Dear Carlos Silva de Souza,

Thank you for taking the time to review the manuscript and provide useful feedback.

We have rewritten part of the manuscript to address the comments made during this review process.

Attached you can find a PDF with replied to the comments.

The revised version of the manuscript where these comments are included will be uploaded soon.

Dear Carlos Silva de Souza,

Thank you for taking the time to review the manuscript and provide useful feedback.

We have rewritten part of the manuscript to address the comments made during this review process.

Attached you can find a PDF with replied to the comments.

The revised version of the manuscript where these comments are included will be uploaded soon.

Dear Referee #2,

Thank you for taking the time to review the manuscript and provide constructive feedback. This helped us to have a more complete and comprehensive manuscript.

In Section 3 (Participants and Modeling Approach), we have included in the new manuscript a description about how the buoyancy force was modeled by the different participants:

Some participants used a linear hydrostatic stiffness while others used a nonlinear hydrostatic stiffness. Participants using the PF theory (NU) and some participants using a hybrid approach (BVMO), included a linear hydrostatic stiffness computed at the undisplaced platform position. Some participants using the hybrid approach (EDF, TUHH) or the strip theory (DUT1, DUT2), computed the nonlinear hydrostatics at the instantaneous platform position up to the mean sea level. Most participants using the strip theory (CSSC, DNV, DTU, GDG, IFE, MAR1, NREL, PRI, SHELL, UPC, W&UG) and CFD (MAR2) computed the nonlinear hydrostatics at the instantaneous platform position up to the wetted surface.

We agree that there might be a problem with the numerical model used by DUT2-SIMA. According to the description provided by the participant, they use a Morison equation approach. It is not a hybrid model (i.e., there is no potential flow body). Similar issues can be observed for DUT2-SIMA for the fairlead 2 tension and the keel line tensions.

Regarding the high structural responses for the tower, in the original manuscript there were some patricipants (e.g., GDG) with large responses in the first tower bending mode. Many other participants obtained similar tower responses along the project. We understand that this is a limitation of Morison-only models. Our understanding is that in the relatively high frequency of the first tower bending mode (around 0.4 Hz), the Morison approach is able to excite the floating system with a loading that is not realistic. Most participants were able to obtain the expected loads after low-pass filtering the irregular wave spectrum (recommended practive DNV-RP-C205) or applying the MacCamy and Fuchs diffraction correction of the inertia coefficient. For reference, the definition document that the participants used to build up the numerical models includes information about the structural damping for the tower (obtained from hammer impact tests).

For the high response of the mooring line tensions (e.g., DUT2-SIMA), this could be a problem with the structural damping of the line or lack of hydrodynamic damping (i.e., drag coefficient). We have added one paragraph in the conclusions to summarize the issues that have been commented along the manuscript:

It is important to note that it was the first time that project participants built numerical models for this novel floating support structure design. Due to the multi-physics nature of the system, including aerodynamics, hydrodynamics and structural dynamics, participants had to use a coupled aero-hydro-elastic approach. The different participants defined a modeling approach according to the capabilities of the code(s) used. Setting up these numerical models was quite challenging and prone to user error. This is evidenced by the fact that some participants used the same code and a similar modeling approach but obtained different system responses.

 

Regarding the cases with wind, in the experimental campaign there was no load sensor at the tower top. Only one triaxial accelerometer was installed at tower top. The wind in the wave basin was measured during the calibration process by means of an ultrasonic anemometer (near the hub height location) and one hotwire anemometer (located upwind, slightly at one side and lower than the hub height). During the testing, the ultrasonic anemometer was removed, but the hotwire was still in place to ensure that the wind generated by the fans was the expected one. For the numerical models, participants considered spatially uniform unsteady winds based on the measured hub-height wind speed in the X-direction. This means that they took the recorded wind at the hub height and assumed that the same wind was applied over the whole rotor plane. For reference, the turbulence intensity during the testing was quite low.