Casing Burst Strength Degradation Due to Casing Wear

Abstract

Casing integrity management is crucial, especially in wells experiencing severe casing wall degradation. Knowledge of stress distribution in worn casing helps predict where a casing failure occurs first. In industrial practice, a common method is to estimate the reduction of the casing burst strength in worn casing using API burst strength equation with a linear reduction in the remaining wall thickness or wear percentage equivalent to a “uniform-worn” casing model. This study focuses on building a rigorous engineering model for burst strength degradation prediction based on “crescent shape” casing wear. This model calculates the hoop strength directly, including the local bending in the thinner portion of the “crescent-worn” casing. This paper has developed a mathematical model to calculate the hoop strength of worn out casing with force and moment balance equations. This study finds the calculation of reduced strength using the linear wear model to be overly conservative because it only focuses on the stress at the thinnest portion of the worn casing. The stress predicted in this paper is similar to the results obtained from the finite element method (FEM), which validates equations and results obtained from this paper. The developed model is generic and can apply to casings, risers, and tubings.