Simulation Testing on the Shutdown and Safe Restart of Crude Oil Pipelines

The plan repair or rush-to-repair of a hot oil pipeline may shut it down. As the pipeline is shut down, the oil's temperature will decrease continuously and the heavy organic molecules will crystallize or aggregate and suspend from the oil, which can strengthen their interaction and worsen the oil's flowability, even form a network structure or freeze the pipeline. Once the pipeline is frozen during its shutdown, it is likely that the pipeline will have to be trenched to unblock and restart the pipeline. Hence, it is necessary to focus on the experimental study of the shutdown and restart behavior of a hot oil pipeline to provide a safe and effective restart proposal for the shutdown pipeline. Considering the potential frozen risk of the shutdown pipeline, this paper takes BZ28-2S crude oil and the mixture of BZ19-4 oil and produced water as the study objects, and the change laws of restart pressure of the shutdown pipeline as the shutdown time and temperature for different study fluids were simulated by using a loop system. The main factors affecting the restart pressure of the shutdown loop with studied fluids were analyzed and the relationship between the pressurizing speed and the restart pressure of the shutdown pipeline were investigated and then an effective method to ensure the safe restart of a shutdown hot oil pipeline was presented. The results indicate that the longer of shutdown time and the lower of shutdown temperature, the higher of the restart pressure; and the restart pressure also increases as the restart speed due to a higher instant pressure, accordingly the restart time becomes shorter. The low-temperature flowability of BZ28-2S oil and BZ34-1N oil are poor, thus the restart pressure should be higher during the pipeline's restart; the yield value of BZ28-2S blended oil is high at low temperatures, hence the temperature drop during shutdown shouldn't be below 25°C. Under the same shutdown temperature and time, the restart pressure of the BZ19-4 oil-water flow decreases while its water cut is higher than that of the phase inversion point and the pumping behavior of the restarted oil-water flow worsens.