Numerical analysis of the heat transfer associated with freezing/solidifying phase changes for a pipeline filled with crude oil in soil saturated with water during pipeline shutdown in winter

A flow and heat transfer model for a crude oil pipeline buried in soil saturated with water during shutdown, where ambient temperature is below the freezing point of water, has been established. Phase changes involving water freezing in the soil and crude oil solidifying in the pipeline and the influence of initial temperature and flow field were included in the model, and the natural convection of water in soil and crude oil in the pipeline were also taken into account. Temperature and flow fields, and interfaces of water freezing in soil and crude oil solidifying in the pipeline were obtained through numerical simulation. Numerical results show that the temperature gradient in the soil is greatest near the top of pipeline, and that natural convection of water and oil occurs from bottom to top along a vertical symmetrical line in both soil and pipeline due to the temperature distribution. The freezing interface for water in soil and solidification interface of oil in the pipeline advance to greater depths with increasing time of shutdown. The rate of increase in the depth of the freezing interface is slower in the soil near the top of the pipeline than in that far from the pipeline.