Application of MFL on Girth-Weld Defect Detection of Oil and Gas Pipelines

Abstract

Globally, the integrity of girth weld (GW) of oil and gas pipelines has increased as a concern due to failures with high consequences. A primary integrity issue to pipelines considers defects originating during field construction but over time may also be subject to external loads and stresses due to earth movement. GW defects in newly built pipelines are also assumed to exist but would be much smaller in size, and more difficult to detect, which motivated the investigation into minimum defect detection levels of the inspection technologies. Research objectives of this paper are to characterize and summarize the applicability of inline inspection (ILI) technology of magnetic flux leakage 4 (MFL4) for inspection of defects related to pipeline GWs. Pull-through test and infield site excavations of operational pipelines have been collected and used for detection, internal/external (int/ext) identification, and sizing quantification. It can be concluded that the MFL4 technique is generally sensitive to lack of fusion or penetration, deep undercut, local thinning with results of both pull test and field data; while not sensitive to closed or very narrow small dimension cracks (narrower than 1 mm, less than 2 mm deep) as was observed in this study.