Von Mises stress envelope is a valuable tool to performing tubular stress analysis. However that pretty ellipse that you put your load calculations in has some math behind it, and understanding how it is derived helps understand why it is used.
The Von Mises envelope equation provides the interaction between the different stresses that will equal to the yield stress of the tubular. When other stresses are 0, then clearly the remaining stress will be the same as yield. However what happens if you start pulling tension on the pipe? Does that additional stress strengthen or weaken the pipe? It actually strengthens it, and can be valuable in eking out maximum performance in your tubing and casing through design. Trying to draw a single envelope can be difficult with up to 4 variables, however this workflow illustrates how some terms can be eliminated or made in terms of one other, and identify the extremes to plot with.
Von Mises works well for extending the ability of the pipe under tension and burst conditions, however collapse is a different story; it is an inherently unstable condition that is fine, until it isn’t. While burst will often cause some cracking and leaks as the pressure increases until total failure happens, collapse can happen extremely quickly, causing catastrophic damage before it can be reacted to. The term pancaking isn’t an exaggeration, pipe can be nice and round in one moment and flattened in the next. I remember an incident with sand screens, the field had been producing great until water influx carried particles that plugged the screen, and wells started going down one after another. The API calculations in 5C3 are the standard for determining collapse, and have a well-proven track history.
I did a separate write up in Word to accompany code on my Github, and have copied it below (email me if you’d like a PDF of it). Head over to https://github.com/jack-charles/VonMisesCalc to check out the code itself. It’s setup out of the box to create and plot multiple VME and API curves with safety factors and derating. It can be modified easily to integrate into your own workflows for quick stress checks, without breaking out the big software packages and needing to re-input all your data.
Next in the series – how do we calculate the tubing and casing loads, especially with buckling or torsion? And how do we know which envelope the load should be plotted against?
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