Dear community,
I’m currently calculating the modal shape polynomial coefficients using WISDEM for the IEA-15-240-RWT (for the monopile + tower). While attempting to include soil-structure interaction, I activated soil_springs in FixedBottomSE and modified sevral times the soil_shear_modulus in the geometry input file. However, when plotting the modal shapes, the results appear unchanged — as if the soil stiffness has no influence.
This seems unexpected, as the theory (e.g., [Suresh C. Arya, Design of Structures and Foundations for Vibrating Machines, p.57]) suggests that WISDEM should compute an equivalent stiffness for the monopile-soil interaction.
I looked through the forum but couldn’t find any posts directly addressing this issue.
Am I missing a required input or coupling step for the soil stiffness to be accounted for in the modal analysis? Is there a way to verify if the soil stiffness is effectively used in the modal calculation?
Any insight would be greatly appreciated!
Best regards,
Adele
Please see the WISDEM Issue on this topic here.
Currently, the modeled springs offer a nearly-clamped boundary condition for the monopile model, so that is likely why you aren’t seeing a different in the mode shapes. It is a bit of an open question as to whether those soil stiffness constants are accurate or fully implemented correctly and we would welcome input on those details.
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I would expect the mode shapes being quite similar between the two boundary conditions (i.e., clamp condition vs compliance from the soil stiffness). For the second bending mode, you would likely see a small translation and rotation at the seabed. This was also observed in the code-to-code verification performed as part of the OC6 phase II project: https://onlinelibrary.wiley.com/doi/epdf/10.1002/we.2698
Despite the mode shape not being significantly different, it is expected a drop in frequency (assuming that the soil stiffness really introduces some flexibility in the system).
I hope that helps!
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Dear @Garrett.Barter,
Thank you for your fast response. I understand better now.
Best regards,
Adele
Dear @Roger.Bergua,
Thank you so much for that piece of information. I will thoroughly check what you sent me!
Best regards,
Adele
