An experimental investigation on the vortex-induced vibration mode transition and response interaction of a fixed-hinged catenary flexible riser

This paper reports the experimental results of the vortex-induced vibration (VIV) of a catenary flexible riser placed in the concave orientation. The flexible riser with an aspect ratio of 125 was hung by its own weight with its top end hinged and bottom end fixed. The non-intrusive optical measurement with high-speed cameras was employed to capture the in- and out-of-plane responses simultaneously. Compared to the fixed-fixed case, the change of top boundary leads to the alterations of response amplitudes and excited mode ranges, and the influence on the in-plane response is greater than that on the out-of-plane one. The coupling response between the top joint and flexible riser is examined in terms of the coincidence of the dominant vibration frequency, which is closely related to the spatial mode competition. The mode competition is distinct in mode transition cases, presenting the time-varying response profile and vibration frequency as well as the spatial evolution of dominant frequency and modal weight. The mode transition is out-of-sync for the in- and out-of-plane responses in spite of the coincidence of vibration frequency. To illustrate the response interaction, four coupling patterns are proposed: strong coupling+, strong coupling, weak coupling and weak coupling-. The spatial variation of coupling pattern is also associated with the mode competition.