FEA Modeling

Hi All,

I am modeling a stick figure model of an offshore container in a structural analysis software as per DNV 2.7-1. The model is pinned on the 4 legged sling lifting point. As per DNV 2.7-1, I have constrained the 4 bottom corner supports with springs in the horizinonal axis (X and Z). When solving, the model does not solve and converge.
But when I put in spring supports in the vertical Y axis as well (with a minimum spring stiffness of 10 N/m), the model solves. DNV does not comment/advice to put spring supports in the vertical axis. But if I do not put, the model does not converge and hence fails/does not solve. My question is it correct to put in the vertical spring supports? How will it differ (what difference does it make) from DNV's guidelines? How can I justify that to DNV?

Thanks in advance.

TFR

 

[Ussuri]

How are you modelling your slings? What you are doing is common using frame packages (eg staad). The hook point is restrained in all 6DOF and minimum horizontal reactions are applied to the lift object to stabilise it. The slings are then moment released. In all the packages I haved used this is a stable model.

There are other approaches to modelling lifts as well.

 

[thefamousrockstar]

Hi Ussuri,

I am modeling the slings as tension only. But my confusion is that if I apply springs in the vertical direction as well, say with a stiffness of 1 N/m, will I be doing anything wrong? As I have not come across any model where one uses a vertical spring support in such cases.

Regards,

TFR

 

[Ussuri]

If you are putting a reaction/spring in X and Z but nothing in the Y direction are you saying that you have not restrained the model in the Y direction at all? No restraint or spring?

When we do lift analysis we usually use a frame anaylsis package instead of an FE package so cant comment on your specifics. But normally the hook point is fully restrained in all 6DOF. The sling members are moment released to ensure they carry axial loads only.

The structure is then prevented from moving laterally by applying two springs (one X and Z, the other just X say). This allows the structure to deflect vertically relative to the hook point.