Pipe Body Yield Strength Calculator: Casing and Tubing Tensile Rating

Solve the pipe body yield strength of oilfield casing and tubing: find the tensile parting load from outside diameter, wall thickness, and minimum yield strength, compare tensile ratings across steel grades, or solve the yield strength needed to hit a target tensile rating. Everything updates live.

Pipe Body Yield Strength and the Third Design Failure Mode

Casing and tubing design checks three separate failure modes: burst from internal pressure, collapse from external pressure, and parting in tension from axial load. Pipe body yield strength addresses the third mode, the load at which the steel in the pipe wall itself would stretch past its elastic limit and part, independent of the connection joining one joint to the next.

The formula is direct: Fy = Yp x As, where Yp is the minimum yield strength of the steel and As is the nominal cross-sectional area of steel in the pipe wall, As = (pi/4) x (OD^2 – ID^2), with ID = OD – 2t. This is the same nominal cross-sectional area and yield strength figure published in API 5C2 tables for every casing and tubing size, weight, and grade.

Verified Against a Published API Worked Example

For a 9.625in OD, 0.395in nominal wall casing, API 5C2 Table 4 Column 4 publishes a nominal cross-sectional area of 11.454 in2. At 80,000 psi minimum yield strength (N-80 or L-80 grade), API 5C2 Table 1 Column 13 publishes a pipe body yield strength of 916,000 lbf. Computing Fy = Yp x As directly from the OD and wall thickness gives 916,302 lbf, matching the published table value within normal rounding. This confirms the formula against an independent, authoritative industry source rather than only an internal derivation.

Because the formula uses nominal wall thickness rather than the minimum wall thickness allowed under API manufacturing tolerance, real-world casing design applies a design factor, typically around 1.25, to account for the possibility that actual wall thickness runs thinner than nominal. This calculator reports the nominal pipe body yield strength; applying an appropriate design factor for a specific design is a separate step left to the engineer.

Comparing Tensile Rating Across Steel Grades

Because the pipe body yield strength equation is linear in Yp, moving to a higher-strength grade raises the tensile rating in direct proportion, exactly as it does for burst pressure under Barlow’s equation and for yield strength collapse. The grade comparison tab holds OD and wall thickness fixed and varies only minimum yield strength across rows, isolating exactly how much grade selection alone changes the tensile rating of a given pipe body.

Solving for Required Yield Strength

String design sometimes works backward from a required tensile rating, set by the hanging weight of the string below plus any overpull margin, to the minimum yield strength a given pipe size must carry. Rearranging the equation makes that solve direct: Yp = Fy / As. The reverse tab defaults to the 9.625in OD, 0.395in wall example, confirming the required yield strength lands exactly on N-80 or L-80 grade steel.

Common Mistakes

Comparing pipe body yield strength directly to the actual working tension without first applying a design factor, since the nominal calculation does not account for API manufacturing tolerance on wall thickness.

Assuming the pipe body governs string design when the connection is often the weaker link; API and premium connections frequently carry a lower rated tensile load than the pipe body itself, so both must be checked and the lower value used.

Ignoring buoyed weight when summing the tension load a string must carry, since the string hangs in mud rather than air and the buoyancy of the drilling fluid measurably reduces the true axial load at any point in the string.

Neglecting biaxial and triaxial stress interaction, since a string under combined tension and internal or external pressure has a lower effective yield strength than pure uniaxial tension alone predicts.

Who Uses This

Drilling and completion engineers checking tensile design margins for casing and tubing strings during running and cementing operations.

Well engineering teams comparing pipe body rating against connection rating to identify the governing tensile limit for a string design.

Petroleum and mechanical engineering students studying tubular design and the three classical API failure modes of burst, collapse, and tension.

Tubular design engineers cross-checking published API pipe body yield strength tables for standard casing and tubing sizes.

Try It Above

Use the quick solve tab to convert outside diameter, wall thickness, and minimum yield strength directly into pipe body yield strength, move to the grade comparison tab to see how much steel grade alone changes the rating, or use the solve tab when you need the yield strength required to reach a target tensile rating.

Related Calculators and References

This completes the burst-collapse-tension casing design triad; continue with the Casing Collapse Pressure Calculator or the Casing Burst Pressure Calculator. For background on API pipe body yield strength and design factors, see Drillers.com: API Tension and Burst Tutorial.