Problem:

According to AutoPIPE help

Stress

Design pressure P = 6750psi
Nominal outside diameter of the pipe D = 8.625in
Nominal wall thickness t = 0.875in
Hoop stress factor Y = 0
Hoop stress Sh  = [P D / (2 x t)] - (P x Y)

Hoop Stress = 33.268 ksi

Allowable

Specified minimum yield strength - API 5L X65 S = 65ksi
Hoop stress design factor F = 0.72
Longitudinal joint weld factor Ej = 1
Temperature derating factor T = 1
Allowable hoop stress Shallow = S x F x Ej x T

Allowable Hoop Stress = 46.8 ksi

Why are AutoPIPE results showing differently:

Solution:

 In this case, the model was sent in and the following was found on the General Model Options dialog:

Question: Are the correct piping code equations being used?

Answer: No, the wrong equations are being used. Why?

Again, look at the piping code selected on general model options dialog, the original screen image above of the calculations, and the actual definitions under the code compliance calculations description. 

The key word is Offshore.

Note, that the equations shown were for B31.8 onshore not B31.8 Offshore. There is a huge difference between these codes and one must be carful when selecting the piping code in the program. Therefore the correct equations to be referenced should have been :

Stress = [P x (D-t)] / (2 x t)

Stress = 29892.86 psi

Allowable = F1 x S x T

F1 = 0.5

Note: as mentioned in the help, 

Currently since Autopipe cannot assign a different design factor to different section of piping the Default values are conservatively based on risers. These values are be entered under hoop, longitudinal, and combined design factors in Tools> model options> Results dialog.

Update these values manually as needed. 

S = 65 ksi

T = 1

Allowable = 32500 psi

See Also

ASME B31.8 Piping Code Calculation Issues

Bentley AutoPIPE