This paper presents a new analysis method for stress-concentration factor
(SCF) in rotary-shouldered connections (RSCs). The proposed method uses
finite-element analysis (FEA) as a primary tool to explore the maximum
peak-stress behavior in RSCs and calculate SCF to evaluate the connection
performance. Key findings include facts that the maximum peak stress and SCF
value are functions of connector geometry and loading condition. The paper
defines SCF in RSCs and provides a thorough discussion about the SCF-analysis
method and its characteristics in evaluating drillstring-connection designs.
The paper compares maximum peak-stress behaviors and SCF values among various
RSC designs to demonstrate their role in selecting connections for drilling-,
completion-, and intervention-riser applications.
In recent years, drilling programs have become significantly more aggressive
in both onshore and offshore operations. Harsh conditions in deepwater,
extended-reach, and ultradeepwater drilling often place severe axial, lateral,
torsional, and pressure loads on the drillstring and its connections. As a
result, RSCs often experience exceptional elevated-stress conditions. In some
cases, RSCs are being used for applications beyond conventional drilling, such
as completion operations and riser intervention. Meanwhile, advanced RSC
designs incorporate features such as multiple shoulders and metal-to-metal seal
features, adding further complexity to the stress distribution in the
connection design. The need to understand the connection-stress behavior fully,
especially the maximum peak-stress behavior for the various applications, is
critical for properly selecting and safely using the drillstring connections.
(For a comparison of a premium casing connection and a proprietary RSC, see
SCF is a useful parameter in terms of evaluating the connection maximum peak
stress in response to the operation loads. In the past, SCF has been employed
to characterize design strength and fatigue performance of premium casing,
tubing threads, and some riser threads under anticipated operation loads.
However, this has been applied only to RSCs, mainly for two reasons: (1) the
traditional SCF-analysis method is not designed for RSCs with high makeup
preload and (2) a lack of standardization of the SCF-analysis method within the
industry for RSCs.
Because of increasing aggressiveness of drilling conditions and merging of
new applications, SCF analysis to predict stress behavior becomes necessary.
This paper presents a series of evaluations that will explore various aspects
of SCF analysis to find a logical and conservative approach to evaluate the
maximum peak-stress behavior in RSCs, in response to operation loads. This
approach will help to better understand RSC-loading limit, stress distribution,
and fatigue characteristics for various existing and new applications.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
28 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
29 August 2008
- Manuscript approved:
8 September 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009