In-Line In-Situ Direct Measurement of Absolute Biaxial Stress in Pipelines Available to Purchase

This paper reports on recent work on the in-situ determination of absolute levels of biaxial stress in ferromagnetic pipeline materials. The response of pipe-wall materials to an applied alternating magnetic field is measured, and knowledge of the variation of magnetic properties of the material used to determine stress values. A wide variety of means are in use to detect, characterize and measure an equally wide range of defects in pipelines. These may arise from pre-existing conditions, errors in construction, effects of corrosion, accidental damage or other causes. Caliper tools, mapping tools, MFL, EMAT and ultrasonic tools are all used to detect conditions that may result in unacceptable stress in a pipeline. The ultimate aim, for all of these detection techniques, is to prove that the inspected pipeline’s fitness for purpose is maintained; and where this is not demonstrated, to provide accurate and reliable information on which to base a rehabilitation program. Recently attention has been raised by a number of groups working on the development of techniques intended to utilize the dependence of the magnetic response of ferrous materials to applied stress in order to directly measure stress. Most of these techniques have been based on the Barkhausen effect. This paper discusses the use of an alternative technique based on other magnetic properties that have been shown to allow derivation of a quantifiable relation between the level of stress present in material and the magnetic response. This technique, named MAPS by its developers, ESR Technology, has been employed with considerable success out-with the pipeline industry. Recent work has concentrated on applications in dynamic situations, in particular in pig-mounted inspection systems. A series of tests are described with dynamic measurements made inside pipes, covering measurement of applied (bending) stresses and artificially simulated defects. Measurements are described at velocities up to 4 m/s in prototype systems at 24” diameter with stand-offs of 2mm and up. Magneto-dynamic effects from the motion of the sensors past the metal of the pipe-wall have been characterized and variations in signal due to the velocity and instrument stand-off are described. It is considered that this work forms the basis for a pipeline inspection technique using these instruments.