A recent study was carried out by Strong, Adey and Rudas and reported in . In this study the design of a CP system to protect the external surface of floors on a number of large, above ground chemical storage tanks located in close proximity to each other was investigated. This problem offers a significant challenge if CP interference is to be minimized. What makes this task difficult is that, in some cases, the tank floors are uncoated. Thus large CP currents are required to fully protect the steel. The effect of this large current combined with the steel floor being located at ground level and immediately adjacent to buried pipelines and steel foundations creates the ideal situation for CP interference.
Three 56.5 m diameter tanks are positioned 28m apart (84.5 m from tank centre to tank centre) and aligned in a row. The external floors of all three tanks are cathodically protected using four anode groundbeds, as shown in Figure 13. Each tank was protected using its own DC power source employing a current of 80A per tank (i.e. total current 240A). The DC circuit was designed so that the two anodes protected each tank floor diametrically positioned 14 m from the edge of each tank, with each anode contributing to 50% of the total current requirement for the tank. Thus, the two outer anodes carried a current of 40A each while the two inner anodes discharged a current of 80A each.
The anodes were 0.2m in diameter. 10m long, and initially buried vertically so that the tops of the anodes were located 20m below the ground surface. To ensure that the CP interference was not underestimated the soil resistivity was assumed to be uniform and equal to the surface resistivity of 50 ohm m. The pipeline was located immediately above one of the inner anodes and extended in a direction away from both the tanks and the anodes. Once again, to ensure that the CP interference was not underestimated, the coating defects were located immediately above the anode and at the extremity of the pipe, i.e. 65m from the anode.
Figure 14 shows the potential distribution over the tank floor with respect to a saturated Cu/CuSO4 reference electrode, It shows, as predicted, that the centre of the tank receives less protection than do the tank edges. Notably potentials at the centre of the tank are slightly more positive (approximately -820 mV) than the ideal protection criterion of -850mV suggesting that, initially, 80A may not be sufficient to fully protect the tank. The oval shape of the contours is indicative that the anodes are positioned too close to the tanks. To achieve a more symmetrical distribution of protection the anodes would have to be buried at a greater depth.
Computer Simulation as an aid to CP System Design and Interference Predictions, Robert Adey and John Baynham, BEASY