Dynamic Coupled Fluid-Structure Interaction Analysis of Flexible Floating Platforms

Abstract

The paper presents the dynamic response analysis of a flexible floating platform subjected to water transmitted, amplified earthquake accelerations input at its base. The finite element method is used for formulating the unsymmetric, coupled dynamic equations of equilibrium of the fluid-structure continuum. The boundary conditions include the free surface wave and radiation damping. The amplification of the earthquake through the water medium is studied using a linear system of lumped masses, springs, and dashpots. A new procedure is demonstrated to solve the coupled, unsymmetric equations using a specially developed computer program FLUSIN. Depending on the water depth, it is estimated that the vertical accelerations transmitted to the bottom of the floating structure may be amplified considerably. Cavitation is a possibility for greater depths and higher accelerations. Two numerical illustrations are presented—a floating nuclear plant and a liquid petroleum gas storage facility subjected to amplitude earthquake accelerations. The results compare well with those obtained by other investigators using approximate techniques. The procedure can be applied to floating exploration/production-storage/transportation platforms and pipelaying barges subjected to water transmitted earthquake forces. The formulation is easily adaptable to any fluid-structure system as well as for other kinds of dynamic excitation. With the increased focusing of attention to compliant-type tension-legged platforms and semisubmersibles for production/drilling and LPG storage platforms, the project is of considerable significance. The work is a forerunner for much needed experimental investigation, particularly with respect to cavitation. Also, the findings would have considerable spin-off effects to OTEC concepts.