Time-dependent responses and mooring tensions of a moored floating structure in tsunami waves

Abstract

This study evaluates the effects of tsunami waves on the global performance of a spread-moored Floating Storage Unit (FSU) through tsunami-floater-mooring coupled dynamics simulations. The time-domain equations of motions for the FSU used the Cummins equation with a body-nonlinear method, in which the nonlinear Froude-Krylov and hydrostatic restoring forces were evaluated at the instantaneous tsunami elevation and body position at each time step. As input tsunami waves, second-order solitary waves and Tohoku-tsunami-like waves constructed by the superposition of several solitary waves were used. The lumped-mass-line model was used for the coupled mooring-line dynamics. Then, the coupled interactions between the floater and mooring system were solved by applying various tsunamis of different shapes, durations, heights, and incident angles. The body-nonlinear results were systematically compared with the linear analyses to demonstrate the pertinent nonlinear effects. Following a significant initial drift of the vessel induced by tsunamis, the moored floating structure exhibits transient motions at natural periods. Larger tsunami heights with relatively short duration result in significantly higher FSU motions and mooring tensions.