# Effect of Parameters on Vapor Generation in Ship-to-Ship Liquefied Natural Gas Bunkering
## 5 Method of Pressure Control in the Bunker Tanks and Receiving Tank  
As discussed earlier, the amounts of vapor generation and vapor return are significantly affected by the pressure difference between the bunker tanks and receiving tank. When LNG is discharged from bunker tanks, the liquid volume in the LNG tank decreases, resulting in more vapor generated to fill the space left by the discharged LNG, and to maintain the pressure inside [32]. The proposed solution in this study is to maintain the bunker tank pressure as the initial condition by using a pressure control valve in the vapor return line. As is common practice in the LNG industry, the pressure of the LNG receiving tank is a few bars higher than that of the bunkering tanks. Thus, in this study, the initial pressure (temperature) of the bunker and receiving tank were set as 2.94 bar (\(-\)147 \({}^{\circ}\)C) and 3.31 bar (\(-\)145 \({}^{\circ}\)C), respectively. A proportional-integral controller (PIC-105 and PIC-106) was added to control the bunker tank and receiving tank pressure to the set point (2.94 bar) by adjusting the vapor return flow rate, as shown in Figure 10.  
Figure 9: Variation of receiving tank pressure at different LNG composition.  
Figure 11 shows that the pressure of the receiving tank with the pressure controller gradually decreased with time. However, the pressure of the receiving tank without the pressure controller sharply decreased at the beginning of bunkering and remained almost constant after being equalized with the bunker tank pressure. Similarly, the vapor return flow rate in the tank with the pressure controller remained almost constant. However, the vapor return flow rate in the tank without the pressure controller sharply decreased at the beginning of bunkering and remained almost constant after being equalized with the bunker tank pressure, as shown in Figure 12. The amounts of vapor return and vapor generation are summarized in Table 9. The total amount of vapor return to the bunker tanks was reduced from 7392 to 3317 kg when the pressure controller was provided. This can be advantageous in the sizing of vapor handling units such as GCUs, or in re-liquefaction units (if they are applied). In addition, the net vapor generation in the receiving tank (\(m_{net,gg,r}\)) was reduced by up to 4047 kg and the net BOG generation in the overall system (\(m_{net,gg,overall}\)) was reduced by 16.2%, i.e., they were more condensed by applying the pressure control. This indicates that maintaining the bunker tank pressure by using a pressure control valve in the vapor return line can provide efficient vapor management by stabilizing the vapor return flow rate and reducing the overall vapor generation. However, the final pressure of the receiving tank after bunkering is 3.18 bar when the pressure controller is provided, i.e., 0.2 bar higher than that in a case without a pressure controller. As the International Code of Safety for Ship Using Gases or Other Low-Flashpoint Fuels states that the tank pressure should be maintained below the set pressure of the tank pressure relief valves for a period of 15 days, assuming a full tank at normal service pressure and idle conditions, the increased pressure of the fuel tank after bunkering may be a disadvantage. However, by carefully reviewing the operating and design pressures of the receiving tank and the BOG consumption in the engine, this solution may benefit both bunkering and receiving ships in terms of BOG generation and the capacity of BOG handling equipment, such as re-liquefaction system, GCU, and compression unit, if those are used. From an economical point of view, considering that the above BOG handling equipment are higher in cost than other ship systems, an overall cost-saving of bunkering ships can be achieved by reducing those systems' size.  
Figure 10: Dynamic process modeling with bunker tank pressure controller.