(eBook
version)
Need:
Electric resistance welded (ERW) pipe has been used in the
natural gas transmission industry for many years. The Department
of Transportation (DOT) has recently expressed interest
in the integrity of the weld seam in pipelines made from
ERW pipe that was manufactured prior to 1970. Specifically,
the DOT has requested that natural gas transmission and
hazardous liquid pipeline operators determine whether or
not their pipelines that meet this description require hydrostatic
proof testing. The initial concern from the DOT was for
seam weld selective corrosion, although reference has since
been made to growth of manufacturing discontinuities in
the ERW seam. Early ERW pipe was manufactured using either
direct current or low-frequency alternating current, processes
that were prone to producing incomplete fusion discontinuities.
These discontinuities (also referred to as cold welds, penetrators,
etc.), if present in a pipeline, can grow under normal service
or under upset operating conditions resulting in leaks or
ruptures. There exists a need for a method that a pipeline
operator could use to demonstrate the integrity of pipelines
that were made from this older ERW pipe other than hydrostatic
testing, which is expensive and potentially harmful to pipeline
integrity. The use of a nondestructive-examination (NDE)
technique would enable an operator to sample the integrity
of a suspect pipeline during other routine in-service maintenance
operations without the need for hydrostatic testing.
Result:
The detection of incomplete fusion discontinuities is difficult
if not impossible with conventional NDE techniques. The
intimate mechanical contact between compressed surfaces
causes a small amount of reflection and a large amount of
transmission of ultrasonic signals. Recently however, several
ultrasonic techniques have been introduced to study these
types of discontinuities in resistance spot welds and inertia
friction welds. These new techniques are based on ultrasonic
spectroscopy, that is, the frequency dependence of the various
ultrasonic interfacial parameters. These parameters include
reflection and transmission coefficients and the frequency
dependence of ultrasonic volumetric parameters (e.g., velocity
and attenuation).
Benefit:
This report describes the adaptation of these techniques
to incomplete fusion discontinuities in the seam of ERW
pipe.
