the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Converging Profile Relationships for Offshore Wind Speed and Turbulence Intensity
Abstract. This paper reduces uncertainty in the quantification of offshore wind speed and turbulence intensity. A primary application is estimation of extreme wind speeds for design of wind energy generation systems, including turbines and fixed or floating support structures. Results of significance to normal winds, for resource assessment, engineering design or operations, can also be derived. This research is part of wider efforts to bring together the long established, but traditionally separate, onshore wind energy and metocean technical disciplines. A range of industry standard relationships, including those from International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO), are compared with an extensive set of met mast data collected offshore Northwestern Europe over recent decades. Analysis initially focused on over 1000 independent storm peak events identified within the 10 minute mean wind speed time series. Time series and coherent vertical profiles were subjected to detailed scrutiny and analysis, considering wind speeds at various averaging periods, turbulence intensity and gust factors. Most peak events were associated with neutral atmospheric conditions, so were well represented by the ISO Frøya profile, with shear close to the IEC power law exponent of α = 0.11. A new pragmatic framework for classification of wind profiles in terms of relative shear is outlined, bringing together key elements of the IEC and ISO standards. This relative shear framework, which extends over the full range of measured wind speeds, is related to atmospheric stability and used to distinguish different classes of ambient turbulence intensity. New empirical relationships to quantify offshore wind ambient turbulence intensity are described. This study highlights the critical role of turbulence intensity in the estimation of gust factors, with a range of relevant relationships from IEC, ISO and other sources assessed using the extensive set of measured storm peak events. A simple generalised form of gust factor relationship is adopted, with a coefficient that varies with averaging period. A similar, but distinct, analysis yields recommendations for estimating peak 10 minute mean winds from peaks in 1 or 3 hour mean winds. Finally, a simplified workflow for the estimation of extreme offshore winds is outlined.
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RC1: 'Comment on wes-2023-35', Anonymous Referee #1, 06 Jun 2023
Review of wes-2023-35: Converging Profile Relationships for Offshore Wind Speed and Turbulence Intensity
The submitted manuscript quantifies profiles of wind speed and turbulence intensity based on data from 11 offshore meteorological masts in northwestern Europe, focusing mostly on extreme conditions with strong wind speeds. The results are compared to industry standards, such as relationships provided by IEC and ISO. Further, gust factors and different stabilities of the marine atmospheric boundary-layer are considered and an improved workflow to estimate the wind speed in extreme conditions is presented.
Although the text is well written and figures are clearly presented, the manuscript comes across to me more like a technical note and I’m afraid I have to suggest the manuscript to be rejected for Wind Energy Science based on the following three reasons:
- The manuscript lacks in scientific quality. There are only a few scientific citations in the manuscript, the results are not put into context comparing with other studies, and I’m missing both a thorough scientific introduction and discussion section.
- In Sect. 9, the author mention LiDAR data and comment that it was beyond the scope of the study to include such data because of uncertainties in data quality under strong wind conditions. However, as modern offshore wind turbines are approaching hub heights of 150 m, with blades sweeping almost up to 300 m, I see no other choice than to include vertical profiles reaching at least up to hub height (and preferably higher) to ensure that this study is relevant. The met masts in the study ranges from 46 — 116 m in height, thus only covering approximately the lower third of modern offshore wind turbines.
- In addition to the relatively simple approaches of describing the wind speed and turbulence intensity presented by the equations, I would request results from a more statistically advanced method for comparison. There are numerous machine learning algorithms that could be applied to the data set at hand.
Finally, as mentioned earlier: I believe that the technical nature of the manuscript makes it suitable for submission as a technical paper/note to another journal. However, if the three comments presented above are met (which I of course realize requires a lot of work), I’m positive that this would make an excellent contribution to Wind Energy Science.
Citation: https://doi.org/10.5194/wes-2023-35-RC1 -
AC1: 'Reply on RC1', Gus Jeans, 12 Jun 2023
Many thanks for taking the time to read and comment on my work, the handling editor(s) have so far found it difficult to find anyone else to do so in the short timescale first suggested. Thanks also for the opportunity to encourage more technical discussion across our community. I selected WES to disseminate these valuable industry application focused results, because the public peer review mechanism resonated with my wider effects to promote discussion across the wind energy community. This has not been successful so far. Perhaps the relatively small number of industry practitioners who recognise the benefits of this applied research have been too busy with operational projects to participate in the online review process. I encourage them to do so while there is still time. I therefore request an extension to the public discussion period, while I also begin to explore other journal options.
Considerable effort has been made to prepare the manuscript according to WES conventions and secure permission to publish these proprietary results from six major offshore wind energy developers. So, I hope the editors can still consider publication within WES. There is no option for doing much more work for this paper, the objective is to share useful results from an already complete proprietary study. Publication of any additional extracts from the Joint Industry Project would need to be approved by all six participants, which would be time consuming. I can understand why an academic might consider the manuscript to be like a technical note. Indeed, I am not a pure academic, I am an industry practitioner. I do not get paid for the time required to work on this paper. I have offered WES the opportunity to share these results, but will of course accept the editor’s decision. This may be the wrong journal for such engagement between industry and academia. If so, I would be grateful for suggestions of suitable alternative journals.
In specific response to the three points raised:
1. I am surprised you suggest rejection due to insufficient scientific quality, as the handling editor(s) evaluated this before releasing into public review. The essential background theory is already well covered by the two textbooks I cited. There is no need to duplicate this in an already extensive paper. The IEC and ISO relationships are also already well documented in the corresponding standards and widely used, but to my knowledge not brought together in this way before, then evaluated using most available relevant measured data. The journal encourages a multidisciplinary approach, but this reviewer has apparently not recognised these original contributions to the wind energy community. The journal invites concise narratives which I have strived to provide, including application focused discussion throughout. I believe I have already included every reference needed to support the narrative, but would be happy to consider any specific suggestions that the reviewer thinks are missing.
2. The reviewer has misinterpreted this point. LiDAR data are not included because the focus of this already extensive study is on met mast data. I am not saying that all LiDAR data are unreliable at high wind speeds, just that they need to be treated with caution, in accord with scientific and engineering best practices. Of course LiDAR is needed for wind profiles above the height of met masts. The paper recognises this, but it could perhaps be made a little clearer. This is not a valid reason to reject an extensive detailed analysis of met mast data, which still provide the only widely trusted measure of turbulence intensity. Furthermore, quantifying shear in the lower 100m is critical for relating winds within the turbine blade swept area to near surface winds. These are essential to understand other metocean processes, including waves and surface currents. The paper provides valuable assessment of industry standard relationships, widely used by practitioners for estimation of extreme wind conditions over these heights.
3. As noted above, this paper is disseminating results from an already complete proprietary study with permission from all six major industry participants. These results are of practical value as they presently stand, with further statistical analysis beyond the scope of this already extensive study. The focussed discussion and conclusions section attempts to makes this clear. This paper may assist other practitioners or researchers to conduct further related study on the mainly public domain data. It is not a requirement of the WES journal to include application of machine learning techniques.
I believe I have convincingly rebutted all three reasons given by this anonymous reviewer for rejection of this manuscript. I hope the journal editors will give this due consideration, but of course I will respect their decision.
Citation: https://doi.org/10.5194/wes-2023-35-AC1
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CC1: 'Comment on wes-2023-35', George Forristall, 12 Jun 2023
The review entirely misses the point of this paper. The author has performed a thorough analysis using a very large data set of met mast data to calculate wind speed and turbulence profiles over the ocean. Knowledge of those profiles is vital for both siting and design of offshore wind turbines. The profiles recommended by different standards and different research communities do not agree with one another, and it is very important to muster as much data as possible to decide which is most accurate. It would be very helpful for this paper to be published in a journal that encourages online discussion in order move closer to a consensus of what profile forms should be used.
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.Citation: https://doi.org/10.5194/wes-2023-35-CC1 -
AC2: 'Reply on CC1', Gus Jeans, 12 Jul 2023
Many thanks for sharing these comments George. There are few metocean perspectives in the community with such authority. As lead author of the planned companion paper, your views on appropriate journal for our work are a key consideration. Fortunately, there has been much more balanced discussion here since the first invited formal review.
Citation: https://doi.org/10.5194/wes-2023-35-AC2
-
AC2: 'Reply on CC1', Gus Jeans, 12 Jul 2023
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CC2: 'Comment on wes-2023-35', Ian Wade, 20 Jun 2023
Firstly, my apologies for my slow responses. Due to a recent heavy workload, I have not been able to give this as much attention as it deserves. Please find my initial, albeit brief, comments in support of this paper. I hope to contribute more in the near future.
This is a detailed analyses that makes a valiant attempt to address some of the inconsistencies present in turbulence and shear assessments arising from the various industry standards. As a practitioner in this field, these inconsistencies create unwanted ambiguity between different interpretations of what is required. These differences are often significant and any attempt to promote discussion towards an integrated approach is to be welcomed.
The paper is generally well written and is based on extensive datasets. The dissemination of the proprietary study results (extended ISO) is a particularly useful extension to the Frøya based original. The practical suggestions for gust scaling and averaging period ratios etc. are also extremely helpful and the presented analyses help explain the sensitivity of changes to various coefficient values. I welcome this pragmatic approach in the full understanding that sites will, unfortunately, often need to be configured on a case by case basis.
I believe this is a useful contribution to the field and hope that it will be published and promote the much required discussions, now and in the future.
Ian Wade
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.Citation: https://doi.org/10.5194/wes-2023-35-CC2 -
AC3: 'Reply on CC2', Gus Jeans, 12 Jul 2023
Many thanks for making the time to provide these comments Ian. Your perspective as a very experienced metocean practitioner, busy working on this subject matter for many offshore wind energy projects, is very important. I look forward to discussing further beyond this medium.
Citation: https://doi.org/10.5194/wes-2023-35-AC3
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AC3: 'Reply on CC2', Gus Jeans, 12 Jul 2023
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CC3: 'Comment on wes-2023-35', Rémi Gandoin, 22 Jun 2023
I am hereby attaching comments I have made to the manuscript.
This paper fits well with the "interdisciplinary perspective" and "fundamental" (in the sense that the topic need to be addressed for every project) scope of WES. It ressembles other papers that summarize outcomes of engineering projects, like https://doi.org/10.5194/wes-5-199-2020,It also fits well with the "Fostering wind energy science and education" motto: the necessary overlap and ajdustements required between "wind" and "metocean" engineering fields is a recurrent topic in the engineering practice which not all know about.
I recommend this paper to be published after revisions are applied, see the list below.- The paper could be shortened (to by focusing on large wind speeds conditions only, see suggestions in the commented document.
- The derivation of the TI values using the IEC 61400-1 and IEC 61400-1-3 should be explained in more details (see my comment in the document)
All the best
Rémi Gandoin
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.-
AC4: 'Reply on CC3', Gus Jeans, 12 Jul 2023
I’m delighted that you could find time to carefully review and comment on this paper Remi. Your very active and widely respected wind resource perspective is exactly the type of discussion I was hoping to encourage! You raise very good points that the primary focus of this paper is on strong winds and that it could be shorter. I will not respond to your specific suggestions further here. I will also seek views from other relevant colleagues, importantly the ENOW participants who approved release of this publication. Hopefully you and I can both help encourage wider discussion of related relevant topics in future WRM meetings, now these results are public domain.
Citation: https://doi.org/10.5194/wes-2023-35-AC4
-
RC2: 'Comment on wes-2023-35', Rémi Gandoin, 11 Jul 2023
I am hereby attaching comments I have made to the manuscript.
This paper fits well with the "interdisciplinary perspective" and "fundamental" (in the sense that the topic need to be addressed for every project) scope of WES. It ressembles other papers that summarize outcomes of engineering projects, like https://doi.org/10.5194/wes-5-199-2020,It also fits well with the "Fostering wind energy science and education" motto: the necessary overlap and ajdustements required between "wind" and "metocean" engineering fields is a recurrent topic in the engineering practice which not all know about.
I recommend this paper to be published after revisions are applied, see the list below.- The paper could be shortened (by focusing on large wind speeds conditions only), see suggestions in the commented pdf document.
- The derivation of the TI values using the IEC 61400-1 and IEC 61400-1-3 should be explained in more details (see my comment in the document)
All the best
Rémi Gandoin
-
AC5: 'Reply on RC2', Gus Jeans, 12 Jul 2023
I’m very pleased to see that your detailed community comment has been upgraded to a formal review Remi.
Citation: https://doi.org/10.5194/wes-2023-35-AC5
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AC6: 'Comment on wes-2023-35', Gus Jeans, 12 Jul 2023
I have now submitted responses to all comments and again express my thanks for all of them. I think my responses clearly outline my position, focussed on encouraging application focussed multidisciplinary discussion. My objective is to help accelerate the energy transition, via rapid coherent growth of offshore wind energy. I also thank all journal editors involved in this review for their time and dedication. I look forward to seeing all community comments properly considered in any further decisions.
Citation: https://doi.org/10.5194/wes-2023-35-AC6
Status: closed
-
RC1: 'Comment on wes-2023-35', Anonymous Referee #1, 06 Jun 2023
Review of wes-2023-35: Converging Profile Relationships for Offshore Wind Speed and Turbulence Intensity
The submitted manuscript quantifies profiles of wind speed and turbulence intensity based on data from 11 offshore meteorological masts in northwestern Europe, focusing mostly on extreme conditions with strong wind speeds. The results are compared to industry standards, such as relationships provided by IEC and ISO. Further, gust factors and different stabilities of the marine atmospheric boundary-layer are considered and an improved workflow to estimate the wind speed in extreme conditions is presented.
Although the text is well written and figures are clearly presented, the manuscript comes across to me more like a technical note and I’m afraid I have to suggest the manuscript to be rejected for Wind Energy Science based on the following three reasons:
- The manuscript lacks in scientific quality. There are only a few scientific citations in the manuscript, the results are not put into context comparing with other studies, and I’m missing both a thorough scientific introduction and discussion section.
- In Sect. 9, the author mention LiDAR data and comment that it was beyond the scope of the study to include such data because of uncertainties in data quality under strong wind conditions. However, as modern offshore wind turbines are approaching hub heights of 150 m, with blades sweeping almost up to 300 m, I see no other choice than to include vertical profiles reaching at least up to hub height (and preferably higher) to ensure that this study is relevant. The met masts in the study ranges from 46 — 116 m in height, thus only covering approximately the lower third of modern offshore wind turbines.
- In addition to the relatively simple approaches of describing the wind speed and turbulence intensity presented by the equations, I would request results from a more statistically advanced method for comparison. There are numerous machine learning algorithms that could be applied to the data set at hand.
Finally, as mentioned earlier: I believe that the technical nature of the manuscript makes it suitable for submission as a technical paper/note to another journal. However, if the three comments presented above are met (which I of course realize requires a lot of work), I’m positive that this would make an excellent contribution to Wind Energy Science.
Citation: https://doi.org/10.5194/wes-2023-35-RC1 -
AC1: 'Reply on RC1', Gus Jeans, 12 Jun 2023
Many thanks for taking the time to read and comment on my work, the handling editor(s) have so far found it difficult to find anyone else to do so in the short timescale first suggested. Thanks also for the opportunity to encourage more technical discussion across our community. I selected WES to disseminate these valuable industry application focused results, because the public peer review mechanism resonated with my wider effects to promote discussion across the wind energy community. This has not been successful so far. Perhaps the relatively small number of industry practitioners who recognise the benefits of this applied research have been too busy with operational projects to participate in the online review process. I encourage them to do so while there is still time. I therefore request an extension to the public discussion period, while I also begin to explore other journal options.
Considerable effort has been made to prepare the manuscript according to WES conventions and secure permission to publish these proprietary results from six major offshore wind energy developers. So, I hope the editors can still consider publication within WES. There is no option for doing much more work for this paper, the objective is to share useful results from an already complete proprietary study. Publication of any additional extracts from the Joint Industry Project would need to be approved by all six participants, which would be time consuming. I can understand why an academic might consider the manuscript to be like a technical note. Indeed, I am not a pure academic, I am an industry practitioner. I do not get paid for the time required to work on this paper. I have offered WES the opportunity to share these results, but will of course accept the editor’s decision. This may be the wrong journal for such engagement between industry and academia. If so, I would be grateful for suggestions of suitable alternative journals.
In specific response to the three points raised:
1. I am surprised you suggest rejection due to insufficient scientific quality, as the handling editor(s) evaluated this before releasing into public review. The essential background theory is already well covered by the two textbooks I cited. There is no need to duplicate this in an already extensive paper. The IEC and ISO relationships are also already well documented in the corresponding standards and widely used, but to my knowledge not brought together in this way before, then evaluated using most available relevant measured data. The journal encourages a multidisciplinary approach, but this reviewer has apparently not recognised these original contributions to the wind energy community. The journal invites concise narratives which I have strived to provide, including application focused discussion throughout. I believe I have already included every reference needed to support the narrative, but would be happy to consider any specific suggestions that the reviewer thinks are missing.
2. The reviewer has misinterpreted this point. LiDAR data are not included because the focus of this already extensive study is on met mast data. I am not saying that all LiDAR data are unreliable at high wind speeds, just that they need to be treated with caution, in accord with scientific and engineering best practices. Of course LiDAR is needed for wind profiles above the height of met masts. The paper recognises this, but it could perhaps be made a little clearer. This is not a valid reason to reject an extensive detailed analysis of met mast data, which still provide the only widely trusted measure of turbulence intensity. Furthermore, quantifying shear in the lower 100m is critical for relating winds within the turbine blade swept area to near surface winds. These are essential to understand other metocean processes, including waves and surface currents. The paper provides valuable assessment of industry standard relationships, widely used by practitioners for estimation of extreme wind conditions over these heights.
3. As noted above, this paper is disseminating results from an already complete proprietary study with permission from all six major industry participants. These results are of practical value as they presently stand, with further statistical analysis beyond the scope of this already extensive study. The focussed discussion and conclusions section attempts to makes this clear. This paper may assist other practitioners or researchers to conduct further related study on the mainly public domain data. It is not a requirement of the WES journal to include application of machine learning techniques.
I believe I have convincingly rebutted all three reasons given by this anonymous reviewer for rejection of this manuscript. I hope the journal editors will give this due consideration, but of course I will respect their decision.
Citation: https://doi.org/10.5194/wes-2023-35-AC1
-
CC1: 'Comment on wes-2023-35', George Forristall, 12 Jun 2023
The review entirely misses the point of this paper. The author has performed a thorough analysis using a very large data set of met mast data to calculate wind speed and turbulence profiles over the ocean. Knowledge of those profiles is vital for both siting and design of offshore wind turbines. The profiles recommended by different standards and different research communities do not agree with one another, and it is very important to muster as much data as possible to decide which is most accurate. It would be very helpful for this paper to be published in a journal that encourages online discussion in order move closer to a consensus of what profile forms should be used.
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.Citation: https://doi.org/10.5194/wes-2023-35-CC1 -
AC2: 'Reply on CC1', Gus Jeans, 12 Jul 2023
Many thanks for sharing these comments George. There are few metocean perspectives in the community with such authority. As lead author of the planned companion paper, your views on appropriate journal for our work are a key consideration. Fortunately, there has been much more balanced discussion here since the first invited formal review.
Citation: https://doi.org/10.5194/wes-2023-35-AC2
-
AC2: 'Reply on CC1', Gus Jeans, 12 Jul 2023
-
CC2: 'Comment on wes-2023-35', Ian Wade, 20 Jun 2023
Firstly, my apologies for my slow responses. Due to a recent heavy workload, I have not been able to give this as much attention as it deserves. Please find my initial, albeit brief, comments in support of this paper. I hope to contribute more in the near future.
This is a detailed analyses that makes a valiant attempt to address some of the inconsistencies present in turbulence and shear assessments arising from the various industry standards. As a practitioner in this field, these inconsistencies create unwanted ambiguity between different interpretations of what is required. These differences are often significant and any attempt to promote discussion towards an integrated approach is to be welcomed.
The paper is generally well written and is based on extensive datasets. The dissemination of the proprietary study results (extended ISO) is a particularly useful extension to the Frøya based original. The practical suggestions for gust scaling and averaging period ratios etc. are also extremely helpful and the presented analyses help explain the sensitivity of changes to various coefficient values. I welcome this pragmatic approach in the full understanding that sites will, unfortunately, often need to be configured on a case by case basis.
I believe this is a useful contribution to the field and hope that it will be published and promote the much required discussions, now and in the future.
Ian Wade
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.Citation: https://doi.org/10.5194/wes-2023-35-CC2 -
AC3: 'Reply on CC2', Gus Jeans, 12 Jul 2023
Many thanks for making the time to provide these comments Ian. Your perspective as a very experienced metocean practitioner, busy working on this subject matter for many offshore wind energy projects, is very important. I look forward to discussing further beyond this medium.
Citation: https://doi.org/10.5194/wes-2023-35-AC3
-
AC3: 'Reply on CC2', Gus Jeans, 12 Jul 2023
-
CC3: 'Comment on wes-2023-35', Rémi Gandoin, 22 Jun 2023
I am hereby attaching comments I have made to the manuscript.
This paper fits well with the "interdisciplinary perspective" and "fundamental" (in the sense that the topic need to be addressed for every project) scope of WES. It ressembles other papers that summarize outcomes of engineering projects, like https://doi.org/10.5194/wes-5-199-2020,It also fits well with the "Fostering wind energy science and education" motto: the necessary overlap and ajdustements required between "wind" and "metocean" engineering fields is a recurrent topic in the engineering practice which not all know about.
I recommend this paper to be published after revisions are applied, see the list below.- The paper could be shortened (to by focusing on large wind speeds conditions only, see suggestions in the commented document.
- The derivation of the TI values using the IEC 61400-1 and IEC 61400-1-3 should be explained in more details (see my comment in the document)
All the best
Rémi Gandoin
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.-
AC4: 'Reply on CC3', Gus Jeans, 12 Jul 2023
I’m delighted that you could find time to carefully review and comment on this paper Remi. Your very active and widely respected wind resource perspective is exactly the type of discussion I was hoping to encourage! You raise very good points that the primary focus of this paper is on strong winds and that it could be shorter. I will not respond to your specific suggestions further here. I will also seek views from other relevant colleagues, importantly the ENOW participants who approved release of this publication. Hopefully you and I can both help encourage wider discussion of related relevant topics in future WRM meetings, now these results are public domain.
Citation: https://doi.org/10.5194/wes-2023-35-AC4
-
RC2: 'Comment on wes-2023-35', Rémi Gandoin, 11 Jul 2023
I am hereby attaching comments I have made to the manuscript.
This paper fits well with the "interdisciplinary perspective" and "fundamental" (in the sense that the topic need to be addressed for every project) scope of WES. It ressembles other papers that summarize outcomes of engineering projects, like https://doi.org/10.5194/wes-5-199-2020,It also fits well with the "Fostering wind energy science and education" motto: the necessary overlap and ajdustements required between "wind" and "metocean" engineering fields is a recurrent topic in the engineering practice which not all know about.
I recommend this paper to be published after revisions are applied, see the list below.- The paper could be shortened (by focusing on large wind speeds conditions only), see suggestions in the commented pdf document.
- The derivation of the TI values using the IEC 61400-1 and IEC 61400-1-3 should be explained in more details (see my comment in the document)
All the best
Rémi Gandoin
-
AC5: 'Reply on RC2', Gus Jeans, 12 Jul 2023
I’m very pleased to see that your detailed community comment has been upgraded to a formal review Remi.
Citation: https://doi.org/10.5194/wes-2023-35-AC5
-
AC6: 'Comment on wes-2023-35', Gus Jeans, 12 Jul 2023
I have now submitted responses to all comments and again express my thanks for all of them. I think my responses clearly outline my position, focussed on encouraging application focussed multidisciplinary discussion. My objective is to help accelerate the energy transition, via rapid coherent growth of offshore wind energy. I also thank all journal editors involved in this review for their time and dedication. I look forward to seeing all community comments properly considered in any further decisions.
Citation: https://doi.org/10.5194/wes-2023-35-AC6
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