# Effect of Parameters on Vapor Generation in Ship-to-Ship Liquefied Natural Gas Bunkering
## 6 Conclusions  
Ship-to-ship LNG bunkering is getting more attention in the maritime sector. It is quite different from conventional onshore LNG loading/unloading in regard to several features, such as the tank type, scale, and bunkering system configuration, etc. In the present study, the effect of the main parameters  
Figure 11: Variation of tank pressure with time (solid line: bunker tank, dotted line: receiving tank). Figure 12: Variation of vapor return flow rate with time. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Without Pressure Control** & **With Pressure Control** \\ \hline Total amount of vapor return (kg) & 7392.55 & 3317.15 \\ Receiving tank net vapor & 3352.41 & \(-\)695.32 \\ generation, \(m_{net,g,g^{\prime}}\) (kg) & \\ Net vapor generation (Overall & \\ system), \(m_{net,g,g^{\prime},overall}\) (kg) & \(-\)380.61 & \(-\)442.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Comparison of the evaluated values at the condition of with/without pressure control. on the performance of ship-to-ship LNG bunkering was studied via dynamic simulations. Our main findings are summarized as follows:  
* When the initial temperature of the receiving tank increases from \(-150\) to \(-145\)\({}^{\circ}\)C at a fixed temperature of the bunkering tank (\(-147\)\({}^{\circ}\)C), the amount of vapor return is proportionally increased. The optimum temperature difference (and pressure difference) should be determined in consideration of tank pressure management. * The net vapor generation in the overall system has a positive value when the receiving tank temperature is colder than the bunkering LNG temperature. In contrast, it has a negative value when the receiving tank temperature is warmer than the bunkering LNG temperature. * Increasing the bunkering flow rate decreases the amount of heat ingress through the pipes and tanks, although the heat dissipated by the pump shaft power increases. In addition to the heat ingress effect, the pressure increase in the receiving tank from pumping is also an important factor. The optimum flow rate must be determined while considering the vapor return and bunkering time limit. * The influences of different insulation materials on the vapor return and vapor generation are minor; however, the influence of the insulation depends on the layout of the bunkering system (i.e., the results may be different for longer pipe lengths). * The composition of the LNG affects the vapor return and vapor generation. Different LNG compositions in the bunker tank lead to changes in the initial pressure of the bunker tank, influencing the vapor return and vapor generation in the receiving tank. When the nitrogen content is varied, the pressure and vapor return are significantly affected. A pressure control method is proposed to compensate for the pressure decrease by the LNG discharged from the bunker tank, and to maintain the bunker tank pressure at the initial condition. With pressure control, the total amount of vapor return to the bunker tanks is reduced from 7392 to 3317 kg. The net vapor generation in the receiving tank (\(m_{net,pg,r}\)) is reduced by up to 4047 kg and the net vapor generation in the overall system (\(m_{net,pg,overall}\)) is reduced by 16.2%, although the receiving tank pressure is 0.2 bar higher than that without pressure control. The limitation of this study is that a constant heat transfer coefficient, instead of a temperature-dependent heat transfer coefficient which will slowly change depending on the in/out temperatures, was assumed. This caused an amount of vapor return of 0.25% offset in the base case simulation. In the future study, this should be reasonably considered. Overall, the results of the parametric study can lay a reliable foundation for ship-to-ship LNG bunkering operations and provide insights for establishing vapor management strategies for bunkering. It is shown that the proposed concept of pressure control in a bunkering system can significantly reduce the amounts of vapor return and net vapor generation.