JPE - Sept09 - cover2-4.pmd - Pipes & Pipelines International ...

178
Camisea system, including the subcritical, stable behaviour
prior to unstable fracture or plastic collapse.
In order to quantify the impact of ground movement,
fracture mechanics was used to relate an assumed defect
size to the failure load and to gain some insight into how
quickly tearing can occur. To assess the soil loading and
observed crack growth behaviour, an elastic-plastic fracture
analysis was conducted for the tearing process that led to
the fifth spill incident at KP 125+950, where a
circumferential crack initiated on the outside of the pipe
and grew by progressive tearing to a through-wall 275-mm
long circumferential crack; Fig.2 shows the failed pipe
segment after complete rupture of this crack. The fracture
surface of the crack has three distinct stages of crack
propagation, which are shown in Fig.3; in the figure, the
top is the outside wall of the pipe and the blue arrows
marking the transition between stages.
In this spill incident, the mechanism of both crack initiation
and propagation was ductile tearing caused by soil
movement. It should be noted that under high axial loads,
the biaxial stress state of the pipe could effectively increase
the axial load needed to cause general yielding and allow
tearing of the weld material prior to the general yielding of
the pipe. After this ductile tearing through the majority of
the pipe wall, the final slant fracture occurred by plastic
collapse of the remaining ligament. The difference in the
fracture surface in the three stages of crack growth shown
in Fig.3 may be due to the differences in the rate of ductile
tearing.
In ductile materials, plastic tearing ahead of a crack initiates
when the driving force of the crack, J, reaches J IC . However,
once tearing begins there can be stable crack advance due
to the increasing resistance of the material to crack advance,
The Journal of Pipeline Engineering
which is typically summarized with a J-R curve. A J-R curve
for API 5L X70 steel is shown in Fig.4 [7].
As can be seen in Fig.4, as the length of the crack, Da,
increases, the driving force required for further crack
advance increases. Unstable fracture will occur when the
rate of change of the crack driving force with crack length
becomes greater than the rate of change of the resistance
curve with crack length for a given loading or the remaining
ligament fails by plastic collapse: for API 5L X70, J is IC
reported to be approx. 400 kJ/m2 [7]. In the initial stages of
crack growth, corresponding to the ductile tearing in stage
one in Fig.3, the resistance curve is relatively steep. As the
crack grows, however, the slope of the resistance curve
decreases and it is possible that a transition to a more rapid
ductile tearing begins when the crack reaches a length of
1.4 mm, which corresponds to the blue arrow indicating
the end of the first stage in Fig.3. This transition may
correspond to this change in slope of the resistance curve
seen in Fig.4. This more rapid ductile tearing continues
until the rate of change of the crack driving force with crack
length becomes greater than the rate of change of the
resistance curve with crack length for a given loading or the
remaining ligament fails by plastic collapse
Sample issue
Fig.4. J-R resistance curve for
API 5L X70.
The load necessary to reach a value of J IC = 400kJ/m 2 was
computed using a SENT (single edge notch tension) with a
crack of length a = 1.4mm. The stress in the wall was found
to be approx. 95ksi, greater than the measured yield stress
(82ksi) of the pipe material. This would be consistent with
the observed plastic deformation and lateral contraction of
the pipe. This wall stress correlates to a 1,205kip load in
addition to the typical operating pressure of 2,320psi. It
should be noted that the calculation of the load based on
J is only approximate because J is very sensitive to the stress
near J IC and tearing would probably occur before the