Time-domain simulation of large-amplitude wave–structure interactions by a 3D numerical tank approach

Abstract

A time-domain 3D Rankine panel method based on a simplified variant of the mixed Eulerian–Lagrangian scheme under certain approximations is developed for studying steep nonlinear waves interacting with practical ship and offshore configurations at zero speed. Appropriate techniques have been developed that enable the method to produce very long-duration simulation results. Two levels of time-domain computations are performed: (1) a fully linear formulation where all external forces are computed on the mean wetted surface, and (2) an approximate nonlinear computation where the hydrodynamics interaction forces (diffraction and radiation forces) are determined on the mean surface and the forces arising from the incident steep waves and hydrostatic restoring forces are determined based upon the exact wetted surface under the nonlinear incident wave. Numerical computations for three practical marine structures, the barge, the S175 hull, and the semisubmersible are presented. The linear computations for which very long-duration simulations are achievable from the present method are validated against results from other available methods. As the method is developed for stationary floating bodies undergoing oscillation about their mean location, it cannot be applied for a fully unrestrained body which can freely drift. In absence of physical restraints, the approximate nonlinear calculation requires imposition of artificial constraints partially or fully restraining the horizontal motions. Very long-duration simulations under the influence of steep nonlinear large-amplitude waves for all the three structures considered could be achieved. Comparative studies between different force and motion components in large-amplitude waves from linear and the approximate nonlinear computations are made to bring out the influence of the incident wave nonlinearities on these structures. It is found that the nonlinearities of the forces and motions are strongly dependent on the above water hull geometry. Compared to a small water-plane area hull (the semisubmersible), or a wall-sided hull (the barge), a flared hull (S175) results in pronounced nonlinear features in the forces and motion time-histories.