Application of the ultrasonic transmission tomography for inspection of the petroleum tank floor

The inspection of a storage tank floor is a time consuming and expensive procedure, mainly due to necessity to empty and to clean the tank before an inspection. For this purpose a long range ultrasonic technique based on Lamb waves may be applied. Such an approach is used for inspection of pipes and can cover the distances up 100m. However the storage tanks are much more complicated structures from the point of view of ultrasonic inspection due to presence of multiple lap joint welds. In order to propose an efficient testing technique regularities of Lamb wave propagation in tank floor plates and welds loaded by sand from a bottom side and by fuel (diesel) on the top side have been investigated. The novel ultrasonic NDT technique based on application of long range guided waves is proposed for detection of defects in a storage tank floor. The technique needs access only to the outer edge of the tank floor and do not require emptying and cleaning inside of the tank. The technique has been developed on the basis of the S0 mode Lamb wave transmission tomography. The propagation of the guided waves in the tank floor has been carried out using numerical simulation and experimental verification. The results obtained on the reduced size tank model and results of in situ experiments are presented as well.