# Effect of Parameters on Vapor Generation in Ship-to-Ship Liquefied Natural Gas Bunkering
## 1 Introduction  
Owing to increasing pressure to reduce emissions in the maritime sector, the International Maritime Organization (IMO) has adopted several environmental regulations regarding nitrogen oxides (NO\({}_{\text{x}}\)), sulfur oxides (SO\({}_{\text{x}}\)), and carbon dioxide (CO\({}_{2}\)). The IMO Tier III regulations require an 80% reduction in NOx emissions from Tier I in NOx emission control areas, and global sulfur caps that limit the sulfur content of ship fuels to 0.5%, which became effective worldwide on January 1, 2020. In April2018, the IMO adopted an initial strategy for reducing the total annual greenhouse gas emissions from shipping by at least 50% by 2050 (relative to 2008). These environmental regulations to reduce CO\({}_{2}\) and other emissions are driving research into alternative fuels. Among several potential alternative fuels, liquefied natural gas (LNG) has been recognized as a technically feasible and proven solution in the maritime sector [1; 2; 3]. Using LNG as a ship fuel can provide a 95-100% reduction in SO\({}_{x}\) emissions, a 45-80% reduction in NOx, and a 25-30% reduction in CO\({}_{2}\) emissions, depending on the engine technology [3]. With these environmental advantages, the number of ships using LNG as fuel is increasing, and thus, the demand for LNG bunkering is also increasing. There are several LNG bunkering methods available, such as truck-to-ship, ship-to-ship, and terminal-to-ship, as illustrated in Figure 1.  
A suitable bunkering method should be selected in consideration of the LNG fuel storage capacity and any potential time constraints for bunkering operations [4]. Truck-to-ship bunkering has been the most commonly used bunkering approach so far, and provides good operational flexibility; however, the limited capacity of the trucks can be a disadvantage in multi-truck operation. In port-to-ship bunkering, a larger volume of LNG can be delivered; however, it is often difficult for LNG-fueled ships to reach a terminal. In the ship-to-ship bunkering method, the LNG is delivered to an LNG-fueled ship by another ship, i.e., an LNG bunkering ship. This method has an advantage in that LNG bunkering can be conducted simultaneously with other transfers, such as cargo handling operations. In addition, it provides a larger delivery capacity and higher bunkering rate than that of the truck-to-ship method, and better operational flexibility than that of the terminal-to-ship method [3; 4]. Therefore, the ship-to-ship bunkering method is considered the most efficient bunkering method [5]. Several LNG bunkering ships are under construction or operation, as can be seen in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Vessel Name**} & \multicolumn{3}{c}{**Cargo Tank (Bunker Tank)**} & \multicolumn{2}{c}{**Bunkering**} & \multicolumn{3}{c}{**Boil-Off Gas Handling**} & \multirow{2}{*}{**Reference**} \\ \cline{2-2} \cline{4-9}  & **Task** & **Cargo** & **Design** & & **Rate** & & \\  & **Type** & **Capacity (m\({}^{3}\))** & **Pressure (Barg)** & & **(m\({}^{3}\)/h)** & **As fuel** & **Burning** & **Re-Liquefaction** \\ \hline Engle Zeebrugge & Type C & 5100 & 4 & 600 & Yes & Yes & No & [6; 7] \\ Coralius & Bi-lobe & 5800 & 4.5 & 840 & Yes & Yes & No & [7; 8] \\ Cardissa & Type C & 6500 & 3.5 & 1100 & Yes & Yes & Yes & [7; 9] \\ Karios & Type C & 7500 & 3.75 & 1150 & Yes & No & No & [7] \\ Coral Methane & Type C & 7500 & 3.2 & 900 & Yes & - & Not for LNG & [10] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of liquefied natural gas (LNG) bunkering ships. Figure 1: Schematic representation of the different bunkering methods. During the storage of LNG, boi-off gas (BOG) generation is inevitable owing to heat ingress, even though the storage tanks are well-insulated [5]. The typical LNG boi-off rate (BOR) under storage conditions is 0.1-0.5% per day [3].