In one of our plants, 20” steam pipe displaced from its supports due to some (at the moment – unknown) forces. Investigation is on. A few facts for the fallen line:
1) Pipe is on a rack and has many expansion loops.
2) All shoe supports fell from rack beams between two expansion loops (Say from loop A to loop B).
3) Pipe displaced axially around 550mm (this is measured at a location between loop A and B).
4) Between loop A and B, the pipe is resting on some of the beams (shoe displaced) while it is not touching down on other places on beams at its (originally designed) support locations. At one of these locations where pipe is not touching the beam, it is measured to be having 75mm clearance from the beam.
5) Some of the axial supports are sheared.
6) Shoe height is 150mm for this line.
Now the main question is how to model this “displaced” piping in Caesar II.
One opinion is that the line to be modelled with 75mm gap and +Y supports for all supports between expansion loop A to B and axial displacement of 550mm at one place where it is measured.
Other opinion is that from loop A to B supports shall be modelled as +Y with 150mm gap and 550mm displacement where it is measured.
Which one is correct?

How you model depends on why you're modelling it. What is it you want to get out of the analysis?

The stresses in this displaced condition of the line.

Personally, I wouldn't even include those support locations, gap or otherwise, unless you believe that under normal operating conditions the pipe will settle down onto the supports during operation - i.e. displacement is equal to or greater than the gap at these particular supports locations.

Generally speaking, you do not need to model shoes into CAESAR, unless you believe that the shoe has an anchor or guide that is applying (significant) moment to the pipe.

If you're attempting to perform an analysis on what the stresses could have been within the pipe before this failure, then you should consider anchors that have sheared off as your biggest culprits in this matter.

Dear Michael,
First I know (and am not trying to) that shoes are not modelled in CAESAR II.
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Second, what I am trying to simulate is:
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"When line displaced horizontally, it fell from shoes on to the structural beams below on few of these supporting points due to sagging (bending) of the pipe. But some end points in pipe are not resting on the structure which (at one location) have 80 mm gap (where it is measured). So, the simulation of these supports in CAESAR II would be 1) removing the axial stops on the line which have sheared. 2) Providing the horizontal displacement on the point where it is measured. 3) For all the supports where shoes have fallen the supports shall be modelled as +Y supports with gap equal to the shoe height."

Dear Shahid,

Find below my comments:

1. Your purpose for doing this analysis is not logical. The analysis of already failed, deformed and dislocated piping does not make any sense, Neither it can be performed correctly through CAESAR II. However, if you want to find out the root cause of problem, it makes sense. Then you have to model original piping with original isometrics and go on.

2. Giving gap in +y support, where it is not resting will also not serve the purpose as normal project based pipe support span length is much much less than CAESAR II failure results. For example, project spec says that horizontal span in 20 pipe is 12 meters. However, your CAESAR II results will show that 20" pipe will not fail even if the span becomes 35 meters.

3. I am sorry but imposed displacement in your case is entirely incorrect approach. It is normally used for equipment thermal displacement.

4. Similarly removing axial stop in between two loop will show no significant results.

Now what you have to do..

1. Do analysis as per original isometrics using CASEAR.
2. Review CAESAR II results for any failure
3. Observe the deformed support to find any difference in original isometrics and site condition.
4. Diagnose the issue , modify the iso, if required. And ask construction team to construct as per your design.
5. You may also check from your process department about probability of happening of any slug, surge, pressure fluctuation in your line.

Regards,
Ali

My guess? You had some sort of transient event (steam hammer?) that pulled your system.
So now your line is "sprung" - it is unable to return to its operating position because some of those Y supports are now acting as line stops. Without knowing the added load that your system is currently holding and because of possible yielding, there is little chance that you can estimate the load distribution.
One thing you could try - define displacements (replacing the restraints) at all those points where the pipe is newly restrained. You might be able to recreate the deflected shape of your system.

Thanks Ali Raza for your inputs.
As a first step, we did all what you have recommended to see if the line had some design issues. But line is perfectly safe in design (and as-built) condition.
Dave, yes some transient loading was there, which broke two axial stops and moved the line horizontally so much that the line fell from shoes. Now it is hanging from its end points (the points where supports are as per intended design.) The line is resting on two of the supports with pipe bottom touching the beam top and at three location it is showing sag and is not touching the beam. Still lifted.
When the transient force was there, the line moved horizontally at the +Y support locations and when shoe bottom moved away from beam, it was like pipe is 150mm away from supports, so it bent down (fell) due to gravity and is now resting on two of these supports as stated earlier.
This is the reason for the modeling +Y supports with 150mm gap.
Dave, good point about Y supports acting as line stops is interesting and logical. This may be happening now.

One more thing, the line has displaced, but no rupture.
This line feeds almost two big plants, for which shutting down is too costly. (I hope delaying its shutdown does not become MORE costly than shutdowns). So we are trying to evaluate the line's stresses by modeling displaced condition to decide about shutdown.

Providing loops alone on the pipe rack does not give stress engineer to control the pipe thermal expansion. You should also provide adequate line stoppers or anchor type supports between or ends of loops, restraints somehow depending on the pipe arrangement. Entire pipe under the thermal/seismic load will float on the rack if it does not see adequate resistance axially and laterally, and you will see all the shoes will leave the structural supports unfortunately.

If the case as I explained above you do not have any guaranty on the stresses on the piping. If all the supports moved off the supports the problems may be concentrated at the beginning and end of the rack on the piping. If some stays on shoes , some still on the supports the problem is more complicated which depend on the on/off support locations. If you are lucky and very large gap between allowable and analysis stresses it may still work up to until the planned shotdown, but there is no guaranty for creep, stress, strain and integrity of the piping. I would not rely of it at all and get prepared to replace the section under consideration. If you decide to continue to operate take all the precautions by assuming it will fail any time soon where the stresses are more concentrated. I believe you do not have time to get real stresses by analysis since this will take a long time if the problem gets more complicated, and the results may not be satisfactory either. However you'd better get everything documented, prepare markups on the isometrics for thermally expanded and new locations of the off-support shoes ASAP to make decision.

If the lateral supports (guides) are connected by design onto the shoes the lateral supporting action may not be there anymore, so problems are more complicated for modelling on the pipe as it is.

I would not make the decision myself, advice the management for the problems and make decision all together and document.

Good luck.

It seems that I did skip reading last responses earlier. However I still would like to see the entire piping isometrics on the pipe rack with adequate markups on the condition of the shoe support (axial and lateral displacement, off-support etc...) to be able to see the condition on the pipe on the rack in addition to pressure, operating and design temperature, wall thicknesses and material. You only talk about the expansion loop area only despite there are many expansion loops as you explained. Is the area that you consider can be isolated for the discussion? Verbal explanation would not show the full picture, a markup is essential for this kind of discussion, I would say.

This is what I wanted to model.

I would model it thusly (for code compliance:

1. As installed with all supports. If it wasn't installed per code to begin with, you can't really expect to make things better while removing supports, and this is to defend yourself and your decision.

2. With the intended thermal displacements as though the shoes were incapable of falling off. Again, if it wasn't passing before this event, you're not making it any better, again to defend yourself and your decision.

3. With the "lift-off" supports completely removed. You never specified piping code, but B31.3 wants you to run any long term "lift off" in the SUS case. If it can't pass this, then it can't really pass this while under duress of permanent deflection.

While I have not performed any fitness for service analyses personally, I would be inclined to expect you can use values from 3 above within such analysis to determine length of life left with permanent deflection and lack of support.

Thanks Mr. Fletcher,
Please remember that line displaced horizontally and then fell 150mm down as shoe height is 150mm. The lift off is misnomer here. Actually the line fell down and bent most in the middle portion and that portion is resting on rack beams. While end points (not bent as much) are not touching the supports.

If the piping itself is touching the rack, then I can agree with a support with gap recommendation.

You also need to model the bends in in order to accurately calculate loads onto the beams from the pipe that's quasi self-supporting.

When you model the piping in the temporarily unsupported case, you'll have bending moments applied to the piping for which you to perform your bend calculation.

There's more to this than modelling restraints. In fact, since you can see what restraints are currently "active" and "inactive", there are no nonlinearities in the current state of the system.
Like I suggested earlier, a transient may have upset your system resulting in one of your shoes (a +Y support) turning into a line stop.
You must somehow model the spring load (as in springing the system) so that your "operating" state matches the current state of your system.
Here's another guess; probably only one of those shoes is carrying all that spring load. I suggest you model a lateral imposed deflection at what appears to be the one, new, "lateral support" that carries the remaining spring load. If you can replicate the current position of all the nodes, then your model is representing the system.
(I referenced a line stop in describing the "new" boundary condition. But that transient load may have also developed a lateral load component that may also be carried by your new support.)

It's a really interesting application, and I definitely agree with Dave that at some point some shoes became anchors. Let's revisit the steps of what we think happened:

1) The line grew hot. (normal analysis)
2) The line displaced axially/laterally due to expansion.
3) The line fell off its supports in some locations (displaced vertically). This act could have bent the pipe.
4) The line presumably cooled down, but remained vertically displaced in some locations, but is now moving axially/laterally in the reverse direction.
5) The line shrank, but encountered new "anchors" in a displaced condition. Some shoes sheared off. This also could have bent the pipe, or made any previous bend worse. It's noteworthy that these anchors will apply a moment and a force.
6) Additional shoes in other locations may have contacted their supports after the previous shoes sheared off as the line continued to cool. They may have sheared, as well.
7) Present day configuration.

And what's being requested:
Try to estimate what current stresses are and will be if it is turned off, presumably possibly shearing off additional shoes or rupturing the pipe. Verifying the local disruption of supports on this loop does not cause other loops to react similarly.

I don't recall seeing anything regarding pipe guides, but if there are expansion loops, one should conclude that some kind of stress analysis was done. Perhaps the pipe was allowed to walk off its supports (repeat thermal cycles causing the pipe to displace over time, little by little until...).

I look forward to hearing how you ultimately proceed.

Thanks.

The 20" pipe is very unusual shape after the very effective transient event. The sketch/ the model does not give the distances between supports. I will assume that support distances are same as B31.1 tabled.

I am just trying to imagine where the transient load was generated;

1. I will assume that there is no condensation removal pot on the lower portion of the loop, and slug was generated and hit the loop many times.

2. Transient load was occurred in the different part of the piping and the pipe pushed axially one side.

In both cases I cannot imagine how pipe changed its shape as given. There must be enormous forced generated on the pipe and sheared all the supports unless supports were not designed to take this kind of forces. By looking at the support displacement and lift-offs, I believe, there are some plastic hinges produced on the pipe. I hope this is not the case but if this is true I am not sure how you are going to deal with them.

I guess Dave gave you the most possible modelling scenario. However first you need to determine the exact cause of the transient to be able to understand the pipe behavior under the current condition. Is the pipe still under axial load (thermal and/or transient event) or relaxed with the current shape? If the pipe is relaxed with current shape the pipe has already developed the plastic hinges under the occasional load. If the pipe still under the transient event axial load there the problem might be concentrated where the pipe holding restraint location is.

I wish you good luck, it is a difficult job, you will need more than stress analysis.

It seems to me that this could be simplified to a cold-sprung system.

Once it cools down, it'll have a force component on the supports it fell off of, and a vertical displacement.

Back calculate the loads of the pipe axially resisting these directional anchors as it cools down and apply those forces and displacements in the sustained case. In the operating case, only apply the displacements.

Dears,
1) Definitely there was an occasional impact load which threw the pipe off the supports. That force is unknown as operations did not report any upset in the operation of this line.
2) Line is safe as far as its operating and design conditions are concerned.
3) I modeled these dislocations as "Axial displacement" at one node in the pipe and by giving gap in +Y support (Not equal to shoe height as before resting on supports, it would compress the cladding/insulation.)
4) It would be interesting to know how to model the load on beams now as the line is definitely "trying" to restore to its "pre-occasional load" conditions and beams are resisting it.
Any idea?

for the line shown in maroon and blue color, supporting appears to be wrong there must be an anchor support between these two loops.

Shahid,

• Measure the pipe (or approximate the dimensions) and model pipe in its thermally expanded state with its displacements that you see while operating, complete with shoe "limits."
• Under Environment>Special Execution Parameters set the ambient temperature to the operating temperature of the pipe.
• Set T1 to the actual ambient temperature you wish the system to cool down to, or other temperatures colder than operating (that the system is now using to approximate ambient conditions).

I agree with Michael, those "sprung" restraints should be changed into displacements where you define your measured displacements (e.g., a horizontal displacement to the edge of the pad and a vertical displacement up the pad thickness). Adding gaps to your +Y supports will not give you what you want. Those displacements will carry load just like restraints and the restraint report will show the load at these points that prevent the system from returning to is regular operating position.
Michael's second two bullets will give you an estimate of the loads when you cool down the piping. If I recall, you wanted to know the current set of operating loads. For that you can just use your original model with displacements replacing the appropriate restraints with maybe a few more displacements for any "new" sprung positions.
If your model results "match" what you currently see in the field, you should have high confidence that your model is performing appropriately.

Dear Dave, Michael and Sridhar,
Thanks for input and very useful suggestions.
Regards,