Thaw bulb formation surrounding warm-oil pipelines and evaluation of the cooling performance of a new air convection pipeline embankment structure

The first China-Russia crude oil pipeline (CRCOP-I) and the second China-Russia crude oil pipeline (CRCOP-II) went into operation in January 2011 and January 2018, respectively. On-site monitoring data have indicated that the seasonal thawing depth (STD) of a borehole 2 m away from CRCOP-I reached 10.8 m by the end of 2021. To improve mitigation measures of thaw settlement, an inverted T-shaped crushed-rock pipeline embankment (ITCPE) is proposed in this study. The long-term thermal activity of three types of pipeline embankments is predicted and compared with the underground structure. This study finds that none of the current pipeline structures has solved the thaw settlement issue for the CRCOP, whether with insulation layers, in an unprotected pipeline embankment (UPE), or in a traditional horizontal crushed-rock pipeline embankment (THCPE). However, the cross-ventilation attained in the proposed approach did mitigate thawing. It is estimated that after 50 years of operation, the natural permafrost table will be 2.9 m deep, whereas the proposed ITCPE central pipeline embankment could maintain the artificial permafrost table at a depth of 0.7 m. This study provides technical support for the CRCOP and provides a reference for the design and maintenance of other pipeline projects in permafrost areas worldwide.