Free Vibration of Neutrally Buoyant Inflatable Cantilevers in the Ocean Environment

Abstract

Free vibration analysis of the neutrally buoyant inflated cantilevers, made of plastic sandwiched films, is presented, accounting for the added inertia and nonlinear hydrodynamic drag. The significant feature of the analysis is the reduction of the shell equations (the membrane, Fliigge's, and Herrmann-Armenakas') into a single equation which is similar in form to that for a vibrating beam with rotary inertia effects. The natural frequencies obtained are compared with the experimental results and those predicted by the Rayleigh-Ritz method in conjunction with the Washizu and membrane shell theories. The analyses show, and the experimental program confirms, that Fltigge's shell equation in reduced form is capable of predicting free vibration behavior quite accurately. However, the reduction technique should be applied with care, since in several cases it leads to misleading results, e.g., in the case of the Herrmann-Armenakas theory, generally accepted to be one of the most elaborate.