Page 264 - Pipelines and Risers

Remaining Strength of Corroded Pipes 237

The effect of axial tension is beneficial. The tension load is not significant when it is less
than 60 percent of yield strength of the pipe section. This effect is significant when the
axial tension is larger than 60% of the yield strength.

The dominant effect of bending stress, on the other hand, is the reduction of the hoop stress in
the corroded region. Therefore, the B31G criteria of burst pressure that considers internal
pressure alone may lead to unconservative results when large axial and bending stresses are
coupled with corrosion.

14.3 Development of New Criteria
In this section, a new criterion is developed for longitudinally corroded pipelines.

For longitudinally corroded pipe, pit depth exceeding 80% of the wall-thickness is not
permitted due to the possible development of leaks. General corrosion where all of the
measured pit depths are less than 20% of the wall-thickness is permitted, without further burst
strength assessment. If the ratio of maximum pit depth and wall-thickness is between 0.2 and
0.8, the following equations are recommended.

The Maximum Design Pressure Level P
The maximum allowable design pressure in the new criterion is the same as that of the
original B31G criteria:
p =- 2sMYs oFt (14.18)
D

where F is the usage factor for intact pipe which is 0.72 according to the B31G criterion.

The Safe Maximum Pressure Level P'
1 2o,,,t 1-QAREAI ARE&
p' =- ( 14.19)
y D I-M-~AREAIAREA~

where:
P = safe maximum pressure level
onow = flow stress of the material
t = wall-thickness of the pipe
D = outside diameter of the pipe
AREA = Lt
AREAo = original area prior to metal loss due to corrosion within the effective are
which is Lt
L = defect length of corrosion profile
M = Folias factor