the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Sep 2023
Preliminary Techno-Economic Study of Optimized Floating Offshore Wind Turbine Substructure
Abstract. Wild fires and excessive floodings have been seasonal climatic changes across the globe in the past decade. The need for clean energy to fight against the climate changes observed as a result of excessive green-house emission over the years is driving the development of the offshore wind sector. This drive is pushing the exploitation of rich wind resources in deep waters with water depth greater than 60 metres requiring a deviation from the commercialized fixed bottom foundation offshore wind technology. Resolving the issue of exploiting rich wind resources requires the use of floating foundation offshore wind technology satisfying stability and durability requirement in any environmental condition.
Floating offshore wind turbines (FOWTs) are still in the pre-commercial stage and although different concepts of FOWTS are being developed, cost is a main barrier to commercializing the FOWT system. This is evidence in the comparison of the CAPEX (capital expenditure) for a fixed bottom platform and a floating platform with the fixed bottom foundation CAPEX representing 13.5 % of the total CAPEX of the system while the floating platform CAPEX represents about 29 % of the total CAPEX of the system leading to an increasing cost.
This article aims to use a shape parameterization technique within a multidisciplinary design analysis and optimization framework to alter the shape of the FOWT platform with the objective of reducing cost. This cost reduction is then implemented in a 30 MW floating offshore wind farm (FOWF) designed based on the static pitch angle constraints (5 degrees, 7 degrees and 10 degrees) used within the optimization framework to estimate the reduction in the levelized cost of energy (LCOE) in comparison to a FOWT platform without any shape alteration – OC3 spar platform design. The optimal platform design variants and the OC3 platform are also deployed in a scaled up 60 MW farm to see the impact of platform geometric shape optimization in a scaled-up scenario.
Key finding in this work shows that an optimal shape alteration of the platform design that satisfies the design requirements, objectives and constraints set within the MDAO framework contributes to significantly reducing the CAPEX cost and the LCOE in the 30 MW floating wind farm. This is due to the reduction in the required platform mass for hydrostatic stability when the static pitch angle is increased. The FOWF designed with a 10 degrees static pitch angle constraint provided the lowest LCOE value while the FOWF designed with a 5 degrees static pitch angle constraint provided the largest LCOE value barring the FOWT designed with the OC3 dimension which is over designed and over dimensioned. The total cost and LCOE is further reduced in a scaled up 60 MW farm for each design assessed. This further reduction is due to combination of the geometric shape parameterization and optimization of the platform with the economics of scale of the wind farm.
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RC1: 'Comment on wes-2023-96', Anonymous Referee #1, 26 Sep 2023
General comments:
The authors have performed a shape optimization for a spar floating wind turbine, with different static pitch constraints, and studied the effect on the overall LCOE of a floating wind farm. This is an interesting topic which deserves attention; however, the manuscript has some major weaknesses which needs to be addressed. Most importantly, it is not clear what the scientific contributions of the current study are, or where the novelty of the work lies. The literature review refers to many papers, but does not mention any research gap or needs that the study tries to address. Also, all the referenced design optimization studies (Hall et al. (2013), Karimi et al. (2017), Hegseth et al. (2020) and Dou et al. (2020)) consider significantly more complex and realistic optimization problems than the one in the current study, where only the static pitch angle due to a constant wind load is taken into account. While it is impossible to consider all aspects of a floating wind foundation design in a numerical optimization process, an overly simplified representation of the design problem will not yield any realistic results and will thus be less interesting also in terms of potential cost savings.
Specific comments:
Abstract, first paragraph: I suggest that this paragraph is removed, or moved to the beginning of the introduction. The abstract should be more straight to the point, in my opinion.
Introduction: The introduction is too long and discusses topics that are unrelated to the study. For example, a spar optimization study does not need to discuss and list pros and cons of semi-subs and TLPs.
Several places: The authors claim to use an MDAO framework; however, the analysis which is performed just solves a simple static pitch angle equation. Can the authors elaborate on why this is an MDAO framework?
Sec. 3.1: The static pitch angle of a spar exposed to a constant wind force is something that easily could be calculated very efficiently in a Python or Matlab script, why did the authors decide to use Genie and WADAM?
Sec. 4.2.2.2: What material is used as ballast in the analyses? Wouldn’t it be easier and cheaper to adjust the ballast density (e.g. by replacing olivine by iron ore) to modify the static pitch angle, rather than the shape of the spar?
Line 95-98: Sentence «With the completion of…» is awkward and difficult to understand, I suggest to rewrite.
Line 103-104: «The most efficient offshore foundations…». What is meant by «efficient» in this context, and are floating foundations really always the most efficient? I suggest that this statement is removed or rewritten in a clearer and more nuanced way.
Line 105-107: «Second, they make it easier to set up». Reference?
Line 172/174: 5 and 4 euro per kW?
Line 179: Just shape optimization? I think there are other things with the design than just the shape that can be optimized (control system, structural properties, etc.)
Sec 3.3: Why is the overturning moment in Eq. 7 calculated about the center of buoyancy (z_cb)? Is that the elevation where the mooring lines are connected? If so, that should be specified.
Table 9, 10, 11: Why does OPEX and DECEX decrease when the CAPEX for the floaters decrease? I don’t think you’ll have lower operational costs just because there is less material.
Sec 4.3.2: The text in this section is repetetive and I suggest that it is revised.
Technical corrections:
Line 93: Remove «(FOWT)» after «clean energy»
Table 8, 9, 10: The numbers should be in a format that is easier to read, e.g. as in Table 11. I suggest to remove the decimals as well.
Line 359: DPBP <<< t?
Line 267: Castro-Santos et al. (2020a)Castro et al.
Line 289: «Floating Offshore Wind Turbines (FOWT)». Can just write FOWT, the acronym is already established earlier in the text.
Citation: https://doi.org/10.5194/wes-2023-96-RC1 -
AC1: 'Reply on RC1', Adebayo Ojo, 31 Oct 2023
Dear Referee#1,
Please see the detailed answers to your comments.
Manuscript: wes-2023-96
The authors would like to thank the anonymous referee#1 for the insightful comments and also recognizing the importance of this work by highlighting the attention it deserves. We also do appreciate the quality of comments provided by the referee to improve this work. We have made adequate effort to effectively address the comments provided in order to improve the quality and readability of the original manuscript while also highlighting our perspective to the anonymous referee.
We have dissected the responses to the comments into three sections which are: General comments response, specific comments response and technical corrections. Please see the respective responses below.
General Comments Response
Most importantly, it is not clear what the scientific contributions of the current study are, or where the novelty of the work lies.
The main contribution of this work is to prove that the concept of shape optimization for a Spar. This is achieved by altering the control points of the spline with an optimization algorithm for a set of static pitch angle constraints. This results in design product that meets the standard requirements of stability, allowable motion response and also reduced material required for manufacturing. This study also shows that increase in the static pitch leads to reduction in the platform’s mass and the authors have explored the economic implications at a high-level. Although this concept of spline shape optimization is in use in the oil and gas industry and other sectors, the authors believe this is novelle in the floating offshore wind foundation sector.
The literature review refers to many papers, but does not mention any research gap or needs that the study tries to address.
Most of the papers referenced are based on traditional modelling of floating offshore wind platform. The research gap we have tried to address is to autonomously alter the shape of the platform using state-of-the-art derivative-free optimization algorithm. The benefits of this approach are: capability to search a large design space with reduced computational time and the reduction in mass observed at different static pitch constraints which contributes to reducing CAPEX cost of FOWT.
Also, all the referenced design optimization studies (Hall et al. (2013), Karimi et al. (2017), Hegseth et al. (2020) and Dou et al. (2020)) consider significantly more complex and realistic optimization problems than the one in the current study, where only the static pitch angle due to a constant wind load is taken into account. While it is impossible to consider all aspects of a floating wind foundation design in a numerical optimization process, an overly simplified representation of the design problem will not yield any realistic results and will thus be less interesting also in terms of potential cost savings
We do agree with the referee that some of the studies referenced in this work are complex optimization problems, we also believe that the novelty of our approach which is exploiting the local propagation capability of spline curve to autonomously model and alter the geometrical shape of a platform’s panel and conduct hydrostatic assessment within a single framework is a unique approach capable of producing bespoke designs. In this high-level work, we have locally altered the geometry, analysed and optimized the design within the optimization framework in the frequency domain. Most geometry optimization studies of floating platforms are altered globally i.e., the entire diameter, length or draft. This work will evolve to a detailed phase which will span different disciplines (structural, aerodynamics, hydrostatics and hydrodynamics) in frequency domain and time domain. The phase we are at is a technoeconomic assessment of our shape optimization concept on a floating offshore wind farm.
Specific Comments Response
Abstract, first paragraph: I suggest that this paragraph is removed, or moved to the beginning of the introduction. The abstract should be more straight to the point, in my opinion.
The authors are in agreement with the referee and have moved this paragraph as advised to start the introduction chapter. It reads better there than in the abstract section. Thanks for the suggestion.
Introduction: The introduction is too long and discusses topics that are unrelated to the study. For example, a spar optimization study does not need to discuss and list pros and cons of semi-subs and TLPs.
We thank the author for this valid point that makes an article’s readability and structure to be improved. We have revised the introduction chapter accordingly and removed sections that do not improve the context of the article.
Several places: The authors claim to use an MDAO framework; however, the analysis which is performed just solves a simple static pitch angle equation. Can the authors elaborate on why this is an MDAO framework?
This issue has been addressed in the document as we have developed an optimization framework rather than an MDAO. We have geometrically altered the model using a derivative-free optimization algorithm and the B-Spline curve to explore design space using the static pitch angle and stability of the system as a constraint. This is a geometric shape optimization study exploring the economic impact of the approach and not an MDAO study.
Sec. 3.1: The static pitch angle of a spar exposed to a constant wind force is something that easily could be calculated very efficiently in a Python or Matlab script, why did the authors decide to use Genie and WADAM?
We agree with the reviewer that static pitch angle can be estimated with python. However, we have gone for DNV tool due to the following reasons:
- DNV embodies recognized standards for offshore structures design and solution and the suite’s modelling module (Sesam Genie) has substantial libraries of polynomial curves and splines with local propagation properties, slope and curvature continuity that are very effective in handling complex shapes. These models from an industry body yield an accurate estimation of steel mass from the model design than a MATLAB or python script.
- We estimated the static pitch angle from the state-of-the-art DNV tool, used it as a constraint within an optimization algorithm and selected the optimal design based on the constraint and objective function which is minimizing mass of steel. We also assessed the system’s motion response in surge heave pitch and the nacelle which is better assessed with a state-of-the-art tool from a standard and certification body like DNV. Also, other tools are validated against DNV software to ensure they meet the required standards. For example, OrcaWave has just been recently validated with DNV WADAM to ensure it comply with industry standard. Also, the University of Strathclyde has access to the Licence for students to carry out research, hence reason why we chose to use it.
- This work is part of a complex framework still in development that will not do optimized design and analysis in frequency domain but in time domain. And DNV tools have the capability to assess the hydrodynamic response amplitude operator for use in time domain tools.
Sec. 4.2.2.2: What material is used as ballast in the analyses? Wouldn’t it be easier and cheaper to adjust the ballast density (e.g. by replacing olivine by iron ore) to modify the static pitch angle, rather than the shape of the spar?
The ballast material used in the compartment model of the spar is seawater. The anonymous referee has raised a valid point on how we could adjust the ballast material. However, we assessed different ways of modifying ballast and due to the nature of our design problem, we had to integrate a python function to work out the ballast filling fraction within the compartment model at the hydrostatic stage (using the data from the java script platform on which the DNV HydroD software works). The iterative design loop goes through several models (panel and compartment) novelle geometric shape changes as different design variables are passed to control points in Sesam Genie from the optimization algorithm. This ensures that for every static pitch angle constraint specified within the optimization algorithm, thousands of design iterations and subsequent hydrostatic assessment are autonomously conducted and feasible designs with the correct ballast adjustment from the correct fill fraction estimates that ensures the system’s stability are selected.
Line 95-98: Sentence «With the completion of…» is awkward and difficult to understand, I suggest to rewrite.
This text has been re-written to improve the understanding of what the author is saying and improve the quality of the article.
Line 103-104: «The most efficient offshore foundations…». What is meant by «efficient» in this context, and are floating foundations really always the most efficient? I suggest that this statement is removed or rewritten in a clearer and more nuanced way.
We agree with the anonymous referee’s opinion as efficiency can be seen from different perspectives. This statement has been rewritten to highlight how floating offshore wind complement its fixed bottom counterpart with capabilities of exploring deeper waters with richer wind resource and less invasive actions on the seabed et.al.
Line 105-107: «Second, they make it easier to set up». Reference?
These lines have been rewritten in a clearer and much more nuanced approach which also includes adequate reference supporting the statement.
Line 172/174: 5 and 4 euro per kW?
These values have been reviewed to the thousands of euros reported in the referenced document.
Line 179: Just shape optimization? I think there are other things with the design than just the shape that can be optimized (control system, structural properties, etc.)
We agree with the referee that disciplines like the control system and structural components and turbine aerodynamics can be optimized. We have included these other disciplines in the text accordingly.
Sec 3.3: Why is the overturning moment in Eq. 7 calculated about the center of buoyancy (z_cb)? Is that the elevation where the mooring lines are connected? If so, that should be specified.
Yes, this is a correct observation. The overturning moment is around the centre of mooring line connection as shown in sketch in fig.2. This has been adjusted adequately in the equation 7 and in the figure2 respectively.
Table 9, 10, 11: Why does OPEX and DECEX decrease when the CAPEX for the floaters decreases? I don’t think you’ll have lower operational costs just because there is less material
We understand the referee’s point of view as in reduction in material might not result in reduction in OPEX and DECEX. However, intangibles like the logistic planning, transportation cost, storage cost for less materials can reflect in the reduction in the OPEX and DECEX.
For the purpose of this high-level technoeconomic study, we have used data from literature as stated in Table 7 with the references included to estimate the DECEX and OPEX values. According to the reference from Maienza et.al 2020, 19% of total expenditure makes up the OPEX while 4% of total expenditure makes up the DECEX. These percentages have been used to estimate the OPEX and DECEX in this study.
Sec 4.3.2: The text in this section is repetitive and I suggest that it is revised.
Section 4.3.2 has been reviewed accordingly and effectively improved its readability. This is done by adhering to the referee’s suggestion of removing the repetitiveness of the texts.
Technical Corrections
Line 93: Remove «(FOWT)» after «clean energy»
The FOWT acronym has been removed as advised by the referee.
Table 8, 9, 10: The numbers should be in a format that is easier to read, e.g. as in Table 11. I suggest to remove the decimals as well.
The tables have been adjusted and formatted adequately as advised by the referee to improve the readability of the article.
Line 359: DPBP <<< t?
The correct mathematical operator has been used to describe the scenario meaning for the discounted pay-back period for this line.
Line 267: Castro-Santos et al. (2020a)Castro et al.
Many thanks for this detailed review. The sentence has been adjusted accordingly to show only reference from a bibliography software.
Line 289: «Floating Offshore Wind Turbines (FOWT)». Can just write FOWT, the acronym is already established earlier in the text.
The established acronym have been used accordingly not only in this line but all through the document.
Citation: https://doi.org/10.5194/wes-2023-96-AC1
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AC1: 'Reply on RC1', Adebayo Ojo, 31 Oct 2023
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RC2: 'Comment on wes-2023-96', Anonymous Referee #2, 27 Oct 2023
General Comments
The authors have conducted basic shape optimization on a spar-type floating wind turbine, and performed a brief cost analysis for the optimized design compared to the baseline. The review of existing literature is fairly thorough, however fails to make clear the gap that the current work addresses. Several of the referenced studies apply much more advanced optimization models or include the capability to consider floating substructures that are more advanced than spars. The techno-economic analysis consists of cost estimations for the final, optimized designs. As a result, the key finding is that reduced steel mass from an increased constraints on static pitch angle ceiling leads to lower costs. There is extensive background information included through care should be taken to avoid statements made without qualifying or supporting information, and to focus the background on the most relevant material for the current work.
Specific Comments
The abstract provides too much background information that isn’t directly related to this work.
Line 103-104: If floating foundations are the most efficient, why have they not been favored worldwide to-date? Avoid superlative statements such as this.
Line 110: Please provide a reference for the claim that floating foundations have environmental advantages, or remove this statement.
Line 134: Consider providing actual estimated values. When only percentages are given it is impossible to know the total project costs are the same – therefore it is impossible to compare these.
Table 1 and Table 2: Several points here require more qualification or context. For example, “Suitable for severe seastate” is listed as a benefit for spar platforms, however all three architectures listed have been installed in locations that experience severe conditions. Consider removing these tables and slight expanding the discussion in Line 117-133 about the different types.
Line 169-174: These cost trends need more thorough explanation – especially in how cost estimates were made for floating wind farms before 2017, when the first floating wind farm was installed.
Line 272: This appears to start a new section of the literature review focused on optimization studies, consider adding a new section here.
Section 2.1: Throughout the literature review avoid statements like ‘groundbreaking,’ and consider how many of the reviewed publications are truly relevant for this work.
Section 2.2: Only the LCOE is mentioned later in this work, so there is no need to include details on NPV, IRR, and DPBP as these are relatively standard and well-known metrics.
Line 393: The cited study makes no mention of ocean biodiversity. Please provide a reference for this claim or remove it.
Line 405: Here the methodology states the farm consists of two platforms and 10MW total capacity, but later results are given for 30MW and 60MW farms. Please clarify.
Line 415: Consider renaming this section, as it discusses both hydrostatics and upright stability.
Line 470: It is very strong to suggest reduced LCOE will ‘ensure commercial viability,’ what about other factors that affect viability such as permitting, supply chains, existing infrastructure?
Section 4.2.2: The section title does not make sense, and there is significant extra information in this section that could be removed to focus solely on the optimization algorithm chosen.
Table 5: This data is unnecessary in the main text, especially as the illustrations are given.
Line 563: If wall thickness is determined with this ratio, please comment on how that affects the optimal designs.
Line 565: Is ballast mass included? If so, how is it calculated?
Line 593: This is a significant omission to neglect all structural assessments. Especially when the optimal designs presented are likely to present significant structural challenges.
Section 4.3.1: It is not clear that the cost estimates are dependent on anything other than total farm capacity and steel mass per platform. If this is not the case, please describe it more explicitly.
Table 7-11: Consider how these tables could be consolidated to avoid an entire page devoted to tables in the main text.
Line 717-730: If the data are mentioned in text, there is no need for Table 13.
Line 731: It appears the reduction in LCOE due to the platform mass optimization is roughly equal to reductions due to farm size, please be more explicit about this. Please also justify why this reduction (~5%) is significant, considering the limited constraints applied to the design in optimization.
Figure 5-9: These figures appear to duplicate data presented in tables, if this is the case please remove the duplication or explain more clearly what novel information these figures present.
Line 770: To what extent can this approach be considered MDAO if the optimization considers only mass and hydrostatics, but omits the true end objective – LCOE?
Line 793: As previously mentioned, the structural assessment is a very significant portion of this work.
Technical Corrections
Throughout acronyms are redefined – this is unnecessary given that a table of abbreviations is given, unless a specific acronym is very unclear in context.
Line 161: “tecno”
Line 171: “The new farms so designed…”
Line 267: “Castro-Santos et al. (2020a)Castro et al.”
Line 314: “… select the optimal design in a quick duration.”
Line 408: “is shown in Fig2abc”
Line 416: “any type pf floating”
Line 476: “The technicality involved…”
Citation: https://doi.org/10.5194/wes-2023-96-RC2 -
AC2: 'Reply on RC2', Adebayo Ojo, 31 Oct 2023
Dear Referee#2,
On behalf of my co-authors, I will like to thank you for your comments. Please see our prepared response to your comments below.
Manuscript: wes-2023-96
The authors would like to thank the anonymous referee#2 for the insightful comment. We also do appreciate the quality of comments provided by the referee to improve this work. We have made adequate effort to effectively address the comments provided for better readability and improvement in quality of this article.
We have addressed the responses to the comments in three sections which are: General comments response, specific comments response and technical corrections. Please see the respective responses below.
General Comments
The authors have conducted basic shape optimization on a spar-type floating wind turbine, and performed a brief cost analysis for the optimized design compared to the baseline. The review of existing literature is fairly thorough, however fails to make clear the gap that the current work addresses. Several of the referenced studies apply much more advanced optimization models or include the capability to consider floating substructures that are more advanced than spars. The techno-economic analysis consists of cost estimations for the final, optimized designs. As a result, the key finding is that reduced steel mass from an increased constraints on static pitch angle ceiling leads to lower costs. There is extensive background information included through care should be taken to avoid statements made without qualifying or supporting information, and to focus the background on the most relevant material for the current work.
General Comments Response
The review of existing literature is fairly thorough, however fails to make clear the gap that the current work addresses
We have emphasized the research gap in the article accordingly. The research gap we have tried to address is to autonomously alter the shape of the platform using state-of-the-art derivative-free optimization algorithm. The benefits of this approach are: capability to search a large design space with reduced computational time and the reduction in mass observed at different static pitch angle constraints which contributes to reducing CAPEX cost of FOWT
Several of the referenced studies apply much more advanced optimization models or include the capability to consider floating substructures that are more advanced than spars
We do agree with the referee that some of the studies referenced in this work are more advanced optimization models that can handle floating substructures more advanced than spars. The novelty of our approach which is exploiting the local propagation capability of spline curve to autonomously model and alter and optimize the geometrical shape of a platform’s panel and conduct hydrostatic assessment within a single framework is a unique approach capable of producing bespoke designs. This methodology will be used on other types of floating substructures other than the Spar.
Most importantly, it is not clear what the scientific contributions of the current study are, or where the novelty of the work lies.
The main contribution of this work is to prove that the concept of shape optimization for a Spar. This is achieved by altering the control points of the spline with an optimization algorithm for a set of static pitch angle constraints. This results in design product that meets the standard requirements of stability, allowable motion response and also reduced material required for manufacturing. This study also shows that increase in the static pitch leads to reduction in the platform’s mass and the authors have explored the economic implications at a high-level. Although this concept of spline shape optimization is in use in the oil and gas industry and other sectors, the authors believe this is not so much in use in the floating offshore wind foundation sector.
The literature review refers to many papers, but does not mention any research gap or needs that the study tries to address.
Most of the papers referenced are based on traditional modelling of floating offshore wind platform. The research gap we have tried to address is to autonomously alter the shape of the platform using state-of-the-art derivative-free optimization algorithm. The benefits of this approach are: capability to search a large design space with reduced computational time and the reduction in mass observed at different static pitch constraints which contributes to reducing CAPEX cost of FOWT.
Also, all the referenced design optimization studies (Hall et al. (2013), Karimi et al. (2017), Hegseth et al. (2020) and Dou et al. (2020)) consider significantly more complex and realistic optimization problems than the one in the current study, where only the static pitch angle due to a constant wind load is taken into account. While it is impossible to consider all aspects of a floating wind foundation design in a numerical optimization process, an overly simplified representation of the design problem will not yield any realistic results and will thus be less interesting also in terms of potential cost savings
We do agree with the referee that some of the studies referenced in this work are complex optimization problems, we also believe that the novelty of our approach which is exploiting the local propagation capability of spline curve to autonomously model and alter the geometrical shape of a platform’s panel and conduct hydrostatic assessment within a single framework is a unique approach capable of producing bespoke designs. In this high-level work, we have locally altered the geometry, analysed and optimized the design within the optimization framework in the frequency domain. Most geometry optimization studies of floating platforms are altered globally i.e., the entire diameter, length or draft. This work will evolve to a detailed phase which will span different disciplines (structural, aerodynamics, hydrostatics and hydrodynamics) in frequency domain and time domain. The phase we are at is a technoeconomic assessment of our shape optimization concept on a floating offshore wind farm.
Specific Comments Response
The abstract provides too much background information that isn’t directly related to this work.
The authors are in agreement with the referee and have rewritten the abstract to address the comment to ensure the information within the abstract are strictly related to the work conducted.
Line 103-104: If floating foundations are the most efficient, why have they not been favored worldwide to-date? Avoid superlative statements such as this.
An important observation by the referee and we will adhere to avoiding superlative statement throughout the document. This statement in line 103-104 has been rewritten and supported with an adequate reference.
Line 110: Please provide a reference for the claim that floating foundations have environmental advantages, or remove this statement.
We have provided adequate reference for this claim.
Line 134: Consider providing actual estimated values. When only percentages are given it is impossible to know the total project costs are the same – therefore it is impossible to compare these.
We have included some estimated CAPEX estimates and included the reference articles related to the values.
Table 1 and Table 2: Several points here require more qualification or context. For example, “Suitable for severe seastate” is listed as a benefit for spar platforms, however all three architectures listed have been installed in locations that experience severe conditions. Consider removing these tables and slight expanding the discussion in Line 117-133 about the different types.
We agree with the referee that several points require more qualification or context. However, to keep focus on the spar floated that this study is about, we have removed these tables and expand the discussions as advised.
Line 169-174: These cost trends need more thorough explanation – especially in how cost estimates were made for floating wind farms before 2017, when the first floating wind farm was installed.
We agree with the referee and have rewritten this section in the article for clarity. The cost trends are taken from review papers in which total installed costs for offshore wind farms were initially evaluated based on the costs of existing shallow water farms and extrapolated to deeper waters for deep water offshore farms by researchers. The extrapolation approach resulted in the trends highlighted in the article.
Line 272: This appears to start a new section of the literature review focused on optimization studies, consider adding a new section here.
We thank the referee for this good suggestion. It has been implemented in the article as Optimization review.
Section 2.1: Throughout the literature review avoid statements like ‘groundbreaking,’ and consider how many of the reviewed publications are truly relevant for this work.
We have removed these kinds of statements from the document and used words that simply help to convey the meaning of the texts appropriately.
Section 2.2: Only the LCOE is mentioned later in this work, so there is no need to include details on NPV, IRR, and DPBP as these are relatively standard and well-known metrics.
This section has been rewritten as suggested by the referee. We have focused the narrative on the LCOE as it’s what was assessed in the study.
Line 393: The cited study makes no mention of ocean biodiversity. Please provide a reference for this claim or remove it.
We agree with the referee on this observation. It was an implied thought put into writing and the lead author has removed it from the sentence.
Line 405: Here the methodology states the farm consists of two platforms and 10MW total capacity, but later results are given for 30MW and 60MW farms. Please clarify.
This mistake has been corrected by the lead author. It’s a 30MW and a scaled up 60MW farm that was assessed in the article.
Line 415: Consider renaming this section, as it discusses both hydrostatics and upright stability.
We thank the referee for this suggestion. The section has been adequately renamed to capture the stability requirement of FOWT.
Line 470: It is very strong to suggest reduced LCOE will ‘ensure commercial viability,’ what about other factors that affect viability such as permitting, supply chains, existing infrastructure?
We agree with this observation from the referee. The statement has been rewritten to include other aforementioned factors that have the potential to increase the commercial viability of a FOWF.
Section 4.2.2: The section title does not make sense, and there is significant extra information in this section that could be removed to focus solely on the optimization algorithm chosen.
We have adjusted the title of this section accordingly. The high-level description of the B-Spline curve is essential as it compliments the shape optimization conducted in this study. We have used the local curve alteration capabilities of the B-Spline at the control points with the optimization algorithm delivering the design variables (radii) at the different control points to alter the shape at every iteration.
Table 5: This data is unnecessary in the main text, especially as the illustrations are given.
We thank the referee for this comment. We have deliberated on this and would like to keep the table in the document as it shows the local propagation properties of the B-Spline curve at the control points which alters the shape of the spar. We believe this is more informative for potential readers as to having only the figures in the document.
Line 563: If wall thickness is determined with this ratio, please comment on how that affects the optimal designs.
This wall thickness is determined based on buoyancy to mass ratio of 0.13 selected from literature to improve buoyancy, weight and hydrodynamic performance. This wall thickness provides the estimate of the platform’s mass from the volume of steel used and the optimal design is one with the minimal mass.
Line 565: Is ballast mass included? If so, how is it calculated?
The ballast mass is included for stability and it goes into the compartment section of the model. We have used seawater as ballast material in the compartment model and a numerical code was used to calculate the required amount of ballast material to ensure stability in the hydrostatic stage. Adequate description of this process has been included in the article.
Line 593: This is a significant omission to neglect all structural assessments. Especially when the optimal designs presented are likely to present significant structural challenges.
We agree with the referee on the need of structural assessment. Ongoing studies for future publications will include the structural assessment of the shape optimized platform.
Section 4.3.1: It is not clear that the cost estimates are dependent on anything other than total farm capacity and steel mass per platform. If this is not the case, please describe it more explicitly.
We thank the referee for this point and have addressed it in the section. Yes, the cost estimate depends on the total farm capacity and the steel mass per platform. However, more importantly, the main driving force of the cost estimate is the static pitch angle constraint used within the optimization algorithms. The larger the static pitch angle used, the smaller the estimated mass of steel required to design the platform and the concept of economy of scale makes this significant when the farm capacity increases.
Table 7-11: Consider how these tables could be consolidated to avoid an entire page devoted to tables in the main text.
We have consolidated the tables accordingly.
Line 717-730: If the data are mentioned in text, there is no need for Table 13.
We have rewritten this part to remove the use of the data in text and reference the data in the table in our discussion.
Line 731: It appears the reduction in LCOE due to the platform mass optimization is roughly equal to reductions due to farm size, please be more explicit about this. Please also justify why this reduction (~5%) is significant, considering the limited constraints applied to the design in optimization.
We thank the referee for this comment. We have explicitly shown that the reduction in LCOE based on the static pitch angle constraints mass optimization is not far off the reduction we see when scaling the farm. This has been done by including tables in the section to highlight the point. It is worth noting that the percentage reduction values will vary depending on the size of the FOWF. Also, the value 5% is quite significant as it highlights the effect the cost of steel has on costing a FOWT system. The article has been adequately updated to reflect these points.
Figure 5-9: These figures appear to duplicate data presented in tables, if this is the case please remove the duplication or explain more clearly what novel information these figures present.
The figures have been removed from the main section of the article and moved to the appendix section..
Line 770: To what extent can this approach be considered MDAO if the optimization considers only mass and hydrostatics, but omits the true end objective – LCOE?
This approach describes the effect a geometric shape optimization of spar platform has on the LCOE of a hypothetical wind farm. The main objective is minimizing the steel material and assessing the impact on the LCOE. We agree with the referee that this is not a full MDAO study and this point has been reflected in the article.
Line 793: As previously mentioned, the structural assessment is a very significant portion of this work.
We agree and thank the referee for this point. We recognize the importance of structural assessment and we have ongoing research assessing this for future articles.
Citation: https://doi.org/10.5194/wes-2023-96-AC2
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AC2: 'Reply on RC2', Adebayo Ojo, 31 Oct 2023
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EC1: 'Comment on wes-2023-96', Erin Bachynski-Polić, 04 Nov 2023
Please note that the reviewers raise significant concerns about the novelty of the presented work. The initial responses from the authors do not provide sufficient evidence of scientific novelty in the approach, and a very significant revision to the scope of work would be required for acceptance.
Citation: https://doi.org/10.5194/wes-2023-96-EC1
Status: closed
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RC1: 'Comment on wes-2023-96', Anonymous Referee #1, 26 Sep 2023
General comments:
The authors have performed a shape optimization for a spar floating wind turbine, with different static pitch constraints, and studied the effect on the overall LCOE of a floating wind farm. This is an interesting topic which deserves attention; however, the manuscript has some major weaknesses which needs to be addressed. Most importantly, it is not clear what the scientific contributions of the current study are, or where the novelty of the work lies. The literature review refers to many papers, but does not mention any research gap or needs that the study tries to address. Also, all the referenced design optimization studies (Hall et al. (2013), Karimi et al. (2017), Hegseth et al. (2020) and Dou et al. (2020)) consider significantly more complex and realistic optimization problems than the one in the current study, where only the static pitch angle due to a constant wind load is taken into account. While it is impossible to consider all aspects of a floating wind foundation design in a numerical optimization process, an overly simplified representation of the design problem will not yield any realistic results and will thus be less interesting also in terms of potential cost savings.
Specific comments:
Abstract, first paragraph: I suggest that this paragraph is removed, or moved to the beginning of the introduction. The abstract should be more straight to the point, in my opinion.
Introduction: The introduction is too long and discusses topics that are unrelated to the study. For example, a spar optimization study does not need to discuss and list pros and cons of semi-subs and TLPs.
Several places: The authors claim to use an MDAO framework; however, the analysis which is performed just solves a simple static pitch angle equation. Can the authors elaborate on why this is an MDAO framework?
Sec. 3.1: The static pitch angle of a spar exposed to a constant wind force is something that easily could be calculated very efficiently in a Python or Matlab script, why did the authors decide to use Genie and WADAM?
Sec. 4.2.2.2: What material is used as ballast in the analyses? Wouldn’t it be easier and cheaper to adjust the ballast density (e.g. by replacing olivine by iron ore) to modify the static pitch angle, rather than the shape of the spar?
Line 95-98: Sentence «With the completion of…» is awkward and difficult to understand, I suggest to rewrite.
Line 103-104: «The most efficient offshore foundations…». What is meant by «efficient» in this context, and are floating foundations really always the most efficient? I suggest that this statement is removed or rewritten in a clearer and more nuanced way.
Line 105-107: «Second, they make it easier to set up». Reference?
Line 172/174: 5 and 4 euro per kW?
Line 179: Just shape optimization? I think there are other things with the design than just the shape that can be optimized (control system, structural properties, etc.)
Sec 3.3: Why is the overturning moment in Eq. 7 calculated about the center of buoyancy (z_cb)? Is that the elevation where the mooring lines are connected? If so, that should be specified.
Table 9, 10, 11: Why does OPEX and DECEX decrease when the CAPEX for the floaters decrease? I don’t think you’ll have lower operational costs just because there is less material.
Sec 4.3.2: The text in this section is repetetive and I suggest that it is revised.
Technical corrections:
Line 93: Remove «(FOWT)» after «clean energy»
Table 8, 9, 10: The numbers should be in a format that is easier to read, e.g. as in Table 11. I suggest to remove the decimals as well.
Line 359: DPBP <<< t?
Line 267: Castro-Santos et al. (2020a)Castro et al.
Line 289: «Floating Offshore Wind Turbines (FOWT)». Can just write FOWT, the acronym is already established earlier in the text.
Citation: https://doi.org/10.5194/wes-2023-96-RC1 -
AC1: 'Reply on RC1', Adebayo Ojo, 31 Oct 2023
Dear Referee#1,
Please see the detailed answers to your comments.
Manuscript: wes-2023-96
The authors would like to thank the anonymous referee#1 for the insightful comments and also recognizing the importance of this work by highlighting the attention it deserves. We also do appreciate the quality of comments provided by the referee to improve this work. We have made adequate effort to effectively address the comments provided in order to improve the quality and readability of the original manuscript while also highlighting our perspective to the anonymous referee.
We have dissected the responses to the comments into three sections which are: General comments response, specific comments response and technical corrections. Please see the respective responses below.
General Comments Response
Most importantly, it is not clear what the scientific contributions of the current study are, or where the novelty of the work lies.
The main contribution of this work is to prove that the concept of shape optimization for a Spar. This is achieved by altering the control points of the spline with an optimization algorithm for a set of static pitch angle constraints. This results in design product that meets the standard requirements of stability, allowable motion response and also reduced material required for manufacturing. This study also shows that increase in the static pitch leads to reduction in the platform’s mass and the authors have explored the economic implications at a high-level. Although this concept of spline shape optimization is in use in the oil and gas industry and other sectors, the authors believe this is novelle in the floating offshore wind foundation sector.
The literature review refers to many papers, but does not mention any research gap or needs that the study tries to address.
Most of the papers referenced are based on traditional modelling of floating offshore wind platform. The research gap we have tried to address is to autonomously alter the shape of the platform using state-of-the-art derivative-free optimization algorithm. The benefits of this approach are: capability to search a large design space with reduced computational time and the reduction in mass observed at different static pitch constraints which contributes to reducing CAPEX cost of FOWT.
Also, all the referenced design optimization studies (Hall et al. (2013), Karimi et al. (2017), Hegseth et al. (2020) and Dou et al. (2020)) consider significantly more complex and realistic optimization problems than the one in the current study, where only the static pitch angle due to a constant wind load is taken into account. While it is impossible to consider all aspects of a floating wind foundation design in a numerical optimization process, an overly simplified representation of the design problem will not yield any realistic results and will thus be less interesting also in terms of potential cost savings
We do agree with the referee that some of the studies referenced in this work are complex optimization problems, we also believe that the novelty of our approach which is exploiting the local propagation capability of spline curve to autonomously model and alter the geometrical shape of a platform’s panel and conduct hydrostatic assessment within a single framework is a unique approach capable of producing bespoke designs. In this high-level work, we have locally altered the geometry, analysed and optimized the design within the optimization framework in the frequency domain. Most geometry optimization studies of floating platforms are altered globally i.e., the entire diameter, length or draft. This work will evolve to a detailed phase which will span different disciplines (structural, aerodynamics, hydrostatics and hydrodynamics) in frequency domain and time domain. The phase we are at is a technoeconomic assessment of our shape optimization concept on a floating offshore wind farm.
Specific Comments Response
Abstract, first paragraph: I suggest that this paragraph is removed, or moved to the beginning of the introduction. The abstract should be more straight to the point, in my opinion.
The authors are in agreement with the referee and have moved this paragraph as advised to start the introduction chapter. It reads better there than in the abstract section. Thanks for the suggestion.
Introduction: The introduction is too long and discusses topics that are unrelated to the study. For example, a spar optimization study does not need to discuss and list pros and cons of semi-subs and TLPs.
We thank the author for this valid point that makes an article’s readability and structure to be improved. We have revised the introduction chapter accordingly and removed sections that do not improve the context of the article.
Several places: The authors claim to use an MDAO framework; however, the analysis which is performed just solves a simple static pitch angle equation. Can the authors elaborate on why this is an MDAO framework?
This issue has been addressed in the document as we have developed an optimization framework rather than an MDAO. We have geometrically altered the model using a derivative-free optimization algorithm and the B-Spline curve to explore design space using the static pitch angle and stability of the system as a constraint. This is a geometric shape optimization study exploring the economic impact of the approach and not an MDAO study.
Sec. 3.1: The static pitch angle of a spar exposed to a constant wind force is something that easily could be calculated very efficiently in a Python or Matlab script, why did the authors decide to use Genie and WADAM?
We agree with the reviewer that static pitch angle can be estimated with python. However, we have gone for DNV tool due to the following reasons:
- DNV embodies recognized standards for offshore structures design and solution and the suite’s modelling module (Sesam Genie) has substantial libraries of polynomial curves and splines with local propagation properties, slope and curvature continuity that are very effective in handling complex shapes. These models from an industry body yield an accurate estimation of steel mass from the model design than a MATLAB or python script.
- We estimated the static pitch angle from the state-of-the-art DNV tool, used it as a constraint within an optimization algorithm and selected the optimal design based on the constraint and objective function which is minimizing mass of steel. We also assessed the system’s motion response in surge heave pitch and the nacelle which is better assessed with a state-of-the-art tool from a standard and certification body like DNV. Also, other tools are validated against DNV software to ensure they meet the required standards. For example, OrcaWave has just been recently validated with DNV WADAM to ensure it comply with industry standard. Also, the University of Strathclyde has access to the Licence for students to carry out research, hence reason why we chose to use it.
- This work is part of a complex framework still in development that will not do optimized design and analysis in frequency domain but in time domain. And DNV tools have the capability to assess the hydrodynamic response amplitude operator for use in time domain tools.
Sec. 4.2.2.2: What material is used as ballast in the analyses? Wouldn’t it be easier and cheaper to adjust the ballast density (e.g. by replacing olivine by iron ore) to modify the static pitch angle, rather than the shape of the spar?
The ballast material used in the compartment model of the spar is seawater. The anonymous referee has raised a valid point on how we could adjust the ballast material. However, we assessed different ways of modifying ballast and due to the nature of our design problem, we had to integrate a python function to work out the ballast filling fraction within the compartment model at the hydrostatic stage (using the data from the java script platform on which the DNV HydroD software works). The iterative design loop goes through several models (panel and compartment) novelle geometric shape changes as different design variables are passed to control points in Sesam Genie from the optimization algorithm. This ensures that for every static pitch angle constraint specified within the optimization algorithm, thousands of design iterations and subsequent hydrostatic assessment are autonomously conducted and feasible designs with the correct ballast adjustment from the correct fill fraction estimates that ensures the system’s stability are selected.
Line 95-98: Sentence «With the completion of…» is awkward and difficult to understand, I suggest to rewrite.
This text has been re-written to improve the understanding of what the author is saying and improve the quality of the article.
Line 103-104: «The most efficient offshore foundations…». What is meant by «efficient» in this context, and are floating foundations really always the most efficient? I suggest that this statement is removed or rewritten in a clearer and more nuanced way.
We agree with the anonymous referee’s opinion as efficiency can be seen from different perspectives. This statement has been rewritten to highlight how floating offshore wind complement its fixed bottom counterpart with capabilities of exploring deeper waters with richer wind resource and less invasive actions on the seabed et.al.
Line 105-107: «Second, they make it easier to set up». Reference?
These lines have been rewritten in a clearer and much more nuanced approach which also includes adequate reference supporting the statement.
Line 172/174: 5 and 4 euro per kW?
These values have been reviewed to the thousands of euros reported in the referenced document.
Line 179: Just shape optimization? I think there are other things with the design than just the shape that can be optimized (control system, structural properties, etc.)
We agree with the referee that disciplines like the control system and structural components and turbine aerodynamics can be optimized. We have included these other disciplines in the text accordingly.
Sec 3.3: Why is the overturning moment in Eq. 7 calculated about the center of buoyancy (z_cb)? Is that the elevation where the mooring lines are connected? If so, that should be specified.
Yes, this is a correct observation. The overturning moment is around the centre of mooring line connection as shown in sketch in fig.2. This has been adjusted adequately in the equation 7 and in the figure2 respectively.
Table 9, 10, 11: Why does OPEX and DECEX decrease when the CAPEX for the floaters decreases? I don’t think you’ll have lower operational costs just because there is less material
We understand the referee’s point of view as in reduction in material might not result in reduction in OPEX and DECEX. However, intangibles like the logistic planning, transportation cost, storage cost for less materials can reflect in the reduction in the OPEX and DECEX.
For the purpose of this high-level technoeconomic study, we have used data from literature as stated in Table 7 with the references included to estimate the DECEX and OPEX values. According to the reference from Maienza et.al 2020, 19% of total expenditure makes up the OPEX while 4% of total expenditure makes up the DECEX. These percentages have been used to estimate the OPEX and DECEX in this study.
Sec 4.3.2: The text in this section is repetitive and I suggest that it is revised.
Section 4.3.2 has been reviewed accordingly and effectively improved its readability. This is done by adhering to the referee’s suggestion of removing the repetitiveness of the texts.
Technical Corrections
Line 93: Remove «(FOWT)» after «clean energy»
The FOWT acronym has been removed as advised by the referee.
Table 8, 9, 10: The numbers should be in a format that is easier to read, e.g. as in Table 11. I suggest to remove the decimals as well.
The tables have been adjusted and formatted adequately as advised by the referee to improve the readability of the article.
Line 359: DPBP <<< t?
The correct mathematical operator has been used to describe the scenario meaning for the discounted pay-back period for this line.
Line 267: Castro-Santos et al. (2020a)Castro et al.
Many thanks for this detailed review. The sentence has been adjusted accordingly to show only reference from a bibliography software.
Line 289: «Floating Offshore Wind Turbines (FOWT)». Can just write FOWT, the acronym is already established earlier in the text.
The established acronym have been used accordingly not only in this line but all through the document.
Citation: https://doi.org/10.5194/wes-2023-96-AC1
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AC1: 'Reply on RC1', Adebayo Ojo, 31 Oct 2023
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RC2: 'Comment on wes-2023-96', Anonymous Referee #2, 27 Oct 2023
General Comments
The authors have conducted basic shape optimization on a spar-type floating wind turbine, and performed a brief cost analysis for the optimized design compared to the baseline. The review of existing literature is fairly thorough, however fails to make clear the gap that the current work addresses. Several of the referenced studies apply much more advanced optimization models or include the capability to consider floating substructures that are more advanced than spars. The techno-economic analysis consists of cost estimations for the final, optimized designs. As a result, the key finding is that reduced steel mass from an increased constraints on static pitch angle ceiling leads to lower costs. There is extensive background information included through care should be taken to avoid statements made without qualifying or supporting information, and to focus the background on the most relevant material for the current work.
Specific Comments
The abstract provides too much background information that isn’t directly related to this work.
Line 103-104: If floating foundations are the most efficient, why have they not been favored worldwide to-date? Avoid superlative statements such as this.
Line 110: Please provide a reference for the claim that floating foundations have environmental advantages, or remove this statement.
Line 134: Consider providing actual estimated values. When only percentages are given it is impossible to know the total project costs are the same – therefore it is impossible to compare these.
Table 1 and Table 2: Several points here require more qualification or context. For example, “Suitable for severe seastate” is listed as a benefit for spar platforms, however all three architectures listed have been installed in locations that experience severe conditions. Consider removing these tables and slight expanding the discussion in Line 117-133 about the different types.
Line 169-174: These cost trends need more thorough explanation – especially in how cost estimates were made for floating wind farms before 2017, when the first floating wind farm was installed.
Line 272: This appears to start a new section of the literature review focused on optimization studies, consider adding a new section here.
Section 2.1: Throughout the literature review avoid statements like ‘groundbreaking,’ and consider how many of the reviewed publications are truly relevant for this work.
Section 2.2: Only the LCOE is mentioned later in this work, so there is no need to include details on NPV, IRR, and DPBP as these are relatively standard and well-known metrics.
Line 393: The cited study makes no mention of ocean biodiversity. Please provide a reference for this claim or remove it.
Line 405: Here the methodology states the farm consists of two platforms and 10MW total capacity, but later results are given for 30MW and 60MW farms. Please clarify.
Line 415: Consider renaming this section, as it discusses both hydrostatics and upright stability.
Line 470: It is very strong to suggest reduced LCOE will ‘ensure commercial viability,’ what about other factors that affect viability such as permitting, supply chains, existing infrastructure?
Section 4.2.2: The section title does not make sense, and there is significant extra information in this section that could be removed to focus solely on the optimization algorithm chosen.
Table 5: This data is unnecessary in the main text, especially as the illustrations are given.
Line 563: If wall thickness is determined with this ratio, please comment on how that affects the optimal designs.
Line 565: Is ballast mass included? If so, how is it calculated?
Line 593: This is a significant omission to neglect all structural assessments. Especially when the optimal designs presented are likely to present significant structural challenges.
Section 4.3.1: It is not clear that the cost estimates are dependent on anything other than total farm capacity and steel mass per platform. If this is not the case, please describe it more explicitly.
Table 7-11: Consider how these tables could be consolidated to avoid an entire page devoted to tables in the main text.
Line 717-730: If the data are mentioned in text, there is no need for Table 13.
Line 731: It appears the reduction in LCOE due to the platform mass optimization is roughly equal to reductions due to farm size, please be more explicit about this. Please also justify why this reduction (~5%) is significant, considering the limited constraints applied to the design in optimization.
Figure 5-9: These figures appear to duplicate data presented in tables, if this is the case please remove the duplication or explain more clearly what novel information these figures present.
Line 770: To what extent can this approach be considered MDAO if the optimization considers only mass and hydrostatics, but omits the true end objective – LCOE?
Line 793: As previously mentioned, the structural assessment is a very significant portion of this work.
Technical Corrections
Throughout acronyms are redefined – this is unnecessary given that a table of abbreviations is given, unless a specific acronym is very unclear in context.
Line 161: “tecno”
Line 171: “The new farms so designed…”
Line 267: “Castro-Santos et al. (2020a)Castro et al.”
Line 314: “… select the optimal design in a quick duration.”
Line 408: “is shown in Fig2abc”
Line 416: “any type pf floating”
Line 476: “The technicality involved…”
Citation: https://doi.org/10.5194/wes-2023-96-RC2 -
AC2: 'Reply on RC2', Adebayo Ojo, 31 Oct 2023
Dear Referee#2,
On behalf of my co-authors, I will like to thank you for your comments. Please see our prepared response to your comments below.
Manuscript: wes-2023-96
The authors would like to thank the anonymous referee#2 for the insightful comment. We also do appreciate the quality of comments provided by the referee to improve this work. We have made adequate effort to effectively address the comments provided for better readability and improvement in quality of this article.
We have addressed the responses to the comments in three sections which are: General comments response, specific comments response and technical corrections. Please see the respective responses below.
General Comments
The authors have conducted basic shape optimization on a spar-type floating wind turbine, and performed a brief cost analysis for the optimized design compared to the baseline. The review of existing literature is fairly thorough, however fails to make clear the gap that the current work addresses. Several of the referenced studies apply much more advanced optimization models or include the capability to consider floating substructures that are more advanced than spars. The techno-economic analysis consists of cost estimations for the final, optimized designs. As a result, the key finding is that reduced steel mass from an increased constraints on static pitch angle ceiling leads to lower costs. There is extensive background information included through care should be taken to avoid statements made without qualifying or supporting information, and to focus the background on the most relevant material for the current work.
General Comments Response
The review of existing literature is fairly thorough, however fails to make clear the gap that the current work addresses
We have emphasized the research gap in the article accordingly. The research gap we have tried to address is to autonomously alter the shape of the platform using state-of-the-art derivative-free optimization algorithm. The benefits of this approach are: capability to search a large design space with reduced computational time and the reduction in mass observed at different static pitch angle constraints which contributes to reducing CAPEX cost of FOWT
Several of the referenced studies apply much more advanced optimization models or include the capability to consider floating substructures that are more advanced than spars
We do agree with the referee that some of the studies referenced in this work are more advanced optimization models that can handle floating substructures more advanced than spars. The novelty of our approach which is exploiting the local propagation capability of spline curve to autonomously model and alter and optimize the geometrical shape of a platform’s panel and conduct hydrostatic assessment within a single framework is a unique approach capable of producing bespoke designs. This methodology will be used on other types of floating substructures other than the Spar.
Most importantly, it is not clear what the scientific contributions of the current study are, or where the novelty of the work lies.
The main contribution of this work is to prove that the concept of shape optimization for a Spar. This is achieved by altering the control points of the spline with an optimization algorithm for a set of static pitch angle constraints. This results in design product that meets the standard requirements of stability, allowable motion response and also reduced material required for manufacturing. This study also shows that increase in the static pitch leads to reduction in the platform’s mass and the authors have explored the economic implications at a high-level. Although this concept of spline shape optimization is in use in the oil and gas industry and other sectors, the authors believe this is not so much in use in the floating offshore wind foundation sector.
The literature review refers to many papers, but does not mention any research gap or needs that the study tries to address.
Most of the papers referenced are based on traditional modelling of floating offshore wind platform. The research gap we have tried to address is to autonomously alter the shape of the platform using state-of-the-art derivative-free optimization algorithm. The benefits of this approach are: capability to search a large design space with reduced computational time and the reduction in mass observed at different static pitch constraints which contributes to reducing CAPEX cost of FOWT.
Also, all the referenced design optimization studies (Hall et al. (2013), Karimi et al. (2017), Hegseth et al. (2020) and Dou et al. (2020)) consider significantly more complex and realistic optimization problems than the one in the current study, where only the static pitch angle due to a constant wind load is taken into account. While it is impossible to consider all aspects of a floating wind foundation design in a numerical optimization process, an overly simplified representation of the design problem will not yield any realistic results and will thus be less interesting also in terms of potential cost savings
We do agree with the referee that some of the studies referenced in this work are complex optimization problems, we also believe that the novelty of our approach which is exploiting the local propagation capability of spline curve to autonomously model and alter the geometrical shape of a platform’s panel and conduct hydrostatic assessment within a single framework is a unique approach capable of producing bespoke designs. In this high-level work, we have locally altered the geometry, analysed and optimized the design within the optimization framework in the frequency domain. Most geometry optimization studies of floating platforms are altered globally i.e., the entire diameter, length or draft. This work will evolve to a detailed phase which will span different disciplines (structural, aerodynamics, hydrostatics and hydrodynamics) in frequency domain and time domain. The phase we are at is a technoeconomic assessment of our shape optimization concept on a floating offshore wind farm.
Specific Comments Response
The abstract provides too much background information that isn’t directly related to this work.
The authors are in agreement with the referee and have rewritten the abstract to address the comment to ensure the information within the abstract are strictly related to the work conducted.
Line 103-104: If floating foundations are the most efficient, why have they not been favored worldwide to-date? Avoid superlative statements such as this.
An important observation by the referee and we will adhere to avoiding superlative statement throughout the document. This statement in line 103-104 has been rewritten and supported with an adequate reference.
Line 110: Please provide a reference for the claim that floating foundations have environmental advantages, or remove this statement.
We have provided adequate reference for this claim.
Line 134: Consider providing actual estimated values. When only percentages are given it is impossible to know the total project costs are the same – therefore it is impossible to compare these.
We have included some estimated CAPEX estimates and included the reference articles related to the values.
Table 1 and Table 2: Several points here require more qualification or context. For example, “Suitable for severe seastate” is listed as a benefit for spar platforms, however all three architectures listed have been installed in locations that experience severe conditions. Consider removing these tables and slight expanding the discussion in Line 117-133 about the different types.
We agree with the referee that several points require more qualification or context. However, to keep focus on the spar floated that this study is about, we have removed these tables and expand the discussions as advised.
Line 169-174: These cost trends need more thorough explanation – especially in how cost estimates were made for floating wind farms before 2017, when the first floating wind farm was installed.
We agree with the referee and have rewritten this section in the article for clarity. The cost trends are taken from review papers in which total installed costs for offshore wind farms were initially evaluated based on the costs of existing shallow water farms and extrapolated to deeper waters for deep water offshore farms by researchers. The extrapolation approach resulted in the trends highlighted in the article.
Line 272: This appears to start a new section of the literature review focused on optimization studies, consider adding a new section here.
We thank the referee for this good suggestion. It has been implemented in the article as Optimization review.
Section 2.1: Throughout the literature review avoid statements like ‘groundbreaking,’ and consider how many of the reviewed publications are truly relevant for this work.
We have removed these kinds of statements from the document and used words that simply help to convey the meaning of the texts appropriately.
Section 2.2: Only the LCOE is mentioned later in this work, so there is no need to include details on NPV, IRR, and DPBP as these are relatively standard and well-known metrics.
This section has been rewritten as suggested by the referee. We have focused the narrative on the LCOE as it’s what was assessed in the study.
Line 393: The cited study makes no mention of ocean biodiversity. Please provide a reference for this claim or remove it.
We agree with the referee on this observation. It was an implied thought put into writing and the lead author has removed it from the sentence.
Line 405: Here the methodology states the farm consists of two platforms and 10MW total capacity, but later results are given for 30MW and 60MW farms. Please clarify.
This mistake has been corrected by the lead author. It’s a 30MW and a scaled up 60MW farm that was assessed in the article.
Line 415: Consider renaming this section, as it discusses both hydrostatics and upright stability.
We thank the referee for this suggestion. The section has been adequately renamed to capture the stability requirement of FOWT.
Line 470: It is very strong to suggest reduced LCOE will ‘ensure commercial viability,’ what about other factors that affect viability such as permitting, supply chains, existing infrastructure?
We agree with this observation from the referee. The statement has been rewritten to include other aforementioned factors that have the potential to increase the commercial viability of a FOWF.
Section 4.2.2: The section title does not make sense, and there is significant extra information in this section that could be removed to focus solely on the optimization algorithm chosen.
We have adjusted the title of this section accordingly. The high-level description of the B-Spline curve is essential as it compliments the shape optimization conducted in this study. We have used the local curve alteration capabilities of the B-Spline at the control points with the optimization algorithm delivering the design variables (radii) at the different control points to alter the shape at every iteration.
Table 5: This data is unnecessary in the main text, especially as the illustrations are given.
We thank the referee for this comment. We have deliberated on this and would like to keep the table in the document as it shows the local propagation properties of the B-Spline curve at the control points which alters the shape of the spar. We believe this is more informative for potential readers as to having only the figures in the document.
Line 563: If wall thickness is determined with this ratio, please comment on how that affects the optimal designs.
This wall thickness is determined based on buoyancy to mass ratio of 0.13 selected from literature to improve buoyancy, weight and hydrodynamic performance. This wall thickness provides the estimate of the platform’s mass from the volume of steel used and the optimal design is one with the minimal mass.
Line 565: Is ballast mass included? If so, how is it calculated?
The ballast mass is included for stability and it goes into the compartment section of the model. We have used seawater as ballast material in the compartment model and a numerical code was used to calculate the required amount of ballast material to ensure stability in the hydrostatic stage. Adequate description of this process has been included in the article.
Line 593: This is a significant omission to neglect all structural assessments. Especially when the optimal designs presented are likely to present significant structural challenges.
We agree with the referee on the need of structural assessment. Ongoing studies for future publications will include the structural assessment of the shape optimized platform.
Section 4.3.1: It is not clear that the cost estimates are dependent on anything other than total farm capacity and steel mass per platform. If this is not the case, please describe it more explicitly.
We thank the referee for this point and have addressed it in the section. Yes, the cost estimate depends on the total farm capacity and the steel mass per platform. However, more importantly, the main driving force of the cost estimate is the static pitch angle constraint used within the optimization algorithms. The larger the static pitch angle used, the smaller the estimated mass of steel required to design the platform and the concept of economy of scale makes this significant when the farm capacity increases.
Table 7-11: Consider how these tables could be consolidated to avoid an entire page devoted to tables in the main text.
We have consolidated the tables accordingly.
Line 717-730: If the data are mentioned in text, there is no need for Table 13.
We have rewritten this part to remove the use of the data in text and reference the data in the table in our discussion.
Line 731: It appears the reduction in LCOE due to the platform mass optimization is roughly equal to reductions due to farm size, please be more explicit about this. Please also justify why this reduction (~5%) is significant, considering the limited constraints applied to the design in optimization.
We thank the referee for this comment. We have explicitly shown that the reduction in LCOE based on the static pitch angle constraints mass optimization is not far off the reduction we see when scaling the farm. This has been done by including tables in the section to highlight the point. It is worth noting that the percentage reduction values will vary depending on the size of the FOWF. Also, the value 5% is quite significant as it highlights the effect the cost of steel has on costing a FOWT system. The article has been adequately updated to reflect these points.
Figure 5-9: These figures appear to duplicate data presented in tables, if this is the case please remove the duplication or explain more clearly what novel information these figures present.
The figures have been removed from the main section of the article and moved to the appendix section..
Line 770: To what extent can this approach be considered MDAO if the optimization considers only mass and hydrostatics, but omits the true end objective – LCOE?
This approach describes the effect a geometric shape optimization of spar platform has on the LCOE of a hypothetical wind farm. The main objective is minimizing the steel material and assessing the impact on the LCOE. We agree with the referee that this is not a full MDAO study and this point has been reflected in the article.
Line 793: As previously mentioned, the structural assessment is a very significant portion of this work.
We agree and thank the referee for this point. We recognize the importance of structural assessment and we have ongoing research assessing this for future articles.
Citation: https://doi.org/10.5194/wes-2023-96-AC2
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AC2: 'Reply on RC2', Adebayo Ojo, 31 Oct 2023
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EC1: 'Comment on wes-2023-96', Erin Bachynski-Polić, 04 Nov 2023
Please note that the reviewers raise significant concerns about the novelty of the presented work. The initial responses from the authors do not provide sufficient evidence of scientific novelty in the approach, and a very significant revision to the scope of work would be required for acceptance.
Citation: https://doi.org/10.5194/wes-2023-96-EC1
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