DNV GL © 2013 SAFER, SMARTER, GREENER DNV GL © 2013

Development of Over-Line Survey Systems for Monitoring Pipelines within an Alternating Electric Field

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23rd Colloquium SAFETY AND RELIABILITY OF GAS PIPELINES - PRAGUE

Day 2, 15 May 2014

David Simmonds and Sam Orton, DNV GL Software

DNV GL © 2013

Abstract

The impact of AC (alternating current) induced corrosion is well known in the

industry. This presentation will provide initial findings from research, development

and field trials undertaken by DNV GL and National Grid to provide an over-line

AC survey system for the initial identification of areas where the levels of AC

interference on pipelines may require mitigating action.

The over-line survey system encompasses:

– measurement hardware and peripherals,

– field operating procedures, and

– survey software incorporating mathematical models.

The models improve the accuracy of AC current density measurements, which

otherwise will be under-reported unless the effects of AC interference on the

measurement equipment are taken into account.

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Acknowledgements

This project is funded and supported by National Grid’s UK Transmission business

– The UK Transmission business owns, maintains and operates the national gas

transmission system in Great Britain (Scotland, England and Wales)

~7,600km of high pressure pipelines and 26 compressor stations

– It also owns and maintains the high voltage electricity transmission system in

England and Wales, together with operating the system across Great Britain

~7,200km of overhead line and 690 kilometres of underground cable and 337 substations at 241 sites

• This project is partially funded by the Network Innovation Stimulus – a scheme

introduced by the UK gas and electricity regulator (Ofgem) to help develop crucial

knowledge and expertise which is being shared across the industry

For more information on network-related innovation projects in the UK, see

http://www.smarternetworks.org/

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Content

Project Background

Smart Survey Device (SSD) Overview

Problem Definition and Key Calculations

Laboratory Tests and Field Trial Results

Survey Wire Arrangements

Preliminary Findings

Next Steps

Q&A

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Background - AC corrosion

AC corrosion has been well documented in the UK, mainland Europe and North

America

Through-wall failures have been recorded and corrosion rates calculated as high

as 1.4 mm/year

AC corrosion occurs at small coating holidays on well coated pipelines when the

pipeline suffers from induced AC voltages

Increasing installation of power lines, rail transit systems and improvements to

pipeline coating quality will all continue to increase AC corrosion instances

The threat posed through induced AC may be assessed by monitoring the levels

of AC voltage and AC current density along the pipeline at risk

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Background – Project goals and likelihood of success

This project will deliver a suitable over-line AC survey system that will be used for

the initial identification of areas where the levels of AC interference on gas

pipelines may require mitigating action

When implemented, the survey system will enable the improved detection and

assessment of AC induced corrosion in gas pipelines, thereby reducing the

likelihood of leakage or failure though this particular corrosion process

Based upon data capture and analysis from preliminary field trials, the solution

can identify areas of high current density, therefore the potential for this project

to achieve expected benefits is high

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Background – Project deliverables and methodology

Main deliverables

– Field test procedure for close interval AC surveys, including proposals for method,

equipment and survey wire arrangement

– Smart survey device and identification of recommended peripherals

– Software to support close interval AC surveys

– Project report and industry presentation of findings and recommendations

Project approach (stage gates and iterations not shown)

– Specify device requirements

– Review market for suitable devices

– Enhance existing survey device

– Develop survey procedure

– Perform laboratory tests , field trials and analyse results

– Produce stage gate reports

– Publicise project findings

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Smart Survey Device (SSD) Overview – Test Post Measurements

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• Automatic switching detection

• DC with AC rejection

• True RMS AC

• +/- 50V DC

• 33V AC RMS

• DC with AC rejection

• True RMS AC

• Up to 10A

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Smart Survey Device (SSD) Overview – DC Close Interval Surveys

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• A second SSD can be left

at the test post to record

the reference voltage

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Smart Survey Device (SSD) Overview – AC Close Interval Surveys

• 30V AC RMS

• 1A AC RMS

• Switch inputs

to take trigger

V or I readings

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Readings captured during AC survey procedure

Pre-survey checks (at starting test post)

– Consistency between Reference Cells used in survey

– Consistency between Smart Survey Devices (static and moving)

– AC Current, AC Voltage and Resistance of survey wire (𝑅𝑊𝑖𝑟𝑒)

Static SSD (during survey)

– Monitors AC Voltage every 2 seconds

Moving SSD (during survey)

– AC Voltage (𝑉𝐴𝐶)

– AC Current (𝐼2)

Post-survey measurement – repeat pre-survey checks

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Problem overview and key concepts

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Surface

Pipe

Test Post

Mobile Measurement

Device Trailing Wire

Current

Probe

Measurement of AC Voltage and AC Current

Up to 1km

Wire coil

Reference

cell

Test

Post Measurement

Device Trailing

Wire

Pipe

Wire Coil

Electromagnetic field

Surface

Pipe

Test Post

Mobile Measurement

Device Trailing Wire

Current

Probe

Measurement of AC Voltage and AC Current

Up to 1km

Wire coil

Reference

cell

Main factors affecting

reading accuracy:

• Resistance of Wire

• Inductance of Coil

• Inductance of Trailing

Wire

• EMF Pickup

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Problem overview and key concepts

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When measuring AC voltage, the input impedance of the measurement device is

very high, causing the current that flows through the trailing wire to be very low.

Therefore changes in resistance of the trailing wire will have a negligible

impact upon the AC voltage measurement.

Significantly however, for AC current measurements, the impedance of the

measurement device is very low, and as such, the inductance of the wire will

reduce the measured AC current. Consequently any AC current, and in turn,

current density measurements calculated, will not accurately reflect the true

levels.

Whilst cables and measurement equipment are exposed to the electromagnetic

field, the affect of this has been demonstrated to have a negligible affect on the

readings.

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Test measurements on survey wire

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0

150

300

450

600

750

900

0 200 400 600 800 1000 1200 1400 1609

Oh

ms

Length of Survey Wire on a Coil in AC Conditions

Reactance

Resistance

Impedance

Length (m) Reference

V Measured

V Difference

% Reference

mA Measured

mA Difference

% Reference

A/m² Measured

A/m² Difference

%

50 8.55 8.55 0% 13.42 13.40 -1% 113.73 113.56 -1%

100 8.00 7.93 -1% 13.13 12.24 -6% 111.27 103.73 -7%

200 8.04 7.88 -2% 13.07 11.85 -9% 110.76 100.42 -9%

50 + Coil 7.66 7.72 -1% 12.50 5.97 -48% 105.93 50.59 -52%

100 + Coil 7.54 7.37 2% 12.06 4.64 -61% 102.20 39.32 -62%

200 + Coil 15.85 15.86 -1% 22.77 10.02 -56% 192.97 84.92 -56%

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Key calculations

Inductance of straight wire

Inductance of coiled wire

Reactance of wire

Impedance of wire

Corrected AC current

AC current density

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𝑍𝑤𝑖𝑟𝑒 = 2 × 𝜋 × 𝑓 × (𝐿𝑐𝑜𝑖𝑙 + 𝐿𝑤𝑖𝑟𝑒)

𝑅𝑊𝑖𝑟𝑒2 + 𝑍𝑊𝑖𝑟𝑒

2

𝐼1 = 𝑉𝐴𝐶𝐼2

(𝑉𝐴𝐶 − 𝐼2 𝑅𝑊𝑖𝑟𝑒2 + 𝑍𝑊𝑖𝑟𝑒

2 )

𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝐴𝐶 𝐶𝑢𝑟𝑟𝑒𝑛𝑡

𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 𝐷𝑒𝑣𝑖𝑐𝑒

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Length (m) Reference

V Measured

V Difference

% Reference

mA Calculated

mA Difference

% Reference

A/m² Calculated

A/m² Difference

%

50 8.55 8.55 0% 13.42 11.677 -13% 113.73 116.76 2%

100 8.00 7.93 -1% 13.13 10.961 -16% 111.27 109.61 -1%

200 8.04 7.88 -2% 13.07 11.201 -14% 110.76 112.00 1%

50 + Coil 7.66 7.72 -1% 12.50 12.72 1% 105.93 107.80 2%

100 + Coil 7.54 7.37 2% 12.06 11.45 -5% 102.20 97.04 -5%

200 + Coil 15.85 15.86 -1% 22.77 24.03 5% 192.97 203.65 5%

Applying adjustments for inductance

Percen

tag

e o

f E

rro

r

Length of Survey Wire (Coiled)

Measured Current Density

Calculated Current Density

Measured Voltage

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Survey Results Comparison – Raw Data vs. Calculated Data

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Survey Wire Comparison – Raw data

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Conclusions to date

Attenuation of AC current density readings along a typical survey can be in the

order of up to 60% using a standard single strand copper wire (as used in DC

CIPS); Similar effects on AC voltage readings are negligible.

The attenuation of AC current density may be reduced to less than 40% through

the use of a coax cable

The majority of attenuation during an AC current density reading can be credited

to induced AC within the survey while the wire is coiled during a survey; due to

practicality, the transportation of wire during a survey is unlikely to change.

The AC induced effect on the coil of survey wire can be modelled in order to

estimate true AC current density readings

Estimates via the modelling technique are within 10% of true AC current density

readings; these predictions, more importantly, over-estimate the true AC current

density.

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Conclusions to date

Temperature change during a survey, pipeline depth and depth of the current

probe used were identified as factors that do not significantly affect AC current

density or AC voltage measurements taken along a typical survey, however,

gathering of further evidence is recommended to prove this prediction more

conclusively.

These trials have enabled the project to collect valuable data that has helped to

qualify the theoretical physics behind induced AC voltage / currents and its impact

on both the pipeline and the measurement technique of AC CIPS.

It has also provided some solid data that has enabled the refinement of the data

analysis approach and more fundamentally the complex equations that form the

core to understanding the real impact of the induced voltages and currents on the

pipeline.

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Next Steps and Q&A

Additional field trial planned

– Static logging of future field trials is to be completed at both ends of the survey section (to

highlight any noticeable differences in AC Voltage between each end of the section)

– Static logging of AC current density is to be completed at both ends of the survey section

(to monitor fluctuations during the survey)

– Further investigation to specifically quantify the effect of current probe depth and the effect

of pipeline depth on survey measurements

Additional research proposed

– The behaviour of AC current and AC voltage readings is yet to be investigated on varying

types of pipeline coating. Research into the influence that each coating type has on AC

measurement readings is proposed.

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SAFER, SMARTER, GREENER

www.dnvgl.com/software

Thank you for your attention

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For further details, please contact:

David Simmonds

david.simmonds@dnvgl.com