Dynamic performance of a passively self-adjusting floating wind farm layout to increase the annual energy production

Mahfouz, Mohammad Youssef; Lozon, Ericka; Hall, Matthew; Cheng, Po Wen

doi:https://doi.org/10.5194/wes-9-1595-2024

Articles | Volume 9, issue 7

https://doi.org/10.5194/wes-9-1595-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/wes-9-1595-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 9, issue 7

Research article

|

29 Jul 2024

Research article |

| 29 Jul 2024

Dynamic performance of a passively self-adjusting floating wind farm layout to increase the annual energy production

Mohammad Youssef Mahfouz, Ericka Lozon, Matthew Hall, and Po Wen Cheng

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Final revised paper (published on 29 Jul 2024)
Preprint (discussion started on 16 Oct 2023)

Interactive discussion

Status: closed

RC1:
'Comment on wes-2023-135', Anonymous Referee #1, 13 Nov 2023

General comments:

- it is suggested to shorten the paper to focusing on the main results only as in the current form it is hard to follow and read.

- it is suggested to change the writing style from the first to the third person

- the power gain increase achieved by the authors is insignificant, so it is hard to justify the application of the proposed mooring system design to industry
Technical comments:

- line 30, the statement "... provides stiffness in surge, sway, and yaw degrees of freedom" is only valid for slack-mooring and is not valid for TLPs

- - line 110, the authors demonstrate a gain of 6.1% at 10 m/s, it would be good to know the power gain using the entire wind probability at a particular deployment location

- Figure 8 and 9 - use the power of 10 to show Frequency of the FOWT, and the chosen color-scheme does not demonstrate the variation in frequency range

- Figure 11 is mentioned first on page 6 while it appears on page 14

- Section 3.2 - the authors refer to the natural frequency of FOWT but nor clear which DOF

- Figure 8 and line 230 - the authors state that the natural frequency does not change with the wind speed and wind direction while in reality it is. It has been shown in https://asmedigitalcollection.asme.org/OMAE/proceedings-abstract/OMAE2023/86908/1167328 that the natural frequency in surge changes with wave and wind directions

- line 265 - the choice of the sea state parameters should be explained

- line 275 - the gain of 1.4% might be within the modelling error and is insignificant

Citation: https://doi.org/10.5194/wes-2023-135-RC1
- AC1: 'Reply on RC1', Mohammad Mahfouz, 06 Dec 2023
  
  We thank the reviewer for the valuable comments and suggestions, which we consider very important and help us to sharpen and improve the manuscript.
  
  Please find a detailed response for all the comments attached.
  
  Citation: https://doi.org/10.5194/wes-2023-135-AC1
RC2:
'Comment on wes-2023-135', Anonymous Referee #2, 30 Nov 2023
The paper describes a methodology to design mooring systems that passively adjusts the position of floating wind turbines in a farm to avoid wakes from upwind turbines. The thinking is innovative, and the authors seek to demonstrate the energy yield gains in applying the methodology. The paper is well written and very detailed, perhaps too detaled, as it is on the long side and sometimes difficult to follow. The discussions of the observed differences in results between FAST/MoorPy, FAST.Farm/FLORIS are important and clarifying. However, descriptions of these two could be presented in a way that gives a better overview for the reader that is not familiar with these softwares.
It is also unclear why 10 m/s was chosen. Something about at which wind speeds the highest gains are expected should be mentioned. Also, it is mentioned in the conclusions that "a more realistic" case will be studied in the future, but it would be useful to know at this point what the design strategy would be if one had to design mooring systems for more than one wind speed. It would also be interesting to know the author's thoughts on how significant the observed increase in energy yield is for the total AEP.
The description of the mooring system database in sec 2.3 is very detailed, and quite confusing. I understand that the concepts of "mooring system watch circle" is descripbed in a previous paper, but it would be helpfulif the concept was described better in the current paper.
It is stated that the allowable displacement of the mooring system is 1D. For a 15MW turbine at 200 m water depth, this is 120% of the water depth. Normal offset requirements to secure cable integrity is in the range of 10-30% of the water depth. This criterion is also why one ends up with fairly stiff mooring systems. Soft systems like the one in the design here, also could have other issues that are not addressed here, such as snap loads. Please comment on this.
Continuing on soft mooring systems. It is weel established that soft mooring systems experience less fatigue than stiff systems, thus the difference in fatigue damage between the base case and the adjusted system is not necessarily related to the fact that it uses passive position adjustment. It is therefore not fair to compare the fatigue damage to a base case that was designed for a 12%WD offset.
Specific comments:
Sec 2.4: What is "brute force optimization"?

Sec 3.2: It is stated that the base case design is a linear mooring system. Is it not catenary? Please explain.

Fig 8 and 9: It would be helpful if the text transfers these frequency ranges to periods.

Sec 3.3: Are tower top deflections really that significant for platform offset, compared to (the mentioned) platform rigid body motions?

Fig 11: Please include units in these figures.
Citation: https://doi.org/10.5194/wes-2023-135-RC2
- AC2: 'Reply on RC2', Mohammad Mahfouz, 06 Dec 2023
  
  We thank the reviewer for the valuable comments and suggestions, which we consider very important and help us to sharpen and improve the manuscript.
  
  Please find a detailed response for all the comments attached.
  
  Citation: https://doi.org/10.5194/wes-2023-135-AC2
AC3: 'Comment on wes-2023-135', Mohammad Mahfouz, 29 Feb 2024

Additional comment on the potential of considering a softer mooring system design to decrease the LCOE while designing the mooring system.

Citation: https://doi.org/10.5194/wes-2023-135-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Mohammad Mahfouz on behalf of the Authors (06 Dec 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (14 Dec 2023) by Maurizio Collu

ED: Referee Nomination & Report Request started (02 Jan 2024) by Maurizio Collu

RR by Anonymous Referee #2 (29 Jan 2024)

RR by Anonymous Referee #1 (12 Feb 2024)

RR by Anonymous Referee #3 (25 Mar 2024)

Suggestions for revision or reasons for rejection

The present work summarizes the previously published approach for optimization of a passively adjusting floating wind farm and examines 1) the application of DWM rather than static methods and 2) the dynamic responses of the FWTs. The results are believable and the work is generally understandable, and the results are basically as expected (allowing larger mean deflections gives a softer mooring system, which takes up fewer dynamic loads than a stiffer system and thus appears to perform “better” than a “conventional” system).

In general, it would be good to include some more caveats about the results. For example, claiming improved fatigue life based on only simulations of a single environmental condition is a bit misleading. The fact that the baseline design allows for a conventional power cable design, while the passive repositioning system requires a power cable that can tolerate much larger offsets, should also be highlighted. I would furthermore argue that a chain-only design is not exactly state-of-the-art for floating wind, where lighter systems with lower environmental impact are also being examined.

When examining the dynamics of the system, it is also very relevant to discuss the control system applied in the simulations. Most reference control systems are actually unstable in surge, and would in principle be “more unstable” for longer natural periods. Did you deal with this issue, or simply ignore it?

The issues related to wake deflection prediction are quite interesting. It is possible to tune the FAST.Farm parameters to achieve similar wake deflection as LES simulations for this turbine, as shown in https://doi.org/10.1016/j.renene.2023.119807. On the other hand, it wasn’t possible to match both yaw and tilt-induced deflections simultaneously. I would also note that DWM may not accurately model the boundary flow around the farm very accurately as the number of turbines increase (especially with pitched floaters causing upward wake deflections).

Line 26: why does “open” need to be specified here?
Line 92-93: Is a minimum distance constraint actually needed? Why doesn’t the optimizer capture the fact that larger losses occur and avoid this area anyhow?
Line 102: wouldn’t it be even better to design a passively adjusting layout from scratch, rather than assuming that it is simply a correction to the optimized fixed layout? This is probably too computationally expensive in practice, but I don’t see a reason to believe that the results would be identical.
Line 125: I’m having a hard time following “the energy gain was reduced by 40% to 30% of the gain calculated at 10 m/s.” Is 30-40% a range, or do these percentages refer to two different things?
Line 168: Do I understand correctly that the yaw angle is calculated, but nonetheless not included in FLORIS?
I like the concept of figures 9 and 10, but have a hard time reading actually frequencies with these colors. Would it be possible to give a range of natural periods along the 10 m/s watch circle in the text? This would be useful in understanding, for example, how different we expect the behavior to be, or whether or not 3600 s is sufficient to capture a sufficient number of oscillations at the natural frequency.
Line 292: “OWFL when coupled to the OWFL” – I understand what you mean, but could be worded better.
Line 315: Is the rotor-average wind speed defined in rotor coordinates?
Line 320: when discussing wake meandering, it is confusing to me whether you refer to the deficit part of the model, deflection part, meandering part, or all 3.
Line 328: How large is the effect of the mooring system on the mean pitch angles?
Line 353: It would be interesting to know whether the dynamic motions or just the mean motions matter in the comparison. You could run the baseline model with the tilt corresponding to the tilt+pitch of the floater and a yaw angle equal to the mean yaw angle to see wthis.
Line 358: wouldn’t it be easier to write 0.17D?
Figure 15: Check figure title
Line 375 and 397: were/where
Line 413: note that the reduced stiffness will in principle reduce the platform motion responses to first-order wave forces (as the system is even farther from resonance). This difference is probably small in this case, though.

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ED: Publish subject to minor revisions (review by editor) (08 Apr 2024) by Maurizio Collu

AR by Mohammad Mahfouz on behalf of the Authors (12 Apr 2024)

EF by Vitaly Muravyev (12 Apr 2024) Manuscript Author's response Author's tracked changes

ED: Publish subject to minor revisions (review by editor) (11 May 2024) by Maurizio Collu

AR by Mohammad Mahfouz on behalf of the Authors (13 May 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (31 May 2024) by Maurizio Collu

ED: Publish as is (31 May 2024) by Carlo L. Bottasso (Chief editor)

AR by Mohammad Mahfouz on behalf of the Authors (03 Jun 2024)

Short summary

As climate change increasingly impacts our daily lives, a transition towards cleaner energy is needed. With all the growth in floating offshore wind and the planned floating wind farms (FWFs) in the next few years, we urgently need new techniques and methodologies to accommodate the differences between the fixed bottom and FWFs. This paper presents a novel methodology to decrease aerodynamic losses inside an FWF by passively relocating the downwind floating wind turbines out of the wakes.