Page 260 - Pipelines and Risers

Remaining Strength of Corroded Pipes 233

The B31G criterion has several problems for corrosion defects in real applications. It can not
be applied to spiral corrosion, pitstgrooves interaction and the corrosion in welds. For very
long and irregularly shaped corrosions, the B31G criterion may lead to overly conservative
results. It also ignores the beneficial effects of closely spaced corrosion pits.

Spiral Corrosion
For defects in other orientations, the B31G criterion recommends that the defect is projected
on the longitudinal axis of the pipe to be treated as a longitudinal defect. This
recommendation appears to be adequate for short defects. It is conservative for long spiral
defects (Bai et al. (1994)).

Mok (Mok et al. (1990, 1991)) conducted extensive tests in the applicability of the B31G
criteria to long spiral corrosion. For spiral defects with spiral angles other than 0 or 90
degrees, the study found that B31G underpredicted the burst pressure by as much as 50%. The
effect of spiral angle is illustrated in Figure 14.2. (Mok et al. (1990)).

0 2 0 4 0 6 0 80 90
Defect Spiral Angle
Figure 14.2 The Effect of Spiral Angle.

Based on the experimental and numerical studies, Mok et al. (1990) recommended the spiral
correction factor in determining the burst pressure for w I t 5 32 as:
l-Q1W
Q= 32 y+Qi (14.13)

in which W is the defect width, and coefficient Q1 is a function of the spiral angle cp
(cp = goofor longitudinal corrosion, cp = 0' for circumferential corrosion):
1 0.2 for 0' <cp<20'

-
Q1 = 0.02~~ 0.2 for 20' < cp< 60' (14.14)
1.0
for cp > 60'
for w I t z 32, the value of Q must be taken as 1 .O

Pits Interaction
Corrosion in pipelines often results in colonies of pits over an area of the pipe. For closely
spaced corrosion pits, a distance oft (wall-thickness) is used as a criterion of pit separation for