The spherical tanks for the Moss-Rosenberg type of liquefied natural gas (LNG) carriers are to be fabricated from rolled and forged nine percent nickel (9Ni) steels, with the former steel being used as tank shell-plate material and the latter as tank equator-ring material. Accordingly, the authors carried out a series of experimental studies to evaluate the fatigue strength, resistance to fatigue crack propagation, fracture toughness, etc., of these tank steels having due regards to the “leak-before-failure” premise of the Moss-Rosenberg LNG containment system.
Various forms of plain and welded test specimens were prepared from actual shell-plate and equator-ring materials for fatigue tests; and, the effects of stress ratio and surface finish of reinforcement weld on the tank-steel fatigue strength were closely studied.
Wide-plate test specimens were fatigue tested under repeated tensile and bending loads, and configurations of through cracks immediately after penetration and crack propagation rates were examined.
Notched plain and welded wide-plate test specimens of different thicknesses were prepared from tank steel plates produced from different heats and submitted for tension tests to determine the critical stress intensity factor (Kc value) at the cryogenic temperature of LNG.
Using data obtained on material properties of the rolled and forged 9Ni steels, the authors evaluated the structural safety of the spherical LNG tank and confirmed that the 9Ni steel LNG tank completely satisfied the leak-before-failure criterion.