# Effect of Parameters on Vapor Generation in Ship-to-Ship Liquefied Natural Gas Bunkering
## 3 Simulation and Input Preparation  
In this study, the LNG bunkering process was modeled using Aspen HYSYS (Aspentech, Bedford, MA, USA). The process was switched to Aspen HYSYS Dynamic to investigate the dynamic behaviors of ship-to-ship LNG bunkering. ASPEN HYSYS is a powerful process simulator that was developed by Aspentech for modeling processes from upstream, through gas processing and cryogenic facilities, to a refining and chemicals plant. It contains comprehensive thermodynamic databases having physical and chemical properties of mixtures at different temperature and pressures. ASPEN HYSYS has subroutines for calculation of vapor-liquid phase equilibrium, heat and mass balances, and simulation of many kinds of plant engineering equipment. The Peng-Robinson equation of state was adopted for the process simulation, as it was found to be appropriate for the LNG bunkering process [15; 16; 17; 19; 24]. The liquid-vapor phase equilibrium was iterated and solved in the simulation. The simulation was validated via comparison with a study by [PERSON] et al. [17] analyzing ship-to-ship LNG bunkering. Validation was performed at the same condition and it was found that total mass difference of BOG during bunkering was within 2%. Two IMO type C tanks with a total capacity of 6500 m\({}^{3}\) and filled with 80 vol\(\%\) LNG were modeled on the bunkering ship side. In each bunker tank, a deep well-type pump was modeled by using performance curves generated in Aspen HYSYS. The LNG was transferred at 500 m\({}^{3}\)/h by two cargo pumps to the fuel tank through an 8-inch header pipe, hose, QC/DC, and ERC. The valves and fittings were counted based on the general configuration of the bunkering system, as shown in Figure 2. The QC/DC and ERC were modeled as butterfly valves with fully open positions for simplicity. The pressure drop of the 8-inch hose was less than 0.1 bar/m at cryogenic conditions and a maximum allowable flow speed of 14 m/s [25]; thus, we set the hose pressure drop as 1 bar/m for a conservative design of the LNG bunkering system. A receiving fuel tank with a capacity of 1000 m\({}^{3}\) filled with 20 vol\(\%\) LNG as a heel was considered. Pump performance curves generated in HYSYS based on flow rate, efficiency, and head are used. HYSYS generates three curves based on three different speeds: user-specified speed, user-specified speed multiplied by low speed %, and user-specified speed multiplied by low low-speed %. By controlling the pump speed, the LNG transfer flow rate was reduced from 500 to 200 m\({}^{3}\)/h when the liquid volume percentage of the receiving tank reached 85% of the tank volume, for \"topping-up\" purposes. Figure 2: Schematic isometric view of ship-to-ship bunkering. When the liquid volume % in the fuel tank reached the loading limit, defined as the maximum allowable liquid volume percentage that the tank could be loaded, the on-off valve was closed to avoid overfilling of the receiving fuel tank. The loading limit was determined from the following formula [26]:  
\[LL=FL\frac{\rho_{R}}{\rho_{L}} \tag{1}\]  
In Equation (1), \(LL\) denotes the loading limit, and \(FL\), \(\rho_{R}\), and \(\rho_{L}\) denote the filling limit (normally taken as 98%) and relative densities of the LNG at the reference and bunkering temperatures, respectively. The reference temperature corresponds to the saturated vapor pressure of the LNG at the set pressure of the pressure relief valves. The loading limit of LNG fuel tanks is typically between 85% and 95% of the tank volume, although it depends on the tank type, pressure relief valve setting, and other factors [14; 22; 27]. In this study, the calculated loading limit of the receiving tank was 93%; however, we adopted 90% as the loading limit, for a conservative approach. The excessive BOG generated in the receiving fuel tank was returned to the bunker tank in the bunkering ship via a 6-inch vapor return line, to reduce the pressure of the receiving fuel tank.