the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Apr 2024
Low Uncertainty Wave Tank Testing and Validation of numerical methods for Floating Offshore Wind Turbines
Abstract. The accurate simulation of loads and motions of Floating Offshore Wind Turbines (FOWT) in operation is key to the commercialisation of this technology. To improve such load predictions, a critical assessment of the capabilities and limitations of simulation methods for FOWT is mandatory. However, uncertainties arise during the whole validation process of a numerical method. These can drastically impair the quality of the validation. In the case of FOWT, the interaction between aerodynamic, hydrodynamic and mooring loads on the one hand and platform motions on the other hand causes a high level of uncertainty in the measurement data acquired in model tests. This also applies to comparing a numerical model to the test data, as these interactions make the distinction between cause and effect challenging. To address these challenges, several improvements to the validation process aiming at the reduction of the uncertainties are proposed and evaluated in this work. The major improvements are the measurement of the rotor thrust force excluding the tower top inertia loads, a significant improvement of the wind field quality in the wave tank and the utilisation of hybrid simulations based on the measured platform motions. These steps are applied to wave tank tests of a FOWT utilising a single point mooring and the subsequent validation of the numerical panel method panMARE. The improvements allowed for a considerable decrease in the random and systematic uncertainty of the model tests and made a valuable contribution to the distinction between cause and effect regarding the deviations between measurements and simulations.
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RC1: 'Comment on wes-2024-46', Anonymous Referee #1, 17 May 2024
Several improvements to the validation process aiming at the reduction of the uncertainties were proposed and evaluated in this work. The major improvements were the measurement of the rotor thrust force excluding the tower top inertia loads, a significant improvement of 10 the wind field quality in the wave tank and the utilization of hybrid simulations based on the measured platform motions. These steps were applied to wave tank tests of a FOWT utilizing a single point mooring and the subsequent validation of the numerical panel method panMARE. The improvements allowed for a considerable decrease in the random and systematic uncertainty of
the model tests and made a valuable contribution to the distinction between cause and effect regarding the deviations between measurements and simulations.This paper covers a lot of contents, so the introduction and analysis of each part is not very full. The research content of this paper is very meaningful, but it is hoped that the author can find the main expression content of the paper, accurately refine it, and condense it into a journal article with academic value. In addition, some suggestions are as follows:
1.The structure of the first and second chapters of the paper is inconsistent with the common articles. The first chapter is basically the author's statement, and the second chapter is the previous research work. In my opinion, the content of the first and second chapters should be properly integrated, and the research review of the second chapter should be used to support the views stated in the first chapter, so as to solve the sense of separation in the first two chapters of the paper.
2.In Chapter 2, so many previous works are introduced, all of which are supported by text. The authors should choose some important content to accompany the figure to illustrate.
3.The title of Chapter 3 does not summarize the content of Chapter 3 clearly, it is too concise.
4.Chapter 4: It is best to provide a layout diagram here.
5.As stated in the abstract: The major improvements: a significant improvement of the wind field quality in the wave tank. This section describes the advantages of this wind generating system, but I think the advantages should be reflected by comparing the quality of the wind generating system with that of other existing basin tests. Therefore, I think a table should be used to compare some important parameters to verify the advantages of this wind generating system.
6. Table1: What is the scale ratio of the test? What scaling criteria do periods, wave heights, and wind speeds follow?
7. The conclusion is too long, requiring extensive cuts and rigorous generalizations
Citation: https://doi.org/10.5194/wes-2024-46-RC1 - AC2: 'Reply on RC1', Christian Schulz, 01 Aug 2024
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CC1: 'Comment on wes-2024-46', Andrew Goupee, 18 Jun 2024
This reviewer finds this manuscript by C.W. Schulz et al. to be of interest to the FOWT community. The article is also fairly well written and organized.
That said, this reviewer has several comments the authors may wish to consider addressing in a revised version of the manuscript. They are as follows:
1) There is surprisingly little quantitative information on the tested FOWT system (dimensions, mass properties, mooring geometry, etc.). This would appear to make the work difficult to reproduce. There is a mention of some information in Appendix B, but very little can be found there.
2) On a related topic, there is very little specific information on the numerical modeling inputs (e.g., aerodynamic properties, hull drag coefficients, mooring stiffnesses, etc.). Again, without providing this information, the work is difficult to reproduce. Also, adding detail in the modeling inputs may help the reader better understand discrepancies observed between the tests and simulations.
3) The authors provide a fairly nice literature review, and do a good job of justifying their tank testing choices for the purposes of reducing uncertainty. However, no quantitative uncertainty information is provided for any of the measurements, nor is there any quantitative evidence provided that the approaches employed leader to less uncertainty than competing approaches. Based on the title of this manuscript, this reviewer thinks it is reasonable that a reader may expect this information to be present in the manuscript.
4) Several choices are made to reduce uncertainty in the testing (wind machine design, wind turbine reduced size, focusing on regular waves, etc.). However, these choices deviate somewhat from the physical properties and design load case environments of usual interest in FOWT design. The authors may want to better defend the choice of reducing uncertainty for these ‘off-design’ scenarios as opposed to attempting more complicated, uncertain, but more realistic tests (closer to Froude-scale turbines, active turbine control, irregular wave environments, etc.). The former may reduce uncertainty, the latter will provide data that can be used to exercise numerical models in areas more relevant for modern FOWT design. As an example, the environments considered in this work will not induce any second-order wave drift forces which can significantly impact certain FOWT dynamic responses. In addition, these are often the hardest to capture with numerical models, and as such, are of great interest in tank testing campaigns.
5) In Figure 4, the amplitude of the dynamic thrust force is approximately 1/3rd of the anticipated full scale amplitude. An explanation and derivation is provided, which is appreciated. However, is the uncertainty in this dynamic thrust force reduced by at least 1/3rd as well? If not, perhaps other approaches that can capture not only the mean thrust force, but the full-scale variation in the thrust force would be better to pursue as they better represent the desired physics and the uncertainty as a percentage of variation would be no worse than the proposed approach (consider reviewing some of the other works produced on the recent FOCAL test campaign).
6) Several qualitative descriptions of the size of the wind machine relative to the wind turbine are provided. Consider precisely quantifying the room the rotor has to move in heave/sway while still remaining in the low spatial variation, low turbulence intensity portion of the wind machine jet.
7) In Figure 6, it would be nice if the color bar variation was focused more on the rotor area; by including the pieces outside of the jet, it is hard to visually pick up on the turbulence intensity and spatial variation trends in the rotor plane.
8) The last paragraph on page 14 discusses a modeling approach where an angle of attack offset is used as a viscous correction. Is this a standard approach? If so, can a reference be provided? This reviewer has not seen this method used before in a model correlation study.
9) No information is given on conditions to reach the ‘steady-state’ responses shown in the plots, nor how many cycles are included in the plots.
10) This reviewer has seen other works that perform ‘hybrid’ simulations of mooring systems with better comparisons between experiment and simulation. The quality of the comparisons depends significantly on the numerical approach being employed in the mooring analysis (quasi-static, lumped mass, FEM, etc.) and mooring line hydrodynamic properties (e.g., transverse drag coefficient). The author M. Hall that has been referenced in this work has a good article that may be worth reviewing.
11) For the full simulations, why aren’t the actual recorded waves used as inputs? Without doing so, it is hard to understand which discrepancies are due to modeling deficiencies/test uncertainties, and which are due to incorrect model inputs. This reviewer thinks it is common practice to use the as measured wave in model correlation studies, and am surprised that this is not done here.
12) Is there a reason the results are not provided at full scale? Results are usually more intuitive when presented at full scale in this reviewer’s opinion.
13) There are some other minor issues that should be addressed: There are some widows/orphans, the paragraph indenting is inconsistent, and the figures are often not located very closely to their first mention in the text.
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.Citation: https://doi.org/10.5194/wes-2024-46-CC1 - AC3: 'Reply on CC1', Christian Schulz, 01 Aug 2024
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RC2: 'Comment on wes-2024-46', Anonymous Referee #2, 20 Jun 2024
This work has a significant scientific contribution, as it deals with a very relevant issue: the testing of floating offshore wind turbines at wave basins. It brings attention to the main uncertainties associated with this type of experiments and proposes some potential improvements on the processes particularly oriented to use of the experimental data for computational tools validation. It also shows how these proposals have been tried out in a particular experiment, showing and analysing the results.
The paper theme is well explained and is supported by referenced previous works. The description of different testing methods is presented highlighting pros and cons. The motivation and objectives of this work are presented in a clear and justified way. The experiment performed is described with detail and the results are shown including a throughout analysis. The writing of the text is clear and well structured.
In general, it is a very well-presented work of great interest due to its subject matter and the results it teaches.
Getting into the different scientific issues addressed in the paper, I have the following comments and questions:
- The wind generator is one of the key aspects for obtaining accurate results on wave basin tests of scaled down floating offshore wind turbines. Obtaining an homogeneous air flow with a wind generator on a non-controlled open space is not straightforward. Description of the wind generator is well performed, and a velocity field measurement is presented at one section.
- Have these measurements been performed using only a Prandtl probe?
- How was the turbulence been measured? Any high frequency measuring instrument, as hot wire, has been used.
- Has cross-flow (y, z directions) been measured?
- One of the main questions that arises to me is the way how the wind turbine rotor has been scaled. What I understand from the text is that the rotor is not a scaled down version of a real wind turbine rotor, but an existing model rotor designed for wind tunnel tests as described in reference Shultz 2022. Then, there are some questions about how the full-scale rotor is defined:
- It is stated that the scaling factor for the wind turbine rotor is 150. Does this mean that the full-scale rotor is a direct geometrical scale-up of the model rotor?
- It is not clear whether tip speed ratio is maintained between full-scale and experiments. It is stated that the tests are performed at a constant rotational speed and different constant wind speeds. May be stating what was this rotational wind speed at the experiments and the equivalent full-scale one would solve the question.
- It is not clear to me either how the wind speed has been scaled down. From table 1, it seems that it has been scaled down by a factor of 2 (from 10 m/s to 5 m/s). But criterium for this value is not explained.
- I don’t understand on Appendix A, the part of equation A4 that states that ulaero is determined by the ratio of aerodynamic and hydrodynamic scaling factors: ulaero = ulhidro (laero / lhydro) . May be this can be explained from the answer to previous question.
- The results show inaccuracy on the thrust measurements, specially at the case with smallest wave period. It is stated that inaccuracy would come from inertial and weight measurements, as derived from the waves only tests, but also the thrust measurement introduces some variability. Has this been more deeply analysed?
May be a thrust measurement of the rotor with the plataform fixed (to separate this from inertial and gravitational forces measurement) could be done. Also, if thrust measurements of the rotor were performed at wind tunnel at same conditions, that could be an interesting analysis, to see possible effects of the flow quality
- All the results show the surge, heave and pitch behavior of the model. Since it is a single point mooring platform, and therefore has yaw as free degree of freedom, I wonder if you have anlalysed the yaw behavior and its potential influences.
- The platform has an airfoil shaped passive yaw mechanism covering part of the tower. It is not clear whether it is completely laying on the wake of the wind generator.
- Could you confirm about this point?
- Although the influence should be small, have you been considering it on the full-scale computations?
Finally, as particular technical correction I would suggest to show in a figure the axis conventions used to derive the results in terms of motions directions (surge, heave, pitch) and forces directions as well.
Citation: https://doi.org/10.5194/wes-2024-46-RC2 - AC1: 'Reply on RC2', Christian Schulz, 01 Aug 2024
- The wind generator is one of the key aspects for obtaining accurate results on wave basin tests of scaled down floating offshore wind turbines. Obtaining an homogeneous air flow with a wind generator on a non-controlled open space is not straightforward. Description of the wind generator is well performed, and a velocity field measurement is presented at one section.
Status: closed
-
RC1: 'Comment on wes-2024-46', Anonymous Referee #1, 17 May 2024
Several improvements to the validation process aiming at the reduction of the uncertainties were proposed and evaluated in this work. The major improvements were the measurement of the rotor thrust force excluding the tower top inertia loads, a significant improvement of 10 the wind field quality in the wave tank and the utilization of hybrid simulations based on the measured platform motions. These steps were applied to wave tank tests of a FOWT utilizing a single point mooring and the subsequent validation of the numerical panel method panMARE. The improvements allowed for a considerable decrease in the random and systematic uncertainty of
the model tests and made a valuable contribution to the distinction between cause and effect regarding the deviations between measurements and simulations.This paper covers a lot of contents, so the introduction and analysis of each part is not very full. The research content of this paper is very meaningful, but it is hoped that the author can find the main expression content of the paper, accurately refine it, and condense it into a journal article with academic value. In addition, some suggestions are as follows:
1.The structure of the first and second chapters of the paper is inconsistent with the common articles. The first chapter is basically the author's statement, and the second chapter is the previous research work. In my opinion, the content of the first and second chapters should be properly integrated, and the research review of the second chapter should be used to support the views stated in the first chapter, so as to solve the sense of separation in the first two chapters of the paper.
2.In Chapter 2, so many previous works are introduced, all of which are supported by text. The authors should choose some important content to accompany the figure to illustrate.
3.The title of Chapter 3 does not summarize the content of Chapter 3 clearly, it is too concise.
4.Chapter 4: It is best to provide a layout diagram here.
5.As stated in the abstract: The major improvements: a significant improvement of the wind field quality in the wave tank. This section describes the advantages of this wind generating system, but I think the advantages should be reflected by comparing the quality of the wind generating system with that of other existing basin tests. Therefore, I think a table should be used to compare some important parameters to verify the advantages of this wind generating system.
6. Table1: What is the scale ratio of the test? What scaling criteria do periods, wave heights, and wind speeds follow?
7. The conclusion is too long, requiring extensive cuts and rigorous generalizations
Citation: https://doi.org/10.5194/wes-2024-46-RC1 - AC2: 'Reply on RC1', Christian Schulz, 01 Aug 2024
-
CC1: 'Comment on wes-2024-46', Andrew Goupee, 18 Jun 2024
This reviewer finds this manuscript by C.W. Schulz et al. to be of interest to the FOWT community. The article is also fairly well written and organized.
That said, this reviewer has several comments the authors may wish to consider addressing in a revised version of the manuscript. They are as follows:
1) There is surprisingly little quantitative information on the tested FOWT system (dimensions, mass properties, mooring geometry, etc.). This would appear to make the work difficult to reproduce. There is a mention of some information in Appendix B, but very little can be found there.
2) On a related topic, there is very little specific information on the numerical modeling inputs (e.g., aerodynamic properties, hull drag coefficients, mooring stiffnesses, etc.). Again, without providing this information, the work is difficult to reproduce. Also, adding detail in the modeling inputs may help the reader better understand discrepancies observed between the tests and simulations.
3) The authors provide a fairly nice literature review, and do a good job of justifying their tank testing choices for the purposes of reducing uncertainty. However, no quantitative uncertainty information is provided for any of the measurements, nor is there any quantitative evidence provided that the approaches employed leader to less uncertainty than competing approaches. Based on the title of this manuscript, this reviewer thinks it is reasonable that a reader may expect this information to be present in the manuscript.
4) Several choices are made to reduce uncertainty in the testing (wind machine design, wind turbine reduced size, focusing on regular waves, etc.). However, these choices deviate somewhat from the physical properties and design load case environments of usual interest in FOWT design. The authors may want to better defend the choice of reducing uncertainty for these ‘off-design’ scenarios as opposed to attempting more complicated, uncertain, but more realistic tests (closer to Froude-scale turbines, active turbine control, irregular wave environments, etc.). The former may reduce uncertainty, the latter will provide data that can be used to exercise numerical models in areas more relevant for modern FOWT design. As an example, the environments considered in this work will not induce any second-order wave drift forces which can significantly impact certain FOWT dynamic responses. In addition, these are often the hardest to capture with numerical models, and as such, are of great interest in tank testing campaigns.
5) In Figure 4, the amplitude of the dynamic thrust force is approximately 1/3rd of the anticipated full scale amplitude. An explanation and derivation is provided, which is appreciated. However, is the uncertainty in this dynamic thrust force reduced by at least 1/3rd as well? If not, perhaps other approaches that can capture not only the mean thrust force, but the full-scale variation in the thrust force would be better to pursue as they better represent the desired physics and the uncertainty as a percentage of variation would be no worse than the proposed approach (consider reviewing some of the other works produced on the recent FOCAL test campaign).
6) Several qualitative descriptions of the size of the wind machine relative to the wind turbine are provided. Consider precisely quantifying the room the rotor has to move in heave/sway while still remaining in the low spatial variation, low turbulence intensity portion of the wind machine jet.
7) In Figure 6, it would be nice if the color bar variation was focused more on the rotor area; by including the pieces outside of the jet, it is hard to visually pick up on the turbulence intensity and spatial variation trends in the rotor plane.
8) The last paragraph on page 14 discusses a modeling approach where an angle of attack offset is used as a viscous correction. Is this a standard approach? If so, can a reference be provided? This reviewer has not seen this method used before in a model correlation study.
9) No information is given on conditions to reach the ‘steady-state’ responses shown in the plots, nor how many cycles are included in the plots.
10) This reviewer has seen other works that perform ‘hybrid’ simulations of mooring systems with better comparisons between experiment and simulation. The quality of the comparisons depends significantly on the numerical approach being employed in the mooring analysis (quasi-static, lumped mass, FEM, etc.) and mooring line hydrodynamic properties (e.g., transverse drag coefficient). The author M. Hall that has been referenced in this work has a good article that may be worth reviewing.
11) For the full simulations, why aren’t the actual recorded waves used as inputs? Without doing so, it is hard to understand which discrepancies are due to modeling deficiencies/test uncertainties, and which are due to incorrect model inputs. This reviewer thinks it is common practice to use the as measured wave in model correlation studies, and am surprised that this is not done here.
12) Is there a reason the results are not provided at full scale? Results are usually more intuitive when presented at full scale in this reviewer’s opinion.
13) There are some other minor issues that should be addressed: There are some widows/orphans, the paragraph indenting is inconsistent, and the figures are often not located very closely to their first mention in the text.
Disclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.Citation: https://doi.org/10.5194/wes-2024-46-CC1 - AC3: 'Reply on CC1', Christian Schulz, 01 Aug 2024
-
RC2: 'Comment on wes-2024-46', Anonymous Referee #2, 20 Jun 2024
This work has a significant scientific contribution, as it deals with a very relevant issue: the testing of floating offshore wind turbines at wave basins. It brings attention to the main uncertainties associated with this type of experiments and proposes some potential improvements on the processes particularly oriented to use of the experimental data for computational tools validation. It also shows how these proposals have been tried out in a particular experiment, showing and analysing the results.
The paper theme is well explained and is supported by referenced previous works. The description of different testing methods is presented highlighting pros and cons. The motivation and objectives of this work are presented in a clear and justified way. The experiment performed is described with detail and the results are shown including a throughout analysis. The writing of the text is clear and well structured.
In general, it is a very well-presented work of great interest due to its subject matter and the results it teaches.
Getting into the different scientific issues addressed in the paper, I have the following comments and questions:
- The wind generator is one of the key aspects for obtaining accurate results on wave basin tests of scaled down floating offshore wind turbines. Obtaining an homogeneous air flow with a wind generator on a non-controlled open space is not straightforward. Description of the wind generator is well performed, and a velocity field measurement is presented at one section.
- Have these measurements been performed using only a Prandtl probe?
- How was the turbulence been measured? Any high frequency measuring instrument, as hot wire, has been used.
- Has cross-flow (y, z directions) been measured?
- One of the main questions that arises to me is the way how the wind turbine rotor has been scaled. What I understand from the text is that the rotor is not a scaled down version of a real wind turbine rotor, but an existing model rotor designed for wind tunnel tests as described in reference Shultz 2022. Then, there are some questions about how the full-scale rotor is defined:
- It is stated that the scaling factor for the wind turbine rotor is 150. Does this mean that the full-scale rotor is a direct geometrical scale-up of the model rotor?
- It is not clear whether tip speed ratio is maintained between full-scale and experiments. It is stated that the tests are performed at a constant rotational speed and different constant wind speeds. May be stating what was this rotational wind speed at the experiments and the equivalent full-scale one would solve the question.
- It is not clear to me either how the wind speed has been scaled down. From table 1, it seems that it has been scaled down by a factor of 2 (from 10 m/s to 5 m/s). But criterium for this value is not explained.
- I don’t understand on Appendix A, the part of equation A4 that states that ulaero is determined by the ratio of aerodynamic and hydrodynamic scaling factors: ulaero = ulhidro (laero / lhydro) . May be this can be explained from the answer to previous question.
- The results show inaccuracy on the thrust measurements, specially at the case with smallest wave period. It is stated that inaccuracy would come from inertial and weight measurements, as derived from the waves only tests, but also the thrust measurement introduces some variability. Has this been more deeply analysed?
May be a thrust measurement of the rotor with the plataform fixed (to separate this from inertial and gravitational forces measurement) could be done. Also, if thrust measurements of the rotor were performed at wind tunnel at same conditions, that could be an interesting analysis, to see possible effects of the flow quality
- All the results show the surge, heave and pitch behavior of the model. Since it is a single point mooring platform, and therefore has yaw as free degree of freedom, I wonder if you have anlalysed the yaw behavior and its potential influences.
- The platform has an airfoil shaped passive yaw mechanism covering part of the tower. It is not clear whether it is completely laying on the wake of the wind generator.
- Could you confirm about this point?
- Although the influence should be small, have you been considering it on the full-scale computations?
Finally, as particular technical correction I would suggest to show in a figure the axis conventions used to derive the results in terms of motions directions (surge, heave, pitch) and forces directions as well.
Citation: https://doi.org/10.5194/wes-2024-46-RC2 - AC1: 'Reply on RC2', Christian Schulz, 01 Aug 2024
- The wind generator is one of the key aspects for obtaining accurate results on wave basin tests of scaled down floating offshore wind turbines. Obtaining an homogeneous air flow with a wind generator on a non-controlled open space is not straightforward. Description of the wind generator is well performed, and a velocity field measurement is presented at one section.
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