**1. Introduction**

Today, humanity is in the age of the most prosperous history. The advancement of industrial technology has allowed people to share goods with no barriers. Seaborne trade significantly contributes to this trend. In 2017, about 10.7 billion tons of products were traded through water, which represents an enormous amount of energy consumption, thereby producing emissions recklessly [1].

Given this, the International Maritime Organization (IMO) has developed a set of stringent regulations on emission control. In particular, MARPOL Annex VI Reg. 14 introduces a progressive reduction in sulfur content contained in marine fuels by 1 January 2020 when the sulphur content in those fuels should be reduced to 0.5% m/m (mass/mass) in the non-ECAs (emission control areas) as illustrated in Figure 1. For the ECAs, the emission levels have been curbed to 0.1% m/m since 2015.

**Figure 1.** IMO Sulphur Regulation (MARPOL Annex VI Reg. 14).

Since conventional marine petroleum products cannot meet these regulations, marine engineers and shipowners are turning their attention to alternative fuel sources. Currently, liquefied natural gas (LNG) is considered as one of the most credible alternative marine fuels that can meet the upcoming air pollution regulations with respect to minimizing sulfur oxides (SOx), nitrogen oxides (NOx), and particulate matter (PM). As a result, LNG-fueled vessels have been gradually introduced in the marine industry and their number now reaches to over 100 ships [2].

Various studies have shown the environmental benefits of using LNG in ship operations [2–6]. However, the mainstream of those past studies largely focused on onboard emission levels at sea, whereas a considerable amount of emissions ranging from extraction to the transportation to the end user was ignored. To acquire better understanding of the impact of LNG on the emission levels as a whole, there still needs of developing systematic and comprehensive approach to evaluate the environmental impact of LNG fuel from a lifecycle perspective.

Meanwhile, the current marine environmental calculators, known as Energy Efficiency Design Indicator (EEDI) and Energy Efficiency Operating Indicator (EEOI), are considered not practicable in terms of understanding the holistic environmental impacts of marine fuels. It is because those indicators are only focused on calculating the emissions produced from onboard fuel combustion. In other words, those indicators are technically ignorant emissions generated from other life cycle processes of marine fuels.

This analytic limitation not only leads to inaccuracies in the calculation, but also mis-guide us to wrong conclusions when choosing clean marine fuel. For instance, they may indicate the hydrogen applied to marine fuel cells as the cleanest fuel source simply because it produces the least level of emission during ship operation. Interestingly, the hydrogen may be generated from LNG which is regarded more harmful fuel source over the hydrogen. If extending our view from the final use to the fuel production and supply chains, the results cannot be answered as simple as the conventional indicators tell us.

To remedy this issue, the International Maritime Organization (IMO) and member states have been determined to develop new guidelines for estimating the life cycle environmental impacts of marine fuels through the document of IMO MEPC.308 (73) [7]. To respond to this resolution, the initial idea was proposed with the document of IMO ISWG GHG, 5/4/5 [8], which requires far extensive follow-up research and case studies as future works.

Given this background, this paper was motivated to introduce an approach of LCA and to demonstrate its effectiveness through a comparative LCA of LNG with MGO in practical supply chain cases.

## **2. Literature Review**

In effort to investigate the holistic environmental impacts of shipping-related issues, the concept of the life cycle assessment (LCA) has been applied to various studies over decades. There are some remarkable researches worth being mentioned in a methodological point of view. Guinée [9] presented a handbook for guiding to apply International Organization for Standardization (ISO) for LCA analysis and Finnveden et al. [10] discussed the recent development and trends of the LCA applied to industrial studies. Dynamic LCA in consideration of time domain was introduced by Levasseur et al. [11]. These literature provide high-quality basement for life cycle assessment. Nevertheless, Woods et al. [12] pointed out the lack of the LCA studies on investigating the marine environment impacts, addressing the scarcity of LCA modelling to quantify the effects of products and processes on marine biodiversity.

Not surprisingly, there are voluminous LCA studies evaluating the environmental impacts of LNG. Some representative examples are noteworthy. Bengtsson et al. [13] applied the LCA for a comparative study across crude oil, LNG and other competitive marine fuels. The research results revealed that LNG would have a relatively lower GWP compared to other candidates. Thinkstep [14] also provided comparative life cycle assessment for the natural gas with other marine fuels in terms of GHG intensity. Similar study was carried out by El-Houjeiri et al. [15] which compared Saudi Crude oil to the natural gas in other regions in terms of GHG emission. Also, Sharafian et al. [16] provided research about GHG and other air pollutants for natural gas. A localized GHG emission taking into account upstream life cycle was studied by Liu [17] where climate change impact by supply of natural gas in Western Canada was studied. This study provides a suggestion to find the prospect that localized impact for the same fuel can vary in different industries. However, those research were largely focused on the GHG impact and lacked discussion of other impact potentials of local pollutants such as AP, EP, POCP and PM. In addition, the diversity of supply routes was not included in the research scope.

Some interesting LCA studies were also conducted to evaluate local pollutants associated with LNG. A study by Brynolf et al. [18] compared the environmental impact of LNG to future marine fuels: liquefied biogas, methanol and biomethanol. An improved method to reduce net climate change was suggested in the research. Life cycle inventory and analysis of fuels in Singapore was presented in Tan et al. [19] that discussed several types of emissions from Singapore power plants compared between LNG and diesel oil. Tagliaferri et al. [20] investigated LNG transport from Qatar to UK with detailed and diverse scenarios. Jeong [21] provided the holistic research for HFO, MGO and LNG in terms of GWP, AP, EP, POCP.

In particular, it is noteworthy that LCA research has been extended not only to fuel types, but also to the ship building field. Hua et al. [22] analyzed the total life cycle emissions of a post-Panamax container ship running in both HFO and natural gas. Jeong et al. [23] presented the excellence of using LNG-fueled engines based on economic and environmental viewpoints. In the research, the life cycle cost assessment (LCCA) was advised as a useful tool for decision making across industries. It offered the possibility of extending the LCA in the economic point of view, since cost impacts cannot be a negligible issue for the marine industry. Rocco et al. [24] studied the purification process of LNG in the LCA point of view. Miksch [25] analyzed the shipping routes from United States (U.S.) to Asia transporting LNG. This research showed that the LNG supply chain would be sensitive to economic and environmental impacts of fuels. In addition, Dong and Cai [26] discussed several ways to reduce the environmental impact by reducing the fuel consumption rate.

Studies on future marine fuels integrated with advanced technologies rather than LNG cannot be neglected. Alkaner and Zhou [27] presented comparative life cycle analysis for molten carbon fuel cells and conventional diesel engines. This research highlighted the environmental benefits of the new power source. Smith et al. [28] suggested that solid oxide fuel cells can be a solution for marine fuel to prevent climate change. However, the study was more or less limited to operational phase. Evrin and Dincer [29] provided thermodynamic analysis and the assessment of an integrated hydrogen fuel cell for ships. In the study, the GHG emission during operation was analyzed. Hansson et al. [30] in the Swedish marine fuel research recommended the hydrogen as the most optimal and the methanol as the second optimal alternative fuels. Nevertheless, LNG and HFO were marked the optimal fuels in the performance and economic criteria.

The literature above provides multi-disciplinary advice for analyzing LCA for marine fuels. On the other hand, it was found that there is still a shortage in case-specific analysis of LNG application to marine vessels when it comes to contributing to developing lifecycle impact of alterative marine fuels. To narrow this gap, this paper was motivated as a preliminary study to evaluate the environmental benefits of using LNG overall by conducting case studies with a newly-constructed LNG-fueled bulk carrier engaged in domestic services of South Korea, one of the world's top five crude oil importers as well as top three LNG consumers in 2018 [31,32]. It is also to introduce a practical approach to evaluate the life cycle emissions from LNG, thereby achieving useful results for the future regulatory framework on the enhanced standardization for maritime emission calculation.
