INSTRUMENTS ETC

Chart recorder with facility to mark the chart with the times of specific events.

2 Cu/CuSO4 electrodes.

High resistance voltmeter.

Trailing conductor mounting .

METHOD

5.1.1 Use Procedure 2 to establish a remote reference electrode.
This will be the potential against which all voltages will be measured.

5.1.2. Carry out procedure 3 to establish the facility to remove errors caused by fluctuations during the period of this test.
This is necessary as the errors caused by other electrical influences will mask the influence of each of the cathodic protection systems as they are switched off.



5.1.3 Connect the negative terminal of channel 4 to the subject cathodic protection test facility.

5.1.4. Connect the positive terminal of channel 4 to an electrode placed in the standard test position.

5.2.1. Mark the time, on the chart paper, that all CP installations are switched on.

5.2.2. Switch off each TR at pre-arranged times, working progressively away from the subject test facility.

5.2.3. Note each time of switch off on the chart paper and look for an event to be recorded.

5.2.4. Continue switching off the TRs, progressively more distant from the subject test facility, until no event is registered on the chart recorder paper.

5.2.5. The furthest TR that influences the reading, should then be put into the 'on/off' mode to confirm the final event.


INTERPRETATION

A pipeline network represents a unique integrated circuit because no coating is totally electrically resistant and the influence of the cathodic protection groundbed spreads into 'remote earth' at which stage the current is conducted by so many resistances in parallel that the ground can be regarded as infinitely low in resistance.

However, the resistance in each cathodic protection circuit is determined by the interface resistance at the anode and the decreasing size of the 'shells of resistance' as the current re-enters the metal through coating faults (cathodes).

The resistanceless conductor (the ground) allows the influence of each impressed current cathodic protection system to spread for many hundreds of miles depending on the coating condition of the pipeline, which is it's negative or return path.

In any pipeline network we have a variety of coating qualities, pipe wall thicknesses and diameters, all buried or submerged in a common electrolyte.

It is possible to model the behaviour within any network using such software as developed by the Dynamic Project, and the results of this procedure can be used as confirmation of the integrity of the computer predictions.



A computer or data logger may have the capability to correct the readings on channel 4 from the data being fed in to the other three channels but it is still an advantage to record the uncorrected readings as these will give an indication of the degree of accuracy to be expected from simple historical data.

This will indicate the condition of the pipeline at coating faults and will give a guide to exploratory excavations necessary to repair coating and re-inforce the pipeline metal.

It will also give a guide to the effectiveness of any supplimentary cathodic protection (which is applied) and aid future monitoring.



The true "off" reading is the lowest value recorded during this procedure, and the true "on" reading is the highest.

This is because de-polorisation cannot commence until there is no cathodic protection current flowing onto both anode and cathode of corrosion cells on the pipeline.

There is still discussion as to the true nature of 'polorisation', but there is no disputing that there is a time element in the difference between the status of equilibrium when the cathodic protection current is operating and when it is not.

The 'normal status' of the pipeline is with the cathodic protection system switched on and this is the state of equilibrium which must be ascertained in order to assess whether the corrosion reactions have been stopped.



This procedure is not a method to obtain the polarised potential for the purposes of establishing the effectiveness of the cathodic protection, as it does not record the 'instant off' voltage at which the subject pipeline would be polarised under normal operating conditions.



This procedure can confirm or disprove the validity of conducting an 'instant off, polorised potential survey', sometimes refered to as a 'CIPS'. The results can demonstrate that the readings are voltages which may or may not be used to assess the reaction potential at the interfaces between the pipeline metal and the electrolyte.