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02079 STRESS EFFECTS ON MFL SIGNALS

Picture for 02079 STRESS EFFECTS ON MFL SIGNALS
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Product Number: 51300-02079-SG
ISBN: 02079 2002 CP
Author: Lynann Clapham and David Atherton
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Magnetic Flux Leakage (MFL) inspection tools are the most cost effective way to monitor corrosion on in-service oil and gas transmission lines. The MFL signal is used to derive defect depth and extent, from which calculations of Maximum Allowable Operating Pressure (MAOP) are made. Unfortunately MFL signals depend not only on defect geometry but also tool speed, tool configuration, pipe wall magnetic properties and stress. The combination of these makes accurate depth predictions difficult. Of these various factors, stress is the most complex and the least understood. Pipe wall operating stresses may exceed 70% of the yield strength, but much higher local stress levels are present around defects because of stress concentrations. Understanding how these stresses affect MFL signals is crucial to accurate defect depth predictions. We have conducted a number of experimental studies that investigate the effects of bulk, local and residual stresses on magnetic behaviour and MFL signals. MFL and Magnetic Barkhausen Noise (MBN) techniques were used to examine and characterize the magnetic behaviour samples in response to stress. In addition to experimental studies, we have conducted finite element analysis (FEA) to model the MFL signals from typical defects. Stress alters the magnetic behaviour of the pipe wall, making it anisotropic and also causing it to vary with position in the defect vicinity. The FEA models therefore are extremely complex, involving 3D modelling and incorporating anisotropic permeability that is spatially non-uniform. The results of experimental and FEA work will be presented and their relevance to MFL inspection discussed.
Magnetic Flux Leakage (MFL) inspection tools are the most cost effective way to monitor corrosion on in-service oil and gas transmission lines. The MFL signal is used to derive defect depth and extent, from which calculations of Maximum Allowable Operating Pressure (MAOP) are made. Unfortunately MFL signals depend not only on defect geometry but also tool speed, tool configuration, pipe wall magnetic properties and stress. The combination of these makes accurate depth predictions difficult. Of these various factors, stress is the most complex and the least understood. Pipe wall operating stresses may exceed 70% of the yield strength, but much higher local stress levels are present around defects because of stress concentrations. Understanding how these stresses affect MFL signals is crucial to accurate defect depth predictions. We have conducted a number of experimental studies that investigate the effects of bulk, local and residual stresses on magnetic behaviour and MFL signals. MFL and Magnetic Barkhausen Noise (MBN) techniques were used to examine and characterize the magnetic behaviour samples in response to stress. In addition to experimental studies, we have conducted finite element analysis (FEA) to model the MFL signals from typical defects. Stress alters the magnetic behaviour of the pipe wall, making it anisotropic and also causing it to vary with position in the defect vicinity. The FEA models therefore are extremely complex, involving 3D modelling and incorporating anisotropic permeability that is spatially non-uniform. The results of experimental and FEA work will be presented and their relevance to MFL inspection discussed.
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