This paper presents a new sloshing assessment procedure based on regular motion scenarios to evaluate the structural integrity of the membrane type LNG containment system. After making a pressure contour map based on a series of 2-D sloshing simulations with 1- degree of freedom (DOF) regular tank motions, 6-DOF regular design motion scenarios in each surge, sway, roll and pitch mode can be obtained. Then from the 3-D sloshing simulations and local dynamic structural analysis considering fluid-structure interactions the structural safety of the system can be assessed. The proposed procedure is found to be promising by the comparison with the results from the existing irregular procedure.
The sloshing assessment technology is critical for the development of the large membrane type LNG cargo containment system. Due to severe nonlinearities of the sloshing impact load the long-duration model test with statistical post-processing to predict the most probable maximum impact pressure has been recognized as the most reliable methodology to assess the safety of the cargo tank within LNG industries (Pastoor et al., 2005). Recently, Kuo et al. (2009) reported that the most probable maximum pressure during three hour exposure would be insufficient to cover the highly stochastic character of the sloshing impact pressure peak and thus extremely long duration sloshing model test should be performed to ensure the statistical consistency of the impact load. The assessment procedure based on the above methodology demands high cost and considerable time. These situations make it difficult for shipbuilders to evaluate several cargo tank candidates which are subject to minor changes at the early design stage. In principle, the most probable maximum sloshing impact load should be defined during the vessel's lifetime and every feasible sea state on a wave scatter diagram needs to be reviewed in terms of the long-term statistics.