Offshore wind profile characteristics and their impact on floating wind turbine power production

Angelou, Nikolas; Dubreuil-Boisclair, Camille

doi:10.5194/wes-2026-20

Preprints

https://doi.org/10.5194/wes-2026-20

Preprints

16 Feb 2026

| 16 Feb 2026

Status: a revised version of this preprint was accepted for the journal WES and is expected to appear here in due course.

Offshore wind profile characteristics and their impact on floating wind turbine power production

Nikolas Angelou and Camille Dubreuil-Boisclair

Abstract. In this study, we investigate the impact of vertical wind shear and wind speed inversions on the power production of a floating offshore wind turbine. Using nacelle-mounted wind lidar data from a 6 MW turbine at the Hywind Scotland wind farm, we analyse inflow conditions and turbine performance during summer and autumn. The wind climatology shows that 33 % of examined cases exhibit non-standard wind profiles within the rotor-swept area, including negative shear and wind speed inversions. These conditions significantly affect power production, particularly below rated wind speeds, with negative shear profiles causing reductions of up to 20 % compared to the reference power curve. Our findings demonstrate that deviations from the logarithmic wind profile at the operating height range of modern wind turbines, are frequent in deep-water offshore environments and can introduce substantial bias in power curve verification. Nacelle-mounted wind lidars provide critical insight into these inflow characteristics, enabling improved performance assessment of floating offshore wind turbines. The results highlight the need for measurement strategies that capture wind conditions across the full rotor-swept area, which can be achieved through nacelle-mounted wind lidar instruments.

Received: 21 Jan 2026 – Discussion started: 16 Feb 2026

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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Nikolas Angelou and Camille Dubreuil-Boisclair

Status: closed

RC1:
'Comment on wes-2026-20', Anonymous Referee #1, 19 Mar 2026
This paper investigates offshore wind profile characteristics and their impact on floating wind turbine power production using real measurement data. The study reveals complex vertical wind profiles in the deep sea and highlights the limitations of using hub-height wind speed to assess the power performance of a floating turbine. The findings provide important scientific contributions to wind energy research, particularly for the development of offshore floating wind.
Several issues and suggestions are raised by the reviewers that could be further addressed to improve the manuscript.
Major comments:
Line 55: It may also be worth mentioning that nacelle-mounted lidar systems are generally simpler to install and more cost-effective than floater-based lidar systems. In addition, the lidar beams can be aligned with the longitudinal (mean wind) direction, which minimizes mismatch with longitudinal wind fluctuations. This is particularly important for accurately estimating freestream turbulence intensity at hub height (Pls. see https://doi.org/10.1016/j.marstruc.2026.104019).

Line 124: There appears to be a significant reduction in the available lidar data. Could the authors clarify the data selection criteria? For example, is there a minimum data availability threshold within each 10-minute interval (e.g., for all range gates), and how is this applied?

Figure 6: It is recommended to clarify how the power curve is calculated. For example, is it based on bin-averaged statistics? Additionally, the reference power curve is not clearly defined. Is it obtained from an aero-hydro-servo-elastic simulation assuming a rigid rotor and tower, or does it include all dynamic effects of the system?

Figure 6 conveys important information from the current study. It would be interesting to plot the power ratio as a function of the wind shear gradient to examine whether the power discrepancy is related to the magnitude of negative shear.

In Figure 6, it appears that negative shear leads to higher power standard deviations. It would be interesting to plot the power spectra to identify the differences in fluctuations. Under negative shear conditions, wind speed decreases closer to the sea surface due to surface friction. A blade operating in a low-level jet (LLJ) can experience enhanced 2P excitations: First period: the mean wind speed varies from maximum at hub height to lower values upward and then returns to maximum at hub height. Second period: the mean wind speed varies from maximum at hub height to lower values downward and then returns to maximum at hub height. (The wind speed is lower downwards due to the effects of tower shadow and positive shear near the surface) . Therefore, it would be insightful to determine whether the additional fluctuations in power are caused by these 2P effects or are primarily due to high ambient turbulence intensity at lower frequencies.

Based on the reviewer’s understanding, even under positive or negative shear, the theoretical power curve of a floating wind turbine would generally be lower than that of a bottom-fixed turbine at below-rated wind speeds. As mentioned by the authors, the thrust force can induce turbine tilt of several degrees. Combined with rotor tilt, the total tilt of the rotor relative to the mean wind direction could approach 10 degrees, which affects the net longitudinal wind speed experienced by the rotor. In addition, turbine structural flexibility also influences the theoretical power curve. Therefore, it is important for the authors to clarify how the reference power curve is obtained. For example, are shaft tilt, pre-cone, and blade and tower flexibility considered in the calculation?

Conclusion: The current conclusion states that negative wind shear and wind speed inversions influence turbine power production, but it is not clear how they affect it. Based on the authors’ observations, negative shear or wind speed inversions can result in maximum or elevated wind speeds at hub height. However, the turbine does not rely solely on hub-height wind speed to produce power; it responds to the rotor-equivalent wind speed (REWS). If hub-height wind speed is used to derive the power curve, which may be higher than the actual REWS, a weaker-than-expected power curve will result. It is recommended that the authors emphasize in the conclusion that using hub-height wind speed for power performance verification offshore may have inherent limitations.

Minor comments:

Line 25: The statement “only at two locations in the North Sea” appears to be too absolute. There are other platforms, such as the Inch Cape Meteorological (Met) Mast in the UK, that should also be considered.

Line 55, maybe also mention that the induction effect to nacelle lidar needs to be compensated as well

Line 55: In addition to different measurement heights, the measurements in different radial positions are possible.

Line 65: There is prior work using motion-compensated lidar measurements to assess the power performance of a 2 MW barge-type floating wind turbine (DOI: 10.1088/1742-6596/2265/4/042016). It would be beneficial to reference this study.

Lines 78, 80, 86, 143: The citation format is inconsistent (e.g., should follow “Jacobsen and Godvik, 2021”). The authors are encouraged to check and standardize all citations throughout the manuscript.

Line 79: It would be clearer to explicitly specify “platform pitch” to avoid confusion with blade pitch.

Line 158: In the Trondborg and Meyer model, the induction effect also depends on the radial position; this could be clarified.

Equation (3): Does the formulation account for the mean platform pitch angle in the vertical wind profile subject to induction? Additionally, there appears to be a missing bracket in the expression for .

Line 182, Fig. 2(c): There is a typographical error in the symbol “;”.

Figure 2: Readability would be improved if the legends clearly distinguish between solid lines and scatter points.

Line 191: Equation (1) appears to provide an analytical expression for RMSE, but the calculation procedure is not clearly explained. It is recommended to clarify how RMSE is computed. Additionally, this seems to be the first occurrence of RMSE, while its definition appears later in the text.

Equation (1): The variables and should be clearly defined.

Line 205: LLJ is defined multiple times; it would be better to define it once and use the abbreviation consistently thereafter.

Line 245: Is the sonic-based wind speed corrected using a nacelle transfer function? It is recommended to clarify whether the induction effect is also considered for the sonic measurements.

Line 267: The nacelle anemometer is typically mounted above hub height. Is this measurement treated as the ground-truth hub-height wind speed? Please clarify.

Figure 5: The black solid and dashed lines should be clearly explained in the legend or caption.

Line 315: For continuous-wave (CW) lidar focusing at short distances, induction effects may also be significant and should be discussed.

Conclusion: It is recommended to specify the number of cases analyzed in the study.
Citation: https://doi.org/10.5194/wes-2026-20-RC1
- AC1: 'Reply on RC1', Nikolas Angelou, 07 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2026-20/wes-2026-20-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/wes-2026-20-AC1
RC2:
'Comment on wes-2026-20', Anonymous Referee #2, 16 May 2026

Please find the referee comment in the attached PDF-document.

Citation: https://doi.org/10.5194/wes-2026-20-RC2
- AC2: 'Reply on RC2', Nikolas Angelou, 07 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2026-20/wes-2026-20-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/wes-2026-20-AC2

Status: closed

RC1:
'Comment on wes-2026-20', Anonymous Referee #1, 19 Mar 2026
This paper investigates offshore wind profile characteristics and their impact on floating wind turbine power production using real measurement data. The study reveals complex vertical wind profiles in the deep sea and highlights the limitations of using hub-height wind speed to assess the power performance of a floating turbine. The findings provide important scientific contributions to wind energy research, particularly for the development of offshore floating wind.
Several issues and suggestions are raised by the reviewers that could be further addressed to improve the manuscript.
Major comments:
Line 55: It may also be worth mentioning that nacelle-mounted lidar systems are generally simpler to install and more cost-effective than floater-based lidar systems. In addition, the lidar beams can be aligned with the longitudinal (mean wind) direction, which minimizes mismatch with longitudinal wind fluctuations. This is particularly important for accurately estimating freestream turbulence intensity at hub height (Pls. see https://doi.org/10.1016/j.marstruc.2026.104019).

Line 124: There appears to be a significant reduction in the available lidar data. Could the authors clarify the data selection criteria? For example, is there a minimum data availability threshold within each 10-minute interval (e.g., for all range gates), and how is this applied?

Figure 6: It is recommended to clarify how the power curve is calculated. For example, is it based on bin-averaged statistics? Additionally, the reference power curve is not clearly defined. Is it obtained from an aero-hydro-servo-elastic simulation assuming a rigid rotor and tower, or does it include all dynamic effects of the system?

Figure 6 conveys important information from the current study. It would be interesting to plot the power ratio as a function of the wind shear gradient to examine whether the power discrepancy is related to the magnitude of negative shear.

In Figure 6, it appears that negative shear leads to higher power standard deviations. It would be interesting to plot the power spectra to identify the differences in fluctuations. Under negative shear conditions, wind speed decreases closer to the sea surface due to surface friction. A blade operating in a low-level jet (LLJ) can experience enhanced 2P excitations: First period: the mean wind speed varies from maximum at hub height to lower values upward and then returns to maximum at hub height. Second period: the mean wind speed varies from maximum at hub height to lower values downward and then returns to maximum at hub height. (The wind speed is lower downwards due to the effects of tower shadow and positive shear near the surface) . Therefore, it would be insightful to determine whether the additional fluctuations in power are caused by these 2P effects or are primarily due to high ambient turbulence intensity at lower frequencies.

Based on the reviewer’s understanding, even under positive or negative shear, the theoretical power curve of a floating wind turbine would generally be lower than that of a bottom-fixed turbine at below-rated wind speeds. As mentioned by the authors, the thrust force can induce turbine tilt of several degrees. Combined with rotor tilt, the total tilt of the rotor relative to the mean wind direction could approach 10 degrees, which affects the net longitudinal wind speed experienced by the rotor. In addition, turbine structural flexibility also influences the theoretical power curve. Therefore, it is important for the authors to clarify how the reference power curve is obtained. For example, are shaft tilt, pre-cone, and blade and tower flexibility considered in the calculation?

Conclusion: The current conclusion states that negative wind shear and wind speed inversions influence turbine power production, but it is not clear how they affect it. Based on the authors’ observations, negative shear or wind speed inversions can result in maximum or elevated wind speeds at hub height. However, the turbine does not rely solely on hub-height wind speed to produce power; it responds to the rotor-equivalent wind speed (REWS). If hub-height wind speed is used to derive the power curve, which may be higher than the actual REWS, a weaker-than-expected power curve will result. It is recommended that the authors emphasize in the conclusion that using hub-height wind speed for power performance verification offshore may have inherent limitations.

Minor comments:

Line 25: The statement “only at two locations in the North Sea” appears to be too absolute. There are other platforms, such as the Inch Cape Meteorological (Met) Mast in the UK, that should also be considered.

Line 55, maybe also mention that the induction effect to nacelle lidar needs to be compensated as well

Line 55: In addition to different measurement heights, the measurements in different radial positions are possible.

Line 65: There is prior work using motion-compensated lidar measurements to assess the power performance of a 2 MW barge-type floating wind turbine (DOI: 10.1088/1742-6596/2265/4/042016). It would be beneficial to reference this study.

Lines 78, 80, 86, 143: The citation format is inconsistent (e.g., should follow “Jacobsen and Godvik, 2021”). The authors are encouraged to check and standardize all citations throughout the manuscript.

Line 79: It would be clearer to explicitly specify “platform pitch” to avoid confusion with blade pitch.

Line 158: In the Trondborg and Meyer model, the induction effect also depends on the radial position; this could be clarified.

Equation (3): Does the formulation account for the mean platform pitch angle in the vertical wind profile subject to induction? Additionally, there appears to be a missing bracket in the expression for .

Line 182, Fig. 2(c): There is a typographical error in the symbol “;”.

Figure 2: Readability would be improved if the legends clearly distinguish between solid lines and scatter points.

Line 191: Equation (1) appears to provide an analytical expression for RMSE, but the calculation procedure is not clearly explained. It is recommended to clarify how RMSE is computed. Additionally, this seems to be the first occurrence of RMSE, while its definition appears later in the text.

Equation (1): The variables and should be clearly defined.

Line 205: LLJ is defined multiple times; it would be better to define it once and use the abbreviation consistently thereafter.

Line 245: Is the sonic-based wind speed corrected using a nacelle transfer function? It is recommended to clarify whether the induction effect is also considered for the sonic measurements.

Line 267: The nacelle anemometer is typically mounted above hub height. Is this measurement treated as the ground-truth hub-height wind speed? Please clarify.

Figure 5: The black solid and dashed lines should be clearly explained in the legend or caption.

Line 315: For continuous-wave (CW) lidar focusing at short distances, induction effects may also be significant and should be discussed.

Conclusion: It is recommended to specify the number of cases analyzed in the study.
Citation: https://doi.org/10.5194/wes-2026-20-RC1
- AC1: 'Reply on RC1', Nikolas Angelou, 07 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2026-20/wes-2026-20-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/wes-2026-20-AC1
RC2:
'Comment on wes-2026-20', Anonymous Referee #2, 16 May 2026

Please find the referee comment in the attached PDF-document.

Citation: https://doi.org/10.5194/wes-2026-20-RC2
- AC2: 'Reply on RC2', Nikolas Angelou, 07 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2026-20/wes-2026-20-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/wes-2026-20-AC2

Nikolas Angelou and Camille Dubreuil-Boisclair

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Short summary

We examined how variations in vertical wind profiles influence floating offshore wind turbines. Using data from a turbine in Scotland, we found that wind speed can change unexpectedly with height. These conditions occur frequently and, if not fully characterised, can complicate power curve verification. Our findings highlight that measuring wind across the entire rotor using nacelle-mounted lidars is essential for improving performance assessment and turbine efficiency.


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