Lee J., Park J., Bae J.

The anti-vibration design of pump tower in LNG carrier is very important because it is the main equipment for discharge and loading of LNG. Its structure is very slender and flexible, so the natural frequencies are relatively low, which may cause resonance problem with the propeller excitations around the normal operation range. Therefore a thorough investigation on the vibration characteristics of pump tower is needed at the initial design stage. However an accuracy of the predicted natural frequencies greatly depend on boundary conditions at the upper and lower parts of pump tower, which are very difficult to reflect properly in analysis modeling because of their structural complexity such as guiding system, thermal insulation material and so on. These boundary conditions can be simplified into a translational and rotational springs with an equivalent stiffness in finite element modeling. So a dynamic tuning was performed to obtain a reasonable boundary stiffness using the vibration measurement results of actual ships. In this study, a vibration analysis for pump tower of both type LNG carriers, namely Moss and Membrane, was carried out and reviewed their own vibration characteristics. Also the vibration measurements at sea trial were followed to confirm the analysis results. Lastly the structural safety of Membrane pump tower due to propeller excitations is evaluated analytically.