1st Q: depend on your seabed undulations, you can model seabed as a rigid surface and use node-to-surface contact model. in this case you don't need to define any gap element explicitly.
2nd Q: In Abaqus, you can define both node-to-surface and surface-to-surface contact as well.
In ANSYS and OFFPIPE, the software just multiply the provided SCF by the stress. It doesn't mean that your are calculating SCF. As you said, the best way to do that is using 3D elements. There is also another way which can give reasonable results with beam elements. to do that, define two...
Understand. I meant if you considered CSF you should consider it in FEM as well.
Another question, don't forget to apply static deflection and axial force for on your model prior to natural frequency extraction.
RP-F105 does not consider the effect of stress concentration for natural frequency calculation, it considers the effect of additional stiffness due to concrete coating. So, if you considered this effect in you analytical calculation you should consider it in your numerical model.
I would like to...