Critical Buckling Load Analysis on Floating objects

[GersonMachado8699]

Hello everyone,

I would like to receive suggestions on how to improve my analysis' reliability when I am designing a floating barge and wish to get results on it's critical buckling load.

On my stress analysis I use springs on the bottom of the barge, simulating the water effect because I cannot add hydrostatic pressure to the bottom. That would be impossible, as the balance of forces would be different of zero, resulting in an analysis failure.

So far it's OK, I'm able to perform stress analyses of floating barges with my "method" perfectly, My results on stress analysis are very reliable.

The problem comes when I try to perform a critical buckling load analysis. When I have all my model supported by springs, I can't get any result; at the end of the analysis I only get a result similar to the result when I perform a "Check Model".

When I add some Nodal General Constrainsts on my model the critical buckling load analysis works fine. The problem is, it becomes unreliable, as my model should be floating and not being supported by anything but water.

Again, any suggestions on how to make my model reliable on critical buckling load analysis?

Thanks

Best Regards,

Yossef Sami El Didi

[AstroJohnPE · Accepted solution]

Hi Yossef,

If the critical buckling analysis is giving no results (which is different than giving results of 0), there should be some type of message in one of the analysis logs. Use the Report tab to view the Log and Summary files.

Also, you might be able to learn something by running a Natural Frequency analysis or Natural Frequency with Load Stiffening analysis. There could be something "strange" occurring with the frequency that the Critical Buckling analysis is having problems with. If any of the frequency results are 0, then your model is not constrained in all directions. Are you using springs to hold it in X, Y, and Z?

On a separate issue, how many springs are you using in your model? More than 3 nodes? To have a statically stable model in all 6 directions (X, Y, Z translation, rotation about X, Y, Z), you only need to pin 3 nodes not in a straight line. So you might be able to apply your hydrostatic pressure like you suggested and use  springs in X, Y, Z at 3 (or more) nodes to balance out the hydrostatic pressure. After you get the results, check the forces in the springs to confirm that the load is sufficiently close to 0; that is, the hydrostatic pressure almost balances the gravity load.

[GersonMachado8699]

Hello John,

You were right, my problem was not restraining my model on both X and Y directions. Once I have done this, I could obtain reliable results on my critical buckling load analysis. I will try to identify problems by running Natural Frequency analysis when I face other problems like this in the future to check if it is tied up enough. Thank you for your help and advices.

About the springs, I fill the entire barge's bottom with them. In my model they simulate buoyancy, a distributed load which has the same unit as springs (N/mm). That way I guarantee my barge's bottom is subject to the correct force and my analysis will converge. If I put hydrostatic pressure, All the difference between buoyancy and weight will be applied by those springs. As you said, I can try to improve my aproximation of hydrostatic pressure in a way that it comes nearer and nearer to the model's weight and forces applied; I just find it easier the other way, but thank you again for your suggestions!