Evaluating pressure integrity of polymer ring seals for threaded connections in HP/HT wells and expandable casing

This paper presents new technology for evaluating high-pressure gas-seal integrity of polymer ring seals used as secondary or backup pressure seals in casing and tubing threaded connections. This new technology may also enable the further consideration of API connections with ring seals, as an alternative to premium connections, for appropriate applications. A nonlinear elasto-viscoplastic constitutive model for the behavior of polymers and elastomers has been developed and extended to the specific application of analysis of casing and tubing connections with fiberglass-filled polytetrafluoroethylene (PTFE) ring seals. Procedures for modeling makeup of a connection including a fiberglass-filled PTFE ring seal have been developed using a finite-element model (FEM) of 103/4-in. OD, 45.5 lb/ft, P-110 API buttress thread casing-seal ring groove (BTC-SRG). The results of finite-element analysis (FEA) of makeup, followed by the application of thermal, axial, and internal pressure loads are presented in this paper. In addition, based on the interest in the development of gas-tight threaded connections for expandable casing, the FEM was subjected to a radial expansion of a 20% increase in the outside diameter. In this paper, the theory of the constitutive model is summarized and calibration of the model with experimental test and published data are presented. The focus of the FEA results is on the contact pressures between the ring seal, coupling groove, and pin threads.