When the bottom of an oil tank corrodes, there is the potential for massive leaks if a hole develops or the tank is damaged by an earthquake. At present, the diagnosis of oil tank corrosion is performed once every ten years by emptying the tank and then using an ultrasound to measure the thickness of the steel plates in the tank's base. However, this method not only requires the oil tank to be emptied, it also requires a great deal of effort in finding all points of corrosion.

In response to this, we are developing a method for diagnosing corrosion at the entire bottom of oil tanks using acoustic emission (AE) waves. Such a method would have the potential for rapid diagnoses to be made, without having to empty the tank. If such diagnostic technology were to be established, highly effective and efficient inspection systems would become possible. This would include the inspection of oil tanks while they are still operating, the rapid and detailed inspection of only those tanks in which advanced corrosion is detected, and the postponement of inspections in tanks in which no corrosion has occurred.

AE waves are faint elastic waves emitted in response to corrosion activity. They are believed to result from the peeling off and splitting of the substances formed by corrosion on various parts of the tank. These AE waves can be detected using sensors attached outside the tank. We have been continuously performing measurement experiments on AE waves using actual oil tanks. Our results have enabled us to quantitatively identify the relationship whereby AE activity grows as the risk of corrosion on the tank base increases. Based on this relationship, we are able to estimate the degree of corrosion using AE wave measurements.

Crack that occurred at the bottom of an oil tank during the 1978 Miyagi-ken-oki earthquake.It is believed the crack resulted from the strong shaking of the corroded oil tank.AE wave sensors attached to the shell plate of an oil tank

Software used to locate sources of AE waves by means of a neural network that we have developed