Conventional wisdom suggests that buckled pipe first forms a plane buckled
configuration, then transitions to a helical shape as the axial load increases.
The helical configuration has been studied extensively using analytic solutions
that allow calculation and analysis. Lateral buckling has received much less
attention because no analytic solution exists for smooth pipe. Most conclusions
about lateral buckling are just guesses. Tool joints have received some
analysis, and buckling calculations, both lateral and helical, have been made
for pipe with connectors, but these calculations have received little attention
from the industry.
Buckling tests (Weltzin et al. 2009) were performed in the Ulrigg U2, a
2020-m research well with a buildup and 60° tangent geometry. These tests
represent a major advance in the understanding of drillstring buckling.
Although there have been many laboratory-scale studies and a few full-scale
tests of limited value, this study is the first full-scale study to acquire a
large amount of high-quality buckling-displacement data for analysis.
This paper presents the calculated drillstring positions for two loading
tests performed in build and inclined sections of the Ulrigg U2 test well. Both
tests were loaded until drillstring lockup. By lockup, we mean that no increase
in load could be transmitted to the load sensor at the bottom of the
drillstring with further decreases in hookload at the surface. In both cases,
lateral buckling was the primary buckling mode, with only a short interval in
the build-section tests showing actual helical buckling.
The results from the Ulrigg U2 tests show that connectors appeared to have
primary importance in the buckling behavior of drillpipe, and lateral buckling
was the primary mode of behavior. These results, which were not expected, will
require considerable revision of the conventional buckling models with much
more focus on the impact of connectors and a deeper understanding of lateral
buckling. In particular, the analysis of casing running in extended-reach
horizontal wells will be impacted significantly.
© 2011. Society of Petroleum Engineers
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- Original manuscript received:
10 March 2009
- Meeting paper published:
2 March 2011
- Revised manuscript received:
12 July 2011
- Manuscript approved:
16 July 2011
- Published online:
1 September 2011
- Version of record:
15 September 2011