Fatigue Failure Investigation of Natural Gas Transmission Pipelines under Train-Induced Vibrations

In countries with expansive natural gas transmission networks, such as Iran, pipelines frequently traverse beneath roads and railways, exposing them to vibrational loads induced by passing vehicles, including trains, trucks, and subway systems. These pipelines vary in diameter, wall thickness, and gas pressure, complicating assessments of structural integrity. This study investigates the fatigue behavior of buried and exposed gas pipelines subjected to traffic-induced vibrations, with particular emphasis on the Alborz region. Field evidence has previously highlighted the vulnerability of flange connections, prompting the replacement of such joints with more robust alternatives. However, the fatigue response of the pipe body under long-term vibrational loading remains less understood. To address this, critical pipeline segments near major transportation corridors were identified and instrumented with strain gauges and triaxial accelerometers. The challenge lies in accurately capturing and evaluating the nonperiodic, multifrequency, and variable-amplitude nature of the vibrational loads. The study applies established fatigue assessment methods, including Soderberg’s criterion for equivalent stress calculation, Rainflow cycle counting for analyzing variable-amplitude loading, and the Palmgren–Miner rule for predicting fatigue life. Finite-element modeling using Abaqus was also utilized to account for geometrical complexities and to validate the stress distributions observed in the field measurements. Results demonstrate that although traffic-induced vibrations do generate transient stresses within the pipe walls and connections, the magnitudes are insufficient to cause fatigue failure over the expected service life of the pipes. Nonetheless, vibrational loosening of flange connections was observed and has been identified as a critical issue warranting design modifications.