# Platform motion vs. the forces

**URL:** <https://forums.nlr.gov/t/platform-motion-vs-the-forces/4866>\
**Category:** Computer-Aided Engineering Software Tools\
**Created:** [April 25, 2023, 7:48am UTC](https://forums.nlr.gov/t/platform-motion-vs-the-forces/4866 "2023-04-25T07:48:23Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Xiaoqin.Zhang](https://avatars.discourse-cdn.com/v4/letter/x/ba9def/32.png) [@Xiaoqin.Zhang](https://forums.nlr.gov/u/Xiaoqin.Zhang)\
**Post date:** [April 25, 2023, 7:48am UTC](https://forums.nlr.gov/t/platform-motion-vs-the-forces/4866/1 "2023-04-25T07:48:23Z")

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Dear Jason,

I am simulating six DOF motion of a semisubmersible FOWT under wind spectrum with mean wind speed 57 m/s, irregular wave and current. The wind turbine is not operating, nacelle yaw is -20 degrees (20 deg clockwise in top view), blades are feathered to 86 degrees.

AeroDyn: WakeMod =0, AfAeroMod=1  
ElastoDyn: only six DOF motion enabled  
ServoDyn disabled  
Platform reference point is at COG (0.29 m above SWL)

Below is the list for six DOF, aero forces and hydro forces for measurement (wind+wave+current), OpenFAST (wind+wave+current) and OpenFAST (wind only). I got roll and yaw in opposite signs with measurement. I have trouble to understand how come roll and yaw are negative from OpenFAST given Mx and Mz from aero and hydro are all positive. Is there any other forces that contribute to the six DOF motion?

For my other cases when the wind turbine is in operation, six DOF motion and thrust can be well predicted by OpenFAST, but not for this case, especially for roll and yaw. Any suggestions or comments are greatly appreciated.

 ![image](https://europe1.discourse-cdn.com/nrel/original/2X/6/6c449ded5cec04e252652e8923979fabdbb493fe.png)

Best regards,  
Xiaoqin

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**Author:** ![Jason.Jonkman](https://dub1.discourse-cdn.com/nrel/user_avatar/forums.nlr.gov/jason.jonkman/32/3086_2.png) [@Jason.Jonkman](https://forums.nlr.gov/u/Jason.Jonkman)\
**Post date:** [April 25, 2023, 12:41pm UTC](https://forums.nlr.gov/t/platform-motion-vs-the-forces/4866/2 "2023-04-25T12:41:07Z")

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Dear @Xiaoqin.Zhang.

I’m not sure why OpenFAST is not matching your measurements, but from your applied aerodynamic loads, I’m guessing the negative floater roll is driven by the large `RtAeroFyh` force (which is likely large because of the nacelle yaw). The negative floater yaw is also driven by `RtAeroFyh`, as well as the thrust (`RtAeroFxh`) combined with the moment arm associated with negative floater roll.

Best regards,

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**Author:** ![Xiaoqin.Zhang](https://avatars.discourse-cdn.com/v4/letter/x/ba9def/32.png) [@Xiaoqin.Zhang](https://forums.nlr.gov/u/Xiaoqin.Zhang)\
**Post date:** [April 27, 2023, 7:34am UTC](https://forums.nlr.gov/t/platform-motion-vs-the-forces/4866/3 "2023-04-27T07:34:18Z")

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Dear Jason,

Thank you for your valuable comments.  
RtAeroFyh \* hubheight is indeed larger than RtAeroMxh+HydroMxi, it can explain the negative roll. Considering the presence of roll and pitch, calculation shows that there is negative Mz, thus negative yaw.

Regards,  
Xiaoqin