SACS Structural Modeling Help - Dynamic Analysis of Offshore Transport

[davidmay84]

This is an offshore-related post. I realize I am in the wrong forum but here is where I figure I find the most advanced SACS users. I explain some of the terminology for your convenience, not to patronize. Please let me know if you have questions, want to see drawings, etc., and I'll do what I can do.


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We are using SACS to try to design seafastening (steel pipe to help the cargo resist dynamic forces) for a 4000mt module (8-legged offshore deck) on a 350' cargo barge.

Generally you load the module onto the barge grillage (I-beam structure to spread the weight of the module to the structural members of the barge) and then fit/weld roll braces (diagonal pipe generally 45 degrees to x & y & z axis) to the barge grillage.

The problem is, when you model roll braces in SACS the roll braces get pre-loaded with a portion of the self-weight of the module. This is cannot be possible in reality because the braces are welded onto the structure AFTER loadout, i.e. self weight of module is spread to the grillage where the 8-legs are sitting. These braces should only take dynamic forces (tension & compression) from roll, heave, pitch, etc., when offshore.

Does anyone know how to model members in SACS where they resist dynamically induced forces but do not carry the self weight of the module? I guess the best corollary I can think of is guy-wires on towers (except our pipes can take compression) where they help the tower resist environmental forces, but do not take vertical-load from the weight of the tower (because they are wires of course, but just replace "wires" with "pipes" and you may get my point).

If you need further information I can supply.

Thx, David

 

[TomasAron]

I have worked with SACS a couple of years. But mostly on smaller projects. 200 ft of water.

Did you try putting a shim at the connections.

I would group over ride the weight of the pipes, make them .001. And although the pipes can support a compression load. I would make them tension only members because SACS will probably give you problems about that.

 

[davidmay84]

Thanks for your reply.

We thought about this solution already, making the roll braces tension-only. However, we have roll braces on both sides of the barge. Therefore, in a roll case, half the braces on one side of the barge would work to resist roll in tension, and the other half of the braces on the other side of the barge would work to resist the roll in compression.

If the members are tension only, only half of the braces work at a time which would be an inaccurate assumption.

Damn, have to figure out a way to get SACS to not pre-load these braces with the weight of the structure

 

[dhengr]

David:
I know nothing about SACS and can only imagine what your structure really looks like. But, can you run the analysis in two steps? First provide nodes for the braces in your original modeling, but give them almost zero area and stiffness, and run the analysis to get the DL and LL stresses and displacements, and stopping the analysis; then change the area and stiffness of the braces, and add the dynamic loads and rerun or continue the analysis run to completion.

Wouldn’t this be akin to running the analysis once to find stresses and displacements for DL and LL; and then saying, I want to check another load condition, or conditions, and am going to add some reinforcements to the original structure to take these new loads? And, then using the results from the first run as your starting point for the second run. This is in keeping with your doing most of the loading and then adding the braces for a new set of loads. If I had a loaded WF, simple beam and wanted to add more load to it and knew it would then be over stressed, I might shore the existing beam, but not completely unload it back to zero stress, then add cover plates to take the new added loads, and re-analyze the beam. Some thought and care must be taken when welding any reinforcement to a highly stressed member or you could cause plastic deformation in the immediate area of the welding during that process and that will change the stresses and deformations in the original structure too. Maybe that means shoring the existing member you are adding the brace welding to, so as to partially unload or un-stress that member during the welding.