Coupling Effects Between Tank Sloshing and Motions of a LNG Carrier

Free liquid surfaces inside the cargo tanks of seagoing ships not only reduce the initial stability but also influence the sea-keeping characteristics, in particular roll motions. Due to the increasing demand of liquefied natural gas (LNG), the concept of offshore processing or receiving terminals will gain in importance. Since pipelines are not economic for long distances, LNG is transported by shuttle carriers from remote marine locations for onshore energy supply. The loading/unloading procedure of an average carrier vessel takes 18 to 24 hours — a time span where the tanks are partially filled and feature free liquid surfaces. Depending on the filling level, the wave incident angle as well as the phase shift of tank sloshing and body motions, the characteristics of the vessel’s response amplitude operators (RAO), especially the roll motion, are altering. This is ascribed to coupling effects between the moving liquid and the hull motions. Hence, the seakeeping characteristics have to be reappraised for each case. A 138,000 m³ LNG carrier with four membrane tanks is numerically analysed with a potential theory based software at different filling levels and wave incident angles in frequency domain. For experimental validation, the tanks are filled with water. Optical motion sensors and transient wave packages are used to determine the RAOs in six degrees of freedom. Additionally, the forces and moments induced by the sloshing liquid are measured by six-component force sensors. On this basis, further computations with LNG are conducted. The focus of the investigations lies on the influence of sloshing on the surge and roll motions of the exemplarily chosen LNG carrier. Instead of one single resonance peak as for the solid filling case, the roll RAO of the dual-mass system features two peaks, whose magnitude and position depends on the filling level of the cargo tanks. But coupling effects due to resonant sloshing also affect longitudinal body motions. The knowledge of the altered seakeeping characteristics of LNG carriers due to free liquid surfaces is essential for save offshore transfer operations.