Abstract
At present, three liquefied natural gas (LNG) carriers of Moss Rosenberg Verft (MRV) spherical tank type are being built at Nagasaki Shipyard & Engine Works of Mitsubishi Heavy Industries, Ltd. (MHI). The first of the three carriers has been launched and fitted with tank insulation, and outfitting work is being carried out. These spherical tanks have been built by welding aluminium alloy 5083-0 plates. Accordingly, non-destructive testing of welded joints of the spherical tanks is one of the most important factors to assure the security of the tank system. In this paper, the detectability of radiographic test (RT), ultrasonic test (UT) and penetrant test (PT) is investigated using welded joint specimens with various kinds of defects. Main conclusions are as follows. (1) Detectability of UT for planar defects such as lack of fusion is much superior to that of RT. (2) Generally speaking, detectability of RT for blowholes is superior to that of UT. However, detectability of UT for clustered blowholes is equal to that of RT. (3) Surface cracks greater than 20μm in depth can be detected by dye-penetrant test. (4) In order to conduct UT with high accuracy for welded joints of ultra-thick plates, it is necessary to control internal defects such as microporosities and to take account of the attenuation of ultrasonic in the base metal.
