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.

 

[rconner]

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?)

 

[LittleInch]

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.

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[loilfan]

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.

 

[canadapipe]

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.

 

[loilfan]

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.

 

[LittleInch]

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.

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[Gator]

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

 

[brimmer]

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

 

[loilfan]

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

 

[LittleInch]

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?

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[bimr]

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.

 

[loilfan]

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.

 

[LittleInch]

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.

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[europipe]

Or use underground shoes.

 

[loilfan]

Europipe, what are underground shoes?

 

[europipe]

I'm sorry , forgot the

 

[MJCronin]

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

 

[LittleInch]

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....

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Also: If you get a response it's polite to respond to it.

 

[loilfan]

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?