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Gas Research Institute's research
and development program on in-line
inspection of natural gas pipelines
Haines, Harvey, Bubenik, Tom, Nestleroth, J. Bruce
Harvey H. Haines, Tom A. Bubenik, J. Bruce Nestleroth, "Gas Research
Institute's research and development program on in-line inspection of natural
gas pipelines," Proc. SPIE 2454, Nondestructive Evaluation of Aging Utilities,
(12 May 1995); doi: 10.1117/12.209372
Event: Nondestructive Evaluation of Aging Infrastructure, 1995, Oakland, CA,
United States
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Gas Research Institute's research and development program
on in-line inspection of natural gas pipelines
Harvey. H. Haines
Gas Research Institute
Chicago, illinois, U.S.A.
Thomas A. Bubenik and J. Bruce Nestleroth
Battelle Memorial Institute
Columbus, Ohio, U.S.A.
ABSTRACT
The Gas Research Institute (GRI) has been supporting a comprehensive research and development
program on in-line inspection techniques for natural gas transmission pipelines. This program contains
assessments of state-of-the-art nondestructive evaluation methods, improvements in current approaches, and
developments of advanced inspection technologies. The elements of the GRI Nondestructive Evaluation
Program range from laboratory evaluations of the capabilities of inspection technologies to large-scale
measurements in simulated pipeline settings. Each level of research stresses a quantification of both the limits
of detection and the accuracy of characterization of pipeline imperfections that are found by in-line inspection
tools.
The overall goal of GRI's Nondestructive Evaluation Program is to develop and improve technologies
that will help gas pipeline companies maintain the physical integrity of their transmission systems, prevent
pipeline shutdowns, and reduce maintenance costs. This paper summarizes the results of the GRJ program to
date in relationship to their direct application to in-line inspection of gas transmission pipelines. The program
consists of three main elements: facilities development, research on current inspection technologies, and
research on future inspection technologies. The facilities development is centered around the Pipeline
Simulation Facility; the research on current inspection technologies is aimed at improving magnetic flux leakage
analyses; and the research on future inspection technologies is centered on stress-corrosion crack detection and
characterization.
Keywords: Pipelines, nondestructive inspection, in-line inspection, natural gas pipelines, magnetic flux leakage
1. INTRODUCTION
The GRI Nondestructive Evaluation Program was established in 1987 to advance the state-of-the-art of
inspection technologies for gas transmission pipelines. In-line inspections are one important tool being used by
pipeline operators to ensure the integrity and safe operation of their pipeline systems.(1)(2) In the United States
alone, the major components of the natural gas system include nearly 450,000 km (280,000 miles) of
transmission pipelines, 145,000 km (90,000 miles) of gathering lines, 1,345,000 km (835,000 miles) of
distribution mains, and 450,000 km (280,000 miles) of service lines. This pipeline system provides gas to over
fifty million customers.
The gas-transmission industry has an enviable safety record as a result of good design and construction
practices and from good operating and maintenance procedures. Nearly $2.7 billion per year is spent in the
United States on operating and maintenance activities.(3) Continuing advances in inspection and monitoring
will assist the gas industry in continuing to optimize and improve its operating and maintenance programs.
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0-8194-1807-2/95/$6.00
Monitoring is au important consideration in maintaining an aging infrastmcture. The GRI
Nondestructive Evaluation Program is aimed at meeting the needs of pipeline operators in this area. In
particular, the program is improving the technologies used in in-line inspection tools. The long-range goal of
the program is to offer technology improvements that can be put into practice by inspection vendors worldwide.
The GRI Nondestructive Evaluation Program, like other programs at GRI, supports GRI's mission to
discover, develop, and deploy technologies that measurably benefit gas customers and the industry. It addresses
one of GRJ's three main objectives: to ensure a reliable, long-term, competitively priced supply of natural gas
that meets demand in sufficient quantities and in a safe and environmentally benign fashion.
GRI's Nondestructive Evaluation Program consists of three elements: facilities development, research on
current inspection technologies, and research on future inspection technologies. Each of these areas is described
in this paper.
2. BACKGROUND
In-line inspections are one important tool being used by pipeline operators to ensure the integrity and
safe operation of their pipeline systems. In-line inspection of gas pipelines is accomplished by instruments
vehicles commonly referred to by the pipeline industry as "pigs". These instrumented vehicles must be selfcontained with data recorders and power to operate sensors and store nondestructive measurements. Ultrasonic,
magnetic flux leakage, and eddy current technologies have been successfully applied for various pipeline
inspection applications. The vehicle is propelled through the pipeline by the flow of the product (oil, natural
gas, etc.). Often the flow of product must be slowed when the preferred inspection speed is 5 miles per hours
or less to achieve accurate inspection results. A single in-line inspection often exceeds 50 miles and the
durations can be days.
.
The use of in-line inspection began around 1965 when Tuboscope introduced the first Linaloga corrosion
survey service. The original Linalog® tool used a magnetic flux leakage system patterned after the equipment
used in oil field tubular inspections. Since that time, many competing technologies and designs, as well as a
wide variety of other in-line inspection equipment, have been introduced by various suppliers. In addition to
corrosion tools, prototype tools were developed to inspect for cracks, coating disbondment, curvature, and other
conditions. Some of these tools have been offered commercially; others are still under development or have
been abandoned.(2)
In-line inspection tools for pipeline inspections are completely self-contained units containing sensors,
data recorders and many other systems that enable the safe, reliable and repeatable nondestructive examinations.
To propel the tool through the line, a drive system made of cups that seal against the pipe wall are used. The
force exerted by gas pushing on a cup or set of cups at the front of the tool pulls the tool through the line.
Differential pressure acting between the front and back of the drive cups provides a force along the pipe axis.
This force propels the drive cups, which in turn pull the rest of the tool. The driving force on a tool must
overcome the friction between the tool and the pipe and the magnetic drag that the tool exerts. The pressure
differential required to move most tools is small; one measurement program showed the pressure differential for
a 24-inch tool to be under 10 psi.
Odometer systems are used to determine the location of indications from the start of the inspection or
from recognizable features along the pipeline route. Recognizable features include pipeline connections, valves,
wall thickness changes at road crossings, and girth welds. Pendulum-type orientation-measuring systems are
used to determine where an indication is located around the circumference of a pipe. Pressure vessels for
electronic and batteiy components prevent damage to tool components from the line pressure, harmful gas
components, and moisture. Vibration and shock mounting systems to isolate the electronic components and the
battery systems from possibly harmful shock and vibration are usually required.
An inspection tool is inserted into a pressured pipeline through a pig launcher or through a pipeline spool
piece. A pig launcher is typically a pipe attached to the main run pipe. Valves isolate the launcher from the
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line pressure when the launcher is opened and the tool Installed. After the tool is in place, a set of valves is
used to pressurize the launcher. Then gas pressure moves the tool from the launcher into the run pipe. After
the tool is in the run pipe, flowing gas propels the tool through the line. While the tool is running, the velocity
of the flowing gas is usually reduced from normal pipeline conditions to the range required by the tool. The
velocity of a tool varies at bends, valves, tees, and elevation changes. Control of the gas and tool velocity is
important in providing good inspection results.
Retrieving an MFL tool is similar to launching, and a pig receiver is similar to a pig launcher. The tool
is stopped with a bypass flow system or with valves when it approaches the receiver. Valves allowgas pressure
from the line to move the tool into the receiver. After the tool is in place, the receiver is isolated from the line
pressure and the pressure in the receiver is released. Finally, the received is opened and the tool removed.
After a tool is removed from a receiver, the inspection data record is removed.
2.1 Industry Use of In-Line Inspection Tools
There have been several industry surveys to collect data on the use of in-line inspection tools in
transmission pipelines. The Pipeline Research Committee reported that 17 out of 43 lines with structural
integrity concerns had been in-line inspected.(4) The Department of Transportation reported that at least five
major pipeline companies have formal programs for conducting in-line inspections.(5) The United States
General Accounting Office reported that 12 of 18 companies that responded to a questionnaire had used in-line
inspection tools. Of these, about half had used in-line tools only once or on an as-required basis only.(6)
Hence, in-line inspection is used by some but not all pipeline companies, at least in the United States.
There are a number of reasons why in-line inspection tools are not used on more pipelines today. Some
of the most common obstacles to in-line inspection include tight bend radii, changes in pipe thickness or
diameter along a line, branch connections, and obstructions due to valves.(2) Roughly 31 percent of the natural
gas pipelines and 55 percent of hazardous liquid pipelines in the United States cannot accept in-line inspection
tools as is.(5) Twenty seven percent of the gas lines and 34 percent of the liquid lines could be made
inspectable with modifications, while 41 percent of the gas pipelines and 10 percent of liquid lines are
considered not inspectable, even with modifications.(5)
The percentage of pipelines in the United States that cannot accept in-line inspection tools is probably
greater than elsewhere in the world because the United States system is older. Newer pipeline construction
practices have recognized and incorporated capabilities for in-line inspection.
3. PLPELJr'E SIMULATION FACILITY
The first phase of GRI's Nondestructive Evaluation Program, the Pipeline Simulation Facility, has been
built to advance the state-of-the-art of gas pipeline inspection technology. The facility consists of four elements.
The first is a nondestructive evaluation laboratory and a linear test rig. This laboratory allows detailed
analytical modeling and carefully controlled experiments for studying basic technology effects. The second
element is an outdoor pull rig. The pull rig consists of four 91.4 m (300 foot) lengths ofpipe with diameters of
305, 610, 762, 914 mm (12, 24, 30, and 36 inches). Test bed vehicles or inspection tools can be pulled
through these pipe lengths, which contain carefully controlled defects.
The third element is a flow loop. The flow loop, which will be available for use in mid-1995, is a
1,433 m (4,700 foot) long, 610 mm (24 inch) diameter pipeline where both pressure and flow can be controlled.
The last element is a set of test bed vehicles. These vehicles are modular systems upon which inspection
equipment can be mounted and used in the pull rig or flow loop.
The relationships among the Pipeline Simulation Facility components are summarized in Figure 1. The
facilities have been designed to accommodate research that extends from detailed analyses of defects in flat
plates under idealized conditions to the same types and sizes of defects in a pressurized pipeline operating under
flowing gas conditions.
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NDE LABORATORy
- Parametric finite-element studies of
idealized NDE signals from defects in
plates and pipes.
- Linear Test Rig for experimental
measurement of signals from machined
and real defects in pipe segments.
- Primary Uses:
- Fundamental studies of basic effects.
- Verification of finite-element
calculations.
- Studies on the effects of velocity and
sensor position relative to a defect.
- Tool and component developments.
PULL RIG
- Measurements of NDE signals from
many defects in full pipe lengths
- Primary Use:
- Statistical data generation and
comparisons of NDE signals from
multiple defects under identical
conditions
- Studies on eddy current effects
- Tool developments
TEST BED VEHICLES
- Generic modular inspection tools for use
in the pull rig or flow loop
- Primary Use:
- Data collection of MFL or other
advanced NDE methods
- Studies on the effects of tool design
parameters
FLOW LOOP
- Full-scale testing on simulated operating
pipeline conditions
- Primary Use:
- Verification of pull rig, linear test rig,
and laboratory results.
- Studies on the effects of stress,
bends, accelerations, "real-world"
pipeline conditions
- Tool development and durability
testing
Figure 1. GRI Pipeline Simulation Facility Components
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3.1 Nondestructive Evaluation
Laboratory and Linear Test Rig
The nondestructive evaluation laboratory and linear test rig provide facilities for detailed analytical
modeling and carefully controlled experiments in a laboratory setting.(3)(7)(8)(9) In the nondestructive
evaluation laboratory, detailed calculations can be made and verified with experiments. These calculations
include detailed parametric studies that cannot easily or economically be conducted with experiments.
Finite-Element Analysis Capabilities. The nondestructive evaluation laboratory includes a computer
workstation that has been used to calculate magnetic flux leakage fields using finite-element analysis. These
calculations have been performed for metal loss defects under static and dynamic conditions.
Commercial software for magnetic finite-element analysis is available from many vendors. The software
can examine both magnetostatic (direct current) and eddy current (alternating current) problems in both two and
three dimensions. The static software being used on the GRI program is Magnet 2-D and 3-D from Infolytica,
Montreal, Canada. This software calculates leakage fields from defects. Sensors are simulated using additional
software developed on this project.
Finite-element modeling software that can examine velocity effects has been evaluated in the
nondestructive evaluation laboratory. This dynamic software, from Vector Fields of Newcastle, England, is
capable of analyzing the leakage fields from two- and three-dimensional defects. The computational complexity
of the two-dimensional velocity calculations is similar to the three-dimensional static solutions. Additional
three-dimensional velocity software is being separately evaluated by Iowa State University as part of another
GRI research program.
Linear Test Rig. The linear test rig is a movable platform on which sensor systems can be installed and
used to take detailed measurements. The rig was designed with the assistance of Pipetrornx, Ltd., and it was
constructed by Mecon Industries Ltd. of Scarborough, Canada, under license to Pipetronix.
The linear test rig can be used to measure the effects of velocity on defect-signal relationships, and it can
support virtually any nondestructive evaluation sensor technology. For a given experiment, analytical modeling
can be used to predict results, the linear test rig used to collect data, and the predicted and experimental data
compared. The linear test rig is being used to study dynamic effects under repeatable controlled laboratory
conditions.
The linear test rig contains an inspection platform that can be pulled by a cable along a 7.3 m (24 foot)
guide rail either under a partial-diameter pipe section or through a full-diameter pipe. The speed of the
inspection platform remains constant up to 12.9 km/hr (8 miles per hour) in the middle 3 m (10 feet) of pipe
with a magnetic load and brush friction and at speeds up to 16 km/hr (10 miles per hour) without a magnetic
load and friction. The speed control is
16 km/hr (0. 1 miles per hour) with a maximum acceleration
distance of 2.4 m (8 feet).
3.2 Pull Rig
The pull rig is an outdoor facility in which fUndamental studies can be conducted to determine the basic
relationship between defects and signals.(3)(7)(1O) The pull rig bridges the gap between the laboratory
setting of the linear test rig and full scale testing in the flow loop and actual application in pipelines. It provides
a full pipe diameter section and full vehicle configuration for developing technologies while still closely
controlling the test environment. An inspection tool or test bed vehicle can be passed by a large number of
known defects and a large amount of data generated in a timely and cost effective manner.
The pull rig is a set of pipe runs with removable defect sections through which in-line inspection tools
and test bed vehicles can be pulled. The pull rig contains
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Figure 2. Flow Loop Layout
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0.55 MPa (80 psi) under some conditions.
The flow ioop will allow continuous operation without relaunching. That is, a tool can be repeatedly
propelled around the loop without stopping and restarting. An important benefit of continuous operation is the
ability to collect data over a long period of time. The loop as designed can be operated for over 50cycles to
simulate an inspection run of 80+ km (50+ miles) without stopping. Continuous runs allow repeatability and
durability to be assessed under field conditions.
3.4 Test Bed Vehides
Two test bed vehicles have been built as test platforms for use in the pull rig and flow loop. These
vehicles remove the need to develop a test platform for each research scenario. Both test bed vehicles were
designed to make it easier for independent researchers to attain field realism, which is something only vendor
laboratories had been able to do previously. Both vehicles were built for use in 610 mm (24 inch) pipe.
The first test bed vehicle is called the advanced sensor test bed vehicle. This vehicle was designed and
fabricated by Southwest Research Institute and T. D. Williamson to support development and evaluation of
ultrasonic and other nondestructive testing methods.(12)(13)(14)(15)
The vehicle can be used for
experiments involving ultrasonics, eddy currents, electromagnetic acoustic transducers, or hybrid techniques
(multiple inspection technologies). The versatility of the advanced sensor vehicle permits a wide range of
experiments because an experimenter can essentially replace any one of the vehicle sections with one of his own
design.
The magnetic flux leakage test bed vehicle was designed and built by Pipetronix and
Battelle.(16)(17)
The main difference between the magnetic flux leakage test bed vehicle and the
advanced sensor vehicle is the data acquisition system and the sensor module. The sensor module contains full
magnetizing and sensing equipment. The magnetic flux leakage vehicle uses slower acquisition rates than the
advanced sensor vehicle; slower data acquisition allows more channels of data to be taken. Also, although the
design of the vehicle was oriented toward research in magnetic flux leakage, its use is not limited. The test bed
vehicle can be used for any research which does not require extremely fast data acquisition rates.
4. RESEARCH ON CURRENT INSPECTION TECHNOLOGIES
The second phase of GRI's nondestructive evaluation program is research on current inspection
technologies. The most commonly used in-line inspection tools in gas pipelines are magnetic flux leakage
tools.(9) GRI's program addresses magnetic flux leakage inspection technology.
The goal of any magnetic flux leakage inspection is to both reliably detect defects and accurately
characterize their geometries. Detection includes locating defects and discriminating them from naturally
occurring conditions or pipeline features. Characterization involves defining the defect's geometry with
sufficient accuracy for repair decisions or failure calculations.
GRI's research program is aimed at improving detection and characterization of defects using magnetic
flux leakage. Since detection reliability is generally considered good in current tools, the main emphasis is on
improving characterization accuracies. Typical inspection tools report enough data on defect depth and length
for use in basic failure calculations, such as with the widely used ASME B31G criteria.(18) More advanced
and more accurate failure criteria require additional information on defect geometry, though, such as the defect
profile. This type of data is not generally available from current tools or, when it is reported, its accuracy is
largely unknown.
The GRI program on magnetic flux leakage technology has two aims. The first aim is to develop
improvements to the analysis methods used in existing tools. The second aim is to develop replacement or nextgeneration analysis methods. Ongoing research in these areas is summarized below. The program is being
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jointly conducted by Battelle, Southwest Research Institute, Iowa State University, Halliburton, Pipetronix, and
Vetco Pipeline Services.
5. RESEARCH ON FUTURE INSPECTION TECHNOLOGIES
The last phase of GRI's nondestructive evaluation program is research on future inspection technologies.
Here, the primary emphasis is on developing and improving techniques for detecting and characterizing stresscorrosion cracks. Detection and characterization of stress-corrosion cracking is a growing area of interest
worldwide.
The GRI program on stress-corrosion cracking is aimed at complementing the prior work by British Gas
and other researchers. While final work plans are not yet complete, the plans are likely to emphasize using two
or more inspection technologies in a synergistic fashion.
6. CONCLUSIONS
GRI's Nondestructive Evaluation Program was established in 1987 to advance the state-of-the-art of
inspection technologies for gas transmission pipelines. The goal of the program is to develop technologies that
will help gas pipeline companies maintain the physical integrity of their transmission systems, prevent pipeline
shutdowns, and reduce maintenance costs. The GRI Nondestructive Evaluation Program consists of three main
elements: facilities development, research on current inspection technologies, and research on future inspection
technologies.
The GRI facilities development program is centered around the Pipeline Simulation Facility. All
facilities, are complete and available for use by third parties. GRI's research on current inspection technologies
is aimed at improving current magnetic flux leakage analysis techniques and developing new and more powerful
analysis techniques. These studies are being used to improve defect characterization analyses and to develop
next-generation analysis techniques. GRI's research on future inspection technologies is centered on stresscorrosion crack detection and characterization. The GRI cracking program will evaluate and incorporate worldwide technological advances to develop improvements that can be used by the in-line inspection industry.
7. ACKNOWLEDGEMENTS
The work described in this paper resulted from the efforts of many individuals at a number of
organizations. In particular, Ted Willke, Tom Steinbauer, and Greg Ridderbusch of GRI, and Terry Boss of
Interstate Natural Gas Association of American have provided excellent advice and direction during the
program. Organizations that have substantially contributed include Southwest Research Institute, Vetco Pipeline
Services, Pipetronix Ltd., J-Tech Consulting, Iowa State University, and Battelle.
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