Numerical Calculation of Forces on Very Large Floating Structures by Tsunami

Abstract

When a very large floating structure (VLFS) is designed, it is quite important to consider the forces by tsunami especially in our country that is often attacked by big earthquakes. The forces on ocean structures by long waves such as tide, surge, and tsunami have been usually estimated by semi-empirical way, for example, using the drag coefficient due to the shape of the structure in constant flow. However, for VLFS like an ocean airport, currents may be different at the different position of the structure. Also, the hydrodynamic interaction between the structure and currents must be taken into account that can be ignored in cases of normal size structures. In this study, a new procedure to estimate the forces on VLFS by tsunami was proposed, in which distribution of currents and waves around the structure were estimated by multilevel model. The initial condition of surface water level of tsunami was given by the sea bed fault model with static parameters, and Reynolds equations using the hydrostatic approximation were used for calculating the propagation of tsunami. The deformation of VLFS and the field of currents and waves were calculated simultaneously, then the pressure force and frictional force on the structure were estimated by numerical way. The method was validated by comparison with the results of some other numerical calculations. The comparison with hydraulic model tests in experimental tank was also made and it is found that the present model can roughly estimate wave propagation and deformation of a solitary wave, motion responses of an elastic structure, and mooring forces on the floating body. Moreover, the forces on VLFS located in Tokyo Bay were estimated for some sea bed fault models with different characteristics.