I have a case of an underground line; it is analyzed by B31.8 with a design factor of 0.6. In a operative case with the presion and temperature of design, the system fail in almost all nodes (including the straight pipe ,Is estrange for me), I think that may be it is a mistake of generation in the underground model, because I did a no-underground model, with the same restrictions but with simples rests and guides (+Y, GUIDES) , and I consider it should be more critical , but in the final result I get a overstress just in the bends at not in the straight pipe like in the underground model.

¿Why I get overstress in the straight pipe in the underground model?

I attached the models.

Thank for you concepts.

Your T2 is very high for buried pipe - 121C. That might be a valid temperature right out of compression but I don't think that temperature would hold over 700m away.
This T2 gives a very high compressive stress in the line and this, I believe, is the major source of your overstress.

Hi
Like Mr Dave said, 121C is high value for buried pipeline,and also your material is X42 with SMYS 42KSI,its true that the pressure is not very big(around 34.5 bar),but you need to check with your material team that the X42 can support 121C(maybe you need X60 and over for such case)!!!!!

Best Regards

In the example above sent (see. TEST.zip) we got some doubts about the criteria used by caesar II 5.0 of buried pipe. In our case we are avaluating a long straigth pipe where its suposed there aren´t transversal displacements that could generate overstresses due to thermal expansion along the pipe.

Why do you say that 121°C is a high value for a buried pipe? under wich criteria do you say that?

If we evaluate the same case in CAESAR II 4.5, the system doesn´t fail.

Thank you

B31.8 is a gas transportation code. For a typical buried gas pipeline, I wouldn't expect that line to keep such a high temperature so far away from a heat source such as a compressor station. But you're correct, I don't know how hot your gas line is.
I took another look at your input.
You have an anchor at both ends of this single run of pipe and only one 45 degree turn. With all the "guides" (horizontal soil bearing), your system cannot deform so you have no flexibility to absorb the axial thermal load. This thermal load is the source of most of your operating stress.
But even when I remove the anchors off both ends, the high temperature run still fails in the middle of that long straight run after the bend.
Other than that temperature, your data appears reasonable.
You ask about an "unburied" run with +Ys and Guides. You do not mention axial restraints. It's the axial term that's kiling you.