Polarisation modelling as part of galvanic anode upgrade of a hybrid offshore cathodic protection system

This paper describes the development of a computer model to evaluate the performance of replacement galvanic anodes for a 25-year old offshore North Sea platform. The platform had a hybrid cathodic protection system comprising impressed current anodes mounted around the structure as well as aluminum - zinc - indium galvanic anodes. Over a period of time a number of the impressed current anodes had failed which together with natural consumption of the galvanic anodes had started to result in under polarization for sections of the structure. Consequently a cathodic protection upgrade was started, incorporating retrofitting of new galvanic anodes to the structure. In order to determine the optimum location for the new anodes a computer model was developed utilizing an iterative approach with an industry standard earthing (grounding) software model and application of polarization data. The model was first set up with the as-found condition and calibrated against subsea survey data. Once calibrated the impact of the proposed galvanic anode locations was assessed as well as various other "what-if" scenarios, including failure of remaining impressed current anodes. The paper highlights problems encountered with the modeling exercise, in particular due to lack of reliable data in some areas, and how this was overcome. © 2007 by NACE International.