Hi all,

I have the following situation:

The hydrotest header ruptured during a hydrotest of a pipeline running on a pipe rack, aboveground to B31.4 (not design error…contractor, wall thickness error on the header for those interested).
As you can imagine this created very large forces and a great deal of damage to the pipe, supports and other lines (no one was hurt). An analysis has been done (third party) to calculate/estimate the hydraulic forces generated by the rupture and the results are reported as Forces vs Time (impulse). The report calculated forces along “significant elbows” and straight pipe sections. I have no reason to question the validity of the report so I now need to calculate the stresses those forces generated on the system.

My problem is that I have doubts of how to use this data into an analysis with CAESAR II. The way I see it this can be done by a static analysis since we know at which moment in time was the worst case (analysis as a snap shot moment if you will). Since this was a sudden change in mass/momentum (no increase in pressure so no hoop stress increase) my thinking is that the straight runs of pipe experienced the least stresses (although the hydraulic report shows the highest forces at the straight runs of pipe which is part of my confusion!) since the pipe would displace axially and “transfer” the loads to the elbows so the worst case scenario are the forces seen by the elbows (change in direction).
1. Is this thinking correct?
2. If so, then do I input the worst case force into the elbows as a static analysis for that moment in time? (with a dynamic load factor of 2 for short duration impulses from what I’ve read)
3. If not, I’m not sure how to use the data provided.

This is my first time dealing with a catastrophic event like this so I apologize if my explanation is not precise or lacking detail.
I would really appreciate any inputs.

Thank you

This is typical water hammer behavior. There is plenty of material out there to find on it, so I'll let you investigate further on that, and just answer your questions directly.

1. Sort of. In typical stress analysis, we're most concerned about the imbalanced force that causes flexure of the bends.
2. Input the loads into the elbows, opposite the direction of flow out of the elbows. If static, you should use a DLF of 2. Dynamic may become obligatory if:
a. This DLF overstresses your pipe but without it, it does not.
b. You have many more than 9 different forces to apply, and inputting them statically can result in loads falsely cancelling each other (although you could have multiple static models for remainder loads, too if >>9 forces)
c. If the individual forces interact in such a way that dynamically you will calculate stresses greater than individually, even with a DLF of 2.

I would suggest looking up Aft-impulse videos. For what it's worth, there's a video that covers an example of importing aft-impulse data to CAESAR that does a decent job at explaining the situation.

Michael, thank you very much for the straight to the point answers.
Lots more to explore and learn that's for sure! (as always).

I've approached it from a static point of view and evenly distributed the hydraulic forces in the straight sections of pipe over the length of each section (i.e 100K lbs over a distance of 300 LF for one section, etc.)
The results are matching the real world end displacements and bends very well actually (very pleasantly surprised about that)

The overall conclusion (which may not be a surprise to the very experienced on this forum) is that all the areas around the bend/elbows were over-stressed but after a few feet of straight pipe after the bends the pipe survived quite well. Obviously, the pipe can take a great deal more axially than any other direction. Just a very basic summary for those interested or future readers.

Thank you