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How to Calculate Actual Compression Stresses Acting on the Buried Pipe

04/11/2017 11:11 PM

Hello everyone ! may i know how can i calculate actual compression stresses in longitudinal (axial) direction. The pipe is buried below the ground about 25 feet and i would like to calculate the actual stress acting on that pipe by the surrounding.

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#1

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 12:45 AM

If you do not mean radial stress:

Axial stress might be the difference from conditions when buried compared with running conditions: mainly temperature. So you would be looking for stretch or compression due to temperature changes.

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#2
In reply to #1

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 1:10 AM

Thank you for your reply! May i know is there any formula for that radial stress ?

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#3
In reply to #2

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 6:07 AM

Yes. Carry out a force balance on a longitudinal section through the pipe. It's only arithmetic.

One way to consider the compressive stress on the pipe is to use a hypothetical fluid of the same density as the surrounding subsoil and cause it to act uniformly on the pipe outer surface.

Of course the outer stress is compensated by the stress caused by the pressure of the contents.

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#7
In reply to #3

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 8:35 PM

Thank you for your reply !

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#4

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 6:25 AM

This is a civil engineering problem.

Is it not the weight or load of the soil with in the cross section along the horizon?

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#8
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Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 8:40 PM

yap! this is civil engineering problem but i found out somewhere it can consider with axial stress also in order to calculate the critical buckling stress of a buried pipe.

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#5

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 7:44 AM

Axial stress is along the pipe length.

You first need to know the pipe consruction/asembly method. Flanges create a rigid sructure, while rubber ring joints (RRJ) enable some movement at the end of each pipe length. Then you need to know whether the pipe is constrained by being captured in cast concrete or similar.

If RRJ is inserted too far, then there are problems, even with Ductile Iron.

Then you need to understand whether it's running up/down grade. Unrestrained pipe can "slide" downhill over time if in loose fill.

Then you need to consider what the contents are doing. Friction from steady state flow will create "drag" in one direction.

Then you have problems when change in flow rates happen. I've seen 18" pipes move the covering ground during flow shut-off situations and abrade through the pipe material.

You do have a real challenge though, "calculate" implies some prediction based on assumptions and observaitons while "actual" implies measurement of the situation.

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#6
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Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 10:24 AM

I concur. There is no model that could possibly be accurate enough to simulate your actual situation, although you might get order of magnitude estimates, by considering the type of fill material, grade, flow conditions, anchoring of the pipe, etc.

Why are you suddenly asking this question? Were put in charge of a pipeline after it was built, after a failure? Are you a regulating body representative performing an audit on a pipeline company?

We would enjoy having more information on the precise situation involved, as some of us here could probably give you a better answer as a starting point.

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#9
In reply to #5

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 8:48 PM

Thank you for your reply ! I understand that actual ground implementation is very different and challenge but i am just started study of designing the underground pipe laying.

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#10
In reply to #5

Re: How to calculate actual compression stresses acting on the buried pipe

04/12/2017 8:56 PM

I realized that it is not a easy going calculation but i need to figure out the critical buckling pressure of a buried thermoplastic pipe (eg: PE100 pipe). This is my new topic and i am from mechatronic background. So all the usages are very strange.

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#11
In reply to #10

Re: How to calculate actual compression stresses acting on the buried pipe

04/13/2017 7:17 AM

One way is to carry out a practical test:

https://www.youtube.com/results?search_query=rail+tank+car+implosion

Another way is to ring the pipe manufacturer, and ask.

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#12
In reply to #10

Re: How to calculate actual compression stresses acting on the buried pipe

04/13/2017 11:41 AM

Why would buckling pressure be more of a focus than crush pressure? You cannot guarantee in any such situation the pipe will always be under pressure.

Buckling pressure will have so many variables, makes me think you need to find out if lines such as the one you are focusing on are buried with a slight tension on them.

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