Experimental study on flow regime classification and riser pressure fluctuation characteristics in the long-distance pipeline S-shaped riser system

Flow instability in the pipeline-riser is closely related to the fluctuation frequency and amplitude characteristics of the riser's differential pressure. The gas–liquid overall and local flow regime distribution and flow characteristics in the long-distance pipeline S-shaped riser system with 46 mm ID and 1722 m length are investigated. An overall flow regime classification method based on the time-domain features, probability density distribution and power spectrum distribution of the riser's differential pressure signal is proposed. The overall flow regimes SS, OSC, and ST1 at lower gas velocities (V_SG0 < 3.00 m/s) correspond to riser's local flow regimes that exhibit a composite morphology of multiple single flow regimes in the vertical upward pipe, whereas the overall flow regime ST2 at higher gas velocities (V_SG0 ≥ 3.00 m/s) shows the same single flow regime as that in the vertical upward pipe. Based on the gas phase Strouhal number and the no-slip liquid volume fraction, a correlation for predicting the fluctuation frequency of riser's differential pressure is established. The fluctuation amplitude, minimum, maximum and asymmetry along the mean value of the riser's differential pressure are associated with the overall flow regime, and a quantitative division criterion is proposed for predicting the unstable flow in the long-distance pipeline-riser system.