Monitoring the integrity of buried ageing ferromagnetic pipelines is a significant problem for infrastructure operators. Typically inspection relies on pig surveys, Direct Current Voltage Gradient (DCVG), Close Interval Potential Survey (CIPS) and contact Non-destructive testing (NDT) methods that often require pipes to be excavated and exposed at great expense. This paper outlines recent developments in a novel remote sensing technique to detect corrosion, metal defects and the effects of ground movement on pipeline stress by mapping variations in the earth’s magnetic field around pipelines. Magnetostriction and its inverse, the Villari effect, is the process by which randomly oriented magnetic domains develop inside the structure of ferromagnetic materials, such as carbon steel alloys, in response to localised stress resulting in detectable magnetic anomalies in the background magnetic field. Corrosion, metallurgical defects and ground movements result in areas of increased localised stress in pressurised pipelines, which in turn generates a localised magnetic field. Measurement of the remote magnetic field around a pipeline due to magnetostriction allows the measurement of stress and determines the location of defects in the pipe wall. The paper first summarizes magnetostriction in ferromagnetic materials and then describes how measurements of remote magnetic field can be applied to define the location of defects in operational pipelines along with the benefits and limitations of using this technique. These include considerable cost savings since no modification to the line is required, no input of energy or change to its operational parameters is needed and, crucially, no excavations are needed to calibrate preliminary results.
The Company set out to develop an inspection method that does not touch the pipe, does not need to input any energy into it and does not require subsequent excavations or conventional NDT applications to guarantee accurate results. Named Stress Concentration Tomography (SCT) the technique searches for and detects localized stress concentration zones (SCZs) by detecting and interpreting from above the ground the magnetic characteristics of the pipeline buried beneath the ground. The industry now has a generic term for this technique, which is known as Large Standoff Magnetometry (LSM).