Understanding Quantitative Performance of Large Standoff Magnetometry in Detecting Live Gas Pipeline Anomalies With Stress Estimation

Large standoff magnetometry (LSM) is an emerging non-intrusive, above-ground, passive geo-magnetization flux leakage measurement technology to detect pipeline features or anomalies associated with elevated stresses. Although many promising field trial results have been reported in the past, its overall performance still has not reached sufficient consistency and reliability. This paper presents PG&E’s effort in gaining some fundamental understanding of the current LSM technology and its qualitative & quantitative performance. Specifically location accuracy of girth weld, casing end, dent and landslide damage is analyzed with references to inline inspection (ILI) and excavation data. In addition, basic physics of LSM stress quantification is examined using references of a full-scale finite element stress analysis on selected plain dents. The outcomes indicate advanced global navigation satellite system (GNSS) tool plus capability of identifying girth weld are important to achieve good anomaly location accuracy especially as LSM tends to report more indications than other inspection technologies in current practice. The LSM stress estimation and its comparison to pipe’s specified minimum yield strength (SMYS) may be only good quantitatively within magneto-elastic regime where localized stress concentration zones (SCZs) are under elastic stress loading only and without presence of residual plastic stress.