practical considerations for users of guided wave ultrasonic testing (51300-07164-sg) (1)

8/11/2019 Practical Considerations for Users of Guided Wave Ultrasonic Testing (51300-07164-SG) (1)

1/6

PRACTICAL CONSIDERATIONS FOR USERS OF GUIDED WAVE ULTRASONIC TESTING

Joseph M. Galbraith and George

C.

Williamson

BP America Production Company

501 WestLake Park Boulevard

Houston. Texas 77079

ABSTRACT

Over the last decade, commercialization of Guided Wave Ultrasonic Testing (also

referred to as Long Range UT) has provided industry with a powerful new technique for

ascertaining the integrity of piping systems. The major advantage of the technique is it s

capability to screen inaccessible piping over long distances from a single exposed location.

The information developed allows the user to identify the locations in a pipe that have suffered

potentially injurious corrosion and gain an understanding of the significance of the damage.

Guided Wave Ultrasonic Testing (GWUT) provides this and other advantages when

compared to more conventional non-destructive testing (NDT) techniques. As with all NDT

techniques, though, GWUT has limitations that must be kept in mind by the user to properly

understand and utilize the results. This paper will discuss these advantages and limitations to

assist the user in utilizing this impottant inspection echnology.

Keywords: Guided wave ultrasonic testing, long-range ultrasonic testing, inspection of

inaccessible pipe, cased pipe inspection, buried pipe inspection, pipe integrity

1

07164

Paper No.

2007 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACEInternational, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper aresolely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

Copyright


2/6

INTRODUCTION

Until the 1950s, detection of internal features in piping could only be accomplished with

radiography. At that time the ultrasonic method of non destructive testing UT) was developed

and is now one of the most widely used methods to detect thickness of piping and define the

extent of corrosion, erosion or other forms of metal wastage. Ultrasonic waves are mechanical

waves, and in conventional testing normally have a frequency range from

1

MHz to

5

MHz.

For measurement of wall thickness the pulse-echo technique utilizing longitudinal

compression) waves is generally employed. These waves are generated by an oscillating

crystal, travel through the material under test at a known velocity, and when reflected off the

far surface travel back through the material and are detected by a receiving probe I

Although this method is widely used and well accepted, there are disadvantages with

compression wave ultrasonic wall thickness testing. A major limitation is only the thickness of

the material directly under the probe is determined. Although computerized automated

ultrasonic scanning systems are now available that are capable of measuring the thickness of

material in many thousands of locations in a given area often referred to as corrosion

mapping), again, the wall thickness is determined only at spots that are directly under the

probe or transducer. Therefore, direct access to one surface of the pipe to be

inspected is

required. Furthermore, the transducer must be coupled to the pipe to eliminate air gaps, and

the contacted surface of the material being tested must therefore be relatively smooth.

These limitations compromise the usefulness of compression wave ultrasonic testing to

determine the condition

o

long lengths of piping, particularly piping that is difficult to access

insulated, in elevated pipe racks or drops down the sides of tall vessels, directly buried or

encased under road crossings, and so forth). This problem created the impetus to develop

other techniques that can provide a more cost effective solution for inspecting long lengths of

piping.

In the 1990s work began at Southwest Research Institute on development of guided

wave inspection utilizing

magnetostrictive sensors, while in the same period work was

conducted by The Welding Institute that focused on generating guided waves using arrays of

piezoelectric crystals. Both techniques are now commercially available, and provide the

capability

to inspect long lengths of piping from a single access point, and detect and locate

both internal and external corrosion damage.

DISCUSSION

Due to the fact that guided waves travel through the bulk of the material, GWUT testing

of pipe can locate both internal and external corrosion damage. The results provide accurate

information regarding the location of any damage detected, and can provide a relative rank of

severity based on the strength and characteristics of the returned signal. Long distances of

pipe can

be

inspected from a single sensor location, which reduces the amount of pipe that

needs to be accessed. This can significantly reduce the cost of inspecting long lengths of pipe

that are elevated, buried, insulated or difficult to inspect with other non-destructive testing

techniques. Furthermore, this technique can be used on in-service piping without disrupting

the transported product.

2


3/6

Guided wave ultrasonic testing can be used to rapidly establish the condition of piping

during a single survey. However, an even more powerful technique is use of the system for

monitoring of changes in the pipe. This is accomplished by permanently establishing the

sensor location and preferably leaving the sensor in-place.

Sequential inspections with the

same equipment settings and sensor position allow the analyst to subtract the most recent

dataset from the older dataset(s), thereby establishing whether active corrosion or erosion is

occurring. The detection of changes over time can be much more sensitive than the

identification of metal loss in a single survey.

Guided wave ultrasonic testing utilizes mechanical waves; however, a much lower

frequency typically ranging from 30 to 75 KHz is used than in conventional ultrasonic testing.

These waves travel within the bounded surface of the test piece parallel to its surface, thus,

they are referred to as guided waves. The longer wave lengths result in much more complex

signals than are typical in compression wave ultrasonic testing. In pipe the guided waves

exist in three modes: longitudinal, torsional and flexural. More than one mode can exist in a

given pipe at a single frequency. These factors combine to create a response that is much

more difficult to analyze than are conventional UT results. Sophisticated computer-based

signal processing has provided a solution to this difficulty. However, to develop reliable

results operators require extensive training and experience in the setup and operation of the

equipment and the analysis of the results produced.

These complex signals travel through the full cross-sectional area of a pipe, and are

scattered by changes in the cross-sectional area. The location of the scattered signal is known

with a sub-foot accuracy, but the profile and depth of corrosion cannot be directly determined

from the results. This has a major implication relative to the use of the results. Since the

methods used to evaluate the remaining strength of corroded pipelines such as

SME

B31G

require that both the maximum pit depth and the longitudinal length of corrosion

be

determined, guided wave ultrasonic tests do not provide the data needed to directly evaluate

the effect of an area of corrosion on the integrity of the damaged pipe. Thus, guided wave

inspection is a screening tool that will identify the location of corrosion, but further inspection

with other non-destructive testing techniques is required to provide the information necessary

to determine the integrity of the pipe.

The providers of guided wave inspection generally use ten percent change in cross-

sectional area as the limit of detectability that will be identified as moderate corrosion in field

tests while stating that changes of as little as two percent change in cross-sectional area can

be detected in ideal situations. By using the monitoring technique, changes as small as one-

half percent of the cross-section can be seen.

In a single survey, though, a two percent

change in cross-sectional area can actually be a complete perforation if the corrosion occurs

as shallow pitting. Using an example of a 34 inch (86.4 cm) standard walled pipe which has a

wall thickness of 0.375 inch (9.52 mm) and a cross-sectional area of 39.6 square inches (255.5

sq cm), one can see by reviewing Table 1 hat corrosion that has a profile with a pit diameter to

depth ratio of 6, which is actually a shallow pit, can be a complete perforation of the pipe and

still be below the ideal detection limit of guided wave inspection.

Using the ten percent cross-

sectional change, a wall loss of sixty-five percent with a diameter to depth ratio of 100, which is

general corrosion, is just at the detection limit.

Certainly, corrosion that jeopardizes the

integrity of a piping system can escape detection by a guided wave ultrasonic test.

3


4/6

Furthermore, there is no absolute method to calibrate the data. The signals are

calibrated using Dynamic Attenuation Curves DAC) with the primary curve established based

on the symmetric responses from girth welds. Inconsistent weld profiles caused by lack of

penetration, improper alignment, high-low, and use of backing bars or a change in the root or

cap profile are not uncommon in pipelines. This can create errors in the assumed DAC shape.

as there is no way to adjust for these irregular reflectors without detailed knowledge of the girth

weld profiles up and down the pipe. In addition, the shape of the feature that scatters the

waves has a strong influence on the amount of energy that is reflected back and received by

the sensors. Also, the combination of two or more areas of damage in close proximity can

alter the response by constructive or destructive interference. All of these factors compromise

the ability to precisely size the reflectors and determine a specific percentage of cross-

sectional or volumetric loss from GWUT data.

It is possible to inspect hundreds of feet of piping from one sensor location. This is

commonly accomplished in situations where neither a dense product nor the environment in

contact with the outer surface of the pipe causes attenuation of the mechanical waves.

However, many conditions exist that do limit the distance that can be reliably interrogated and

can create artifacts in the data. These include:

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

In such cases, the amount of pipe inspected can be reduced to only tens of feet, and due to

the reduced inspection distance and r the creation of data artifacts the usefulness of guided

wave ultrasonic testing can be compromised.

Finally, interpretable results are not produced for some distance on either side of

sensor, creating a dead zone of several feet centered on the sensor. In addition, since the

scattering occurs at changes in the cross-section of the pipe or changes in the acoustic

impedance that exist at cracks and so forth, corrosion that exists as grooves that pass under

the sensor location or that results in a gradual variation can exist without being detected.

4


5/6

CONCLUSIONS

Guided Wave Ultrasonic Testing provides a powerful new tool to locate and estimate

the severity of corrosion in piping systems that are difficult to access.

With only a single

access point for the sensor, hundreds of feet of pipe in both directions can be inspected, even

around bends. However, due to a number of factors the results can be difficult to interpret and

significant damage can be overlooked. These facton include:

1.

Complicated evaluation of data is required due to signal complexities

2 Dimensions of corrosion wall loss, longitudinal length, profile) cannot be directly

determined

3. Significant corrosion can be missed

4. The reflected signal cannot be equated to a specific area or volume of loss due to a lack

of an absolute calibration standard

5. Many field conditions exist that limit the distances that can be effectively inspected and

that cause artifacts with can complicate analysis

Guided wave ultrasonics is an important screening tool that provides the capability of

quickly surveying long lengths of piping. It is particularly useful to screen pipe that is difficult to

access, locating both internal and external corrosion that can be evaluated further with other

more conventional

N T

techniques. By keeping its capabilities and limitations in mind the

results can be effectively utilized to help assure the integrity of piping systems.

ACKNOWLEDGEMENTS

Many knowledgeable individuals and experts have provided guidance to us in this

subject matter. In particular we would like to acknowledge Mr. Joe Brophy of B

E

Limited,

who has graciously provided a great deal of valuable knowledge and experience that he has

gained in developing and fielding the services he provides. We would also like to thank Mr.

Richard Cook and Mr. Kelly Smith of Petrochem Inspection Services, and Mr. Grady Ferguson

of ImPro for the information they have shared.

TABLE

.

- . .

.

. ~

....

~ ~~

~

~ e p t h bh

by

GWT @wed

nt

parahe~ic

tiap

with varyily ph

d t p ~ ~ ~

m

diamercq

5


6/6

REFERENCES

1. ASM Committee on Ultrasonic Testing, ASM Metals Handbook, Volume 11

Nondestructive Inspection and Quality Control, 8Ih ~d ition, ages 161-198, American

Society for Metals,~etals ark, 0hid 44073

2.

J.

Galbraith, J. McMillan, Assessment of Piping Integrity by Automated Ultrasonics ,

Proceedings of the First International Symposium on Process Industry Piping, 1993

3

H. Kwun, S. Kim, G. Light, Long-Range Guided Wave Inspection of Structures Using

the Magnetostrictive Sensor , Southwest Research Institute, San Antonio, Texas 78238

USA

4 P. Mudge,J. Harrison. TELETEST Guided Wave Technology case histories ,

Nondestructive Testing, I iddle East Conference and Exhibition, Bahrain. 2001

5

Supplement to

SME

Code for Pressure Piping, Manual for Determining the

Remaining Strength of Corroded Pipelines. ASME B31G, The American Society o

Mechanical Engineers, New York, NY 10017

6

practical considerations for users of guided wave ultrasonic testing (51300-07164-sg) (1)

Documents