In this application the performance of the cathodic protection systems protecting two pipelines placed in a right-of-way with 0.6 m of separation is investigated. One of the pipes leaves the right of way at a 45° angle halfway along the length of the pipes modelled. Each pipe had an outer diameter of 1.8 m that included a coating thickness of 6.0 x 10-4 m. The depth at which the pipes were buried was 3.0 m.
A series of remote anode groundbeds were allocated to each separate pipe CP system and staggered at intervals along the length of the pipes. The anodes had a length of 18.3 m, a diameter of 0.3m and were modelled as being 79.2 m below the surface of the soil. This distance was sufficiently far for the anodes to be considered remote.
The area of interest in the analysis was near the bend in the 45° pipe (Figure 3). To model the current and potential distributions around the circumference of the two pipes, 3-dimensional boundary elements were used to discretise the surface of the two pipes for a length of 13.7 m on either side of the 45° bend, as can be seen in Figure 4. Beyond this region, BEASY 'tube' elements were used which assume a uniform radial current density distribution. Tube type elements are adequate for situations in which the average distribution of potential and current density along the pipeline is sufficient. (I.e. the variation around the circumference of the pipe is not required) The potential levels and current demand can be used for design purposes but the impact of very localised defects cannot be represented with this type of element. The pipes were enclosed in a far field boundary modelling zero normal flux density as is shown in Figure 2.
Computer Simulation as an aid to CP System Design and Interference Predictions, Robert Adey and John Baynham, BEASY