Greenhouse Gas Emissions from Decommissioning Manmade Structures in the Marine Environment; Current Trends and Implications for the Future

Round 1

Reviewer 1 Report

The paper discusses Greenhouse gas emissions (GHG) from decommissioning manmade structures in marine environment. This study reports a 200-fold increase in GHG emissions due to the growth manmade structures in offshore wind industry and oil & gas industry.

They have then mentioned policies of few countries such as Northeast Atlantic Region and Norway, discussing few regulatory and legal requirements. They have emphasized upon revisiting the methods which are used for calculation of GHG in the said scenario.

The authors used GHG emission data for 44 decommissioning programs. The pre-decommissioning data was compared with post-decommissioning data and then they determined emissions gap. This emissions gap was subsequently applied to other pre-decommissioning emissions for more accurate model of the post-decommissioning emission costs.

Following require consideration:

(1) Error bars have been shown in Fig 4, accounting up to 30-35% error. How the authors have found this error, and how they have validated this value?

(2) Line 210: “pe-decommissioning” may kindly be corrected.

(3) A suggestion is that the authors may consider using Statistical tolls, such as Regression, for developing models for Figs 7, 9, 10, 11, 13.

(4) Also two figures have been numbered as 8 which may be corrected.

Author Response

Dear Reviewer,

Thank you very much for taking the time to read and review our work. I hope you find the responses to your suggestions suitable.

Yours Truly

Abigail Davies                                            

(1) Error bars have been shown in Fig 4, accounting up to 30-35% error. How the authors have found this error, and how they have validated this value?

The error bars are from the methods used by the Oil & Gas Industry (OGI) in calculating their greenhouse gas (GHG) emissions as per the Institute of Petroleum (IOP) guidelines. This is mentioned in the caption of figures 4 and 5 as such no validation of this error was undertaken.

(2) Line 210: “pe-decommissioning” may kindly be corrected.

Thank you. This has been corrected in the document.

(3) A suggestion is that the authors may consider using Statistical tolls, such as Regression, for developing models for Figs 7, 9, 10, 11, 13.

Thank you for the suggestion, however we feel that the lack of good quality data would invalidate any statistical tool results at this stage. However, this is certainly something that should be attempted in the future.

(4) Also two figures have been numbered as 8 which may be corrected.

Thank you. This has been corrected.

Reviewer 2 Report

1. The article is well-written and interesting, the following aspects are proposed before publication.

2. It would be interesting to enlarge the size of figure 2, to improve its reading and comprehension.

3. Please cite the figure when appropriate, there are figures between one and other citations.

4. Parts of the text are with justified paragraph typology and others are not.

5. Could you include a more conservative and lower estimate of total expected emissions?  

6. The titles of the graphs (for example figure 7) are not necessary since they appear at the bottom of the figure.  

7. What criteria has been chosen to select bibliographic sources? For example, to find an emissions cost ratio for offshore wind and offshore hydrocarbon decommissioning. Why those costs and not others?  

8. It is recommended to explain in this article a large part of the methodology used in https://doi.org/10.1016/j.enpol.2021.112717, since it is referenced multiple times (15 times).  

9. The main limitation of the work is the incomplete calculation or the lack of bibliographic data of the GHG emissions of the OGI decommissioning programs in the North Sea, how could you solve this problem?

10. Could the authors show the most optimistic and the most pessimistic scenario?----Although the work is complete and exhaustive, it is limited to boundary conditions, which may vary with the advancement of technology, needs, and economies of scale.

Author Response

Dear Reviewer,

Thank you very much for taking the time to read and review our work. I hope you find the responses to your suggestions suitable.

Yours Truly

Abigail Davies                                                                                 

1. The article is well-written and interesting, the following aspects are proposed before publication.

Thank you.

2. It would be interesting to enlarge the size of figure 2, to improve its reading and comprehension.

Thank you, this has been corrected.

3. Please cite the figure when appropriate, there are figures between one and other citations.

I’m not sure what this means. I have checked through the citations in the figures and can find no errors.

4. Parts of the text are with justified paragraph typology and others are not.

Thank you. This has been corrected.

5. Could you include a more conservative and lower estimate of total expected emissions?  

Thank you for the comment. Whilst we agree that a range of potential GHG emissions would reflect reality more accurately, the lack of data does not make this currently possible. 

6. The titles of the graphs (for example figure 7) are not necessary since they appear at the bottom of the figure.  

Thank you. This has been corrected.

7. What criteria has been chosen to select bibliographic sources? For example, to find an emissions cost ratio for offshore wind and offshore hydrocarbon decommissioning. Why those costs and not others?  

GHG emissions data are almost non-existent and very few researchers are working in this field. As such no other studies like this one have been attempted to date.  Cost data is also very difficult to get, especially future costs as there is a great deal of uncertainty because of the timescales involved (the political, economic and environmental will invariably change over the next 25 years).

8. It is recommended to explain in this article a large part of the methodology used in https://doi.org/10.1016/j.enpol.2021.112717, since it is referenced multiple times (15 times).  

Thank you, I have expanded the methodology section to include this recommendation. Please see to lines 100-105.

9. The main limitation of the work is the incomplete calculation or the lack of bibliographic data of the GHG emissions of the OGI decommissioning programs in the North Sea, how could you solve this problem?

I’m unsure what calculations you feel are incomplete, could you expand on this?

Data is very limited both globally and in the North Sea. It is impossible currently to include any more emissions data than presented here because the only publicly available data has been included.

All available North Sea GHG emissions data to date has been used and is listed in table 3 and includes citations.

To address this would require a change in policy to make complete data publicly available or agreement with both the oil and gas industry and renewables industry to provide data for further study. This would have to be done individually for each government, policy group, and individual companies. 

Data is one of the most limiting factors in our understanding of GHG emissions from decommissioning and may explain why no models have been undertaken previously.

10. Could the authors show the most optimistic and the most pessimistic scenario?----Although the work is complete and exhaustive, it is limited to boundary conditions, which may vary with the advancement of technology, needs, and economies of scale.

Lack of data means it is impossible to provide a range of values for emissions. This may become possible in the future but requires significantly more detailed data to become available.

Predictions about changes and advancement in technologies, needs and economies of scale are out with the scope of this study and the lack of data makes it currently not possible. This would certainly be required in the future.