Yield Stress and Minimum Pressure for Simulating the Flow Restart of a Waxy Crude Oil Pipeline

International audienceWaxy crude oils are likely to develop a yield stress when cooled to a temperature below its wax appearance temperature. In a subsea pipeline, when the flow stops and the oil cools down, a minimum differential pressure is required to restart the flow. In order to estimate that pressure, it is crucial knowing the yield stress distribution in the pipe at restart time. However, the yield stress of waxy crude oils depends on the histories of shear and temperature experienced by the oil. In this paper, we present the parameters that play a major role on the yield stress determination by measuring creep tests in different conditions. For the analyzed oil sample, final temperature and shear while cooling are the most important yield stress parameters. Based on those results, we introduce a correlation between the oil cooling conditions and its yield stress at low temperature. Then, by calculating the oil cooling history during steady-state flow and cool-down time at rest, it is possible to correlate those histories to the yield stress developed by the oil. That is an essential information for calculating the minimum flow restart pressure. Here, 2D axisymmetric calculations were performed. Results for a given oil sample show that at restart time the oil-gel is likely to break at the wall, concentrating shear at that region and forming a plug flow with unsheared material at the center of the pipe