# Output hydrodynamic forces

Dear sir,

I have two questions about the hydrodynamic forces calculated in FAST for floating offshore wind turbine.

1. I want to output the individual contribution of hydrodynamic forces, eg. diffraction force, added inertia force, hydrostatic force, and radiation damping force. But I didn’t find the channel in FAST for this individual output.
I tried to use HydroDyn as a standalone code, but I think I cannot get the added inertia force and radiation damping force since the motion of platform is not considered in HydroDyn. Is it right ?
So, what is your suggestion if I want to output those individual contribution of hydrodynamic force ?
2. About the impact of Substructure motions on loads explained in HydroDyn_Manual 6.3.
As the manual said, When HydroDyn coupled to FAST, the hydrodynamic loads by HydroDyn are applied to the displaced position of the substructure, but are based on wave kinematics at the undisplaced position.
Does it mean FAST always use wave kinematics at the undisplaced position to calculate hydrodynamic force no matter where the platform is after motion ? If this is the case, I think wave kinematics is the function of position, is it reasonable to ignore the effect of motion even the motion of platform is relative small (small enough)?
I wish someone could help me fix this problem or give me some suggestions.
Thanks a lot.
``Shining.Zhang``

Dear Shining.Zhang,

In terms of your first item, the HydroDyn manual, https://wind.nrel.gov/designcodes/simulators/hydrodyn/HydroDyn_Manual.pdf, Appendix C has a complete list of available HydroDyn outputs.

There are channels for all the various load contributions due to potential flow theory as well as strip-theory. In addition, you can output the total integrated hydrodynamic loads from both potential flow and strip theory at the WRP using: HydroFxi, HydroFyi, HydroFzi, HydroMxi, HydroMyi, HydroMzi

With regards to your second question: “Does it mean FAST always use wave kinematics at the undisplaced position to calculate hydrodynamic force no matter where the platform is after motion ?”

Greg Hayman

Dear Greg Hayman,
One more thing, I know that FAST can output the viscous drag force acting on the nodes of prescribed element in MEMBER OUTPUT LIST Table.
I wonder is it possible to output the total viscous drag force acting on platform(WRP) at once ?

``````Best regards,
Shining.Zhang``````

Dear Shining,

You can output the total of all hydrodynamic loads at the WAMIT reference point (WRP) from HydroDyn, as indicated by Greg (via outputs HydroFxi, HydroFyi, HydroFzi, HydroMxi, HydroMyi, HydroMzi). These will include the viscous-drag loads, as well as other hydrodynamic loads summed together. At this time, it is not possible to output the total contribution of viscous drag at the WRP distinct from the other hydrodynamic loads. (But you can see the contribution of distinct hydrodynamic load components at the node level.)

Best regards,

Dear Jason,

``````Thanks for you kind help.
1) I wonder if it is reasonable to get the total viscous drag force as follows:
Fd=HydroF-WavesF-RdtnF-HdrStcF.
Fd is the total viscous drag force at WRP.
2) One more thing. I know RdtnF accounts for radiation damping force and Added mass force in total.
From the HydroDyn manual P36. Whether the sign '-' is missing in the general form of the convolution integral for radiation damping force calculation? Or the sign '-' is actually considered when outputing the Rdtnf as follows:
RdtnF=F_AM-F_RD.