抄録
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 as scheduled. In all these ships, aluminium alloy A 5083-0 plates have been used for the spherical tanks, and especially ultra-thick plates have been used for equatorial rings of the tanks. Various properties of many samples of aluminium alloy A 5083-0 plates, which were manufactured on trial by two domestic manufacturers, are investigated in order to select suitable aluminium alloy plates for the spherical tanks. This paper describes a summary of the investigation results on three typical samples of the ultra-thick aluminium alloy A 5083-0 plates for the equatorial rings of the spherical tanks. These investigations are focused on the short transverse properties of the ultra-thick plates which are the most inportant for the spherical tanks. The chemical composition, macro-and micro-structure, mechanical properties, fatigue strength and micro-defects distribution of three samples are examined, and the following conclusions have been obtained. (1) The short transverse (ST) mechanical properties of these three samples are excellent compared with the published data of Shipbuilding Research Association of Japan, and there is no remarkable difference among the mechanical properties of these three samples. (2) The fatigue strength for ST direction of sample -1 and sample -3 are excellent compared with the published data of Shipbuilding Research Association of Japan. There is, however, a distinct difference in fatigue strength among three samples. It may be considered that the difference in fatigue strength is caused by the micro-defects found at the middle of the plate thickness. (3) These micro-defects can be detected by the ultrasonic flaw detector with an increased sensitivity. The fatigue strength for ST direction is to be estimated by the height of defect echo in the ultrasonic inspection with a proper sensitivity.
