technotoy stage 4

Technotoy stage 4

Modelling Ground Potential Gradient Surveys.

Actual survey 3D graph.

The voltages

2D plot

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.

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.

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.

Moving the Cu/CuSO4 electrode

• Moving the ground contact electrode caused a variation of the displayed voltage.

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.

Ground current flow.

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.

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.

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.

Kirchhoffs Law

Field observations

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.

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.

Real 3D plot using Excel

Electronic model

Computer model

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.

DCVG

Step the electrodes

We now use ‘walking sticks’

At 1 meter intervals

Alternatively

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.

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.

Report of DCVG dated 1982

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.

TR

TR/power supply connections

DC to circuit

DC positive that supplies energy to remote earth.

Two corrosion cells

Timer

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.

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.

Remote earth is the copper plate.

Shells of resistance• The charges have to pass through shells of

resistance from remote earth to the point of entry into the metal.

•

Actual measurements can be made

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.