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 sym¬metrically 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).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
7 June 2004
- Revised manuscript received:
1 October 2007
- Manuscript approved:
7 October 2007
- Version of record:
20 March 2008
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