1. Understanding Above Ground Tools ACCA and ACVG
Gord Parker
CTW 2005 – Calgary

2. Introduction NACE approves 4 tests for determining Coating Condition
AC Current Attenuation
AC Voltage Gradient
DC Voltage Gradient
Pearson Method
‘old school’
Note CIS is NOT one of them
Does not find small defects.

3. Introduction
Before joining the Pipeline / Corrosion industry, my schooling was Electronics Engineering Technology.
This ppt focuses on 2 of the tests ACCA/VG
Both these inspection techniques are very electrical in nature and are well explained by simple Ohm’s law.

4. ACCA Introduction AC Current Attenuation
Sometimes called the EM method BUT it doesn’t do anything really tricky with electromagnetics.
I personally think RP0502 should have this reworded
Simply uses Gauss & Faraday’s well proven laws that the strength of an EM field is proportional to the current through a conductor.
Working backwards, knowing the EM strength & conductor depth, we can calculate the current.

5. ACCA Application Apply a known and consistent I.
Measure the current at points along the line.
Calculate a loss/distance ratio
Alternately factor in diameter for loss/*area* ratio to compare different diameters
More loss/distance = worse coating.
Current loss can also indicate unknown connections, insulator and casing problems.

6. dB Current
Expressing current loss as a ratio to current before the loss is more accurate.
deciBels are a ratio
Next chart: Loss 1 = ~1000 mA, 2~400, and #3~150.
Loss 1 *looks* much worse but expressed in dB all are the same. Hence all three features (T’s to similar service mains) are the same size/length.

7. Current Attenuation Graph
3 steps are different looking in mA but nearly identical in dB
Note dB mA

8. Common Problems Current goes UP as you walk away from Tx
Current returning to ground point is sufficiently concentrated near Tx ground stake to create an EM field that cancels out some of the EM field on the pipe.
Returns to normal attenuation after ~100m
Current fluctuates down the line
* Can’t GAIN Current walking away from Source
We are detecting EM fields and they can be distorted by large metallic objects and other EM fields
Reduce bonds / bleed-over
Some places you just can’t take a reading.
Deep >10-18’ pipes can sometimes be a problem.
If you can’t take a reading, the graph will show more loss across more distance, ratio will still be accurate.

11. Using ‘TrendLine’ function in Excel

12. Bad Example #1 Example of current that goes up @ reading #47
Also, survey is using <20mA, small errors look big, better to use 100mA+
Survey done with locate frequency, not low (ie:4Hz)

13. Bad Example #2 Current can’t go up by any amount, never mind double
Same data but shown on Log vert. axis.
See how first 60% now looks more even compared to linear graph previously.
Segments 0-10 are worse then 10-40

14. AC Frequency
Even through a perfect coating, an AC signal will bleed off due to capacitive coupling to the ground outside the coating.
Higher f = higher attenuation.

15. Bad Example #2 Same graph as previously.
In my opinion, the high attenuation near the beginning is only due to the higher frequency (remember this was done at a locate freq, not 4Hz).

16. Shielded defects ….. Higher frequency I will bleed off faster.
Capacitive impedance drops as f
Remembering we are detecting current loss, does disbonded coating have a lower impedance?
i.e.: does the pipe-coating-dirt model lose current faster then the pipe-electrolyte-coating-dirt ? Why ? (the electrolyte isn’t grounded)

17. Distribution of current. 1kHz vs. 4Hz
Because of an overall lower impedance at higher frequencies, current will travel down stubs and short mains (and attenuate faster on all runs).
It also bleeds through what are actually good (DC) insulators.
Surveying at a lower frequency will more accurately mimic where your CP current is going.
1A (4Hz)
1A (1KHz)
40mA
400mA
600mA
960mA
900mA
200mA
fault
60mA
400mA

18. Low frequencies go to the short
Avoid distorted readings

19. Troubleshooting Loops
Current Direction can be seen above ground without contact.

20. Benefits and Strengths
Easy to operate by one single operator
No direct contact to ground necessary
Near DC fault detection signal makes capacitive effects insignificant
Low frequency for pipe detection: long distances
Current Direction facility to enable short identification
With addition of earth-frame, often same hardware can be used for an AC Voltage Gradient survey.
Most of these benefits all boil down to cost savings.

21. Benefits and Strengths II
Previously Difficult situations:
Casings – Now, just check if here is a current loss
Insulators – If both zones have similar potential, there could be a question as to if insulator is shorted. But in Current Mapping, if there is current getting through to far side piping, there is a short.
Bonds – large current loss and/or direction reversal show below grade bonds very easily.
Most of these benefits all boil down to cost savings.

22. Difficult Applications
We are locating current flow and current does not travel into an open circuit.
Insulating flanges and fittings (ZapLock, Bell&Spigot, Dresser) will limit current flow along a length of pipe.
This is still valuable information to locate these and verifies their proper operation.
Congested areas – confusing EM
Older equipment was subject to 50/60 Hz interference
Newer circuitry ‘sees’ PCM current through other EM noise.

23. The Effect of Insulators (i. e
The Effect of Insulators (i.e.: Dressers™, ZapLock, Bell/Spigots in some conditions.)
When low resistance connection is in effect, current distribution is uniform
PCM shows 50/50 split of current
When joint goes high resistance, current splits unevenly
PCM (& CP) current drops to undetectable level ahead of open
(Is still protected – potential is maintained, just takes less current)

24. Mg Anodes
Anodes will provide a path to ground for the AC signals we are detecting (4Hz and locate freqs.).
It may look like a loss but likely a big one
Any current this big is a short (casing, other utility, insulator), so look for 4 Hz somewhere else as current has to go somewhere. If nowhere else, is a Mg.
Amount of signal loss will be proportional to anode quality / life expectancy.

25. Typical data gathering in field.

26. Underground Gas Distribution Short to Water
Quite prevalent, especially in warmer climates (water lines more shallow)
Hard to find without excavation
PCM method walked technician right to it.
Picture of site on next slide

27. Underground Gas Short
Black pipe is gas line, copper is poorly installed water line.
Current flowed down gas to water, then direction showed current up both sides of water line.

28. CASE I N MSA not insulated - 875 ma found on gas serviceline
Area Rectifier - Anodes used as PCM ground & PCM 3 amperes output DE
Insulator
875 ma
110 ma N
450 ma
375 ma
94 ma
578 ma DE
475 ma DE
700 ma
100 ma DE
805 ma DE
400 ma
500 ma
950 ma
1.60 A
2.60 A
No Signal
2.50 A
98 ma
600 ma
No Signal
100 ma
78 ma
Typical Mag. anode - connected
Mainline Insulator DE

29. 2nd Distribution Example

30. Transmission Coating Survey (and bad insulator)
Started as just a demonstration, no known problems on line.
Upon connecting transmitter, 85% of signal going in one direction !?!
Drive to next road crossing (only ~1200m) and had already lost 90% of remaining current, thus problem is between road and transmitter
Perform survey towards transmitter, find 2 areas of higher than average loss
Found major loss across one point, investigation found a shorted insulator in underground T connection to foreign compressor site.
If both systems had similar protection levels, a CIPS may not have shown any defect.
If short had gone unnoticed, stray currents could develop causing problems and/or rectifier power bills /current rating could be higher than necessary.
Several important points on this page.

31. Transmission Coating Survey
Perhaps I shouldn’t have included all the info here, this looks like a scary graph.
Includes mA, dB, depth and loss/distance ratio.
Loss/distance is most important trace and depth is handy.

32. ACCA Questions ?

33. ACVG AC Voltage Gradient
Like DCVG / Pin-to-pin / similar but uses a transmitter, not the CP DC
Sizing similar to DCVG
Empirical testing now underway

34. ACVG
An improvement though is to tightly tune the voltmeter to the transmitter frequency
A Fluke® on DC can find other sources:
Stray current, sacrificial anodes, dissimilar metals
A Fluke® on AC isn’t tuned:
Any AC frequency from ~5 Hz - 25kHz
60 Hz.AC currents/faults, telephone/scada noise

35. Is AC, but at any instant in time, there is a direction.
Pool of Potential
Is AC, but at any instant in time, there is a direction.

36. ACVG Sometimes part of an ACCA tool Is a big AC voltmeter
Voltmeter Leads are the probes
Results in dB gives very wide dynamic range
Calibrated amplification reduces a variable
Consistent lead spacing (a-frame) removes one other variable

37. ACVG Receiver Theory

38. ACVG
Faults (on an otherwise good cable) have been found exceeding 6.8 MΩ
Very sensitive as it has an extremely high signal-noise ratio

39. AC Voltage Gradient Can be part of Current tools Becoming very popular
Extreme sensitivity
Rejection of interference
Very accurate location of faults
typically better then 6"
Sometimes part of Current Attenuation equipment
This method deserves to be considered as a solid tool for integrity and the ECDA process.

40. ACVG in Operation
Both signal strength and direction arrows lead user to holiday.
Fault value is proportional to holiday size and soil resistivity.

41. In this case the next fault was quite close (20-30m) which is why the left side of graph climbs quickly.
Other cases may show 100s of meters of signal at 30 and under..

42. ACVG Tuning Older systems used a simple DMM
Does not tune to any one frequency
60 Hz, cable earth faults, telecom noise Rx’d
Very tight tuning in the signal generator and receiver effectively increases sensitivity as it ignores current from other sources
SNR improves

43. ACVG Find Transversal A-frame dB readings: 80 (right) and 85 (l)
dB (from PCM) before fault= 49.85,after= 49.48
Distance between readings = 15 meters
37mB/15m (actually just fault)
Pipe dia.= 12”/PE in two layers
Soil Ohm x m
Fault size = 60 mm length x 2 mm wide
Note the higher trans dB reading on left side that is the same side where valve station is located

44. Dig Pictures

45. Jordanian Fire Water Line 16”/30yr old
“We are using this equipment for locating coating defects on the coal tar epoxy coated pipelines. While doing survey on the pipeline we got 63 dB microvolt with A-Frame. The epicenter of the defect was located by taking readings above and in line with the pipeline and perpendicular to the pipeline. From all four sides direction of arrow was indicated towards the defect. The defect location was excavated.
“Upon excavation, we could not find bare pipe at this location. What we found was that the coating has degraded badly and it has become permeable. This has happened at two locations.
“Please advise if the current is flowing through the permeable coating or there are other reasons.”

46. Sarvjit Singh
Corrpro Companies Middle East L.L.C
Client : Jordan Petroleum Refinery Company

47. Sarvjit Singh
Corrpro Companies Middle East L.L.C
Client : Jordan Petroleum Refinery Company

48. My Favorite ACCA/VG Story
2 mile long distribution main, 25 yr old
For last 4 years, potentials have been dropping
Several surveys showed no shorts
ACCA: in each 100’ span, current was dropping like a rock but no single point of trouble
ACVG: BAM… BAM… every 40’
Had spent 40+ person-hours with no resolution. In 45 minutes we found and quantified the problem. The customer could budget digging up 6’ of every 40’ to replace the joint coating.
Do the math, 40 hours of a techs time or 40 minutes? This one application paid for a kit.
In addition, reduced power consumption and likelihood of further coating damage from high current density.

49. Thank You Questions? Gord Parker, C.E.T.
Spectrum XLI (Calgary, Alberta)
www. spectrumxli.com