Page 290 - Pipelines and Risers

Residua[ Strength of Dented Pipes with Cracks 263

The stress ratio S, is defined as the ratioof net section stress on to flow stress onow:

S,=- (5" (15.24)
(Jflow
15.3.2 Material Toughness, Kmt
Several statistical correlation exists between standard full-size C, (the Charpy V-notch) and
Kmt. Rolfe and Novak (1970) developed the following correlation for upper shelf toughness
in steels:
0.6459 C,
- 0.25 (1 5.25)

with Kmt is in MFa(mm)In, C, is in mm-N, and cry is in MPa.

15.3.3 Net Section Stress, (5"
Following PD6493, the net section stress for pipes with surface flaw is:

(1 5.26)

where:
(5b= bending stress
om = membrane stress
a = (2a / t) / (1 + t /c) (15.27)
M
(5 -- (15.28)
b- t2/6

where M is given by Equation (15.12) substituting OH by nominal axial stress OM.
15.3.4 Maximum Allowable Axial Stress

The critical stress at failure is obtained by iteratively solving the Level-2 FAD of PD6493
(Equation (15.20)) including safety factors.

15.4 Reliability-based Assessment and Calibration of Safety Factors
Due to uncertainties involved in the fracture assessment of damaged pipes, the conventional
approach has its limitations whereas structural reliability theory provides a rational and
consistent way to deal with those uncertainties in loading, strength and modeling.