In-line inspection (ILI) by Magnetic Flux (MFL) Intelligent Pigging of pipelines is perhaps the most convenient method to assess the internal condition of an operating pipeline. Even though ILI pigging data provides a sound assessment of the internal corrosion status of the pipeline, it may miss some indications which may lead to failures even after conducting the ILI assessment. Hence judicious interpretation of the ILI data is needed. This paper highlights the findings of an investigation of a ruptured high-pressure gas line which failed in a region, where the ILI data had not shown indications of any significant corrosion damage or reduction in thickness. The possible reasons for the discrepancy in the ILI data are discussed.
A 20 inch (508 mm) diameter pipeline, HP057, transports dry gas at a pressure of about 400 psi (27.5 bar) from a gathering center manifold (GCMN) to a gas booster station (BS). Only one gathering center (GC) is presently connected to the GCMN and hence the process stream in this line is essentially the same as that the export gas from this GC. In-Line Inspection (ILI) survey using intelligent pig based on the principle of magnetic flux leakage (MFL) was carried out in August 2006. In August 2010, another ILI was carried out. Concentration of internal corrosion anomalies were detected in the lower half of the pipelines though scattered internal corrosion anomalies were found on top half of the pipeline as well. On September 19th, 2010, at a distance of 6070.5 m (19916 feet) from GCMN, a leak was observed. This incident, which occurred after an ILI has been carried out, provided the basis for an in-depth study into the possible causes of an ILI survey not being able to detect extensively internally corroded regions in a pipeline.
