Need: Longitudinal defects, including stress-corrosion
cracking (SCC) and selective seam weld corrosion, can occur
in natural gas transmission pipelines. Previously developed
nondestructive testing techniques (including angle beam
ultrasonics, electromagnetic acoustic transducers (EMATs),
and remote field eddy current) are capable of detecting
longitudinal defects. Two angle beam ultrasonic syatems
are commercially available, and an EMAT system is being
commercialized. But there are factors that limit the applicability
of these techniques, such as minimum pipe diameter and maximum
inspection speed. A new more cost effective nondeastructive
testing technique for pipeline inspection is needed to identify
and evaluate the potential hazards from SCC.
Benefits: The complexity of angle beam ultrasonics
and EMAT's and others makes the cost of inspection higher
than MFL corrosion inspection. Circumferential MFL is more
economical has the potential to to more accurately size
axially oriented corrosion than the more common axial implementation.
Though at present the method contained within this study
has some deficiencies, a circumferential MFL in-line inspection
tool could serve as a screening tool to determine whether
a pipeline has cracks and seam weld defects as well as other
longitudinal defects such as third-party mechanical damage.
Result: Circumferential magnetic flux leakage
(MFL), a new implementation of a commonly used technology,
was investigated as an alternative to other nondestructive
testing techniques and hydrostatic proof testing. The results
of this study show that both cracks and seam weld defects
can be detected with circumferential MFL. However, many
small cracks with lengths less than 1 inch and depth of
less than 20 percent of the wall thickness were not detectable.
Since the initial publication of this work in 1996, two
implementations of circumferential MFL have been used to
inspect pipelines for axially oriented corrosion and stress
corrosion cracking. Both implementations were considered
successful since they produced new information on defects
that could affect the servicability of the pipeline.
