More and more transient gas-liquid operations in pipes has been successfully applied in the oil and gas industry. Pigging operation in two-phase pipelines to remove liquid accumulation or for cleaning purpose is an important transient operation. Another important operation is the injection of gas to transport the accumulated liquid in the pipeline to process facilities. Analysis of such transient two-phase flow in a pipeline is necessary not only for designing the liquid and gas handling facilities, but also for safe operating procedure. In pipeline-riser system such operations cause even more severe change in flow conditions. A two-fluid model has been developed to determine the transient behavior of fluids during these operations. The derived one-dimensional set of equations for each flow pattern describe the flow of fluids in all regions. Semi-implicit finite difference schemes were used to solve the initial and boundary value problem for each phase of the pigging process: gas/pig injection, gas shut-in, slug production and gas flow out of the system. An extensive experimental program has been carried out to acquire two-phase transient flow and pigging data on a 67 m long, 2 in diameter pipeline-riser system. A computer based data acquisition system has been utilized to obtains rapidly changing and detailed information of the flow behavior during the transient tests. The model compares well with the experimental data for characteristics such as inlet pressure, hold-up and pig velocity.

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