Strength Assessment of Type ‘C’ LNG Fuel Tanks

The objective of this study is to present a complete procedure for the strength evaluation of Type ‘C’ liquefied natural gas (LNG) fuel tanks. The fuel tank is designed independent of the ship’s hull and consists of double tanks with inside and outside supports. The inner tank, which holds the LNG cargo, is held within the outer tank using strap and cone supports at two ends. The outer tank is kept in vacuum and the space between the inner tank and the outer tank is filled with insulation materials. There are two saddle supports connecting the outer tank to the hull. The major components of the fuel tank in FE model include inner tank, outer tank, strap support, cone support, as well as two saddle supports. All these structures are designed to withstand all ship and service design loads. The inner tank is subjected to the internal pressure and the outer tank is loaded by external pressure. Due to the ship motion, the inertia forces caused by cargo accelerations will be applied to the tank structure. In this study, Finite Element (FE) analyses were performed in various load cases in which load components include design internal/external pressures, longitudinal / transverse / vertical accelerations, and gravity. Additionally, a Computational Fluid Dynamics (CFD) sloshing simulation on the fuel tank with splash baffles was performed to determine sloshing loads. Accidental load cases, including collision and flooding, were also considered in the structural analysis. Meanwhile, a buckling analysis on the evacuated outer tank was conducted to identify the critical collapse pressure. Acceptance criteria corresponding to yielding and buckling failure modes have been detailed for the strength evaluation of the Type ‘C’ LNG fuel tank. Thus, a complete procedure has been developed for the structural integrity assessment of tank and supporting structures for Type ‘C’ LNG fuel tanks.