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high pressure gas pipeline stress analysis
3

high pressure gas pipeline stress analysis

high pressure gas pipeline stress analysis

(OP)
Hi,
I've been assigned the calculations on a high pressure gas pipeline, but have no idea where to start! Can anyone guide me?
Thanks in advance.

RE: high pressure gas pipeline stress analysis

(OP)
sorry...just re-read that...need to rephrase...I should have said the stress analysis calculations for the pipeline - it's a cross country buried one.  Thanks.

RE: high pressure gas pipeline stress analysis

2
Get out your design code and find the limiting stresses for your pipe materials, all area class design factors, wall thicknesses and yield strengths, soil density, soil expansion and cohesive properties, and minimum and maximum ambient and product temperatures and then identify all possible loading conditions to which your pipeline will be exposed.

Stress on a pipeline can come from,

1.  Hoop stress from internal and external pressures.
2.  Axial tension from internal pressure (closed end effect)
3.  Axial compression from expansion against anchor points.
4.  Bending stress from unsupported spanning at weak soil
5.  External loads due to traffic or soil pressures above
6.  Temperature changes  
7.  Uplift axial compressions from pipe expanding upwards at overbends, and conversely, tension from pipe contracting downwards and hanging up on the overbend.
8.  testing pressures
9.  liquifaction and loss of support by underlying soils
10. unstable landslide areas and lateral bending from soil slide loads
11.  seismic accelerations and/or fault zone displacements
12.  construction installation stresses
13.  tie-in temperature stresses
13.  other environmental loadings such as flowing water forces, snow or ice loads, buoyant uplifting, etc. just to name some of the more common ones I can think about right now.  Get you your old Strength of Materials texts and start reviewing combined stress analysis.

Out on the pipeline, you will be able to identify areas where some of the above loadings will obviously apply, but many are general loading conditions that may have no effect in some areas and great effects on others, or not even exist now but develop into significant loadings at a later date, such as fault subsidence areas, landslide loadings and the like.  If the line is more than a few kilometers long, it may be difficult to identify all the specific areas where you will have loading combinations well known enough to do precise calculations, so it is easier to set up limiting conditions that will not produce an overstress and then design and construct within those safe limits you have identified.  You should develop installation details according to your findings, typical valve supports, blowdown support details, uncased and cased crossing requirements, general guidelines for anchor block locations, bending limits, ambient temperature limitations wor welding in any tie-ins, etc., then design within those limits whenever possible.  Don't forget to put those limits in the construction and installation contract.  As you do detailed design for installation, you will find specific areas which cannot be constructed within those pre-identified safe limits and you can do a specific and more exact stress analysis for those actual conditions and locations as they are identified.

Specifically known fault and seismic areas and other areas presenting geological hazards should be identified ASAP and allow you to start specific deisgns that include provisions to allow the expected movements over the life of the pipeline to occur without producing overstressed conditions.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: high pressure gas pipeline stress analysis

(OP)
Thanks biginch - I've read your post twice and think I'm a little overwhelmed!  To start off with, by design code presumably you mean the code to which I'll be designing the pipeline and then to find limiting stresses of the materials, I'd need to go to the likes of API5L(?) - is this right?

RE: high pressure gas pipeline stress analysis

Pipeline design code might be one of several.  ANSI B31.8 / CFR Title 49 Part 192 in the USA, but could be different elsewhere.  There you will find the minimum loading conditions, Area Classification Factors and pre-approved pipe material yield stresses, and the limiting stresses.  With Area Class Factors, internal pressure, pipe diameter, wall thickness and yield stress you will be able to determine an initial allowed design pressure for various wall thicknesses using the Barlow based formula (PD/2tDF <= yield stress), or given the pipeline design pressure, you will be able to determine the minimum wall thickness required for installing the pipe through each different class factor area you have along the pipeline.

API 5L is the pipe production specification typically used for a range of strength in pipeline steels, so you will find the yield stress for all grades of material in API 5L, which will also be listed under the same material designation in ANSI B31.8, but in B31.8, you will find the class factors which must be applied to calculate your hoop stress allowable.  


The first step is to use the Barlow formula to determine the minimum wall thicknesses you will need for each class factor through which your pipeline will be installed.  That wall thickness will technically only be sufficient for hoop stress, as Barlow's formula does not address combined stresses.  There is a higher allowable for combined stresses, so you can have additional axial and bending stresses, analyze those in combination with hoop stress and find the combined stress to check against the higher combined allowable stress.  So total stress is not just limited to checking hoop stress against the Barlow allowable.  If you find areas where total stesses cannot be limited to the combined stress allowable using your initial wall thickness determinations, including hoop stress, you may have to increase the wall thickness for those specific areas.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: high pressure gas pipeline stress analysis

Hi brid,

First of all, read BigInch's narrative several more times.

I am sorry brid, but from your questions you seem to be in way over your head.  Please seek help from a more experienced engineer. For such an important pipeline, for the sake of everyone's safety, a very experienced person must be involved.  I would recommend to you that you first learn exactly how complicated an analysis you are facing.

You must determine what DESIGN Code is required by the contract and the jurisdiction (your reference to API 5L is a pipe material specification, not a design Code).  Get a copy of ASME B31.8 and absorb everything in the design section and then all the rest.  Find a textbook that addresses pipeline design (buried systems present a very different set of challenges to the designer/analyst).  Look at Mohinder L. Nayya's latest edition of the "Piping Handbook".

Now, read BigInch's narrative several more times.

Regards, John.

RE: high pressure gas pipeline stress analysis

(OP)
Hi,

Sorry, I think I've been misunderstood somewhat and think I can see why......

I've already designed my pipeline to the applicable code, so I already know my inlet/outlet conditions, pipeline length (approximate), wall thickness and material - my problem is purely the stress analysis.  There are so many types to choose from!! hoop, longitudinal etc. etc. - how do I know which are applicable to my pipeline and where can I find all the formulae to calculate/design the stesses out of my pipeline?

I hope I've asked the question a bit better this time and thanks so much for your help so far - really really appreciated.

RE: high pressure gas pipeline stress analysis

i will re-emphasize JohnBreen comments - get a skilled, saavy, knowledgeable, and competent engineer in this field to conduct the analysis and learn from that engineer.  doing this on your own is risky and most likely time-consuming.

meanwhile, good luck!
-pmover

RE: high pressure gas pipeline stress analysis

You mean you've done the preliminary design.  You probably have the wall thickness determinations based on hoop stress only.  Now its time to verify, and find out if you can add all the rest of the stresses to hoop stress and come out squeaky clean and still under the combined stress allowables.

OK here's a link to a basic pipe stress course I found on the web a year or so ago.  I zipped up for you and put it here so you can download and see what you've been missing up to now,

http://rapidshare.com/files/29275388/pipe_stress_course.zip.html

Its a start in the right direction.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: high pressure gas pipeline stress analysis

(OP)
thanks so much biginch - I'll get onto this right away and as you say it's a start in the right direction.  You're right - my design thus far only allows for hoop stress and therein lies the problem - I didn't know what else to expect.  To anyone else who's worried, I will of course have my design checked by an engineer experienced in the stress analysis area.  This is my first time out on the whole stress analysis thing and college was too long ago to go trawling through the books once more!
Cheers & thanks again.

RE: high pressure gas pipeline stress analysis

Hi again brid,

ALL the stresses that you have enumerated apply to your pipeline design and that is why everybody is recommending that you become very familiar with the DESIGN Code is required by the contract and/or the Code prescribed by the jurisdiction in which this system is to be built.  

You must recognize that the structural analysis of piping systems is tied to various sets of Codes and Standards. You have to determine what Codes and Standards are mandated by the jurisdiction in which the piping system you are to design will be located. Then you must simply sit down and read the Codes and Standards mandated by that jurisdiction – the cited books will help by providing some of the background for the rules you see in the C&S’s.

The Code will tell you HOW you must calculate the relevent stresses (the equations) and what criteria you must satisfy. The Code typically will tell you what minimum wall thickness your pipe must have to satisfy the requirements for circumferential (hoop) stress.  Longitudinal pressure stresses are combined with other sustained (primary) stresses (e.g., due to weight) and a maximum allowable stress for this combined loading stess is prescribed by the code.  Stresses caused by thermal expansion/contraction (secondary stresses) must also be calculated and compared to the Code allowable stress range.  These are fundamental stress assessments that all the piping Codes address but in the case of buried piping systems you have some unique loading conditions that you must address.  You will have to address all the issues that BigInch described for you.

Please download the manual you will find at:

http://www.americanlifelinesalliance.org/pdf/Seismic_Design_and_Retrofit_of_Piping_Systems.pdf

read this document from beginning to end and keep it handy for reference.

What computer program are you intending to use for the structural analysis of the piping system?  Go into to manuals for that program and determine how you must build the analysis model to include the interaction of the piping with the soil - this is very, very importaant.  The soil supports the piping and it restrains the movements of the piping during expansion/contraction.  Look for examples in the program's manuals and study them.

Please look for help!

Regards, John.

RE: high pressure gas pipeline stress analysis

brid,

You said:

"I've already designed my pipeline to the applicable code, so I already know my inlet/outlet conditions, pipeline length (approximate), wall thickness and material - my problem is purely the stress analysis."

As BigInch has said, you design is preliminary.  You have NOT "designed my pipeline to the applicable code" until you demonstrate that the design complies with all the requirements of the Code at issue (including all the requirements for stess limits).  Until you demonstrate complete compliance you are not finished with design - layout and drafting maybe yes - but not design.  

John

RE: high pressure gas pipeline stress analysis

(OP)
thanks for that johnbreen - I've had a quick look and think it should give me a good start too - although seismic loading isn't one I've to worry about (thankfully!).  Getting the equations seems to be my biggest problem - the code doesn't list any, just that adequate stress analysis must be carried out.  At the moment, I've got B31.1 Power Piping on my desk and so will flick through this.

RE: high pressure gas pipeline stress analysis

brid,

Your optimism will help you achieve results for your work.  There are tools and answers for what is needed to complete the pipeline design.  However, the advice of BigInch, John Breen, and pmover should impress on you that it is not a simple task to perform a pipeline stress analysis.  Which is the stress to consider? All must considered, and the criterical loads analyzed to avoid failures and problems.  The stress analysis results are also required for defining loads to civil and structural designers.

Sure, you could purchase a stress analysis software package - look at coade.com website for CaesarII software at cost of $11k.  You could even go to training for a week for $2K plus expenses.  There is the coade website with bulletin board posting questions of pipe designers to learn from their problems.   Does all this make you into stress analyst capable of producing a good design?  Are you responsible for specifying field examination and testing? Would you be able to put a P.E. stamp on the design drawings?  Or would it better to pay 1% of the cost of materials to get a piping design verified to be in Code Compliance by a competent engineer?   

The buried portion of gas pipeline may not have the most critical stress requirements.  If there are compressor stations along the pipeline, the above ground piping and loads on the compressor station equipment will require more attention to design and installation than the buried pipe.       

RE: high pressure gas pipeline stress analysis

brid

I'd like to circle back on something...

You started off by saying you were

Quote:

assigned the {stress analysis}calculations on a high pressure gas pipeline
but didn't know where to start.  A few posts later, you stated

Quote:

To anyone else who's worried, I will of course have my design checked by an engineer experienced in the stress analysis area.
I'm taking this to mean that you're not the only engineer in your company and that this is somewhat of a learning experience for you.  As both biginch and John Breen have emphasized, this is not a trivial design exercise.  Besides the help you get here, and having the final product checked, you need to be checking in with your senior engineer/mentor along the way.

Your company is ultimately responsible for the design.  If this high pressure gas line is installed and fails due to  design errors, no court is going to be very sympathetic to you telling them that you relied on an Internet forum on how to do your design (even though you're getting some excellent advice from two very knowledgeable engineers).

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: high pressure gas pipeline stress analysis

(OP)
Thanks again everyone.
Biginch, my pipeline won't be governed by American Codes however I do find them very useful.
vpl - you're correct - I work with a firm of consulting engineers and will, as I've already stated, have my design checked by an engineer experienced in stress analysis.  I think some of you may be taking my post a little too literally - I am fully conversant with pipeline sizing etc. however have not carried out a full stress analysis before.  This is the only problem (albeit a large one) that I have - it's not insurmountable - I'm not entirely stupid and realise that the courts wouldn't take too kindly.  I'm using this website purely as a starter in order to get a pointer in the right direction.  As soon as I've had a chance to go through what you've suggested (particularly the links offered - thanks) - I'll then revert to my seniors in order to get their advice on the next steps.

RE: high pressure gas pipeline stress analysis

If you follow the CFR's you won't get too far from having a B31.8 compliant result, but B31.8 does give more insight into the methods.

If you run into any "insurmountables", feel free to write to me.  You'll find my e-mail on my virtualpipeline MSN spaces pages... in the contacts segment.

vpl... i like your initials.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: high pressure gas pipeline stress analysis

(OP)
Thanks biginch - you're a star and for that, you get a star! I'll certainly take you up on your offer of help should I hit a wall along the way.

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