Collapse happens when differential between external and internal pressure exceeds a collapse rating pressure of material. This situation can happen due to several cases, such as pressure testing in annulus, trapped pressure in the annulus or well fully evacuated with gas, etc.
Collapse pressure equations come from experiments from test specimens and the full details can be found in API Bulleting 5C3, Formulas and Calculations for Casing, Tubing, Drillpipe, and Line Pipe Properties. From the experimental results, there are 4 collapse regimes based on Diameter of pipe/Wall thickness (D/t) and yield strength of material which are yield strength collapse, plastic collapse, transitional collapse and elastic collapse.
Yield Strength Collapse
Yield strength is based on yield at the inner wall by applying the Lamé thick wall elastic solution.
- D/t < ± 15
- Tangential stress is over the yield strength of material before a collapse instability failure occurs.
- The formula for yield strength collapse is shown below;
- The applicable D/t ratios for yield strength collapse are illustrated in Table 1.
Plastic collapse equation is derived from empirical data of K-55, N-80, and P-110 seamless casing.
- The formula for plastic collapse is shown below;
The factors A, B, and C and applicable D/t range for the plastic collapse formula are shown in Table 2.
Transitional collapse is derived from a curve fitting between the plastic and elastic regions.
- The formula for transitional collapse is shown below;
The factors F and G and applicable D/t range for the transition collapse pressure formula, are shown in Table 3.
The elastic collapse is applicable for thin wall pipe (D/t> ±25).
- The formula for elastic collapse is shown below;
The applicable D/t range for elastic collapse is shown in Table 4.
D = nominal outside pipe diameter, in.
D/t = slenderness ratio, dimensionless
Most of oilfield tubing and casing are within the plastic and elastic region and the simple way to find the collapse pressure of each pipe is to look at a casing specification table.
Example: Determine the tensile strength of the following pipe.
4-1/2” casing, weight 9.5 ppf, grade J-55
Pipe ID = 4.09”, Wall thickness 0.205 “
Collapse pressure (psi) =3,310 psi.
Download the API tubular data sheet here – https://www.drillingformulas.com/oilfield-casing-data-sheet-free-download/
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Wan Renpu, 2011. Advanced Well Completion Engineering, Third Edition. 3 Edition. Gulf Professional Publishing.
Ted G. Byrom, 2014. Casing and Liners for Drilling and Completion, Second Edition: Design and Application (Gulf Drilling Guides). 2 Edition. Gulf Professional Publishing.
PEH:Casing Design. 2015. Casing Design. [ONLINE] Available at: http://petrowiki.org/PEH%3ACasing_Design. [Accessed 1 June 2016].