Hello everyone,

I construct a spar type floated wind turbine model with multibody formula. I am comparing the response of my model with response in openfast to make sure the numerical model is correct. I have some questions.

- Body reference point offset

The origin of the coordinates system (CS) is 78.01m underwater, so I make some changes in the fast as below:

**-78.01 PtfmRefzt** - Vertical distance from the ground level [onshore] (in ElastoDyn.dat file);

**78.01 PtfmRefzt** - The zt offset of the body reference point(s) from (0,0,0) (meters) ( in HydroDyn.dat file);

I use moordyn to calculate mooring force in my model. I check the moordyn document(Input Files — MoorDyn 2.1.0 documentation) and found the information as below:

X, Y, Z – Coordinates of the point (relative to inertial reference frame if “fixed/point/free”, or relative to platform/body reference frame if “vessel” or “body#”).

I change the paraments as below:

---------------------- CONNECTION PROPERTIES --------------------------------

Node Type X Y Z M V FX FY FZ CdA CA

(-) (-) (m) (m) (m) (kg) (m^3) (kN) (kN) (kN) (m^2) (-)

1 fixed 853.87 0.0 **-241.99** 0 0 0 0 0 0 0

2 fixed -426.94 739.47 **-241.99** 0 0 0 0 0 0 0

3 fixed -426.94 -739.47 **-241.99** 0 0 0 0 0 0 0

4 vessel 5.2 0.0 **8.01** 0 0 0 0 0 0 0

5 vessel -2.6 4.5 **8.01** 0 0 0 0 0 0 0

6 vessel -2.6 -4.5 **8.01** 0 0 0 0 0 0 0

But it doesn’t help, so the question is that how should I deal with the moordyn system?

- Morison formula

I calculate the hydrodynamic loads with Morison formula, so I reset some parameters of HydroDyn.dat in openfast as below:

**0 PotMod**

1 ExctnMod

1 RdtnMod -

60 RdtnTMax

0.02 RdtnDT

1 NBody

1 NBodyMod

“…/5MW_Baseline/HydroData/Spar” PotFile

1 WAMITULEN

0.0 PtfmRefxt

0.0 PtfmRefyt

**78.01 PtfmRefzt**

0.0 PtfmRefztRot

8029.21 PtfmVol0

0.0 PtfmCOBxt

0.0 PtfmCOByt

I couldn’t make sure the changes are correct, and the other question is that when **PotMod** is set to be **0**, the PLATFORM ADDITIONAL STIFFNESS AND DAMPING are not used. I don’t use the PLATFORM ADDITIONAL STIFFNESS AND DAMPING either in my model, however the response of yaw become very large, do you have some suggestions on how to deal with the stiffness for yaw degree of the platform?

Thank you in advance

Best regards