Influence of Residual Stress on Casing Collapse Performance under External Pressure

Abstract

Abstract Collapse performance of a casing string subjected to external hydraulic pressure is one of the most important parameters in the deep water well casing design. It often becomes one of the governing factors in a casing design program. This paper investigates the quantitative effect of inherent circumferential residual stress (RS) on casing collapse resistane to external pressure through parametric Finite Element Analysis (FEA). Nonlinear elastic-plastic FEA study on casing collapse calculations have been performed. Different magnitudes and orientations of the initial circumferential residual stress were considered. Modified Riks method was used to predict the casing onsite collapse pressure as well as the post-collapse response. Parameters considered in the analysis include: (i) two material grades (L80 and Grade 135); (ii) for each material grade, ratio of outside diameter to wall thickness (i.e. D/t) ranging from 10 to 50; (iii) for each D/t ratio, initial circumferential residual stress at casing inner diameter (ID) ranging from -40% to +50% of the material yield strength (YS). Analysis results indicate that there is a highly nonlinear, parabolic-like dependence of collapse strength on the initial circumferential residual stress. Comparison of the FEA results to the predictions by Klever-Tamano Ultimate Limit-State (KT ULS) equation indicates that the KT ULS equation in the current API Technical Report 5C3 (API TR 5C3) does not capture this nonlinear relationship. This is probably due to the limitation of the API historical collapse test data upon which the KT ULS equation was calibrated to. The majority of the API historical collapse tests had the compressive residual stress at the casing ID. Moreover, the relationship between casing collapse strength and residual stress may be used as a guideline for potentially developing new high collapse casing by full-length residual stress control in mill production.