opening, is analyzed for demonstration. Keywords Ultimate Strength; Residual Strength Index; Damaged Ship; Collision; Grounding Introduction Hull girder strength is the most fundamental strength of ship structure. Generally, ships have been designed to resist all loads expected to arise in their seagoing environment.58678
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The objective in structural design has been to maintain a ship structural integrity for normal operating conditions. A combination of the most severe loads is usually selected as the normal design load. However, the catastrophes which cannot be avoided will induce the loss of the ships, cargo and life at sea, even the oil pollution which require expensive clean-up and cause long-term environmental damage. The total losses of all ships during the years 1995-1998 are 674 in number and 3.26 million in gross tonnage (GT), according to LR of Shipping’s World Causalty Statistics. Total losses in GT during the years 1995-1998 are present in following figure. Grounding (wrecked/stranded) accounts for total losses amounting to 17% in number and 24% in GT. Collision accounts for total losses amounting to 11% in GT. A ship may collapse after an accident (such as the collision and grounding) because of inadequate longitudinal strength or suffering from the severe seagoing environment. The impact that damage has on the overall residual strength of the structure is a function of its extent, mode of failure and relative shipboard location. Fig. 1: Total losses of ship in GT during the years 1995–1998. Until now, many research works focused on the ultimate strength of intact ship hull are conducted in both theory and experiment. The methods applied by the researchers can be classified into: 1. Simple methods: Initial yielding; Elastic analysis; and Assumed stress distribution,
2. Advanced methods: Progress collapse analysis with idealized relationship of stress-strain; Progress collapse analysis with computed relationship of stress-strain; ISM; and Non-linear FEM. In the ISSC report (2000), the methods are assessed from the viewpoint of applicability. Each method was quantitatively graded with respect to 15 capabilities by scoring 1-5. It was also done qualitatively by showing the consequence of omitting capabilities by low, medium and high. Of these methods, the method based on Initial Yielding is an empirical one. Methods based on Elastic Analysis and Assumed Stress Distribution are direct methods, whereas the remaining methods have the capability to trace out the full sequence of progressive collapse behavior of the hull girder. It is seen that the most effective among all methods is Progressive Collapse with Calculated ε σ − Curves, that involves the use of numerical methods to determine the stress-strain cures of inpidual plate and stiffened plate elements, which are then integrated following the assumptions of simple beam theory in order to trace out the progressive collapse curve (ISSC, 2000). In recent years, the focus of ship hull ultimate strength turns to the damaged ship. In the Report of Ship Structure Committee (1995), the following tasks are performed: 1. collecting and evaluating the damage of marine structure; 2. summarizing the state of the art technology and methods available in marine and non-marine industry for quantifying residual strength; 3. recommending future work to investigate current engineering procedures in the areas of crack growth, permanent deformation and global ultimate strength to assess residual strength of damaged marine. Zhang (1996) proposed a semi-analytical method of assessing the residual longitudinal strength of damaged ship hull. Paik et al (1998) analyzed the residual strength of bulk carrier in grounding and residual strength of ships after collision and grounding. In the paper, the location and amount of collision and grounding damage were prescribed. The possibility of hull collapse was explored by a comparison of the applied extreme bending moment and the ultimate hull strength which were estimated using design oriented methods and formulas. Two types of residual strength index, namely the section modulus based residual strength index and the ultimate bending strength based residual strength index, were defined. Wang (2002a) first reviewed the state-of-the-art research on collision and grounding. It focused on the three issues that a standard for design against accidents needs to address: definition of accident scenarios, evaluation approaches, and the acceptance criteria. Laterly, Wang(2002b) investigated the longitudinal strength of damaged ship hulls for a broad spectrum of collision and grounding accidents. Both the hull girder section modulus and hull girder ultimate based on the appropriate approach are calculated.