FAILURE ANALYSIS OF AN IN-SERVICE NATURAL GAS PIPELINE

A natural gas pipeline which had been serviced nearly 10 years burst and led to fire. Analysis on samples taken from the pipeline had been made, the mechanism of crack initiation and propagation was discussed, and the cause of burst and fire of the pipeline was put forward. The analysis results showed that, the pipeline had some grooves along the axial mechanical damages, which caused the stress concentration. Except that, the coating in the mechanical damage area was broken, this caused the failure of cathodic protection (CP). Under the combined action of the stress concentration and the corrosion environment of the soil, localized corrosion in the mechanical damage area was caused, where micro-cracks initiated. Furthermore, in the stage of cracks propagation, the stress corrosion cracking (SCC) occurred due to the corrosion environment of the soil. Finally, with the cracks expanded, connected and grew, the residual wall of the pipe was small, and the accident arose because of insufficient strength.