Effectiveness of Bypass-Pigging Solutions in Multiphase-Flow Pipelines With Waxy Crude Oil: Evaluation and Innovative Solution
Bypass pigging, compared with conventional pigging, reduces the damaging effects of the pig-generated liquid slug by redistributing gas and liquid in the pipeline. Oil- and gas-production rate, high liquid-slug flow to the slug catcher, high pipeline backpressure, and the capacity of the slug-handling facility at the receiving end are major considerations when designing a bypass-pigging solution. Various operational and engineering challenges are encountered while implementing the commonly known bypass-pigging solutions, and empirical correlations are developed on the basis of experimental results and compared with simulation results. This paper suggests an innovative bypass-pig geometry as a solution. The Thornhill-Craver equation is introduced to calculate the bypass-flow quantity and the pig velocity. A comparison between transient-flow simulation and field results showed some devia- tions. Empirical correlations are developed for prediction on the basis of experimental results. A new convergent/divergent bypass pig geometry/profile is developed, followed by simplified model development. Through this innovative design, critical and constant gas-flow rate is achieved at lower pressure ratio through the bypass hole, where the gas enters through a nozzle, stabilizes at the throat, and recovers pressure through a diffuser section. At a predefined inlet pressure and area of cross section of the hole, a properly designed convergent nozzle with throat section will give maximum critical flow rate at the exit by reducing the gas pressure to the critical pressure ratio. However, with help from the diffuser section, the high-velocity energy is converted back into pressure energy, and the line pressure regains up to 90% of the upstream pressure. Adopting such a bypass-hole profile with suitable geometry can ensure required bypass-gas quantity through the pig and can avoid pig stalling and minimize process upset, thus ensuring better pipe- line cleaning.
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07 May 2018