The flow behavior in hilly terrain pipelines with low flow rates of gas and liquid is gravity dominated. For these conditions, steady or unsteady state flow can prevail, even for constant input gas and liquid mass flow rates. The transient behavior of the flow is due to pipeline geometry and the compressible nature of the gas phase. The result is the occurrence of terrain induced slugging which can cause operational problems.

Severe slugging in a pipeline-riser system can be considered as a special case of terrain slugging in hilly terrain pipelines. For this case, the system consists of with a downward inclined section followed by an upward inclined section. Recently, an improved quasi equilibrium model has been developed to simulate the transient flow behavior in pipeline-riser systems.

A new model is developed to simulate low velocity flow in hilly terrain pipelines by extending the improved severe slugging analysis. The model is based on fundamental principles and incorporates the physical phenomena. Flow instabilities can inherently be predicted by the model, and a separate stability analysis is not required.

The new model developed for low velocity flow in hilly terrain pipelines can be utilized to predict the flow behavior under a wide range of flow conditions, including zero net liquid and gas flow, pipeline start up and shut down practices, the effects of pipeline rupture, and variations of inlet flow rates or separator pressure. Simulation of a full scale field pipeline is presented to demonstrate the capabilities of the proposed model. The results show that large amounts of liquid and gas can be produced in a short period of time due to the occurence of terrain slugging, creating a potential operating problem at the outlet of the pipeline.

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