Scour Protections for Offshore Foundations of Marine Energy Harvesting Technologies: A Review

Round 1

Reviewer 1 Report

The paper presents a very wide review about the scour protection around maritime foundations. The authors report the majority of the recent studies (some references should be added) and critically analyze such findings, by highliting the latest advances and the unsolved aspects. The paper faces up to a lot of issues about the scour protections: the different design levels, the physical and numerical analyses, the field investigation, etc. The paper is very useful and only few aspects need to be revised or some comments added in the revised manuscript based on the following comments/remarks.

REMARKS/COMMENTS

• Section 2.1. Lines 120-121. Surely the rip-rap protections are the most common type, however the availability of the roubble-mound material depends also on the median design size of the stone material: by using a static scour protection design approach the stone size could be significant in specific wave-current conditions. Moreover, the availability and the costs (material, transport, etc.) could change from country to country (depends on the maritime foundations location), and ,hence, a rip-rap protection is not always the most affordable solution. Please add some comments in order to analyze such problems.
• Lines 143-145. The dynamic scour protection approach generally leads to a reduction of a median stone size, hence the availability and the cost could be reduced. However, the optimitation of the scour protection made by using a smaller median stone size "may require an increase of the thickness to sustain the accaptable damage level without failure occurence". I have some doubts about an optimitazion design made with a larger protection thickness with a smaller median size, because I believe that the needed repairments will have to take place more frequently and , hence, such scour protection could be more expensive than a more cautelative one in a life cycle assesment. The authors should add some comments about it.
• In the design of a scour protection it is important to know the scour depth and also the scour pattern in order to estimate the minimum needed extension of the protection. In some experimental studies the scour pattern has been analyzed for different wave charateristics, as reported in Corvaro S., Marini F., Mancinelli A., Lorenzoni C., Brocchini, M. (2018): Hydro- and Morpho-dynamics induced by a vertical slender pile under regular an-d random waves. Journal of Waterway, Port, Coastal, and Ocean Engineering, ASCE, 144(6), 04018018, https://doi.org/10.1061/(ASCE)WW.1943-5460.0000470
• Lines 214-219. Please better explain these sentences.
• Lines 271-272. How has the equilibrium scour depth been evaluated?
• It is also not clear if the equilibrium scour depth is referring to the scour protection or the mobile seabed. Please add some details in the manuscript in order to clarify these aspects.
• Lines 284 and 287. I believe that a single wide-graded material cannot be evaluated by using the same damage level used for multiple protection layers. Therefore, is it feasible a single wide-graded layer as a protection of maritime foundations? I suggest the authors to add some comments about it.
• Line 297-298. The sinking has been analyzed in the study of Nielsen, Probst, Petersen, Sumer (2015) "Sinking of armour layer around a vertical cylinder exposed to waves", Coastal Engineering, vol. 100, pp. 58-66. Please add this reference.
•  Line 122. Recently an experimental study on the effectivness of alternative scour protection around a monopile is made by using geotextile sand container.  Corvaro S., Marini F., Mancinelli A., Lorenzoni C. (2018). Scour protection around a single slender pile exposed to waves. Coastal Engineering Proceedings, 1(36),  https://doi.org/10.9753/icce.v36.papers.6
• Section 2.4. The authors should highlight the importance of the knowledge of the amplification factor in order to better design a scour protection (static approach).
• Section 3. Please specify the wave charateristics of 3000-5000 or more waves: one wave spectra made of a wave train of 3000 waves (5000 or more), a repetition of the same sequence of a wave spectra for which 3000 waves (5000 or more) were reproduced; a repetition of different sequences of wave train.
• As done for the fluvial scour, the effect of sequential smaller storm events can produce a different response on the scour and on the scour protection. Please add some comments.
• In Section 5 the damage level has been reported. Surely as reported in Fig. 2 the position of scour protection exposure is important. Moreover, I believe that the location of the exposure with respect to the distance with the monopile is important in the evaluation of the scour exposure. Two different risk areas can be identified. If the exposure of the scour protection occurs in the area closer to the monopile (also due to the different structure responce), the safety of the foundations is reduced with respect of the same expsoure which occurs in the further area.
• In section 6. Add some references about the scale effects, for example: Ettema, R., Melville, B., and Barkdoll, B. (1998). “Scale Effects in Pier-Scour Experiments.” Journal of Hydraulic Engineering, 124(6), 639–642.
• In section 7. Line 621. The scour edge has been studied both numerically and experimentally by Petersen, Sumer, Fredsøe, Raaijmakers, Schouten (2015)" Edge scour at scour protections around piles in the marine environment — Laboratory and field investigation". Coastal Engineering, 106, pp 42-72.https://doi.org/10.1016/j.coastaleng.2015.08.007
• In section 7. Lines 693-695. I suggest to reformulate such sentence. It seems that the authors believe that the experimental models is less accurate than the numerical ones. I suggest to highlight the importance of a combined use of both the numerical and experimental models.
• In Miozzi et al. (2019) the complex soil-structure-fluid interaction has been experimentally analyzed by means of a DDPIV technique. The coherent structure, the sediment transport, the pressure at the seabeds were analyzed [Miozzi et al. (2019) "Wave-induced morphodynamics and sediment transport around a slender vertical cylinder", Advances in Water Resources Volume 129, Pages 263-280]. In order to better understand and shed light to complex phenomena such experimental study are needed in combination with numerical model.
• In section 8 I suggest to add some details about the monitoring systems used for the scour and scour protection.
• In section 8. I also suggest to add some details about the different type of scour protection installation.
• In section 8: Add this reference: Asgarpour M. (2016) "Assembly, transportation, installation and commissioning of offshore wind farms". Offshore Wind Farms. Technologies, Design and Operation. pp 527-541

Minor issues

• In line 277 remove m after [4]
• in line 595 Many instead of Manny

Author Response

The paper presents a very wide review about the scour protection around maritime foundations. The authors report the majority of the recent studies (some references should be added) and critically analyze such findings, by highlighting the latest advances and the unsolved aspects. The paper faces up to a lot of issues about the scour protections: the different design levels, the physical and numerical analyses, the field investigation, etc. The paper is very useful and only few aspects need to be revised or some comments added in the revised manuscript based on the following comments/remarks.

A: The authors kindly appreciate the comment made, and highlight that the new references suggested in the remaining revision did contribute to improve the manuscript's quality. The authors thank for this particular aspect.

REMARKS/COMMENTS

• Section 2.1. Lines 120-121. Surely the rip-rap protections are the most common type, however the availability of the roubble-mound material depends also on the median design size of the stone material: by using a static scour protection design approach the stone size could be significant in specific wave-current conditions. Moreover, the availability and the costs (material, transport, etc.) could change from country to country (depends on the maritime foundations location), and ,hence, a rip-rap protection is not always the most affordable solution. Please add some comments in order to analyze such problems.

A: It is indeed true, despite the fact that it is a very commonly applied solution, it may not always be the "best option". The authors thank for this remark and have made changes between the lines 120 to 129 to accommodate this remark for the benefit of the reader and to avoid giving the impression that rubble-mound protections are always the best choice.

• Lines 143-145. The dynamic scour protection approach generally leads to a reduction of a median stone size, hence the availability and the cost could be reduced. However, the optimitation of the scour protection made by using a smaller median stone size "may require an increase of the thickness to sustain the accaptable damage level without failure occurence". I have some doubts about an optimitazion design made with a larger protection thickness with a smaller median size, because I believe that the needed repairments will have to take place more frequently and , hence, such scour protection could be more expensive than a more cautelative one in a life cycle assesment. The authors should add some comments about it.

R: Former studies (e.g. Schoesitter et al. 2014 or Ferradosa et al. 2018) do indicate that significant reduction of the stone size can be achieved for dynamic scour protections at a small increase in the thickness. In these cases the money savings are there. Typically, the increase in thickness allows for larger scour depths to occur, which will eventually reach a point of stabilization given the loss of strength of scour mechanisms as the depth increases. However, the reviewer is right, if the thickness is too large, or if the if maintenance (refilling) operations become too common, then dynamic solutions might not be the best. A new comment was added to include the importance of analysing the actual benefits of reducing the stone size in comparison with the increase in the thickness. This was added in the new lines 151 to 156.

• In the design of a scour protection it is important to know the scour depth and also the scour pattern in order to estimate the minimum needed extension of the protection. In some experimental studies the scour pattern has been analyzed for different wave charateristics, as reported in Corvaro S., Marini F., Mancinelli A., Lorenzoni C., Brocchini, M. (2018): Hydro- and Morpho-dynamics induced by a vertical slender pile under regular an-d random waves. Journal of Waterway, Port, Coastal, and Ocean Engineering, ASCE, 144(6), 04018018, https://doi.org/10.1061/(ASCE)WW.1943-5460.0000470

R: The authors thank for this reminder, this aspect was addressed in the introduction, near the reference 16. For the benefit of the reader and given the importance of the work, this reference was also added to this paragraph and is now reference 17.

• Lines 214-219. Please better explain these sentences.

R: In these sentences the authors wanted to convey the idea already expressed in the comment the reviewer made for former lines 143-145. In addition to the clarification already added, the authors rephrased this sentences to make them more clear. Changes are implemented in lines 220-224 and 228.

• Lines 271-272. How has the equilibrium scour depth been evaluated?

R: This detail was not added, because the reference is in place to allow the reader to further dive into this aspect of the work itself. For the knowledge of the reviewer, it is hereby mentioned that in this reference, the scour depth evaluated at the wide-graded protections mentioned in this part of the paper were obtained by means of digital elevation models (DEMs) of the bed topography, which were measured with a laser distance sensor (OADR 2016480,Baumer, Friedberg, Germany; resolution 0.015–0.67 mm). A vertical accuracy of approximately 1.0 mm was achieved by calibration preliminary tests.

• It is also not clear if the equilibrium scour depth is referring to the scour protection or the mobile seabed. Please add some details in the manuscript in order to clarify these aspects.

R: it refers to the scour protection. This detail was added to the manuscript to clarify this aspect. Lines 282 and 284.

• Lines 284 and 287. I believe that a single wide-graded material cannot be evaluated by using the same damage level used for multiple protection layers. Therefore, is it feasible a single wide-graded layer as a protection of maritime foundations? I suggest the authors to add some comments about it.

R: In Chavez et al. (2019) and Wu et al. (2020) wide graded scour protections are analysed with damage levels introduced by De Vos et al. (2012) for dynamic scour protections. However, since the failure introduced by De Vos et al. (2012) relates to the exposure of the filter layer, the interpretation for wide-graded scour protections might not be straightforward. However, its prospective applicability has been consistently indicated as possible by the literature (albeit scarce), as it is discussed throughout all section 2.4. Given the importance to analyse the damage number for wide-graded protections, Fazeres-Ferradosa et al. (2020) - ref 16 - does state clearly that the damage scale may require a redefinition in terms of its failure boundaries. Eventually, for wide-graded protections, as there is no filter, the exposure of the sand-bed instead of the filter layer could be used as a criterion. A comment to accommodate this remark nicely pointed by the reviewer was included in the manuscript Lines 302-307.

• Line 297-298. The sinking has been analyzed in the study of Nielsen, Probst, Petersen, Sumer (2015) "Sinking of armour layer around a vertical cylinder exposed to waves", Coastal Engineering, vol. 100, pp. 58-66. Please add this reference.

R: This reference was added and the authors appreciate the suggestion made, as it improves the quality of the references mentioned. 

•  Line 122. Recently an experimental study on the effectivness of alternative scour protection around a monopile is made by using geotextile sand container.  Corvaro S., Marini F., Mancinelli A., Lorenzoni C. (2018). Scour protection around a single slender pile exposed to waves. Coastal Engineering Proceedings, 1(36),  https://doi.org/10.9753/icce.v36.papers.6

R: this reference was added to this line.

• Section 2.4. The authors should highlight the importance of the knowledge of the amplification factor in order to better design a scour protection (static approach).

R: The authors thank the reviewer for this reminder and have added a comment in Lines 348 to 350 to highlight the importance of the amplification factor, which as been discussed in the literature.

• Section 3. Please specify the wave charateristics of 3000-5000 or more waves: one wave spectra made of a wave train of 3000 waves (5000 or more), a repetition of the same sequence of a wave spectra for which 3000 waves (5000 or more) were reproduced; a repetition of different sequences of wave train.

R: At the end of section 3, this was already mentioned in point ii of the last paragraph of this section, where it was stated that severe damage may occur under different sequential storms. However, to make it more evident, this remark was also added in Lines 396-398

• As done for the fluvial scour, the effect of sequential smaller storm events can produce a different response on the scour and on the scour protection. Please add some comments.

R: The authors entirely agree with the reviewer. This aspect was addressed with the new remark made to accommodate the previous correction suggested by the reviewer. Moreover, in the last paragraph of section 3 this was already addressed, as mentioned before, thus in this case no changes were made.

• In Section 5 the damage level has been reported. Surely as reported in Fig. 2 the position of scour protection exposure is important. Moreover, I believe that the location of the exposure with respect to the distance with the monopile is important in the evaluation of the scour exposure. Two different risk areas can be identified. If the exposure of the scour protection occurs in the area closer to the monopile (also due to the different structure responce), the safety of the foundations is reduced with respect of the same expsoure which occurs in the further area.

R: Indeed, the distance to the foundation may have more influence on the structural safety than in if damage occurs at the outer regions of the protection. However, note that damage occurring at the outer regions, e.g. as in edge scour, has been shown to propagate towards the inside of the protection and once the protection's registers such propagation there is a high chance of further damage to occur at the structure's proximity. Thus on the long run all damage at the protection is important. Still given the importance of damage near the foundation on the short term, the authors appreciate the reviewers suggestion and added a new comment on this aspect in Lines 526 to 530.

• In section 6. Add some references about the scale effects, for example: Ettema, R., Melville, B., and Barkdoll, B. (1998). “Scale Effects in Pier-Scour Experiments.” Journal of Hydraulic Engineering, 124(6), 639–642.

R: In this section the classic reference, which focus on marine environment scour, of Whitehouse and Sutherland (1998) had been added, because it provides an extensive discussion of the scale effects and model effects along with other references. Nevertheless, for the benefit of the reader the reference suggested was also included.

• In section 7. Line 621. The scour edge has been studied both numerically and experimentally by Petersen, Sumer, Fredsøe, Raaijmakers, Schouten (2015)" Edge scour at scour protections around piles in the marine environment — Laboratory and field investigation". Coastal Engineering, 106, pp 42-72.https://doi.org/10.1016/j.coastaleng.2015.08.007

R: Looking at this reference the authors found no numerical results, as the study addresses an extensive set of experimental and field data. In this paper, the images similar to numerical simulations are in fact three dimensional images derived from the tests, e.g. see Fig. 17 of the reference suggested. These images are a sequence of three-dimensional images of the bed morphology obtained in Test no. 23 measured by bed-profiler system. They do not correspond to a numerical study of edge scour. Nevertheless the authors appreciate the suggestion given, which could be added to the section of physical modelling. The only reason why it was not done is because the suggested paper is derived from the extensive study performed by Petersen in : Petersen, T. Scour around offshore wind turbine foundations. Ph.D. thesis. Technical University of Denmark, 2014. Thus to avoid a replication of information between references, the authors kept the paper as it is, since the Ph.D. thesis of Dr. Petersen was already discussed in the manuscript.

• In section 7. Lines 693-695. I suggest to reformulate such sentence. It seems that the authors believe that the experimental models is less accurate than the numerical ones. I suggest to highlight the importance of a combined use of both the numerical and experimental models. In Miozzi et al. (2019) the complex soil-structure-fluid interaction has been experimentally analyzed by means of a DDPIV technique. The coherent structure, the sediment transport, the pressure at the seabeds were analyzed [Miozzi et al. (2019) "Wave-induced morphodynamics and sediment transport around a slender vertical cylinder", Advances in Water Resources Volume 129, Pages 263-280]. In order to better understand and shed light to complex phenomena such experimental study are needed in combination with numerical model.

R: The authors kindly appreciate these remarks. The idea that the authors wanted to convey was that experimental models are more accurate than numerical models. Numerical ones are yet to reach a similar level of maturity. This sentence was re-phrased and the importance of combining both physical and numerical models was highlighted in Lines 719-724.

• In section 8 I suggest to add some details about the monitoring systems used for the scour and scour protection.

R: The authors appreciate the suggestion, however the review is lengthy given the wide range of topics already covered. The topic of monitoring systems and techniques is important, but the main idea of section 8 is to analyse the contents of field reported data and not the way the data is acquired. However, for the benefit of the reader the following reference was added, where different techniques are described and discussed and studied by means of physical modelling research as well:

Byuna, Yong-Hoon, Parkb, Kiwon, & Lee, Jong-Sub. (2015). Scour-monitoring techniques for offshore foundations. Smart Structures and Systems, 16(4), 667–681. https://doi.org/10.12989/SSS.2015.16.4.667

• In section 8. I also suggest to add some details about the different type of scour protection installation. In section 8: Add this reference: Asgarpour M. (2016) "Assembly, transportation, installation and commissioning of offshore wind farms". Offshore Wind Farms. Technologies, Design and Operation. pp 527-541

R: The installation technique may indeed play a role in the performance of the scour protection. This is not however the key topic of the review which already covers a lot of knowledge gaps and is quite lengthy. However, the authors kindly appreciate the suggestion and have added some comments on this aspect between lines 831 and 838, including the reference of Professor Asgarpour.

Minor issues

• In line 277 remove m after [4]

R: This was corrected, the typo "m" was supposed to be a comma signal.

• in line 595 Many instead of Manny

R: The authors thank the reviewer for the careful review. This typo was corrected as well.

That authors would like to thank the reviewer for the detailed review and feel that the suggestions improved the overall quality of the work, including the new references added.

Reviewer 2 Report

This is a through review paper on the key challenges and most up to date advances related to offshore wind turbine scour protection. This paper will be interesting and beneficial to the readers who work on offshore wind project or research. 

Comments for author File: Comments.pdf

Author Response

This is a through review paper on the key challenges and most up to date advances related to offshore wind turbine scour protection. This paper will be interesting and beneficial to the readers who work on offshore wind project or research. 

A: The authors appreciate the positive comment from the reviewer and they agree with the fact that this paper might be helpful to those working on this topic, due to the wide range of topics covered. The authors kindly appreciate the reviewer's evaluation.

Reviewer 3 Report

The paper "Scour protections for offshore foundations of marine energy harvesting technologies: A review" reports a wide and detailed summary of a large part of the literature available on the topic of scour protction for several different marine fundation, such as the ones for wind turbines, but also wave energy converters and tidal turbines.
The paper is clear and the language is rather correct. The literature span over an abundant set of articles and identifies the significant aspect of the topic, more or less studied and/or requiring further research.
However, in my opinion the paper is not still suitable for publication for the follwoing reasons:

- the review paper should aim at a synthesis of the problem, and/or at least a classification of the available research in literature. The large set of literature used is not classified, neither an overall view of the problem is provided by the authors.

- the organization of the referred litarature should be explained using syntethic sketches, tables or graph, that helps the reader in understanding the classification and strengthen the view of the authors on the topic.

- the introduction should be shorter and provide the overall view of the topic, in addition to the motivation (LCoE minimization) and to the summary of the subsequent chapters (that should be summarized)

- several different aspects affecting the LCoE are reported, but no one is quantitavely related to the LCoE. The authors should ranks the most/less imporant aspects and suggest future perspective for the research (or policy, or anything else) in this fields.

In addition, I suggest the authors to:

- check the text, since there are some typos;
- reconsider their conclusion (L1212-1215) "numerical modelling studies [...] are clearly identified as the fields with largest potential for near future development, given the considerably reduced number of studies available in the literature." In my opinion the lack of studies is nod a good motivation. Further, the numerical models are still characterized by an high computational cost, or a poor sediment transport representation.

Author Response

The paper "Scour protections for offshore foundations of marine energy harvesting technologies: A review" reports a wide and detailed summary of a large part of the literature available on the topic of scour protction for several different marine fundation, such as the ones for wind turbines, but also wave energy converters and tidal turbines.
The paper is clear and the language is rather correct. The literature span over an abundant set of articles and identifies the significant aspect of the topic, more or less studied and/or requiring further research.

A: The reviewers would like to appreciate the reviewer’s time and attention to perform a detailed review on this manuscript. In addition, the authors kindly thank the reviewer for recognizing that this manuscript includes “a wide and detailed summary of a large part of the literature available on the topic of scour protection for several different marine foundation, such as the ones for wind turbines, but also wave energy converters and tidal turbines”. Also, the authors note that the reviewer agrees that this review goes over an abundant span of articles which enabled the identification of important aspects to be studied in future.

However, in my opinion the paper is not still suitable for publication for the follwoing reasons:

- the review paper should aim at a synthesis of the problem, and/or at least a classification of the available research in literature. The large set of literature used is not classified, neither an overall view of the problem is provided by the authors.

R: In this matter, the authors would like to highlight that an attempt was made to not merely identify the key knowledge gaps but also to discuss such gaps based on the literature available. This is a lengthy review which was focused on providing a broad overview on the advances concerning “scour protection research”. It happens that in reviews that aim at a broad overview, while some references are analysed in more detail, others are included for the benefit of the reader, to serve as examples or as additional information that the reader may use for further details. Both types of references are useful. Note that the latter can be used by different researchers or professionals that are looking for a compilation of literature that can tell where they should start reading for a better perception of a specific topic. Additionally, such references are addressed properly in each topic, which helps the readers to understand, where such references contribute the most, among other important aspects. As a result, in such a broad review, not all references can be analysed with the same level of detail. Note that the manuscript has almost 21 000 words which is quite a lot, even for a review. Other side topics, such as the design of eco-friendly scour protections and the installation of scour protections, where left for future research. The authors felt that increasing the size of the manuscript could contribute to disperse the reader’s attention from a set of topics that could be considered as being some of the first to tackle in near to mid-term research. Furthermore, the reviewer requests that a synthesis of the problem needs to be made. The authors agree with this point of view; however, they feel that perhaps the notion of overall synthesis was somehow diluted in the sense that several synthesis are being made in each section. That is in fact the reason why this paper is separated into different and well-defined sections. Although relationships between topics exist and are provided in the paper, each section focus a specific topic, where the authors tried to analyse important references and contributions made from several research teams across the globe. Additionally, the reviewer states that a classification of the literature is not made for a large portion of the references and that an overall view of the problem is not provided. The authors would like to highlight that the original form of the manuscript included 91 references. These were carefully analysed in each well-defined section, these works were analysed based on their contents and most relevant contributions. If the reviewer is suggesting that the works’ contribution should be classified regarding the quality of the findings provided by each work, the authors feel that would be rather inappropriate, as all the works analysed contributed in a positive manner to the state-of-the-art, even in the cases where the findings are not easily generalised or unequivocal. Note that such works raise important questions that are often addressed by subsequent literature. On the other side, if the classification requested by the reviewers concerns the nature of the works, then the authors believe that this is clearly made, as there are very well-defined sections across the paper. For example, the works whose nature is more related to design are addressed in section 2, papers about physical modelling come in their own section 7 and the same occurs for other sections and references. Of course this does not mean that the references are solely analysed or mentioned in one section uniquely. This occurs because some works address important aspects that vary in nature, e.g. physical modelling, numerical modelling, design, scale effects etc… In spite of agreeing on the importance of such classifications, the authors believe that in this case, in such a broad review, it is important to keep in mind the big-picture, which in this case is a discussion and compendium of different challenging knowledge gaps that are yet to be fully developed by the literature.

- the organization of the referred litarature should be explained using syntethic sketches, tables or graph, that helps the reader in understanding the classification and strengthen the view of the authors on the topic.

R: Generally, the authors appreciate and agree with the comment from the reviewer. However, the authors are convinced that in some of the sections, where the literature available is manifestly small, e.g. the studies related to scour at wave or tidal energy converters, or the numerical modelling specifically addressing scour protections, such sketches or tables are not providing any additional information or a benefit in terms of organising the works formerly made. Such sketches or table would consist in a rather reduced amount of information, that deviates the reader from the detailed discussion provided in the text. For example, if a Table was created for the numerical modelling applied to scour protections’ physics, roughly only 2 works would be included the work from Nielsen et al. (2013) and Arboleda et al. (2019). The other ones discussed in this section, e.g. the ones from Dr. Mayall, are more related to the numerical modelling of the structural behaviour of the foundation in presence of scour or the protection. Other sections, as the 10.1 for WECs or 10.2 for tidal energy converters, the scarcity of references is too evident and the amount of information is rather small to produce meaningful tables or similar material. In these cases, as it is evident in section 10.1, the discussion is raised based on examples, which help the reader to have insights on the important topics that should be analysed in future research. In sections where such information is abundant, e.g. physical modelling, a Table was created where different works are separated according to their characteristics. In others where the analysed concepts are of a more theoretical nature, schemes and figures have been created, e.g. in section 2 or 5. Finally, as in section 2 and others, the separation is done by means of subsections, which are quite evident and help the reader to understand the different aspects studied in the respective main section. For example, in section 2, one subsection is created per design concept. The authors appreciate the reviewer comment and tried to review the manuscript to identify places where new summarised information could be included, without extending the manuscript to even lengthier one. As a result, and to accommodate the reviewer’s suggestion in a better manner, a new table was created in section 8 with examples of relevant studies addressing monitoring and field data. Additionally, this table includes observations on the aspects that make such works useful to the reader.

- the introduction should be shorter and provide the overall view of the topic, in addition to the motivation (LCoE minimization) and to the summary of the subsequent chapters (that should be summarized)

R: The authors thank for the present comment. The introduction was made shorter, while at the end a paragraph was added for the benefit of the reader to outline and summarise the contents of the subsequent chapters. The overall view of the problem, which is the lack of studies and knowledge gaps in particular aspects of scour protection research that may contribute to a more competitive LCOE, was given in several parts of the introduction, which the reviewer perhaps did not grasp due to the length of the introduction (now shorter). Several passages outline general challenges that are then addressed in further sections, for example, the complexity of applying scour protections to complex geometries and other harvesting technologies than offshore wind, the lack of in sity and large scale data, among others. Also a mention is given on the importance of these to dropdown the LCoE. Additionally, in several passages the main goal of the paper and its contribution was clearly stated. For example, in the former last paragraph of the introduction:

“Aiming to contribute to a deeper notion on the existing knowledge gaps that contribute for an over-conservative design and large costs of scour protections, the main goal of this article is to summarize and highlight the most recent research on scour protections, with particular emphasis on the dynamic and wide-graded rip-rap systems.

The present review approaches different optimization perspectives, from the conceptual improvements to the application of scour protections in complex and hybrid offshore foundations. This research ultimately provides a broad notion on the fields that benefit from the most recently acquired knowledge on the topic and the prospective developments that may also enhance the most recent trends shown in offshore foundations. In addition, synergies between scour and scour protection research and related research fields are also identified and discussed.”

As per request from the reviewer, the introduction was shortened and a new paragraph was added to explain subsequent chapters. The authors believe that introduction is now in better conditions.

- several different aspects affecting the LCoE are reported, but no one is quantitavely related to the LCoE. The authors should ranks the most/less imporant aspects and suggest future perspective for the research (or policy, or anything else) in this fields.

R: The authors understand the reviewer’s point of view. However, it was not the paper’s main goal to quantify the effects on the dropdown of the LCoE. To do that financial information on costs and return on investment related to the scour protections would be required. Such information was not available to the authors, and such detailed quantification and further relationship with the solutions of the identified knowledge gaps would lead to a new publication per si. The authors recognise the importance of quantifying the costs reduction arising from solving the existing knowledge gaps and hope to address that in future research. However, at this point, this seems to correspond more to a consequent work to the paper presented, more than just a matter of adding isolated remarks to each section provided in the manuscript. The reviewer also suggests a ranking between most and less important aspects of the research to tackle, but the underlaying idea of the paper is that the address topics are the ones considered most relevant challenges and gaps derived from the literature. Since these are the most relevant (note that as mentioned before other side topics were left out of the paper) it would be rather unfair to ranked them, as this could provide the false notion that some of them are less important. Eventually a ranking based on the impact on them LCOE could be made, but as explained, this was not the goal of the paper neither the information available allows for that in this point in time. At the end, in the conclusions sections the most relevant ideas are stated per section and this somehow wraps up the priorities per field. On the future perspective research, all sections identify fields for future research this was made per section, the authors think that adding a mere suggestion of a policy or an isolated paragraph on the topic does not provide additional information to the one already conveyed in throughout the paper and could deviate the reader’s attention from the main conclusions.

In addition, I suggest the authors to:

- check the text, since there are some typos;

R: All manuscript was reviewed and the identified typos were eliminated.

- reconsider their conclusion (L1212-1215) "numerical modelling studies [...] are clearly identified as the fields with largest potential for near future development, given the considerably reduced number of studies available in the literature." In my opinion the lack of studies is nod a good motivation. Further, the numerical models are still characterized by an high computational cost, or a poor sediment transport representation.

R: The authors felt that this sentence was somehow misunderstood. This paragraph was re-phrased for the sake of clarity and to avoid misinterpretations.

The authors kindly appreciate this reviewers remarks that generally contributed to improve the overall quality of the manuscript.

Round 2

Reviewer 3 Report

The authors satisfied mostly all of my questions and now I think the paper is suitable for publication after some minor corrections, suggested in the following:

-I understand the point of the authors and after their explanation I agree with their position. My main suggestion was on the summarizing of the introduction, since it was rather long and it did not list the point studied in the subsequent section. The introduction have been improved now.

The emphasis on the cost is provided largely by the authors in this article, that’s why a reader would aspect a more detailed analysis. I suggest the authors to reduce the emphasis on the LCOE in the paper, maintaining this argument only in the introduction/conclusion.

In relation to the conclusion,  I still do not agree with the position of the authors on the numerical modelling. Hence, I suggest the authors to change “clearly identified” (L1256) with “considered by the authors”.

Author Response

The authors satisfied mostly all of my questions and now I think the paper is suitable for publication after some minor corrections, suggested in the following:

-I understand the point of the authors and after their explanation I agree with their position. My main suggestion was on the summarizing of the introduction, since it was rather long and it did not list the point studied in the subsequent section. The introduction have been improved now.

R: The authors would like to deeply acknowledge the reviewers' willingness to accept the justifications provided in the previous review. The authors also express that they feel that manuscript quality improved due to the comments provided.

The emphasis on the cost is provided largely by the authors in this article, that’s why a reader would aspect a more detailed analysis. I suggest the authors to reduce the emphasis on the LCOE in the paper, maintaining this argument only in the introduction/conclusion.

R: The LCOE references and arguments were now kept to the introduction and conclusions. In the remaining locals where LCOE was referred, either it was erased from the text or solely a mention to general costs is kept instead of a specifically reference to the LCOE.

In relation to the conclusion,  I still do not agree with the position of the authors on the numerical modelling. Hence, I suggest the authors to change “clearly identified” (L1256) with “considered by the authors”.

R: this was rephrased to include the expression "considered by the authors" instead of the former expression.

The authors would like to acknowledge the reviewer's time and attention to the manuscript.