| ABSTRACT: | USING THE MAGNETIC FLUX LEAKAGE TECHINIQUE TO DETECT MECHANICAL
DAMAGE IN PIPELINES
L. Clapham, V. Babbar, J. Byrne, A. Rubinshteyn
Department of Physics, Queen's University, Kingston, Ontario, Canada
Magnetic flux Leakage (MFL) inspection tools represent the most cost-effective solution for detecting
corrosion defects in operating gas and oil transmission pipelines. MFL tools also have the potential to locate
and characterize mechanical damage, however, signal interpretation is problematic because:
1) the MFL signal will be a superposition of geometrical and stress
effects,
2) the stress distribution around a mechanically damaged region is very
complex, consisting of plastic deformation and residual (elastic) stresses,
3) the effect of stress on magnetic behaviour is not well understood.
This paper summarizes recent results of experimental and modeling studies of MFL signals resulting
from mechanical damage. In experimental studies, mechanical damage was simulated using a tool and die
press to produce dents of varying depths in plate samples. MFL measurements were made before and after
selective stress-relieving heat treatments. These annealing treatments enabled the stress and geometry
components of the MFL signal to be separated. In general, geometry effects scale with dent depth and tend
to dominate in deep dents, while stress contribution to the MFL signals is relatively constant and is most
significant for shallow dents. The influence of other parameters such as flux density and topside/bottomside
inspection was also quantified. In the finite element analysis work, stress was incorporated by modifying
the magnetic permeability in the residual stress regions of the modeled dent. Both stress and geometry
contributions to the MFL signal were examined separately. Despite using a number of simplifying
assumptions, the modeled results matched the experimental results very closely, and were used to aid in
interpretation of the MFL signals.
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