Revealing the hidden threats: How low field MFL is transforming pipeline integrity

by T.D. Williamson, USA

Matt Romney Product Line Director

Mike Kirkwood Director, Integrity Engineering Solutions

Understanding the Challenge: CSCC and Girth Weld Complexity

Circumferential Stress Corrosion Cracking (CSCC) is a form of environmentally assisted cracking that occurs when tensile stress, corrosive environments, and susceptible materials converge. Unlike axial SCC, which forms perpendicular to hoop stress and is more readily detected by conventional axial crack tools, CSCC manifests in off-axis orientations—often triggered by unintended bending, geohazards, or residual construction stresses.

Girth welds are, generally, not a threat to pipeline integrity unless they contain defects. When girth weld defects are present, they might include burn-throughs, misalignments, porosity, and start-stop anomalies and metal loss. These features are difficult to assess with standard MFL tools, which often miss or mischaracterize the anomalies due to their orientation and metallurgical complexity.

The LFM Advantage: Seeing What Others Miss

Figure 1: Example MDS™ Pro tool featuring (technology order left-to-right) SpirALL® MFL (SMFL), axial MFL (MFL), low-field MFL (LFM), deformation (DEF), internal/external discrimination (IDOD), and inertial mapping (XYZ)

MDS™ Pro (Figure 1) is a multi-technology inline inspection platform designed to deliver the most comprehensive assessment of pipeline integrity threats. By simultaneously collecting data from six independent technologies—including axial MFL, SpirALL® MFL (SMFL), deformation (DEF), internal/external discrimination (IDOD), inertial mapping (XYZ), and low-field MFL (LFM)—MDS™ Pro enables operators to detect, classify, and prioritize complex anomalies with unmatched precision. This integrated approach allows for reliable identification of interacting threats such as corrosion coincident with dents, hard spots, and stress corrosion cracking (SCC), which are often underreported or missed by single-technology tools, making MDS™ Pro a cornerstone in modern pipeline integrity management.

Among these technologies, Low-field MFL (LFM) stands out for its unique sensitivity to changes in steel microstructure, permeability, and stress—especially around sharp features. Unlike traditional axial MFL, LFM operates at a lower magnetic energy state, making it particularly effective at identifying circumferential stress corrosion cracking (CSCC), hard spots, and strain-induced anomalies. As industry specifications evolve, LFM is increasingly recognized not just as a detection tool, but as a critical asset for anomaly prioritization and material verification in high-risk pipeline environments.

Case Study: Girth Weld Analysis with MDS™ Pro

During the analysis of a pipeline inspected by MDS™ Pro, the Data Analyst noticed several locations where the LFM channel showed unusually strong indications with minimal corresponding metal loss signal on the standard MFL. These locations also coincided with unintended bends in the route (picked up by the IMU mapping unit), suggesting the possibility of elevated bending stress. Suspecting something lurking, the operator excavated those sites. Sure enough, they discovered colonies of CSCC right where the LFM had indicated. In contrast, a conventional axial MFL-only inspection earlier had not raised an alarm in those locations. Thanks to LFM, the cracks were identified and repaired under controlled conditions – averting a potential in-service failure.

Figure 2: MDS™ Pro data with exaggerated LFM response indicating CSCC

In another case, MDS™ Pro was leveraged to assess an aging girth-weld that had corrosion nearby. The axial MFL data suggested metal loss stopped at the weld boundary, perhaps implying the weld was unaffected. However, the combined SpirALL MFL and LFM data told a different story – LFM indicated a permeability anomaly extending into the weld area. Upon investigation, it became clear that the metal loss continued into the weld zone, potentially impacting the weld integrity. This critical insight, only evident by comparing all available datasets including LFM, prompted the operator to perform a targeted weld repair. By catching the full extent of the defect, LFM helped avoid leaving a weakened weld in service, thereby mitigating a significant integrity threat that might have been overlooked.

Industry Momentum and Future Outlook

The growing body of evidence supporting LFM has led to its inclusion in specification development and industry uptake. Today, LFM is being leveraged for material verification, to enhance mechanical damage, hard spot, CSCC, and girth weld anomaly detection, identification and prioritization.

As the industry continues to refine crack detection and sizing—especially for gas pipelines—LFM stands out as a cornerstone technology. Its ability to detect subtle metallurgical shifts and correlate them with mechanical stress makes it indispensable for modern pipeline integrity management.