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https://doi.org/10.5194/wes-2025-56
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wes-2025-56
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
19 May 2025
| 19 May 2025
Status: this preprint is currently under review for the journal WES.
Feasibility of up-tower repair concepts for pultruded carbon spar cap planks in wind turbine blades
Abstract. While modern wind turbine blades utilize pultruded carbon fiber-reinforced polymer (CFRP) planks for structural integrity in spar caps, these materials can sustain damage from operational stresses, leading to potential failures if unaddressed. Traditional down-tower repairs result in significant costs related to dismantling and transportation, especially for offshore installations, emphasizing the need for efficient up-tower repair methods. The research utilizes a finite element model of an 81.6 m rotor blade designed for a 7 MW offshore turbine, subjected to aeroelastic simulations to evaluate load conditions during maintenance. The analysis focuses on a step-wise increased repair zone, assessing susceptibility to buckling, cyclic strains, and permissible wind speeds. Results indicate that while substantial repairs can endanger structural stability, turbulence-induced strain amplitudes are manageable. Recommendations include installing temporary pretensioning and buckling support structures to enhance safety during repairs. Various innovative support designs are proposed for installation from both inside and outside the blade, aimed at improving structural integrity during up-tower repairs.
How to cite. Rosemeier, M., Saathoff, M., Kolsch, J., Buriticá, P., Petersen, E., and Beer, C.: Feasibility of up-tower repair concepts for pultruded carbon spar cap planks in wind turbine blades, Wind Energ. Sci. Discuss. [preprint], https://doi.org/10.5194/wes-2025-56, in review, 2025.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
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Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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CC1: 'Comment on wes-2025-56', Wolfgang Holstein, 22 May 2025
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AC1: 'Reply on CC1', Malo Rosemeier, 19 Sep 2025
Dear Wolfgang,Thank you for your practice-based contribution. We fully agree that repair decisions are multi‑criteria and not purely technical.Your remarks on the considerable effort to install stiffening aids as an exoskeleton or endoprosthesis without a crane - from a platform or on ropes - highlight the operational challenges. Likewise, the choice between up‑tower and down‑tower repair is strongly context‑dependent: offshore it hinges on the availability, mobilization time, and day rates of jack‑up vessels, as well as suitable metocean windows; onshore it is shaped by the accessibility of remote sites (road and crane access, crane logistics, permits, seasonal constraints).Your input is very valuable and will be carefully considered in our revision.Kind regards,
The Authors
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AC1: 'Reply on CC1', Malo Rosemeier, 19 Sep 2025
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RC1: 'Comment on wes-2025-56', Anonymous Referee #1, 06 Aug 2025
The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2025-56/wes-2025-56-RC1-supplement.pdf
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AC2: 'Reply on RC1', Malo Rosemeier, 19 Sep 2025
Dear Anonymous Referee #1,Thank you for your constructive assessment. We understand your main concern is the clarity of novelty. Our manuscript’s intent is to contribute a structural feasibility assessment rather than to claim new field procedures or cost benefits. Specifically, we investigate feasibility via (i) simulations and (ii) practical considerations: an FE model of an 81.6 m commercial blade for a 7 MW offshore turbine under a maintenance load case, with loads from aeroelastic simulations, evaluating the weakened structure (step‑wise increased repair zone) for buckling, cyclic strains, and permissible wind speeds for conducting the repair.We confirm that the support‑structure content reflects brainstorming-concept proposals to enable up‑tower repairs, not mature designs. These proposals should be developed further and validated in more detail.We also agree that cost and asset availability drive the decision, and we do not present proof of O&M cost reduction; as you note, no such evidence is provided or claimed.Regarding Section 4.2, our intent is to keep it as part of the discussion because it documents the proposal space (and is only an excerpt of the related patent application), which helps contextualize the feasibility analysis. We will consider your recommendation to streamline it in a revision. We may also consider modifying the title to better reflect a conceptual, structural feasibility assessment.Regarding Section 2.1 “the proposed repair concept,” our focus is not to claim novelty for individual repair steps - which are already performed by repair companies (as cited in Fig. 2) - but to combine established practices into a coherent up‑tower sequence and assess its structural feasibility (limits) for pultruded CFRP spar caps under maintenance loads. We appreciate your detailed points on FE modeling; all relevant aspects will be considered in a revision.Thank you again for your valuable input.Best regards,
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AC2: 'Reply on RC1', Malo Rosemeier, 19 Sep 2025
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RC2: 'Comment on wes-2025-56', Anonymous Referee #2, 14 Aug 2025
Dear Authors,
Thank you for this interesting submission that already has decent quality.
The submitted manuscript addresses an interesting question with regard to feasibility of up-tower repairs for wind turbine rotor blades. Most of my comments address minor revisions in the manuscript. However, one major problem of the underlying technical challenge has not been addressed.
Please revise according to my comments in the attached *.pdf.
Best regards.
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AC3: 'Reply on RC2', Malo Rosemeier, 19 Sep 2025
Dear Anonymous Referee #2,Thank you for highlighting this major point. We agree that a permissible cyclic strain (or stress) level is required for the structural validation. Our current results quantify cyclic strain/stress amplitudes under a maintenance load case, but the permissible level of cyclic loading during curing of the matrix along with movement of fibers for safe up‑tower repair must be derived experimentally.We will consider stating this explicitly in the outlook and framing our current cyclic assessment as indicative pending experimental verification.Best regards,
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AC3: 'Reply on RC2', Malo Rosemeier, 19 Sep 2025
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Malo Rosemeier
CORRESPONDING AUTHOR
Department of Rotor Blades, Fraunhofer IWES, Fraunhofer Institute for Wind Energy Systems, Am Seedeich 45, 27572 Bremerhaven, Germany
Matthias Saathoff
apera engineers GmbH, Wendenstrasse 130, 20537 Hamburg, Germany
Johannes Kolsch
Renertech Betriebsführungs GmbH & Co. KG, Graf-Zeppelin-Strasse 68, 33181 Bad Wünnenberg, Germany
Pablo Buriticá
DNV Renewables Certification GmbH, Brooktorkai 18, 20457 Hamburg, Germany
Enno Petersen
Department of Rotor Blades, Fraunhofer IWES, Fraunhofer Institute for Wind Energy Systems, Am Seedeich 45, 27572 Bremerhaven, Germany
Christopher Beer
apera engineers GmbH, Wendenstrasse 130, 20537 Hamburg, Germany
Short summary
This research explores a method for repairing wind turbine blades made from carbon fiber while they remain installed, eliminating costly disassembly. It addresses the challenges of repairing damaged blades and proposes a practical solution to improve repair efficiency. Through structural analysis, we found that this approach can ensure safety during repairs providing significant benefits to the wind energy industry.
This research explores a method for repairing wind turbine blades made from carbon fiber while...
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Besten Dank Malo für die Möglichkeit an der Diskussion zur Veröffentlichung teilzunehmen.
Die Erfahrung aus meiner Praxis zeigt, daß Reparaturentscheidungen aufgrund mehrdimensionaler Kriterien erfolgen, die nicht alleine technischer Natur sind. Die Verfügbarkeit von Ersatzteilen, Material und geeignetem Personal, sowie meteorologische Bedingungen spielen hier mit eine Schlüsselrolle.
Gleichwohl zeigt das Verbundprojekt die Leistungsfähigkeit der Simulation und ingeniöse Kreativität durch versteifende Komforten als Exosketett oder Endoprthese die erforderliche Positionierung der Fügepartner herzustellen. Welch ein Aufwand am hängenden Blatt diese Hilfsvorrichtungen ohne Kran aus der Bühne oder dem Seil anzubringen.
Soviel zu dem ersten Eindruck, den ich weiter vertiefen möchte. Ich bin gespannt auf Kommentare, die ich ebenfalls zu sehen bekomme.
Wolfgang Holstein