Flow pattern transition and void fraction prediction of oil-gas two-phase flow in a pipeline-riser system

Oil-gas two-phase flow in pipeline-riser systems exhibits complex hydrodynamic behaviors characterized by varying flow patterns and void fractions, which fundamentally govern the hydraulic performance of offshore transportation systems. This study experimentally investigated oil-gas two-phase flow through a long-distance pipeline-riser system featuring a 46 mm internal diameter and a 1722 m total length. Temporal variations of both gas-liquid interfacial structure and void fraction in the riser were quantified through high-speed cameras, pressure transducers, and electrical capacitance tomography. At low gas and liquid superficial velocities, the riser exhibited composite flow patterns characterized by periodic alternation among multiple flow patterns. Increasing gas superficial velocity induced transitions from these composite patterns to slug or churn flow. The critical void fraction for slug-to-churn transition decreased markedly with increasing liquid superficial velocity. Compared to the upward flow in simple vertical pipes, liquid slug stability was significantly reduced at the same liquid slug void fraction. The non-uniform phase distribution and high-velocity discharge of gas and liquid at the inlet of the riser resulted in considerably enhanced slip ratios and drift velocities throughout the riser. Accounting for inlet effects, an improved drift flux correlation was proposed to predict void fraction in pipeline-riser systems, achieving a root mean square error (RMSE) of 3.19 % and a mean absolute percentage error (MAPE) of 6.94 %.