On the Ultimate Limit State Strength Models and Its Application for Casing Design

Abstract

It is proposed a new methodology for calculating safety factors in triaxial casing design, regarding Ultimate Limit State (ULS) models. The definition of an equivalent axial force allows an unified graphical approach, in which both ULS and Serviceability Limit State (SLS) envelopes are represented in the same space. A review of the ULS casing design proposed in the API TR 5C3 (2008) standard is presented, and some considerations are made on the usage of Klever-Stewart and Kever-Tamano models. Both are defined for combined load analysis but are not effective for triaxial casing design routine, for they only compare the applied internal/external pressures to the design limit. Thus, two additional limit states are verified: axial and triaxial. Furthermore, the ULS envelopes (Klever-Stewart and Klever-Tamano) are plotted in the same load space as the SLS envelopes, allowing for an analysis on both limit states in a single graphic. During studies of the Klever-Stewart and Klever-Tamano formulations it was verified that, despite both being combined models (the external/internal pressure and axial load influence the internal/external pressure resistance), the consideration of their uniaxial safety factors might not be enough to ensure if the tubular is about to fail. There are some special load cases where other failures might occur prior to burst/collapse. The ULS envelopes are revisited, using the very same equations proposed by the API TR 5C3 (2008), but in a different load space, which also allows the representation of SLS envelopes, providing a complete evaluation of the tubular strength against the load scenario. The studies also revealed some drawbacks inherent to the Klever-Stewart and Klever-Tamano models, as the existence of inadmissible pressure resistance beyond the axial ultimate tensile strength. The new paradigm of ULS is challenging and it is not well established in terms of design practice. In the light of this, it is presented a new methodology for computing safety factors in ULS analysis, effectively comparing them to the SLS approach, in order to support the decision-making process in casing design and the well integrity verification.