Except for coiled tubing, most tubular goods used for downhole operations
(such as drillpipe and sucker rod) have connectors. Because a connector and the
pipe body have different outer radii, the deformation and buckling behavior of
a pipe with connectors constrained in a wellbore is much more complicated.
However, most buckling models were established by neglecting the existence and
effects of connectors.
In this paper, buckling equations of a pipe with connectors in horizontal
wells were derived with application of elastic-beam theory. The axis of an
unbuckled pipe is a 2D curve in the vertical plane and has three
configurations--no contact, point contact, and wrap contact. We derived the two
critical distances between connectors, Lc1 and
Lc2, beyond which a pipe changes its configuration
from one to another. The authors proposed an algorithm to determine the
critical force (Fcrs) of buckling by numerically solving the
buckling equations using the fourth-order Ronge-Kuta method.
Both the distance between two adjacent connectors (Lc) and
the radius difference between a connector and the pipe body
(Δrc) have significant impact on the critical force, in
addition to net clearance between a pipe and wellbore (r0),
bending stiffness (EI), and weight per unit length (w) of pipe.
When Lc is small, radial deflection is negligible.
Fcrs increases as Δrc increases. However,
when Lc is close to Lc1, effects
of radial displacement become significant, and Fcrs decreases
dramatically as Δrc increases. Fcrs
decreases as Lc increases when Lc <
Lc1, and it reaches its minimum at
Lc = Lc1. When
Lc > Lc1,
Fcrs fluctuates as Lc increases. Some
curves of Lc1, Lc2,
and Fcrs, all in dimensionless forms, were calculated and
presented in this paper for practical applications.
Our numerical results show that the critical force may reduce by 20 to 60%
for commonly used drillpipes and sucker rods with centralizers, which indicates
that a pipe string designed without considering the effects of connectors may
be risky. The results presented in this paper may provide some practical
guidance for optimal design of centralizers for sucker-rod strings, or may
avoid some risks because of improper design of drillpipe strings.
© 2012. Society of Petroleum Engineers
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- Original manuscript received:
26 June 2011
- Meeting paper published:
31 October 2011
- Manuscript approved:
11 November 2011
- Published online:
31 August 2012
- Version of record:
12 September 2012