Considerations on the Effect of Slow-Steaming to Reduce Carbon Dioxide Emissions from Ships
Round 1
Reviewer 1 Report (Previous Reviewer 2)
Thank you for your answers. Most of the comments have been satisfactorily addressed. The paper still lacks specific information on the results of studies presented in other papers. Specifically: by how much of a percentage reduction in fuel consumption and carbon dioxide emissions was achieved for other papers? In the conclusion, there should be specific conclusions on the research results obtained. Specifically: by how much fuel consumption and carbon dioxide emissions were reduced for the studies carried out? Please review the formatting and editing requirements for the text (including tables) and include them in the paper.
Author Response
I thank you for your comments. We have taken them into account. Explanations of percentage savings have been added at the end of Section 3 and in the conclusion. The formatting of the text (including the text) has been changed to meet editorial requirements.
Reviewer 2 Report (New Reviewer)
The paper uses an industrial software to calculate the total FOC of two power plants (of two different ships), when slow steaming occurs. Different modes of operation are examined.
The main contribution of the paper is the consideration of the additional fuel needed when operation at low ME load and the inability of the power plant to operate efficiently (e.g. not enough steam to run the TC)
Line 36: CO2 emissions affect global warming and do not have only a local impact. So, I do not understand the comment about ports and coastal waters.
Line 82: need to add a description of these papers
Line 101-102: provide some references
Line 120: Not correct. An automatic data acquisition system can be much more reliable. Furthermore, engineering simulators entails the uncertainty of the models used.
Line 126: provide some reference
Line 168-169: It is not clear. As part of the shaft power generated by the ME is used by the shaft generator, how the ship' s speed remain the same?
Line 183-189: As the time duration differs significantly, the case where DGs are not operating in both cases, eliminates the chance to study the increase of the DG power needed (e.g. in case of reefer cargo units) due to the extension of time duration when slow steaming occurs and subsequently of the DG fuel consumption. So not a realistic mode of operation.
Line 265: This is an interesting point that is not examined in the paper. There is probably a power-speed curve of the type P=a*V^b assumed behind the simulations. Is it the same power b, irrespective of the type of ship examined?
Furthermore, and especially for the containership this curve is very sensitive on the speed range considered. Besides it is known only for a limited speed range from sea trials and usually no extrapolation is allowed for the lower speed as the results fails. How you derived this curve?
The whole paper is based on the calculations through the software utilized, which is a black box method, that can be considered adequate for industry applications but not sufficient for a journal paper.
Author Response
Thank you for your remarks. The first two sentences correctly describe in short form the main subject of the work.
Line 36. CO2 emissions are a global problem, but countries are taking actions to protect their own waters. To do this, the first step is to identify harmful gas concentrations carried in by ships. These papers were studied to examine the care that coastal countries take to protect their environment.
Line 82. A description of the two papers was added.
Lines 101 - 102. This is a quote from paper [4].
Line 120. I disagree with this. Measuring the outflows from the appropriate tanks is as accurate as possible. It is the same as measuring the fuel levels in the tanks. The ME flow meter gives a ME consumption but (usually) not the DGs and steam generator(s) consumption.
Line 126 is a subhead. Did you mean line 125? It is my opinion that the validity of data collected on board may be questionable due to human error, deliberate misrepresentation to cover up own errors, etc.
Line 168 - 169. Main engine 'telegraph' handle sets the propulsion shaft number of revolutions (RPM). When SG is on, a certain amount of ME power is transferred to SG's shaft, which leads to ME speed of rotation decrease, resulting in a propeller shaft decrease in RPM, but fuel flow to the ME is increased to return to the set RPM.
Line 183 - 189. I agree it is not a realistic mode, but only because it is unlikely a ship will have TG, SG and a number of DGs. In our case there is one powerful TG. However, it is mentioned that further studies should follow to cover as many power plants and modes as possible.
Line 265 and onward. The power calculation formulas used in the Norcontrol’s simulator were not known to the authors. However, if one considers modern marine diesel engines with common rail fuel systems, which are known to require very precise fuel injection control, an electronic version of closed-loop indicator diagrams is readily available, and consequently the indicator power of each cylinder can be readily measured (calculated). Therefore, I cannot agree with your last statement. Anyway, the accuracy of the power measurement was not an issue. You explained the objective of the work at the beginning of your comments, and we believe that the results are correct. If there are errors in power measurement due to simulator limitations, they are the same for ME and DG (s). And the goal of the paper was to point out that slow steaming is a measure of limited utility.
Reviewer 3 Report (New Reviewer)
The manuscript "Considerations on the effect of slow-steaming to reduce carbon dioxide emissions from ships" described some of the current issues related to exhaust emission from marine vessels in relation to effectiveness of fuel consumption. The work which test and summarized some of the methods which was recently suggested as a potential fuel effective alternative during transportation. English language was fine, and the manuscript was adequately written.
The method of approach seems persuasive and I don't have doubts with the results as the authors have described some of the main drawbacks with the suggested method in their conclusions. There is no doubt that one of the major limitations or potential point of error with the estimation of CO2 emission is caused by the simultaneous emissions of other constituting compounds from engine combustion such as SO2, NOx, and etc. All of which can cross-interact with each other during chamber studies or even in real environments. Nevertheless, I think the authors should described slightly more in regard to a broad exhaust emission rather than just CO2 emissions.
General Comments:
In the introduction, the authors should also include other emission reduction controls such as usage of lower density fuels and implementation of filters / scrubbers before emissions. These are very commonly techniques which has been continuously implemented on most transportation vehicles.
The authors should also refer to and describe about the IMO 2020 by MARPOL, which recommends a cleaner air quality policy especially when marine vessels are cruising nearby coastal regions. Although not globally compulsory, most countries have lawfully enforced these standards to improve air quality in their respective countries.
As most marine vessels use HFO (density of MF180) for larger vessels and diesel / bunker A (density < MF80), I am worried with the potentially misleading information of methane / methanol usage as fuel for vessel combustion which the authors described in their introduction.
Specific Comments:
CO2 should always be CO2. The authors should correct this.
Font for images are inconsistent and have low-resolution. I think the authors should improve image quality.
Author Response
Thank you for your kind comments.
Due to the size of this paper, we have not addressed the issue of NOx or other harmful gasses emissions. We will address NOx and other emissions in subsequent papers. We also did not consider it relevant to write about scrubbers and similar equipment, since we only analyzed CO2 emissions. As for the IMO and coastal country regulations that we indirectly mentioned, that is exactly what we are talking about: the effectiveness of such measures (e.g., slow steaming). We mentioned methane and methanol because one of the reviewers asked for a better description of the references.
The CO2 has been changed to obvious CO2.
As for the resolution of the images, the problem is that the images with the printscreen option were taken directly on the simulator with a resolution of 96dpi. The blurring occurs when the size of the images is adjusted to the Word document. Therefore, the resolution of the images that are part of the text cannot be improved, but we will upload a separate file with the images in landscape format.
Round 2
Reviewer 2 Report (New Reviewer)
Line 120. I disagree with this. Measuring the outflows from the appropriate tanks is as accurate as possible. It is the same as measuring the fuel levels in the tanks. The ME flow meter gives a ME consumption but (usually) not the DGs and steam generator(s) consumption.
Reply: I do not disagree with the reply but my comment was for the following: “On-board measurement generally means the investigators use data collected by crew members. The quality and validity of such data could be, at least, questionable.”
Generally, flow meters provide the best measurements especially when the analysis is sensitivity in time, e.g. transients etc. DG and boiler can be also measured using flow meter in the appropriate location in the fuel line.
Fuel level in tanks is usually measured by the crew and not frequently e.g. per day so it cannot be appropriate for time sensitive analysis. So my suggestion is that on-board measurements is not only the ones performed by the crew, but appropriate sensors with high -frequency sampling are also a very good choise for performance analysis.
Line 126 is a subhead. Did you mean line 125? It is my opinion that the validity of data collected on board may be questionable due to human error, deliberate misrepresentation to cover up own errors, etc.
Reply: See the previous reply. I agree when the crew is involved. However, there are the automated data collection systems.
The comment for reference goes for the NorControl system.
Line 168 - 169. Main engine 'telegraph' handle sets the propulsion shaft number of revolutions (RPM). When SG is on, a certain amount of ME power is transferred to SG's shaft, which leads to ME speed of rotation decrease, resulting in a propeller shaft decrease in RPM, but fuel flow to the ME is increased to return to the set RPM.
Reply: I agree with your reply, but this means that the ME power shall be increased (as the fuel flow to ME is increased) to sustain the 11.89 kn ship’s speed.
Line 265 and onward. The power calculation formulas used in the Norcontrol’s simulator were not known to the authors. However, if one considers modern marine diesel engines with common rail fuel systems, which are known to require very precise fuel injection control, an electronic version of closed-loop indicator diagrams is readily available, and consequently the indicator power of each cylinder can be readily measured (calculated). Therefore, I cannot agree with your last statement. Anyway, the accuracy of the power measurement was not an issue. You explained the objective of the work at the beginning of your comments, and we believe that the results are correct. If there are errors in power measurement due to simulator limitations, they are the same for ME and DG (s). And the goal of the paper was to point out that slow steaming is a measure of limited utility.
Reply:
My comment was about the hydrodynamic and resistance part of the problem. This mean that given a hull shape and the propeller, there is a relationship that connects the ME power and ship’s speed for a specific ship’s draft and weather conditions (e.g. calm water). This is very roughly a third power law between power and ship’s speed (line 266), but this differ for each ship. For example, it will be quire different between the crude oil tanker and the containership. I understand that for your analysis that its emphasis is on the power plant but when slow-streaming effect is examined, the simulator shall somehow consider this issue. I would accept to provide some comment on that point referring to that issue.
Author Response
Lines 120, 126. An explanation is added. Hopefully, this answers your secondary coments.
Line 168 - 169. A footnote is added, explaining that with term power we mean the propeller shaft power or, preciselly the percentage of RPM. The fuel flow is in fact increased.
Line 265. Again, I cannot provide the mathematical model that Norcontrol used in their simulator, but if there is an approximation, it would be the same for both ship models. We've not found that the ships have the same flow resistance, especially at very different speeds, but the models used by Norcontrol are the same. Therefore, regardless of the errors that the model may have caused, the results of the investigation should be valid.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
The paper holds the investigation of CO2 emissions for ocean-going vessels with speed reduction measure. The authors used a commercial simulator for creating the results. There is a concern about the validity of the result because neither the method used in the simulator nor validation data has been presented. It also lacks the reference for the simulator in scientific literature. This is a major drawback of the paper that must be improved to be published in this journal.
Author Response
The authors disagree with the statement that 'neither the method used in the simulator nor validation data has been presented'.
Rather, the method was explained in detail. The modes of operation were explained: the main engine was operated at full speed or its output was reduced; the power generators were operated in various arrangements; the steam generator was activated as needed. The investigation was conducted as follows: While the main engine was operating at full speed or at reduced power and other equipment was running in automatic mode, fuel flows were measured and carbon dioxide emissions were calculated from fuel consumption.
The conclusion derived are consistent with the data.
The papers on the use of simulators in scientific papers are not included in the reference list because they are not related to this research. Nevertheless, the authors believe that engine room simulators are a powerful tool for thermal analysis of ship power plants and have been used in scientific research in the past.
Reviewer 2 Report
Notes to the Authors:
1. The topic of the paper is: "Considerations on methods to reduce carbon dioxide emissions from ships". The paper actually presents one method of reducing carbon dioxide emissions from ships. The paper does not present different methods to reduce carbon dioxide emissions. The different methods to reduce carbon dioxide emissions are not described in the introduction. In my opinion, the title of the paper is not appropriate to its content.
2. The abstract should clearly state what the purpose of the research was and what research was carried out as part of the work done. In a general way, it should be written what results were obtained.
3. In the paper, all abbreviations, symbols and designations should be explained. For example, the following designations are missing from Table 1: 'ME', 'SG', 'TG', 'DG1', 'DG2'.
4. The introduction lacks a literature review on the topic of the paper. There is no information on what studies, on carbon dioxide reduction, have been carried out by other researchers. What research results they obtained. The literature review is insufficient.
5. The NorControl engine room simulator should be described in more detail in the paper. This is the test stand used to carry out the research presented in the paper.
6. All tables should be described in detail in the body of the paper.
7. Descriptions of the test facilities and the operating modes tested should be more detailed. Why these modes of operation of marine power plants were chosen and not others.? Whether other modes of operation are possible?
8. The paper needs to be formatted as required. For example, the description style of figures number 1 to number 6 is not correct.
9. How was the mass of carbon dioxide emitted calculated?
10. There are no units in the graphs presented in Figures 7 and 8.
11. The literature review is insufficient. The list of literature includes only sixteen items. The authors of fifteen literature items are employees of the same University. There is no literature review on the subject. Has no one so far studied the effect of ship speed on fuel consumption and carbon dioxide emissions? Pelić, V. is the co-author of ten works included in the reference list. Radonja, R. is also a co-author of ten works included in the list of references.
Author Response
Point 1. The title has been changed to match the text of the paper.
Point 2. Although the authors do not agree that the results should be fully explained in the abstract, this was changed. Exactly what was measured was explained and a reference to the results was also added.
Point 3. All abbreviations are explained in footnotes.
Point 4. and point 11. New papers have been studied and included in the reference list. Not that they bring new knowledge, but it was asked for.
Point 5. Few sentencies were added explaining this two engine rooms. Few sentencies were added explianing how the log operates.
Point 6. A paragraph is inserted before Table 1, also explaining Table 2. A paragraph explaining this table is inserted before Table 3.
Point 7. These two engines and ships were chosen because they are typical (crude oil tanker and a container ship). Of course, many other engines, ships and modes of operation are conceivable and should be the subject of further investigation. The authors do not consider it necessary to explain power plants in detail.
Point 8. The authors do not understand this remark. What style of description is required?
Point 9. From the data given in the 7th column of the Table 3 (Btotal) approximately 83% is the carbon mass fraction. This number is multiplied with the coefficient 3,67 (resulting from the carbon dioxide molecular mass - 44 increase over carbon atom number - 12). This explanation is added to the text.
Point 10. The units need not be given because these graphs were created using the data in Table 3 and the units are given there. In Graph 7, total fuel consumption is given in tons, as are carbon emissions in Graph 8.
Point 11. See point 4.
Reviewer 3 Report
The title of the paper does not accurately reflect the research work carried out by the author.The author is suggested to adopt this paper title: Considerations on methods to improve Ship Energy Efficiency for reducing carbon dioxide emissions;
The author is suggested to add the latest papers on ship decarbonization in the introduction of the paper,The author may consider the following papers
1.A comprehensive review on countermeasures for CO2 emissions from ships,https://doi.org/10.1016/j.rser.2020.110222;
2.Decarbonisation of shipping: A state of the art survey for 2000–2020,https://doi.org/10.1016/j.ocecoaman.2021.105936.
3.Route to zero emission shipping: Hydrogen,ammonia or methanol? https://doi.org/10.1016/j.ijhydene.2021.06.066.
Author Response
The title was changed, not as suggested by the reviewer, because it has little to do with improving energy efficiency. Hopefully, it would reassure the reviewer.
Some other papers are studeid and included in the reference list.
Round 2
Reviewer 1 Report
I wanted to see the improvement regarding the validity of the simulator since not everyone is familiar with the simulator that you have used and it is authors’ responsibility to provide the credibility of the results that are presented in the paper. Again I encourage you to provide relevant references regarding the mathematical models used in the simulator or validation using real-world data.
Author Response
I have added new references, mine and those of other authors, in which engine room simulators were used. As for the mathematical models used in the engine room simulator we used, unfortunately it is not possible to provide them. I would have to crack NorControl's proprietary file and probably break some copyright laws. Even if it were possible, what would be the purpose of that. Hundreds of mathematical models would have to be used. For example: a complete fuel oil system is modeled, every pump, flow resistance, heat exchange, etc. Then there's the fuel oil system on the engine: high-pressure pumps, high-pressure lines, and fuel injectors. And then the model of the engine begins: fuel injection, air compression, fuel combustion, forces acting on the engine mechanism, heat dissipation through the cylinder liners, heat generation by friction in the bearings, etc. Just to mention few.
Anyway, from my experience and that of many of my colleagues and NorControl simulator users who are experienced marine engineers and/or long-time simulator users, I can assure you that the simulator is as realistic as possible.
Some changes have been made in the Introduction section and Conclusion.
Reviewer 2 Report
Thank you kindly for your responses to my comments on the article so far. Unfortunately, not all my comments have been taken into account. Some have remained without satisfactory answers to me.
Notes to the Authors:
1. In my previous comments, I did not write that in the abstract, the results of the research should be explained in detail. There should not be a detailed analysis of the results in the abstract. I wrote that there should be a clear objective and the main results of the research should be stated in general terms. The current abstract is adequate.
2.1. A literature review in scientific articles is done to show what others have done on a given topic. The results of the research done in this work to be published should be compared with those of other researchers. So please write clearly what new knowledge has been contributed by the paper submitted for publication? What has the Authors of the reviewed paper done that is new compared to other researchers? If other researchers have done similar studies then please compare Your results with those obtained by other researchers? Is it new in Your work that if a ship travels at a much lower speed it will use less fuel. In my opinion, this is obvious.
2.2. The literature review has been slightly improved. In my opinion, it is still modest. There is a lack of analysis of the findings of other researchers and their comparison with the findings presented in the reviewed article.
3. Lines 115 and 116 state: „The abbreviations in Table 2 are the same as in Table 1.”. In the description of tables in the text, it is not just a matter of describing abbreviations, but above all of describing what the table contains.
4. I believe that the study objects should be described quite thoroughly in the paper. The basic technical data of the objects studied should be given. Why were such modes of operation of marine power plants chosen and not others? Are other modes of operation of the investigated ship power plants possible?
5. Please refer to the formatting requirements for articles for the journal in which you plan to publish an article. I give examples below. The authors write: „Figure 1 – Fuel flows for mode 1.1”. It should be: „Figure 1. Fuel flows for mode 1.1.”. The authors write: ,,Figure 7. Fuel oil consumptions”. It should be: ,,Figure 7. Fuel oil consumptions.”. The authors write: ,,Table 3. Fuel oil consumptions and CO2 emissions calculations”. It should be: ,,Table 3. Fuel oil consumptions and CO2 emissions calculations.”.
There are also other errors in the work regarding the formatting of the text. For example: distances between text and table description, distances between table description and table. Literature items in the literature list should be arranged according to the citation order. To my knowledge, in an article for publication in this journal, all abbreviations should be explained in the text and not in footnotes. Please check and take this into account.
6. The graph is a self-contained piece of work. Graphs should always give units if the quantities presented are expressed in specific units. This is the first time I have encountered the statement that units are not given on graphs if the units are in a table.
Author Response
Point 1. #
Point 2.1. The relation power reduction – fuel consumption reduction is not straightforward because the ship usually has other fuel consumers. The explanation has been added to the main text. Also, reduced power lengthens the trip and increases specific consumption.
Point 2.2. It has been addressed. The results of other authors were described in more detail.
Point 3. A paragraph was added to explain Tables 1 and 2. The abbreviations used in these tables are well known in marine engineering circles. It seems counterproductive to explain these abbreviations in the main text, but I did it as requested. I hope that Table 3 has been explained to your satisfaction.
Point 4. Of course, other modes are possible. These modes were chosen because we wanted to make a comparison between operation at full sea speed and operation at slow speed as a basis. The other modes were chosen because they involve turning on and off power generators and oil-fired boilers, the operation of which could affect fuel consumption and carbon dioxide emissions. As stated in the conclusion, further research should be conducted that considers different loads, propulsion engines, and environmental influences.
Point 5. I sincerely apologize for this inconvenience. I was under the impression that work could be submitted with and without templates. And in the case of the second model, the paper would be formatted by the editor. These errors would be corrected.
The reference list was reorganized and papers on the use of simulators in scientific papers (as requested by another reviewer) were added.
Point 6. This item is corrected.
