Towing Analysis and Validation of a Fully Assembled Floating Offshore Wind Turbine Based on an Experimental Study

Round 1

Reviewer 1 Report

The research was meaningful, however there were still several issues need to be noticed.

(1) The unsolved problems of previous researches are not clearly introduced in the introduction section.

(2) Compared to the existent research, the innovation of this paper is not claimed clearly.

(3) The results and conclusions should be more focused.

Author Response

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Reviewer 2 Report

The manuscript is well written, with a clear message delivered to the reader. The abstract resumes the important points of the work: general and specific background, methods, key results, though key concrete values are missing. Literature review is complete and up to date. Adopted materials and solvers adopted are well explained, results and related discussion are clearly given.

The state of art is comprehensive. I would suggest considering and discuss some recent reports, e.g.:

10.1016/j.oceaneng.2023.116064

10.5194/wes-8-1659-2023

At the end of the Introduction, after declaring the manuscript's novelty, I would recommend stating its structure, e.g., In Section 2, the experimental setup is described…

Figure 9. Towing force seems to be quadratic with speed, or at least, more than linear: a discussion is welcomed.

Table 8. Natural periods are giving of the order of seconds. Why not a greater precision?

LINE 418. Potential solver gives good results for low towing speeds. Full CFD approaches could be taken into account to predict RAOs for high speeds as well. Further insights are welcomed.

Conclusions support the obtained results.

DOIs are encouraged in the References. “World’s largest floating offshore wind farm under tow” is double cited.

Author Response

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Reviewer 3 Report

The manuscript must be rewritten. The sizes of the real structure and the experimental setup must be distinct. In chapter 2, which deals with experiments, the values given in the text refer to the real structure and not to the model. The scale of the model affects both experimental and numerical results and this is not clear in the manuscript.

Figure 1 refers to the 10 m dimension in the lower right figure. Give an explanation of its usefulness. Also the Oxyz is needed.

Table 1. Give an explanation for L. The Kxx, kyy, kzz, and Ixx, Iyy and Izz are from the centre of gravity of the structure or from the SWL?

Figure 2. How you calculate the GZ curve? Experimentally or by using a software. If you the GZ curve is calculated via experiments are also needed and comparisons with i.e. Tribon, MaatHYDRO or similar software’s for the hydrostatic calculations.

2.1.1. Determination of towing draught.

This chapter is sub-chapter of “2. Experimental Configuration”, but here you use values from the real structure and not from the model. Something is missing here. Please place this chapter where the real structure is referenced and not the model.

The stability criteria are given by… Please give references for these criteria.

2.3. Towing setup and configurations

Please change with platform (plat-form).

2.5. Test campaign

“speeds ranging from 0.514 m/s to 3.086 m/s”. For these values give the Froude, Strouhal and Reynolds Numbers.

Also for the velocities of 1.029, 2.058, and 2.572 m/s give the Froude, Strouhal and Reynolds Numbers.

The tests covered a range of speeds, 0.514, 1.543, 2.572 and 3.086 m/s and encompassed significant wave heights of 1, 2, 4 and 6 meters. For higher speeds, tests were 243

conducted for significant wave heights of 1, 2 and 4 meters only.

These values are for the real structure and not for the experiment.

3.2.1. Towing forces and added resistance in waves

With the term “added resistance” do you mean the mean second order wave drift forces? If this is the case, the Struhal number should be checked, which should be much less than unity. Has such a check been made? If this is not the case, you must change the view that has been taken in the programs and see what happens when the body moves with forward speed (resistance case, i.e. Maxsurf).

Appendix B

The code Seacal removes the irregular frequencies (B6, b7)?

Give an explanation between the big differences between Seacal – Orcaflex and experiments in appendix B.

Also, Near to 0.4 rad/s there is a cancelation frequency that only appears in Seacal. Maybe more omegas are needed in the Orcaflex simulations near 0.4.

More details for the Secal and Orcaflex are needed (number of elements, min-max omegas, step of omega etc.). More details about the theory behind the wave and (small?) forward speed are needed.

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Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Table 1. Give an explanation for L. Is the characteristic diameter or radius from a cylinder or the total length of the structure (see next comment for Fn)?

The numerical tool BEMRosetta does not calculate the GZ curves. Is that correct? Please write the correct numerical tool that you use in this manuscript for the calculation of the GZ curves.

https://mural.maynoothuniversity.ie/16257/1/C401EW21IZ.pdf

Table 2. For the calculation of the Fn. You use L=12m (the characteristic Diameter of the cylinder of the floating structure and not L=63.7m – see Table 1). Is that correct? Make a comment and give some references here. Also, explain the calculation of Reynolds number.

Table 2. You write that the Strouhal number is 0.2 in all cases. The Strouhal number equals to τ=ωU/g. Ηow is it possible for the Strouhal number to be the same in all cases while it depends on ω?

Appendix A

Why didn't you use roughly the same discretization in both programs? That may account for the differences between the results in the RAO's comparisons between the two programs.

Did you do a convergence check of the results for the grids you used?

Appendix C

 Did you check the mean second-order drift forces (for the fixed body) to see if the grid converges? The reviewer suggests that this comparison be made to somehow clarify the differences between the two programs.

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Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Generally the towing of floating structures is done at low forward speed. Therefore the slow-drift motions and slow drift forces of the structure require special attention and study.

It is not clear to the reviewer whether the authors have understood the difference in methodology and theory applied to solve the hydrodynamic problem for a body moving at forward speed and for a body moving at low forward speed or, equivalently, the interaction of the body with a sea current. These issues have been dealt with in detail by:

1.      Chen X.B. and Malenica, S. Uniformly valid solution of the wave-current-body interaction problem, Proceedings, 11th International Workshop on Water Waves and Floating Bodies, Hamburg, Germany, 1996.

2.      Emmerhoff, O.J and Sclavounos, P.D. The slow drift motion of arrays of vertical cylinders, Journal of Fluid Mechanics, 1992 (242), 31-50.

3.      Faltinsen, O. M. Wave and current induced motions of floating production systems, Applied Ocean Research, 1994, Vol. 15 pp.351-370.

4.      Grue, J. and Palm, E. The mean drift force and yaw moment on marine structures in waves and current Journal of Fluid Mechanics, 1993 (250), 121-142.

5.      Grue, J. and Biberg, D. Wave forces on marine structures with small speed in water of restricted depth, Applied Ocean Research, 1993(15).

6.      Kinoshita, T. and Bao, W. Hydrodynamic forces acting on a circular cylinder oscillating in waves and a small current, Journal of Marine Science and Technology, 1996(1), 155-173.

7.      Matsui, T., Lee, S.Y. and Sano, K. Hydrodynamic forces on a vertical cylinder in current and waves, J. Society of Naval Architects Japan, 1991 (170) 277-287.

8.      Mazarakos TP, Mavrakos SA. Wave–current interaction on a vertical truncated cylinder floating in finite-depth waters. Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment. 2013; 227(3):243-255.

9.      Nossen, J., Grue, J., Palm. E. Wave forces on three dimensional floating bodies with small forward speed, Journal of Fluid Mechanics, 1991 (227), 135-160.

10.  Zhao, R., Faltinsen, O.M. Interaction between current, waves and marine structures, Proceedings, 5th International Conference on Numerical Ship Hydrodynamics, Hiroshima, Japan, 1989, 87-99.

The reviewer's opinion is that if only the experiments were presented, the publication would be accepted because the experimental investigation is truly original.

The theoretical background is weak.

Comparisons of experimental and numerical results are not necessarily comparable because the limits to which numerical codes are accurate are not specified.

For example ANSYS - AQWA (wave and small forward speed) describes that the reliability of numerical results is for Fn<0.3.

Do the programs used have a limit?

Is this limit met in your comparisons with the numerical results and if so, what are the limits?

More references for the wave and small forward speed are needed (see above references 1-10).

More references for the wave- current interaction are needed (see above references 1-10).

Introduction

“particular forward speed” or small forward speed?

“ships moving with a forward speed” or small forward speed?

“to the second intercept.;” please formulate differently.

“green water and slamming loads can be avoided”. Give an explanation what happens for the air gap.

For the BEMRosetta:

You use:

Y axis rot. (pitch) from 0 to 150 deg, delta 1. Plane XZ rot. Around Z (yaw) from 0 to 90 deg. With delta 90. BEMRosetta plots two curves the GZ=0 and the GZ= 90. Which one have you used in the publication and why?

To do the parametric analysis of the GZ you changed the depths. How did you do that? Through BEMRosetta or by using a CAD program?

Write more information about the grid convergence as you did in your answer to the reviewers (5372, 3072, 1972).

Coming back to the Strouhal number:

Making use of the relation τ=ωU/g (valid for values much smaller than unity, about ¼=0.25, see above references 1-10). I note that:

For U=0.514m/s, τ=[5.2*10^-3(ω=0.1rad/s), 0.07(ω=1.4rad/s)] pass

…

For U=3.086m/s, το τ=[0.03(ω=0.1rad/s), 0.44(ω=1.4rad/s) ] no pass

Similarly, a check must be made for all values of ω and all speeds.

I believe that for those frequencies where τ>1/4 the results should not show up on the graphs.

Also Fn=0.284 is marginally acceptable.

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Author Response

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Author Response File: Author Response.pdf