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THE SAFETY AND RELIABILITY of gas and oil pipeline systems are dependent upon the effectiveness of current monitoring and inspection techniques. This paper outlines the present methods used for constant, or on-line, monitoring of pipeline systems, including a discussion of their reliability, sensitivity, and response time for detecting system irregularities. Periodic inspection and monitoring systems are discussed to highlight the techniques used to supplement on-line monitoring techniques and the capabilities for accurately locating defects and degradation before pipeline failure occurs. Future technologies are then discussed to provide insights into the potential for overcoming the limitations of current systems.
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Pipeline Science and Technology Vol.2, No.3 (2018)
A review of pipeline monitoring
and periodic inspection methods
by K. Sachedina and A. Mohany*
Faculty of Engineering and Applied Science, University of Ontario
Institute of Technology, Oshawa, ON, Canada
THE SAFETY AND RELIABILITY of gas and oil pipeline systems are dependent upon the
effectiveness of current monitoring and inspection techniques. This paper outlines the
present methods used for constant, or on-line, monitoring of pipeline systems, including
a discussion of their reliability, sensitivity, and response time for detecting system irregularities.
Periodic inspection and monitoring systems are discussed to highlight the techniques used to
supplement on-line monitoring techniques and the capabilities for accurately locating defects
and degradation before pipeline failure occurs. Future technologies are then discussed to provide
insights into the potential for overcoming the limitations of current systems.
Key words: pipeline, monitoring, inspection methods, ILI, MFL, bre optic
1. Introduction
Oil and gas pipelines are critical components of the world’s energy distribution infrastructure.
The consumption of oil and natural gas is expected to increase over the next several decades
[1], and the safety, efciency, and perceptions of energy use and distribution using pipelines
is therefore critical. There are several risks associated with pipeline operation that should be
mitigated and, if possible, eliminated. Regular pipeline degradation, i.e. corrosion and wear,
can lead to pipeline failure and leaks. Unintentional third party damage, especially of buried
pipelines, can cause also pipeline damage. Intentional damage of piping systems can occur as
a result of theft or terrorism. Operation outside of the design limits, or beyond the capabilities
of piping that is compromised due to corrosion, will lead to premature failure. Furthermore,
extreme environmental events, such as earthquakes or hurricanes, can severely impact pipeline
The consequences of pipeline damage or failure are manifold [2]. Pipes convey hazardous
chemicals and that can pose a serious threat to life and wellbeing. Second, there are direct
nancial costs associated with pipeline failure, which include the loss of product, downtime,
and clean-up costs. Third, indirect nancial costs can arise in the form of nes and lawsuits.
Additionally, other consequences may result which are not easily quantiable, such as
environmental damage and negative effects on public perceptions. From a risk management
perspective, it can be extremely difcult to accurately predict the magnitudes and contributions
of these various costs. However, the enormity of these potential consequences of pipeline failure
show the need to minimize all risks associated with pipeline operation.
Pipeline systems are designed to resist wear and measures are taken to help prevent
degradation. Manufacturing processes are chosen to minimize defects that could accelerate
pipeline deterioration. Cathodic protection and coating materials are used to protect pipelines
Vol. 2, No 3, September, 2018
*Corresponding author: Atef Mohany, email:
© 2018 Pipeline Science and Technology. Published by Pipeline Transport Institute (PTI)
in association with Technical Productions Ltd. All rights reserved.
Received:15 May, 2018
Revised: 6 June, 2018
Accepted: 11 June, 2018
Pipeline Science and Technology Vol.2, No.3 (2018)
and reduce corrosion rates [3]. Numerous factors complicate pipeline monitoring and inspection
techniques, including the fact that pipelines exist in multiple congurations and convey various
uids. Pipeline systems can be installed above ground, buried, or offshore, and can convey gas
or liquid media of various pressures, ow rates, and chemical composition. Pipelines can span
thousands of kilometres, be routed through harsh and inaccessible terrain, and be installed
in locations that may be vulnerable to natural threats such as landslides or earthquakes.
Furthermore, some pipeline systems may convey multiphase ow, which cannot be accurately
monitored using many of the methods currently relied upon in industry. While several standards
and industrial practices exist for pipeline monitoring and inspection, these multiple factors make
it impractical to develop a single standard or guide which can be applied to all piping systems.
While the design, operation, and maintenance phases of pipeline systems are all important,
this paper focuses on the monitoring and inspection techniques used for pipeline integrity
management. Much of the literature categorizes leak detection systems (LDS) according to
‘hardware’ and ‘software’ based systems [4, 5, 6, 7], which refer to monitoring techniques that
rely on special, external sensors, or that use internal measurements of ow parameters such as
ow rate, pressure, and temperature, respectively. Some authors using this classication also
consider biological, or non-technical, methods as the third category, which includes procedures
such as having trained personnel patrol the pipe to observe changes that are indicative of a
leak. Other authors have categorized systems into optical and non-optical methods [8], which
refer to techniques that involve visually evaluating the condition of piping or using non-visual
sensors and software to detect leaks, respectively. Both of these classication schemes organize
leak detection techniques according to their technical operation. More recently, LDS have been
grouped into automated, semi-automated, or manual detection methods, according to the degree
of human intervention required for operation [9]. This groups the various methods according
to one aspect of their applicability, i.e. the level of intervention or expertise required from an
operator. Another potentially useful classication is to consider which methods are available for
continuous, ‘on-line’, monitoring, and those that are used on a periodic basis for inspection or
monitoring. This type of classication has been adopted in the current work in order to highlight
the availability of the various systems.
On-line monitoring techniques are methods that are used to constantly monitor and detect
pipeline failure once it has occurred, namely by detecting leakage. Sensors that are inserted
into or attached to the piping are connected to software systems, so that pipeline parameters
can be continuously monitored and analysed. Monitoring systems vary in sensitivity, accuracy,
and response time, and may have the ability to nd the location of a leak. Periodic inspection
methods, on the other hand, are practices used on an intermittent basis for leak detection or to
evaluate the condition of pipeline systems. These are generally more intensive than monitoring
techniques, and can involve physical inspection using various specialized tools. Inspection
techniques are generally better able to pinpoint the location of a leak than many monitoring
methods, but this may occur long after a failure has occurred because they are not constantly
available. Inspection can also be used for preventative maintenance, so that the condition of a
pipeline can be evaluated before failure occurs.
An overview of current on-line monitoring and periodic inspection methods is provided, which
provides an overview of each method as well as a discussion of the strengths and limitations of
each technique. First, on-line monitoring techniques are discussed, which consist of methods that
allow continuous monitoring of piping systems. Next, periodic inspection methods are discussed,
which include in-line inspection tools and other practices which can be used at intervals to
monitor pipeline degradation or to localize pipeline defects identied via on-line monitoring.
Emerging and future technologies are then presented, followed by a discussion that hopefully
provides insights into the best practices and importance of continually developing methods to
improve pipeline integrity management.
Pipeline Science and Technology Vol.2, No.3 (2018)
2. On-line monitoring
On-line monitoring techniques consist of methods that can be used to monitor piping systems
on a continuous basis without interfering in the operation of the pipe. These methods require
sensors, instrumentation, data acquisition, and analysis systems. They are therefore sensitive
to the calibration and sensitivity of sensors, the integrity of the instrumentation, and the
accuracy of the analytical practices. The following discussion provides an overview of the current
methods that are used for on-line pipeline monitoring. The rst few methods – line ow balance,
pressure change monitoring, and real time transient monitoring are typically classied as
software-based methods since they rely on simple sensors to monitor ow variables (ow rate,
pressure, and/or temperature). The other monitoring methods discussed can also be classied as
hardware-based methods since they require specialized sensing devices.
2.1. Line ow balance
Simple line ow balance utilizes ow meters at the inlet and outlet of pipeline segments
to ensure that the volume or mass of uid into the pipe equals the amount at the outlet [5]. If
there is a variance in the amount of uid entering a segment compared to the amount at the
exit beyond an established threshold, a leak alarm is generated. Simple volume or mass balance
methods are relatively cheap since they require simple sensors, i.e. ow meters, and can achieve
good accuracy in steady-state systems. However, they are inaccurate during transient operation,
cannot detect small leaks, cannot identify the location of leaks, and can be prone to false alarms.
To overcome some of these limitations, advanced versions of line ow balance can be used that
utilize advanced computational tools as well pressure and temperature sensors [10]. However,
this adds to the complexity and cost of an otherwise simplistic and cheap method.
2.2. Pressure change
Pressure gauges are installed at certain intervals along the length of the pipe and are
continuously monitored, usually in conjunction with ow meters and/or temperature sensors.
Pressure along the pipe is monitored and signicant changes in measurements are attributed
to leaks [5]. Statistical analysis can be used to monitor when the pressure measurements drop
below a certain predened threshold. Some pressure analysis techniques have been shown to
be effective for underwater and arctic environments for leak rates less than 0.1 % of the ow
rate [7]. It is also a relatively cheap monitoring technique, since it requires simple sensors and
analytical methods. However, similar to the line ow balance method discussed above, it is not
a reliable technique during transient ow. Also, while it may be able to provide some indication
of the location of a leak, it cannot accurately locate leaks with high precision.
2.3. Real time transient modelling
Real time transient modelling (RTTM) involves computational dynamic modelling of pipeline
ows using uid mechanics equations and data from ow, temperature, and pressure sensors,
to model the ow in a pipeline. The equations to model uid ow (such as conservation of mass,
momentum, and energy, and equation of state) and the data from measurements taken at
certain points are solved using various computational techniques [6]. Discrepancies between
the modelled uid ow and measured parameters is indicative of a leak. Various packages are
available from different suppliers, which can utilize different instrumentation and algorithms
to model the pipeline uid ow and evaluate whether or not there is a leak.
Dynamic modelling generally has a better sensitivity for detecting small leaks compared with
the simple ow monitoring methods discussed above. It also has a greater reliability because
Pipeline Science and Technology Vol.2, No.3 (2018)
it is less likely to falsely signal that there is a leak and is capable of detecting leaks during
operational changes, i.e. for transient ow conditions. Furthermore, it can give an indication of
the location of the leak. However, it is a more costly method because relatively complex software
packages are used in conjunction with measurement instrumentation that is used to collect data
in real time. This also means that trained operators are required for analysis and interpreting
2.4. Acoustic monitoring
Multiple measurement devices are placed along the length of the pipeline. Since noise is
generated as uid escapes due to a leak, the acoustic waves will propagate away from the leak at
a speed associated with the properties of the uid. Acoustic measurements can therefore be used
to detect and localize a leak. Figure 1 shows a schematic of this generalized acoustic monitoring
technique. Several types of sensors can be used, including pressure transducers, accelerometers,
and microphones, which are selected depending on the application [9, 11]. The sensors can be
attached to the pipeline itself, as shown in the gure, or placed outside the pipeline at specied
distances from the pipe and one another. A large number of sensors is required for extended
pipeline systems, which can make this monitoring technique expensive. This method can only
detect leaks that produce acoustic signals signicantly higher than background noise levels,
which limits its application in systems with high environmental noise levels and/or pipes with
high ow rates. Additionally, the sounds associated with operational changes cannot be isolated
from leak sounds, so it can only be used accurately during steady state operation.
2.5. Pressure wave monitoring
As a leak occurs, a rarefaction wave is produced and propagates in both directions along the
pipe. If a pressure transducer is installed on the pipeline on both sides of the leak, i.e. one at
the upstream and one at the downstream end of a pipeline segment, the pressure uctuations
with respect to time can be measured [12]. From this, the onset of a leak and its location can
be determined with good accuracy. This is commonly referred to as negative pressure wave
monitoring because it involves detecting the rarefaction wave that is produced at the onset of a
leak. Figure 2 shows a schematic illustrating the concept of negative pressure wave monitoring.
The response time using this method is very low because it is dependent upon the speed of
sound. However, similar to the general acoustic monitoring discussed above, it is not accurate
during operational changes.
The frequency response method [13] is a technique that involves purposely generating
pressure waves and monitoring them to detect leaks. The periodic opening and closing of a valve
will generate pressure waves, and pressure measurements are transformed from the time to
the frequency domain to determine the frequency response of the system. A system with leaks
will have additional resonant peaks compared with those of a non-leaking system because of the
reections that occur at the leak location(s). These additional resonant peaks are analysed to
determine if there is a leak.
Fig.1. Schematic showing concept of leak detection using
acoustic sensors.
Fig.2. Schematic of negative pressure wave monitoring.
Pipeline Science and Technology Vol.2, No.3 (2018)
2.6. Fibre optic sensing
Optical bres typically consist of a bre core
surrounded by a cladding material, and are
used to transmit light from one end of the bre
to the other. Physical or chemical disturbances
of the bre will alter the characteristics of the
guided light, affecting parameters such as the
amplitude, phase, wavelength, polarization,
modal distribution, or time of ight [14].
These changes are observed at the receiving
end and interpreted to identify issues affecting
the pipeline. Optical bres can therefore be
used to monitor several physical or chemical
properties along a pipeline system. If a bre
optic cable is installed along the length of
pipe, in close proximity to the pipe, it can be
used to detect leaks via temperature changes.
Gas escaping from a pipeline will result in
temperature changes, which affects the signal
in the optical bre. Also, special optical bres have been developed with cladding materials that,
when exposed to hydrocarbons, change their refractive index [7].
Fibre optics are non-metallic and not susceptible to electromagnetic interference. Optical
bres have a greater bandwidth and capacity compared to conventional cables, and have the
capabilities to act as both sensors and signal carriers [15]. There is a high cost associated with
installing bre optic systems, and some uncertainty regarding the robustness of the coating
materials used. Additionally, retrotting bre optic cables to buried pipelines is an intensive
and expensive process.
2.7. Summary
Some key features of the on-line monitoring techniques discussed is presented in Fig.3.
Multiple criteria are considered, including sensitivity, which refers to the ability to detect
Fig.3. Summary of key features of on-line monitoring
Software-based Hardware-based
Line ow
change RTTM
Sensitivity H M H M H H H
Accuracy M M H M H M H
Reliability L L M L M M H
Robustness L L H L L L M
time M M F F F F F
Ease of use H H L M H M M
Type of ow Liq, G Liq,G Liq,G,MP Liq,G Liq,G Liq,G Liq,G,MP
Cost $ $ $$$ $$ $ $$ $$$
Table 1. Comparison of features of different on-line monitoring techniques
Pipeline Science and Technology Vol.2, No.3 (2018)
small leaks. Accuracy refers to the method’s accuracy in leak sizing and location, if applicable.
Reliability considers the technique’s ability to detect a leak when there is one without providing
false alarms in the absence of a leak. Robustness refers to the capability to work under
changing, i.e. transient, ows. The ‘$’ symbols are used to indicate the costs associated with
each technique. More features are summarized in Table 1, which uses abbreviations for low (L),
medium (M), high (H), fast (F), liquid (Liq), gas (G), and multi-phase (MP) to ll the table. Ease
of use considers the level of operator training required for the method; a ‘high’ ease of use means
a ‘low’ level of operator expertise is needed. The type of ow shows whether the method can be
applied to gaseous, liquid, or multiphase ows.
The summary table shows that the simple software-based systems are prone to spurious
alarms and lack the ability to effectively monitoring transient ows. They are also limited to
either gaseous or liquid ows. RTTM, on the other hand, provides an improvement in reliability
and robustness, and advanced models can be applied to multiphase ows [7]. However, this
is the most expensive software-based system. The hardware-based systems consist of acoustic
methods, which are relatively cheap, and more expensive bre optic techniques. These acoustic
systems, especially negative pressure wave sensing, can be highly accurate, however they are
not able to work well in transient conditions or multiphase ows. Optical bres can be used to
overcome these limitations but are expensive and difcult to retrot to existing systems. They
are also still being developed to enhance their monitoring capabilities, which is discussed later
in this paper in the section about emerging technologies.
3. Periodic inspection and monitoring
Although on-line monitoring techniques are always available and can be monitored remotely,
current methods are not capable of detecting and locating leaks with total accuracy. Another
important consideration is that on-line methods can only detect a fault once it has occurred;
they cannot pre-emptively detect pipeline degradation or a fault before failure. Therefore, other
methods are required to supplement the currently available monitoring capabilities. These are
referred to as periodic inspection and monitoring methods in this paper, which comprise of
methods for leak detection and localization and non-destructive testing techniques, such as in-
line inspection. Inspections of the pipeline can be used to accurately detect and locate defects
before pipeline failure occurs. Some of these methods can only be used periodically because they
interfere with pipeline operation, some are too laborious to be constantly applied, while others
are dependent upon environmental factors and therefore are not constantly available.
3.1. Non-technical inspection
Non-technical inspection methods include the methods for pipeline inspection which involve
the physical presence of personnel. Non-technical inspection methods are the relatively basic
approaches to physical pipeline inspection. Trained personnel can patrol pipeline systems to
observe any unusual visual, auditory, or olfactory changes that could indicate a leak, sometimes
with the aid of trained dogs [5, 9]. This method is obviously expensive and depends on the
experience of the individuals involved. Also, it is an inefcient process since it can take a long
time to inspect a relatively short segment of pipe. As with all periodic inspection techniques,
leaks may also develop after an inspection, since this method cannot determine or evaluate
pipeline degradation before a leak occurs.
3.2. Soil monitoring
In addition to observation, soil can be monitored in the environment around buried pipeline
systems. Soil monitoring involves introducing a small amount of a tracer chemical into the
pipeline uid. Any leakage in the piping will result in this chemical being released, which can
Pipeline Science and Technology Vol.2, No.3 (2018)
then be detected using instrumentation that is dragged along the surface of the ground, above
the pipeline. While some literature considers this method for gas systems, it can also be used
for liquid systems [16], since the tracer chemical is meant to evaporate when released in a
liquid medium. This method has a high sensitivity and low rate of false alarms but can be very
expensive because chemicals must constantly be added to the uid. Also, the soil above the
pipeline must have some degree of air permeability so that the tracer vapours from a leak can
be detected.
3.3. Vapour sampling and monitoring
Gas sampling typically involves using a handheld or vehicle-mounted probe to detect methane
or ethane to detect leaks from a natural gas piping system [8]. This method is very sensitive to
small concentrations of gases, allowing small leaks to be detected with good condence. However,
the detection is slow and limited to the area that was sampled from, which means that a leak
can be localized but that monitoring long lengths of pipe is very expensive.
Vapour monitoring systems detect leaks in buried pipelines using a sensor tube that
is installed parallel to the pipeline [6]. This sensor tube is permeable to the pipeline gas,
which means that in the event of a leak from the pipe some of the gas diffuses into the tube.
Periodically, the contents of the tube are pushed past a monitoring sensor, which measures the
gas concentration. At the beginning of each periodic inspection, a test gas is injected at the end
of the tube. The concentration of gas is used to determine the magnitude of the leak, and the
arrival time of the pipe gas compared to the test gas is used to determine the leak location. This
is shown schematically in Fig.4. This method has a very low detection threshold, reportedly 0.5
% of the ow, but is very costly to install. It also cannot be used for above ground or high-depth
3.4. In-line inspection (ILI) tools
In-line inspection (ILI) tools are cylindrical tools that are inserted into the pipeline in order
for non-destructive examination. Figure 5 shows a simplied schematic of a modular ILI tool,
which consists of modules for power supply, data acquisition and storage, and sensors [17]. ILI
tools are also known as ‘pigs’, a term used either because it is an acronym for ‘pipeline inspection
gauge’, or because of the squealing sound that the rst pipeline cleaning tools used to make
during operation. The term has gained such widespread acceptance that it has subsequently
achieved status as a verb – the term ‘pigging’ is now used to describe the use of ILI tools.
These tools are integral for monitoring and classifying defects, welds, and fractures, which is
important for determining remedial measures [18]. This section gives an overview of the kinds
of ILI tools that are commonly used to detect pipeline defects.
3.4.1. Ultrasonic testing
Ultrasonic testing (UT) ILI tools use multiple ultrasound probes that transmit pulses of
Fig.4 (left). Schematic of leak detection and localization
method using a vapour sensing tube.
Fig.5 (above). Schematic of simplied modular ILI tool.
Pipeline Science and Technology Vol.2, No.3 (2018)
acoustic energy and detect the reected
pulses from the pipe walls [19, 20]. UT tools
can be used for wall thickness measurement,
detection of mid-wall aws, and crack
inspection. Probes oriented perpendicular to
the pipe wall are used to directly measure the
wall thickness. This is shown conceptually in
Fig.6. A UT probe transmits an acoustic pulse
towards the pipe wall. Part of the pulse is
reected from the inner pipe wall, while the
remainder enters the wall and is reected at
the outer surface of the pipe. Since the speed
of sound in the uid medium and the pipe wall
are known, the time between the sent pulse
and the subsequent reections can be used to provide quantitative and accurate measures of the
wall thickness. Since this method is able to evaluate for the offset between the sensor and the
wall and the wall thickness itself, i.e. by measuring t1 and t2 as shown in Fig.6, it can distinguish
between metal loss on the inside and the outside of the pipe. Furthermore, it can be used to
detect internal defects such as laminations or inclusions, since these can result in an additional
(a) No metal loss
(b) Internal metal loss
(c) External metal loss
Ultrasonic testing methods require a clean pipe wall because the accumulation of material
on the inside pipe wall can affect measurements. Furthermore, most available UT tools utilize
piezoelectric transducers as actuators and sensors, which can only be used in a liquid medium.
This is because a liquid medium will allow a sufcient amount of the ultrasonic signal to enter
the wall, whereas very little will enter the pipe wall from a gaseous medium. This issue can be
overcome by using electromagnetic acoustic transducers (EMAT) [21], which excite ultrasonic
vibrations at the pipe wall itself, so that both gas and liquid piping systems can be inspected.
Additionally, the applications with UT sensors placed perpendicular to the pipe wall are not
suited for crack detection. For this reason, probes are placed at an angle so that the refracted
acoustic signals inside the pipe wall are sufciently disturbed by the presence of cracks that
may be perpendicular to the pipe wall, as shown in Fig.7. In this conguration, the cracks will
result in reections that can be detected by the probes. ILI tools using ultrasonic technology can
therefore utilize sensors with different orientations so that both wall thickness measurements
and crack inspections can be performed.
(a) No metal loss
) Internal metal loss
) External metal loss
Fig.6. Schematic of ultrasonic testing method for wall
thickness measurement: (a) wall without metal loss, (b)
internal metal loss, and (c) external metal loss.
Fig.7. Schematic of ultrasonic testing method for crack
Pipeline Science and Technology Vol.2, No.3 (2018)
3.4.2. Magnetic ux leakage
Magnetic ux leakage (MFL) tools induce a saturated magnetic eld into the pipe. If there
is no defect, i.e. no missing material, the magnetic ux lines pass through the piping material,
as shown in Fig.8a. However, if there is missing material, the magnetic eld around the defect
becomes distorted; the magnetic ux ‘leaks’ from the pipe wall, as shown in Fig.8b. Sensors are
used to detect the leaked magnetic ux and analysis is performed to determine the size of the
metal loss [22]. The magnets used to induce the magnetic eld in the piping exist in multiple
arrangements, so that defects in different orientations can be detected.
Most MFL tools can determine the axial and radial location of defects in the pipeline. However,
MFL tools cannot directly give a quantitative measure of wall thickness, unlike UT tools. Also,
standard MFL tools are generally only capable of detecting metal loss of at least 20% wall
thickness, although higher resolution options are available at greater cost [21]. Additionally,
this method may result in permanent magnetization of the pipe, which can affect subsequent
measurements using MFL tools and make it difcult to repair piping because high levels of
magnetization can affect welding processes [23].
3.5. Optical inspection
Some research for natural gas leak detection has categorized systems according to optical and
non-optical methods [8]. Using this classication, the methods discussed in this paper to this
point are non-optical, since they rely on non-visual sensors and computing systems to monitor
or evaluate the condition of the pipeline. Optical monitoring, on the other hand, utilizes special
sensors to visually map the pipeline and detect leaks, and consists of active and passive optical
3.5.1. Active optical methods
Active optical methods require a radiation source to illuminate the inspection area. Light
detection and ranging (LIDAR) systems are an active optical technique that typically use a
pulsed laser to illuminate the pipe and its surroundings [8]. The absorption of the laser energy
along the length of the pipeline is monitored to evaluate for possible leaks. This technique
can be used for remote monitoring using aircraft, mounted on moving vehicles, or installed on
location. It can be used to inspect an extended range of piping and is able to visualize leaks
without temperature differences between the gas and surroundings. LIDAR monitoring is very
expensive, requiring expensive laser sources and skilled operators, and cannot be used for
Fig. 8. Schematic of MFL inspection on (a - left) pipe wall without metal loss, and (b - right) pipe wall with metal loss.
Pipeline Science and Technology Vol.2, No.3 (2018)
continuous monitoring. LIDAR systems can
also have signicant numbers of false alarms.
Some of these costs can be reduced by using
diode lasers or low-cost lamps as broadband
sources instead of pulsed lasers, but these
systems are still prone to spurious alarms [9].
3.5.2. Passive optical methods
In contrast to active optical methods,
passive ones do not require a radiation source.
Instead, they utilize the background radiation
or radiation from the leaking gas itself.
Thermal imaging methods detect and analyse
the temperature gradients around pipes to
detect leaks. This can be used from vehicles,
portable systems, or aircraft, and can cover
large distances of piping in relatively small
amounts of time. However, thermal imaging
is ineffective when the temperature of the leaking gas is the same, or close to, the temperature
of the surroundings. Another common passive optical method is multi-spectral imaging, which
utilizes either an absorption mode or an emission mode [8, 9]. In emission mode, it requires the
temperature of the gas to be higher than that of the surrounding air. Absorption mode imaging
utilizes the absorption of background radiation at several wavelengths to create a map of the
gas concentration, and does not require a temperature gradient between the escaping gas and
ambient air. Multi-wavelength imaging is highly accurate and can be used remotely without
constant operator input. However, the sensitive imaging sensors used are very expensive.
3.6. Summary
A summary of key features of periodic inspection and monitoring methods is shown in Fig.9.
Regular operation during inspection is shown to indicate if the pipe can operate uninterrupted
by the inspection/monitoring method. Leak detection and sensitivity refer to the ability to detect
small leaks and the accurate identication of leaks without giving false alarms, respectively.
Condition monitoring considers the techniques that are used to detect and evaluate defects
and metal loss, such as corrosion or cracking. Additional criteria are highlighted in Table 2.
Abbreviations of yes (Y), no (N), slow (S), medium (M), and fast (F) are used along with those
dened for Table 1. Defect and metal loss detection refers to whether or not the technique is
capable of detecting defects and metal loss, such as corrosion. Inspection time considers the time
taken to evaluate or monitor a segment of piping.
By the denitions given in this paper, these methods are only used on periodic bases
for inspections or monitoring. Apart from the in-line inspection tools, these techniques are able
to operate without interfering in pipeline operation. However, ILI tools are the only reliable
method used to evaluate the condition of pipelines, which is helpful in preventing a leakage
event. Condition assessment using ILI can also be used to determine the residual strength,
residual lifetime, and probability of failure [24], and operational measures can be taken to reduce
piping stresses if required. Leak detection using non-technical, soil, and vapour monitoring
is sensitive to small leaks and can identify the location of leaks accurately without spurious
alarms. Also, non-technical and soil monitoring are the only periodic methods that are effective
for multiphase ows. However, these are intensive processes that involve trained personnel,
long inspection times, specialized equipment, and, in the case of vapour monitoring, high up-
front costs for system installation. Optical methods can be applied for above-ground gas systems
and are capable of inspecting long segments of piping systems in relatively small amounts of
Fig.9. Comparison of main features of periodic inspection
and monitoring methods
Pipeline Science and Technology Vol.2, No.3 (2018)
time. However, they are either prone to false alarms or require specialized systems that can be
very costly. Additionally, by their very nature they are sensitive to environmental conditions,
since rain or excessive dust can affect measurements.
4. Emerging technologies
Many of the discussed methods for on-line monitoring and periodic inspection are constantly
being developed to improve their sensitivity, accuracy, and reliability. Updating existing
systems with faster and more accurate sensors, modern servers, and updated algorithms can
overcome the shortcomings of out-of-date systems [25, 26]. Improved versions and variations of
existing systems are always emerging alongside the development of new methods. This section
highlights a few of the developing technologies that show promise for overcoming the current
limitations in pipeline monitoring and inspection practices.
4.1. Fibre optics
Fibre optic technology has already been discussed, however there is ongoing research into
expanding the capabilities of this technology because of its potential. Because of the sensitivity
of bre optics, research is being done to increase the scope of its use in pipeline monitoring. A
bre optic sensor can replace several discrete, or point, sensors, because the cable is sensitive
to changes over its entire length. Moreover, not only can optical bres replace multiple point
sensors, they can also be used to replace different types of sensors, i.e. to measure different
Table 2. Comparison of features of various periodic inspection methods
Soil monitoring
Vapour monitoring
ILI tools
Diode laser ab-
Thermal imaging
Regular operation
during inspection Y Y Y Y Y Y Y N N
Defect and metal
loss detection N N N N N N N Y Y
Inspection time S S M F F F F S S
Leak detection
sensitivity H H H M M M H - -
Leak detection ac-
curacy H H H L L L H - -
Ease of use H H H L L M L M M
Type of ow
Liq, G,
Liq, G,
MP G G G G G Liq,G Liq,G
Cost $ $$$ $$$ $$$ $$ $$ $$$ $$$ $$$
Pipeline Science and Technology Vol.2, No.3 (2018)
parameters. For example, optical bres potentially have the capabilities to monitor changes in
strain, displacement, cracking, vibration, pressure, temperature, and liquid levels in pipeline
systems [15]. The ability to accurately monitor all of these parameters on a continuous basis will
be useful for improving pipeline safety systems. Fibre sensing can be used to monitor up to 60
km of piping with a single instrument, which can be extended to 300 km using optical ampliers
4.2. Eddy current
One of the emerging in-line inspection tool methods is known as eddy current technology.
Use of eddy currents (EC) for ILI can be viewed as an extension of MFL techniques. A magnetic
eld that changes with respect to time is placed near a second conductor, in this case a pipe,
where an electrical current is induced that takes the form of small circular paths, also called
eddies. These eddies create their own magnetic elds, which are then measured and analysed
to evaluate for material aws such as cracks [28]. Eddy current ILI tools are currently being
developed, and can overcome some of the limitations of traditional UT and MFL tools. EC tools
can be used in liquid or gas applications, and are especially well-suited for high speed gas lines
[29]. These tools are physically similar to other ILI tools, so that once they are developed and
become commercially available they can integrate into the modular design of current in-line
inspection devices. The currently identied disadvantage of EC methods are that they are
sensitive to coupling variations, which are dependent upon the lift off distance between the pipe
wall and the tool [21].
4.3. Articial neural networks
Articial neural networks (ANNs), as the name implies, are computing systems that are
inspired by biological neural networks. These systems ‘learn’ tasks via training examples,
developing outputs based on their own developed set of characteristics, i.e. without any a
priori knowledge of relevant characteristics. For pipeline monitoring, this can involve training
an ANN with examples of pipeline data (measurements of ow, pressure, temperature, etc.)
for systems with and without leaks. The system will process and analyse the data, creating
criteria which can then be used on other systems, in real time, to determine whether or not
the system parameters are indicative of a leak. ANNs have been developed that are based on
process variables routinely measured during pipeline operation, which allow these systems to be
used with basic hardware and incorporated into existing systems without the need for physical
intrusion on the piping. While ANNs are sensitive to the training received and noise levels of the
sensor signals, developed models have shown promising results. Some literature has reported
the ability of ANN-based systems to detect leaks as small as 1% of the ow rate without false
alarms [7, 30]. These systems also reported good accuracy under transient conditions and for
the location of leaks which were 5 % or more of the ow rate. Additionally, ANNs can be used for
other aspects of piping integrity management, such as condition prediction models that predict
the deterioration of pipeline systems [31]. These models consider data from piping systems such
as diameter, age, cathodic protection monitoring, and results from MFL inspections to train the
ANN system.
5. Discussion
The selection and application of various monitoring and inspection methods for a given
pipeline system is based on a compromise between their strengths and limitations. Leak detection
systems vary in sensitivity, accuracy, reliability, robustness, response time, applicability, and
cost. Moreover, the level of operator expertise, application to various ow types, and use in
different pipeline environments can severely limit the techniques that are available for a
certain system. For example, the systems that are available for multiphase ow are very costly,
Pipeline Science and Technology Vol.2, No.3 (2018)
with the exception of non-technical inspection, which is limited to readily accessible pipelines
and very time consuming. Multiphase ow exists in crude oil gathering lines, where oil and
gas are simultaneously extracted or where water ooding is used for extraction. Multiphase
mixtures are common in deep-water, subsea, and arctic pipelines, which transport unprocessed
mixtures of oil, water, and natural gas [7], which makes monitoring multiphase pipelines a
crucial aspect of pipeline integrity management. Another example of limitations of current
techniques include the sensitivity and calibration of instrumentation. The sensors that are
needed for the various techniques are designed for certain operating conditions and require
calibration at certain intervals. Sensors and communications systems can be adversely affected
by harsh environments, and specialized, robust instrumentation is expensive. Additionally,
some methods are limited to pipeline location. Optical techniques are restricted to above-ground
gas pipelines because of their mechanisms of operation. Similarly, vapour monitoring can only
be applied to buried pipes.
In-line inspection tools provide a method for evaluating pipeline system conditions so that
measures can be taken before failure occurs. The detection of defects and the evaluation of
corrosion from ILI tools can be used to change the operating points of piping systems, to ensure
that operation does not exceed the limits imposed by degradation. ILI data can also be used in
conjunction with corrosion growth rate models [32] and other analytical tools to predict pipeline
lifetime and the potential remedial measures that should be taken. Recently developed tools offer
high resolution sensors and can utilize both ultrasonic and magnetic ux leakage technologies.
While ILI tools are essential for non-destructive testing of pipes, they are intrusive, costly and
slow. Furthermore, the accuracy of the results is dependent upon the types of sensors and their
sensitivity, as well as the analytical techniques employed to interpret the data.
Current technologies for constant monitoring and periodic inspections can be used to
considerably reduce the risk of pipeline failure. This depends on the available techniques that
can be used for specic applications, such as the type of uid(s) being conveyed, the physical
conguration of the pipeline (buried, above ground, offshore), and so on. It will also depend
upon the combination of the methods employed. For example, use of multiple on-line monitoring
methods, i.e. redundancy, can be used in conjunction with periodic inspections using in-line
inspection tools and optical methods. This combines the ability to accurately and quickly identify
leaks with the periodic assessment of pipeline condition, so that defects can be identied and
dealt with before failure. Moreover, the emerging technologies for sensing, in-line inspection, and
data analysis will undoubtedly improve pipeline monitoring and inspection capabilities. While
the use of many of the techniques discussed is expensive, and the total costs are compounded
when multiple systems are employed, the mitigation of pipeline events curtails the multitude of
costs of a failure that goes undetected.
6. Conclusion
A review of the currently available techniques for pipeline leak detection/integrity
management has been presented according to the availability of the method, i.e. whether it is
available for continuous monitoring or can only be used on a periodic basis. This was done to
provide insights into the applicability of the various technologies, which can be used to help
evaluate the methods that should be used for a given pipeline system. Some of the emerging
techniques for monitoring and inspection were also presented, which show that various sensors,
tools, and software-based systems are being developed to overcome some of the limitations of
current practices. Since several pipeline congurations exist, which, for example, vary in size,
physical location, conveyed uid(s), accessibility, and environmental conditions, there is no one-
size-ts-all approach to pipeline leak detection or inspection. Therefore, each pipeline system
should be evaluated given its unique operating conditions, and a combination of robust design,
preventative measures, predictive models, on-line monitoring, and periodic inspection should be
used to mitigate the risks associated with pipeline failure.
Pipeline Science and Technology Vol.2, No.3 (2018)
Conicts of interest
All authors have no conicts of interest to declare.
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