It is estimated that there are more than 500,000 Above ground storage tanks (ASTs) in the U.S. These tanks can leak gradually (more likely) or may collapse suddenly (low probability). The loss of tank content can lead to water contamination or may lead to a fire in case of a hydrocarbon.

The most common failure mode for storage tanks is corrosion and usually most ASTs have some kind of corrosion protection. It is a common practice to have a tank inspection program as a part of preventive inspection. How frequently should one inspect? You should carefully evaluate inspection frequency during following situations:

• If tank inspection is adversely affecting production
• If you have fewer resources

API 301, Aboveground Storage Tank Survey, was conducted in 1989 and covers all segments of petroleum industry, namely marketing, refining, production, and transportation. The failure rate for tanks is based on data reported for 572,620 production tanks. There were 8,389 tanks replaced or repaired in an average year due to visible indication of leaks. If we simply divide the number of failures by the number of tanks we obtain the average failure frequency – 1.5 x 10^-2 failures/yr or 1 failure every 67 years. You should take into account storage material since it will impact corrosion rates; however, for this article I’m assuming you are storing oil or liquid hydrocarbons. Furthermore I will assume that there are no errors in tank installation or fittings.

We can therefore conclude that a tank failure will be observed once every 67 years.

This high failure frequency of 1 in 67 years provides a significant discretion in choosing inspection interval. One can now argue that periodic inspection frequency for a tank can be increased beyond the usual 10-years period.

If you want to decide how much can the periodic frequency be increased, you will have to take into account the tank MOC in consideration, material stored and carefully look at the various failure modes.