Optimal Ship Fuel Selection under Life Cycle Uncertainty

Round 1

Reviewer 1 Report

<General comments>

This paper deals with future uncertainty in selecting alternative fuels for ships. The uncertainty is one of the factors that make it difficult for shipowners and shipping companies to select which fueled ships should be constructed. This paper, which attempts to solve the issue, is interesting and useful. However, in this paper, there are some ambiguities and not enough explanations, which make it difficult to fully understand the paper so I rated the paper as a major revision.

I would like to make comments and questions to make the paper more understandable as follows. After the manuscript is modified appropriately, I want to try to read and understand the paper and evaluate the paper’s content again.

<Comments and questions>

About equations in section 2. Table 1 shows that sf means fuel options. First, “fuel options” is ambiguous because I cannot understand whether the word means fuel utilized in shipping(feed-stock + fuel label in Table 3), fuel group (only fuel label in Table 3), or fuel type of ship (system option described in Table 1). It is also confusing that in equations (3) and (4), f is utilized similarly to sf, and set S is newly used. I could not differentiate whether they were just editorial mistakes or not. For your paper, those equations are the most important to understand the model so please review and revise your formulas with great care for accuracy.

Question about equation (1). What is the lost opportunity cost? I want to know what elements are taken into account for the opportunity loss, and how it is set up in the case study. Please add more explanation.

Comments about the case study. Does Table 3 cover all the information required to reproduce the case study? Especially for me, I couldn’t understand how you set ρ_c, ρ_f, and ρ_fg in Figure 1. Table 3 provides some information about uncertainty like the upper and lower bound of fuel cost, but I want to know how the probability distribution can be set by the information. I understand Tables 4 and 5 are hints, but in the tables, words like EVPI, VSS, ECIU, and gamma scenario are utilized without any definitions. Authors describe “Beta variate probability distribution developing over time” but I could not find an explanation of how it will develop. I understand it is tough to contain all the information in the paper, but please add a bit more explanation.

Questions about L 241~246. In the paragraph, the authors describe “The large conservativeness against carbon pricing shifts fossil and biofuel options with higher emissions beyond the Pareto front. This shows that robust optimization advocates switching focus toward starting with methanol and LNG ships instead,…”. I understand the Pareto font was shifted, but on the Pareto front, the ammonia ship (red line) appears as well between 160 - 180 mUSD expected total cost of ownership. Am I looking at the graph wrong?

L 250 ~ 255. The paragraph just repeats the same thing before. I want to read the explanation of Figure 4.

Comments about 4.2.1. The authors set the EU target and IMO target. I guess the target means the exact value of GWP utilized for optimization or the percentage of GWP reduction. Please add a more precise explanation because the IMO and target will be changed over time.

Question about L 301. “Biofuels are found in many pathways on the Pareto fronts.” How can we find the facts in the Figures? The figures don’t provide any information on whether the fuels are from bio or not.

Comments about L 373. I don’t think subsection 3.3 shows any result of the base case. Please fix it to the exact section number.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

1. In equation 3, you must specify what "s.t" means, I understand that it refers to the acronym, "subjected to", but there can be no doubt. The same in (5), (6) and (7).
2. In equation 5, there is a symbol in red that I don't know why it is like that, if you want to highlight something you should say why and if it is a mistake correct it.
3. Figure 1 needs to improve its resolution, it comes out blurry.
4. Is Figure 2 yours? If it is not, you should reference it asking for the appropriate copyright and if it is yours, it would be good if it had the same format as the rest of the images in the manuscript. Its resolution must be improved. The same with figures 3 and 4
5. I notice that the font changes in each image, it would be good if there was uniformity in the font format throughout the manuscript.
6. I see a lot of confusion in section 4, with many sub-sections and a sub-section for discussion of each of them, which creates a bit of confusion for the reader. I suggest you make that section more understandable and also give a brief summary of how you have organised the paper at the end of the introduction section to help the reader.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Problem of the ship fuel selection is very important in real life ship exploatation. Taking into acount uncertainties and giving optimization procedures for the fuel type selection may have a wide range of practical applications. The total impression after reading the work is good and this manuscript addresses many scientifically significant aspects. But there are several parts, that in my opinion should be refined:

1. I suggest supplementing and expanding the introduction, beacuse it does not place your work compared to other scientific works and does not point the nowelty of the presented solution. Please have a look at fuel optimization and prediction models, like ex."A two-step strategy for fuel consumption prediction and optimization of ocean-going ships", "Prediction and optimisation of fuel consumption for inland ships considering real-time status and environmental factors", "Speed and Fuel Ratio Optimization for a Dual-Fuel Ship to Minimize Its Carbon Emissions and Cost". There are also available manuscripts considering fuel price uncertainties, like ex. "Optimal Selection of Multi-Fuel Engines for Ships Considering Fuel Price Uncertainty".

2. Please rewiev Table 2, where for "Set" description and modeling comment are the same. Moreover, there is only one modeling comment in the whole table. There is also lack of "Xsf' " decision variable description.

3. In Eq. (3) I suggest unification of the notation " ' " for all variables.

4. In my opinion, to ensure more readablitity for the "non-specialist" reader theta should be described in Eq. (5) and (6).

5. Could you please explain how support function (L.97) is defined? In my opinion it needs some clarification in this part of the manuscript.

6. In line 147 there shoupl be "P" or "p", which is now and in my opinion is inconsitent with the equations placed below it.

7. In line 202 Subsection caption should start with capital letter.

8. In tabe 4 I suggest making some spaces between each aspect to ensure better readability.

9. I have a question if in Table 5 change should start with capital letter or not?

I have noticed only few punctuation mistakes. Please have a look at punctuation in the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I would like to thank you for the excellent revision, in which all the comments were reflected and questions were clearly answered.

Author Response

Thank you for your feedback. Your comments and suggestions have been very helpful in improving our paper. We also appreciate the compliments on the revision.

Reviewer 3 Report

Thank you for the done manuscript corrections. Now readability has increased and context of your work has been described in a good manner. I have only one remark. Legend and axest desriptions in Fig. 7 and Fig.8 are very small and hard to read. I suggest enalrging them. This remark does not affect manuscript content, but only the way of results presentation.

Author Response

Thank you for your comments – both now and in the previous round. Below, we repeat your comment and address the corrections we made in the manuscript:

"Thank you for the done manuscript corrections. Now readability has increased and context of your work has been described in a good manner. I have only one remark. Legend and axest desriptions in Fig. 7 and Fig.8 are very small and hard to read. I suggest enalrging them. This remark does not affect manuscript content, but only the way of results presentation."

We have increased the font sizes in Figures 7 and 8 to improve readability.