Research On Dynamic Calcualtion Of Ship Structure Subjected To Underwater Explosion And Wave Loads

During the whole period of operation and navigation, ship should be affected by various forces. According to the rate of change, the forces can be classified into static and dynamic loads. By comparing with each other, the effect of dynamic load should be stronger. Thus, the response of ship under dynamic load should be more paid attention to and studied. The evaluation standard on dynamic load includes force magnitude and acting time. Therein the loads with high impulse, which include not only small amplitude and long time but also big amplitude and short time, are usually considered to be relative stronger. Under these high loads, the characteristic and rules of ship responses should be studied in this paper.1. The free vibration of3D submerge structure is studied. In the traditional method, by assuming the cross section to be rigid plane, the added mass is computed. In the present method of this paper, by considering the interaction of fluid and elastic structure, added mass matrix of submerge structure is calculated by3D BEM method. Then the results show that the natural frequency of submerge structure is lower than structure in air by20-25%, and the vibration modes of each other are different. Finally, the scale effect of submerged structure are calculated by3D added mass matrix, and the results show that the scale effects of structure in water and air satisfy the similarity law of elastic force simultaneously.2. The ship elastic and rigid responses under shock wave have been studied. Here the popular DAA method is studied and programmed to compute the resonance of vessel structure by underwater explosion. Then the results show that the response on ship side towards explosion wave is max, and the peak of acceleration response isn’t the nearest position but spherical distribution on ship structure. Furthermore, a theory method about rigid motion of ship under shock wave is deduced to evaluate the ship response, which can be used in the initial phase of design and validated to be efficient and feasible. On this basis, by comparing the influence of ship section parameters (width, draft and section shape) on ship response, for the small impulse load, the bigger the B/T, the higher ship response. However, while the impulse load is relative big, the response may decrease with the B/T increase. Furthermore the section shape of obtuse triangle has max effect on motion performance, and semi-circular section has min influence.3. The elastic and rigid ship responses under the effect of Bubble arc discussed. In this method, the ship is simplified into a hull girder under the Bubble which can be described by Vemon model. Meanwhile the bubble floating effect, floating drag effect, water surface effect and energy loss are considered. The results show that the hull girder has not only rigid displacement but also elastic deformation. Moreover, the response of hull girder should be over-evaluated by neglecting energy loss of bubble. On this basis, the elongated degree of vessel is calculated and analyzed, the results show that the amplitude of whipping motion should be max with the10-11length-width ratio, and this situation is very dangerous.4. The rigid motion of ship in wave is studied, and new special problems about high speed vessels (such as trimaran) are further analyzed, which include the large change of navigation attitude and abnormal peak due to numerical hydrodynamic disturbances of free surface. Therein for the large error of high-speed motion response, the navigation attitude is considered in the relative calculation. On the other hand, by theoretical analysis, the reason of numerical hydrodynamic disturbances should be the resonance of false stationary wave due to pulsating source method. Thus the artificial viscous is introduced into the free surface between main and side hulls to solve the problem of abnormal peak. By the calculation of2D cross section, it can be found that the abnormal crest value can be successfully solved. Finally, the motion responses of a trimaran model are calculated and compared with experiment, which shows that the results are in good agreement with experimental data and this method has good ability to accurately and effectively solve the "numerical hydrodynamic disturbances" and the large error of high-speed motion problems.