Stress assessment and control measures for deformed tunnel-crossing pipeline section

Objective It is common for long-distance oil and gas pipelines to traverse mountain tunnels. The sections of pipelines passing through tunnels are susceptible to significant deformation due to internal pressure and temperature effects during the transmission process, especially when the constraints or compensatory mechanisms for the pipelines within the tunnels are inadequate. This tendency for deformation underscores the importance of conducting stress assessments on deformed pipeline sections crossing tunnels.

Methods The study's proposed strategy begins with stress calculations for deformed tunnel-crossing pipeline sections using three approaches: theoretical calculations, ultrasonic stress detection, and finite element analysis. Subsequently, the process involves dynamically monitoring stress changes in these deformed pipelines at high frequencies, employing monitoring points strategically positioned based on calculation outcomes. The dynamic monitoring data is then utilized to develop control measures entailing the staged release of pipeline stress. Additionally, the pre-control pipeline stress calculation results and stress variations during the control process (i.e., pipeline stress monitoring data) are combined to determine the post-control calculated pipeline stress. These calculation outcomes serve as the foundation for assessing the pipeline's safety margin.

Results The methodology was applied to the deformed pipeline section crossing a tunnel at PipeChina West-East Gas Pipeline Co. Ltd. The stress assessment revealed high stress levels in the deformed pipeline section, showing stress concentration at the tunnel entrance and exit. Under the design conditions with an operating pressure of 12 MPa and an operating temperature of 40 ℃, the equivalent stress in pipeline welds at the tunnel exit exceeded 93.87% of the yield stress, falling short of the safety limit of 0.9 times the yield stress for equivalent stress levels in pipelines. Following non-hot work treatment, the safety margin for equivalent stress at the tunnel entrance increased from 68.50 MPa to 181.14 MPa, while at the tunnel exit, it rose from 47.50 MPa to 183.66 MPa, showcasing a significant stress relief effect.

Conclusion The research findings are valuable for establishing design and monitoring schemes for the tunnel-crossing sections of long-distance oil and gas pipelines.