Multi-mode flow-induced vibrations of two side-by-side slender flexible cylinders in a uniform flow

Flow-induced vibration (FIV) of multiple marine risers frequently occurs in deepwater applications and might result in serious structural failure due to fatigue damage accumulation. It is known that long marine risers may experience high modes of vibration and behave multi-mode vibration features. Moreover, the interactions of multiple risers subject to FIV are very complex and still unclear. In this paper, a series of experimental tests were carried out to investigate FIV of two side-by-side flexible cylinders with high aspect ratio (length to diameter, L/D = 350) in a towing tank. Four cases of different spacing ratios (center-to-center separation distances to cylinder diameter, S/D = 3.0, 4.0, 6.0 and 8.0) were adopted to examine the effect of spacing on the multi-mode FIV of the two flexible cylinders. The maximum dominant modes are 4th and 6th in cross-flow (CF) and in-line (IL) directions for both side-by-side cylinders, as well as the single one. In the switching region of the adjacent modes of vibration, higher-order mode vibrations are less difficult to excite for side-by-side cylinders. The IL displacement amplitudes of the two cylinders could be enhanced by the remarkably strong interaction between cylinders, even with a center-to-center distance of up to 8.0 cylinder diameter. In addition, the IL FIV behaviors are much more complicated than those in CF direction, for instance the response spectra in IL direction exhibit several large peaks and lots of small spikes around. The IL and CF interactions of the two side-by-side flexible cylinders were also investigated by using the response trajectories collected from seven measurement points at different reduced velocities.