## 5 Conclusions and Recommendations  
The prospect of LNG as a marine fuel in Indonesia is growing because LNG is the most advanced technology for alternative marine fuel compared to other alternatives such as hydrogen, methanol and LPG. Furthermore, in terms of investment, LNG has shown good cost efficiency in long-run operations. In vessel design, the ship owner has the option to choose to retrofit technology for their current fleet instead of purchasing new vessels. The life cycle analysis of the retrofit 600 TEU container showed that the retrofit will bring low operational costs for vessel owners. This is aside from the investment, which is mostly for the FGSS on board the vessel. It helps owners to know when to use a retrofit to purchase a new build during the acquisition of a vessel. The FGSS with gas combustion unit is the first option to consider during the selection of technology. However, three comparisons made between retrofit and the other two new build shows that retrofit was the recommended option; the cost of retrofit of USD 6,156,058 is lower than the other two options for LNG fueled 600 TEUs container ship's new build (USD 11,547,860) and MFO fueled 600 TEUs container ship's new build (USD 10,702,872). Other savings that shipowners can obtain from retrofit is less time in dry dock for conversion from current MFO fueled to LNG fueled. The more time that the shipowner can save will provide an opportunity cost for the vessel to return to operation and generate income. LCCA is a tool used for life cycle and low-cost analysis with respect to retrofit investment. This analysis will be affected by LNG prices, especially when the uncertain price of LNG will bring a change in analysis.  
LNG is one of the advanced technologies of alternative fuel and several studies proved that it is the most reliable energy source. From the economic analysis, it was discerned that LNG as a marine fuel reduces maintenance and spare part costs. With variable interest rates, the capital recovery factor shows a decrease in payment. The maintenance cost takes significant consideration due to the usage of LNG as fuel. The 600 TEU container vessel capital recovery result served as a reference or guide to vessel owners to be committed to using green fuels such as LNG.  
This study already provided information about the challenges that Indonesian vessel owners face when they want to implement green alternative fuels. Some of these challenges are centered on technology, investment and potential profit. However, this is an opportunity for vessel owners to consider the use of LNG as marine fuel due to its long-term impact on cost efficiency and operating activities.  
Conceptualization, R.B. and R.O.S.G.; methodology, R.B.; software, R.B.; validation, R.B., R.O.S.G. and S.; formal analysis, R.B.; investigation, R.B.; resources, R.B. and R.O.S.G.; data curation, R.B.; writing--original draft preparation, R.B.; writing--review and editing, R.B. and R.O.S.G.; visualization, R.B.; supervision, R.O.S.G.; project administration, R.B.; funding acquisition, R.B. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.