Thermal–hydraulic coupling model and operating conditions analysis of waxy crude oil pipeline restart process after shutdown

To realize the safe and economic operation of a waxy crude oil pipeline, a thermal–hydraulic model in an unsteady state was established for the restart process of waxy crude oil. The model is based on the thermodynamic and hydraulic characteristics of crude oil within a pipeline intercoupling with media outside the pipeline during the restart process of the crude oil pipeline. The characteristics of waxy crude oil are considered, and the model includes the continuity equation, momentum conservation equation, and energy conservation equation. This article suggests a method for developing the model based on shockwave theory combined with the characteristic line method, numerically simulates the restart process of a certain buried oil pipe in summer and winter, and analyzes how the oil temperature at the beginning and terminal of the pipeline and the pressure at the beginning of the pipeline change during restart. In addition, this article confirms the safe duration of shutdown in two seasons for a waxy oil pipe, which are 37 h in summer and 33 h in winter.