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 ?”

The answer is simply, yes.

I hope this addresses your questions/concerns.

Greg Hayman

Dear Greg Hayman,
I got your point. Thanks for your kind help.
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 ?
I will appreciate for your any help and comments.

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.
  F_AM is the added mass loads.
  F_RD is radiation damping force.

 Best regards,
 Shining.Zhang

Dear Shining,

Here are my answers to your questions:

  1. Yes, that would be a reasonable calculation if you have a hybrid model where you don’t have any platform additional stiffness/damping and the only loads calculated at the nodes of the strip-theory solution are viscous-drag loads. (For a hybrid model, the strip-theory solution can also include the loads from fluid-filled (ballasted) members.)

  2. Yes, you are correct. I see that the minus sign is missing from the right-hand side of F_RD. (I checked the code and it is correct, but the manual is wrong.) The warning placed at the beginning of Chapter 7 in the draft “HydroDyn User’s Guide and Theory Manual” should be taken seriously :slight_smile:: “This is a preliminary draft of the HydroDyn theory and should be considered a work in progress.

Best regards,