Title : Systematic troubleshooting and restoration of a failed ICCP system on an onshore storage tank bottom: A case study aligned with API RP 651
Abstract:
Storage tank bottom plates represent one of the most critical assets in onshore oil and gas facilities, where loss of cathodic protection can rapidly escalate into a High Safety Critical Element (HSECES) concern. This paper presents a structured troubleshooting and remediation case study in which routine Cathodic Protection (CP) monitoring — conducted per API RP 651 requirements — identified a complete absence of protective potential readings on a storage tank bottom plate under an Impressed Current Cathodic Protection (ICCP) system.
During a scheduled CP survey, potential readings at all perimeter test points and the center reference electrode returned zero values, immediately triggering an HSECES classification requiring urgent corrective action. Following API RP 651 diagnostic methodology, a systematic fault-tree investigation was initiated. The Transformer-Rectifier (TR) unit was first inspected and confirmed fully operational. Fault isolation was then extended to the power feeder network through a step-by-step cable continuity and insulation resistance testing sequence, which successfully identified a severed feeder cable as the sole root cause of ICCP system failure.
As an interim protective measure, and in accordance with API RP 651 provisions for temporary CP coverage during system outages, sacrificial galvanic anodes were deployed to maintain minimum protection criteria on the tank bottom while permanent repairs were engineered. Cable replacement was subsequently executed in a coordinated manner, strategically aligned with the upcoming scheduled off-stream internal inspection to minimize operational disruption and asset risk exposure. Following cable reinstatement and system re-energization, CP potential readings were fully restored to the required protection criterion of -850 mV (Cu/CuSO4 CSE) across all monitoring points.
This case study highlights the critical value of structured ICCP fault diagnostics per API RP 651, the importance of interim CP protection measures during system failures, and the benefits of integrating remediation activities with planned maintenance windows. Lessons learned and recommendations for enhanced CP monitoring, feeder cable protection, and HSECES response protocols in onshore storage tank facilities are discussed.
