(eBook Version)
Need: The development
of stress corrosion cracks (SCC) in buried gas pipelines
has posed a serious threat to pipeline integrity for many
years. It can be reliably detected by magnetic particle
techniques in the field or by laboratory studies using low
frequency eddy currents. It is also possible to find and
measure the depths of the cracks from the ID by careful
scanning with an ultra-sonic angle beam probe but the transducer
must be manipulated by a skilled operator. All of these
approaches are not very satisfactory for in-line in-spections
because they are not suitable for covering the total area
of a pipeline and they are too labor intensive to be automated.
In order to address this problem with new technology, the
PRCI requested proposals for any technique that seemed to
be practical and the Electromagnetic Acoustic Transducer
(EMAT) was suggested as very promising because it has already
demonstrated operation in the environment of the inside
of a gas pipeline. Magnasonics, Inc., of Albuquerque, New
Mexico, was chosen from many respondents to conduct an in-vestigation
of the use of EMATs for overcoming the problems expected
to arise from in-line operation and to incorporate the latest
developments in ultrasonic inspection with EMATs.
Benefit: The
objective of the program described in this report was twofold.
First, to apply the most recent developments in EMAT technology
to the problem of detecting and sizing stress corrosion
cracks (SCC) in operating gas pipelines and second to exploit
the ability of EMATs to generate and detect a wide variety
of ultrasonic waves in the walls of a pipeline under operating
conditions.
Result:
To achieve these objectives, the PRCI provided Magnasonics
with four coupons cut from pipe that contained colonies
of SCC and one complete pipe 24" in diameter and 8"
long containing one isolated colony of cracks near its center
and away from the weld line. Magnasonics assembled a variety
of EMATs capable of inspecting these samples with various
kinds of ultrasonic waves at several frequencies in the
100 kHz to 3 MHz range. Electronic transmitters and preamplifiers
specifically designed for these transducers were assembled
and computer programs were written to extract as much information
as possible from the ultrasonic signals produced by these
EMATs. By the end of the program, a mechanical scanning
device had been constructed to scan an EMAT sensor past
the SCC colony in the 8" pipe while a personal computer
printed out graphs of several properties of the ultrasonic
signals that had interacted with the flaws in the pipe.
