Three structures and seven pipelines at Santa Ynez Unit (SYU) in the Santa Barbara Channel require cathodic protection (CP) life extension. The primary post-retrofit corrosion risks are expected to be overpolarization of the pipelines and underpolarization of the jacket interiors. Due to high current requirements and installation limitations, retrofit Impressed Current Cathodic Protection (ICCP) systems comprising high output remote anode sleds were specified. The polarization risk associated with remote ICCP systems is affected by sled position, especially when high current output sleds are used. Cable length is also controlled by sled placement and represents substantial project cost.
Given the complexity of SYU, historical methodologies such as "voltage rise" and "remoteness" criteria are insufficient tools for quantifying polarization risk. Polarization risk was instead defined with a 3D model that accounted for anode mutual interference and interactions between structures and pipelines. Model parameters were calibrated with CP survey measurements and historical environmental data.
The model was evaluated many times in a multi-objective optimization of anode sled placement to simultaneously reduce polarization risk and cable cost.
A tradeoff curve comparing system cost and polarization risk was constructed from which stakeholders and subject matter experts made an informed decision on the final configuration.
The life of offshore jacket structures can be extended up to 30 years towards the end of their design life by retrofits. For larger structures, and in unusual cases, semi-remote ICCP anode sleds are a favorable option compared to sacrificial anode cathodic protection (SACP) due to the high current output from each individual installation, reducing the time to install substantially. Anode sled locations are determined based on site plans, experience, survey data, and equations evaluating "remoteness" or "voltage rise" criteria.1 The assumption is that if they are placed at a maximum distance, the system would provide a better current distribution and eliminate over-polarizing nearby pipelines. However, computational models have indicated that maximizing the distance between the ICCP anode sled(s) and surrounding assets is not guaranteed to minimize the risk to both the pipeline and the overall protection of the structure.2 The use of 3D computer models to optimize the ICCP retrofit may result in better decision making and performance and will be enhanced by multi-objective optimization algorithms commonly applied to problems in other fields.
