Slug flow's intermittent feature affects VIV responses of flexible marine risers

Abstract

Slug flow is characterized by the variations of internal flow parameters with time and position. This study aims to investigate the slug flow effect (SFE) on the cross-flow Vortex-Induced Vibration (VIV) of a flexible riser. A steady slug-flow model is adopted using several assumptions and empirical correlations. A distribution of van der Pol oscillators is employed to create the VIV effect. A novel approach has been proposed to capture the intermittent feature which is validated by comparisons with experiments. The SFE is explored by comparisons of the dynamic behaviors of a flexible riser subject to two uniform currents, assuming the pipe is filled with still liquid, transporting a single-phase liquid flow and a two-phase slug flow. It has demonstrated that the SFE is much more profound than the single-phase internal flow effect. A flexible riser conveying a slug flow can exhibit quite distinguished dynamics owing to the intermittent feature. Apart from triggering new modes, the combination of slug flow frequency and VIV frequency can generate an amplitude-modulation phenomenon of excited modes. Most distinctively, beating behaviours which corresponds to the resonant synchronization have been captured. This study can contribute to fatigue analysis of a flexible riser conveying a two-phase slug flow.