Theory of Floating Seismic Isolation System

Abstract Floating-type nuclear plants have been studied and, in some cases, deployed as marine systems. Although horizontal seismic waves must be naturally isolated from the floating plant due to little transfer of shear forces in water, the vertical component can propagate through water to the plant from the seafloor in what is called a “seaquake” phenomenon. The authors have proposed a theoretical concept for a Floating Seismic Isolation System (FSIS). The FSIS is a unique seismic isolation mechanism having a number of cavities and orifices embedded in the bottom of the floating structure. The cavity is designed to reduce peak vibration response of the structure by vertical seismic input whereas the orifice is used to add damping for residual vibration reduction. This paper presents an analysis of the mechanism and theory of vertical seismic isolation of the FSIS, which are analyzed in detail using equations of motion. In addition, the seismic isolation effect of the FSIS is quantitatively clarified using equations of motion and analysis. Furthermore, the seismic isolation effect due to differences in parameter characteristics such as the area of the cavities and the damping of the orifice used in the FSIS is also described. This paper validates the potential of the FSIS.