Water Slug Simulation for Transient Multiphase Process Flow

Abstract

Current paper represents water slug simulation details applied to realistic transient scenario of pigging gathering line that transfers oil and gas production to Tengiz Second Generation Plant. Pigging as well as any dynamic multiphase flow creates a big challenge for receiving facility causing not only inlet separators level fluctuations, but also carrying a risk of process upset and plant facilities damage downstream due to water flooding. Big diameter long distance transportation lines significantly contribute to this risk. One of the triggers to start using OLGA dynamic simulation tool in TCO on constant basis, was water flooding event that damaged stabilizer internals. Modeling is able to predict water slug, considering conditions for water settling in the pipeline prior to transient event, and thus allows to prevent or minimize potential upset. Several scenario results were reviewed and compared: one versus the other, including scenario with mitigation measures. Hilly underground profile and low flowrate through the line over certain amount of time appeared to be key factors in obtaining reasonable accumulated water content, whereas transient event speed defines how quickly this water will arrive as a slug. Recognizing processing facility handling limits, the control over impact could be maintained by increasing flowrate prior to event and/or reducing speed of change (pig velocity in this particular case). Besides getting realistic water slugs due to rigorous pipeline geometry and flow preconditions, slower actual historical pig movement (against earlier simulated with flat geometry) was finally explained. Present research was done to demonstrate how accurate and detailed inputs and preconditions to the simulation case can improve software predicted water slugs and create safe guidelines to overcome negative consequences. This practically discovered simulation strategy established itself as credible over few years, is allowing to manage all dynamic operations safely.