Crude oil transportation pipelines depend on Computational Pipeline Monitoring (CPM) systems for leak detection. Accurate prediction of the volume of vapor phase in the pipeline is very challenging when crude oil goes though phase change (column separation) in the pipeline. It is also challenging to accurately predict the vapor phase volume when the pipeline is started from extended shut-in period during which thermal cooling or heating can occur depending on the season of the year. Pipeline operators rely on the accuracy of CPMs to make decisions on column separation and to avoid the masking of a leak during column separation. The column separation can happen due to heating and/or cooling during extended pipeline shut-in, or due to elevation changes or due to flow transients.

New methods of approach to address the hydraulics are necessary when dealing with a pipeline during shut-in period. Particularly a shut-in pipeline has no longitudinal motion of fluid, however phase change occurrence attempts to set the stationary fluid inside a pipe into motion and overcoming this difficulty was not available in the literature perhaps due to lack of encumberment with similar problems. This paper explains mechanism of column separation and its transients in pipelines during extended shut-in period. The results for a 90 Km long-pipeline shut-in over a 78-hour period will be presented to show the evolution of flow field and column separation (vapor phase change) prediction and hydrodynamic pressure in the whole pipeline over the shut-in period. This paper will also critically review the current approaches available in the literature to predict the column separation.

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