Summary
Conventional material specifications and test methods were developed to
support load-based designs in which inelastic deformations are relatively small
and yield strength is the primary material factor governing design. However, in
strain-based designs where substantial portions of the structure soften under
post-yield deformation, more detailed characterization of the post-yield
material behavior is required. This paper presents a framework for describing
the post-yield properties of metals (including strain-rate dependence of yield
strength) a testing method for measuring post-yield strength in terms of strain
and strain rate, and an analytical basis for extrapolating measured properties
to static conditions for strain-based design and quality assurance (QA).
Introduction
Typical test specifications for determining the mechanical properties of
oil-country tubular goods (OCTG) were developed to provide an index of
mechanical strength to support common load-based design methods. Advancing
recovery techniques impose conditions on many well structures that exceed the
limits of these methods and the material characterizations on which they are
founded. Among these new techniques are those used to recover heavy oil. While
typical conditions in heavy-oil reservoirs appear benign, enhanced-oil-recovery
(EOR) methods such as thermal stimulation and ultrahigh sand production create
some of the most challenging conditions for well structures. Imposed
deformations commonly exceed the yield limit of the material, therefore
post-yield material characteristics govern much of the structural response.
Industry-standard material tests provide only limited characterization of
post-yield behavior, particularly at strain levels near the yield point (both
pre- and post-yield). Furthermore, test strain rates can affect the measured
material strength significantly. Field loading usually occurs at much lower
rates and is then sustained for extended periods. A method for characterizing
post-yield material properties is, therefore, desired to adequately support
designs for such applications.
This paper proposes a new basis for characterizing mechanical steel
properties that provides the static strength and stiffness over the post-yield
strain range. Relaxation characteristics are interpreted from testing, and
local stiffness properties are provided. Although static properties are
inferred, the test and interpretation basis allows the tests to be executed in
a relatively brief time frame, making it possible to apply the method in QA
programs to confirm post-yield properties for strain-based designs. A test
apparatus built to implement the material-characterization protocol is
presented, and sample results are provided to demonstrate the method.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
30 August 2005
- Meeting paper published:
1 November 2005
- Revised manuscript received:
20 August 2008
- Manuscript approved:
11 September 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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