HydroDyn lumped loads

Dear all,

First of all, I thank you for the possibility to use such a great tool as FAST.
I spent many weeks looking into the manuals and into the blog topics for an answer to my question but I couldn’t find it and for this I need to ask what I hope it’s not a stupid question.
I’m a civil engineer currently working on his MSc thesis regarding integrated analysis of offshore wind turbines on the OC4 deep water jacket substructure founded on suction bucket foundation. The thesis is focused on the effect of the soil-structure interaction.
In order to study this I worked with the commercial software DIANA FEA with a specified subroutine to model the foundation as a lumped node.

Now, as DIANA FEA environment doesn’t allow me to define wave and current loads in a specific way such FAST, I decided to try and use FAST in order somehow to extract a time-history at each node of the jacket substructure and, hence, apply these time-histories to DIANA as lumped loads.

I saw there is a function in the HydroDyn module that allows to do so, but I am not getting realistic results. More specifically I defined a sea state with waves acting in one direction ( x direction ) with a JONSWAP/Pierson-Moskowitz spectrum. After that I gave to the axial coefficient standard values :

---------------------- AXIAL COEFFICIENTS --------------------------------------
             1   NAxCoef        - Number of axial coefficients (-)
AxCoefID  AxCd     AxCa     AxCp
   (-)    (-)      (-)      (-)
    1     1.00     1.00     1.00[/code]

My idea, from reading the HydroDyn manual, was to take every component of the total lumped force vector and sum it on by one.
What I have in output is, however, strange as I have same values for components X and Y of drag and inertia forces when I was expecting a dominant x component.
In the next block codes I want show you a few second of the output of my simulation. I printed here just the components of the Viscous-drag force, fluid-inertia force and  hydrodynamic added-mass forces at J3 node ( I don't think is really relevant the node position but of course is under the MSL ).

[code]Time  	J3FDxi    	J3FDyi    	J3FDzi    	J3FIxi    	J3FIyi    	J3FIzi    	J3FAMxi   	J3FAMyi   
(s)   	(N)       	(N)       	(N)       	(N)       	(N)       	(N)       	(N)       	(N)       
20.0000	-3.239E-01	-3.239E-01	-1.069E-02	 1.383E+02	 1.383E+02	 4.565E+00	-6.792E-02	-6.792E-02
20.0500	-3.212E-01	-3.212E-01	-1.060E-02	 1.370E+02	 1.370E+02	 4.524E+00	-1.690E-02	-1.690E-02
20.1000	-3.182E-01	-3.182E-01	-1.051E-02	 1.358E+02	 1.358E+02	 4.482E+00	-3.404E-03	-3.404E-03
20.1500	-3.157E-01	-3.157E-01	-1.042E-02	 1.345E+02	 1.345E+02	 4.441E+00	-6.688E-02	-6.688E-02
20.2000	-3.136E-01	-3.136E-01	-1.035E-02	 1.333E+02	 1.333E+02	 4.400E+00	-1.035E-01	-1.035E-01
20.2500	-3.116E-01	-3.116E-01	-1.029E-02	 1.320E+02	 1.320E+02	 4.359E+00	-5.906E-02	-5.906E-02
20.3000	-3.052E-01	-3.052E-01	-1.008E-02	 1.299E+02	 1.299E+02	 4.290E+00	-1.017E-02	-1.017E-02
20.3500	-2.981E-01	-2.981E-01	-9.840E-03	 1.278E+02	 1.278E+02	 4.220E+00	 4.455E-02	 4.455E-02
20.4000	-2.905E-01	-2.905E-01	-9.591E-03	 1.257E+02	 1.257E+02	 4.151E+00	 9.582E-02	 9.582E-02
20.4500	-2.830E-01	-2.830E-01	-9.343E-03	 1.236E+02	 1.236E+02	 4.081E+00	 6.951E-02	 6.951E-02
20.5000	-2.759E-01	-2.759E-01	-9.109E-03	 1.215E+02	 1.215E+02	 4.011E+00	 1.144E-02	 1.144E-02
20.5500	-2.658E-01	-2.658E-01	-8.776E-03	 1.186E+02	 1.186E+02	 3.914E+00	-7.265E-03	-7.265E-03
20.6000	-2.559E-01	-2.559E-01	-8.450E-03	 1.156E+02	 1.156E+02	 3.817E+00	 1.748E-02	 1.748E-02
20.6500	-2.456E-01	-2.456E-01	-8.108E-03	 1.127E+02	 1.127E+02	 3.720E+00	 8.522E-02	 8.522E-02
20.7000	-2.350E-01	-2.350E-01	-7.759E-03	 1.097E+02	 1.097E+02	 3.623E+00	 1.154E-01	 1.154E-01
20.7500	-2.250E-01	-2.250E-01	-7.428E-03	 1.068E+02	 1.068E+02	 3.525E+00	 2.380E-02	 2.380E-02
20.8000	-2.129E-01	-2.129E-01	-7.029E-03	 1.030E+02	 1.030E+02	 3.401E+00	-7.488E-02	-7.488E-02
20.8500	-2.018E-01	-2.018E-01	-6.663E-03	 9.928E+01	 9.928E+01	 3.277E+00	-1.019E-01	-1.019E-01
20.9000	-1.911E-01	-1.911E-01	-6.308E-03	 9.552E+01	 9.552E+01	 3.154E+00	-6.864E-02	-6.864E-02
20.9500	-1.800E-01	-1.800E-01	-5.943E-03	 9.177E+01	 9.177E+01	 3.030E+00	-5.695E-03	-5.695E-03
21.0000	-1.690E-01	-1.690E-01	-5.580E-03	 8.802E+01	 8.802E+01	 2.906E+00	-4.497E-03	-4.497E-03
21.0500	-1.562E-01	-1.562E-01	-5.157E-03	 8.353E+01	 8.353E+01	 2.758E+00	-7.620E-02	-7.620E-02
21.1000	-1.443E-01	-1.443E-01	-4.765E-03	 7.904E+01	 7.904E+01	 2.609E+00	-1.058E-01	-1.058E-01
21.1500	-1.328E-01	-1.328E-01	-4.383E-03	 7.455E+01	 7.455E+01	 2.461E+00	-2.881E-02	-2.881E-02
21.2000	-1.210E-01	-1.210E-01	-3.996E-03	 7.006E+01	 7.006E+01	 2.313E+00	 5.650E-02	 5.650E-02
21.2500	-1.095E-01	-1.095E-01	-3.616E-03	 6.557E+01	 6.557E+01	 2.165E+00	 4.818E-02	 4.818E-02
21.3000	-9.701E-02	-9.701E-02	-3.203E-03	 6.043E+01	 6.043E+01	 1.995E+00	 2.931E-02	 2.931E-02
21.3500	-8.534E-02	-8.534E-02	-2.817E-03	 5.529E+01	 5.529E+01	 1.825E+00	 3.050E-02	 3.050E-02
21.4000	-7.419E-02	-7.419E-02	-2.449E-03	 5.015E+01	 5.015E+01	 1.656E+00	 7.628E-02	 7.628E-02
21.4500	-6.372E-02	-6.372E-02	-2.104E-03	 4.501E+01	 4.501E+01	 1.486E+00	 8.891E-02	 8.891E-02
21.5000	-5.415E-02	-5.415E-02	-1.788E-03	 3.987E+01	 3.987E+01	 1.316E+00	 2.786E-02	 2.786E-02
21.5500	-4.451E-02	-4.451E-02	-1.469E-03	 3.419E+01	 3.419E+01	 1.129E+00	 1.264E-03	 1.264E-03
21.6000	-3.591E-02	-3.591E-02	-1.185E-03	 2.852E+01	 2.852E+01	 9.414E-01	 1.286E-02	 1.286E-02
21.6500	-2.818E-02	-2.818E-02	-9.304E-04	 2.284E+01	 2.284E+01	 7.541E-01	 1.344E-02	 1.344E-02
21.7000	-2.136E-02	-2.136E-02	-7.050E-04	 1.717E+01	 1.717E+01	 5.667E-01	-1.088E-03	-1.088E-03
21.7500	-1.562E-02	-1.562E-02	-5.158E-04	 1.149E+01	 1.149E+01	 3.793E-01	-6.504E-02	-6.504E-02
21.8000	-1.056E-02	-1.056E-02	-3.485E-04	 5.416E+00	 5.416E+00	 1.788E-01	-1.349E-01	-1.349E-01
21.8500	-6.484E-03	-6.484E-03	-2.140E-04	-6.601E-01	-6.601E-01	-2.179E-02	-9.712E-02	-9.712E-02
21.9000	-3.332E-03	-3.332E-03	-1.100E-04	-6.736E+00	-6.736E+00	-2.224E-01	-1.177E-02	-1.177E-02
21.9500	-1.182E-03	-1.182E-03	-3.902E-05	-1.281E+01	-1.281E+01	-4.229E-01	 6.748E-03	 6.748E-03
22.0000	-1.213E-04	-1.213E-04	-4.005E-06	-1.889E+01	-1.889E+01	-6.235E-01	-2.190E-02	-2.190E-02
22.0500	 1.805E-04	 1.805E-04	 5.960E-06	-2.521E+01	-2.521E+01	-8.323E-01	-6.351E-03	-6.351E-03

I do not belive this is the best way to do so, and if you have any suggestion I would be really grateful.
Please find attached the setting of my hydrodyn input file which was taken from the testing folder, test21 regarding OC4 deep-water jacket substructure, and then slightly modified.

I tried to be as clear as I could to explain my problem. Thanks you all for your time and consideration.

Best regard,
Davide Moretti
NRELOffshrBsline5MW_OC4Jacket_HydroDyn.txt (31.7 KB)

Dear Davide,

I think you are misunderstanding what is meant in the HydroDyn documentation by “lumped loads at joints”.

The strip-theory solution in HydroDyn calculates hydrodynamic loads as a combination of distributed loads (per unit length) at nodes distributed along the member and lumped loads at member-end nodes (joints). The lumped loads at the member ends (joints) are the hydrodynamic loads applied to the exposed (wetted) surface at the end of members. I think this theory is best explained in a presentation I gave at our last extensive modeling workshop. See slides 11-14 of the following presentation: wind.nrel.gov/public/jjonkman/Pr … onkman.pdf (the lumped loads at member ends are explained on slides 13-14).

It sounds like what you want is the integrated value of all hydrodynamic loads (distributed + lumped) lumped to various nodes within the substructure that you can output and apply to your DIANA FEA software. This integration of the hydrodynamic loads is performed within the FAST source code (in the mesh-mapping load transfer between HydroDyn and SubDyn), but without modification of the source code, it is not possible to output these integrated loads at all nodes directly.

Best regards,

Dear Dr. Jonkman,
Thanks for your reply, it clarified the matter.

I actually knew I couldn’t extract the total hydrodynamic loads for all the nodes in one way at once.
What I grasped from the hydrodyn manual was that I could output each component of the distributed loads on the slender members for 9 element at the time and then sum each component, for example, with a python script.
Is it possible work in this way, or am I still getting wrong?

Thanks again for your time,

Kind regards,
Davide Moretti

Dear Davide,

Yes, you are correct. You can output the various components of the hydrodynamic loads for up to 9 nodes of 9 members and up to 9 joints for any given FAST/HydroDyn simulation. But what you are describing may be difficult if you have many more nodes/members/joints than 9, in which case it would probably be better to modify the source code to output exactly what you want.

Best regards,