A fully integrated optimization framework for  designing a complex geometry offshore wind  turbine spar-type floating support structure

Leimeister, Mareike; Collu, Maurizio; Kolios, Athanasios

doi:https://doi.org/10.5194/wes-7-259-2022

Articles | Volume 7, issue 1

https://doi.org/10.5194/wes-7-259-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/wes-7-259-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 7, issue 1

Research article

|

04 Feb 2022

Research article |

| 04 Feb 2022

A fully integrated optimization framework for designing a complex geometry offshore wind turbine spar-type floating support structure

Mareike Leimeister, Maurizio Collu, and Athanasios Kolios

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Final revised paper (published on 04 Feb 2022)
Preprint (discussion started on 26 Jun 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Reviewer's comment on the manuscript', Anonymous Referee #1, 15 Jul 2020
- AC1: 'Answers to RC1', Mareike Leimeister, 03 Aug 2020
RC2: 'Review', Anonymous Referee #2, 16 Jul 2020
- AC2: 'Answers to RC2', Mareike Leimeister, 03 Aug 2020
EC1: 'Additional comments', Katherine Dykes, 28 Jul 2020
- AC3: 'Answers to EC1', Mareike Leimeister, 03 Aug 2020
EC2: 'Additional comments', Katherine Dykes, 28 Jul 2020
- AC4: 'Answers to EC2', Mareike Leimeister, 03 Aug 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Mareike Leimeister on behalf of the Authors (08 Aug 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (01 Sep 2020) by Katherine Dykes

RR by Anonymous Referee #2 (23 Sep 2020)

RR by Anonymous Referee #3 (02 Nov 2020)

ED: Reconsider after major revisions (07 Mar 2021) by Katherine Dykes

AR by Mareike Leimeister on behalf of the Authors (20 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (10 Apr 2021) by Katherine Dykes

RR by Anonymous Referee #4 (27 Apr 2021)

Suggestions for revision or reasons for rejection

The manuscript contains a presentation of a simulation and optimization framework for optimization of some geometric properties of a spar-type floating support structure. The considered problem is modelled as a small-scale nonlinear optimization problem and is solved by an existing genetic algorithm. The work uses a simulation and optimization framework presented in previously published articles with some extensions to the design variables etc.

The manuscript contains the statement “advanced spar-type“. In my opinion, there is nothing advanced with either the spar or the optimization problem and the authors should moderate these statements. The proposed approach can perhaps be used for conceptual design studies, but both the optimization problem and the simulations lack capabilities to perform more detailed design. For example, manufacturing considerations and cost, structural requirements, and mooring system design are all excluded. The authors are encouraged to extend the work in at least one of these directions. It may be that the argument that less restrictive feasible sets can provide unexpected designs, but it is clear from the figures that several of the presented designs have structural integrity issues (this is also acknowledged in the manuscript).

Unsuccessful simulations are encountered frequently according to the manuscript. This is an important topic within simulation-based optimization and appropriate actions must be taken. When unsuccessful simulations are flagged, they are dealt with in the implementation by considering the design under study as infeasible. The frequency of unsuccessful simulations, the type, and severity of failures are not reported. This could lead to a very conservative approach, with possibilities of disregarding good designs. The manuscript should be extended to report on the unsuccessful simulations and the authors are encouraged to investigate the reasons, types, and severities for the failures.

The description of the optimization problem is unnecessarily verbose and spread out over several sections. It is for example not necessary to describe bounds on design variables in detail like in Table 3. The same holds for the other linear constraints. The focus should be on the constraints that require simulations. For the presented optimal designs, it is notable that almost none of the constraints are active, which suggests that the problem without simulations should also be solved and the designs compared. The authors should also investigate if the solver has actually found an optimal design.

Several of the bounds in the optimization problem are physically unrealistic and additionally likely to cause numerical issues in the simulations. The problems should be re-solved with realistic values.

The reported optimization (wall-clock) time is more than 31 days. This is clearly far too much for conceptual design studies. The authors should make an effort to reduce this or at the very least explain the reasons.

The found mass is compared to an existing design and it is reasoned that significant material reductions can be achieved. It is however not reported if the two designs are subject to the same design requirements and have both been assessed in the same way. The authors should address these topics in detail before stating conclusions.
The manuscript can with advantage be drastically shortened, sharpened, and re-organized. Several sections contain information already presented.

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ED: Reconsider after major revisions (27 Apr 2021) by Katherine Dykes

AR by Mareike Leimeister on behalf of the Authors (05 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (23 Jun 2021) by Katherine Dykes

RR by Anonymous Referee #2 (06 Jul 2021)

Suggestions for revision or reasons for rejection

It has been a while since I last reviewed this paper and while it does meet a minimum bar for publication, it is not a strong paper and there is still room for significant improvement in the presentation of the material. I'm not sure if there is an appetite for further editing. In case that is a possible path forward, here are some recommended changes:

There is a mismatch between the many simplifying assumptions and the claims of a "fully integrated framework for designing complex geometry". The fact that the turbine, mooring system, and structural analysis are all excluded and that some of the hydrodynamic constants are not updated with design changes means that the work oversells itself. Hence the unfamiliar designs in the results seem even more surprising given the claims earlier in the paper.

With that idea in mind, the Introduction does a poor job of setting up the paper. Too much text is devoted to a market survey and not enough to explaining the premise of the paper. For instance, the Introduction makes statements that assume spars are difficult to handle (line 47) and that "advanced" spars can address those shortcomings. However, these concepts are not explained until he beginning of Section 2. Also, given the prior publication by the authors (https://www.sciencedirect.com/science/article/pii/S0029801820302286) I would also recommend including the explanation of how this article goes beyond it. I would state explicitly what the contribution is to the literature in this paper.

The article is still unnecessarily long and many statements are repeated section by section. For instance, both sections 2.3, 2.4, 3.*, explain the design variables and constraints. These should be condensed and combined. For Sections 2 & 4, if the approach or methods are the same as in prior publications, it might be easiest to provide a very terse description and cite the prior work. Finally, the paper could be much more succinct by reading the paper through and determining if each sentence/paragraph contributes to core narrative or not. For instance, some of the comments about the computational execution history (first paragraph in Section 5) are extraneous and the plots of the GA results (Fig 3,4,6) say very little to the reader. I could list many more examples. If professional editing is available, that could help quite a bit as well.

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ED: Publish subject to minor revisions (review by editor) (06 Jul 2021) by Katherine Dykes

AR by Mareike Leimeister on behalf of the Authors (27 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (28 Nov 2021) by Katherine Dykes

AR by Mareike Leimeister on behalf of the Authors (08 Dec 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (27 Dec 2021) by Katherine Dykes

ED: Publish as is (29 Dec 2021) by Gerard J.W. van Bussel (Chief editor)

AR by Mareike Leimeister on behalf of the Authors (06 Jan 2022) Author's response Manuscript

Short summary

Floating offshore wind technology has high potential but still faces challenges for gaining economic competitiveness to allow commercial market uptake. Hence, design optimization plays a key role; however, the final optimum floater obtained highly depends on the specified optimization problem. Thus, by considering alternative structural realization approaches, not very stringent limitations on the structure and dimensions are required. This way, more innovative floater designs can be captured.