ASME B31.8 Chapter VIII stress check in CaesarII

[jaysid]

have observed in a current model which is 36 meters gas lift riser lines inside a buoyancy tank 100 meteres below sea surface. The code used for the analysis is ASME B31.8 chapter VIII.
As the Caesar II states it uses Equivalent stress Se.
Se=2[((Sl-Sh)square/2+Stsquare]sqrt
should be less tha equal to 0.9times yield stress.

When checked in the CII, found that the code stress calculated in CaesarII doesnt matches with the manually calculated combined stress or quivalent stress.

Wanted to know if anyone knows about this as this is the first time I am using 31.8 apart from 31.3
cheers.
sid.

 

[richay]

Be careful here. B31.8 VIII has three stress equations and three corresponding allowables. CAESAR II computes all three stresses and allowables.

Because you can analyze a system with multiple parts governed by more than one piping code, the CAESAR II stress report has one value for "code stress" and one value for "allowable stress", to keep the reports uniform.

So in the case of B31.8 VIII (and B31.4 Ch9, and DnV), CAESAR II determines which of the 3 stresses is governing. This is done by looking at the stress/allowable ratios:

a) Shoop / Shoop allowable
b) Slong / Slong allowable
c) Seq / Seq allowable

Which ever ratio is largest determines what stress and allowable are displayed in the report as "code stress" and "allowable stress". So for one node "hoop" might govern while for the next node "equivalent" might govern. You can tell which stress you're dealing with by taking the allowable and dividing by "Sy", since the three stress equations are all a "constant" function of "Sy".

In addition, watch what you do with corrosion. As per note 1 of Table A842.22, the equivalent stress is computed using a corroded wall thickness. The Sh and Sl equations are not corroded. Therefore, Sh and Sl must be recomputed using the corroded wall for use in the Se equation.

If you can't figure it out, send the job to "Tech Support" at COADE.

Richard Ay
COADE, Inc.