Research On New Types Of Ship Structures Based On Crashworthiness

Ship collision is a kind of marine damage accidents of ships contacting with other objects. Catastrophic aftermaths will be brought by ship collision, such as personnel loss, vessel sinking, environment pollution and so on. There are many reasons that will cause a ship collision accident. Researchers of many countries are trying to find the method avoiding ship collision accidents.Nowadays, due to factitious effect, ship collision accidents cannot be avoided entirely. From the point of ship structure, a lot of single-skinned ships fall into disuse or are refitted to double-skinned ships. From the statistics of accidents, large numbers of accident reports do not show that the structures of conventional vessels have good crashworthiness. In order to enhance security, the conventional ship structure needs to be improved or reformed.The ship collision problem is involved in a lot of subjects and domains. The collision analysis of ship structure needs many kinds of knowledge, such as structural dynamics, fracture mechanics, plastic mechanics, hydrodynamics, etc. The FEM technology and theoretical analysis are also useful for the research. The process of ship collision has various non-linear conditions, such as geometric, kinetic, contact, material, etc. The duration of the whole process is very short, which is normally between 0.1 and 10s. The collision region of ship hull is probably under the water, therefore, ship collision is a problem of fluid-structure coupling or hydroelasticity, and whose theory is helpful for the research.The methods of ship structure collision research are mainly categorized into three kinds, which are experiment, simplified analysis and numerical simulation. The experiment method includes accident investigation of real ships and collision model tests. Its results are clear and simple, which play an important act for theoretical methods. However, the experiment method is very expensive and costs a lot. Simplified analysis method has relatively low precision, which deals with the ship in collision by a lot of simplification, and semi-analysis and semi-empirical mechanics equations of ship structure are established. The numerical simulation method has the advantage that the real ship collision scenario can reappear. With the help of some FEM analysis software, all kinds of physical variables in the ship collision process can be outputted. The simulation of ship collision mainly uses non-linear FEM, and some commercial FEM software has well shown that they can simulate this process with good precision. Establishing a simulation model needs certain knowledge, experience and skills, and sometimes computation precision is not satisfaction, so verification by experiment or other researcher's calculation is necessary.In this paper, the universal concept of ship collision is defined as dynamic contact under instantaneous impulse loads between ships, between a ship and other marine structures, or between a ship and other obstacles, which mainly includes ship-ship collision, ship-bridge collision, ship-platform collision, ship grounding, etc. According to structural crashworthiness, this paper tries to find the method that can reduce the loss of ship collision accident by simulation of the collision process. There are mainly two approaches. One is crashworthiness optimization based on certain collision analysis, and the other is to improve the conventional ship structure and present novel concepts of structural design. Research of this paper makes it clear that mechanism of structural response and energy absorption in ship collision, and provides design and manufacture criteria with new crashworthiness data.The major research work and conclusions of this paper are summarized as follows:⑴The research status of ship collision is summarized and analyzed, which includes several aspects, such as ship-ship collision, ship grounding, ship-platform collision, novel anti-collision structure, residual strength and ultimate strength of ship hull, technology of numerical simulation, structural optimization of anti-collision, risk assessment of ship collision, etc.⑵The development process and basic theory of FEM is reviewed and summarized. The key technology and modeling skills of ship collision simulation are discussed.⑶The problem of dangerous points of side structure collision is analyzed by numerical simulation. There are three kinds of dangerous points, which is the intersection point of girder and transverse frame, the mid-span point of girder or transverse frame and the center of plate lattice. The research shows that the ultimate status of collision is not essentially related to energy absorption of structure for bow-double side scenario, but to resistance and deformation of cross frames and inner side shell plating. For ship side structure of conventional size and thickness, the cross frame formed by girder and transverse frame is always advantageous to reduce collision damage. When the thickness of the cross frame exceeds certain critical value, the collision force will make the inner shell fracture before the bow of the striking ship touches the inner shell. This paper gives the method to evaluate the critical thickness of the cross frame.⑷Based on numerical simulation of double side structure, the simplified method of assessing energy absorption of collision is analyzed and summarized. For the scenario of bow-side vertical collision, several calculation conditions are considered, such as impacting location, side structure size, mesh density, region of computational model, etc. The Minorsky's and Paik's equations can evaluate energy absorption in ship collision. However, they do not consider the effect of bow curvature, side girder and transverse frame on collision damage, and the paper revises them.⑸According to comparison of numerical simulation, the effect of bottom girder on energy absorption of grounding is analyzed, and Zhang's empirical equation is revised. Based on the new grounding equation of assessing energy absorption, double bottom structure is optimized by the method that can deal with both discrete and continuous variables. The variables of optimization are the size of ship bottom, and restriction function is the weight of bottom structure.⑹The collision capability of the typical double-skinned side structure is analyzed by the numerical simulation method. Three novel side structures aimed at anti-collision are presented, which are box girder structure, trough stiffener structure and sandwich panel structure with inclined stiffeners. These novel structures are derived from simply changing conventional double side structure, and their common design concept is to increase components of energy absorption in the area of possible contact. According to comparative studies, crashworthiness of the novel structures is evidently better than the conventional structure.⑺The grounding of typical double bottom structure is analyzed by numerical simulation method. The conventional double bottom structure is improved from three aspects: transverse, longitudinal and plate. Three new anti-grounding bottoms are presented, which are double bottom with double-trough floor, double bottom with trough stiffeners and double bottom with ripple stiffeners and sandwich panel. The research shows that these novel structures can enhance the capability of ship anti-grounding. Moreover, according to energy absorption of ship grounding, improvement of transverse components and panel structure have better effects than improvement of longitudinal structure.⑻Collision damage of bulbous bow and double-skinned side structure is analyzed simultaneously. As bow has less damage than side structure in most ship collision accidents, the paper presents the new design of multi-bulkhead bow, whose function is to decrease longitudinal stiffness of bow. According to the comparative studies, the novel structure design can transfer some damage from side structure of the struck ship to bow structure of the striking ship, which is good for preventing oil spill.⑼A novel structural design concept of anti-collision, additional deck structure, is presented, which is a box structure above the main deck of bow. Mechanism of ejecting or sliding is used to make the additional deck structure fall and prevent direct contact of both sides of collision. The new structure can absorbs a part of whole collision energy and protect main ship structures.The main innovations of this thesis include three aspects:⑴The problem of dangerous points of side structure collision is presented and analyzed;⑵Against grounding, double bottom structure is optimized by the method that can deal with both discrete and continuous variables;⑶Some novel structures and new structural design concepts of anti-collision are presented.Altogether, this paper researches the damage of ship hull during collision accidents, and some new types of structure design are presented. The mechanism of ship collision, especially structural response and deformation mode in the damage area, are understood to a certain extent. In order to protecting ship, structural behavior under some accidental loads, such as collision, explosion, slamming and so on, should be embodied in the new structure design rules. In this thesis, the numerical simulation research of different collision scenarios is helpful for the rule revision, and the new structures of anti-collision or structure design concepts will provide reference for other researchers to innovating in ship structure design.