Large standoff magnetometry (LSM) as a non-intrusive NDE technology has been used in commercial trial many years for above-ground detection of underground pipeline anomalies associated with stress concentration zones (SCZs). As a passive geo-magnetization flux leakage measurement method, it has been mainly targeting common anomalies such as corrosion/metal loss, gouges/dents and cracks that are often very localized in small scale. Insufficient consistency and reliability are still the major concern due to technical challenges in getting high resolutions and signal strength at large standoff distance. In comparison, geohazard related external forces induce much large-scale elevated stresses/strains with stronger stress-magnetization signals. The lack of economically viable solutions for pipeline screening and monitoring under geohazard conditions provides a good opportunity for LSM technology to build up its first market position.

This work is part of PG&E’s effort in gaining better fundamental understanding of the current state-of-the-art LSM technology and its potential to enhance the current industrial practices of pipeline assessment under geohazard conditions. Specifically, 3D mapping of pipelines including depth of cover (DOC) measurement, locating girth welds and peak stresses/strains with risk rating, verification of stress/strain relief operation and monitoring afterwards. Inline inspection (ILI) and geotechnical analysis data together with field excavation and strain-gage-on-pipeline data are utilized as references to cross-check the LSM results.

All three active LSM technology providers participated in this small-scale field trial that includes a total of 9 field sites subjected to ground movement/landslides or erosion in Northern California. State-of-the-art positioning technologies including total automatic tracking station and real-time GNSS/GPS correction through base station cellular network are tested as well. The outcomes demonstrate LSM technology’s capability of 3D mapping including DOC measurement with reasonable error when high accurate GPS coordinators can be acquired and DOC is 6 feet or less.  Capture of significant peak strains (≥ 0.4%) is demonstrated including those associated with major field bends, though monitoring small amount of change may be still a challenge.  The locating accuracy is largely dependent on the adopted positioning technologies as well as the on-site satellite and/or cellular network receiving condition, which can be a challenge at remote fields.   In comparison to excavation and strain gage data, ILI peak bending strain data have more than expected errors at some of the sites where LSM outcomes show better matching that could have led to better mitigation decision, confirming the value of integrating LSM technology into the current practices.


Large Standoff Magnetometry as a Practical Screening and Monitoring Tool for Pipelines Under Geohazard Condtions

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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