Chaotic response is a generic feature of vortex-induced vibrations of flexible risers

We show through analysis of experimental data that the vortex-induced vibrations of long flexible risers are characterized by time intervals of chaotic response, followed or preceded by periods of statistically stationary response. Regions of chaotic response have been ignored in past analyses, while they contain distinctly different response features and have significant implications on riser fatigue analysis. Whereas periods of statistically stationary response are characterized by nearly mono-frequency traveling waves, with small standing wave contributions, near the ends of the riser, and possibly accompanied by sharply peaked third and fifth force harmonics, the chaotic response is characterized by a rather wide-band spectrum with several individual peaks and a mix of traveling and standing waves. Phase-plane plots and Poincaré maps show typical features of chaotic response for the latter, while the statistically stationary response can be classified as periodic or quasi-periodic. Focusing on the Strouhal region of the response spectrum gives adequate results for the statistically stationary response, provided the higher force harmonics are also accounted for, but is inadequate for the chaotic parts of the response, whose fatigue properties are influenced by the entire broader-band spectrum. It is remarkable that both sheared and uniform current profiles cause both quasi-periodic and chaotic responses.