Evaluating Pressure Integrity of Polymer Ring Seals for Threaded Connections in HP/HT Wells and Expandable Casing
Authors: Hilbert L.B. , Bergström J.S. 
Published: December 2007

Abstract
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 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 of 10–3/4-inch OD, 45.5 lb/ft, P-110 API Buttress Thread Casing with a Seal Ring Groove. The results of finite element analysis 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 finite element model was subjected to a radial expansion of a 20 percent 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 finite element analysis results is on the contact pressures between the ring seal, coupling groove and pin threads.

Historical Perspective
Finite element analysis has become an integral 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 5C51 and ISO 13679:2002.2 Major advances have been achieved in design of premium connections through analysis of metal-to-metal seal contact stresses computed from finite element models.3
Analysis and verification of the performance of threaded connections which include polymeric or elastomeric ring seals has been limited to full-scale physical testing.4 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 an 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 finite element analysis, have been used, the pressure generated by entrapment of the ring seal has been estimated and then these estaimted pressures have been applied to the groove and pin thread surfaces to simulate the effect of the actual ring seal. The developments presented in the paper were motivated to reduce the costs of connection qualification by reducing the number of tests and to improve the process of ring seal designs.

Properties of PTFE
Polytetrafluoroethylene (PTFE) is a thermoplastic fluorocarbon derived from the monomer tetrafluoroethylene (TFE). Its molecular structure, depicted in Figure 1, consists of long chains of carbon atoms symmetrically surrounded by fluorine atoms. This structure imbues PTFE with many 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 Dr. Roy Plunckett of Du Pont discovered TFE when he was conducting experiments to develop nonflammable, nontoxic, colorless and odorless refrigerants.5