ABSTRACT:

In this study, experimental and numerical investigations on dynamics of flexible free hanging riser are presented. Two riser models made of polyethylene and Teflon (PTFE) of different size are used for the experiments, in which models are forced to oscillate at the top end. For three-dimensional motion measurement, twelve CCD cameras are used to measure horizontal- and vertical- motions of the model along the longitudinal axis; six for in-line motion and six for transverse motion. The top end of the riser models is oscillated horizontally in a sinusoidal mode in still water. The largest response amplitude of vortex-induced vibration is induced at the bottom part of the model for all excitation frequencies and amplitudes. A numerical simulation method for dynamics of vertical risers is developed. The equations of motion of riser are derived based on Hamilton's principle. Galerkin method and Newmark-β method are employed for integration of the equation of motion in space and time, respectively. The results of the numerical simulation are validated by comparing with experimental results and showed a good agreement.

INTRODUCTION

Marine risers are widely used in various offshore activities such as ocean thermal energy conversion (OTEC), deep-sea water exploration, oil exploration and production. As the ocean resource developments are moving toward much deeper seas, dynamics of a long slender marine riser is now becoming more important than before. These slender and long marine risers for ultra-deep water developments may be highly flexible due to the increase of length over diameter ratio, so their dynamic motions induced by various external loads become more complex. Thus it is necessary to carry out more exact dynamic analysis and experiments for understanding the behavior of a long flexible marine riser. There have been many numerical and experimental researches on the dynamics of a slender marine riser.

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