is your pipeline clean enough for in-line inspection?
TRANSCRIPT
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Oil and gas pipelines represent a major national and
international asset. Globally there are in excess of 2 million
kilometres in production, and the UK Continental Shelf (UKCS)
oshore network alone comprises some 14,000 km. The average
age of the main export pipelines, representing 63 per cent of the
entire network, is 17.2 years, compared with typical 25-year design
lifetimes, with operators looking to extend beyond this. As a
result proving the integrity and monitoring the condition of these
assets is of vital importance if they are to stay in service. To do
this, pipeline operators primarily use inspection tools (intelligent
pigs) to provide data on the condition of the pipe material.However, for these to work eectively they require the lines to
be clean to ensure good sensor contact with the metal surface.
It typically costs $US400,000+ to survey a pipeline in this way,
with some oshore lines costing over $US2 million to inspect.
However, all too often these surveys are severely compromised
because of the poor quality data that is captured caused by the
pipeline being dirty.
Pipeline operators contract specialist pipeline cleaning
companies to prepare and clean their lines prior to running
inspection tools. This typically involves running a variety of
cleaning pigs to remove debris, such as waxes and sands. At
present there are no measurement tools available to pipeline
cleaning companies that enable them to measure how eectivethey have been and whether they have conditioned the line to
the standard required. Those tools that do exist (data loggers and
calipers) cannot measure the residual debris with a sucient
resolution to provide this information. As such judgement
decisions are made regarding when a pipeline is deemed clean.
These decisions are frequently wrong, resulting in very expensive
failed inspection runs or more worryingly inaccurate data
resulting in lines either being allowed to continue to operate
when potentially unsafe to do so, or having to be down-rated
when this isnt actually necessary.
Pipeline Engineering (PE) recognised that there was a need for
an inspection tool capable of measuring residual debris deposits.If this could be included within the companys existing cleaning
service, it would be possible to measure the eectiveness of a
cleaning job whilst in progress. To achieve this, PE proposed
using intelligent calipers as the base technology for the product.
These combine proximity sensors to directly measure sensor
lift-o with a more traditional multi-channel caliper to measure
residual pipe bore.
It was this concept that has resulted in the PECAT the pipeline
cleanliness assessment tool the development of which isdiscussed in this article.
BackgroundThe rst step required in the design of a practical cleanliness-
measuring pipeline inspection tool is the identication of
suitable instrumentation to make the measurement. The outline
conceptual design provided parameters that potential solutions
would have to satisfy, namely that the pig should have the ability
to measure the build-up of wax and scale on the inside of the
pipeline. The obvious route to achieve this is with something
that would have similar dynamics to existing in-line inspection
detector ngers. It was felt to be neither necessary nor desirablefor these to replicate the functionality of such mechanisms,
as in order to keep the build cost of the system within targets,
individual arms need to be relatively inexpensive.
Intelligent inspection plays a key role in the continuing monitoring of pipeline integrity. It is widely recognised thatone of the main causes of the failure of such surveys is the unexpected presence of unacceptable quantities of debris
in the line being surveyed, leading to a degradation of data quality. As a result, the design of a suitable pre-cleaningprogramme, and the reliable assessment of its results, plays an important role in the overall success of the inspectionprocess. However, since techniques for assessing pipeline cleanliness have remained relatively primitive, it is generallyrequired to design these pre-inspection cleaning programmes in a conservative way, for example by aiming to clean untilsatised with the volume of debris produced with a pigging tool. This extends the timescale for the operation, whilenevertheless all too frequently failing to prevent surveys which deliver data compromised by the presence of largequantities of debris.
Recent advances in intelligent calipering technology have resulted in improvements in the tools used to carry out theseassessments, allowing for far greater accuracy when measuring the depth of debris present on the pipe wall. This paperlooks at the techniques currently adopted for assessing the cleanliness of pipelines and in particular describes the useof Hall Effect based sensor technology, congured to detect proximity to the pipe wall. The paper considers how thishas been used in a tool with a suitable arrangement of pig mounted sensors which has the capability to measure the
circumferential prole of debris to an accuracy of +/-0.5mm.
Is your pipeline clean enough for in-line inspection?By David Russell and Gordon Short, Pipeline Engineering, Catterick, UK
Figure 1: Proof-of-concept sensor testing.
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Possible solutions that suggested themselves at this stage included:
Some type of non-contacting measurement, possibly
ultrasonic in nature;
Magnetic proximity sensors (with various technologies
possible here); and,
High resolution calipering.
The choice between these represents a choice between two
competing philosophies on making the measurement. Firstly,
using a proximity measurement (i.e. measuring the sensor
lift-o from the metal of the pipe wall) is a local measurement,
which should be relatively accurate. The drawback is that no
information is then provided about dents and other out-of-
roundness features. On the other hand, an adaptation of a
callipering solution would measure only the internal diameter.
Accuracy in this type of system is likely to be limited by the
relatively loose tolerances on the internal diameter of API
specied linepipe.
As far as magnetic sensors are concerned, this technology has
formed the basis of magnetic ux leakage (MFL) tools for manyyears, and its operation in pipelines is well understood. Ironically,
the fact that MFL survey results are compromised by the presence
of wax and other debris in the pipeline, was a good indication
that the instrumentation is sensitive to the debris. The challenge
was to nd a conguration that would allow the amount to be
accurately quantied. MFL systems generally use measurements
of axial ux to detect defects; however, the radial ux can be
shown to be strongly dependent on the distance to magnetic pipe
material.
A set of proof-of-concept tests was performed, both to conrm
that the Hall-eect sensor idea was sound, and to select the best
combination of sensor and magnet. This work demonstrated
that the proposed sensor concept was feasible, and that a system
could be assembled from readily available components. The
required performance was clearly demonstrated over the range
of interest. Figure 1 shows the experimental set-up, and Figure 2
shows the sensitivity demonstrated over the required range.
With the Hall element mounted on a dummy sensor sledge and
a magnet separation of approximately 10 mm, clear sub-millimetre
resolution over the rst 20 mm of lift-o was readily obtained.
Tool specicationThe rst action in the full engineering programme of work was
the preparation of a detailed specication for a prototype tool,
intended to be suitable for both testing and eld-trial purposes.
The specication falls into four major areas, each of which wasconsidered separately, before a nal specication was prepared
that was felt to be realistically achievable, while meeting the
previously identied market needs.
Sensor systemThe debris-measurement system was specied to have a
maximum angular separation between sensor arms of 15, with an
Figure 2: Proof-of-concept results for Hall sensor.
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accuracy of measurement of +/-0.5 mm over a range of 020 mm.
Measurement resolution was specied to be 0.1 mm. Separationvalues were based on an understanding of the number of arms
required to give a good picture of the circumferential prole of
any debris in the pipeline, tempered against the practicalities of
how many arms could physically t around the circumference of
the tool. The accuracy of measurement is a compromise between
the desire to provide as accurate a measurement as possible, and
the realistic expectations raised by the preliminary study.
Readings are to be taken every 5 mm, which is in line with high-
specication caliper pigs and other intelligent inspection systems.
A fall-back time-based data acquisition system was specied
in order to ensure that data was captured even if the odometer
system suered complete failure.After preliminary consideration of the likely nature of the
mechanical design, the decision was taken to include caliper
measurements in the system specication given that the tool
was going to include wall-contacting arms, the extra engineering
to include a measurement of the angle between the pig body
and the arms was thought to be worthwhile, particularly as it
can provide a conrmation of the prole of any debris on the
interior of the pipe wall. The specication of measurement for the
caliper system was prepared so as to be consistent with the debris
measurement system.
The decision having been taken to construct the initial
demonstrator tool at 10 inch, the range of arm movement to be
measured was specied as 75137 mm, with other values identical
to the oset sensors.
It has to be accepted that a practical system cannot currently be
engineered that is capable of making an accurate measurement
of the total volume of debris in a pipeline, as the integration of
values that may be in error by even 0.5 mm over the length of a
pipeline will lead to very wide error bands on any such estimate.
Mechanical system
The mechanical carrier system was required to be capable ofpassing through a 1.5D bend at maximum wall tolerance limit,
and of passing an 80 per cent reduction in bore.
ElectronicRange was to be a minimum of 100 km in distance-driven mode,
and 24 hours in time-based mode, powered by a rechargeable
battery. The system was to be capable of acquiring data at the
specied 5 mm interval at speeds up to 7 m per second, equivalent
to 1,400 Hz. Given the importance of this aspect of the design, it
was also recognised that sucient modularity to allow ranges to
be extended should form part of the specication.
EnvironmentalThe initial specied environmental conditions were a maximum
operating pressure of 150 bar, and a temperature range from 0 to
Figure 3: Polyurethane spring under test at 70Celsius.
Figure 4: Polyurethane spring load characteristics.
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80Celsius. The tool was to be designed to operate in both liquid
and gas, and to be able to resist the aggressive uids to be found
in hydrocarbon pipelines.
As had been identied in the market study, the tool had to bedesigned to meet the requirements of Atmosphere Explosibles
(ATEX). These requirements form a set of guidelines intended
to provide a uniform set of safety standards for the operation
of electronic equipment in explosive atmospheres across the
European Union. New tools intended for operation in the oshore
environment must meet these guidelines, both in design and
manufacture.
Originally introduced on an advisory basis in the mid-1990s,
ATEX is now compulsory for new products in the European
Union. Some ambiguity exists regarding whether the in-pipe
environment is covered by the regulations, although it is
clear that the area around the trap during launch and receive
operations is. On the other hand, pipeline inspection tools are not
designed to operate in this area. Given the dierences of opinion
that exist, it was considered prudent to engineer PECAT to meetATEX requirements.
While ATEX remains a purely European issue, International
Electrotechnical Commission Explosive (IECEx) certication
constitutes a direct equivalent with a growing global relevance.
The expectation is that a global standard will emerge over the
next few years.
For a system of the type under consideration here, consisting of
multiple powered sensors mounted external to a pressure vessel
containing a power source, and logging electronics, the obvious
design constraint introduced here is the necessity of avoiding the
existence of any eective source of ignition. This needs to cover
Figure 5: 10 inch PECAT tool. Figure 6: Test spool in line.
Figure 7: Preliminary results from test spool.
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not only the obvious risks arising from failures of electronics or
batteries, but also the avoidance of the possibility of mechanicalstriking of sparks, and electrostatic discharge. This causes some
slight restriction in choices of materials. Achieving ATEX or
equivalent consists of three activities:
Design to avoid sources of ignition;
Third-party testing to demonstrate compliance with
applicable standards; and,
Quality assurance to demonstrate that manufactured tools
are complaint with the design.
Following the achievement of being certied by an external
body, the tool needs to carry appropriate markings.
In the present case a route combining explosion proong of
the pressure vessel with intrinsic safety in the sensor design
has been pursued. The PECAT system has been designed to becompliant with ATEX from the beginning, and formal certication
is currently in progress.
Prototype build
Relatively few variations from this original specication wererequired in light of the design engineering process. A few minor
changes were made, principally the following:
Time above 50C limited to a few hours; and,
Passage of tool through 80 per cent crash bore limited to be
at a 3D (rather than 1.5D) bend.
The suspension system for the arms was designed using
polyurethane springs, rather than conventional springs. These
springs in a range of dierent hardnesses were extensively
tested in order to ensure that concerns regarding durability and
performance at elevated temperatures were adequately addressed.
In order to obtain reproducible results, a test jig was produced
consisting of an aluminium channel, and angle sections, and was
used to contain the prototype sensor assembly. The bottom of thesensor assembly was mounted on two rollers so that there was
no restraint on horizontal movement as the parallelogram was
compressed. The base of the jig was independently mounted to
allow free movement with respect to the top. The whole jig could
be mounted in a hot-water bath in order to perform testing at
elevated temperatures. Both loading and unloading curves were
generated.
Figure 3 shows one of the springs under load at 70C, and
Figure 4 shows the small variation in the load deection
characteristic compared to that at ambient conditions. The utility
of the design lies in designing the arms system so that the spring
lies on the at part of its characteristic throughout its expectedrange of compression.
Design and construction of the 10 inch tool uncovered relatively
few unexpected issues. The requirement to pass 1.5D bends, and
Figure 8: Petrofac test loop.
Table 1: Petrofac loop specication.
Parameter Unit10 inchpipeline
OD m 0.273
WT m 0.0078
ID m 0.2574
Length of pipeline m 1000
Unit volume cubic metre/
metre
0.052
Total pipeline volume cubic metre 52.035
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80 per cent restriction led to a dual module design being required.
Figure 5 shows the demonstrator tool.
Pig testing
Test facilityIn order to test the tool, a simple pipe spool was manufactured,
replicating the type of feature the tool is designed to measure
(Figure 6). A debris coating was simulated by bonding a series of
polyurethane inserts into a pair of short anged pipe sections.
Each insert is approximately 50 mm in length, and a total of
four such inserts were used, with thicknesses of 5, 10, 15 and
20 mm. A series of dents were created in a further short section
of heavy wall pipe. These ranged in size from around 2 per cent ofinner diameter (approximately 5 mm intrusion), up to 25 per cent
(approximately 60 mm intrusion). This was intended to test both
the ability of the tool to detect and size relatively small features,
and to test its ability to transit relatively large features while still
making useful measurements.
This spool was initially used in small-scale pigging trials to
conrm functionality of the tool. Figure 7 shows some preliminary
(uncalibrated) results from these tests, which were intended only
to demonstrate the basic functionality of the tool.
Full testing was performed at a facility belonging to Petrofac at
Montrose, UK, with the test spool described above inserted into
a 10 inch, 1 km long test loop. Table 1 gives the overall test loop
specication, while Figure 8 shows the layout. Performance of thePECAT tool was veried at speeds from 0.25 m per second up to
1.4 m per second. The loop contains a short elevated section, and
20 5D bends, and can be operated at pressures up to 7 bar, and a
maximum ow rate of approximately 350 cubic metres per hour.
Test programmeThe main objects of this testing were twofold. Firstly it
was desired to prove the mechanical robustness of the tool,
and particularly of the sensor system, in a relatively onerous
service. The second objective was to generate sucient data to
demonstrate that the accuracy and repeatability of data generated
by the tool met the required specication.
In order to achieve this, tests were repeated for a range of
velocities from 0.25 up to 1.5 m per second. Table 2 summarises
the tests performed. Tests were repeated a number of times, in
order to give a total of 15 km of data.
Test resultsInitial results were considered to be promising. The tool arms
coped well with both the general pigging duty through the pipe
and round the 5D bends, and the more dicult transit past the
larger features in the test spool. Over the course of the testing
only minor wear was noted to the contacting parts of the sensor
arms, and both the polyurethane springs and the fabric hinges
used to retain the sensor heads showed no signs of damage.
With the exception of a single run where user-error caused
a minor issue, the logger unit was successful in recording data
throughout the course of the test work. The raw data generated
in the test spool shows good agreement throughout with theexpected readings with the polyurethane inserts clearly visible in
all data sets.
Figure 10: Typical sensor data in polyurethane inserts.
Speed (metres per second)Flowrate
(cubic metres per hour)Volume
(US bbl/d)Time to pig 1 km
0.27 50 7552 62 min 27 secs
0.48 90 13593 34 min 41 secs
0.64 120 18125 26 min 01 sec
0.75 140 21145 22 min 18 secs
1.06 198 29906 15 min 46 secs
1.43 268 40478 11 min 39 secs
Table 2: Pig loop test matrix.
Figure 9: Typical sensor data at dent.
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Figure 9 shows a typical raw data trace for one arm for the
spool with dents, both oset and caliper data. It can be noted how
the oset data shows a peak to either side of the dent, indicative
of the sensing arm lifting up as it crosses the peak of this dent.
The caliper reading indicates where the arm is pressed in, with
the oset sensor showing a slight peak on either side where its
sled is lifted o the pipe metal.
By contrast in Figure 10, which shows the coated spools for
both caliper and sensor, the data pattern is similar for the two
sensors, as the arm is pressed in, and the polyurethane inserts liftthe oset sensor in the arm away from the metal.
Applying a preliminary calibration to the oset sensors allows
the data to be presented in a more intuitive format. Figure 11
shows the readings for all 28 arms, while Figure 12 shows readings
for all 28 arms combined into a single image, with the5 mm steps clearly visible.
ConclusionsHaving identied a requirement for a tool capable of
quantifying the amount of debris left in pipeline prior to
performing an intelligent inspection, PE has built and tested a
tool designed to be capable of measuring up to 20 mm of wax,
scale, or similar material on the inside of a pipe. Test work
on a 10 inch prototype tool has demonstrated that the PECAT
tool oers a realistic prospect of providing a genuine solution
to the technical challenge. Engineered solutions have already
been prepared for larger pig sizes, and the tool is currently
undergoing eld trials.
AcknowledgementsPipeline Engineering acknowledges the assistance of Yorkshire
Forward during the course of this project.
Figure 12: Integrated data for polyurethane insert spools.
This paper was presented at the 22nd International Pipeline
Pigging & Integrity Management (PPIM) Conference
organised by Tiratsoo Technical and Clarion Technical
Conferences and held in Houston in
February 2010. For more information on future PPIM
conferences visit www.clarion.org
Figure 11: Oset sensor data in test spool.