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Hydrodyn Forces

Posted: Tue Mar 03, 2020 9:23 am
by Andres.Piedra
Hello everyone,

I am new to FAST, and i'm trying to get all the hydrodynamic forces calculated in FAST, and compared them with the tower base reaction (TwrBsFxt) because I assume that to achieve static equilibrium these forces most be equal.

So I obtained the hydrodynamic forces distributed in each member, transformed them in point loads, sum them and compare them with the resulting hydrodynamic forces calculated by hydrodyn (HydroFxi), and the results were quite similar.

Finally I add all the acting forces in X (hydrodynamics, inertial (-M * PtfmAxT) and moorings), and compared them with the Tower base reaction(TwrBsFxt) and got the next graphic:
Image

The broken line is the tower base reaction, and the other one is the resultant of all my acting forces.

Can someone help me and tell me where my mistake is or if I need to consider another force?

Sorry If my english is not very good i hope you can understand what i'm asking.
Thank you,

Andrés

Re: Hydrodyn Forces

Posted: Tue Mar 03, 2020 12:27 pm
by Jason.Jonkman
Dear Andres,

If I understand correctly, you are trying to compare the tower-reaction loads found by-processing the hydrodynamic applied loads, mooring applied loads, and platform acceleration with the tower-base reaction loads output directly. In general, I would agree that F = m*a for the platform, where F includes applied loads from the tower, hydrodynamics, moorings, as well as the weight of the platform itself, and m*a is the inertia of the platform. The challenge is likely ensuring that all terms are in the same coordinate system--at the platform / tower base, some terms are in the inertial-frame coordinate system (i) and some are in the platform / tower-base coordinate system (t), which orients with the platform as it rotates in roll, pitch, and yaw.

Best regards,