Hull girder ultimate strength and fracture toughness of damaged marine structures
This thesis addresses the structural performance of the hull structure of a vessel in extreme response state of a combined loading consisting of vertical and horizontal moments for intact and damaged conditions, where fracture mechanics of ships with large damage openings are investigated in great detail. The motivation for conducting this research task is the importance of the ultimate and post-ultimate strength in relation to the reliability of intact as well as damaged vessels. Moreover, in an emergency situation, knowledge of the structural capacity of the vessel in whatever damaged or intact condition it may be in, is crucial to allow for a rational decision to be made regarding possible salvage, or at worst disembarkation of the crew, in the interest of safety for human life, environmental protection, and capital investment. To facilitate these requirements, the overall objective of the present research therefore becomes to build a rapid computer based analysis tool for calculation of the ultimate and post-ultimate capacity of the hull girder in intact and damaged states of combined bending moment loading. Then, to verify the accuracy of the procedure developed against available experimental results and by comparison with other theoretical results. To meet this objective essentially requires five sub-tasks to be completed. Hence, the scope of the work to be carried out becomes - A study of literature for the available methods usable to evaluate the ultimate and post-ultimate capacity of the stiffened plates - Developing the new simple design equations for predicting the ultimate strength of stiffened plates with imperfections in the form of imperfections, where a non-linear finite element method is employed on a wide range of typical ship panel geometries. The new simplified analytical method must be generalized to deal with such combined load cases. The performance of the proposed equations must be also verified by experimental results. Establishing a rapid computer code based on simplified formulas developed for calculation of the ultimate capacity of the hull girder. This program must be capable of considering initial imperfections and must be compared with other theoretical approaches developed. - The influence of the coupled bending moment on the intact & damaged ship structures must be extensively investigated. The rational interaction equations must be developed, which based on the extensive ship types. - Accidents such as collision, grounding and explosions occur and can have major consequences to the ship structure. The effect of such large damages on the hull girder must be assessed under time pressure. The recent accident with the PRESTIGE shows the importance of being able to reliably assess the longitudinal strength of damaged vessels. The procedures for this must be developed. A major drawback is lack of knowledge of the failure mechanism that may play a role in such circumstances. Therefore, the fracture mechanism of damaged ships is essential. Key effects must be identified and if possible, included in the procedure. When these five points have been addressed, and the objectives solved, all that is needed for the development of a computer code for the ultimate strength. The remaining final task will thus to be benchmark the code against available experimental results and other theoretical methods.