This document describes the development of ground potential gradient surveys to locate coating faults on buried pipelines. It discusses: - How moving a copper/copper sulfate electrode causes voltage variations that can identify coating faults. - The technique was developed in the 1970s in the UK to use differences in ground potentials rather than assuming the electrode was a reference point. - Interpreting the results shows highest voltages locate where cathodic protection current returns to the pipeline, indicating coating faults.

1. Technotoy stage 4
Modelling Ground Potential
Gradient Surveys.

2. Actual survey 3D graph.

3. The voltages

4. 2D plot

5. Voltages between two electrodes
• The voltages are measured between two
Cu/CuSO4 ground contact electrodes.
• One of these electrodes is in a fixed position
during this type of survey.
• The cathodic protection current is interrupted
and the difference between the switched on
reading and the switched off reading is recorded
at a grid of locations.
• This can be done with a transformer rectifier or
sacrificial anode.

6. Devised in 1973 in the UK.
• In 1973 I realised that the Cu/CuSO4 electrode
could not be a reference potential as suggested.
• I found that I could achieve any voltage I wanted
by simply moving the electrode without altering
the electrical equilibrium between the pipeline
metal and the electrolyte.
• I used the difference in potentials of the ground
itself to identify the exact position of coating
faults.

7. Low resistance of pipeline
• The resistance of the pipeline metal between
two test posts is so small that it cannot be
measured on most instruments.

8. Moving the Cu/CuSO4 electrode
• Moving the ground contact electrode caused a
variation of the displayed voltage.

9. Ground current flow
• My experiments about the detection of
direct electrical currents flowing in the
ground (that I had carried out in the UK
and in Iran) had resulted in a method of
detecting the presence of high electrical
potential zones based on the simple
assumption the current would flow from
areas of high potential towards areas of
low potential.

10. Ground current flow.

11. Galvanometer
• When I put a low resistance path between the two, then
the charges would use this path and I could see the
needle deflect in the direction of the current.

12. Calculations from field data.
• The negative pole of the transformer rectifier sucks
charges from the pipeline and that is why the connection
is known as the 'drain point'. We can conveniently say
that this is at zero potential for the purposes calculations.

13. Visualising charge distribution
• The next few pictures show the steps in logic
that helped in visualisation of the electrical
equilibrium in the ground when charges are
impressed.

14. Kirchhoffs Law

15. Field observations

16. Visualisation mistake
• The problem is that if you visualise cones then
you might mistakenly think that current in the
earth path is directional, but this is not true.

17. Ground potential profiles
• This can be dramatically demonstrated in the
field where the impressed current system uses
horizontal anodes in trenches about 2 meters
deep.

18. Real 3D plot using Excel

19. Electronic model

20. Computer model

21. Switching is essential
• The reason why I used switching was to identify each
source of energy and thus get further information to add
to the plan of the area.

22. DCVG

23. Step the electrodes

24. We now use ‘walking sticks’

25. At 1 meter intervals

26. Alternatively

27. Interpretation
• The largest voltages are obtained where the potential
gradient is caused by the CP current returning to the
pipeline.
• It follows that the marked locations are over coating
faults which allow contact between the backfill and the
pipe metal.
• In the mid 1980's I held a Cathodic Protection Course for
Graduate Corrosion Engineers during which the students
were required to carry out two-half-cell techniques and
later these same students were given the opportunity to
carry out field work on pipelines owned by the Severn
and Trent Water Authority.

28. Acceptance
• By the 1990's DCVG had been
established as a way to locate coating
faults on buried pipelines and a form of
DCVG had been adopted for offshore
inspection of submerged pipelines.

29. Report of DCVG dated 1982

30. Electronic model
• Our electronic model has a TR and an
interrupter.
• These can be adjusted to replicate the
electrical equilibrium that we experience in
field work.
• Technotoy is designed to enable us to
calculate corrosion and corrosion control.

31. TR

32. TR/power supply connections

33. DC to circuit

34. DC positive that supplies energy to
remote earth.

35. Two corrosion cells

36. Timer

37. On and Off voltages.
• We can now measure voltages with the
impressed current switched on or off at any
intervals we need to investigate.
• The built in capacitor in Orac shows the system
decays as soon as the system is switched off.
• The two corrosion cells show if we have
achieved cathodic protection and the Alexander
Cell shows the exact criterion for cathodic
protection in this configuration.

38. Electronic coating fault
• The following pictures show the shells of
resistance that are inherent to any coating
defect.
• Charges follow the path of lowest
resistance according to Kirchhoffs laws.
• We can position the contact point of our
Cu/CuSO4 probe to measure the effect
that the current has on the corrosion
reaction.

39. Remote earth is the copper plate.

40. Shells of resistance
• The charges have to pass through shells of
resistance from remote earth to the point of
entry into the metal.
•

41. Actual measurements can be made

42. Ready to go
• We can now apply energy and measure
the effects on corrosion that we can not
only measure but observe.
• We can record everything we do and build
our software to accurately calculate the
settings required to stop corrosion.
• We can use the oscilloscope to trigger
adjustments in response to events.