LC Peng û Stress analysis methods for underground pipe lines

[auzie5]

Just reviewing a great paper published by LC Peng that presents basic calculations for the stress analysis of underground pipelines. I’m sure most of you are familiar with this paper but here’s the link anyways:

http://www.pipestress.com/papers/UnderGrd-1.pdf

Page 4 of 5 works through an example where LC Peng demonstrates that the equivalent tensile stress at the fully restrained portion of the pipeline exceeds 0.9*SMYS (i.e. 47,993 psi > 0.9 * SMYS). The equivalent stress combines the hoop stress and compressive longitudinal stress (owing to the line being fully retrained and therefore unable to elongate axially due to temperature effects). Pressure elongation of the pipeline is arrested by soil friction and is therefore not considered in the analysis.

LC Peng goes on to suggest some methods to make the design work:

a) Use semi-restrained construction such as placing offsets at regular intervals;
b) Increase installation temperature by burying pipe at midday or running hot air through it before back filling;
c) Increase wall thickness.

I do not understand how option a) works. Even with expansion offsets on our below ground pipeline, will there not be sections between these offsets that are restrained therefore requiring inclusion of compressive longitudinal stress? The formula for compressive longitudinal stress is independent of pipe length so if there exists any single point on our below ground line where the line is “fully restrained” by soil friction, are we not required to include the compressive longitudinal stress (which would result in us exceeding 0.9*SMYS)?

Or is LC Peng saying that one possible solution is to include enough offsets at appropriate intervals such that the design of the below ground pipeline can be considered “unrestrained”? If this is the case, then is the idea to allow the pipeline to grow longitudinally between expansion offsets (due to temperature and pressure elongation) and use the longitudinal strain values to calculate the bending moments of the pipe at the offset bends?

I guess I’m very unclear on what “semi-restrained” means. Isn’t a line either “restrained” or “unrestrained”? And in real life, are there actual applications where below ground pipelines can be accurately designed to be “unrestrained” (perhaps in heavy muskeg or river crossings where the pipeline lies on the river bed – i.e. in locations where arresting longitudinal strain in not possible)?

Thanks in advance for any insight offered.

KDW

 

[BigInch]

Yes use enough bends such that any remaining straight run is less than the virtual anchor length. Offsets should be around 20 to 30 pipe diameters.

I hate Windowz 8!!!!

 

[BigInch]

Any buried pipeline is "partially restrained" just after it goes from above to underground and before traveling a length >= its virtual anchor distance. Partially restrained areas also occur when bends are made, as axial movement is no longer immediately resisted by equal and opposite directional forces from one side of the bend to the other. Again where soil types change and soil cohesion values are different. Partial restraint may occur during rainy seasons where same soil type becomes wetter in one patch and is not as cohesive as in other patches. It may also occur where ice has formed adjacent to the pipe wall and it melts. It may also occur during seismic loading when soil may liquify. It may also occur under a high wave, where hydrostatic pressures at the mud line and within the soil below may reach liquification pressures. It may also occur where pipe leaves the soil through an unanchored passage through a basement wall. It may occur where two parallel pipes join. Twice the load on one side of the joint as on the other side. That should open your imagination to additional possibilities.


I hate Windowz 8!!!!

 

[LittleInch]

Big Inch knows more than me, but the difficulty, as you allude to, is knowing how far the pipeline remains semi-restrained. Depending on your burial depth and pipe soil friction the length before your virtual anchor can be from 100 to 5-600m. Even at a nominal 200m introducing offsets on a straight pipeline becomes very costly in terms of bends and construction, such that usually some other means is found to reduce any longitudinal stresses.

The other factor to consider in hot lies is upheaval buckling which even if you solve the stress issue is a problem when you have any form of vertical undulation.

Your offsets also need to be able to move horizontally and only for a small distance or you run into issues of soil compaction and potential for filling in in the void and getting into issues over ratcheting of the line.

As BI rightly says, pipeline movement under pressure and thermal load is a max at the exit point, providing no ones put a bloody great anchor block in the way, gradually reducing to zero at the virtual anchor. If you look at a readout from a stress analysis, that's normally how you figure out were the virtual anchor is - first node where there is no movement.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[auzie5]

Thanks for the feedback guys. I was able to acquire a demo version of Simflex from Peng engineering and am in the process of working through some pipeline simulation examples to build my intuition towards how stress programs are able to frame these b/g pipeline systems. Incredibly well executed piece of software. I look forward to securing a full working version (current analysis in demo is limited to NPS 6 x Sch 40 although all functions are available).

Again, appreciate the valuable insight.