Research on bypass pigging in offshore riser system to mitigate severe slugging

Abstract

During pigging operations in offshore pipelines, severe slugging can block the pig or cause sudden velocity changes. Therefore, this paper proposed a method involving bypass pigs to mitigate the influence of severe slugging on the pigging process. The impact of different inlet mass flow rates and pig bypass fractions on the characteristic parameters, such as the pressure in the riser bottom, pig velocity, and liquid-phase distribution patterns, were analyzed by combining OLGA and CFD simulations. Furthermore, the accuracy of the simulations was verified by the comparisons of OLGA and CFD, as well as the transient indoor experiments. The results show that the bypass pig significantly reduces pressure fluctuations in the riser bottom and pig velocity in severe slugging conditions, compared with the conventional pig. Consequently, the bypass pig dissipates downstream liquid slugs, reducing the mass flow rate and liquid holdup in the riser outlet. In addition, an increase in bypass fraction reduces the pressure fluctuations in the riser bottom and pig velocity, increases the ability to dissipate the downstream liquid slug, and decreases the mass flow rate and liquid holdup in the riser outlet. Increasing the inlet mass flow rate improves the ability to dissipate the downstream liquid slug. However, the inlet mass flow rate has a lower mitigation effect on severe slugging than the bypass fraction.