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

180
Crack Mouth Opening Displacement (inches)
0.004
0.003
0.002
0.001
δ
0.1 mm
a
σ
t
σ
0.000
0 500 1000 1500 2000 2500
cracks under either the MAOP or an external soil load that
will yield the entire pipe, an analysis must be performed to
evaluate the crack opening for such cracks under such
loads.
To put an upper bound on the crack mouth opening
displacement (CMOD), the crack geometry is taken as the
crack shown in Fig.6 with the pipe cracked all the way
around the circumference of the pipe. The solutions were
performed for a pipe with R/t = 18.2, a hardening exponent
of n = 10, and different depths into the wall (see Fig.6). This
solution should slightly overestimate the crack opening of
a finite length crack in the actual pipe since the crack is not
all the way around the circumference of the pipe.
Figure 6 shows that under the design operating pressure of
2,700psi (72% SMYS), even a crack that has propagated
halfway through the wall is not reliably detectable by the
MFL technique, since the crack mouth opening is still less
than 0.1mm. At an assumed operating pressure of
approximately 2,320psi a circumferential crack that is 15%
of the way through the wall would have a crack mouth
opening that is an order of magnitude less than the detection
limit of the MFL technique.
In this context it is also useful to consider what the crack
mouth opening displacement would be under both the
operating pressure and external loads such as soil loads.
The CMOD versus applied bending moment and versus
applied tension with the same crack configurations and
pipe geometry as given in Fig.6 are plotted in Figs 7 and 8,
respectively. In both cases the applied load is in addition to
the operating pressure of 2,320psi (86% of the MAOP).
a
Internal Pressure (psi)
a=0.15t
a=0.25t
a=0.5t
Cracked Region
t
Operating
Pressure
The Journal of Pipeline Engineering
The loads at which the cracks could fail by plastic collapse
of the remaining ligament are circled in red in Figs 7 and
8: when the crack is 15% of the wall thickness, an external
soil load of 720kips would cause the remaining ligament to
reach the flow stress (76,000psi) of the material. At this
point the crack opening is only 0.03mm and below the
detection capability of the MFL technique. When the crack
is 25% of the wall thickness, an external soil movement
inducing a tensile load of 595kips would cause the remaining
ligament to reach the flow stress (76,000psi) of the material.
At this point the crack opening is only 0.06mm, which is
still below the detection capability of the MFL technique.
Therefore the MFL tool is most likely not capable of reliably
detecting this size of circumferential cracks prior to them
being susceptible to progressive tearing by small increases
in external loadings.
Sample issue
Fig.6. Plastic CMOD of an outside
circumferential for three different
crack depth versus internal pressure
for a pipe with dimensions R/t =
18.2 and t = 0.375in.
The repeat inspection interval required to mitigate this risk
is determined by computing the difference between the
time to detection, T det , i.e., the amount of time needed
under the loading conditions for the crack to grow to a
detectable size at high probability and confidence for the
chosen method, and T c , the time needed under the loading
conditions for the crack to grow to a critical size. This gives
rise to the computation of the safe inspection interval T s ,
which is simply the difference T c - T det . The repeat inspection
interval is then typically chosen to be a fraction of this the
safe inspection. If the length of time is short between when
the crack can be detected and when the crack is critical, a
short repeat inspection interval is obtained that would be
economically and logistically not viable.
Following the above, it is imperative that pipelines in