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188
Specim
en
Maxim um
strain
( % ) Maxim
um
rotati
on
Foot Crest
( dreg
rees)
pipeline operation after its regular shutdown, a rupture
occurred at the foot of the wrinkle similar to the way the
rupture occurred in Specimen 1.
Strains and rotations
As mentioned earlier, a large number of strain gauges were
installed on the middle 300mm long segment of each pipe
specimen to make an effort in acquiring the strain histories
at critical locations of the wrinkle as wrinkle initiates and
grows. The strain history for Specimen 1 is shown in Fig.5.
From this figure, it can be observed that the strain at both
feet of the wrinkle increased monotonically as the axial
loading continued. The strain stabilized at both feet when
the wrinkle came in contact from inside face of the pipe
wall (Figs 3 and 5). However, for the crest, the strain
increased only until when the maximum load capacity
reached (point U in Figs 3 and 5). Then, the strain at the
crest reduced slightly since the crest area relaxed as tension
developed locally in the crest. The maximum compressive
strains obtained from this specimen at the foot and at the
crest of wrinkle are 13.7% and 3.6%, respectively (Table 2).
It is important to note that these maximum compressive
strain values were obtained from the axial load test and all
the strain gauges failed and became non-functional when
the wrinkle came in contact (point C in Fig.3).
Inclinometers were installed on the top end plate of pipe
specimens to measure the rotation that occurred during
formation and growth of wrinkle. Figure 6 shows the
rotation history of Specimen 1. It can be seen that the
increase in rotation until initiation of wrinkle (point U)
was small and it was only 0.37º. The rotation began to
increase rapidly as soon as the wrinkle started to grow and
the total rotation at top end of Specimen 1 reached to 3.3º
until the wrinkle closed from inside pipe wall (point C in
Fig.3). The change in rotation was very small once the
wrinkle closed from inside pipe wall and the maximum
rotation for Specimen 1 was 3.6º (Table 2).
Similar behaviour in strain and rotation were also observed
for Specimen 2. However, the maximum compressive
strains at crest and foot of the wrinkle in Specimen 2 were
4.9% and 13%, respectively. The maximum rotation at the
top end of Specimen 2 was recorded at 2.8º.
Maxim
um
load
( kN)
Ruptu
re
locati
on
1 13. 7
3. 6
3. 6
1151 foot
2 13. 0
4. 9
2. 8
1176 crest
Conclusions
The Journal of Pipeline Engineering
The following conclusions are made based on the
experimental test data obtained from two tests conducted
under the scope of this study. Therefore, these conclusions
are limited to the pipe specimen and loading history that
were applied to these two specimens.
Sample issue
1. Both test specimens produced wrinkle shape that
look similar to the one observed in the field linepipe.
Therefore, it can be concluded that the field NPS10
linepipe was subjected to an eccentric axial load.
The axial load might have developed due to
movement of soil in the unstable slope.
2. The shape of wrinkle and location of rupture of the
field linepipe correlates better with those of
Specimen 1. An axial load test on this specimen was
undertaken in presence of internal pressure. Thus,
it can be concluded that the wrinkle in field linepipe
initiated and grew when it was in operation. The
rupture occurred when the pipeline was being
brought back to its normal operation after its
scheduled shutdown.
Acknowledgements
This work was completed with financial assistance from the
Natural Science and Engineering Research Council of
Canada.
References
Table 2. Strains and rotations.
American Standard (API), 2008. Specifications for Linepipe:
API 5L. American Petroleum Institute, Washington, DC,
USA.
S.Das, J.R.Cheng, and D.W.Murray, 2007. Behavior of wrinkled
steel pipelines subjected to cyclic axial loading. Canadian J.
Civil Engineering, 34, pp 598-607.
S.Das, J.J.R.Cheng, D.W.Murray, S.A.Wilkie, and Z.J.Zhou,
2001. Wrinkle behavior under cyclic strain reversals in
NPS12 pipe. Proc. 20th International Conference on Offshore
Mechanics and Arctic Engineering, ASME, Rio de Janeiro,
Brazil, pp129-138.
S.Das, J.J.R.Cheng, D.W.Murray, S.A.Wilkie, and Z.J.Zhou,