Abstract
After a subsea inspection on a relatively new platform in the North Sea substantial corrosion damage was revealed on the carbon steel seawater pump caissons. The subsequent offshore inspection utilizing a portable Ag/AgCl half cell revealed that the cathodic potential in the caisson was more positive than the free corrosion potential of carbon steel. Hence, the rapid corrosion failure of the caisson was related to galvanic interactions between the 6Mo stainless steel seawater pump and the carbon steel caisson.
A cathodic protection design based on galvanic anodes was performed according to NORSOK /1/. However, previous experience with similar problems had revealed that a design specifying the required anode alloy and amount of anode mass to achieve the requested lifetime would not solve the problem. The narrow annulus between the impeller section and the pump caisson internals introduces a significant larger potential drop than in the rest of the caisson. Hence, a computer model was established to verify that the anode current would pass the narrow annulus and polarize the damaged carbon steel impeller section sufficiently to avoid further corrosion development.
The final solution for distribution and type of galvanic anodes secured 10 years lifetime as requested by the client.
