The application of the coupled acoustic-structural approach (CASA) method on the free vibration of submerged structures

Abstract

Traditionally, the consideration of the surrounding water during the modal analysis of submerged structures has been done by applying constant added mass factors. This method may lose its accuracy for complicated structures, especially those with bends and multi-plane topology. The present study utilizes the coupled acoustic structural approach (CASA) to obtain natural frequencies on submerged structures. The work compares the simulated CASA results with experimental results. Four experiments with objects with different geometry were chosen; a straight pipe, cantilever plates, stiffened cylindrical shell, and a disk. We found that the CASA method produced accurate results for the three first eigenmodes compared to the experimental results. It could simulate bending, twisting, ovality, and stretching mode shapes with an accuracy of less than 10 % difference. Additionally, we applied the CASA method to two submerged jumper structures for the first time. We found that the CASA method gives accurate modal analysis results and is recommended for use.