Cathodic Protection Training Course


Module 15

Computerisation of cathodic protection and corrosion control

COMPUTERISATION OF CATHODIC PROTECTION (CP) DATA

CP is an ideal subject for computerisation as it has always involved the gathering and analysis of enormous amounts of data.

Nowadays, even more data can be gathered, using data-loggers which are themselves related to computers.

As a first step in utilising the power of the computer we have used it to present the data in the more readily assimilated form of graphical display. By this presentation it is possible for the engineer to see the effects of electrical potentials on a larger scale.

This computer function has been utilised for many years by such organisations as British Gas, and offered as a part of service by many companies including Harco and Global.

Specialised instruments are now available and we are in a position to gather and display an almost limitless amount of data in this way.

However, the real power of the computer is in 'number crunching' and this can be used in more ways than simple graphical interpretation. It can execute formulae on a massive scale which would be impossible to do manually.

CP IS NO LONGER SOMETHING OF A MYSTERY

Cathodic Protection is based on confirmed scientific theory, which can be expressed mathmatically. Some areas are contentious but some axioms are well established and beyond dispute, from a practical point of view. For example, no engineer will dispute the credibility of Ohms Law or Kirchoffs Law, whereas there is ongoing discussion relating to the value of certain voltage readings which are recorded within cathodic protection practice.

We have enough indisputable data available to which one can apply computer analysis. The results of this analysis can be tested against the reality of field application and a computer model can be fashioned as a tool for the cathodic protection engineer. If the data and the scientific axioms are correct, then it is possible to apply the computers massive calculating power to analyse the results, before presenting them in a way which will improve the engineers overview.

One region of a major oil production company has already started this next phase of computerisation by applying the most basic laws of DC electricity to a model of the negative side of their CP circuits. This represents the buried and submerged metal of oil and gas production stations and associated pipelines in one region of Nigeria.



THE COMPLEXITY OF THE MODEL

The locations consist of a "spider" of pipelines, bringing oil and gas from wells, which are spread over several square kilometers, to gathering centres, known as 'flowstations'. Some wells are connected to injection pipelines carrying gas or chemical inhibitors from the flowstation to be injected downhole. These pipelines can be over a kilometer long and some are coated, buried and connected to the CP system, while others are bare steel with ground contact, or earthed through their steel supports.

At the flowstation itself, all metal is earthed to a common ground potential, for spark prevention reasons. This earthing system consists of a bare copper ring main which runs round the periphery of the station enclosure.

All buried pipework is coated, but pipe supports and stantion bases are frequently in direct contact with the ground. The result is a very complex area of DC electrical flux which is difficult to visualise and time consuming to analyse.



AVAILABLE DATA

However, convention has demanded that pipe-to-soil voltage measurements be recorded with respect to many of the pipelines coming into the station, as well as the amount of current from these pipelines to the negative terminal of the cathodic protection transformer rectifier.

We therefore have a DC electrical circuit and a certain amount of recorded data and can build a computer model which reacts to the input of the survey results. Using Kirchovs and Ohms laws, the computer is programmed to analyse the readings and display guidance for subsequent field activities.



PROGRESS SO FAR

A recent bi-annual CP survey has been conducted, using print-outs from this software package, and the data is being reported directly into a computer, by the contractors survey team. The overall report will be submitted to the client on a floppy disk, which will hold the spreadsheet, and its analysis. There will be a back-up, hard copy, print out which can can be bound and presented in any form to satisfy the traditional requirements of the client. The engineer can manipulate the computer to guide him for his follow-up activities.



DEVELOPMENT

The program is being developed while "in use" and, at present, is limited to disclosing cathodic protection defects which could have been detected by any competant engineer. However, historical data, from old records, has been subject to this program which has already revealed the causes of leaks that have actually occurred.

It is realised that human hindsight can normally perform this function but, by highlighting all the CP system defects as soon as the data is gathered, this tool would allow the engineer to decide his priorities more clearly, and in many instances, would have prevented the leaks.

It is clear that the real cause of some of these particular leaks, was the huge amount of infomation presented to the engineer, which could not be examined and analysed manually.



WHAT IS MANUAL ANALYSIS?

It is normal practice to examine survey results page by page, looking for individual readings which are below a particular criterion. The next step is to examine related readings which are on the same page, and then to turn from one page to another, examining related readings. The weight of information relating to the leak area, obviously, clouded the issue before the application of the computer analysis.

CP is so complex that it is difficult to follow any line of logic and even more difficult to visualise the knock-on effects of all the readings that are available.



HOW DOES THE COMPUTER HELP?

The program under development has a display which highlights the electrical nodes where the current readings do not, apparently, balance according to the laws of DC electricity (Kirchov). It is clear that this condition cannot exist, and that the measurements must be inaccurate or the circuit diagram incorrect.

A sub-routine is being developed to explore the possibility that polarities have not been noted correctly. This can cope with any node with up to six connected conductors, at the present stage of development. This will define the arrangement of polarities that will give a balanced result and could identify any pipeline that is discharging currentfrom the system.



PIPE-TO-SOIL POTENTIALS

The computer analysis of pipe-to-soil potential readings is being approached on a statistical basis, as there is no international agreement with respect to a definative criterion, at present.

The computer is programmed to display certain conditions graphically, where they suggest that the electrical pressures might cause current to leave the pipelines. Once more, no contentious theories are being utilised, but the massive number of voltage readings that are already available, are being displayed on a simple graphical format which allows more of the relevant data to be examined at one instance.



TESTING A HYPOTHESIS

One notion, that has been applied to historical data, has proved to be accurate. This uses potential shifts, calculated from voltage readings between unconnected steel and other steel which is connected to the negative side of the CP system. Both measurements are normally made, in practice, with reference to an electrode in the same position at each location.

It has been noticed that there is a tendency for corroded pipelines to have fallen into an identifiable group, which is being used as an indicator to identify pipelines which require special attention.

In this case, the supporting theory is too vague to report to this conference, but the results so far, look very promising. The computer is being used to perform boring, repetitive calculations that would be time consuming and may not be of consequence. However, now that this routine is in place, the computer can test the theory against a limitless amount of data at no extra cost.

It is easy to compare the theoretical current flow, deduced from the voltage readings, with the actual current readings,taken in the field, as these are on the same spread sheet. The program simply brings these together in graphical display, on the same screen.



FUTURE POSSIBILITIES.

Future development of this project is to design and implement the integration of all the oil fields and trunk line cathodic protection systems so that one gigantic spread-sheet can handle every voltage and current reading taken in the region. This will be a relatively simple achievement, as spreadsheets can now handle enormous amounts of data.

The more difficult task will be to make it possible to compare data historically, by linking all the spreadsheets containing every reading ever taken so that every reading can be related to every other reading, both topographically and chronologically.



FEASIBILITY

A feasibility study has been carried out in the UK, confirming that it is possible to construct a computern software product which will display a complete map of all pipelines etc. in the whole of an operation region. It will be possible to zoom in on any detail and view this in three dimensions. The computer will be capable of displaying the calculated amount of CP current and direction of flow at any node of the whole circuit.

This display would provide user friendly access to a data base containing tabular or graphical information, relating to the location selected on the map. This data-base could contain all data relating to the asset depicted on the display. When this stage of the development has been reached as a cathodic protection tool, then it would seem opportune to extend the data base to all other information.



USER FRIENDLY GRAPHICS

It is visualised that potentials will be represented by shading and colours and current flow by arrows such as are used on television weather reports.

This would allow for product flow and pressure to be represented in the same way and it would enable this type of computer handling to be integrated with a total condition monitoring and operation control system.

Such a program would take time and expertise to construct, but would be a model on which all future development could be based. It could even contain all the drawings etc. (in CAD) which are presently held in drawing office archives.

The feasibility of such a program may seem far fetched, but there is already a transport planning program in the UK that is based on a similar concept. It consists of a map of most of the roads in the UK through which the user can access a wealth of information relating to any location.