Dear All,

I am performing an underground 12inch gas pipeline stress analysis as per B31.8. The pipeline starts from an above ground manifold and runs in underground for around 2 kms and then connecting to the buried 12 inch pipeline. In between the line raising to above ground for a valve station and then goes to undeground.
Header pipeline has been modelled around 1 km on both side.

I am facing a stress failure in the above ground-underground section. I had tried to change the next available material from API-5L B to API-5L X60 to have more SMYS, still it shows failing. Tried with increasing the bend radius and a routing change in the A/G portion, none is working.

Pipe size 12inch
Temp: 176 deg F
Pressure : 1264 Psig

I am attaching herewith the C-II (ver 5.1) files and a picture shows the above problem.

Could someone please suggest me some solution to reduce the stress.

Also please suggest me the options we should try to qualify, incase of a stress failure in U/G analysis.


Regards

Babu

Hello Mr. Babu,
I hope my reply isnt too late, I just checked your Analysis and I think you should try these;
1. Put a Hold-Down Support (Y-restraint) @ the Mid-span between nodes 6820-6840 (with Mu.=0.3)
2. After converting to Buried Piping Model, Put Axial Stops @ nodes 3640 and 3780

This will do though the code stresses around the bends at node 3700 is still on the high side (98.6%) ! but enough tips to work with, and you can try modifying the soil ppties at the vicinity of the bend (node 3700) eg. increase the soil density (say by 50% if its feasible) from nodes 3640 to 3780, you can also try increasing the soil cover depth.

Please Note:
In Your Soil parameters you specify the temperature as 88F, I think it sould be 144F (176-32) or what dyu think?
Please verify that the Hydrotest Pressure is "really" 1.5*Operating pressure. Note that ASME B31.8 says use 1.25 if the pipeline is at a class 1 location and 1.5 if it's at Class 2,3 or 4 Location.

I hope all these tips helps,

Dear Mr.Caesarian

Thanks for your detailed reply.