THE IMPORTANCE OF NON-DESTRUCTIVE TESTING
AND INSPECTION OF PIPELINES.
Patrick J Garland
ASNT LEVEL III
ACCP LEVEL III
OSG TESTING (PTY) LTD
Johannesburg
South Africa
ECNDT MOSCOW 2010
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1. Introduction
While initial capital outlay is important, the ongoing operating costs and maintenance
of a pipeline can outweigh the savings made by selecting a pipeline with a lower
installation cost but a high risk of failure and a limited working life.
As a result, for the purposes of cost optimization, the need for Non-Destructive
Testing (NDT) and inspection during the manufacturing, construction and the
operation stage is both inevitable and invaluable. Based on a study conducted by
Rentgen Technische Dienst, the NDT industry suffers from a perception that NDT is
viewed, in a best case scenario, as a necessary evil. This perception does hurt the
industry as it impedes technological advancement as well as it being overlooked by
investors as a valued added to a capital investment.
In order to maximize on investments as well as reducing operating costs and
maintenance, adequate NDT and inspection by suitably qualified personnel is
essential during all stages of pipe manufacture, construction and operation.
The use of the latest inspection methods and equipment will assist in obtaining the
maximum life expectancy from a pipeline reducing the overall operating costs.
As a result, the use of fluoroscopy, computed radiography, digital radiography and
automated ultrasonic testing, would, in general, improve the probability of detection
(POD) of discontinuities. In addition the possibility of operator error and the cost of
inspection by saving time and reducing the quantity of consumables used are
reduced. That said, the use of these methods is predicated on budgetary constraints
and the availability of suitably of qualified personnel.
The objective of this paper is to introduce technologically advanced NDT methods for
the inspection of pipelines during manufacture, construction and operation. Some of
these methods are digital radiography, automated ultrasonic testing, time of flight
diffraction (TOFD), phased array, alternating current field measurement, long range
ultrasonic testing, intelligent pigging and three dimensional laser beam measurement
of corrosion.
In summary, the oil and gas industry exhibits a proclivity of reducing costs by
minimizing the need for NDT. This in turn creates market conditions that erode the
prices for NDT services and in turn generates an environment that is more
predisposed to maintaining a status quo then an environment that fosters
technological advancement and long term thinking. This could, however change, if
and when NDT is no longer perceived as a requirement, but as a solution that
produces tangible savings by an increase of quality, safety and time.
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2. Non-Destructive Testing
Over time various NDT methods were developed, each one having advantages and
limitations making it more or less appropriate for a given application. According to
recent market studies, modern NDT methods have become more quantitative and
less obtrusive, which in many scenarios, translates into savings over time. As a
result, these advanced NDT methods have the potential that could lead to significantly
lower repair rates while maintaining existing safety standards as long as adequate
criteria and procedures are adapted.
As indicated, the industry is weary of replacing well-established and conventional
procedures with advanced technology without a time consuming validation effort.
However, several international projects have tackled these issues over time, some of
which produced some tangible changes. For example, digital radiography is now
accepted by ASME and has been used to produce acceptable images on material up
to 100 mm thick using Ir-192. Furthermore, under the direction of the International
Pipe Line and Offshore Contractors Association (IPLOCA), criteria have been
developed for the use of a combination of automated ultrasonic testing and time of
flight diffraction on the testing of weld discontinuities on pipeline welds.
With the variety of NDT methods available, it is important to select the method that
will provide reliable results. A combination of different NDT tests may be applied to
provide assurance that the material or component is fit for use.
2.1. Radiographic Inspection
The equipment required to perform radiographic inspection can be either an X-ray
machine, which requires some electrical input, or a radioactive isotope that produces
gamma radiation. The isotope offers increased portability as no electrical power
supply is required.
Radiation detectors used are image intensifiers in fluoroscopic and real time imaging
systems. Electronic imaging panels and phosphorescent imaging screens are used to
produce digital images for computed and digital radiography.
Real time imaging can be used online close to the welding station and can detect
defects at an early stage thus reducing the amount of faulty welds produced.
The use of phosphorescent imaging plates in digital radiography replaces X-ray film
and processing chemicals. They are reusable and the X-ray images are stored
electronically on optical disc. These images can be electronically enhanced to
increase or reduce density enabling discontinuities which may have previously been
undetectable, to be seen.
Electronic imaging panels produce real time images but the panels are flat and rigid
and are electronically connected to the image processor thus limiting their portability
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for in field use. The phosphorescent imaging screens are flexible and are used
remotely as is conventional x-ray film. The phosphorescent screens store a latent
image which is scanned with an infrared laser scanner. The images are then viewed
on a monitor. Functions such as magnification and measuring tools are available for
further evaluation of images.
The use of phosphorescent screens requires shorter exposure times which can
amount to considerable savings. Lower kV levels are required to produce the images
thereby extending the life expectancy of X-ray tubes and allowing isotope sources to
be used for a longer period of time. As the exposure times are also much shorter
there will be a general reduction in radiation levels.
2.2. Ultrasonic testing
The primary benefit of UT is that it is considered to be a truly volumetric test. That is,
it is capable of determining not only the approximate dimensions and location of a
defect, but it will also provide the testing technician with information as to the type of
defect. Another major advantage of UT is that it only requires access to one side of
the material being tested and it will best detect those more critical planar
discontinuities such as cracks and incomplete fusion which may not be possible with
radiographic testing. Because a variety of beam angles can be used, UT can detect
defect which may not be detectable by radiography.
Portable UT equipment is lightweight and often battery-powered. UT requires highly
skilled technicians because interpretation of indications can be difficult. Reference
standards are required for calibration and setting up of the equipment. Test scans can
be recorded by most equipment providing automated scanning.
This test method is generally limited to the inspection of butt welds in materials that
are thicker than 6 mm.
Automated UT is often found in pipe mills where the welds are inspected soon after
welding, by a multiple array of probes, scanning the entire weld thus detecting any
discontinuities at an early stage. AUT is an accepted in-field test method. An array of
probes mounted in a scanner is placed on the pipe and the weld area is scanned as
the scanner is moved along the weld. An encoder will record the probe position in
relation to the distance traveled, which enables the weld to be tested in a shorter
period of time, giving a complete volumetric test of the weld and reducing operator
error.
Time of flight diffraction is another automated scanning and sizing technique which
produces a permanent record of the test. Multiple channels can be recorded and
displayed simultaneously for evaluation. The probes are set up to scan different zone
areas of the weld in one pass and record the results for evaluation at a later stage. As
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a result, the inspection is completed in a reduced time and most importantly, it
reduces operator error.
Phased Array UT inspection of welds is a relatively new inspection method developed
from medical applications. The transducer unit contains multiple elements which can
be focused by the timing of the ultrasonic pulses of the different elements. This
allows the probe to scan through different angles, scanning the weld in a single pass.
The probe distance from the weld can be maintained at a fixed distance and the
probe moved along the weld, which results in the scanning of the complete weld in
one scan. The results are recorded for later evaluation and this minimizes operator
error and reduces inspection time. Once the sound wave is in the material it behaves
exactly the same as pulse echo. The difference is the probe where the angle of the
sound wave can be steered through a range of angles. The electronics of the
display is a different and can display a three dimensional image which aids in the
interpretation of indications.
2.3. Alternating Current Field measurement (ACFM)
ACFM is an electromagnetic technique that uses induced uniform currents and
magnetic flux density sensors to detect and size surface breaking discontinuities. The
main advantage of this method is the ability to test through coatings several
millimeters thick, the ability to obtain depth information on cracks up to 25 mm (1 in.)
deep, and easier testing at material boundaries such as welds. The intensity of a
uniform field performance does not drop off very rapidly with probe liftoff, so
alternating current field measurement can be used to test through thick nonconductive
coatings.
The second advantage is that the larger inducing coil forces currents to flow farther
down the face of a deep crack. The same feature occurs with an alternating current
field measurement probe but, because the depth of penetration down the crack face
is related to the size of the magnetic field inducing coil, the probe can measure more
deeply, typically 15 to 30 mm, depending on the probe type. The ACFM method
allows for the detection and sizing of fatigue cracks, stress corrosion cracking,
hydrogen induced cracking, and corrosion pitting.
2.4. Pipe coating inspection
The surface preparation prior to coating is of prime importance before the application
of any coating to any surface. If the pre-requisite conditions have not been met, the
durability and life expectancy of the coating may be considerably shortened. The
correct application of the coating to the required thickness is also important. As a
final test, a pinhole detection check is carried out for air entrapped in the coating
which could lead to premature failure of the coating.
The proper and effective preparation of a surface prior to coating is essential. Making
sure that the correct surface roughness or profile has been generated is
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essential. If the profile is too low, the adhesion of the coating to the surface will be
reduced. It is important that a coating is applied to the correct thickness. Applying too
much wet coating will not only waste time and money, but there is also a possibility of
the coating cracking during the curing process. Too little coating and there is a
chance that the substrate will not be sufficiently covered. To control process
variables, it is often desirable to measure the film while it is still wet. Wet film
measurements are also useful for systems where the dry film thickness can only be
measured destructively.
Premature corrosion of a substrate is usually due to the failure of the coating. A major
cause of failure is the presence of flaws in the finished coating. Early inspection for
coating flaws will prevent the expense and inconvenience of a coating failure.
2.5. Adhesion testing
Adhesion testing after the coating process will quantify the strength of the bond
between substrate and coating, or between different coating layers or the cohesive
strength of some substrates. Routine testing is used as part of inspection and
maintenance procedures to help detect potential coating failures. The coating may be
continuous and look good, but how well is it connected to the substrate?
Tests can be made on flat or curved (concave and convex) surfaces. A reusable dolly
is adhered to the coatings surface and the force required to push the dolly from the
surface is applied. The value of the force applied is displayed either on a digital
display or on a dial.
3. In-service inspection
3.1. Detection of in-service corrosion
Pipelines world-wide are subject to corrosion and the early detection and
measurement of this corrosion plays a significant part in their safe operation.
The methods used today to detect corrosion damage and material loss in pipelines
are long-range ultrasonic testing and intelligent pigging which includes magnetic flux
leakage.
3.2. Long Range Guided Wave Ultrasonic Testing
Long range guided wave ultrasonic testing is a non-invasive method used for the
detection of both internal and external corrosion and erosion in thermally insulated,
coated and buried pipelines, corrosion under pipe supports and hidden welded joints.
Use is made of low frequency guided waves to detect corrosion, erosion and material
loss in the pipelines being tested. A unit comprising three rings of piezoelectric
transducers is clamped around the pipe and ultrasound is sent first in one direction
along the pipe and then in the other direction. The signal obtained is similar to a
conventional ultrasonic A-scan, where the horizontal axis represents distance along
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the pipe and the vertical axis represents signal amplitude, which is indicative of the
severity of the corrosion.
Although propagation distances vary according to pipe geometry, contents, coating,
insulation and general condition, in ideal conditions, it is not unusual that a range of
up to 30m in either direction from the transducer belt can be inspected. However care
must be taken as this distance is substantially reduced for buried pipelines and
pipelines with heavily attenuating coatings. The technique is equally sensitive to
internal and external corrosion.
The principal advantage of this technique is that it provides 100% initial screening
coverage, and only requires local access to the pipe surface (i.e. removal of a small
amount of insulation) at those positions where the transducer unit is to be attached. It
is suitable for use on pipe diameters above 50mm (2.0") and on wall thicknesses up
to 40mm.
3.3 Intelligent Pigging
Pipeline pigs are intrusive devices that are inserted into and travel throughout the
length of a pipeline driven by product flow. They were originally developed to remove
deposits which could obstruct or retard the flow through a pipeline. Nowadays pigs
are used during all phases in the life of a pipeline for cleaning purposes and for
internal inspection.
The pigs used as in-line inspection tools provide information on the condition of the
line as well as the extent and location of any problems. Intelligent pigging uses
ultrasonic thickness measurement and magnetic flux leakage methods to determine
areas of corrosion, pitting, erosion and cracks.
As a result, the magnetic flux leakage technique leads to a substantial time and
financial savings, which has been used for the testing of hundreds of kilometers of
piping in the desert. The evaluation of the data has shown that a testing rate of 1 km
per day was easily achieved - a rate far greater than that achievable through
conventional wall thickness measurements.
3.4. Three dimensional laser profile measurement
Three dimensional (3D) laser profile measurement of the corroded surface has
proven to be a rapid and accurate method of measuring corrosion depth and the
related software programs enable on site reporting, corrosion mapping and finite
element analysis.
Once the area of corrosion has been located by long range ultrasonic testing or
intelligent pigging, the pipeline may be excavated in the case of a buried pipeline, or
the insulation removed in the case of a pipeline above ground. The protective coating
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or wrapping must be removed to expose the area of corrosion. The surface must be
cleaned removing all protective coatings, scale, rust and any contamination, which
could interfere with the accuracy of the laser-beam measurements to be made.
Scanning of the corroded surface is rapid and accurate with the scanned area being
viewed on the computer monitor to ensure full coverage of the area being scanned.
The scanning arm has several encoders which are able to give the co-ordinates of
any measured position and thus build up corrosion maps of the surface. The laser
beam scanner takes 69,000 measurements per second.
The benefits of laser beam scanning far outweigh the mechanical methods of
measuring due to accessibility, accuracy and speed. Where older mechanical means
of measuring are not possible or difficult be used on some curved surfaces such as
bends, nozzles and compensating rings this problem is eliminated by laser beam
scanning.
Reports are generated on site. Out of roundness measurements can be taken of the
pipe circumference creating references on the scan data for the determination of the
best fit cylinder enabling finite element analysis calculations to be reported.
Color plots of critical areas are obtained and X and Y cross sectional views can be
obtained from the display of the corroded area indicating the worst case scenario.
Corrosion measurement and analysis can be done on any accessible surface with an
accuracy of <0.07 mm and on site reports generated.
Internal corrosion can be differentiated by ultrasonic thickness measurement of the
internal surface by a thickness measurement probe attached to the mechanical
scanning arm. Automatic importation of ultrasonic thickness measurements with X, Y
and Z probe positioning coordinates on scan data enable the condition of internal
surfaces to be included in the report.
The analysis of the report will allow the best repair method to be determined. When
comparing previous measurements of the same area an accurate rate of corrosion
can be determined by the 3 dimensional data recorded. Determination of corrosion
may be interpreted and analyzed with software and data processing in accordance
with different codes and specifications. Computer generated reports will indicate the
surface corrosion using a color depth scale and also a graphical worst case profile.
3D corrosion measurement used during risk based inspections is a rapid cost
effective method of determining the integrity of the pipeline or plant enabling finite
element analysis to be completed in a short space of time allowing qualified
engineering decisions
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4. Conclusion
Making use of the latest technology, such as digital radiography can result in
considerable savings for the user in time as well as consumable costs and also
enhance the probability of detection of defects.
Automated ultrasonic testing that utilizes TOFD and phased array produces
recordable results and a permanent record, which can be evaluated by third party
inspectors at a later stage. Mostly importantly, this tangible benefit translates into a
reduction of operator error and substantial time saving, which is derived from the high
testing speed.
Long range ultrasonic testing is a good screening tool for surface or insulated
pipelines and will reveal suspect areas for further evaluation by other methods. An
ideal example for what a great money saver this method is the testing of pipe
crossings underneath roads and other short pipe lengths on pipe sleepers which are
unable to be pigged or the use of pigs would be consider an overkill due to the
associated costs.
Intelligent pigging is extensively used for the evaluation of corrosion in pipelines and
can be used on all pipelines fitted with pig launching facilities.
Three dimensional laser beam profile measurement of corrosion is an extremely
accurate tool for the measurement of the corrosion depth and location, finite element
analysis and on site report generation allowing qualified engineering decisions to be
made.
Use of the above methods assisted in the development of risk assessment strategies
and pipeline integrity management programs.
Reference
[1] Non-Destructive Testing: Necessary Evil or Benefit F. H. Dijkstra* and J. A. de
Raad**
[2] IPLOCA, Weld Defect Acceptance Criteria: Final Report of the Study
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International Pipe Line and Offshore Contractors Association (IPLOCA),
Summer 2000.
[3] 3D Corrosion, Belgium
[4] Pigging Products Services Association
[5] de Raad, J.A., T. Bouma and A. Bnisch, "Rapid Corrosion Screening in up to
30 mm Wall Thickness for Plates and Pipes,"
[6] TSC Inspection systems www.tscinspectionsystems.com
[7] ASNT Non-destructive testing hand book Vol. 5
[8] Non Destructive testing hand book, 2nd edition, Volume 7, Ultrasonic testing
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[9] Handbook of Non-destructive evaluation. C. J. Hellier
[10] Silverwing UK Limited, UK
[11] Radiography in modern industry. Eastman Kodak
[12] CIT - Computerized Information Technology, UK
[13] Elcometer Elco news E- zine
[14] US Department of transportation Pipeline info home
[15] General Information: Consulting@MaterialsEngineer.com
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