Using B31.3 or B31.8 I see no option in AutoPIPE to calculate hoop stress using thick wall therory. Is this included "behind the curtains" in the program with an automatic switch when t < ID/20?

Thick wall hoop stress formula is applied for Euro Code EN13480 per equation 6.1-3. It is not applied for other codes. However AutoPIPE uses thick wall formula in calculating pressure extension regardless of the piping code.

For B31.3 code, you can enter a modified Y factor in Tools/Model Options/Result for thick pipes as per section 304.1.1.

B31.3-2008 section 304.1.2 states the following:

(b) For t ≥ D/6 or for P/SE > 0.385, calculation of
pressure design thickness for straight pipe requires special
consideration of factors such as theory of failure,
effects of fatigue, and thermal stress.

For B31.8 regular code, no mention is given for thick wall. In B31.8-2007 offshore section, a new formula is given for thick pipes, but that formula is wrong (section A842.221, Diameter term cancels out). For offshore code, you need to enter the net pressure Pi-Pe in AutoPIPE.

Thick wall hoop stress formula is applied for Euro Code EN13480 per equation 6.1-3. It is not applied for other codes. However AutoPIPE uses thick wall formula in calculating pressure extension regardless of the piping code.

For B31.3 code, you can enter a modified Y factor in Tools/Model Options/Result for thick pipes as per section 304.1.1.

B31.3-2008 section 304.1.2 states the following:

(b) For t ≥ D/6 or for P/SE > 0.385, calculation of
pressure design thickness for straight pipe requires special
consideration of factors such as theory of failure,
effects of fatigue, and thermal stress.

For B31.8 regular code, no mention is given for thick wall. In B31.8-2007 offshore section, a new formula is given for thick pipes, but that formula is wrong (section A842.221, Diameter term cancels out). For offshore code, you need to enter the net pressure Pi-Pe in AutoPIPE.