Optimization of Retrofit CP Systems Using Computational Modeling by Evaluating Performance of Remnant and Retrofit CP Systems, Taking into Account Long-Term Polarization Effects

The retrofitting of sacrificial anode systems to aging structures is regularly performed as anticipated lifetimes are extended. In some cases previous retrofit systems are now being replaced or supplemented. Understanding when a remnant CP system will start to allow depolarization is important, as timely intervention can reduce the overall cost. The design of a retrofit CP system must extend life some required number of years, and needs to take into account existing anodes and the state of calcareous deposits, which may be available from survey.

During the design stage, computational modeling can be used firstly to gain an understanding in a quantitative way of the state of the structure, together with remaining life of the existing anodes and estimated date at which serious loss of calcareous deposits will occur.

Secondly, computational modeling can be used to determine the immediate effect of a new CP system on structural potentials. This information can be used to modify the number and/or positions of new anodes, for example to achieve uniform distribution of potential, or to achieve uniform anode mass loss rates.

Finally, computational modeling can be used to determine the long-term effects of the new and old CP systems, for example to identify when individual anodes will reach their utilization factor (and become inactive) and the consequent effect on the remainder of the structure.

The aim of this paper is to explain and present a case study where computational modeling has been applied to a jacket structure, using “long-term” polarization curves to represent accumulation of calcareous deposits on the structure. Example cases showing the predicted depolarization of the structure are presented.