I understand that you have modelled column as a rigid element in your CAESAR II model.

The natural frequency of a piping model is dependent on the stiffness of the system which is again a function of the model elements crosssection, mass, end conditions( mostly defined by the support locations /type and their stiffness). As you increased the column element thickness ,you added more stiffness in your model and got a better or higher nat frequency. It is very unlikely that this solution will be accepted by your vessels group.


If you have to meet some min natural freq of the overhead line :

1. suggest not using GAPS in your model (if a gap is not closed ,there are chances of the restraint not considered in your modal anlaysis and hence lower frequencies)
2. lower the guide spans (where possible)
3. use a diff model without the vessel elements and the supports defined as restraints. Remember that the support stiffness also plays an important part in modal analysis. If your vessel is thin and supports flexible ,then your natural frequencies could be lower than your requirements.

If the natural frequency has to be achieved in your project, it is the stress engrs responsibility to defined the stiffness and location of the supports to the interfacing discipline(vessels/structures)…


Cheers!!!!!!!!!