Simplified Analysis and Design of Ships subjected to Collision and Grounding

Ship collisions and grounding represent significant potential incidents which may result in very unfavorable consequences, such as economic loss, environmental damage and fatalities. In this context, continuous effort is being made by the research community for the purpose of mitigating or preventing the collision or grounding events from developing into consequences disproportional to the original causes. This continuous concern is exemplified by the successive series of International Conference on Collision and Grounding of Ships (ICCGS), initiated in 1996 in San Francisco and carried over once every three years since 2001 in Copenhagen. The present thesis focuses on the internal mechanics of ships subjected to collision and grounding accidents. Simplified methods for structural analysis are developed as well as their application It is stated in the Foreword of the Proceedings of the first ICCGS, “Improved techniques for structural analysis will have a profound impact on the design of the ships of the future, enhancing both safety and environmental performance.” A significant part of the thesis is dedicated to the development of simplified analytical methods for structural analysis of ships during collision and grounding. In many cases, it is essential that the structural performance of ships during accidents can be evaluated quickly, e.g. in a decision support system for ships in emergency or critical situations or in a rational design procedure, where the structural crashworthiness of ships in a large number of accident scenarios needs to be assessed. Compared to empirical methods, experimental methods and non-linear finite element methods (NLFEM), simplified method based on plastic mechanism analysis is considered the most suitable and advanced method in such circumstances. Web girders represent a category of important components in double and single hull ship structures. Accordingly, the behavior of web girders during collision and grounding is investigated comprehensively. Depending on the loading scenarios, two types of deformation modes are identified, namely local denting and sliding deformation. An improved theoretical model for local denting is proposed, taking into account important deformation features that have not been considered by the existing models. The first plastic mechanism for the sliding deformation mode of web girders is developed. This mode is especially geared to the analysis of ship grounding over blunt type seabed obstructions where longitudinal bottom girders are subjected to continuous sliding process. It is also relevant for side stringers during sliding collision. It is recognized that the shape and size of the striking object are of crucial importance with respect to the structural damage of ships during collisions and grounding. As for ship grounding, three major types of underwater obstructions have been defined according to the characteristics of damage occurred during grounding, namely “rock”, “reef” and “shoal”. Most existing studies of ship grounding to date are concerned with “rock” type, sharp obstruction. An integrated simple tool is established to assess the strength of double bottom grounding over blunt seabed obstructions with large contact surface such as “shoal”. The simple method is composed by the proposed and existing simplified methods for individual structural members. Good correlation between the simplified analysis and numerical simulation is obtained. Though the response of plates under patch load was initially investigated for ships navigating in ice conditions, it is also of interest for ship collision or stranding analysis due to the fact that the damage is generally local. The resistance of patch loaded plates is derived by extending the classical “roof-top” or “envelope” yield line model. Further, a new yield line pattern, “double-diamond”, is proposed. It gives better prediction in the plastic bending phase. More importantly, the present formulation includes the resistance due to membrane effect when significant permanent deformation is developed. This is especially useful for plate design or damage estimation when abnormal/rare actions such as collisions are considered. Numerous crashworthy structural concepts have been proposed, notably in the past two decades. However, any of such novel concepts will have a long way to go from being accepted to being applied. This necessitates studies on ways to improve the structural crashworthiness under the present design regime. Applying the formulation developed for plates under partial lateral loads, a direct and simple design procedure is established for strengthening the side hull against large impact loads. By considering the ductility limit of the material consistently, a simple expression relating the stiffener spacing directly to the allowable permanent deformation has been derived. Eventually, the required plate thickness is simply connected to the material yield strength and the stiffener spacing. The design procedure follows the principles of the accidental limit state (ALS) criterion and the strength design principles adopted by the NORSOK standard for design of offshore steel structures. The attractiveness of the design approach is that it is based on closed-form solutions for plating and stiffeners. As the design load, the collision force is represented by a relatively simple pressure-area relationship. The procedure is demonstrated and verified by the design of a ship-shaped FPSO tank side structure subjected to collision from a 7,500 tons displacement supply vessel.