Experimental investigation on vortex-induced vibration of a flexible pipe with single/bi-frequency parametric excitation

Abstract

Vortex-induced vibration (VIV) of marine risers is a complicated fluid-structure interaction problem in ocean engineering. The parametric excitation caused by the heaving of the platform complicates the VIV of risers. In this paper, a series of model tests were conducted in a towing tank to investigate the VIV characteristics of a flexible pipe with single/bi-frequency parametric excitation. The linear motor was connected to the end of the pipe through a spring, and the movement of the linear motor provided various parametric excitations for the pipe. With parametric excitation, additional frequency components such as excitation frequency, the multiples of excitation frequency and the sum and difference frequencies are excited. Parametric excitation has more significant effect on VIV in IL direction than that in CF direction. With the increase of excitation amplitude, the maximum RMS strain in CF direction decreases while that in IL direction increases in most cases. The phase plane plot and motion trajectory of the flexible pipe becomes less regular with parametric excitation. When bi-frequency parametric excitation is applied, more types of additional frequencies such as sum and difference frequencies between the two excitation frequencies and other frequencies can be observed in the response, and the parametric excitation component with larger weight has more significant effect on the VIV of the flexible pipe.