The two-half-cell survey
(Plotting the electrical potential of the ground using two Cu/CuSO4 electrodes)
I devised this type of survey in 1975, when working for The Shell-BP Development Corporation of Nigeria.
I was faced with a situation of several thousand miles of buried pipelines, manifolds and associated buried pipes of which the drawings had been lost in the conflict that had taken place in Nigeria during the years before I arrived.
There were approximately 40 oil fields, producing through a network of pipelines to an export facility with an offshore SBM system to which the supertankers moored.
I was given responsibility for all corrosion matters in the Eastern Division where re-habilitation of the Biafran War damage was taking place and where there were constant leaks due to corrosion.
The main problem was that manifold drawings had been destroyed and piping had been constructed to re-establish oil and gas production. The new piping was connected to the old manifolds but there were no drawings to show the complete buried layout of configuration.
Digital instruments were not available and the analogue meters were more sensitive to current flow than to potential differences. (details of this can be found in other parts of this website)
I had experimented with the detection of ground flowing direct electrical currents in the UK and in Iran, and devised a method of detecting the presence of high electrical potential zones based on the simple assumption the current would flow at a greater rate between areas of greater potential difference.
The impressed current cathodic protection systems in Nigeria had been connected to every available coated pipeline but the underground layout had not been known at the time. Consequently the circuit drawings were not a true schematic of the actuality.
The coating on much of the underground pipework had been damaged or was not effective due to degradation with age and damage by spilled products during the bombing.
Using the impressed current systems I was able to plot the underground piping and correct many of the drawings prior to excavation where necessary to repair the coating.
I issued several contracts in which I defined the work to be carried out in plotting the direct current flow over several areas with a view to plotting the potential gradients in the ground. One of these contractors was Mark Derefaka who is now a senior consultant to the Nigerian Petroleum industry, and who I met again at the first African Corrosion Conference in Lagos in 1993.
A grid was marked in the ground with pegs at 1 meter intervals over the area to be investigated.
The current directions were plotted by noting the polarity and strength of current through a sensitive ammeter, between two standard copper/copper-suplhate elecrodes placed in the ground at the positions marked by the pegs.
The resulting drawing showed the path and strength of ground currents and showed features underground which were excavated for examination.
At Bomu Manifold we were able to locate valves and pipework which was not known to exist, but that was still connected to the working manifold. This was draining protective current from the cathodic protection system, but once re-coated and added to the drawings was of no further problem.
During the period that I was in Nigeria I developed the two-half-cell procedure of coating fault detection and issued several contracts specifying the procedures to be used.and which are now available on this website. More details, and practical excercises, are available within the on-line cathodic protection course.
In the early 1980's I was engaged in field work on contract to North Thames Gas, which was then part of the British Gas Corporation, under the Gas Council. I joined a team of surveyors working out of Slough and conducting a type of survey which has become known as the CIPS. There is a separate paper describing the development of this technique elswhere on this site, and the procedures developed are incorporated in my list of standard procedures for cathodic protection.
I worked with the co-operation of Mike Foskett, the North Thames Gas Corrosion Engineer and practiced and reported on the results of two-half-cell (DCVG) surveys which I conducted personnally throughout the region whenever accurate location was necessary for excavation.
I used extensive two-half-cell techniques during successful analysis of the problems at Hedgerly Manifold, Taplow Rail crossing and Ammersham Road Manifold as well as over 100 excavations which were carried out to verify the credibility of the survey techniques that were being developed.
My paper on Cathodic Protection Monitoring was published by John Tirratsoo in his two international magazines and mentioned the two-half-cell technique mentioning potential gradients in the soil as an indication of the electrical equilibrium of the pipeline. This paper is reproduced in full on this site together with a paper which supports all the principles and defines potential gradients in the ground.
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.
Cathodic Protection Network Procedure 7a with reference to a paper published in 1982