I have a large bore pipe (40", thickness 5mm) having stiffening rings:
If there is a stiffening ring (lenght 20mm, thikness 10mm) every 1000mm, can i model:

-x-x-x-

where
- : pipe lenght 1000mm, thickness 5mm
x : stiffening ring lenght 20mm, thickness 10mm

or is wrong?

I estimate ~4% increase in (empty) weight per meter due to addition of above stiffening rings, so would not model these in Caesar II (unless there are spring supports/ strain sensitive equipment nearby or calculated stresses are very close to their allowable limit). I would add suitable factor in support loads instead.

if pipe is full vacuum, does stiffening rings modeling does help stress results? (i mean: without rings the pipe would implode?)

No. Caesar II does not address collapse/buckling due to vacuum.

Please refer to earlier post
http://forums.coade.com/ubbthreads/ubbth...=true#Post33223

This is the case of D/t>100. Even if you model this in CAESAR you cannot calculate the SIF. CAESAR will show the warning for this case. because CAESAR II is code base calculation. Any of the piping code will not address D/t>100. better option is FEA analysis for accurate result.

Also stiffening ring for the purpose of to avoid Buckling or collapsing due to vacuum. which cannot be check in CAESAR II.

Please remember that CAESAR II utilizes the "3D Beam" element. Think of this as just a piece of spaghetti. The "3D Beam" element has no concept of shell behavior - this element addresses beam bending, which is what the piping codes are after (from a flexibility stress point of view).

Just found this article, maybe it can be helpfull in this situation...
https://www.whatispiping.com/how-to-perform-the-buckling-analysis-of-vacuum-piping