AIE was recently contracted to support with a Cathodic Protection (CP) Optimization Study for a major onshore operator. The CP system is arranged over approximately 100 pipelines and 80 CP stations. The pipelines were categorized into corridors, with each corridor having its own CP stations.
Using AIE’s optimization methodology, with defined criterion based on industry best practice, an assessment was carried out for the pipeline corridors covering requirements of protection as well as life expectation of CP system power supplies and ground beds. The current requirement for each pipeline was calculated using an optimized current density (as per ISO 15589-1) which was specific for each pipeline coating type (3LPE, FBE and Coal Tar Enamel). The calculated current requirement for each pipeline based on the optimized current density values was compared to the current values from existing operating conditions.
The maximum CP current spread for each pipeline was calculated using a pipeline attenuation calculation (as per ISO 15589-1, Appendix E) to determine the maximum current spread achievable. The coating resistance of each pipeline was estimated by using the current density and the IR drop; the IR drop included the voltage drop through the soil and effectively considered the soil resistivity.
The maximum spread values were used to determine the specific CP stations that were required to be kept ‘ON’ in each corridor. This was done by plotting each pipeline’s route using Google Earth along with the CP stations available in the pipeline’s corridor using GPS coordinates.
It was found that many pipeline corridors’ CP stations had significant amount of excess capacity when the current requirements were calculated using the optimized current density. In many cases, the CP stations had individual rated current outputs exceeding the total current requirement for an optimized design for the whole pipeline corridor.
What this means in practice is that any adjustments to the system, i.e. bonding more pipelines into a CP station and switching off some CP stations, is not limited by the existing CP stations’ individual capacities. As may be expected, the achievable spread exceeded the pipeline length in many cases.
Using this data, AIE specialists were able to make recommendations to optimize the existing CP system by making several CP Stations redundant and ensuring the system operates efficiently.
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