Long-term performance and optimization of two-phase closed thermosyphons for buried large-scale warm-oil pipelines under rising oil temperature

The China–Russia crude oil pipeline (CRCOP) serves as a critical energy supply route for both countries. The pipeline during its operation inevitably dissipates heat to the surrounding permafrost, leading to thaw settlement issues. A full-scale monitoring system has been deployed in key areas affected by thaw settlement along the CRCOP to guarantee the safe operation of the pipeline. This system provides real-time alerts and verifies the cooling mechanism of the vertical two-phase closed thermosyphons (TPCTs) used for mitigating thaw settlement. However, on-site monitoring results indicate that the TPCTs alone cannot maintain the permafrost temperature in the vicinity of the CRCOP, suggesting the need for optimized TPCT arrangements. This study proposes three design schemes to optimize the configuration of TPCTs based on in-situ observations. Numerical simulation results indicate that the composite approach — combining TPCTs with a thermal insulation layer — performs better than the standalone TPCT configuration. Using this composite solution, the artificial permafrost table (APT) can be stably maintained at approximately 2.5 m depth. This strategy is recommended for engineering applications in permafrost environments. Furthermore, the findings of this study provide valuable guidance for the CRCOP and other similar pipeline projects in permafrost regions.