Structural Integrity Assessment of Independent Type ‘C’ LNG Carriers

The objective of this paper is to present a complete procedure for strength evaluation of independent Type ‘C’ LNG carriers. This strength evaluation procedure includes two main parts, global finite element (FE) analysis mainly for hull structure evaluation and detailed FE analysis for tank and saddle supporting structures. A global FE model with three tanks including hull, tank and saddle supporting structures is used for structural analysis. Static and dynamic loads are applied to the global FE model as design load cases for the total strength assessment (TSA). FE analyses on the three tanks model have been performed and verified by the developed acceptance criteria to evaluate the safety adequacy of yielding, buckling, and fatigue of the hull and supporting structures. Additionally, a detailed FE model for a pressurized tank and its saddle supports is made, which consists of major components including tank shell, ring stiffeners, longitudinal central bulkhead, and saddle supports. An FE stress analysis of the pressurized bi-lobe tank with saddle supports considering the hull deflection effect has been carried out under internal pressure, dynamic loads due to ship motion, and thermal loads. Finally, an FE buckling analysis of the bi-lobe tank has been carried out under external pressure following gas carrier (IGC) code requirements. Meanwhile, acceptance criteria are developed for yielding and buckling strength evaluation of pressurized bi-lobe tanks according to the ASME Boiler and Pressure Vessel (BPV) Code. An independent Type ‘C’ bi-lobe LNG vessel has been utilized to implement the presented evaluation procedure. Finally, a complete procedure for evaluating the structural strength of cargo tank structures, supporting members and hull structures has been developed for an independent Type ‘C’ LNG carrier.