Three-dimensional sloshing in a scaled membrane LNG tank under combined roll and pitch excitations

This paper experimentally investigates the three-dimensional sloshing in a membrane-type LNG (liquefied natural gas) tank under combined roll and pitch excitations. Seven groups of roll and pitch amplitudes are studied. For each group, the roll and pitch have the same frequency, and around ten frequencies are tested in a frequency band that ranges from 0.5 times of the resonance frequency in the length direction of the tank to 1.4 times of the resonance frequency in the tank breadth. The characteristics of the sloshing waves and impact pressures are analysed in detail. It is found that the steady-state sloshing waves can be classified into four patterns: the length-dominant wave, swirling wave, diagonal wave, and breadth-dominant wave. Highlighted is the swirling wave that is observed in most of the cases where the roll and pitch amplitudes are significant and the excitation frequency locates between the resonant frequencies in the two directions. It is hypothesized that the rotational motion of the tank imposes necessary initial conditions to trigger the swirling wave. The swirling wave is always associated with wave impingements at tank corners and induce violent impact pressures. The practical implementation is that reinforcements of the membrane layer should be added or the sloshing wave should be suppressed near the tank corner to mitigate the damages to the interior layer of the membrane-type LNG tank.