Hello,

I am trying to verify how accurate FAST models the sub structure (monopile) stiffness.

Is there a way to isolate the simulation (hydrodyn) approximately to one static load?

Also, do you know where I can attain common ocean (hydrodynamic) loads seen on a wind turbine?

Thank you as always!

Kan Ito

Dear Kan,

I’m not really sure what type of static load you want to apply, but you can run SubDyn in standalone mode (uncoupled from FAST/HydroDyn) if you want to examine the response of the substructure in the absence of hydrodynamic loads or coupling to the turbine. In standalone SubDyn, you can analyze the response of the substructure to prescribed motions of the transition-piece reference point (which can be considered as prescribed loads at the reference point).

In your second question are you asking where you can obtain real data to validate the numerical models? If so, you’ll need to be much more specific on what type of data you want; regardless, very few publicly available datasets exist.

Best regards,

Hi Jason,

I just ran the standalone simulation on SubDyn with several acceleration inputs at TP ref point.

However the deflection output for all directions to be zero (m) [IntfTDXss, IntfTDYss, IntfTDZss] while the reaction forces are clearly not zero.

Do you know how this is causing no deflection?

Kan Ito

Dear Kan,

In standalone SubDyn, the motions (displacements, velocities, and accelerations) of the TP reference point are specified by the user. SubDyn does not check for physical consistency between thesr motions; it sounds like you’ve specified nonzero accelerations with zero displacements, which is unphysical.

Best regards,

I see but then I am unsure how I can validate the stiffness of the substructure if I am specifying the deflection and the acceleration as inputs, I am not sure what I can get from the outputs to verify the stiffness. What way can I do this?

Dear Kan,

What do you mean by wanting to “verify the stiffness”? The cross sectional stiffness of the individual members are an input to SubDyn. These data are used within SubDyn to form the element-level stiffness matrices, the global stiffness matrix, and the Craig-Bampton reduced stiffness matrix of the SubDyn model. Do you want to verify that these individual matrices are as you expect or something else? Please clarify what you want to do.

Normally, the first step in validating a structural model is to ensure that the overall mass and center of mass is located properly and to ensure that the first several natural frequencies match measured values. If both are correct, one is typically confident that the mass and stiffness of the model are represented correctly.

Best regards,

Hi Jason,

I apologize if I was unclear. That is correct, I am trying to find if these matrices will respond as I expect.

Under a specified static load (x direction), I was hoping I would attain a deflection response of the substructure so that I can calculate the stiffness from this and compare with the cross sectional stiffness input of the individual members in SubDyn.

How do I verify the overall mass and center of mass is located properly? and the natural frequencies?

Normally, the first step in validating a structural model is to ensure that the overall mass and center of mass is located properly and to ensure that the first several natural frequencies match measured values. If both are correct, one is typically confident that the mass and stiffness of the model are represented correctly.

Dear Kan,

Instead of applying a force and looking at the deflection, you can apply a deflection and look at the force in order to ascertain the stiffness in standalone SubDyn.

The overall mass, center of mass, and natural frequencies of the substructure are written to the SubDyn summary file–see section 4.2 of the SubDyn User’s Guide and Theory Manual for more information.

Best regards,

Thank you so much.

This was extremely, helpful!

Regards,