Rolling stability analysis of high-static-low-dynamic stiffness floating raft vibration isolation systems

Floating raft vibration isolation (FRVI) systems employing isolators with high-static-low-dynamic stiffness (HSLDS) characteristics offer excellent large-scale and multidirectional vibration attenuation performance ideally suited to stealth applications in ocean faring vessels. However, the rolling motions of vessels in heavy seas can produce large displacements and induce instability in HSLDS-FRVI systems, and our present state of understanding of these systems is insufficient to facilitate the design of more stable systems. The present work addresses this issue by establishing an accurate analytical model of a double-layer FRVI system with 12 degrees of freedom to facilitate detailed rolling stability analyses when adopting HSLDS isolators. This model is also adapted to obtain a simplified single-layer HSLDS-FRVI system model and a corresponding single-layer equivalent linear system (ELS) model for investigating the factors contributing to FRVI system stability. The results demonstrate that the single-layer HSLDS-FRVI system obtains increasingly superior rolling stability compared to that of the corresponding ELS as the system loading increases. Also, good rolling stability can be maintained by applying proper mounting locations for the isolators. Additional factors contributing to FRVI system stability are deduced by comparing the results obtained for double-layer HSLDS-HSLDS, HSLDS-ELS, ELS-HSLDS, and ELS-ELS systems. The results demonstrate that the use of isolators with HSLDS characteristics is most beneficial for improving the rolling stability of double-layer FRVI systems when applied between the ground and bottom layers, and maximum rolling stability is obtained when the mass of the lower level is close to that of the upper level.