A calculation method for ultimate and allowable loads in high-pressure thick-walled worn casing

Casing wear has become a critical issue in oil and gas drilling, significantly reducing casing burst strength. As exploration and development progress, the number of high-pressure, ultra-deep wells continues to rise, requiring high-strength, thick-walled casings capable of withstanding ultra-high internal pressures. However, research on the strength characteristics of worn high-strength, thick-walled casings under such conditions remains limited. This study fills that gap by quantitatively evaluating worn casings to determine the maximum allowable internal pressure, particularly for high-pressure, thick-walled casings. A systematic analysis of existing methods for calculating the ultimate load of casings is conducted, and a suitable approach for high-grade steel casings is proposed. A parametric finite element model (FEM) of worn casings was developed to assess the impact of various dimensionless structural parameters, including the thickness ratio (Do/t), D o / t ), wear percentage (h/t), h / t ), wear circle ratio (2ri/Do), r i / D o ), and nominal yield strength (Rp0.2), R p0.2 ), on the ultimate load. The results demonstrate that both wear depth and casing dimensions significantly affect the ultimate load. A new formula is proposed for calculating the ultimate and allowable loads of worn casings based on the results of a finite element analysis. This formula is applicable to high-strength casings of varying wall thicknesses, particularly under ultra-high pressure.