Detection of anomalous stresses in gas pipelines by magnetometer survey (invited)

Abstract

Pipelines constructed on unstable terrain (e.g., permafrost), submarine lines, etc. may shift, producing stresses which can lead ultimately to rupture. We have used magnetometer surveys to detect bending stresses in buried pipelines and to monitor new construction. The perturbations to the earth’s field are large and complex; typically fluctuations of 5–10 kilogamma are found 2 m above 1 m diameter buried line. These signals record initial magnetisations, joints, construction history, etc. and include the results of stress induced magnetisation changes. The magnetostrictive effect in steel depends on field, stress, and cyclic history. There is little basic data on the inverse effect, stress induced changes in magnetization. In a pipeline the bending stress distribution is complex so that fringing field changes due to stress magnetisation changes are not calculable. However, experience now enables us to interpret some of the features of our magnetometer surveys. Since stress effects are large we are optimistic that we can eventually develop the technique for surveying pipelines for unusual stress concentrations.