Galvanic Anode Cathodic Polarization of Steel in Seawater: Part III — Retrofit Cathodic Protection of Offshore Structures

Numerous offshore petroleum production structures have aged to the point that the galvanic anodes on these will expire before the end of their useful lives. Retrofit of the cathodic protection (CP) systems for these may be required in the near future. However, there presently exists no criteria or recommended practice for retrofit CP design other than those for new structures. With this in mind, a recently proposed, first principles-based approach to CP design, one which utilizes parametric features of the linear potential vs current density decay with time, was applied to the case of retrofits to identify the optimum time for retrofit and aspects of the design itself for which this method has utility. Analysis of the CP design for two offshore structures for which CP data, before and after retrofit, were available indicated the existing CP system for these may have provided continued protection despite the fact that the anodes largely were expired. Also indicated was that the retrofit systems that were installed may not be capable by themselves of repolarizing the respective structures if calcareous deposits become removed by storm activity. Parameters identified as particularly important with regard to performance of new and retrofit CP systems were identified as the nature of the long-term sigmoidal potential-current density interrelationship, whether the structure is still polarized or has partially or completely depolarized, the likelihood that any of the remaining original anodes that have passivated or partially passivated will reactivate in the event of storm-related depolarization, and anode capacity at the requisite current outputs.