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Assessment of the Capabilitiesof Long-Range Guided-Wave

Ultrasonic Inspections

Houston, Texas February 14, 2012

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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

WeldMetal loss Metal loss

FlangeConventional

Ultrasonic

Test

Weld Metal loss Metal loss

Guided Wave

100%

Inspection

LocalizedInspection

Convent ional ult rasonic inspect ion prov ides a local th ickness measurement

GWUT Inspect ion p rovides d etect ion of b oth in ternal and

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 3075 KHz, are

introduced into the pipe by one of two systems:

An array of piezoelectric crystals arepositioned 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 andtransmit 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 todays

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

Umbilicalcable

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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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 requiredbecause 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 orpumps

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 misscorrosion 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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Pit at Location +F12 is 25% wall loss, but

only 2% cross-sectional loss

Profile of F12 pit

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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

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 sincethe 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 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 tomiss-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 its 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 ofcorrosion 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.