Pipes with coated surfaces can be modelled in several ways. The coating can be considered to be a perfect insulator, a highly resistive barrier to current or a selective barrier to ionic transport, allowing water, dissolved gases and ionic species to permeate through to the pipe. It was assumed in the analysis that the coating is a highly resistive barrier, with a resistivity of 5x1010 Wm. This effectively modelled the coating as an electrical resistor in series with the IR drop through the soil. The current delivered to the coated portion of the pipe can be expressed by equation 1;
Where f is the potential on the surface of the coating, fcorrosion is the corrosion potential of the metal under the coating, r is the electrical resistivity of the coating measured in air, and d is the thickness of the coating.
For the first case that was modelled, the straight section of pipe had an undamaged new coating along its entire length. The bent pipe was modelled as having an aged coating along the section parallel to the straight pipe and a new coating after the bend. At this stage, a holiday was not modelled. This case was firstly modelled with a soil resistivity of 1x103 Wm and then repeated with a value of 5x102Wm.
The second case was identical to the first except that a holiday was introduced in the straight pipe. The holiday was at the halfway point of the straight pipe, facing the bend in the adjacent pipe. The boundary condition for the holiday took into account corrosion through polarisation data for steel. In each of the first two cases, the two pipelines were modelled as having separate CP systems. Therefore, a third case was modelled that was identical to the second case, except that the pipes were assumed to be electrically connected. The value of soil resistivity that was used was 1x103 Wm.
Computer Simulation as an aid to CP System Design and Interference Predictions, Robert Adey and John Baynham, BEASY