In this paper, a process to verify that the cargo containment system of a membrane LNG ship is acceptable with respect to the application of sloshing impact loads is presented. The following is a summary of the process. A dynamic nonlinear FE analysis of part of the cargo containment system (CCS) is validated through comparison with the structural responses measured from simple dry drop impact tests. Then the validated dynamic nonlinear FE analysis is used to calculate the structural responses of the CCS and hull structure of an LNG ship against various sloshing design loads and the structural responses are compared to the failure criteria to define the structural capacity. Finally, the structural safety margin against sloshing loads estimated for specific sea-state operations with a specific filling condition is assessed by comparing the structural capacities obtained by the application of the proposed dynamic nonlinear FE analysis with the estimated sloshing loads directly.