Root Cause Analysis of NIOC\'s 20" Natural Gas Explosion

Transmission and distribution pipelines have a significant role in the oil and gas industries. Pipelines are facing different threat like corrosion, improper design and incorrect operation. Gas vapor released from a pipeline (owned by NIOC) failure spread out and reached to ignition source. Due to gas outdoor spreading, “flash fire” occurred and flame return caused “vapor cloud explosion” of gas that was gathering in hole has been created around the pipeline. Explosion energy was cutting out 5-meter length of pipeline and throwed it about 20m. In current paper, the technical and root factors causing crack and leakage were investigated. For performing this, first of all, the compatibility of base metal chemical composition with APL 5L standard was investigated. furthermore, soundness of pipe manufacturing was considered. In the next step, a scanning electron microscope was used for fracture analysis. According to microstructure embrittlement and severe inter-granular cracks observed, tensile and impact tests were performed to confirm embrittlement phenomena. Impact test results showed that the absorbed energy was 11 joules which it was much lower than standard value (27 joules). In this regard, ESD crack surface chemical analyzing in pipe cracked area showed out of range sulfur, especially in longitudinal seam weld area. Its gradient determined that pipeline crack initiated from outer wall due to overage operation of this pipeline (more than 40 years). The pipeline coating was deteriorated which let pipe contact with soil, consequently, Stress Corrosion Cracking and rupture were occurred. The root cause analyzing also showed the lack of proper periodic technical inspection, repair and maintenance program for the pipe coating.