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Back to Basics [ click here for the Back to Basics Archive ]
Alternating
Current Field Measurement:
Getting New Technologies Accepted by Old Industries
by Alan Raine*
and Bob Cameron+
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I am
amazed! I have been quoted! The ACFM technique (read the title
of the article if that is not a familiar acronym for you) looks
like an improvement over magnetic particle testing in some
important applications. You be the judge for the questions and
answers raised in this article.
Frank Iddings
Tutorial Projects Editor
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Figures
1-3
Figures 4-5
Introduction
In the
1996 April issue of Materials Evaluation, Frank Iddings wrote
an introductory paragraph for a paper, which said,
"In 1983... an article finished with the
following assessment of MT:' ...there remains a demand for further
improvement in equipment and materials.' This month, 'Back to
Basics' provides some of the basics for improved electromagnetic
testing in and out of the ocean." The article was about the use of
the alternating current field measurement (ACFM) technique and its
ability to detect and size for length and depth surface breaking
cracks on offshore oil and gas installations. The article described
the ACFM technique and its application to the testing of subsea
structures as well as topside pressure systems. It described how the
systems had been tested using a large library of fatigue cracks and
that probability of detection (POD) curves had been produced which
gave better values than other electromagnetic techniques and as good a
value as magnetic particle testing with a lower level of false calls.
In the last five years, improvements have been made in the technology
and its applications have been extended into other fields.
Before
a technician uses the ACFM technique, they should understand how
the techniques works.
Use of Alternating Current Field Measurement
The ACFM technique had been used primarily for the
detection of fatigue cracks in offshore structures, both on the subsea and
topside structural sections. The testing applications were then extended to the
testing of pressurized systems and process plants (for example, pressure vessels
and pipe work). Testing trials have been carried out to prove the detection and
sizing capabilities of the technique compared to the results obtained using more
conventional techniques such as magnetic particle testing, eddy current and
ultrasonic creeping wave. The technique was then used on the testing of
nonmagnetic materials, such as stainless steel and titanium, and proved equally
successful. Since then, ACFM has played a useful role in testing for nonfatigue
cracks at normal and elevated temperatures. Procedures have been produced so
that transverse discontinuities and those at an inclined angle to the toe can be
differentiated from longitudinal discontinuities and cracks detected on the
inner surface from the outer surface can be easily identified (only on certain
materials and within a specific thickness range).
Because the technique has proven so versatile, ACFM is now
being used on a wide number of applications. The technique has been used to test
the steel structures of roller coasters and the people carrying components, with
great cost benefits to the theme park owners. The ACFM technique has also been
used in the testing of bridges with the use of abseilling techniques to test the
more difficult access areas as well as through coatings using conventional
inspectors. The combination of rope access (abseilling - without scaffolding)
and the ACFM technique has also been adapted to the testing of dockside and
other large cranes - once again with great economic benefits (Figure
1). An
example of this is a major crane that can be tested at its most critical areas
in 6 h (Raine and Laenen, 2001).
The oil and gas industry has not been ignored and the
technique is still being used in petrochemical applications. In one recent
application, $500,000 was saved by changing from traditional testing to ACFM.
More examples were given in a recent paper at the ASNT ICPIIT topical in June
2001 (Raine, 2001). The ACFM technique is taking its place in the testing world
as a technique which can reliably detect and size surface discontinuities with
additional economic and time saving benefits.
In service testing can include many different areas of
activity, including the offshore industry, public safety, petrochemical, civil
and mechanical engineering. All of these sectors have their own problems in the
forms of access, such as different forms of coating - from the protective to
the cosmetic - and the type of materials which require testing. In the majority
of cases, fatigue type cracks need to be detected. These are common in the
offshore industry as well as in the civil, mechanical engineering and public
safety sectors. In the petrochemical industry, environmental cracks, such as
stress corrosion cracking, hydrogen sulfide cracking and stress orientated
hydrogen cracks, are required to be detected.
The ACFM technique was originally developed for testing of
carbon steel welds on subsea structures, which were usually nodal welds. A
number of probes were developed such as a general purpose weld testing probe, a
30 degree angle probe for examining tight angle geometries and a pencil probe
specially designed to examine welds that had been subjected to grinding. This
was used to test the bottom of the ground toe of the weld to determine if
discontinuities were present and then determine their length and depth or to
confirm that the discontinuity had been removed.
During a trial organized by University College London,
where samples were produced to reproduce some of the difficult geometries and
access problems located in process plants, it was found that additional probes
were required to gain access and detect and size the discontinuities located
within the samples. A variety of mini and micropencil probes have been developed
with straight and 90 degree access with increased sensitivity. In addition, it
was realized that testing of short lengths of weld also created problems, in
that the communication rate was too slow to produce a good representation of the
weld result on the visual display screen. New software that eliminates this
problem, including communication rates, has also been developed, which allows
scanning speeds seven times faster than before. This allows greater presentation
on the screen for shorter lengths of welds and faster scanning speeds for the
testing of long lengths of weld.
The ACFM technique was also used to test structures that
had been coated with protective or antifouling coatings so that the expensively
applied coatings did not have to be removed and reapplied, thus avoiding costly
preparation and reinstatement. The topside testing engineers also adopted the
technique for the testing of process and pressurized plant, structural steelwork
and crane pedestals. The system was used in conjunction with rope access teams,
proving the usefulness of two man operations (Figure
2). Tests could be carried
out up to 50 m (164 ft) between the ACFM probe and the ACFM unit, while the
operator could be another 30 m (98 ft) from the unit.
The technique has also been applied to testing of drill
threads on casing and drill tools. A special transportable system has been
produced to automatically test the drill thread ends and classify them. This
provides go/no go reporting. The system is based on new ACFM array technology. A
handheld probe has also been produced to test drill threads with the portable
ACFM system.
New materials are being used for components and coatings
on offshore structures, but the ACFM system has now been successfully applied to
ferritic steels, austenitic stainless steels, aluminum, duplex, super duplex,
monel, inconel and titanium (Figure 3). It has also been used to test through
the following coatings:
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flame sprayed aluminum |
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epoxy coating |
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standard paints |
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ferrite based paints |
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copper coated threads. |
Some tests have to be carried out when the plant is
operational. ACFM has been used during tests at 253 K (-4 °F) and up to 773 K
(932 °F).
Because of the above advantages, the ACFM technique has
been used to test coated flare booms, epoxy coated pig traps, painted nozzle
welds, pipe butt welds, pipe and saddle support welds and pressure vessel seam
welds as well as conduct the above mentioned tests.
Since 1996, the ACFM technique has been further developed
following customer requirements. When it was realized that the original probe
coverage was not adequate, new probes were designed. When a particular client
wanted to detect a particular minimum size discontinuity, the probes were
redesigned and this modification has been built into production probes. Some
testing companies found that the scanning speed was not fast enough when long
lengths of weld required testing, or that short weld scans needed to be expanded
so that they were presented on a single scan. The software and the
communications rate were adapted to take these two points into consideration.
Results of Using Alternating Current Field Measurement
When POD trials are carried out on any technique, the
technique is usually compared to magnetic particle testing as that is normally
classed as the industry norm. When POD trials were carried out in the early
1990s on carbon steel welds, ACFM and magnetic particle testing gave similar
results although magnetic particle testing had a greater number of false calls (Rudlin
and Dover, 1993). In a trial carried out in 2000 on axle components in their
dirty condition, ACFM produced a POD of 84% compared to 44% for magnetic
particle testing, even though magnetic particle testing was carried out on a
cleaned surface (Pollard and Lear, 2000). The axles had all been rejected for
use in service. All of the discontinuities present were fatigue cracks produced
during service, which had caused them to be scrapped by the overhauler using
magnetic particle and ultrasonic testing techniques. The trials showed that ACFM
would have rejected all of the axles, whereas magnetic particle testing would
have rejected only four. The ACFM system was set up to detect discontinuities
greater than 0.2 mm (8 x 10 -3
in.) deep, although the standard for the testing
was to detect discontinuities 0.5 mm (0.02 in.) deep. In this case, if magnetic
particle testing had been the only testing technique applied to these axles,
then some axles with discontinuities present would have been returned to service
with the probability that accidents would have occurred. This trial shows that
magnetic particle testing should not be classed as the infallible technique it
is usually made out to be and should not be used as a benchmark against which
other techniques are compared.
Ultrasonic and magnetic particle testing have been used as
standard testing techniques in the past for the testing of welds, and their use
has been historic. As such, it has been difficult to replace their use. The
results of the recent Netherlands Institute of Welding and the Programme of
Assessment of NDT trials show that manual ultrasonic testing gave only a 50%
detection rate from the Netherlands Institute of Welding trials and between 30
to 70% for the Programme of Assessment of NDT trials. The sizing errors were
quite poor with an average length error of 8 mm (0.3 in.) with a standard
deviation of 24 mm (0.9 in.). The depth measurements were worse with a slope of
0.24 and an intercept of 2.7 compared with the ideal of a slope of 1 and an
intercept of 0. The overall result was that all small discontinuities were
oversized and large discontinuities were undersized (McGrath, 1999). This again
illustrates that a technique, which is used every day, may not be the most
suitable technique for the task in hand. Some industries are still using manual
ultrasonic testing for measuring the depth of surface breaking cracks when there
are much more suitable techniques available.
Trials with ACFM on welds show that no discontinuities
were missed below a POD of 80% for all discontinuities longer than 9 mm (0.4
in.). As well, discontinuity depth measurements were accurate to within 1 mm
(0.04 in.) with a POD of 91% for all discontinuities deeper than 1 mm (0.04
in.). Even with this information, magnetic particle and manual ultrasonic
testing are accepted as standard techniques in all of the conventional
industries, not withstanding the fact that, when using these techniques, the
surfaces have to be cleaned prior to testing and couplant or inks have to be
removed after testing. This residue may be classed as hazardous waste and
special disposal procedures have to be used for environmental reasons. This does
not occur when using the ACFM technique.
Training schemes, which are internationally recognized
(such as CSWIP, PCN and the Lloyds Register of Shipping), are in place and many
technicians have now been trained at Levels I and II. A scheme similar to those
mentioned has been submitted to ASNT. ACFM training approved by Lloyds Register,
leading to a Lloyds Certificate, is currently offered by a major US NDT training
school. The ACFM technique has been approved by certifying authorities such as
Det Norske Veritas, Lloyds Register, Bureau Veritas, Germanischer Lloyds and
Offshore Certification Bureau for use on offshore structures and petrochemical
plants after going through trials with these organizations. Much of the ground
work is done by technicians in the field who see other companies using the new
techniques and inform their company engineers using the bottom up transfer of
information.
However, the technician coming across ACFM for the first
time sees it as just another electromagnetic technique. He is probably wondering
whether or not it is eddy current. It is not eddy current. The eddy current
technique uses a compact circular excitation alternating current, which produces
an alternating magnetic field (the primary field). This field then produces eddy
currents in the material, which in turn produce their own magnetic field (the
secondary field). This field opposes and modifies the primary field. This
results in a very sensitive detection capability, but also makes the technique
prone to strong lift off signals and signals due to material property changes.
The secondary field can be affected by variations in conductivity, permeability,
part geometry, thickness and discontinuities. All of these discontinuities,
including the cracks, produce display signals. Some of them are very similar and
it takes a well trained operator to discriminate between a crack and a noncrack
signal.
Unlike eddy current testing, the ACFM technique is
generally insensitive to permeability, metallurgical changes and lift off; the
new probes ensure that edges have little effect. Each probe is delivered with an
individual software file, which sets up the instrument to give the best
available performance. Since the technique is relatively insensitive to lift
off, it can be used to test through coatings.
The ACFM technique uses a uniform electric current which
is induced into the material under test - this produces a magnetic field,
which, if a discontinuity is present, will be disturbed and flow around its
edges. Special techniques are used to induce these electric currents and the
components used are built into the probes. Small detectors or sensors are built
into the probe, which measures the magnetic field disturbances. Because the
alternating current field is uniform, it is possible to model the current flow
and to separate the magnetic field into its three components. The X and Y
components are in the plane of the material surface (Y being in the same
direction as the flow of the current into the weld area and X being parallel
with the weld toe). The Z component is normal to the weld toe (Figure
4).
When no discontinuities are present and the uniform
current is flowing in the Y direction, the magnetic field is uniform in the X
direction perpendicular to the current flow, while the other two components (By
and Bz) are zero. The presence of a crack causes the current to flow around the
ends of the discontinuity and below it. This causes a concentration of the
current at the ends of the discontinuity and a reduction in the current at the
deepest part of the crack. This concentration of the current results in creating
peaks and troughs in the Bz and a reduction in the Bx along the length of the
crack, which, when it is at its smallest value, is at the deepest part of the
crack (Figure 5). The sensors in the ACFM probe measures the values of the Bx
and Bz as it traverses the length of the weld - the former giving crack depth
and the latter crack length. These measurements, together with software
algorithms, are used to give the accurate measurements of length and depth of
the discontinuity. To aid interpretation, the values of Bx and Bz components are
plotted against each other and will produce a closed loop if a discontinuity is
present. This is called a butterfly loop. The right and left edges of the loop
give an indication of where the ends of the discontinuity are present as these
are the locations of the Bz - ve and Bz + ve. These positions plotted on the
weld give the estimated length of the discontinuity. The positions of the Bz -
ve and Bz + ve are located just inside the true length of the discontinuity.
When this length is used with the software algorithm and the Bx background and
minimum values, accurate values of length and depth are produced. The magnetic
field values being measured are absolute and these are used with mathematical
look up tables so that there is no requirement to calibrate the ACFM instrument
before use using a sample piece with artificial discontinuities.
The mathematical model look up tables have been produced
using fatigue cracks rather than slots because slots do not behave the same
electrically as a crack in carbon steel. As such, all of the probe files have
been produced for carbon steel, but there are now probe files available for
stainless steel. Thus, the length and depth of a crack in stainless steel can
now be produced. If any other conducting material is being tested, then a
calibration curve will have to be produced for that material using a block with
discontinuities present. The depth measurements produced with the carbon steel
look up tables will be compared with the actual values of the discontinuities in
the block and a calibration curve will be produced. The length measurements will
not be affected.
Accepting and Understanding Options of Alternating Current
Field Measurement
Before a technician uses the ACFM technique, they should
understand how the technique works. They should also understand all of the
options they have with the equipment before they begin their testing. As well,
they should be aware of all the trials that have been carried out to prove the
technique in case a client asks questions about the validity of the test
results. Examples have been given above of developments that have taken place
over the last five years using the ACFM technique in an attempt to get the
technique accepted more as a general technique instead of some technique which
is brought out to be used on special occasions.
What does it take to get new technology accepted by old
industries? Compared to ultrasonic and magnetic particle testing, any form of
electromagnetic testing has always been looked on with suspicion. In this case,
ACFM has had to undergo more trials and produce better results than manual
ultrasonic or magnetic particle testing because of preconceived ideas. So, why
don't these new technologies get used? In so many cases, when a testing company
is contacted by a client who requires some welds to be tested, it is easy for
the company to quote and supply a magnetic particle testing technician without
thinking about the residual cost to the client. This is the very same client who
has to clean the welds, arrange scaffolding and also arrange for any debris to
be removed as well as recoat the welds after testing. The cost of the testing
itself is quite small. In many cases, the client would rather pay more for a
noninvasive testing technique which would not give him the logistics problem of
carrying out magnetic particle testing. Both the client and the testing company
need reeducating. Articles have been published in Materials Evaluation, papers
have been given at ASNT conferences over the last five years and the equipment
has been shown at a number of ASNT exhibitions in order to address this ACFM
technique.
A number of companies who have heard about ACFM, seen a
demonstration and then have used it in their plants have come to an economic
understanding of the technique and I am sure that this also applies to other new
techniques. When trying to get the technique used, the end user usually wants to
know who can supply the new technique. And when speaking to the testing company,
the testing company wants to know if the user company has specified the
technique that they want used. The circle remains unbroken until a company
becomes a product champion and specifies the technique.
Conclusion
Going back to the first paragraph of this paper, the 1996 paper introduced some
people to ACFM as a method, which provided some of the basics for improved
electromagnetic testing. Hopefully, this update will give an insight to more
people into the development of a technique which could produce great economic
advantages in certain industries if it was applied.
References
McGrath, B.A., Programme for the Assessment of NDT in Industry, AEA
Technology, 1999.
Pollard, M. and A. Lear, "Evaluation of Electro-Magnetic
Non-Destructive Testing Techniques," Engineering Link, Report 11363/03,
December 2000.
Raine, A. and C. Laenen, "Applications Using the
Alternating Current Field Measurement (ACFM) Technique, Using Rope Access,"
Insight, Vol. 43, No. 5, May 2001, pp. 318-321.
Raine, A., "Cost Benefit Applications Using the
Alternating Current Field Measurement Testing Technique," Proceedings -
ASNT's International Chemical and Petroleum Industry Inspection Technology (ICPIIT)
VII Topical Conference, June 18-22, 2001, pp. 111-117.
Rudlin, J.R. and W.D. Dover, "Results of Probability of
Detection Trials," IOCE Conference, Aberdeen, October 1993.
* Technical Software Consultants, Ltd., 6
Mill Square, Featherstone Road, Wolverton Mill, Milton Keynes, MK12 5RB,
England; 44 1908 220 255; fax 44 1908 220 959; e-mail <alan@tscn.freeserve.co.uk>.
+
Hellier South Central, 16631 West Hardy Street, Houston, TX
77060; (281) 873-0980; fax (281) 873-0981; e-mail <bcameron@sprintmail.com>.
Copyright ©
2002 by the American Society for Nondestructive Testing, Inc. All
rights reserved.
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