Page 261 - Pipelines and Risers

234 Chapter I4

a colony of longitudinal oriented pits separated by a longitudinal distance or parallel
longitudinal pits separated by a circumferential distance.

For circumferentially spaced pits separated by a distance longer than t, the burst pressure can
be accurately predicted by the analysis of the deepest pits within the colonies of pits. For
longitudinal oriented pits separated by a distance less than t, the failure stress of interacting
defects can be predicted by neglecting the beneficial effects of the non-corroded area between
the pits. For parallel longitudinal pits separated by a circumferential distance, experiments
suggested that pits could be treated as interacting pits if the circumferential spacing is less
than t @ai et al. (1994)).

Groove Interactions
For the interaction of longitudinal grooves, if the defects are inclined to pipe axis and the
distance x between two longitudinal grooves of length L, and L2 is larger than L1 and L2, the
length of corrosion L is the maximum of L, and L~. If the defects are inclined to pipe axis
and the distance x between two longitudinal grooves of lengths L1 and L~ is less than L~ and
L
L2, the length of corrosion L is the sum of x, L1 and L~, L1 + L2 + X.
=
-

Root Gmve

Corrosion at Toe of Cap
Figure 143 Typical Patterns of Weld Corrosion.

Corrosion in Welds
One of the major corrosion damages for marine pipelines is the effects of the localized
corrosion of weld on the fracture resistance. Figure 14.3 shows typical pattern of weld
corrosion. The B31G criteria did not cover the assessment of corroded welds. The existing
fracture assessment procedures (BSI PD6493) are recommended.

Figure 14.4 Effect of Defect Width.