It will take time to do FEM analysis. For most of such cases,we do not resort to FEM analysis.
If I am in your shoes, I will calculate nozzle flexibilty of WRC 297 in a conservative way.
Firstly, calculate the equivalent diameter of the cone roof. then get the equivalent thickness of roof consdering rafter effect.
The equivalent parameters shall be obtainned reasonably conservative.
Normally, I try do do a accurate analysis if case is critical or profits can be made by doing this way.(e.g. save piping materials or big support or no-reinforced nozzle). But if the activity of accurate analysis take so many man hours, I just make conservative proximation. However, you may carry out a FEM analysis if cost and time are not problems.
Regards
Alvin Zhu