THE INFLUENCE OF INHOMOGENEOUS RADIAL PROFILE OF WAXY CRUDE OIL TEMPERATURE AND SHEAR STRESS IN A STOPPED PIPELINE ON THE RESTART PUMPING PRESSURE FOR «USA — UKHTA» MAIN PIPELINE

Abstract

The purpose of this work is to calculate the shear pressure gradient and shear radius of paraffinic oil in a stopped pipeline as functions of the longitudinal coordinate z along the axis of the stopped section, as well as the minimum shear pressure difference between the ends of the pipeline, taking into account the non-uniform cooling of oil over time in the radial direction, leading to a significant increase in static shear stress with distance from the center to the edge of the pipe. Using a simple mathematical model that describes the non-uniform cooling of oil along the radius r and along the longitudinal coordinate z along the pipe axis due to heat conduction, it is shown that with a smooth increase in start-up pressure at the beginning of a stopped pipeline section, the cooling oil can shift not along the pipe wall, but along its central part, the radius of which depends both on the oil cooling time and on the z coordinate along the pipeline axis. Calculations show that the process of cooling of the stopped oil can be conditionally divided into three stages. At the first time stage of cooling, the oil near the pipeline axis does not cool down to the temperature of the appearance of static shear stress, that is, it is actually Newtonian and there is no problem with starting this oil layer. At the third time stage of cooling, when the oil cools below the temperature of the onset of paraffin mass crystallization, that is, it turns into a viscous-plastic one in the entire volume of the oil pipeline, the calculation of the minimum pressure difference between the ends of the pipe required to start the oil pipeline can be carried out using a method that qualitatively coincides with the method described in [27]. Finally, in the second cooling time step, which occupies an intermediate position between the first and third steps, the pipeline can be divided into two sections in the longitudinal direction. The behavior of oil in the first section, adjacent to the beginning of the pipeline, resembles the behavior of oil in the entire pipeline at the end of the first stage of cooling, and in the second section, extending from the end of the first section to the end of the pipe, the oil behaves similarly to the oil in the entire pipeline in the third temporary stage of cooling. In this case, the boundary between the first and second sections of the pipe during the second temporary stage of cooling is displaced from the end of the pipeline to its beginning. It is shown that calculations of the shear pressure required to resume the pumping of cooled oil, which do not take into account the inhomogeneous cooling of oil in the pipe cross section, can overestimate the desired pressure value. This fact should be taken into account in practice when assessing the restart pressure and the safe shutdown time of a pipeline transporting highly pourable oils using the “hot” pumping method. All laboratory measurements of the thermal parameters of oil, as well as calculations of pressure and shear radius, were carried out on the example of the Usa-Ukhta main oil pipeline of JSC Transneft-North.