1. DNV GL © 2013 SAFER, SMARTER, GREENERDNV GL © 2013
Development of Over-Line Survey Systems for
Monitoring Pipelines within an Alternating Electric
Field
1
23rd Colloquium SAFETY AND RELIABILITY OF GAS PIPELINES - PRAGUE
Day 2, 15 May 2014
David Simmonds and Sam Orton, DNV GL Software

2. 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.
2

3. DNV GL © 2013
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/
3

4. DNV GL © 2013
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
4

5. DNV GL © 2013
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
5

6. DNV GL © 2013
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
6

7. DNV GL © 2013
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
7

8. DNV GL © 2013
Smart Survey Device (SSD) Overview – Test Post Measurements
8
• 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

9. DNV GL © 2013
Smart Survey Device (SSD) Overview – DC Close Interval Surveys
9
• A second SSD can be left
at the test post to record
the reference voltage

10. DNV GL © 2013
Smart Survey Device (SSD) Overview – AC Close Interval Surveys
• 30V AC RMS
• 1A AC RMS
• Switch inputs
to take trigger
V or I readings
10

11. DNV GL © 2013
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 (𝑅 𝑊𝑖𝑟𝑒)
 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
11

12. DNV GL © 2013
Problem overview and key concepts
12
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
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

13. DNV GL © 2013
Problem overview and key concepts
13
 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.

14. DNV GL © 2013
Test measurements on survey wire
14
0
150
300
450
600
750
900
0 200 400 600 800 1000 1200 1400 1609
Ohms
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%

15. DNV GL © 2013
Key calculations
 Inductance of straight wire
 Inductance of coiled wire
 Reactance of wire
 Impedance of wire
 Corrected AC current
 AC current density
15
𝑍 𝑤𝑖𝑟𝑒 = 2 × 𝜋 × 𝑓 × (𝐿 𝑐𝑜𝑖𝑙 + 𝐿 𝑤𝑖𝑟𝑒)
𝑅 𝑊𝑖𝑟𝑒
2
+ 𝑍 𝑊𝑖𝑟𝑒
2
𝐼1 =
𝑉𝐴𝐶 𝐼2
(𝑉𝐴𝐶 − 𝐼2 𝑅 𝑊𝑖𝑟𝑒
2
+ 𝑍 𝑊𝑖𝑟𝑒
2
)
𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =
𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝐴𝐶 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 𝐷𝑒𝑣𝑖𝑐𝑒

16. DNV GL © 201316
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
PercentageofError
Length of Survey Wire (Coiled)
Measured Current Density
Calculated Current Density
Measured Voltage

17. DNV GL © 2013
Survey Results Comparison – Raw Data vs. Calculated Data
17

18. DNV GL © 2013
Survey Wire Comparison – Raw data
18

19. DNV GL © 2013
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.
19

20. DNV GL © 2013
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.
20

21. DNV GL © 2013
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.
21

22. DNV GL © 2013
SAFER, SMARTER, GREENER
www.dnvgl.com/software
Thank you for your attention
22
For further details, please contact:
David Simmonds
david.simmonds@dnvgl.com