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Bent Pipelines

Bent Pipelines

Bent Pipelines

(OP)
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.

RE: Bent Pipelines

Could the soil and piping settled/have attempted to settle relative to the piles (or may the pipe metal not be sufficiently thick to handle such loading etc?)

RE: Bent Pipelines

What in gods name is an "underground guide"???

Buried pipelines should be continuously supported by soil. Introduce hard points /point loads and you have huge trouble..

Can you sketch this or describe it better please.

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

RE: Bent Pipelines

(OP)
The guide is a pipe (piling) with an I-beam on top and two 2" pipes welded on each end to prevent the lateral movement of the line.




RE: Bent Pipelines

I've seen this on numerous occasions on tool runs and it is almost always caused by underground supports. My company expressly refuses to accept the use of underground supports like this to the differential settlement between the support and the bottom of ditch which leads to this issue.

RE: Bent Pipelines

(OP)
canadapipe, thanks for your response. That is what we were thinking too. Since the lines are resting on top of the support I would think that the dents at the bottom of the pipe are likely caused by the support coming up.

The ILI data indicates that it is right where the support is, but we don't have verification of where exactly it is.

RE: Bent Pipelines

I am astounded. I have never seen anything like this before for exactly the reason you've found. What on earth was this monstrosity installed for?? Looks like a piping designer who didn't know a thing about pipelines installed this.

These should be removed forthwith and never installed again.

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

RE: Bent Pipelines

We don't need checkers any more, we have computers!

RE: Bent Pipelines

Just to add confirmation, I have seen this exact scenario before, and guess what, the dents are always right at the I beams. I have even seen this scenario play out, and what happened, oh there are dents, there is not enough support here, better add more I beams, and guess, what, more dents.... anyway, my recommendation would be to remove, you may be able to put a sleeve over the dents resting on the I beam, but not sure that would work in the end, you still have a similar problem

RE: Bent Pipelines

(OP)
Thanks for the responses everyone! What would you say the cause of the dent at the I-beam is? The support lifting?

RE: Bent Pipelines

Not usually. It is the settlement of the soil either side over time which then leaves the pipe attempting to hold up the soil above it. Not surprisingly for such a small pipe it can't. Therefore the pipe bends.

The risk for any point load or differential settlement is a shear failure causing a complete rupture.

I remain absolutely astonished that a few of you seem to think this is somehow "normal practice".

Whoever started this, which then somehow got moved onto the next project as something that is normally done needs to cease pipeline engineering and go back to piping where they belong. Is this a strange Canadian thing? Wherever it comes from it needs to stop - I though anchor blocks were bad enough, but "Underground guides" are far far worse. Think yourself fortunate that you only have a dent - it could easily be much worse.

Point made?

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

RE: Bent Pipelines

Agree that underground pipe supports should not be used.

Differential settlement of a structure to which the pipe is rigidly connected can induce not only high bending moments, but also shearing forces. These forces and moments are set up when the structure and/or the pipe moves laterally with respect to the other. Quantitatively, these induced stresses are not easily evaluated. Effort should be made during design and during construction to see that differential settlement is eliminated or at least minimized. This can be accomplished by the proper preparation and compaction of foundation and bedding materials for both the structure and the connecting pipe.

A realistic design load for a pipe is the prism load, which is the weight of a vertical prism of soil over the pipe. The prism load will be different for the pipe support and the pipe because of the different cross-sectional area. Also, a true trench condition may or may not result in significant load reductions on the pipe since a reduction depends upon the direction of the frictional forces in the soil.

RE: Bent Pipelines

(OP)
Thanks for everyone's responses! What are your opinions on installing a flat plate between the support and the pipe that has a lower moment of inertia than the pipe? The idea being that the plate would re-distribute bending stresses across the length of the pipe that is in contact with the plate instead of having the sharp edge at the I-beam. We can do the FEA to determine what force was required on the pipe to create the dent in the first place and then use that to determine what length/thickness/I of plate would be required to re-distribute the force so that the stress in the pipe is below Code allowable if it were to sink any further.

Since the original design information is unavailable and this line has seen service, I am hesitant to remove the support/guides only to have the loads transferred to the pipe as it comes aboveground or at the aboveground support.

RE: Bent Pipelines

No no no no. Just remove the underground support which should never have been there.

There are no mysterious loads other than in the mind of demented piping engineers.

Design this properly and there are no issues. Don't be beholden to a piping engineers view of buried pipeline design. They really don't understand it, hence this monstrosity you've been landed with.

Dig up the entire thing, remove these point loads, compact the ground again then you won't have an issue ever again.

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

RE: Bent Pipelines

Or use underground shoes.

RE: Bent Pipelines

(OP)
Europipe, what are underground shoes?

RE: Bent Pipelines

I'm sorry , forgot the smile

RE: Bent Pipelines

Ummmmm ..... Underground guides are those evil individual who aid you in your journey ...... to Hades !!!

I like this thread ... it has "demented piping designers" and a strange "Canadian thing" ..... lot of potential for cross-border conflict !!!

MJCronin
Sr. Process Engineer

RE: Bent Pipelines

It's still one of the strangest things I think I have ever seen in pipeline design.

To have a proper isometric and something which some one has clearly thought about and still done is just mind boggling.

BTW, it would be Trans Atlantic conflict....

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

RE: Bent Pipelines

(OP)
The pipeline is built through soft soil (i.e. muskeg). I think the supports were put there to limit the settling of the pipeline which could have caused dents in the riser pipe aboveground.

If we remove the supports, what can be done to ensure that the pipeline doesn't settle excessively and cause dents elsewhere?

RE: Bent Pipelines

This was installed in Alberta, I take it?

RE: Bent Pipelines

(OP)
Correct.

RE: Bent Pipelines

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

RE: Bent Pipelines

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.

RE: Bent Pipelines

(OP)
Peat bog is the closest comparison.

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

RE: Bent Pipelines

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.

RE: Bent Pipelines

(OP)
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?

RE: Bent Pipelines

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.

RE: Bent Pipelines

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.

RE: Bent Pipelines

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.

RE: Bent Pipelines

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.

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