Learning Forum

[catur.indrasukmana]

I am modeling floating jetty with pile guide (separated structure) . The pile guide has rigid joint with floating jetty. The movement of whole structures is only Global Z (heave) and Global RZ (yaw). I activated the deactivated freedoms on pile guide (Global X, Global Y, Global RX, Global RY). The problem is, after 1100 second simulation the structure is moving from the initial position (both Global X and Global Y). Does anybody know how to solve this problem? Thank you

Cheers,

Catur

[Mike Pettit]

Deactivated Freedoms in time domain calculations may not be the most rigorous approach to restricting a structure's motion, due to the way in which the freedom deactivation is applied. The change in position that you see is basically the sum of numerical round-off errors over a large number of time steps. Deactivated Freedoms are usually more appropriate for stability analyses, where we are iterating towards convergence of the forces acting on each structure.
A better approach for this workflow may be to use an arrangement of Fenders between Fixed Points and the pile guide. To restrict motions to Z and RZ only, you will need 8 Fenders on each pile guide, arranged as shown:
Viewed from above:
This approach also allows you to define stiffness and friction terms which may give a more realistic model of the actual pile-jetty connection.
I hope this helps; please let me know if you have any further questions.
Cheers, Mike

[Mike Pettit]

I should also mention that the amount of unwanted movement with Deactivated Freedoms in the time domain is reduced (but still not completely zero) if you reduce the Time Step value in Analysis Settings, so this may be another option.
Mike

[catur.indrasukmana]

Thank you for your explanation. I'll try it first.