Calculation of High-Viscosity Oil Temperature During Pipeline Operation and Shutdown

Daniyar Bossinov, Uzak Zhapbasbayev, Gaukhar Ramazanova

The transportation of high-viscosity and waxy crude oils through long-distance pipelines presents significant thermal and hydraulic challenges, particularly when hot pumping is combined with temporary shutdowns. During normal operation, crude oil is heated to elevated temperatures to reduce viscosity and maintain stable flow. However, once pumping stops, the oil gradually cools due to heat exchange with the surrounding soil. This cooling leads to increased viscosity, higher restart pressures, and potential flow blockage caused by wax crystallization. Therefore, accurate prediction of temperature evolution under both steady-state and transient conditions is essential for ensuring safe and efficient pipeline operation. This study aims to determine how accurately the temperature behavior of high-viscosity crude oil can be predicted during both continuous operation and shutdown periods, and how these predictions can be used to estimate safe shutdown durations. In particular, the research investigates the influence of soil thermal properties, phase change effects, and flow conditions on cooling rates and restart constraints. The analysis is conducted for the Severnye Buzachi– Karazhanbas pipeline in the Mangystau region of Kazakhstan, which has a length of 25 km and an internal diameter of 0.414 m. The pipeline operates under hot pumping conditions, with crude oil heated to approximately 60 °C at the inlet. Due to the large length-to-diameter ratio, a one-dimensional heat transfer model is applied. For steady-state operation, the classical Shukhov formula is used to calculate the temperature distribution along the pipeline. During shutdown, a modified Shukhov-based approach is employed to describe transient cooling, accounting for heat exchange between the oil, pipe wall, insulation, and surrounding soil. The model includes variable heat transfer coefficients that depend on flow regime and material properties. To capture the effects of wax crystallization, the apparent heat capacity method is applied, incorporating latent heat into the thermal model. Oil viscosity is described as a function of temperature using a regression model based on experimental data. Validation is performed using operational data from a supervisory control and data acquisition system collected between 2020 and 2024. The simulation results show strong agreement with measured data. Under steady-state conditions, the temperature deviation at the pipeline outlet does not exceed 0.002 °C. During shutdown scenarios, the difference between calculated and observed temperatures ranges from 0.29 °C to 0.44 °C for shutdown duration 18 hours. The results indicate that soil thermal conductivity is a key factor governing the cooling rate of the pipeline. Higher conductivity leads to faster heat loss and reduced allowable shutdown times. The inclusion of latent heat significantly improves prediction accuracy by slowing the cooling process within the wax formation temperature range. These findings highlight the importance of considering both environmental and thermophysical factors in pipeline design and operation. The proposed modeling approach provides an accurate and physically consistent method for predicting temperature evolution in high-viscosity oil pipelines. It enables reliable estimation of safe shutdown durations and helps prevent operational failures associated with excessive cooling. The methodology can be applied to optimize pipeline operation and improve thermal management strategies in similar systems.