Master thesis : Study on the Vibration of Corrugated Cargo Tank Bulkheads

Abstract

Cargo tankers corrugated bulkheads are constantly subjected to excited vibration due to propulsion system components, such as engines, shafts and gear boxes, and severe vibration can be observed if the bulkheads vibrate in resonance with the excitation frequency. Added to it, the influence of the cargo density and the filling level at the cargo tank can lead to a considerable variation of the natural frequencies, shifting them closer to the excitation frequency and leading the system to a dangerous resonance state. In addition, vibration induced by the propulsion system is known for an extremely high number of load cycles in a short period of time. In a state of resonance, the effects of such cycles combined with huge amplitudes may negatively affect the design fatigue life, leading to fatigue induced cracks. In the course of this study, two main questions are supposed to be answered. First, how to design a proper corrugated cargo tank bulkhead in order to minimise resonance risks due to the excitation frequencies generated by the propulsion system? And second, to assess at which vibration levels fatigue damage is likely to occur. For that purpose, a parametric Finite Elements model is developed in order to analyze how the natural frequencies vary according to the variation of dimension parameters, cargo filling levels and cargo densities. In addition, the model is refined at the stress concentration areas for a fatigue strength analysis in order to determine the vibration levels and the stress range for which the fatigue damage is likely to occur.