Evaluating Pressure Integrity of Polymer Ring Seals for Threaded Connections in HP/HT Wells and Expandable Casing
Authors: Hilbert Lawrence B. , Bergstrom Jorgen 
Published: March 2008

Summary
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 10¾-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.

Historical Perspective
FEA of threaded connections has been used for overcoming challenging well-design problems for many years (Crose et al. 1976). FEA has become an important part of the validation and service evaluation process of API and proprietary casing and tubing threaded connection designs, along with the physical testing procedures documented in API RP 5C5 (1996) and ISO 13679: 2002 (2002). Major advances have been achieved in design of premium connections through analysis of metal-to-metal seal contact stresses computed from FEM (Hilbert and Kalil 1992).
Analysis and verification of the performance of threaded connections that include polymeric or elastomeric ring seals has been limited to full-scale physical testing (Payne 1988). Until now, only costly full-scale gas pressure tests have been used to evaluate ring seal integrity. Ring-seal design has been a trial and error process, with new ring-seal or pin and coupling dimensions prescribed only after failure of the seal in a proof test. In some cases, ring design or the effects of ring dimensions have been based on analytical calculations, relying on the bulk modulus of the material. When more advanced design tools, such as FEA, have been used, the pressure generated by entrapment of the ring seal has been estimated and then these pressures have been applied to the groove and pin thread surfaces to simulate the effect of the actual ring seal. The developments in the paper were motivated by a need to reduce the cost of connection qualification by reducing the number of tests and to improve the process of ring-seal design.

Properties of PTFE
PTFE is a thermoplastic fluorocarbon derived from the monomer tetrafluoroethylene (TFE). PTFE is a semi-crystalline polymer composed of crystalline and amorphous regions. Its molecular structure, shown in Fig. 1, consists of long chains of carbon atoms symmetrically surrounded by fluorine atoms. This structure imbues PTFE with unique mechanical and chemical properties. The straight "backbone" of carbon atoms provides PTFE with a high degree of chemical inertness, stability, and one of the lowest coefficients of friction of any commonly used material. PTFE is more commonly known by the trade name Teflon. In a moment of pure serendipity, in 1938 Roy Plunckett of DuPont discovered TFE when he was conducting experiments to develop nonflammable, nontoxic, colorless, and odorless refrigerants (Ebnesajjad 2000).