An analysis of the ballast voyage of an LNG Carrier. The significance of the loading and discharging cycle

Abstract

• Introduction of the Loading and Discharging Cycle in the analysis of laden and ballast voyages of LNG carriers. • Conjugate heat transfer analysis of the insulation- cargo system. • Projection of the Loading and Discharging Cycle on to the boiling curve. • Explanation of the pressure fluctuations observed during the laden voyage. • Ballast voyage being controlled by thermodynamic equilibrium. • Laden voyage being controlled by the boiling curve. A conjugate heat transfer analysis combined with a non-equilibrium thermodynamic model for LNG ageing during ship transportation has been developed based on experimental data. The laden and the ballast voyages are being considered as a continuous and inseparable process interconnected through the loading and discharging cycle with the thermal dynamics of the insulation of the cargo containment system. The loading and discharging cycle is instrumental not only in the understanding of the ballast voyage behavior but it is also the key for the explanation of the pressure fluctuations during the laden voyage as soon as the insulation temperatures during the cycle are projected on to the boiling curve. Thus, the pressure fluctuations may be attributed to the complex and turbulent vapor release mechanism associated with the various boiling modes imposed by the temperature non-uniformity over the insulation surface of the tanks. The secondary barrier temperature measurements of the insulation show that significant temperature variations exist across the tank walls, suggesting, that different heat fluxes cross the primary insulation barrier and consequently different boiling modes (regimes) simultaneously exist on the tank wall surface which is in contact with the liquid cargo. Moreover, the on-board pressure and temperature measurements in the tank reveal that the gas and the liquid phases during the ballast voyage are almost in local thermodynamic equilibrium, since the tank walls are interacting primarily with the gas phase and the multi-boiling regimes envisaged by the boiling curve are no longer the defining characteristic of the physical process. This is an important finding, resulting in a drastic reduction of the required computational effort, compared with the laden voyage, enabling the use of simpler equations of state and even Raoult’s ideal mixture law.