This paper presents the verification of a newly developed numerical framework that couples the aero-hydro-servo-elastic simulation tool OpenFAST for offshore wind turbines with the open-source CFD package OpenFOAM. In this framework, OpenFOAM is used to perform high-fidelity hydrodynamic CFD simulations of the floating platform while OpenFAST models the aerodynamic loads on the rotor along with the flexible tower and blades. The control of the turbine is also integrated in the simulations through OpenFAST. This framework has great potential in the design and analysis of floating offshore wind turbine systems. However, some additional clarification and investigation are suggested to render the verification presented more convincing. Please see a list of comments and suggestions below:

1. In Table 1, what does “deterministic” mean in the load case description?
2. On page 7 line 190, please add a comma before “and two flexible modes…”
3. Were wave forcing/damping zones included in the OpenFOAM/OF2 simulations?
4. Please provide the total number of cells used in the OpenFOAM/OF2 simulations.
5. In Figure 5, it looks like the CFD mesh does not have a boundary-layer region next to the floater surface. What is the boundary condition on the platform surface? Free slip?
6. In Figures 7 and 8, the agreement between the two approaches for LC 3.1* appears acceptable. However, the results for LC 3.1 with waves show important differences. Apart from the opposite initial transient motion in surge, which the authors already pointed out and should be investigated further, there is also a large phase shift in the periodic steady-state motion. This is somewhat concerning because the same wave time series was used in both simulations. The authors are encouraged to investigate what is causing the phase shift.
7. Between lines 245 and 250 on page 11, the authors state that the mean displacements of the platform are similar between the two approaches; therefore, the mean drift load is consistently captured. In my opinion, the mean displacement observed in this case is primarily driven by the wind. In fact, Figure 8b shows the two methods giving rather different mean hydrodynamic force in surge. It’s also surprising that OpenFAST appears to show a negative mean surge force, opposite of what’s expected.
8. The OF2 results for pitch motion and moment appear to be slowly decaying, whereas the pure OpenFAST results do not. I’m curious if there is any potential explanation for this.
9. Is the larger fluctuation of rotor speed and generator power observed in the pure OpenFAST results simply a consequence of the larger tower-top motion?
10. While the hybrid approach presented in this paper is definitely more computationally efficient compared to full CFD simulations, the comparison of computing time needs to be treated with care because it heavily depends on the targeted level of numerical resolution and convergence. For example, Tran and Kim (2016) reported the use of a prism-layer mesh on the platform surface to help resolve the boundary layer. It can therefore be argued that the CFD simulations of Tran and Kim have a higher level of fidelity compared to the present study. The authors are encouraged to include discussions on these caveats in the comparison of CFD computing time.

This paper develop a new tool coupling OpenFAST and OpenFOAM where the hydyodynamics and dynamic reponses of platfrom is conisdered in OpenFOAM and the aerodynamics of blade and controller in simulated in OpenFAST, to obtain the high-fidelity results, especially the nonlinear hydrodynamic loads. I believe this paper more or less makes contribution to the community. However, there are some technical issues must be addressed to increase the quality of the paper. See the attached file.