PR-652-203801-R05 The Lab study Effect of Earths Field and Line Pressure on Magnetization of Pipeline

Large standoff magnetometry is an emerging non-destructive magnetic test method, which is remote, passive, and non-contact. It is based on the inverse magnetostrictive effect and, therefore, has potential for detection of magnetic anomalies produced by elevated stresses in steel pipelines and ferromagnetic structures. Changes in the local pipeline stress state may arise due to corrosion, cracking, mechanical damage, ground movement (geohazards), or external loading. Laboratory measurements were performed on macroscopic steel samples in the elastic stress regime to investigate the potential for stress detection in steel. The effects of the pipe samples’ orientation when degaussed, the generation of a reproducible magnetization state, rotation relative to Earth’s magnetic north and then pressurization were also investigated. Three of the seven pipe samples were pressurized with water at 21 MPa (3000 psi), and the effects on the magnetic flux density was measured on the samples’ surface in different orientations in Earth’s magnetic field. Four-point bending and pressurization was applied to two pipe samples. The results indicate that Earth’s magnetic field and its relative orientation with respect to the pipe has a measurable effect on the magnetic state of a pipe, and the resulting magnitude of changes in measured flux density when stress is applied. Changing the orientation of a pipe after degaussing and then applying pressure will also cause more significant changes in the measured magnetic flux density. Pressurization is shown to be the cause of larger changes in magnetization when compared to the effects of bending. The implications for detecting potentially damaging elastic stresses on pipelines using above-ground inspection, large standoff magnetometry, are examined.