**5. Interpretation and Discussion**

Although both LNG and MGO fuels are able to meet IMO 2020 sulphur regulations, the environmental impacts of those fuels have been found to significantly vary depending on fuel type and supply chain. Given this, the modelling of each life stage of the fuels has been shown to help identifying the potential processes where we can optimize and reduce emission levels. In this research, the environmental impact of the processes pertinent to the fuel supply chain was proven significantly high. Hence, several ways to reduce emission levels by optimizing transportations can be proposed as follows:


On the other hand, theWtW analysis shows that the emission levels from onboard fuel consumption are dominant in the overall process. Although CO2 emissions from LNG combustion are lower than the MGO case, simulation results from the TtW impact assessment indicate that methane slip from LNG fuel engine operation is an important factor for increasing GWP emissions. It reveals that the life cycle GWP level from the LNG is 9.77 <sup>×</sup> 105 kg CO2 equivalent per 1.0 <sup>×</sup> 107 MJ fuel consumption, whereas that from the MGO is 1.11 <sup>×</sup> <sup>10</sup><sup>6</sup> kg CO2 equivalent per 1.0 <sup>×</sup> <sup>10</sup><sup>7</sup> MJ fuel consumption. These results provide insight into why the marine industry should strive to minimize methane slip from engine combustion.

In short, this research demonstrated the excellence of using LNG as a marine fuel relative to the MGO. However, it cannot neglect emphasizing on the need to resolve the methane slip issues; if successful solutions arrive in the marine industry, the use of LNG as a marine fuel can further contribute to reduction of the GHG emissions.

Results of the impact assessment for local pollutants even more clearly support the superiority of LNG over MGO since the MGO fuel cases are revealed to contribute 5–7 times higher emissions than the LNG cases. In this respect, the LNG is confidently recommended over the MGO as long as we are solely concerned with the environmental perspective.

On the other hand, the demands on addressing the shortcomings of current maritime environmental indicators, EEDI and EEOI, are increasing. Given this, the life cycle assessment can be an effective approach to estimate holistic emission levels attributed by marine fuels from the extraction to the final use. Taking into account that the marine industry is grappling with lowering emission levels more comprehensively, this research is highly believed to be an important primary study that will help to provide practical solutions to assess the all-inclusive environmental impacts of marine fuels. Such solutions will help us to obtain a clear knowledge on the holistic environmental impacts from shipping. To achieve this goal, the future research is recommended to expand the case studies to various alternative fuel types and supply chains to determine the most optimal future marine fuel as well as to obtain general observations on the level of differences and how to improve it.
