## 3 Methodology
### Vessel Design  
In this study, the existing container vessel was used to carry out certain analyses. This is intended to provide an overview of the vessel owners' perspective on the fuel transition strategy. Assuming this is not a new build vessel, the ideal methodology that needs to be adopted is retrofit. The availability of technology reduces the cost of the vessel and improves efficiency. Furthermore, it was stated that zero-emission fuel impacts the already-built vessel [31]. It provides retrofit, which most vessel owners usually consider. The 600 TEU container vessel serves as a retrofit to dual fuel. According to the Retrofit Series (2020), the three vessels subjected to retrofit have significant potential savings, such as lube oil cleaning and other attributes that are often overlooked [32]. This includes potential savings from machine learning. Some other studies carried out on a mini-cape size bulk carrier stated that the payback period for LNG-fueled vessel retrofit is 4.5 years compared to a 0.5% compliant fuel vessel [33]. A retrofit vessel that uses LNG fuel is an attractive option to meet the new regulation. Another study stated that Hapag-Lloyd investigated a 15,000 TEU Sajir retrofitted for LNG fuel. This concept has LNG cylinders contained in open frames with 40-foot containers. The venting system and LNG piping, including the fire-fighting technique, are integrated into the container cell guide structures handling the gas adjacent to the storage. It feeds the low and high-pressure fuel gas system to the current four-stroke dual-fuel engines [22]. In this study, the 600 TEU container vessel has a similar concept with retrofit, as stated by previous studies on 15,000 TEU Sarij by Hapag-Lloyd. [PERSON] et al. designed a three-configuration fuel gas supply system, and as mentioned earlier, FOSS is a critical factor in the LNG fuel system. The three configurations of FOSS are GCU, AE, and reliquefaction schemes with the combustion of boil-off gas-by-gas combustion unit (GCU), supply boil-off gas using auxiliary engine (AE) and reliquefaction boil-off gas by reverse Brayton cycle (RBC) system, respectively. In line with a previous study [20], this present study selected a suitable retrofit configuration for a 600 TEU container vessel dependent on a GCU scheme because the system is reliable, simple, and compact. Figure 6 illustrates the configured FOSS with boil-off gas handled by GCU. The configured FOSS is adapted from [PERSON] et al.'s scheme [19], and the LNG Tank is fitted into a deck with a similar arrangement as a refrigerator container tank. It uses the plug-in system on the LNG tank and container cell, thereby reducing the cost of the conversion vessel [22].  
The retrofitted designed vessel has employed a promising strategy to avoid uncertainty. They defined the retrofit cost using a Pareto-optimal solution, and interestingly, it depends on different alternative fuel types. The retrofit cost was calculated by analyzing certain aspects, namely machinery, tank, piping, shipyard and lost income. Figure 6 shows an illustration formulated by [PERSON] et al. [34] as a reference. Another study that proposed the use of the calibrated method for the fuel substitution ratio, economy and particulate matter emission proved brake-specific consumption for the dual fuel model is higher than the diesel [35]. The generated boil-off gas tends to have certain advantages, such as energy efficiency [36]. The calculated lost income and the time needed during retrofit at  
Figure 6: Configuration of FOSS with BOG handled by GCU.  
the shipyard are perceived as a challenge to Indonesian vessel owners because it requires opportunity costs to compensate for the time lost.