Prediction of sloshing pressure and structural response of LNG CCS

Sloshing pressure that has a short rising time and a large peak value can cause structural damage to the primary membrane or insulation panel of the LNG cargo containment system (LNG CCS). Several experimental and numerical approaches have been suggested to estimate the design sloshing pressures used in the LNG CCS design stage. In this paper, in order to improve the management quality of LNG CCS during the operation of a vessel, methods for prediction of LNG CCS sloshing pressures and hot-spot stresses in real-time from measured innerdeck stresses (the latter not actually measured in the present study but calculated from the numerical simulation) are suggested. The proposed methods are based on the impulse superposition method and mode superposition method. A space superposition concept was introduced to consider the characteristic that the sloshing pressure appears differently at each position of the primary membrane. For prediction of sloshing pressure from inner-deck stresses, the inverse impulse/space superposition method was established, and the accuracy of the overall sloshing pressure prediction was enhanced by minimizing the sloshing pressure prediction error at a specific duration time through the least square method. By applying the impulse/space superposition method to the hot-spot stresses for the unit impulse, the hot-spot stresses for the sloshing pressure were calculated. The inverse process of the mode superposition method was also studied. In order to increase the accuracy of modal amplitude estimation, an additional mode selection process was devised to remove highly correlated modes. The prediction of LNG CCS hot-spot stresses calculated from the modal amplitude only was well performed, whereas the uncertainty of sloshing pressure prediction requiring numerical difference of the modal amplitude was quite high. From a series of numerical simulations, the validity of the proposed methods was verified. Using the realtime sloshing pressure time histories predicted from the proposed methods, the level of sloshing pressures generated in the actual LNG tank can be identified and the validity of the design sloshing pressures used in LNG CCS design can be judged. In addition, from the estimated real-time LNG CCS hot-spot stresses under sloshing pressures, LNG CCS structural health monitoring can be carried out during the lifetime of vessel.