Sacrificial anode retrofitting to aging structures is routinely performed for life extension of offshore assets in matured fields. Sometimes earlier retrofit systems are now being replaced or supplemented. Understanding when remnant CP systems can no longer prevent depolarization is important as timely intervention can reduce overall retrofit cost, which is a significant factor. Life must be extended a required number of years, and design should take into account performance of remaining anodes and state of calcareous deposits. Data for this is generally available from past surveys.
During design, computational modeling can be used firstly to gain quantitative understanding of the state of the structure, remaining life of existing anodes and estimated date at which serious loss of calcareous deposits will occur. Secondly, modeling can determine the short term effect of a new CP system on structural potentials. This information can be used to modify the numbers, positions and mass of new anodes. This optimizes distribution of potential and anode mass loss rates. Moreover, benefits of fewer large versus several smaller anodes can be weighed-up. Finally, modeling can determine the long-term effects of new, old, and combined CP systems, eg to identify when individual anodes reach their utilization factor and consequent effect on remainder of structure.
The aim of this paper is to present a case where modeling has been applied to a jacket structure, using “long-term” polarization curves to represent accumulation of calcareous deposits. Hence, the key benefits achieved are: retrofit requirement reduction, significant cost savings, better CP current distribution despite reduction in number of anodes, maximized life. Other future benefits are possibility to predict future CP survey frequency and improved planning of retrofits requirements; hence saving further cost on needless future surveys.
Keywords: Optimization, CP, Cathodic Protection, Modeling, Retrofit, Jacket, Long-term polarization.
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