Guided Wave Testing

December 27, 2012

The I.A. University of Ahvaz

Mahdi Bavi Hossein Kharazmipour

Guided waves are ultrasonic waves that propagate along the length of a structure, guided by and confined in the geometric boundaries of the structure

DEFINITION

Conventional UT measures the wall thickness at a spot , while GWT 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 measurement

GWUT Inspection provides detection of both internal and external corrosion

The difference in concept between UT and GWT

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

The differences between conventional UT waves and guided waves

Longitudinal

Torsional

Flexural

The waves used in the GWT

Guided waves 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 ,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.

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

Monitor

Field electronics

Umbilical cable

connecting electronics to transducers

Weld

WeldArea of corrosion

Example of graphical data display

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Example of graphical data display

Distance Amplitude

Correction (DAC) CurvesWeld

Minor Anomaly

Welds at two elbows

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

Some Advantages of Guided Wave Ultrasonic Testing

Complicated evaluation of data by highly trained operators is required because of the complex signals involved

Dimensions of corrosion 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.

The tests identify change 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

Some Limitations of Guided Wave Ultrasonic Testing

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 build up 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

The conditions that can limit the distance of a piping segment that can be reliably inspected

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Test on Buried Pipeline

Pit located on 10” Schedule 80 pipe (0.15 in deep x 4.5 in x 3 in)

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-ground and exposed the corrosion probably would have been noted as a minor anomaly

Example of corrosion that would not have been noted with Guided Wave on a buried piping segment

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.

1. Guided wave offers valuable new inspection technology if it’s capabilities and limitations are kept in mind.

2. It is a SCREENING tool. Need to follow up with other NDT techniques to quantify / evaluate possible defects.

3. If used without other verification, GWT cannot provide the level of detail needed to ascertain the integrity of piping.

4. Main advantage is the ability to screen long sections of pipe to determine overall, general condition and locate areas that require more detailed examination.

Conclusions