Burst & Collapse rating of Materials wrt Load

[saqibjahtemuri]

Hi Guys

The above plot of differential pressure and load is used for determining pressure requirement for various pipes.
This plot is of coiled tubing, coiled tubing is a continuous pipe of metal alloy which is coiled on a reel and is used to perform various pumping jobs in oil and gas wells.

At 0 load differential pressure is lower as compared to DP at 20K lbs. Please explain this phenomena as I am not an expert of metallurgy and got no results of this from google. I know that increase in load decreases pressure limitations due to triaxial stresses (tension in CT).

Yield Strength of Pipe is 80ksi with safety factor of 0.8.

 

[EdStainless]

Is the plot a rating of usable conditions or the limits of failure?
Burst (at zero applied load) is a function of UTS, OD, and wall. For thin wall tubing Barlow's formula is a good approximation. You can use Yield strength in the calculation, and then apply a safety factor.
As you apply axial load the stress state in the tube wall changes. Since the axial stress from the internal pressure is only 1/2 of the hoop stress there is no reduction in capacity until you start to approach that stress value.
Collapse is another issue. The strength will enter into how easily the tube buckles, but in classical external pressure equations the strength does not factor in at all. It is strictly governed by the geometry of the tube. Anything that causes deviation from a straight smooth tube will decrease collapse pressure.

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P.E. Metallurgy, Plymouth Tube

 

[saqibjahtemuri]

Thanks for replying.
It is limit of failure of coiled tubing.
Is it possible that a material have more burst rating in axial stress (load) as compared to that when it doesn't have any axial stress ?

 

[EdStainless]

Yes, as it is loaded axially there is some stress trying to collapse the column, this creates a compressive hoop stress that offsets some of the internal pressure. Look at a piece of tube when it is tensile tested, as it starts to yield the diameter decreases.
But in reality it is a very small effect. I wouldn't rely on it.

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P.E. Metallurgy, Plymouth Tube