Vibration of highly flexible free hanging pipe in calm water

Abstract

The paper presents a numerical and experimental investigation on dynamic behaviors of a highly flexible free hanging pipe in calm water. In the numerical study, an implicit finite difference algorithm is employed for three-dimensional pipe equations. Fluid and geometric non-linearity and bending stiffness are considered and solved by Newton-Raphson iteration. In order to verify the numerical results and to see real physical phenomena, an experiment is carried out for a 6 m flexible free hanging pipe in a deep and long towing tank. In the experiment, the free hanging pipe is excited by top end oscillations and in-line displacements of the model are measured. The measured results are analyzed for the first vibration mode and found that the maximum response amplitude occurs at much smaller frequency than its natural frequency and the deviation is much larger than a usual damped vibration system. The large deviation is presumably due to high flexibility of the model where a structural wave speed is rather slow. The comparison between numerical and experimental results shows a good agreement for the upper mid part of the model but with some differences for the bottom part. The difference is due to the large interaction between in-line motion and vortex-induced transverse motion that is largest at the bottom part. In the case of slender marine structures with free end at bottom, the dynamic behavior of the bottom part is very important to safely operate the system but is difficult to analyze exactly in a numerical method due to large vortex shedding. (c) 2005 Elsevier Ltd. All rights reserved.