| The Magnetic Flux Leakage (MFL) non-destructive technique is the most cost-effective way to monitor corrosion on in-service oil and gas pipelines. The method consists of magnetizing the pipe wall and detecting the leakage flux as a defect indication. The MFL signals are used for locating and sizing metal loss defects in the pipe.;The shape and amplitude of the MFL signals do not depend only on defect geometry, but also on tool speed, applied flux density, inspection configuration, magnetic properties, and stress state of the pipe wall. The last two factors cannot be accounted for through calibration measurements. High operating pressures of the pipelines, combined with the local variation of both stress and magnetic properties of the material make the MFL field data difficult to interpret.;The main objective of this project was the study of interacting defects, a term referring to two sufficiently close corrosion pits that the area between them is subjected to stress superposition and magnetic flux shielding. This research analyzed the stress-dependent magnetic flux leakage (MFL) signatures from various single and interacting defects, electrochemically milled on steel samples at no applied stress or at an applied tensile stress of 46% of the material yield strength, and magnetized at moderate and high field amplitudes.;A complementary technique, Magnetic Barkhausen Noise (MBN), capable of concurrently detecting magnetic properties and local stresses on a microscopic scale, was used in interpreting the stress-affected MFL signals. The MBN method was also employed for macroscopic magnetic characterization of the ferromagnetic samples under uniaxial tensile load. Bulk and local MBN experimental results were compared with existing theoretical and numerical models of stress patterns.;The MBN results were used to model the MFL response to stress under low MFL applied field conditions. Under high field MFL conditions, stress effects are small and flux shielding dominates the signal from the interacting defects. An analytical model was developed for the latter case, which relates the normal and tangential MFL components to the defect size and location, respectively. Flux shielding effects were accounted for in the model and good agreement between theoretical and experimental results suggested that this method can be used as an inverse MFL data interpretation technique. |
