Magnetic Flux Leakage Investigation of Interacting Defects: Competitive Effects of Stress Concentration and Magnetic Shielding

The magnetic flux leakage (MFL) method is used for in-line inspection of oil and gas pipelines. Corrosion-induced MFL signals are sensitive to the level of stress and magnetic flux density existing in the pipe wall. Defects act as stress raisers, locally influencing the magnetization of the material, and subsequently the path of flux flow. Two adjacent corrosion pits can furthermore complicate this situation by the superposition of their stress concentrations and by the mutual shielding of the defects from the applied flux density. This type of defect geometry is termed interacting defects. In this study, various degrees of interaction were considered by modifying the separation and direction of alignment of the two defects. The MFL signals generated by these defect geometries were analyzed as a function of applied stress and magnetic flux density in the bulk material. While the overall MFL signal amplitude dependence on stress was minimized at high flux densities, it was observed that the shape of the MFL profile was dictated by the interacting defect geometry. The absolute MFL amplitude was found to be influenced by the local residual stress developed at the defect edges. Introduction: The MFL method relies on calibration runs for correct interpretation of the leakage signals in terms of defect location, size, and depth. Calibration of the corrosion-induced MFL signals as a function of the test object and inspection tool properties had been extensively studied, as reviewed in [1]. Although the effect of stress on the magnetic properties of the pipe ferromagnetic steel is not clearly understood, the most efficient method of reducing stress influences on the MFL signals is by magnetically saturating the pipe [2], minimizing in this way local variations in the permeability of the material. Oil and gas pipelines are operated at pressures as high as 70% of their yield strength [3]. This line pressure generates circumferential (hoop) stress in the pipe wall. Furthermore, much higher stresses are expected in the vicinity of the corrosion defects since these act as stress raisers [4,5] in the material. Three types of stresses are usually developed in a pipeline [6,7]: (i) bulk stress, occurring in the volume of the specimen as a result of the pressure of the transported fluid, (ii) local stress, generated in the neighbourhood of metal-loss defects, and (iii) residual stress, that develops in the sample after the removal of the applied load. When clustering of the corrosion defects occurs-regions …