Stress analysis of cross country piping 1

[nstress]

Hello

I have to do stress analysis of cross country (Fuel gas) piping. Though I had done the stress analysis for in plant piping,this is new for me. Are there any special considerations which I must take care? I am going to do the analysis in Caesar.

Thanks all.

 

[DSB123]

nstress,
Cross country pipelines are different to on plant piping. Caesar for example does not consider buckling aspects of pipework which comes into action when you have long lengths of "cross country pipelines". Is the piping to be buried? if so and the contour of the land is not flat then the piping needs to be checked at all bends to ensure there is sufficient coverage. Caesar is strictly(as most stress packages are) for plant pipework where buckling does not play a major part.

 

[LSThill]

HI nstress (Mechanical)

HI DSB123 (Mechanical) you need to do your home work, Please read below:

Piping Stress Analysis just get better

Guideline for the Design of Buried Steel Pipe - July 2001

This guideline presents design provisions to evaluate the integrity of Buried steel pipe for a range of applied loads. It applies to new or existing buried pipe made of carbon or alloy steel; welded pipe; piping designed, fabricated, inspected and tested in accordance with an ASME B31 pressure piping code; and buried pipe and its interface with buildings and equipment.

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

Leonard Stephen Thill
Leonard@thill.biz

http://www.thill.biz

 

[DSB123]

Hi Isthill,
I know what I said is correct. You have not said otherwise. I know there are techniques for analysis of overland and buried overland piping systems. What I said , if you would be read it again, is that Caesar does not account for buckling aspects which a concern with long lines. do you dispute this???

 

[nstress]

Hi DSB123 , Isthill

The pipeline is overland only and not buried; If Caesar can not do this, is there anyother package?

Thanks for your tips

 

[dbevil]

Cross-country pipelines rarely require a formal stress analysis with programs such as Caesar. An above-ground fuel gas line may be designed as restrained or unrestrained. For either the design should consider the extremes of ambient and operating temperatures. Design and hydrotest pressures also will be considered. For a restrained line, the supports will need to prevent buckling (typically about 3% of the axial force) and the end anchor of course would be designed for the maximum axial force. Changes in direction will require additional calculations to ensure that the lateral forces of the bend are not exceeded. Finally, the wall thickness should be sufficient to ensure that the combined hoop stress and thermal compressive stresses are within allowable.

Unless the line is long and straight, most undulations in the terrain or normal changes in direction will be suitable to prevent excessive movement of of the pipeline, but the end anchors should consider the cumulative effect of friction on the supports.

 

[gramya]

There is another very useful software which takes care of the buried pipeline. It is AutoPIPE Plus now licensed by Bentley ( earlier with rebis USA). The analysis takes care of the earth pressure, frictional longitudinal stresses imposed on the buries pipeline as it tries to expand due to temperature variations when being charges, takes care of the soil support to pipeline by considering the bearing capacity of the soil ( modelled as support at close intervals). You may try to visit

http://www.bentley.com

and view the capability of AutoPIPE Plus under Plant Design/ Engineering softwares.

Narendra K. Roy
Gramya Research Analysis Institute,
PO box 4016, Vadodara 390015, India
Website:

http://www.gramya.com

;

http://www.charismaglobal.com

 

[KevinNZ]

We have used Autopipe for 100's of cross coutry steam lines for geothermal power stations. Autopipe does not consider buckling but it has not been an issue. Tyipcally our lines are large (200mm to 1050mm) and the spans 10 to 18m.

Cheers
Kevin

 

[DSB123]

Hi KevinNZ,
With Steam lines you generally guide these at regular intervals such that buckling (unless you have done something totally stupid) never is a problemm/issue. However for long "ambient" lines which may be buried or not where they are not generally guided at the same frequency as steam lines or anchored at the same frequency buckling could become an issue and as such taking the results from programs such as Caesar and Autopipe as being the sole check could be frought with danger.

 

[Hookem]

For buried cross county petroleum pipelines, look up API 1102 (highway & railroad stresses on pipelines and their casings), and others. One of these others is for lowering existing pipelines. Others deal with horizontal directional drills. Pipeline Tool Box (google search) is a starting point. But, no single program, Triflex, Caesar II, AutoPipe, etc. has all the answers.

Before the battle, one must arm themselves with knowledge.

 

[DrillerNic]

Don't forget that cross country pipelines often have elastic bends put in them whereas on plot piping almost always uses elbows, so the stress regime in cross country lines is different...

Is the line going to be restrained or unrestrained? Again, cross country lines are different to on plot piping as the friction factor with the ground means that even an unrestained line, if it's long enough, will be restrined over a large part of its length, and the axial forces you'll need to consider will only be over the free sections at each end.

It was due to these differences that when I was a pipeline engineer in one of the biggest contractors in the world, the Piping boys used Ceasar and the Pipeliners used AUTOpipe, with a hand calculation as per API RP 1102 done at any road & railway crossings.

Upheaval buckling can be an big issue- I've seen an offshore line which popped up and then got caught by a fisherman's trawlboard and dragged a few hundred metres. Cost a lot of money to do the repair (still the fisherman lost tens of thousands of pounds worth of fishing gear, which serves him right for trawling along a pipeline!).

 

[dan511]

What are the criteria to evaluate the buckling effect for underground piping?

 

[Hookem]

Buckling does not control steel pipes. Look at API 1102. This calculates stresses and compares them against an allowable. The criteria therein are based on limiting the vertical deflection to less than 3% of its outside diameter when subjected to combination of dead and live loads with full internal pressure.