Hi everyone, I am modelling a floating wind turbine (12 MW) and trying to compare OpenFAST results with model test (scale 1:60). I need some help in generating the WAMIT data using AQWA. The case I am modeling is a current only case. The numbers and figures that I provide below are all in full scale for ease of understanding.

Below is the 6Dof that I obtained for current only case (0.82m/s), where the surge has a very large period. From model test, the surge is around 4.8 m fluctuating between 4.3-5.3m. Actually the OpenFAST simulation crashed at a bit over 9000 s and fluctuation of 6DOF is getting bigger and bigger. Here only 6Dof is on in ElastoDyn, I am using hybrid model of potential flow and strip theory, WAMIT data is generated in AQWA and members of the floater are given in HydroDyn. If I simulate potential flow only, the second figure is obtained, where the extreme small frequency is obvious and sway and surge are oscillating violently.

In AQWA, I split the floater at water level, and added point mass, cog and moment of inertial. The simulation is done in hydrodynamic diffraction with wave [-180, 180] for every 10 deg, 99 intervals of frequency in [0.002, 0.8] Hz. Mass difference between displaced volume and input mass is around 4%.

I have some doubts:

- I am modeling the wind turbine and floater in model scale by scaling down the dimension, mass and forces with scaling factor, as I am trying to use the same setup as the model test, is there any issue to model it in model scale with OpenFAST?
- To get the added mass, damping matrices and wave excitation forces, is it right just to simulate floater alone (without tower and RNA), give mass of the whole system, COG of the system in the global coordinate (intersection of tower center at water level), and moment of inertial of the floating system including turbine?
- If anyone has experience with conversion from AQWA to FAST, please advice. When I read AQWA to BEMRosseta and save as .hst, .1 and .3 file, I found that all the values in the converted files are different from the output of AQWA, e.g. there is a scaling difference of rho*g in hydrostatic restoring matrix, and rho in added mass. I had to manually scale the values back. Did I miss any step before I save them to WAMIT files in BEMRosseta?
- May I understand it this way for this case, the potential flow mainly contributes to the frequency of the floater, the magnitude of displacement is mainly from the viscous drag from strip theory? If the extreme small frequency 0.000493Hz of surge is not right (the smallest frequency from model test for surge is 0.0116Hz), then the problem is from WAMIT data not strip theory?
- what can be done to eliminate the small frequency and correct the large fluctuation of surge?

Any suggestions and comments are greatly appreciated, I can post the input parameters if required.

Many thanks,

Xiaoqin