DYNAMICS OF FIXED OFFSHORE CYLINDRICAL SHELL STRUCTURES : Part I; Free Vibration Analysis in Water

Abstract

The free vibration analysis of fixed offshore cylindrical shell structures partially or totally submerged in water is studied by three different numerical methods, Rayleigh-Ritz method, finite element method and matrix progression method, in the consideration of the dynamic interaction between shell walls and water. In order to obtain the free vibration data which is necessary in the dynamic response analyses to wave or earthquake loadings, some numerical calculations are carried out by using these three methods. The modal characteristics of an offshore shell submerged in water are very sensitive to shell configurations and water depths. The natural frequencies decrease monotonically with the increase of water depth and the mode shapes for both displacements and stress resultants are different from those in air, because of the hydrodynamic inertia force due to an interaction with surrounding water. Also, the variation of modal characteristscs are influenced by shell dimensions, such as radius, length and thickness. Hence, it is important to discuss the free vibration data before starting the dynamic response analysis.