COOLING DOWN OF OIL WHEN THE PIPELINE STOPS IN FROZEN SOIL

Abstract

The article presents the formulation and solution of the associated problem of oil cooling when the underground laying oil pipeline stops and the temperature changes in frozen soil in the presence of moss and snow cover on the surface. A physical and mathematical model and an associated computational algorithm for calculating the parameters of oil in a pipeline and soil with covers have been developed. Peculiarities of solidification of oil containing N-fractions of paraffins during heat removal into frozen soil have been studied. In this work, an important solved problem is the approximation of a characteristic diagram of phase equilibrium states during cooling of paraffinic oil in the temperature range from the onset of crystallization of paraffins to the pour point. A feature of the problem being solved is that the temperature field of oil in the pipeline (region A) and the temperature field of the moist soil surrounding the pipeline (region B) have a common border — the pipeline wall, which is assumed to be thin. Through the pipeline wall, the temperature of which is not known in advance, the mutual influence of temperature fields (conjugation) is taken into account. The results of an experimental study of changes in the temperature in the pipeline with time in laboratory conditions are presented. The calculation results are in satisfactory agreement with experimental data on the solidification of high-viscosity paraffinic oil in a model oil pipeline when the oil is cooled from +4.5 to −5.5 °C. On the basis of the physical and mathematical model developed in this article and the coupled algorithm for calculating the parameters of soil and oil, it has been established that in the presence of moss and snow cover, characteristic of the winter conditions of the Middle Ob region of Western Siberia, in an oil pipeline with a nominal diameter of 700 mm, oil freezes in a time of 40 60 hours depending on soil parameters and oil fractional composition.