There isn’t a lot of great information on designing breech openings, but there is a little information. CICIND published some information in the link below.
There is also a book called “Tubular Steel Structures” by Troitsky that has some good information.
Meca's practice is to do the following. Generally if the width of the opening is less than 50% of the diameter of the stack, then we just model it in MecaStack. We model it by entering the opening, and then in the “Stiffener Area” cell of the entry form, We enter the cross sectional area of the Vertical stiffener. The software uses this information to calculate reduced section properties at the location of the breech, which will be evident if you review the “Section Properties” in the output, you should see the values change in the region of the breech. If the stress ratios are still well below unity, then we usually stop there and consider it acceptable. If they are getting close to unity, maybe >= 0.8, then we might consider taking the analysis further or increasing my stiffener size. The reason is that this is an “Approximate” method and we want to be well on the side of conservatism.
If the opening width is greater than 50% of the stack diameter, or if we have a more complex arrangement (Multiple openings, or a very tall opening, etc...) then we will create a finite element model. We will usually create the finite element model using brick elements of the breech and maybe 3 to 4 diameters of stack length (half above and half below the opening), and then we will take the maximum vertical load and overturning moment from the MecaStack model at the center line of the breech (Or bottom of the breech to be conservative) and apply them to the top of the model. The bottom of the shell will be fixed. We apply the moment in such a way that it will try to collapse the opening. If the Von Mises Stress is less than the hot yield stress of the material, then I consider it acceptable. We try to be fairly conservative on this, if we are approaching 0.8 to 0.9 * the yield stress, then we consider adding more reinforcement.
All of the same basic principles are applied as above; however, you usually the reinforcement of a round opening is a reinforcement pad (Repad). In this case we take the cross sectional area of the repad is considered as the "Stiffener". This is handled automatically when you select the option within MecaStack Details / Nozzle (Round).