INTRODUCTION
To control external corrosion of the soil side tank bottom on crude oil storage tanks, impressed current Cathodic Protection system using polymeric cable (linear) anodes have been utilized for over 10 years. For certain tank farms, polymeric cable anodes were placed outside the tank foundation in a ring configuration and for other tank farms, the polymeric cable anodes were placed within the tank foundation. This paper describes the design, configuration, method of installation and assessment of operational parameters of this type of cathodic protection system for protection of tank bottoms of above ground storage tanks (AST?s).
Indian Oil Corporation Ltd. owns & operates above ground storage tanks which have been built on compacted sand foundation contained within the concrete ring foundation or on a sand foundation built on earth reinforced with stone pile columns to increase the load bearing capacity. The tank bottom plates are coated on the soil-side with 100 microns of bitumen based paint or coal tar epoxy prior to construction and then welded together to complete the tank bottom. The heat of the welding process damages the coating in the vicinity of the welds area and these unprotected areas in contact with the soil cannot be repaired and our experience shows are locations where corrosion occurs leading to eventual leaks.
To prevent this soil side corrosion, it was decided to implement an effective cathodic protection system on large diameter crude oil storage tanks of diameter ranging from 60meters to 80meters where the soil resistivity of the tank farm area was less than 5,000 ohm cm.
Cathodic protection of AST bottoms is a real challenge in view of its geometry, level of protection at tank edge versus the tank center and the level of fluid in the tank which influences tank plate to soil interface contact. Accordingly, the design criteria / technical requirements drawn up for implementing the impressed current CP systems were:
- Structure to soil potential (PSP) between -0.85V to -1.2V OFF with respect to Cu/CuSO4
reference electrode.
- Uniform current distribution.
- System minimum design life of 30 years.
- Current density 10mA/m2
- Bare Steel-100%
- Minimal stray current interference with other incidental structures in the vicinity of tank
foundation, earthing grid, pipe supports etc.
- No anode splices or anode to anode cable connection under the tank bottom plate.
- Electrical remoteness to be achieved in the immediate vicinity of the anode cable.
- Overall circuit resistance to be limited to less than 1 ohm.
It was required by the civil department of the company that for foundations built on reinforced stone columns, no foreign material like anodes were to be laid in the sand foundation so as not to have any adverse impact of the sand compaction.
Based on the above technical and design consideration requirements, various type of anodes & anodes configuration were evaluated and the decision was made to use polymeric cable anodes with integral factory filled high performance coke breeze.
3 - 79 meter diameter tanks each having 60,000 KL capacities commissioned in 1994 were built on a sand pad foundation with a circumferential concrete ring wall. For these tanks, polymeric cable anodes were laid in a horizontal grid configuration in the tank foundation. Subsequently, three more tanks were built in a different location, where the earth was consolidated with stone columns and polymeric cable anodes were installed outside the tank foundation in circumferential loops.
