# hydrodynamics

dear jonkman sir,
i as modeling modeling the NREL 5 mw OC3 Hywind to get the added mass, damping, excitatation force and hydrostatic stiffness but i was confused how would you got these from wamit as it can solve only for radiation-diffraction panel method but the spar is slender member and it comes under the morison region.

kindly help me out with your valuable suggestions.
Thanking you

Dear Neeraj,

I’m not really sure I understand your question, but WAMIT can solve for the hydrodynamic loads from both wave excitation and body motion except for viscous effects. (WAMIT captures potential-flow effects and neglects quadratic drag.) In FAST, we augment the hydrodynamic loads from WAMIT with viscous drag from Morison’s equation.

I hope that helps.

Best regards,

Dear jonkman sir,
Thanks for your reply but i wanted to know how you augmented viscous effects seprately in FAST by morison equation and does that not affect the added mass and damping which is previously calculated by wamit from wave excitation and body motion. Also as i am modeling this oc3 spar support platform in ANSYS AQWA, there also added mass and damping in the translational mode is matching but in the rotational modes it is coming different.
sir i wanted to know why this is so as i am giving the same geometrical properties which are listed in your report as:
spar mass(including ballast) = 7466330 kg
cg below MSL = -89.915 m
M.O.I about roll axis = M.O.I about pitch axis = 4229,230,000 kgm^2
M.O.I about yaw axis = 164,230,000 kg m^2
As this hydrodynamic coefficients are not matching I doubt in the spar maas whether this mass 7466330 kg is including the mass of wind turbine and moorings and the COG postion of the complete system is what if it is different fro 89.915m.
kindly help me out with your valuable suggestion sir i wll be very thankful to you please.

Dear Neeraj,

FAST augments the potential-flow loads with viscous effects by taking only the viscous drag term from Morison’s equation and superimposing this load with the added mass, radiation damping, hydrostatic, and wave-excitation loads from potential flow. This should impact the damping, but not the natural frequencies of the response.

The masses, inertias, and CG you report are correct for the OC3-Hywind spar alone, without the tower, nacelle, rotor, or mooring system. However, while the system response certainly depends on the mass/inertia of the system, the added mass and damping coefficients do not. My guess is that you are not referring the rotational modes to the same reference point. When NREL derived the added mass, damping, and wave-excitation loads for the OC3-Hywind spar, we used WAMIT, which references the rotational modes relative to the centerline of the body at mean sea level. What is the reference point for the ANSYS/AQWA results?

Best regards,

Dear jonkman sir,
I am vey thankful to you for your valuable suggestion this will definitely help me.
The ANSYS-AQWA reference point is about the COG postion of the system.

Thanking you the most.

Now I can obtain the results relative to the COG, and I want to know the results relative to the centerline of the body at mean sea level. How to convert between these two results?

Dear Yaqi,

I’ve provided the transformations from the center of gravity (COG or CG) to the tower centerline at still water level (SWL) in my May 04 and May 06, 2015 posts in the following forum topic: http://forums.nrel.gov/t/oc3-hywind-raos/1085/1.

Best regards,

Dear all,
I am using the NREL-MIT TLP 5 MW turbine to perform a fatigue analysis.
After inspection on the WAMIT files for the HydroDyn (.hst file), I realized that he restoring matrix considers only the hydrostatic restoring force for the heave roll and pitch dofs.
In addition, I see in the primary hydrodyn data that the AddF0 and AddClin matrices are zero as well.
My question is, does this particular model consider any mooring pre-tension and if yes where? If not, how should I included to my input files?
Sould I do that to the AddClin and AddF0 in HydroDyn primary input or in the Fx Fy Fz in MAP+ iput file?
Finally, should I include the surge and sway K11=K22=Tt/lo restoring force components to the AddClin?

Dear Georgios,

The FAST model of the MIT/NREL TLP available as Test23 in the CertTest directory of the FAST archive is complete as is. You shouldn’t need to change anything (including AddF0 and AddCLin) to match the specifications of the MIT/NREL TLP. The tendons, including pretension and surge/sway restoring, of the MIT/NREL TLP are fully defined by the MAP++ module. The *.hst file only contains the buoyancy contribution to hydrostatic restoring.

I hope that helps.

Best regards,

Dear Jason Jonkman
That was really helpful thank you very much.
In addition I speculate that the second order effects might be important for a TLP hydrodynamic analysis since they might undergo resonant heave oscillations with periods of a few seconds which might be excited by nonlinear wave effects. However I cannot find the WAMIT output files with the .7,.8,.9… that are needed from the HydroDyn so as to consider the above effects. Do you think I could find those files somewhere?