Abstract
CP retrofitting of offshore structures is a very cost intensive operation. The Claymore 'A' platform has a hybrid CP system and for several reasons, including high maintenance costs of the IC system, the number of IC anodes has been reduced and retrofitting of sacrificial anodes has been required. In order to optimize the retrofit work and reduce cost, CP computer modeling and CP surveys have been utilised. Both computer modeling and surveys confirmed very low current density levels for the platform below 20 mA/m2, considerably lower than traditional design rules. The computer model was used to calculate the retrofit design. The CP potential and current density distribution were taken into account when calculating retrofit anode demand. This made it possible to determine an optimized retrofit programme with useful cost savings. The average difference between predicted and measured potentials was only 23 mV. Excellent calcareous deposit on the jacket, in addition to a relatively good fit of model prediction to actual potentials and still relatively large IC system, provide confidence for maintaining cost effective corrosion protection in the future. The calcareous deposit of mainly CaCO3 and Mg(OH)2 in the ratio of 3:1 was found to have different parameters of formation from scale/deposit compositions measured in the laboratory.
