A Generalized Empirical Expression for Collapse of Worn Tubulars Using Stress Concentration Factors

As wells in modern operations are getting longer and more complex, assessing the effect of casing wear becomes ever more crucial. Degradation of the tubulars through mechanical wear reduces the pressure capacity significantly. In this paper, we use the finite element method (FEM) to analyze the stress distribution in degraded geometries and to assess reduction in collapse strength. A model for the collapse strength of the casing with a crescent-shaped wear groove is developed and its performance evaluated in relation to experimental data. The model was created by using the Buckingham Pi theorem to make generalized empirical expressions for yield and elastic collapse of tubulars. Finite element analysis (FEA) of 135 geometries was used in the development of the model. The results show that the generalized expressions capture the trends observed in the FEA accurately and match the experimental data from six tubular collapse tests with an average relative difference in collapse pressure of 5.2%.