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• Understanding how the CIPS technique measures voltages between the potential of the ground and the potential of a span of the pipeline.
At this present time millions of voltage measurement are being recorded, relating to buried pipeline corrosion control globally. These volyages are being presented in the form of graphs to the pipeline operators and owners with an analysis about the performance of the corrosion control systems.
I was lucky to be party to the very first Close Interval Potential Survey that was devised in the UK in the late 1970's by British Gas Corporation.
Until this time it had been the practice to make voltage measurements at places where metalic contact could be made with the pipeline but there had been many failures where the indication said that the pipeline was protected.
It was proposed that voltage measurements should be taken between test facilities using a trailing connection and stepping the ground contact electrode at a variety of distances.
There was also concern at this time that the current flowing from the cathodic protection system caused the ground to have a potential of its own caaled the IR Drop in the soil. It was believed that this was an error that could be removed by switching the current off immediately before making the measurement.
A current interruptor was to switch the cathodic protection current off and then back on while the survey team recorded voltages between the pipeline metal and a copper/copper-sulphate electrode.
Ground contact was to be made at a variety of spacings over the pipeline and switching time intervals were changed (and discussed in detail) as the survey progressed.
The mainframe computer at North Thames Gas, head office was programmed to show three traces ... on ... off ... and the difference between them.
It was anticipated that this difference would show where there was active corrosion but in fact it did now show anything that could be interpreted correctly so the third trace was quietly dropped and not discussed further.
I had developed Direct Current Voltage Gradient mapping techniques a few years earlier and this was incorporated into CIPS at places where there were features in the data that required further investigation.
Pipe locating was part of the survey but there was no instrument manufactured at that time that would provide hard copy so I devised a method of recording the audible signal of the Pearson pipe locator and making this into a voltage trace that could be displayed on recording voltmeter paper. This is now known as PCM when done electronically.
I also pointed out and demonstrated that each secion of the pipeline was effected by all the other sections of the pipelines, despite the insulation joints that had been previously assumed to isolate each span of each pipeline.
I also demonstrated that making the measurement of the voltage immediately over the pipeline did not achieve the intended result because good coating makes all readings 'remote'. You can detect the switching of the transformer rectifier many hundreds of meters away from the pipeline route.
We found, hen excavating towards a coating fault, that the voltage decreasesas the as the distance between the electrode and the metal decreases. This shows that depth of cover is a feature that must be considered as well as the resistance of the electrolyte at each separate coating fault location.
It used to be assumed that switching off the source of the cathodic protection current would instantly remove the error in the voltage measurement (that was called by some the IR Drop in the soil) but in fact it is clear that it does not.
The removal of the current produces a drop in the voltage that is seen on the wave form using an osciloscope or analogue recording voltmeter.
Once more this is miss-understood as being the graph that is produced by dataloggers
Present day instruments record (and can display) voltages as graphs but the rise and fall in values is NOT the wave form that is required by science to determine the 'polarised potential' of a coating fault.
Synchronised TR switching was attempted during the original CIPS survey in the 1980's but this but this was before satelyte technology was available and was impossible.
A variety of 'off' times was tried and discussed but no decision was made that had any real scientific basis.
Einstein said that if you cannot explain your theory to a child then you do not understand it yourself so it is first necessary to define the words 'electrical potential'.
An electrical potential is how much electricity is in an item.
Everything has an electrical potential as explained by the equations of Gibbs Free Energy.
Electrical energy is transfered from one item to another as seen in the executive toy known as Newtons Cradle.(video clip)
electrical energy
Electrical energy does NOT travel like liquid or gas but water is sometimes used to explain electrical pressure. This next clip uses the water analogy as a visual aid to understanding the problem that we face in analysing the results of CIPS surveys.
(video clip)
water level model of voltage
We cannot see or measure electricity without instruments that measure the difference between two potentials. This includes osciloscopes, data loggers, digital recording voltmeters and analogue meters.
We can only measure the difference between two potentials ... we must have two potential values in order to calculate the voltage.
(video clip)
pipe to soil potential measurements
DCVG uses this understanding to plot the ground potentials of the backfill at surface level along a pipeline route.
(video clip)
The value of the electrical charges (potential) of the ground change when any source of electrical energy changes and we can cause such a variation by switching on and off our transformer/rectifiers.
• Determining how the potential of the ground varies over time and distance due to a variety of electrical disturbances
The following was published many years ago when analogue recording voltmeters had to be used as data loggers were not available.
INSTRUMENTS
- Recording voltmeter.
- High resistance voltmeter.
- 2 Cu/CuSO4 electrodes arranged and connected as per the attached drawing.(walking electrodes)
- 2 Cu/CuSO4 electrodes.
- Reel of trailing conductor and spool.
METHOD
6.1.1. Carry out Procedure 6
6.1.2 Connect the walking electrodes to the trailing conductor on the carrying spool.
6.1.3. Connect the other end of the trailing conductor to the positive terminal of Channel 4 on the recording voltmeter.
6.1.4. Connect the negative pole of Channel 4 of the recording voltmeter to the pipeline test facility.
6.1.5. The timers should be switching the CP current off for a period of 3 seconds and back on for a period of 12 seconds.
6.1.6 Set the chart speed at 1000mm/hr and start recording.
6.2.1. Lift both electrodes clear of the ground for a period of a minute, to leave a clear trace of the start of the survey, and make a note on the chart "Location, start continuous overline survey."
6.2.2. Leave an attendant watching the recorder, and 'walk' the two electrodes, at one meter steps, over the pipeline to the next CP test facility. Each step should take about one second and ground contact must be maintained by either electrode, or downward spikes will appear on the chart trace. 6.2.3. At the next test facility, lift both electrodes clear of the ground for one minute to leave a clear trace on the chart, then place them together on the ground.
6.2.4. Mark the chart with the location, so that the section of the trace can be clearly identified.
6.2.5. Lift both roving half-cells clear of the ground again, mark the chart and survey to the next test facility.
6.2.6. Repeat for the complete length of a section of pipeline between isolation joints.
Note
If there is a risk of the conductor being broken, such as at road crossings, then the chart recorder should be moved to a new, suitable location.
Where this is necessary, then the new remote reference electrode potential should be related to the last remote reference electrode. The chart will bear a trace of the voltage between the old remote reference electrode and the fixed electrode over the pipeline at the beginning of the preceding survey run.
The new position of the remote reference electrode can be established in relation to the last position of the over the pipeline polarized potential survey. It is therefore possible to have a complete section of pipeline with a common reference electrode potential.
INTERPRETATION
The trace will appear as a 'saw tooth', which will undulate along the section surveyed. The on and off readings will normally run parallel, except where passing through the immediate area of influence of the groundbed which is being switched.Any other marked peaks where the on and off readings run parallel, are due to IR influences in the ground, and can indicate interference.
Troughs can indicate coating faults, shielding or interference and their location should be identified and marked on site .
Where the 'off' potential is less than 0.850v (the trace being negative to the top), then investigations should continue at this location.
Each location that is investigated should be identified and have its own file.When all instrument techniques have been carried out, the indications should be checked by excavation and physical examination of the pipe and coating condition.
The location and work should be photographed, or video recorded to provide training material and allow techniques to be developed further.
• Examining how the potential of the pipeline varies one time due to the induction and conduction of charges from the ground and metallic paths connected to the pipeline