LNG carriers have usually operated in the fully loaded condition or with a minimum filling of liquid cargo for the tank cooling-down purpose during the ballast voyage. The typical filling level of the LNG tank is greater than 95 percent of the tank height in the fully loaded condition and less than 10 percent in the ballast condition. Recently however, there has been growing demand for membrane-type LNGCs that can operate with cargo loaded to any filling level. This demand stemmed from the emergence of a spot market for LNGCs. LNG FPSO/FSRUs and their shuttle vessels also must also be able to operate in the partially loaded condition. As a consequence, ABS has undertaken a comprehensive analysis of the new patterns of sloshing loads in the partially filled condition.
LNG carriers have usually operated in the fully loaded condition or with a minimum residue of cargo during the ballast voyage. The typical filling level of the LNG tank is greater than 95 percent of the tank height in the fully-loaded condition and less than 10 percent in the ballast condition. Early designs of membrane LNG tanks experienced a number of incidents involving minor damage at the insulation box when in the high filling condition. Damages in the low filling condition were also reported. These have been very limited, occurring primarily when the No.1 cargo tank was loaded to between 15 and 20 percent of the tank height to provide LNG as coolant for other empty tanks during the ballast voyage. Class societies, containment system designers and ship operators have conducted thorough studies of these damages. In every instance the sloshing of the cargo was identified as the cause of the damage. Simple but effective plans were proposed to counter the sloshing impact.