Page 285 - Pipelines and Risers

258 Chapter 15

15.2 Fracture of Pipes with Longitudinal Cracks
The following assumptions are made for the analysis:

0 Elastic-Plastic Fracture Mechanics is applied.
The dent is assumed to be continuous and to have a constant length.
0 The stress-concentrator is considered to be a notch located at the deepest point of the dent
(infinite length, constant depth). The notch is longitudinal of length, k2c, and depth, a.

15.2.1 Failure Pressure of Pipes with Longitudinal Cracks
Longitudinal surface cracks can occur as isolated cracks or in colonies of numerous closely
spaced and parallel cracks. A procedure based on Maxey et al. (1972) for calculating the
failure stress of longitudinal flaws is as follows:

Folias factor MT is determined from Kiefner and Vieth (1989):
MT = 41 +0.6275x2-0.003375~~ for x 5 7.07 (15.1)
MT = 0.032 x2 + 3.3 for x > 7.07 (15.2)

where:
x= U@t)'"
L= total length of the crack (G2c)
D= pipe nominal outside diameter
t = pipe wall-thickness.

The failure pressure of pipes with longitudinal flaws is calculated as:
4t bfl,
P, = -coi'(exp(- B)) (15.3)
TCDMs

where, oflow is the material flow stress and auxiliary parameters MS and B are given as
follows:

(15.4)

(15.5)

where:
a= crackdcpth
KmFmaterial toughness, estimated from Charpy impact energy tests, as shown later.