the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Control design for floating wind turbines
Abstract. While the feedback control of onshore wind turbines is well-established, applying the same controllers to floating offshore wind turbines causes the turbines to become unstable. Such instability is attributed to the coupling between the fore-aft motion and the wind turbine controller, which makes the wind turbine negatively damped. The non-minimum phase zeros existing in the transfer function from the blade pitch to the generator speed impose a fundamental limitation on the closed-loop bandwidth, posing a challenge to the operation of the floating turbines. This paper gives an overview of the control strategies and their tuning techniques employed for floating wind turbines in the presence of the negative damping instability. It discusses the different available strategies. Moreover, we propose a new controller that can alleviate the adverse effects of the negative damping while preserving the standard proportional-integral control structure. Contrary to the multi-inputmulti-output controllers that have been proposed, the proposed controller is more robust since it does not require additional signals of the floating platform, which makes controllers often sensitive to unmodelled dynamics. The controller is compared against the previously proposed controllers using the non-linear simulation tool, OpenFAST. The proposed controller excels in regulating generator speed, surpassing other controllers in performance. Additionally, it effectively mitigates the platform pitch in addition to the tower and blade loads. However, achieving a balance between power quality, actuator usage, and structural loading presents inherent trade-offs that need to be carefully addressed.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Wind Energy Science.
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 preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 01 Jun 2025)
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RC1: 'Comment on wes-2025-68', Anonymous Referee #1, 20 May 2025
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General comments
Dear Authors,
I was invited by the Associate Editor to review your article, which I have read carefully. I would like to share my feedback below.
The stabilization of floating wind turbines at above-rated wind speeds, through the development of control strategies that overcome the limitations of conventional approaches used in bottom-fixed turbines, is a classical topic in this research field. The article addresses this issue, offering a discussion of existing strategies and proposing a new control architecture and tuning methodology. Overall, the topic is of interest and relevance to the floating wind turbine control community, and the manuscript deserves consideration for publication. However, several aspects require revision.
In particular, the second part of the article, where the SIMO and MIMO controllers are described and the results presented, is generally written better and it is more coherent than the first part, which introduces the problem, derives the model, and discusses the detuning strategy. Sometimes, the first part is inaccurate and has unclear statements. I strongly recommend a thorough revision of this section, considering the comments below.
Another concern I have is about to the writing style, especially in the first half of the paper. While I cannot verify whether AI tools were used, some sentences seem to be written very well but lack clarity or technical accuracy. I use such tools myself to improve clarity, and they can be helpful when used carefully. However, in your manuscript, a few sentences appear artificial and not precise. I have pointed out some of these under Specific Comments, but I ask you to revise the text thoroughly to ensure that all sentences are meaningful and technically sound.
Lastly, a major issue concerns the tuning of the controllers. Proper tuning is very important when comparing different control architectures, as a poorly tuned controller may perform worse even if its structure is theoretically sound. While the manuscript explains general tuning procedures, it is often unclear how the actual tuning was performed: which gains were used in the simulations, which hyperparameters were fixed to compute the gains, and what rationale was applied? Please provide more details in this regard.
Specific comments
- Abstract: "feedback control”: Specify the type, e.g., generator speed or power control.
- Abstract: "applying the same … unstable”: Clarify that only the pitch-to-feather scheme typically induces instability; other feedback control strategies exist that maintain stability in floating wind turbines.
- Abstract: "fore-aft motion”: Specify this is the nacelle fore-aft motion.
- Abstract: "more robust”: Define what is meant by "robustness". Robust to what (e.g., model uncertainties, disturbances)?
- Lines 18–19: Remove “this is not … know that” and “no wonder … wind energy”. These are unnecessary and informal.
- Lines 20–26: Consider whether this section is necessary. Is the article relevance limited to the EU context?
- Lines 37–38: The phrase "while modifications … to reducing the LCOE” is vague. Co-design of turbine and platform may reduce LCOE. Please clarify the mechanism you’re referring to.
- Line 80 and following: Including physical units is not necessary. Models can use different units.
- Line 87: "and a non-linear function f(x) = 0”. Unclear connection with the surrounding text.
- Line 98: Instead of "remains constant … and assumption errors”, state clearly that the control strategy assumes accurate knowledge of kg.
- Line 113: "Notice that the terms irrelevant to the control problem”. Please specify which terms are considered irrelevant and why.
- Line 131: The definition of above-rated operation is incorrect. Rated wind speed is the minimum at which rated power is produced. All higher wind speeds are above-rated.
- Line 133: "The steady-state … generator speed variations”. This appears inaccurate. In above-rated conditions, pitch is increased to limit aerodynamic torque. Clarify.
- Line 135: "The objective is to achieve … differential diminishes”. Unclear. Please rephrase for clarity.
- Line 138: "capturing the critical dynamics”. Specify what these critical dynamics are. Are you referring to adding platform pitch to drivetrain dynamics? State clearly which degrees of freedom are included and which are not, along with your rationale.
- Line 149: "omitting radiation damping … frequency-dependent coefficients”. This choice is questionable. Why are some terms relevant only in certain scenarios? Clarify.
- Line 181: "clearly shows … via the term”. Why is this term important?
- Line 192: "is of the main interest”. Justify why this is the main interest.
- Line 205: "quasi-static … wind speed”. Define quasi-static equilibrium more precisely. What does it mean that system variables are “balanced”?
- Line 208: "modifies the force direction”. Unclear. What force? How is the direction modified?
- Line 229: “the pitch angle … are negative”. I think you are mixing gradients and steady state values. The derivatives of thrust and aerodynamic torque wrt blade pitch are negative. As the wind speed increases, the blade pitch is drecreased to have constant aerodynamic torque (hence to balance the increase it would have because of wind speed). The increase in blade pitch doesn't yield constant thrust but thrust decreases.
- Line 293: "this approach”. Please clarify which approach is being referred to.
- Line 299: The explanation regarding gain tuning and stability is not clear. Rephrase.
- Line 313: "a range of … xi_c”. Define these parameters explicitly.
- Line 321: The effects of omega_c and xi_c on bandwidth and stability are difficult to interpret without definitions. Please clarify.
- Line 326: What is omega_p and how does it differ from omega_c? Define all parameters clearly.
- Line 336: "regularisation terms”. Briefly explain what regularization is used for in this context.
- Lines 371, 377: "blade pitch damping” ???
- Line 400: "should this objective … it will be unstable”. Unclear. Explain what the objective is and why instability would result.
- Line 463: "It was learnt … band-pass filter”. Why is it feasible to retain the PI controller and what is the role of the band-pass filter?
- Line 468: "an inverted notch”. Clarify the purpose of using an inverted notch filter.
- Line 519: "to compensate for … aerodynamic damping”. Rephrase for clarity. What is being compensated, and how?
- Line 522: "appears to be doing slightly better”. Specify the performance metric used for this comparison.
- Line 549: "generator torque actuator”. This is misleading. There is no dedicated actuator. The generator torque is actively regulated.
- Line 568: "torque feedback”. If you mean dynamic generator torque control, please state so clearly. It doesn’t appear that feedback from generator torque is used.
- Line 580: "wave forces”. Are wave forces used as a control input? If so, how are they measured in practice?
- Line 605: "simulation failure”. Generalize this to relate it to operational stability or reliability concerns in real turbines.
Technical corrections
- Line 42: Remove “peak to peak”.
- Line 80: Remove “(kg.m2)” and similar notations. The dot is not conventionally used.
- Line 121: Replace “lie in the vicinity of” with “include”.
- Line 122: Replace “there” with “the”.
- Line 129: Replace “pitch” with “platform pitch”; replace “full load” with “above-rated wind speed regime”.
- Line 272: Remove “a point that is elaborated further in this section”. It’s redundant.
- Line 277: Replace “triggered by” with “associated with”; replace “causing” with “causes”.
- Line 320: Consider replacing “failure” with “instability” if that’s what you meant.
- Line 325: Replace “:” with “,”.
- Lines 367–368: Replace “actuators” with “actuation”.
- Figure 15: Adjust the line style for β to improve readability.
- Figure 15 caption: Explain which transfer function the Bode plot represents.
- Figure 16, magnitude: I think one dot is missing. Figure 16, phase: one vertical line is hard to see.
- Line 510: Replace “how impressive of a difference” with “the significant impact”.
- Line 531: Remove repeated citation. Only cite once unless needed.
- Figure 17: Reduce line thickness to avoid covering differences.
- Line 547: Replace “dramatic” with “large”.
Citation: https://doi.org/10.5194/wes-2025-68-RC1 -
RC2: 'Comment on wes-2025-68', Anonymous Referee #2, 21 May 2025
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The paper consists of two parts, where the first part summarizes the state-of-the-art of floating wind turbine control, and the second part elaborates on an extension of existing controllers, as well as the development of a new SIMO controller. The results show that the rotor speed tracking performance, as well as platform motion differs among the controllers with a clear benefit of the more advanced controllers.
The paper provides a good summary of the state-of-the-art, even though there are multiple other papers already providing such summaries. The new controller is interesting, but requires a clearer description of the control structure and design methods.The following items should be reconsidered:
- Clarify the turbine model used (which platform, rating etc)
- Clarify the site conditions assumed, i.e. wave height, period, wind shear, TI
- Clarify the controller architecture and changes of existing controllers from the literature
- Tabulate used controller gains for reproducibility of results
- Clarify structure of new SIMO controller, especially why the plant is changed. I expected only K to change, not the plant
- Discuss sensitivity to sea state: Often controllers perform well in benign sea states but fail in harsher conditions. It should be proven that the controllers don't amplify first-order wave motions
- Improve comparability of the results: Some controllers use a lot of generator torque actuation, while others don't
- Further comments in the pdf attached
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