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Assessment of the Capabilities of Long-Range Guided-Wave

Ultrasonic Inspections

Houston, Texas February 14, 2012

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Ultrasonic Inspection was developed in the 1950s

“Time of Flight” of an ultrasonic wave is

directly proportional to the thickness of the

material measured

Piezoelectric crystal oscillated

by RF signal

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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

Weld Metal loss Metal loss

FlangeConventional Ultrasonic Test

Weld Metal loss Metal loss

Guided Wave

100% Inspection

Localized Inspection

Conventional ultrasonic inspection provides a local thickness Conventional ultrasonic inspection provides a local thickness measurementmeasurement

GWUT Inspection provides detection of both internal and GWUT Inspection provides detection of both internal and external corrosion typically for 100’ or more down the pipe.external corrosion typically for 100’ or more down the pipe.

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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

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Longitudinal

Torsional

Flexural

Guided Wave Ultrasonics rely on the use and interpretation of far more complex waves than the

compression waves used in conventional UT testing

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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.

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The power and durability of today’s electronics has made it possible to field

the GWUT system in a compact package

Pressurized bladder containing the array of piezoelectric crystals

Laptop computer

Field electronics

Umbilical cable

connecting electronics to transducers

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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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Minor Anomaly

Example of graphical data display

Distance Amplitude Correction (DAC)

CurvesWeld

Welds at two elbows

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Weld

WeldArea of corrosion

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Zoom Shot

Area of corrosion

Welds

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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

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Wave form obtained from uninsulated section of a 10” x 0.594” above-grade pipe

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Largest pit located on 10” Schedule 80 pipe(0.15 in deep x 4.5 in circumferential extent)

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Pit at Location +F12 is 25% wall loss, but only 2% cross-sectional loss

Profile of F12 pit

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Test on Buried Pipeline – loamy, relatively dry soil

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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

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• 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

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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.