Analysing the Near-Field Effects and the Power Production of Near-Shore WEC Array Using a New Wave-to-Wire Model

Round 1

Reviewer 1 Report

GENERAL COMMENTS

 The work is very interesting and well written. Furthermore, it has a clear degree of originality and it is appropriate for publication in the journal, after performing a moderate level revision.

Indeed the topic targeted is very important since the study of the near and far field effects of the marine energy farms represents an issue of increasing importance for both scientific community and industry. From this perspective the proposed work is relevant and has technical merits.

 Some specific comments are given next related to some corrections, which I recommend to be operated before publication.

 SPECIFIC COMMENTS

 ENGLISH LANGUAGE

This is in general OK. However, an additional English grammar and spelling check should be performed. Moreover, some small errors should be corrected all along the work and some long sentences have to be reformulated in order to present in a clearer way the ideas and the findings of the proposed work.

For example,

Page 7 (last line)…..you have water water and the same repetition in page 8 (line 222)

Page 10 (line 292) descritize

Page 14 (line 397) you have another repetition … the the

Page 20 (line 514) you have another repetition … sections Sections

 TITLE

The title is too long and for this reason is somehow confusing. The use of acronyms (OSWEC) in the title is also confusing.  Please consider to reformulate it providing a more concise form.

 KEYWORDS

I have counted 15 keywords. I think that this is too much and also confusing. I would suggest limiting the keywords to 5 or 6, which the authors consider more important.

 SYMBOLS AND EQUATIONS

There are 12 equations presented in the paper and they seem to be OK.

Please check carefully whether all the quanties involved are properly explained and also included in the nomenclature

 ABBREVIATIONS

Please, check carefully whether all the abbreviations and notations considered in the work are explained for the first time when they are use, even if these are considered trivial by the authors. The paper should be accessible to a wide audience.

For example BEM, WSI, NEMOH, DCNS, CANDHIS are neither explained in the text nor given in the abbreviation list.

 FIGURES & TABLES

First, it is not clear why some figures are designed in black and white and some others in colours. Since this is an open access journal it would be indicated to provide all figures in colours.

Furthermore, some corrections are required in relationship with the figures and the figures captions, as follows:

 Figure 1 – please redesign this figure. First the quality seems not to be very good, and then you have inside a combination of French and English text. Furthermore, it will be more illustrative to indicate the longitude and latitude on the axes.

 Figure 2 – please indicate the source of the map in the figure caption, you should indicate also on the axes the units and you should specify in the figure caption that the water depths are also given.

Figure 3 – you should indicate also on the axes the units and some coordinates. In order the reader to understand the bathymetric map, you should also provide a colorbar that explains the colormap considered in designing this figure.

Figure 6 – please provide a color version and with a better resolution of this figure. You should also refer in the figure caption to each subplot.

Figure 8 – please provide a color version

 Table 1 – please indicate the time range of the data processed in this table

REFERENCES

There are still some related works that might be considered as important references for this topic. Please see for example the next titles:

- Assessment of the changes induced by a wave energy farm in the nearshore wave conditions, Computers & Geosciences, Volume 71, October 2014, Pages 50–61;

- Study on the influence of the distance to shore for a wave energy farm operating in the central part of the Portuguese nearshore, Energy Conversion and Management, 2016, 114, 209-223;

- The expected efficiency and coastal impact of a hybrid energy farm operating in the Portuguese nearshore, Energy, Volume 97, 15 February 2016, Pages 411–423;

- Coastal impact induced by a Pelamis wave farm operating in the Portuguese nearshore, Renewable Energy, 2013,58, 34-49.

Author Response

We would like to thank the reviewer for the insightful feedback on our manuscript.

The suggestions are copied below with our response in red:

ENGLISH LANGUAGE

Page 7 (last line)…..you have water water and the same repetition in page 8 (line 222)

Page 10 (line 292) descritize

Page 14 (line 397) you have another repetition … the the

Page 20 (line 514) you have another repetition … sections Sections

We thank the reviewer for this recommendations. All of the above were rectified. A few additional small typos such as spacing were fixed.

TITLE

The title is too long and for this reason is somehow confusing. The use of acronyms (OSWEC) in the title is also confusing.  Please consider to reformulate it providing a more concise form.

We thank the reviewer for this suggestion. New title: Analysing the near-field effects and the power production of near-shore WEC array using a new wave-to-wire model

KEYWORDS

Reduced number of keywords to 5.

SYMBOLS AND EQUATIONS

There are 12 equations presented in the paper and they seem to be OK.

Please check carefully whether all the quantities involved are properly explained and also included in the nomenclature

Double checked and added wavelength symbol λ to the list.

ABBREVIATIONS

Please, check carefully whether all the abbreviations and notations considered in the work are explained for the first time when they are use, even if these are considered trivial by the authors. The paper should be accessible to a wide audience.

For example BEM, WSI, NEMOH, DCNS, CANDHIS are neither explained in the text nor given in the abbreviation list.

Added the aforementioned abbreviations to the list, defined WSI and CANDHIS.
DCNS is a proper name and the authors do not see the benefit of defining the acronym.

Added test explaining NEMOH as an example of BEM in line 271.

FIGURES & TABLES

We thank the reviewer for the recommendations.

Figure 1 – please redesign this figure. First the quality seems not to be very good, and then you have inside a combination of French and English text. Furthermore, it will be more illustrative to indicate the longitude and latitude on the axes.

Removed the French text insert for clarity and improved quality.

 Figure 2 – please indicate the source of the map in the figure caption, you should indicate also on the axes the units and you should specify in the figure caption that the water depths are also given.

Added source reference and water depth indication in caption and removed extraneous units.

Figure 3 – you should indicate also on the axes the units and some coordinates. In order the reader to understand the bathymetric map, you should also provide a colorbar that explains the colormap considered in designing this figure.

Coordinates, depth, figure source, and colourbar added.

 

Figure 6 – please provide a color version and with a better resolution of this figure. You should also refer in the figure caption to each subplot.

Colour added and resolution updated. Caption updated.

Figure 8 – please provide a color version

done

 Table 1 – please indicate the time range of the data processed in this table

done

 

REFERENCES

There are still some related works that might be considered as important references for this topic. Please see for example the next titles:

- Assessment of the changes induced by a wave energy farm in the nearshore wave conditions, Computers & Geosciences, Volume 71, October 2014, Pages 50–61;

- Study on the influence of the distance to shore for a wave energy farm operating in the central part of the Portuguese nearshore, Energy Conversion and Management, 2016, 114, 209-223;

- The expected efficiency and coastal impact of a hybrid energy farm operating in the Portuguese nearshore, Energy, Volume 97, 15 February 2016, Pages 411–423;

- Coastal impact induced by a Pelamis wave farm operating in the Portuguese nearshore, Renewable Energy, 2013,58, 34-49.

We thank the reviewer for this suggestions. The authors are familiar with the listed papers. A detailed comparison between the results of a phase-resolving, coupled model presented in this paper and spectral, phase-averaged models is beyond the scope of this investigation, but we agree that a short reference to the models can be made. Indeed, it will be the topic of future investigations to compare the two approaches. The introductory part has been adjusted to include suggested references in lines 119-121.

Author Response File: Author Response.docx

Reviewer 2 Report

COMMENTS AND SUGGESTIONS FOR AUTHORS

In general, the topic of the manuscript is of great interest to readers of the WATER journal. The authors in their manuscript present numerical simulation in order to analyze the near-field effects and the power production of an array of OSWECs, using a novel wave-to-wire model. These simulations were performed by the authors in a real case study placed in the Baie d’Audierne near Pouldreuzic in Bretagne, France. The manuscript is well organized and well written. Nevertheless, I recommend a minor revision.

Some comments are given next. Please note that they are not exhaustive and a double check by authors is needed before publication.

MAJOR COMMENTS

1. The authors should reduce the number of keywords (maximum 5).

2. The authors used a Mild slope equation model. They should specify why they did not use a depth-integrated numerical model such as Non-linear shallow water equation model. Indeed all the presented simulation were performed in shallow water.

3. The authors used a Pierson-Moskowitz spectrum typically used in ocean deep water. The authors should motivate and comment on this particular choice.

4. The authors, in order to generate an irregular sea state, used a perturbative method. This method used the superposition principle, to sum up, the total wave field. They should motivate this choice and describe what are the limits of it compared to a real irregular sea state.

5. The authors used a Boundary Element Methods (BEM) code dedicated to the computation of first-order wave loads on offshore structures (added mass, radiation damping, diffraction forces). Also, the before mentioned software was developed for offshore studies, what are the limitations of the adopted model in shallow water?

6. The authors should improve the resolution of Figure 1.

7. The authors should check the unit of measurements of geographical coordinates (eg. degree minutes second or decimal degree?)

8. The descriptive part of the numerical simulation should be summarized because it is too long and dispersive.

9. The authors should highlight the limits and advantages of the proposed wave to wire model.

Author Response

We would like to thank the reviewer for the insightful feedback which allows us to improve our manuscript.

The suggestions are copied below with our response in red: (ll= lines)

 

“In general, the topic of the manuscript is of great interest to readers of the WATER journal. The authors in their manuscript present numerical simulation in order to analyze the near-field effects and the power production of an array of OSWECs, using a novel wave-to-wire model. These simulations were performed by the authors in a real case study placed in the Baie d’Audierne near Pouldreuzic in Bretagne, France. The manuscript is well organized and well written. Nevertheless, I recommend a minor revision.

 

Some comments are given next. Please note that they are not exhaustive and a double check by authors is needed before publication.”

 

MAJOR COMMENTS

 

1.            The authors should reduce the number of keywords (maximum 5).

Per the reviewer’s suggestion, the number of keywords was reduced to 5.

 

2.            The authors used a Mild slope equation model. They should specify why they did not use a depth-integrated numerical model such as Non-linear shallow water equation model. Indeed all the presented simulation were performed in shallow water.

We thank the reviewer for this recommendation. We have now adjusted the manuscript accordingly in Sec. 2.3

Although the modelled OSWECs were placed in an intermediate water depth given the constraints of the studied WEC technology, the largest part of the simulated domain is not in shallow water. The flexibility of our coupling approach allows for the modelling in both shallow water and deep water conditions.  Both cases regarding water depth conditions were investigated in our research group utilizing MILDwave and published in a number of research articles.

Given the large number of WECs modelled and the size of the basin, utilizing a non-linear model such as that suggested by the reviewer would result in a much higher computation time, which in its turn would be impractical or necessitate the need for further simplifications of the WEC hydrodynamics. Most importantly, in the studied case and domain the use of a non-linear model is not necessary and a more simple, fast and easy to use mild-slope model (such as the one we used, MILDwave) withstands without introducing significant inaccuracies. This is due to the fact that non-linear phenomena in our case are not important; e.g. while the used linear solver would have limitations in highly non-linear waves that are in the surf zone,  in our investigation the wave regime is far outside of it for all simulated wave conditions. As such, studies such as that performed by Stratigaki et al. 2010 (comparison of wave diffraction around the Ostend breakwaters and wave propagation results between a mild-slope – MILDwave – and a Boussinesq model – SimWave), by Folley et al. 2012 (OMAE paper) and by Folley (BOOK) have pointed out the necessity to choose suitable numerical tools in a strategic way based on the case studies, on the objective of the investigation and on the added value of the use of a specific numerical tool to simulate wave energy converters and other structures interacting with incoming waves.

3.            The authors used a Pierson-Moskowitz spectrum typically used in ocean deep water. The authors should motivate and comment on this particular choice.

We thank the reviewer for this suggestion. We have adjusted now the manuscript in order to explain our choice in a more clear way.

As was mentioned  in ll. 321-323, the proposed site is open to the North Atlantic and thus is not fetch limited.  Although a part of the domain is in the shallow water zone, the location of the OSWECs is in intermediate water depth and the majority of the basin is in deep water.  Furthermore, MILDwave takes into account the transformation of each component of the irregular sea state into account. Therefore, we believe that the choice of the particular spectrum is justified.

4.            The authors, in order to generate an irregular sea state, used a perturbative method. This method used the superposition principle, to sum up, the total wave field. They should motivate this choice and describe what are the limits of it compared to a real irregular sea state.

We thank the reviewer for this comment. We have adjusted now the manuscript in order to explain our choice in a clearer way in ll. 322-333.

In the linear class of numerical models, to which the presented wave-to-wire model suite belongs, the real irregular sea state mentioned by the reviewer is mathematically equivalent to the irregular sea state utilized in this investigation.  It is accurate to the number of frequency components used.  The perturbative method introduced in this paper applies to the modelling of array effects and does not affect the simulation of irregular incident waves.

5.            The authors used a Boundary Element Methods (BEM) code dedicated to the computation of first-order wave loads on offshore structures (added mass, radiation damping, diffraction forces). Also, the before mentioned software was developed for offshore studies, what are the limitations of the adopted model in shallow water?

We thank the reviewer for this recommendation. We have now adjusted the manuscript accordingly on ll. 284-286.

In this investigation, the BEM solver NEMOH is utilized for the small domains at a depth of 10 meters to simulate the component WEC arrays in the WEC farm.  This is safely within the intermediate wave regime for the wave conditions investigated.  While the solver would have limitations in highly non-linear waves that are in the surf zone,  in our investigation the wave regime is safely outside of it for all simulated wave conditions. Please also check our previous reply regarding the choice of the used numerical models.

6.            The authors should improve the resolution of Figure 1.

We thank the reviewer for this recommendation. We have now adjusted the figure accordingly.

The problem has been rectified by removing the low-quality part of the figure which is not essential to the information presented in the paper.

7.            The authors should check the unit of measurements of geographical coordinates (eg. degree minutes second or decimal degree?)

We thank the reviewer for this recommendation. We have now adjusted the figure accordingly.

All units were changed to decimal degrees in Figs.  1  and 3, and in the  body of the paper.  In Fig 2 they were removed as the main purpose of the figure is to show the water depths.

8.            The descriptive part of the numerical simulation should be summarized because it is too long and dispersive.

We thank the reviewer for this recommendation. We have now adjusted the figure accordingly.

Several pieces of modelling information were taken out on ll. 289-297, ll295-296 and ll 315-316 to streamline Sections 2.3 and 2.4.

9.            The authors should highlight the limits and advantages of the proposed wave to wire model.

We thank the reviewer for this recommendation. We have now adjusted the figure accordingly.

A paragraph to this effect has been added in the Conclusions section on ll. 659-668.

 

Author Response File: Author Response.docx