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Premature Failure of API 650 Oil Storage Tank Bottom Plates Due to Soil Side Corrosion

Picture for Premature Failure of API 650 Oil Storage Tank Bottom Plates Due to Soil Side Corrosion
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The above ground storage tanks in the refinery are experiencing bottom plate underside (soil) corrosion at a high rate of 1mm/year. Results: Failure of 4 tanks within a period of 9 years. Observations, details of the CP survey, tests, results and forward path taken up.

 

Product Number: 51317--9025-SG
ISBN: 9025 2017 CP
Author: Naif Abri
Publication Date: 2017
Industry: Petroleum Refining
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The above ground storage tanks (AST’s) in the refinery are facing an unprecedented form of bottom plate underside (soil) corrosion at the rate of 1mm/year. This has resulted in the failure of 4 tanks within a period of 9 years since commissioning (2006-2015). The corrosion morphology is of severe localized corrosion with large deep pits and corrosion lakes.The design of the underside corrosion protection for the AST’s is by providing an impressed current cathodic protection (ICCP) system (MMO grid anode system). The grid anode is placed between the bottom plate and a HDPE (Release prevention barrier) sheet. Recent and past cathodic protection (CP) survey of the tanks clearly reveal a very low (electropositive) polarized potential across the tanks indicating the CP system has not been functioning since commissioning. From the failure analysis it is concluded that the tanks are corroding at an unacceptably high rate and the corrosion is specifically related to the inadequate strategy of CP system which has completely failed in its objective of corrosion control.The priority of the refinery is to apply a short term stratagem by which the corrosion rate is substantially slowed which would enable to distribute these tank turnarounds across as many years as possible. If this is not achieved then a large number of tanks might be expected to perforate in any given year overwhelming maintenance works and disrupting operations.The report summarizes the observations details of the CP survey tests results and forward path taken up by the refinery.Key Words: Storage Tanks Impressed Current CP System Soil Side Corrosion.

The above ground storage tanks (AST’s) in the refinery are facing an unprecedented form of bottom plate underside (soil) corrosion at the rate of 1mm/year. This has resulted in the failure of 4 tanks within a period of 9 years since commissioning (2006-2015). The corrosion morphology is of severe localized corrosion with large deep pits and corrosion lakes.The design of the underside corrosion protection for the AST’s is by providing an impressed current cathodic protection (ICCP) system (MMO grid anode system). The grid anode is placed between the bottom plate and a HDPE (Release prevention barrier) sheet. Recent and past cathodic protection (CP) survey of the tanks clearly reveal a very low (electropositive) polarized potential across the tanks indicating the CP system has not been functioning since commissioning. From the failure analysis it is concluded that the tanks are corroding at an unacceptably high rate and the corrosion is specifically related to the inadequate strategy of CP system which has completely failed in its objective of corrosion control.The priority of the refinery is to apply a short term stratagem by which the corrosion rate is substantially slowed which would enable to distribute these tank turnarounds across as many years as possible. If this is not achieved then a large number of tanks might be expected to perforate in any given year overwhelming maintenance works and disrupting operations.The report summarizes the observations details of the CP survey tests results and forward path taken up by the refinery.Key Words: Storage Tanks Impressed Current CP System Soil Side Corrosion.

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