EVALUATION OF DESIGNS OF SHIPBOARD CATHODIC PROTECTION SYSTEMS USING BOUNDARY ELEMENT MODELLING TECHNIQUE

Boundary element modelling technique was used to evaluate the performance of the underwater hull impressed current cathodic protection (ICCP) system of a steel-hulled vessel, as part of an investigation of the corrosion failure of the steel hull near the ICCP anodes. The same technique was also used to evaluate the designs of an alternate sacrificial anode cathodic protection system. The modelling results indicated that the existing ICCP system was capable of providing adequate cathodic protection to the underwater hull and appendages if the current demand is less than 2.4 A. Small anode size was found to have a profound effect on the potentials immediately adjacent to the anode, stressing the need for an effective, intact anode shield. Application of paint coating on the bronze propellers could substantially reduce the current demand from the anodes, resulting in less negative potentials adjacent to the anodes. The modelling results also showed that the anode design adopted from a vessel of similar size (<5% difference in all dimensions), which consists of 8 hull mounted aluminum anodes and 4 rudder zinc anodes, would provide adequate cathodic protection for the underwater hull structures of the existing vessel for more than 3 years. The performance of a modified anode placement was also studied in comparison with that of the original anode placement. The results indicated that the anodes in the modified anode placement would not only contribute more uniform current than the anodes in the original anode placement, but also result in the potential profiles that have less variations along the hull form. This modified anode placement would potentially result in a longer service life of the anodes.