the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 May 2024
Turbine Repositioning Technique for Layout Economics (TRTLE) in Floating OffshoreWind Farms – Humboldt Case Study
Abstract. This study investigates the farm-level effects of designing a uniform turbine repositioning (TR)-enabling mooring system on the efficiency and economics of a floating wind farm at the south-west Humboldt wind energy area, characterized by its deep waters. Four layout concepts with possibility for shared anchors are investigated. These concepts vary in terms of number of mooring lines per wind turbine. The 2-line configuration allows the platforms to have large excursions around their undisplaced positions when the system is loaded compared to a typical 3-line and 4-line configurations. The relative displacements of the wind turbines as a function of varying wind speeds and directions and their subsequent impacts on wake losses are studied. When allowing TR, wake effects can be reduced. Annual energy production increases of up to 1.3 % are achieved. Furthermore, through strategic farm-level management of mooring orientations for the 2-line setup, the proposed design achieves 50 % and 31 % reduction in anchor count and total mooring length, respectively, for the tetragonal design, compared to a conventional baseline design. The hexagonal design reaches 20 % and 27 % reductions of these quantities. A preliminary cost analysis shows a 27 % cost reduction compared to the baseline mooring system, giving a financial window for increasing reliability to the mooring system.
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Status: open (until 28 Jun 2024)
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RC1: 'Comment on wes-2024-67', Anonymous Referee #1, 18 Jun 2024
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General comments
This article demonstrates several innovative means for reducing the Levelized Cost of Electricity (LCOE) for floating wind farms, applied to the Humboldt south-west wind energy area (WEA). This is achieved by reduction of wake losses by a passive repositioning technique shifting the floating wind turbines away from upstream wakes, and by reduction of the number of mooring lines and anchors.
This work is important because cost reductions are necessary for most components of floating wind farms. The impact of wake losses on the annual energy production (AEP) of course directly affects LCOE. Assuming that the mooring system costs are for example 15% of the total costs, major savings on mooring line length and the anchor count also makes a significant difference on LCOE.
The impact of this article may be visible in future floating wind farms, with new and innovative mooring system configurations.
I find the quality of this interesting and comprehensive article very good. The article is innovative, clear, logical, and easy to follow. My main comments and questions are as follows:
Specific comments
- In the description of the PyWake model, it is stated that the wake superposition from multiple turbines is represented by a linear addition of the individual wakes. I suspect that this might exaggerate the wake losses below rated power, and thereby the reduction of wake losses from the TRTLE procedure relative to the reference configuration. The WeightedSum option in PyWake might be more accurate. If a sensitivity study on the effect of these options have not been done already, I would like to see this included in the article.
- The reference wind farm has the same spacing in the x- and y directions. The wind rose for the WEA has a distinct prevailing wind direction from the north. A wind farm layout would then typically have larger spacing north-south, and tighter spacing east-west. The shape of the WEA also favors this configuration, perhaps allowing more wind turbines, with the same or even lower wake losses than the reference case. The authors also point out that Sx and Sy do not have to be the same, but I miss a discussion how this could affect the comparison between the reference and optimized cases.
- The steady-state computations are based on rotor thrust, ignoring turbulence, current and waves. Although this belongs to future work, I think it is important to mention issues that might show up when dynamics, turbulence, current and waves are taken into account, for example:
a) If I read the reference articles on the rotor and platform right, the drag force on the platform is 40% of the rated rotor thrust for 1m/s of current. How can this affect the results?
b) The two-line configuration allows large platform offsets. This gives a desired reduction of wake losses in the steady-state computations. Could wave drift forces give a different behavior? - For 800m water depth, a taut leg mooring system with polyester or nylon ropes should give a significant reduction in costs compared to a catenary system, and it seems like the industry is moving in that direction. How can this be used for the TRTLE system? Should this be the reference system?
Technical corrections
Attached is a pdf of the article with some minor notes, questions, comments, and edits for consideration.
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RC2: 'Comment on wes-2024-67', Anonymous Referee #2, 19 Jun 2024
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The paper introduces a method to design mooring systems with two lines to relocate the turbines out of the wake, which aims to increase the AEP and decrease the mooring system cost. This type of research is crucial for enhancing the efficiency of floating wind energy and fostering future investment. The innovative approach and potential cost reductions are promising aspects of this study.
General comments:
- I have read the previous work by the authors (Yuksel R. Alkarem et al., 2024), which I found to be an excellent paper with interesting new concepts. While this current work is a good continuation, it does not significantly extend the previous research. It investigates a new site with similar assumptions and the same level of simplicity. The first research question, which aims to investigate the MS’s turbine repositioning ability to mitigate wake effects under various wind speeds and directions, closely mirrors the research question of the prior study. To enhance and extend this paper, it would be beneficial for the authors to clearly highlight the unique contributions and advancements of their current research, distinguishing it more effectively from their earlier work.
- Moving a step further to use more realistic designs for deep water sites would demonstrate the method's applicability with more realistic designs. The passive relocation of the FOWT heavily depends on the mooring system design, which seems oversimplified in the current work.
- The innovative use of two mooring lines could drastically decrease future costs. However, the paper does not discuss the potential challenges, such as low yaw stiffness, which currently limits the adoption of two mooring line designs. Including a discussion on the change in yaw stiffness between different designs and its relationship with wind direction variations would add valuable insights.
Specific comments:
Line 70: The connection between avoiding the most dominant wind direction and slack condition is unclear. Please clarify.
Line 70: Consider revising to "steady wind loads."
Equation 2: Is it π/2 multiplied or added?
Figure 2: Please use this figure to clarify the parameters and how they are defined inside the wind farm: Sb, Sx, Sy, ϕm, ϕm1, ϕm2, ϕF. It is not clear how the angles are measured and how the distances are defined. Using Figure 2 to clarify these parameters on the plots would be helpful.
Sb is defined following Equation 1 for semi-taut mooring. Are the mooring lines in this paper catenary? Or are there loading conditions where there is no section of the mooring line lying on the seabed? Is this considered while choosing the anchor type?
Sections 2.1.1 and 2.1.2: Apologies if mistaken, but it is hard to connect these sections to the rest of the paper. If Sx, Sy, and ϕF are assigned fixed values later, are these equations only used to calculate ϕm and Rm? There is no iteration or optimization over these parameters, correct? If so, it would be easier to define the values of each parameter early on in the text and use Figure to help the reader understand what each parameter means on a farm level.
Equation 6: What is C?
Line 150: Why are Sx and Sy chosen to be 8?
Line 163: "β = 0.1160 is selected" I believe this is for the TRTLE design; is this correct? What are the values for the baseline designs? What was the criterion for their selection?
Figure 4: What are the orientations of the mooring systems for these watch circles? Can a subplot be added next to this plot with the mooring lines to show their orientation?
Line 167: Can you please clarify which line is the bow line using any of the figures?
Line 171: What is the value of tension compared to the MBL of the mooring lines?
From Figure 5: It appears the stiffness of the TRTLE design is higher than the baseline designs in some wind directions. Showing the stiffness of the mooring systems at different wind directions would be interesting.
Figure 6: Please update the x-axis of Figure a.
Figures 7 and 8: I suggest showing these figures side by side.
Figures 10 and 11: Please move the color bar to the side and not with subplot one.
Line 228: Is the amplification factor also multiplied by the mooring lines? I believe the mooring line cost is a function of the mass of the chains. It is not affected by the increase or decrease in tension. The increase in tension can affect the anchor, but this depends on the anchor type.
Section 4.5: What are the types of anchors assumed for each mooring system design?
Line 233: At which wind direction is the AEP gain highest? Are there any wind directions where relocation increases wake losses? Showing AEP gain distribution over wind direction would be insightful.
Line 233: The AEP gain percentage may be low because the wind rose distribution of the site favors higher wind speeds, where there are no wake losses. Discussing this would highlight the method's higher potential for different sites and dependency on the wind rose.
Table 4: Why did the single-line anchor cost change between the baseline and the TRTLE?
Table 4: For multiline anchors, are all anchors connected to 2, 3, or 4 lines in the tetra baseline design summed up here? A more detailed cost calculation and clarification of methods used for each mooring component are suggested.Citation: https://doi.org/10.5194/wes-2024-67-RC2 -
RC3: 'Comment on wes-2024-67', Anonymous Referee #3, 20 Jun 2024
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This paper presents an interesting discussion of an innovative turbine repositioning technique, applied to a Humboldt Case Study, which has the potential to reduce wake losses and mooring system costs. This is an important area of research.
The technique presented in this work is highly dependent on two line mooring systems, which are necessary to allow the large platform offsets and reduction in mooring system materials that lead to the large cost reductions found. However, the mooring system detail design is considered out of scope in this paper and a very simplified catenary model is assumed. This feels like an oversight, because two-line mooring systems are uncommon and much more research is required to understand whether they are feasible (impacts on mooring system, turbine, and tower loads, yaw stiffness, dynamic cables).
The paper by the same author "Passive Mooring-based Turbine Repositioning Technique for Wake Steering in Floating Offshore Wind Farms" presented the start of this work. It's not clear how the present paper adds significant contributions to the previous paper. Some of the main ideas are repeated, including the same simplified mooring model. One of the main new contributions of this paper is that the TRTLE technique is applied to the Humboldt lease area, however a catenary mooring system is again assumed which is not applicable to California deep water sites. While the authors acknowledge this limitation, it remains a weakness.
The methodology section presents many equations, that are somewhat difficult for the reader to follow. It's not clear how these equations are used as the turbine spacing is defined as fixed. The equations may be better defined using diagrams that show relationships between turbine spacing and mooring system orientation, for example. The constraints in section 2.2 are also difficult to follow and may be better explained in words. It's not clear whether these constraints were applied in some sort of optimization. The AEP and power law equations can probably be left out, given that they are well known.
The section on design for redundancy mentions some ideas for improving redundancy, but they aren't supported by any concrete research in the present paper. This is also mentioned in the abstract "giving a financial window for increasing reliability to the mooring system".
Sigma m1 and sigma m2 are not defined.
Where are two line mooring systems attached to the VolturnUS-S platform? How would the suggested bridle configuration be attached?
The tension amplification ratio would be much more valuable if supported by dynamic results. Curious how two-line mooring systems impact peak loading.
The baseline layouts and the TRTLE layout align turbines in the dominant wind direction, which maximizes the benefits of the repositioning technique. Providing another comparison with the turbines not aligned in the dominant wind direction would provide another nice data point.
Citation: https://doi.org/10.5194/wes-2024-67-RC3
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