ABSTRACT
In this paper, we investigated the vortex-induced vibration of a catenary flexible riser under log-law sheared flows. Two high-speed cameras are employed to capture the in-plane and out-of-plane displacements, respectively. The experimental results indicate that the mode transition is out-of-sync in the in- and out-of-plane. Nevertheless, the out-of-plane response affects the in-plane response, and the influence becomes weaker with increasing the reduced velocity.
The dynamic response of a circular cylinder exposed to flows is widely encountered in engineering applications. Particularly, in offshore oil and gas engineering, the elongated flexible riser as a result of the increasing water depth experiences more complicated vortex-induced vibration (VIV) with the presence of multiple frequencies and mode competition. The vigorous VIV response is the main reason of structural fatigue damage, which potentially brings huge economic loss and serious environmental pollution. Thus, the comprehensive understanding of VIV response of flexible cylinders with a large aspect ratio is urgently required to predict and prevent the fatigue failure (Wang et al., 2019; Zhu et al., 2020). VIVs of flexible cylinders in uniform flows have been extensively investigated in past decades. The considered flexible cylinder in the experiments conducted by Song et al. (2011) has a large aspect ratio of 1750. Although the cylinder is subject to uniform flow, asymmetric response was observed with the highest mode reaching the sixth in the cross-flow direction. Sun et al. (2012) reported that the excited mode increased with the incident current velocity.
Due to the mono-frequency response, standing wave pattern was observed by Zhang et al. (2013) in their experimental study on the VIV of a straight flexible cylinder in uniform current, Travelling wave pattern emerges in the presence of multiple frequencies, as reported in Fan et al. (2019) and Feng et al. (2019). In order to meet the nonlinear velocity profile of currents, Srinil (2011) numerically investigated the dynamic behavior of a variable-tension riser in linearly sheared currents. An asymmetric response with mode switching over time was observed. Experimental study conducted by Gao et al. (2015) illustrated the multi-mode characteristics of a straight flexible pipe in linearly sheared flows. The space-varying frequency was observed by Mao et al. (2015) for the response of a drilling riser under sheared flow. As the submarine risers are usually deployed in catenary configuration, Assi et al. (2012) experimentally investigated the VIV of a rigid-curved cylinder in both concave and convex configurations in uniform flows. The distinguished feature was found in the curved cylinder as compared to a straight one. The cylinder in concave configuration exhibited a relatively high amplitude that persisted beyond the typical synchronization region.
