Coupling vibration response of a curved flexible riser under the combination of internal slug flow and external shear current

摘要

Apart from the vortex-induced vibration (VIV) caused by flow over flexible risers, internal slug flow-induced vibration (FIV) is extensively encountered in practice as risers are conveying oil-gas two-phase flow. This paper presents an experimental study of the coupling response of a flexible catenary riser under the combination of internal slug flow and external shear current. The vibration displacement and the internal flow regime are synchronously captured using the non-intrusive technique of high speed imaging method. The experimental results illustrate that the contribution of internal FIV and external VIV depends on the phase difference and the water depth. For the in-plane response, the contribution of internal slug flow is greater than the external flow at the upper part, while the out-of-plane response is dominated by the external current. Unlike the standing wave behavior in the internal FIV case, a traveling wave occurs with the participation of external flow, associated with the multimodal response. Additionally, the response frequencies vary over time, possessing oscillating characteristics. Thus unstable response and mode switching are observed in the deflected shapes of the flexible riser. Due to the vigorous out-of-plane vibration, the longer liquid slugs are split into shorter segments. As the gas-liquid ratio increases, the liquid slug length is decreased gradually, resulting in the reduction of system mass. Both the in-plane and out-of-plane responses are enhanced accordingly. (C) 2019 Elsevier Ltd. All rights reserved.