Experimental Research on Vortex-Induced Vibration of Flexible Catenary Riser Model

Abstract

Vortex-induced vibration (VIV) is one of the most important factors accounted for the fatigue damage of long flexible risers in deep water. In this paper, the VIV response characteristics of flexible catenary riser model with the slenderness ratio of 578 were firstly studied by means of scale physical model experiments. In the experiment, the riser model was installed on a towing carriage, which might move horizontally above a wave basin with constant speed to simulate the working condition of the riser model under uniform current. The tension sensor was used to measure the time-history variation of the top tension. The acceleration sensor was used to measure the accelerations of the riser model in cross-flow (CF) and in-line (IL) directions. And the top tension, vibration spectrum, amplitude and vibration locus of the riser were analyzed in accordance with the flexible riser model experiment, and the VIV law of the model experiment working condition was analyzed; then the hydrodynamic software Orcaflex was used to verify the finite element analysis (FEA) of the experiment. The experimental results show that the flexible catenary riser with lower mass ratio and lower bending stiffness has more complex hydrodynamic characteristics than the steel catenary riser (SCR). When the suspension angle remains the same, the top tension of the riser increases with the increase of flow velocity, and the higher the flow velocity, the faster the growth rate; the natural frequency of the riser increases with the increase of flow velocity; the VIV of the riser is the second-order vibration mode when flow velocities range from 0.2m/s to 0.4m/s. The vibration frequencies corresponding to acceleration sensors at different measuring points along the axis of the riser tend to be the same and increase with the increase of flow velocity, which results in “lock-in” near a certain natural frequency close to the vortex shedding frequency. When V = 0.2m/s, the VIV responses of some measuring points of the flexible riser present a positive “8”-shape or oblique “8”-shape vibration, when the amplitude tends to be the maximum. As the flow velocity approaches to 0.8m/s, the “8”-shape vibration disappears and the VIV vibration locus begins to become cluttered, and the variation rate of the VIV phase angle is faster than that at low speeds. The experimental results show that the physical model experimental results is well matched with that of Orcaflex numerical model. The physical model experimental results can reflect the vibration law of flexible risers under actual working conditions and can be used to predict the actual vibration law and characteristics of the VIV of flexible risers.