Framework for probabilistic leakage resistance envelopes of casing connections

Abstract Probabilistic well casing design is a recent trend in Oil & Gas exploration, especially for ultra-deep and HPHT wells. Reliability-based design allows safety margins to be optimized, while managing the corresponding risks. Statistics for pipe body geometry and strength are today widely available, but most reported incidents are related to connection failures. In this context, a methodology is proposed herein to construct probabilistic leakage resistance envelopes for casing connections. The methodology is based on extensive axisymmetric Finite Element analysis, considering geometric variabilities resulting from manufacturing tolerance. Maximum internal pressure is found for different values of axial load. Multi-dimensional response surfaces are constructed to take into account individual effects, and cross-effects for the most relevant variables. Regression analysis is used to construct failure envelopes for pre-defined probability levels. These are also converted into probabilistic envelopes for arbitrary combinations of axial load and internal pressure. The API 8 Round 5½ J55 14 lb/ft LTC connection is employed as study object, but the methodology can be readily extended to other connections with metal-to-metal seal. To the best of our knowledge, this is the first time such a probabilistic framework for leakage resistance of casing connections is presented in the literature. This framework, and the resulting models, are fundamental for the proper reliability-based design of well casing.