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Table 1 summarises the effect of CP interference on the pipe as a function of the size of the coating defect. As intuition may suggest, locating the pipeline within the influence of the anodic field results in CP current being picked up by the pipeline in the vicinity of the anode and being discharged at the remote end of the pipeline. Assuming the worst case scenario that the corrosion process involves formation of Fe, the current densities corresponding to current discharge can be converted to metal loss per year. The severity of this interference is now readily apparent and the results suggest that it would be prudent to undertake further measures to help minimise the interference.
It is interesting to note that the corrosion rate is predicted to be higher when the coating defects are smaller. This suggests that smaller coating defects tend to focus the interference effects, and thus increase the damage caused by the interference on the pipeline.
One possible measure to help minimise the interference effects is to bury the anodes at a greater depth and thereby minimise the anodic interference. Table 1 summarises the effect of lowering the anodes so that the top of the anodes is located at a depth of 70m the model predicts that repositioning the anodes has major effect on interference levels. It not only increases the effect of CP interference but also relocates the corrosion site. Corrosion that was initially located at the remote end of the pipe is now located immediately above the groundbed. This occurs because lowering the anodes removes the anodic field at the ground level. The pipeline is now subjected only to the cathodic field of the tanks. As result current is picked up at the remote end or the pipeline and conducted along the pipeline before being discharged back into the ground at the point where the pipeline closely approaches the tank floors.
The fact that the current flow inducted by CP interference is reversed by positioning the anodes at greater depths suggests that there is an optimum depth at which anodes could be buried so that interference is minimised. Determining the optimum depth could be evaluated efficiently using computer modelling. This would help to minimise the number of field tests to be undertaken. In order to optimise the position of the anodes, a more detailed resistivity profile of the soil would be required to enable local variation of the soil resistivity to be included in the model.
Computer Simulation as an aid to CP System Design and Interference Predictions, Robert Adey and John Baynham, BEASY