Norton,

Suggestion: Try to employ an overall view of the Piping + Vessel + Steelwork System.

My understanding is you have over-stress issue on PIPING system (pipe spool and/or pipe elbow).

You say: "...my process unit is nor suitable as the OD is too small for the relevant code.".
This is definitely an unclear statement. Probably, by "Process Unit" you mean equipment/vessel, and "a too small OD" would mean that (OD)nozzle/(OD)vessel ratio is too high to yield to a relevant (read "low enough") stiffness of the "nozzle-shell" junction.
Have you tried to compute the stiffnesses by Finite Element Analysis (FEA), or you've used only Caesar II Built-in WRC297 Module? FEA approach is much more realistic and would provide realistic stiffness values.

Anyway, even if "nozzle-shell" junction stiffness is relatively high, you may approximate the actual stiffnesses of the pipe supports and/or of the terminal anchors (e.g. realistic boundary conditions). Please remember that pipe supports' actual stiffnesses are significantly lower than 10^12 order magnitude assumed by Caesar II software.

Finally, if your thermal expansion over-stress is not very high (i.e. less than 20%), you may use a realistic Fatigue Stress Reduction Factor "f" (e.g. f = 1.00 ... 1.20) to increase the Allowable Displacement Stress Range SA, according to ASME B31.3 provisions. Such approach may be employed, based on sound engineering judgement, if Loading-Unloading Thermal Cycles for the System Design Lifetime is lower than 7000.

To conclude, there are not "magic" formulas or "wizard" approaches to make a Piping-Vessel System to match Code compliance requirements. It's just a matter of Code requirements knowledge and sound engineering judgement.

If you are dealing with a newly-designed System, and you encounter such flexibility-lack problem, definitely RE-DESIGN is required. Either an expansion loop or an expansion joint...

Regards,