Assessment of the Capabilities
of Long-Range Guided-Wave
Ultrasonic Inspections
Houston, Texas
February 14, 2012
1
Ultrasonic Inspection was
developed in the 1950s
Piezoelectric
crystal oscillated
by RF signal
“Time of Flight” of an
ultrasonic wave is
directly proportional
to the thickness of the
material measured
2
Conventional UT measures the wall thickness at a
spot, while Guided Wave Ultrasonics can identify
locations of metal loss along a length of the pipe
Flange
Conventional
Ultrasonic
Test
Localized
Inspection
Weld
Metal loss
Metal loss
Conventional ultrasonic inspection provides a local thickness measurement
Guided Wave
100%
Inspection
Weld
Metal loss
Metal loss
GWUT Inspection provides detection of both internal and
external corrosion typically for 100’ or more down the pipe.
3
Some of the differences between conventional
ultrasonic waves and guided waves are:
•
Guided waves are bulk waves; therefore the entire volume of the pipe is inspected
•
Frequencies used in guided wave inspection are much lower than conventional
ultrasonic testing; therefore the wave lengths are much longer and are scattered
instead of reflected from changes in the dimension of the wave guide
•
The pipe acts as a wave guide, permitting the waves to travel long distances
•
The waves can be introduced at a single location:
– When introduced with piezoelectric crystals an array of transducers are used.
– Coils of wire are used to create vibrations in the pipe via the magnetostrictive
effect exhibited by ferromagnetic materials
4
Guided Wave Ultrasonics rely on the use and
interpretation of far more complex waves than the
compression waves used in conventional UT testing
Longitudinal
Torsional
Flexural
5
Guided waves, typically between 30 – 75 KHz, are
introduced into the pipe by one of two systems:
•
An array of piezoelectric crystals are
positioned in modules that typically hold two
transducers each. The modules are spaced
around the pipe under an air bladder which
when pressurized forces the units against the
surface. The individual crystals oscillate at
the frequency at which they are excited and
transmit the wave into the pipe.
•
Coils of insulated wire are wrapped around
the pipe. An alternating current is passed
through the coils, and an oscillating magnetic
field is produced. Due to the
magnetostrictive effect of ferromagnetic
materials, this produces a wave in the pipe
which can be amplified by using a nickel or
cobalt strip bonded to the pipe under the coil.
6
The power and durability of today’s
electronics has made it possible to field the
GWUT system in a compact package
Laptop
computer
Umbilical
cable
connecting
electronics to
transducers
Pressurized bladder
containing the array of
piezoelectric crystals
Field
electronics
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Some Advantages of Guided Wave Ultrasonic Testing
• Can test long distances of pipe from a single access point
• Has developed into an effective screening tool useful in locating and
ranking areas of corrosion; thereby minimizing the amount of follow-up
inspection needed to determine the integrity of piping.
• Can be used on in-service pipelines
• Both internal and external corrosion can be identified
• Current commercial systems are packaged in a small number of durable
components. The systems are easily transported and quickly setup in
the field with preliminary results available at the time of the test
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Example of graphical data display
Distance Amplitude
Correction (DAC)
Curves
Weld
Welds at two
elbows
Minor
Anomaly
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Weld
Area of
corrosion
Weld
10
Zoom Shot
Welds
Area of
corrosion
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Some Limitations of Guided Wave Ultrasonic Testing
• Complicated evaluation of data by highly trained operators is required
because of the complex signals involved
• Dimensions of corrosion (wall loss, longitudinal length, profile) cannot be
directly determined
• Significant corrosion can be missed, especially localized damage
• The scattered signal cannot be directly equated to a specific area or
volume of loss due to a lack of an absolute calibration standard
• Many field conditions exist that limit the distances that can be effectively
inspected and that cause artifacts which can complicate analysis.
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Examples of conditions that can limit the distance
of a piping segment that can be reliably inspected
• various coating such as coal tar epoxies, asphalt-tar wraps,
concrete, etc,
• plastic sleeves, particularly those with internal mastics
• wet insulation, particularly if ice is present
• rough internal or external surfaces
• direct buried pipe, particularly in situations where heavy or wet soil
is encountered
• dense product, internal buildup of solids, and situations with variable
product flow
• system noise created by factors such as turbulent product flow or
pumps
• temperature variations and gradients that can lead to changes in the
wave velocity
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Considerations regarding the type of corrosion that
can be reliably located with Guided Wave Ultrasonics
• Sensitivity is stated to be positive detection of features with a 10% change
of cross-sectional area, with a potential of locating changes of as low as
2% of the cross-sectional area in ideal situations.
• The tests identify CHANGES in cross-sectional area, and can miss
corrosion that is general in nature, is in the configuration of grooves that
pass under the array, or are too small to detect
• A very powerful application of guided wave inspection is using the system
with permanently mounted transducers or excitation coils. In this mode,
repetitive tests are conducted on some frequency (say every 6 months as
an example), and the wave forms compared. Using this technique, the
resolution can improve by an order of magnitude, located changes of as
little as 0.2% to 0.5% of the cross-sectional area.
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Example of resolution of guided wave inspection
relative to the profile of the corroded area for an
ideal situation
15
Wave form obtained from uninsulated section
of a 10” x 0.594” above-grade pipe
16
Largest pit located on 10” Schedule 80 pipe
(0.15 in deep x 4.5 in circumferential extent)
17
Pit at Location +F12 is 25% wall loss, but
only 2% cross-sectional loss
Profile of F12 pit
18
Test on Buried Pipeline – loamy, relatively dry soil
19
Example of corrosion that would not have been noted
with Guided Wave on a buried piping segment
•
This is a photograph of the
corroded area which caused
the leak in a buried 6” line.
•
Along the line drawn, the
cross-sectional area of the
½” walled pipe is
approximately 9.62 square
inches, while the area lost
to corrosion through the
hole is 0.5 square inches.
•
This is a loss of
approximately 5.2% of the
cross-section. It would not
been seen in a scan since
the section was buried.
However, if this line was
above-grade and exposed
the corrosion probably
would have been noted as a
minor anomaly
20
•
A tethered ILI tool run in this 6 inch pipeline located isolated, deep pits separated by thousands of feet
of undamaged pipe. The pit above was 65% of the wall thickness in depth and ½ inch in diameter
21
Weld profiles are assumed to be uniform along the length of the tested
segment, and represent some arbitrary percent change in cross-sectional
area, typically 25% CSC. There is no absolute calibration standard.
This can compromise the accuracy of the results and can even lead to
miss-calls, as in the case below. The high-low condition extended around
approximately one-forth of the circumference, created an asymmetrical
response, and was therefore ranked as a moderate anomaly.
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Conclusions
• Guided wave offers valuable new inspection technology if it’s capabilities
and limitations are kept in mind.
• It is a SCREENING tool. Need to follow up with other NDT techniques to
quantify / evaluate possible defects.
– MAOP calculations per codes require much more detailed knowledge of
corrosion than can be provided by Guided Wave testing
– Significant damage can be overlooked
• If used without other verification, GWT cannot provide the level of detail
needed to ascertain the integrity of piping.
• Main advantage is the ability to screen long sections of pipe to determine
overall, general condition and locate areas that require more detailed
examination.
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