High-quality Smart-pig Inspection of Dents - ROSEN Inspection ...

High-quality Smart-pig Inspection of Dents, Compliant with the
US Code of Federal Regulations
a report by
Thomas Beuker and Dr Florian Rahe
ROSEN Group
The US Code of Federal Regulations (CFR) for
Transportation of Gas and Liquids is very specific
about the minimum geometry required for the
assessment of the size of dents affecting the integrity
of a pipeline. This has changed the operators’
viewpoint on in-line geometry surveys from one of
asset operation and monitoring to one of utilising
them as a basis for a defect remediation process.
Based on 20 years of experience with in-line
inspection (ILI) technology, ROSEN has developed
a novel technology for the characterisation and
sizing of dents that will be presented in this report.
The high-resolution ILI technology combines the
advantage of a touchless electronic measuring system
with the advantages of the well-established caliper
arm tools. The advantage of the touchless system is
its applicability under highly dynamic operational
loads, while the mechanical system provides highly
accurate results under static conditions.
Utilising dynamic compensation technology, a
sufficient accuracy can be provided under
demanding operational conditions. Furthermore,
the ‘mecha tronic’ concept improves the sizing
capabilities for dents due to its insensitivity to scale
or wax debris.
Integrity Management of Pipelines
Integrity management of pipelines in the US is
regulated by federal codes for both liquid and gas
pipelines. In particular, an updated code for gas
pipelines became effective in January 2005. Prior to
this, integrity management of gas pipelines was
incorporated only by reference, e.g. through code
B31.8 from the American Society of Mechanical
Engineers (ASME).
Both rules contain strict prescriptive integrity
management provisions for the pipeline operator.
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These prescriptions define limits for pipeline
geometric anomalies, such as mechanical damage
and dents. For example, the minimum
requirements for the sizing of dents are prescribed:
a high-resolution geometry tool must detect and
size dents with a depth ≥ 0.25 inches (6.35mm).
Additionally, a process for inspecting against
provisions of a management based rule, rather than
inspecting for compliance with a purely
prescriptive rule, is encouraged 1 .
The discussion in recent literature about possible
failure modes of mechanical damage in pipelines is
unanimous: anything but a plain dent must be
analysed very carefully with relevant expertise.
Latest investigations confirm that the dependency of
failure pressure is from the dent shape rather than
the dent depth. 2,3 This is also reflected in the CFR,
recommending engineering analyses wherever they
are needed. A stress riser such as corrosion, gouges
or cracks within dents or between dents, rerounding
of unconstrained dents or shape and
sharpness of dents need to be considered within the
appropriate assessment method. While the failure
assessment process is not prescribed in the federal
rules, the (integrity) management measures and
management consequences are.
High Quality
Technology & Services Section
The ‘classic’ geometry inspection for ovalities and
large deformations does not provide the required
information for a dent assessment as considered
previously. Furthermore, an adequate detection,
characterisation and parameterisation of all anomalies
found will require a higher effort of evaluation than
typically spent or requested. This is also indicated in
a recent study by Olson 4 , who compared the results
of 78 excavations with the corresponding data of the
geometry tools. The probability of detection (POD)
of dents in this case was found to be 32%.
1. Gerard S, “New directions in federal pipeline safety program promote continuous improvement”, OGJ, (2005); Vol 103.2:
pp. 52–55.
2. Dinnovitzer, et al., Geometric dent characterization, Proc. of IPC’02 -27076, (Oct.2002).
3. Leis B, et al., Integrity analysis for dents in pipelines, Proc. of IPC 2004 -0061, (Oct. 2004).
4. Olson M, “Detecting mechanical damage”, PipeLine and Gas Technology (2004); pp. 26–27.
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Technology & Services Section
Figure 1: High-quality Geometry Unit (XGP) in Combination with Different
High-resolution Metal Loss Inspection and Crack Inspection Tools
From left to right: ECD+XGP, AFD+XGP, CDP+XGP and XGP.
Introducing a high-quality in-line inspection process
for dents and mechanical damage can provide the
basis information, such as the geometric data of dents
and stress risers, to start a managed integrity process
for the inspected line. The more high-resolution and
high-quality information that is available about the
anomaly found, the better the subsequent failure
analyses can be.
It is important not only to size and describe the
dents with high accuracy, but also to detect and
characterise the mentioned stress risers. State-of-theart
equipment for characterisation of the stress riser
are high-resolution ILI tools based on the magnetic
flux leakage principle (MFL), circumferential MFL
tools or the recently developed Electro Magnetic
Acoustic Transducer (EMAT) Crack Detection
(ECD) tool based on electromagnetic coupled
ultrasound. In addition, or more preferable in
combination with one of those tools, a highresolution
Geometry Pig (XGP) rendering the pipe
shell in space must be used (see Figure 1).
Circumferential Resolution and
Coverage
The performance of a geometry inspection
configuration for dents can be estimated using an
analytical model. According to the model the
sizing accuracy and the POD for dents can be
determined as a function of the circumferential
resolution and the coverage of the sensing area of
the caliper sensors.
The decisive parameter for the POD is mainly the
coverage of the caliper sensor in circumferential
direction. A geometry tool like the established
ROSEN Electronic Geometry Pig (EGP) with a
circumferential coverage of 100% would perform
with an analytical POD of one for the three types of
dents. The 100% coverage is achieved with the
touchless operating sensor unit. Another example is
the simple caliper tool, equipped with 12 caliper
arms and a typical coverage of 55%. The POD in this
case is reduced to 0.75 for a sharp dent of 0.32 inches
(8.1mm) depth (2% improved detection). Again, this
indicates the importance of coverage.
Another effect that can be studied is the systematical
undersizing of dents as a function of circumferential
resolution and coverage. Assuming that a dent is not
hit at the tip by a caliper or scanning sensor, the
maximum possible undersizing of the dent depth can
be calculated for a geometry inspection configuration.
The maximum possible undersizing of the dent, in
comparison with the real depth, is plotted versus the
circumferential coverage for a different number of
caliper arms. It can be seen that an increase of caliper
arms improves the sizing difference. Nevertheless,
coverage close to 100% is the strongest parameter to
achieve an accurate depth measurement.
The maximum coverage a geometry inspection tool
with a single plane of caliper sensors will achieve is
close to the bore reduction a tool can negotiate,
typically reduced by approximately 15% (due to
required mechanical tolerances). Therefore, a
typical value of 75% passage leads to a maximum
coverage of 60%.
The circumferential resolution determines the
parameterisation capabilities for geometric
anomalies. Recent studies did show that a ‘dent
acuity’ of 0.1d/w must be resolved (acuity=2d/w,
where d=dent depth and w=dent width) 1 .
Including the previously mentioned depth
threshold of 0.25 inches (6.35mm), a dent of 5
inches (127mm) wide has to be resolved.
Based on these theoretical considerations, a geometry
inspection configuration is desired with an acceptable
total circumferential resolution e.g. < 2 inches
(50.8mm) and caliper sensors arranged in two axially
separated planes governing a possible circumferential
coverage of more than 95%.
Mechatronic Principle
Other issues, beside the requirements derived from
the preceding analytical considerations and the
regulatory demands highlighted, have to be
considered for the conceptual design of a geometry
inspection tool.
A disadvantage of the traditional mechanical caliper
tool design, where the mechanical movement of the
caliper is transformed into a position signal, is the
dynamic behaviour of the arms under ‘run’
conditions. This typically leads to inspection speed
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estrictions. Above a critical tool speed, the caliper
arm starts to lose continuous contact with the
internal surface of the pipeline. However, at low
speeds, abrupt changes at the internal pipe surface
may not be monitored correctly. Pure mechanical
designs that try to overcome these problems have to
be lightweight and, hence, are quite fragile.
Therefore, these systems do not extend the
operational range of this inspection task.
A solution for this problem is provided by touchless
measurement technology. To achieve a high
measurement accuracy and a satisfying circumferential
resolution, a mechanic caliper arm system
equipped with a sensor to transform the mechanical
movement into an electric signal and an electronic
distance measurement were combined to create a
mechatronic solution (see Figure 2). The picture
shows the concept of having a touchless electronic
sensor integrated inside the sensor head and a
position sensor attached at the bottom monitoring
the mechanical position of the sensor arm. The
touchless electronic sensor is used to compensate for
data obtained from the dynamic behaviour of the
caliper arm. The unwanted inertia of the caliper arm
is fully assimilated by the touchless measurement.
Sharp transitions at the internal surface, such as a
pipe misalignment at a girth weld, are monitored
well. Since the electronic sensor is insensitive to
non-conductive material, the compensation method
refers always to the metal internal surface of the
pipeline. Scale or wax debris, although detected by
the system, will not affect the geometry evaluation
of the pipeline. A single mechatronic unit is
designed to cope with a tool-speed of up to
11.2mph (5m/s) and to provide an accuracy of 0.02
inches (0.5mm) for radial measurements.
Tool Design
Due to the linear equation connecting the pipeline
diameter with its circumferential perimeter, the
coverage of a single unit tool is linked to the internal
diameter (ID)-passage of the unit. As a rule of thumb,
the coverage for a single plane geometry tool equals
the specified passage minus 15%. Thus a typical
caliper tool with a passage of 75% would cover only
60% of the internal pipe surface.
Therefore, an accurate sizing of dents or other
geometric properties in a pipeline requires 100%
coverage in circumference by the geometry sensors.
To achieve this, the inspection solution must consist
of two inspection planes, where the trailing sensor
covers the gap of the preceding unit. The tool
concept for the ROSEN high-resolution XGP
consists of two sensor units connected to each other
and as a result guarantees 100% coverage of the
internal surface.
High-Quality Smart-Pig Inspection of Dents, Compliant with US CFR
BUSINESS BRIEFING: OIL & GAS PROCESSING REVIEW 2005
Figure 2: XGP Mechatronic Caliper Sensor
Figure 3: XGP Tool and Data Sample Obtained on
Test Joints Containing a Dent and a Wrinkle Bend
To allow a detailed evaluation and characterisation of
the geometric anomalies the circumferential
resolution was set to < 1.2 inches (30mm). This
exceeds the requirements for the lateral
characterisation of a dent as typically required.
Furthermore, caliper arms providing the discussed
accuracy are fragile. In turn, the presented
mechatronic concept, which compensates for the
inertia of the mechanical caliper arm, allows a more
robust design of the caliper arm improving the
survey conditions under which the inspection
system can be operated. The XGP is equipped with
these ‘heavy duty’ caliper arms (see Figure 3).
The XGP unit is also equipped with a Gyro system
measuring three dimensional (3-D) co-ordinate
system (XYZ) that maps an accurate route of the
pipeline and can help to determine bend radii, bend
length and bend direction. The XYZ information is
the basis for a stress/strain analysis on a larger scale,
which in turn can be part of the remaining lifeassessment
procedure carried out for local anomalies.
Geometry inspection is advancing into the high-
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Technology & Services Section
resolution technologies. The assessment of a dent
requires all available information characterising
potential stress risers in the vicinity of the dent.
Therefore it can be useful, from an operational pointof-view,
to combine the XGP unit to another
inspection technology (such as a corrosion survey)
that provides this information (see Figure 1).
Conclusion
The demand for reliable dent detection and sizing
has changed the scope of existing geometry
inspection services with regards to the inspection
technology used, as well as the evaluation process
of the data. Geometry inspection has entered the
‘high-quality’ inspection market. Analytical
considerations emphasise the importance of both
coverage and resolution in the circumferential
direction of the pipe perimeter. Measurement of
the mechanical position of the caliper arm in
combination with a touchless distance
measurement provides better accuracy independent
from the caliper’s inertia and a wider operational
range of the system. ■
BUSINESS BRIEFING: OIL & GAS PROCESSING REVIEW 2005