Bent Pipelines

[loilfan]

An ILI found dents >6% deep near 3 of our underground guides in a double S-bend riser for an NPS 6 pipeline. The dents are located at the 6 o'clock position. Verification digs will be performed in a few months, but we are thinking it is either caused by rocks (not likely to have caused all 3 dents), the support falling off the guide or the support piling has moved.

Has anyone else come across a similar situation with a different cause for the damage? Corrosion does not appear to be a significant factor.

 

[Gator]

This was installed in Alberta, I take it?

 

[loilfan]

Correct.

 

[europipe]

Can't You put a plate (stainl. stl. , hard neoprene, ptfe) between the pipe (fixed to the pipe) and the u.g. t-post?

 

[LittleInch]

Ok, kind of like a peat bog?

Generally the only successful option is to excavate down to something quite hard, build it back with rocks / stone etc and then lay the pipe in or on that surface.

Not great ground normally but then I assume the AG section has some good foundations?

Differential settlement is not good news for any type of pipe, but putting supports there just makes it even worse, especially for small diameter, thin wall pipes like you have.

The key is to put in sufficient material to prevent differential settlement, monitor the situation and allow some flexibility in the incoming pipe.

Either that or extend the supports and make it an above ground pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[loilfan]

Peat bog is the closest comparison.

What do you mean by extending the supports to make it an aboveground pipe?

 

[LittleInch]

Making the supports taller. You either remove the supports and do something else to support the pipe on a contiguous support basis instead of point supports, or lift the pipe clear of the ground.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[loilfan]

Oh i see what you mean. I'm sure that the cost of lifting an entire line out of the ground will work but that there is a cheaper option we can go with.

Does anyone have any experience with the idea of removing the supports but putting a plate (say 3/16"x2'x20') below the pipe to give the pipe a larger area to bend over and also restrict the settlement of the pipe?

 

[LittleInch]

Yes, it's still a terrible idea. Get rid of the supports. That's the root cause of your trouble. Messing about with them won't help.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Muskeg may be quite deep. In Alaska, the depth was as much as 20 feet.

I can see the reason the designer used the pipe supports. A better solution would be to install the pipe above grade on the supports.

 

[LittleInch]

I agree routing pipeline in peat bogs isn't easy. The last one I worked on was up to 5m deep. That was a lot of stone to pour into the ground to get down to something solid, but there isn't really much option, especially when you get close to the below ground / above ground interface.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rconner]

This thread started out advising the readers the pipe had (underlying?) "guides" in the vicinity of an arrangement of off-setting bends, and the figure later supplied appeared to depict the problematic guides were underground. A later comment of the OP stated an apparent different or concurrent function of the supports i.e. to "limit the settling" of the pipeline in "muskeg".
In any case, pipe have of course been supported on intervals for hundreds of years, more commonly aboveground, but also in many cases over the years in unstable soil masses as well. However, the pipe, as well as contact points and the support itself, must be sufficiently strong to take the loads imposed (as well as stiff enough to prevent objectionable deformation). High localized stresses and/or deformation can occur at supports when thin or weak pipes are supported at relatively unyielding, flat supports (i.e. where the contact area may be near infinitely/VERY small) and particularly underground.
I believe all manners of ground and pipeline behaviors, in addition to vertical settlement of the soil and pipe mass (or flotation, if e.g. a line lighter in bulk density than that of the OP is strapped down in liquefaction incidents) of the pipe and soil mass, can contribute to localized loads/reactions. Examples of perhaps further complicating behaviors would be the effects of e.g. at least slight Bourdon (extension) movements of the piping due to thrust at bends near the supports, as well as thermal expansion/contraction etc. In the case of the figure configuration, it would appear even slight thermal expansion and outward and downward thrust forces due to internal pressure would (pivot the riser to a some longer vertical elevation difference and) perhaps press the pipe barrel even more tightly against the nearest support plate as the upbend at the bottom tried to move downward? [Also, what is the span of the piping both between the supports or guides as well as outside of the two supports depicted, that were to "limit the settling" in the muskeg, is there any traffic over the area, and could/do ice lenses form over the line between same and the surface in the winter?]
I think some background of and reference to pipe on supports and research involving same, particularly to calculate maximum localized stress once reaction loads are defined, can be found in many past threads, e.g.

http://www.eng-tips.com/viewthread.cfm?qid=394902.