Objective In the inaugural switching operation between Pipelines Q₁ and Q₂, which were originally used for heated and normaltemperature transmission of crude oil respectively in China, the approach of "transformation while operation" was employed, posing challenges such as complex process flows, demanding engineering implementation, and engagement of numerous offload and injection users.

Methods To ensure a rational transformation and switching mode with minimal impact on normal production and operation, a comprehensive study was conducted on the hydraulic and thermal systems of these transmission pipelines. This study aimed to evaluate various factors influencing preheating and temperature dropping during commissioning, as well as the pros and cons of different switching modes. Simulation accuracy was checked for the preheating process after transforming Pipeline Q₂, originally designed for normaltemperature transmission, into a heated transmission system. These simulation results were then verified with actual engineering data to ensure alignment with the practical requirements. Additionally, valuable field experience was gained from the transformation of Pipeline Q₁, originally intended for heated transmission, into a normal-temperature transmission system. Based on these study findings, reasonable suggestions and recommendations were presented for key nodes in switching operations.

Results The selection of the preheating method should be based on site-specific conditions when transforming in-service normal-temperature oil transmission pipelines into heated transmission systems. For instance, Pipeline Q₂ should be treated by heating Russian oil to be transmitted in advance combined with preheating the pipeline with high-temperature Russian oil, aligning the commissioning process with the transmission requirements of Daqing oil.Likewise, appropriate measures should be taken to gradually reduce the temperature of oil products between stations during the transition from the heated to normal-temperature mode.This can be achieved through the quantitative transmission of oil products with a gradually decreasing temperature by batch.To facilitate this, heating stations along the route can be utilized to ensure a gradual temperature drop of the oil products between stations.

Conclusion The study findings offer valuable insights for commissioning under switching operations of inservice crude oil pipelines.