Abstract
The influences of the coating covered over the butt-welded bottom plate joint of oil tanks on the ultrasonic transmission factor and detectability of surface flaws are summarized in this paper, which were performed as one of the parts of projects for development of rational maintenance management of national oil-stockpiling tanks from 2009 by Japan Oil, Gas and Metals National Corporation.
As the rational examination method to substitute the magnetic particle examination that requires the removal of the present coating, the ultrasonic examinations by the creeping wave technique and the phased array technique using sector scans were discussed in this research.
Existence of coating is rather suitable for the ultrasonic transmission and reception of the creeping wave, and there was no necessity in particular for compensation of transmission loss. However, when the thickness of the coating had irregularity and the incidence angle to a specimen changed from the condition to emit creeping wave, amplitude of transmitted creeping wave was made rapidly small.
In the angle beam method, when the coating became thick, the transfer loss became larger, but it was able to evaluate flaws without underestimating to 1500 micrometers in coating thickness, by performing sensitivity compensation of 10 dB beforehand by using the polystyrene resin for the wedge material.
When flaw detectability was discussed using the butt-welded joints induced EDM notches and simulated natural flaws on the surface of welds and covered with the coating, especially in creeping wave technique, the result inferior to the detection of flaws induced on the reinforcement part was obtained. It was considered as a factor that bringing a result in which the creeping wave should spread the lower part of a notch due to height of the reinforcement of welds, and the propagation distance of a creeping wave became long, and attenuation became large.
On the other hand, according to phased array technique using the sector scans and the focusing ultrasonic beams, all flaws including simulated natural flaw could be detected, and it was evident that the phased array technique examined on the coated surface had sufficient ability of flaw detection for substitution of magnetic particle examination conducted after removal of the coating.
