is your pipeline clean enough for in-line inspection?

7/27/2019 Is your pipeline clean enough for in-line inspection?

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6 PiPelines international digest | aPril 2010

technical

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.

is your pipeline clean enough for in-line inspection?

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