Factors Affecting Stress Concentrations Near Pit Defects, as Monitored by Magnetic Flux Leakage, Magnetic Barkhausen Noise, and Neutron Diffraction

摘要

Oil and gas pipelines are pressure vessels with steel walls operating at up to 70% of their yield strength. They need to be inspected rigorously to avoid failure and for environmental safety reasons. Magnetic flux leakage (MFL) is the most cost effective technique for in-service corrosion inspection of buried gas pipelines [1]. In this method, the pipe wall is magnetized to near-saturation using permanent magnets. If the wall thickness is reduced by a defect, more magnetic flux leaks from the wall into the air inside and outside the pipe. This ‘leakage flux’ can be detected by a Hall probe or an induction coil [1]. The circumferential (hoop) stress generated in the pipe wall by line pressure alters the MFL signal and needs to be accounted for when sizing the defect [1]. Defects also change the local stress distribution, creating stress concentrations which may lead to plastic deformation at the defect edge. As a result, the study of stress concentration around pits of different depths and made under different external conditions is important in estimating the size of the defect. This stress concentration around a defect can be measured directly by neutron diffraction [2] and photo-elasticity measurements [3]. We also measure stress concentrations indirectly using the magnetic Barkhausen noise (MBN) technique [4].