Sloshing Impact of LNG Cargoes In Membrane Containment Systems In the Partially Filled Condition

LNG carriers have usually operated in the fully loaded condition or with a minimum filling of liquid cargo for the tank coolingdown 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. The sloshing load at filling levels other than the fully loaded or ballast condition is the main concern for vessels operated in this manner. In particular, the sloshing impact load on the insulation system and hull and the dynamic load on the pump tower at the low-filling levels is of particular concern in light of the previous history of damages. As a consequence, ABS has undertaken a comprehensive analysis of the new patterns of sloshing loads in the partially filled condition. This has included: • Evaluation of the extreme ship motion in the North Atlantic Ocean with service life of 20~25 years • Determination of ship motion condition that gives most critical sloshing load • Time domain simulation of the sloshing motion • Calibration of the sloshing load for scale effect • Evaluation of the factor of safety of the insulation system • Evaluation of dynamic load factor for hull scantling • Structural analysis of pump tower This paper describes the method of approach to these sloshing issues, with the emphasis on the difference in the sloshing load at the highand low-filling conditions.