Historical inspections of atmospheric storage tanks in a major onshore Oil and Gas terminal identified underside floor corrosion. Investigations suggested that the corrosion was likely caused by a number of factors, namely, issues with the cathodic protection (CP) system and water ingress. Based on previous experience gained from atmospheric storage tanks inspection of similar design, a decision was taken to act on this matter.
The intent of this paper is to share learnings and observations on how potential damages were flagged, repaired and/or mitigated, including improvements made at site to ensure repeat issues do not occur, maintenance costs are reduced, and reliability is maintained for long-term utilization
The terminal is subdivided for Oil, Gas and Produced Water plants. Each of these plants has number of storage tanks. The age of the tanks varies between 15-25 years. The products held within these tanks varies, from crude oil, condensate, produced water, potable water, to off spec oil and diesel fuel.
Most of the tanks within the facility have a similar CP arrangement and design. Each tank base is protected by an impressed current grid mesh anode buried in compacted, clean, sand backfill beneath the tank base and is powered by a transformer-rectifier placed outside the bund wall or within an electrical switch room. Permanent reference electrodes are installed beneath all tank bases to enable accurate potential measurements. Reference electrodes vary from Copper/Copper Sulphate, Silver/Silver Chloride to Zinc.
The general goal of the facility's Tank Corrosion Management program is to identify corrosion threats, define and implement appropriate corrosion control measures, assess performance, and maintain barrier effectiveness. Through execution of the program, inspections revealed underside floor corrosion on many of the tanks. Investigations suggested that the corrosion was likely caused by a number of factors, namely issues with the CP system and water ingress.