Please see attached pdf

Review of WES-2024-130

Sensitivity analysis of numerical modeling input parameters on floating offshore wind turbine loads in extreme idling conditions

General comment:

The manuscript proposes a sensitivity analysis of a multitude of environmental and turbine related input parameters to certain quantities of interest (QOI). Both ultimate and fatigue loads are considered. While similar studies have been carried out in the past (including by some of the authors), the scientific significance lies in the focus on extreme idling conditions (IEC load cases 6.1-6.5) rather than power production load cases. The study is carried out on the IEA15MW RWT atop the U Maine VolturnUS-S platform that is investigated for two sites of varying depth that require different mooring configurations (taut and semi-taut).

An elementary effects (EE) method is used to identify the parameters to which a given QOI is most sensitive. This required a large number of OpenFAST runs to be performed and analyzed in order to assign a sensitivity to a perturbation rather than a random combination of met-ocean conditions. The scientific approach and methods are valid and described extensively.

The manuscript is very well written and structured. The content of the work is presented clearly and reflects the title well.

Key outcomes can be summarized as:

• Ultimate load EE are generally sensitive to load directionality and thus to variations in inputs such as yaw, wave misalignment and current misalignment in idling extreme cases.
• The mooring configuration plays an outsized role as variations in the polyester length drives ultimate events in various QOI for the taut mooring system, while variations in this parameter are not that essential for the semi-taut configuration. Maximum wave heights are not essential.
• Fatigue load EE are also driven by directionality; however, in contrast to ultimate load EE, they are sensitive to wave height and periods as well.

A few remarks aimed to improve the manuscript are mentioned in the following:

• L 61: This sentence seems seams somewhat detached from the rest­ of the paragraph and might be better suited for the introduction.
• L 95: While the decision to exclude dynamic stall models in these idling conditions is current industry standard and therefore reasonable, a lack of a dynamic stall model in idling conditions has been shown to also lead to non-physical instabilities (doi:10.1088/1742-6596/2626/1/012026).It would be valuable to hear the authors opinion on this.
• L 105: Mentioning the wave stretching method used in this study seems relevant as various studies in the past have shown its significance on non-linear excitation.
• L 176: Twist_root is somewhat misleading. I believe what is described here is a pitch error/pitch misalignment
• Figs 4 – 8 would benefit from some more analysis. For example, the 500yr wind speed distribution of Humboldt Bay looks a bit odd and it is not clear to the reader where the lack of bins between 35 and 39 m/s comes from.
• L 290: Are the V_current measurements above 3 m/s interpreted as a measurement error? This parameter already (at the capped value) shows a strong ultimate EE sensitivity which presumably would increase with a larger range of variation
• L 377: Analyzing the separate influence of wave-only heading change and collective heading change is referred to as future work. A reference to this could be added in the conclusion
• Sec. 6 Seed Convergence & Sec. 7 Starting Point convergence: These sections could be combined into a single section. It would also be sufficient to demonstrate convergence on a single site, if the manuscript is to be slightly shortened.

Relevant aspects according to the WES guidelines

1. Does the paper address relevant scientific questions within the scope of WES? yes
2. Does the paper present novel concepts, ideas, tools, or data? Yes (focus on idling LCs)
3. Is the paper of broad international interest? yes
4. Are clear objectives and/or hypotheses put forward? yes
5. Are the scientific methods valid and clear outlined to be reproduced? yes
6. Are analyses and assumptions valid? yes
7. Are the presented results sufficient to support the interpretations and associated discussion? Yes, some figures could be analyzed in more detail
8. Is the discussion relevant and backed up? yes
9. Are accurate conclusions reached based on the presented results and discussion? yes
10. Do the authors give proper credit to related and relevant work and clearly indicate their own original contribution? yes
11. Does the title clearly reflect the contents of the paper and is it informative? yes
12. Does the abstract provide a concise and complete summary, including quantitative results? yes
13. Is the overall presentation well structured? yes
14. Is the paper written concisely and to the point? yes
15. Is the language fluent, precise, and grammatically correct? yes
16. Are the figures and tables useful and all necessary? Yes, some could be excluded to streamline the paper
17. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used according to the author guidelines? yes
18. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? Some minor edits could be implemented, see above
19. Are the number and quality of references appropriate? yes
20. Is the amount and quality of supplementary material appropriate and of added value? yes