the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A New Methodology for Upscaling Semi-submersible Platforms for Floating Offshore Wind Turbines
Kaylie Laura Roach
Matthew A. Lackner
James F. Manwell
Abstract. This paper presents a new upscaling methodology for floating offshore wind turbine platforms. The size and power rating of offshore wind turbines have been growing in recent years, with modern wind turbines rated at 10–14 MW in contrast with 2–5 MW in 2010. It is not apparent how much further wind turbines can be increased before it is unjustified. Scaling relations are a useful method for analyzing wind turbine designs, to understand the mass, load, and cost increases with size. Scaling relations currently do not exist but are needed for floating offshore platforms to understand how the technical and economic development of floating offshore wind energy may develop with increasing turbine size. In this paper, a hydrodynamic model has been developed to capture the key platform response in pitch. The hydrodynamic model is validated using OpenFAST, a high-fidelity offshore wind turbine simulation software. An upscaling methodology is then applied to two semi-submersible case studies, in which the platform pitch angle at rated wind turbine thrust is constrained to a specified value. The results show that platform dimensions scale to a factor of 0.75, and the platform steel mass scales to a factor of 1.5 when the wall thickness is kept constant. This study is the first to develop generalized upscaling relations that can be used for other semi-submersible platforms, in contrast with other studies that upscale a specific design to a larger power rating. This upscaling methodology provides new insight into trends for semi-submersible platform upscaling as turbine size increases.
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Kaylie Laura Roach et al.
Status: final response (author comments only)
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RC1: 'Comment on wes-2023-18', Anonymous Referee #1, 16 Mar 2023
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2023-18/wes-2023-18-RC1-supplement.pdf
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RC2: 'Comment on wes-2023-18', Anonymous Referee #2, 26 Apr 2023
1. Introduction, lines 20-25 - the authors refer to "now" using papers dated 2011, 2013. Please update with the most recent information
2. Introduction, line 35 - after 2009, it was a number of studies prooving that the wind turbine mass does not scale as 'square cube' but closer to square or even less.
3. Introduction, line 40 - LCOE of offshore wind is not twice of onshore, plesae refer to the most recent data
4. The authors cannot claim that their paper is "the first", please refer to https://doi.org/10.1016/j.rser.2022.112477 and similar papers.
5. In-text referencing should be improved - for example, Leimeister et al. (Leimeister et al., 2016) - should be Leimeister et al. (2016)
6. Equation (2) - the authors have not included viscous damping acting on the platform
7. Equations (3) and the following ones should be referenced
8. The authors should make clear where is the origin of their coordinate system, is it the centre of gravity of waterline?
9. The authors use a lot of figures taken from other papers/reports. Please make sure that you have obtained all copyrights to use them in your work.
10. Section 3.2.1 - not clear why the authors decided to upscale the wind turbines if the solutions already exist for 10 and 15 MW?
11. Section 3.3 - it is not clear how exactly the authors have modelled Equation (4). Also, how the authors obtained hydrodynamic parameters, and state-space model of the radiation force for their model?
The reviewer is a bit concerned by the 10% error in natural periods, because to check natural period, you just need to make sure that the mass, added mass and hydrostatic stiffness are correct, and all these numbers are available in the public domain.
12. In Table2, the authors mentioned the CM as -13.46 - but this number is for the platform itself without installed wind turbine. When assembling OC4 with a WT, you will get -9.9 m or so, have the authors taken this into account in their model? Also, the mass matrix will be changed
13. Figures 5, and 6 - the authors need to demonstrate the comparison between OpenFAST and their model
14. Line 245 - error is 36% is not acceptable, the authors should have contacted the authors of UMaine to get more accurate data. Also, it is possible to calculate the assembled mass matrix if the platform and wind turbine are known
15. Section 3.4 - it is not clear why the authors decided to apply the scaling factor to both radius and distance, as the product of these two is correlated to the hydrostatic stiffness. Please provide more explanation on this step
16. Line 275, have the authors assumed that the rated wind speed is the same for all wind turbines?
17. The authors have not mentioned a possibility of peak shaving to decrease the wind turbine max thrust force if needed to the platform design purposes
18. "The added mass coefficient cA comes from the documentation for each semi-submersible case study" - have the authors scaled this parameter as well or used fixed for all platform dimensions?
19. The authors numerically found that their scaling factor for linear dimensions is 0.75. This result can be obtained also using some manual calculations. Say you have R - distance to the column, and d - diamater of the column, hydrostatic stiffness scales as R^2 d^2. Also, Thurst force scales as D^2, hub height as D. Stiffness = Force x height/angle, so stiffness should scale as D^3. If we have stiffness = D^3 and at the same time R^2 d^2, so (Rd) should scale as D^(3/2) = D^1.5. If we scale both R and d at the same time, so each of them will scale as D^(1.5/2) = D^(0.75).
Please revisit all your results and support by simple analysis referring to fundamental equations of the platform stiffness, etc.
20. As was found in one of the review papers, the anticipated linear scaling for combined radius and distance parameter is 1.5 (0.75+0.75), but most recent designs are close to 1 due to applied optimisation of each particular design.Citation: https://doi.org/10.5194/wes-2023-18-RC2 - AC1: 'Author Comment on wes-2023-18', Kaylie Roach, 24 May 2023
Kaylie Laura Roach et al.
Kaylie Laura Roach et al.
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