API 5CT Oil Casing Stress Analysis

API 5CT (American Petroleum Institute Standard) is an internationally accepted standard for the manufacture and use of oil casing, which specifies the materials, dimensions, mechanical properties (such as tensile strength, yield strength, collapse strength, etc.) and stress requirements of oil casing. Oil casing pipe will be subjected to various stresses underground, mainly including:

Main stress types of oil casing:

(1) Axial Stress
Source: casing deadweight, wellhead suspension load, thermal stress caused by temperature changes, fluid pressure, etc.

(2) Radial Stress

Source: wellbore internal and external pressure (such as formation pressure, water injection pressure).

(3) Circumferential (Hoop) Stress
Source: The internal and external pressure difference causes the casing to be subjected to circumferential force, which is a key indicator of casing collapse strength.

(4) Shear Stress
Source: wellbore bending, casing eccentricity, non-uniform formation load, etc.

(5) Combined Stress

Von Mises equivalent stress (used to evaluate casing strength)

API 5CT requirements for casing stress:

API 5CT specifies the minimum yield strength and tensile strength of casing of different steel grades, for example:

Steel Grade	Minimum Yield Strength (psi)	Minimum Tensile Strength (psi)
H40	40,000	60,000
J55	55,000	75,000
K55	55,000	95,000
N80	80,000	100,000
P110	110,000	125,000
Q125	125,000	135,000

Oil casing stress failure mode:

Tensile failure: Axial stress exceeds tensile strength, resulting in fracture.
Extrusion failure: external pressure exceeds the anti-extrusion strength, causing casing deformation or collapse.
Explosion failure: internal pressure exceeds the internal pressure strength, causing casing rupture.
Fatigue failure: alternating stress (such as water injection, acidizing operation) causes crack propagation.

Corrosion failure: corrosive media such as H₂S and CO₂ reduce casing strength.

How to optimize the stress design of oil casing?

Rationally select steel grade (such as P110 for deep wells and Q125 for ultra-deep wells).
Optimize wall thickness (increasing wall thickness can improve anti-extrusion strength).
Use composite casing (such as titanium alloy, corrosion-resistant alloy).
Optimize cementing quality (reduce non-uniform load).

Finite element analysis (FEA): simulate complex stress state underground.

Conclusion:

The stress analysis of API 5CT oil casing involves axial, radial, circumferential and composite stresses. It is necessary to calculate key parameters such as tensile strength, anti-extrusion strength and internal pressure resistance in combination with API standards to ensure the safe service of casing underground. The steel grade, wall thickness, corrosion environment and load conditions must be considered during design to avoid the risk of failure.

Read more: Oil Tubing VS. Casing Pipe or Seamless Casing VS. ERW Casing