Cathodic Protection Training Course


Module 15

Computerisation of cathodic protection and corrosion control



COMPUTER ASSISTANCE IN C.P. ANALYSIS


It is now possible to use a computer to analyse cathodic protection readings and obtain more positive indications of the corrosion control status of a pipeline network. This work has traditionally been carried out by each engineer making an ad hoc assessment base on various criteria and his own personal experience. This has not been totally successful as cathodic protection voltage readings are taken between two variable potentials and it is therefore not possible to analyse the readings until they have been related to each other.



EXAMPLE

A reading of 0.600v may be taken between the metal on a pipeline manifold and an electrode placed close to the pipe. A second reading between the same manifold metal and the electrode placed a few feet away might be 0.450v due to the difference in the ground potential, in which the electrode is placed.

The above illustration shows that it is possible to adjust all readings to show their values relative to a common potential. In this case the reference position of the electrode is the box marked Ref (aa) The reference potential is 1.280v higher than pipeline 'a' and 0.600v higher than the manifold metal.

However, when the electrode is moved to position Ref (bb) it's potential is 0.550v higher than the manifold metal. We know that there cannot be a volts drop of this magnitude in the manifold metal, as it has virtually no resistance, and little current is flowing through it.

The ground at position Ref (bb) must, therefore, be 0.050v lower potential than the ground at position Ref (aa). Any voltage taken with reference to position (bb) will make the subject potential appear to be 0.050v higher than that taken with reference to an electrode placed in position (aa).

If manifold (a) has a common potential it is clear that any difference in voltage between the electrode and each flange, must be in the electrode. Since the electrode is recognised as a standard potential, then the potential of the ground, in which the electrode is placed, must be the undefined variable. This can be demonstrated, on site, by taking all voltages against an electrode in a fixed position, proving that the computer model is realistic.



COMMON REFERENCE POSITION

The illustration shows that it is possible to adjust all readings taken at such a location, to give voltages which are referred to a common potential. In this way they can be analysed to indicate the direction in which currents are flowing and it is then possible to assess the effects that this will have on corrosion. It is also the only way in which to analyse the balance of the complete cathodic protection system because otherwise the voltages have a 'floating zero'.

Computer analysis can be applied to historical records which, at the moment, consist of thousands of unrelated voltage readings. An examination of records will show many instances where a variety of different voltages have been recorded between a manifold and the so-called 'reference electrode' during a single visit. This can only be explained by fluctuations in the manifold potential or the different potentials in the ground in which the electrode is placed. Applying the recorded readings to a computer model can show the true electrical balance which can then be analysed using the basic laws of electricity.



THE ELECTRICAL CIRCUIT

A pipeline network, and it's associated cathodic protection systems, are connected with low resistance bonds and together the pipes and bonds conduct DC current from ground with a high potential to ground with a low potential. The passage of current from pipe to ground, can only occur at coating faults, but we do not know their location or extent. The purpose of the CP system is to lower the potential of the pipeline network so that no current flows from the pipe to the ground. Current is discharged to ground through cathodic protection anodes (either sacrificial or impressed from rectified AC) and passes to 'mass earth', which can be regarded as a resistanceless conductor. When returning to the pipeline, however, the current is subject to the 'shells of resistance' surrounding each coating fault, and each has its own ohmic value. The distribution of current throughout the pipeline network is governed by the arrangement of the resistances at the coating faults, which, in electrical terms are 'in parallel'. It is clear that we cannot evaluate these resistances, but we can observe the effect that they have on the passage of the CP current.

The passage of the CP current causes 'potential gradients' which have become known as the 'IR drop in the soil' and it is this effect that enables us to make extremely accurate surveys to detect coating faults. When this information is applied to the computer model it can give an indication to the balance of the whole system.



COMPUTER SPREAD-SHEET MODEL

The next illustration is from a computer spread sheet (EXCEL FOR WINDOWS) which has been designed to show that it is possible to relate all voltages to a common reference potential on a pipeline network, that is under the influence of one or more cathodic protection systems.

It shows that the readings taken in the field, by traditional methods, can be adjusted to give a more accurate indication of the actual electrical balance of the pipelines and their associated electrolytes.

The model assumes that there is virtually no resistance in the pipeline metal, and that bonded pipelines have virtually no resistance in the connecting leads.

In this second illustration, the electrode has been designated as the reference potential, zero, when in position (aa) and all voltages can be calculated back to this one point.

The circles and lines represent the meter and connections, while the shaded boxes show the locations of the electrode.



The bold (blue) readings are those recorded by the field technicians, using traditional techniques. The readings in the red boxes show the voltage between the ground at that particular location and the ground at the reference location (aa), as well as some adjusted pipeline voltages.



The (green) readings, between the isolation joints, represent the voltage shift between the pipeline metal and the manifold metal, which is caused by the cathodic protection.



The calculations involved are extremely simple but they are cumulative and can become confusing as they progress along the network. The readings on the model can be substituted with readings from records, by the computer user.



The 'manifold' readings are the ones that are traditionally taken on the manifold side of insulation joints and flanges.(These are sometimes known as 'unprotected' readings because the metal is not connected to the cathodic protection system.) Most field workers take the voltages on both sides of the insulation with the electrode in the same position.



Where there are many pipelines,(16 or 32 for example) the records will show groups of identical voltage readings on the manifold side of the insulation. It will be clear that the electrode was in the same position during the reading of each group.



In the computer model above, the voltage from the insulation joint on pipeline (a) is used as a reference and all others are corrected accordingly. For example, the voltage between the manifold metal at this insulating joint and reference electrode (aa) is recorded as 600 mv. We know that the metal at insulating joint (a) has the same potential as that at insulating joint (b) but that the recorded voltage is 550 mv. This is 50 mv less than the voltage recorded when the electrode was in position (aa). All voltages recorded with the electrode at position (bb) must therefore be increased by 50 mv if they are to be compared to voltages taken with the electrode at position (aa).



This notion is easily demonstrated in field work, by taking all voltages with the electrode in the same position using a long conductor to move the meter position around the manifold area, changing the position of metal contact. It will be found that all the voltages taken in contact with the manifold metal are identical. The voltages taken in contact with the various pipelines will differ, showing that the pipelines are at different potentials.



This experiment has been extended to testing the model in relation to a length of pipeline. A long line conductor was used to span between two test posts over a mile apart, on a pipeline, and the difference in readings was thus proved to be in the soil, with no detectable potential difference in the pipe metal itself.



ANALYSIS

The corrected voltages on an electrically bonded pipeline network should therefore be the same throughout the network. This is because the resistance in the metal component is less than the resistance in the electrolytic component by an order of magnitude. For confirmation of this, compare the resistance of metal (0.00001-0.0000001) in Ohms/cm to that of earth (2000-100,000)

There are two possible reasons for the readings to vary, errors in the readings and resistances in the circuit.



1. ERRORS IN THE READINGS.

These can be identified and eliminated using formal procedures in the field.



2. RESISTANCES IN THE CIRCUIT.

If these are accidental, they are probably caused by malfunction, neglect or physical damage. These faults can be traced and corrected by maintenance work.

If variable resistances are designed into the cathodic protection wiring, then they can be included in the computer model and adjusted to ensure that they are not causing an imbalance that is likely to result in current straying from one part of the system to another via the earth. ( This has actually happened, on some networks, and resulted in stray current accelerated corrosion.)



USES OF THE MODEL

This computer model can be tailored to any pipeline network, and used as a tool to assist in the analysis of the cathodic protection balance and to assist the engineer in trouble shooting.

It will identify poor connections and diagnose the cause of erroneous readings, as well as indicating areas for further investigation, for possible corrosion damage.

This cannot be accurately done without the use of a computer.