Page 286 - Pipelines and Risers

Residual Strength of Dented Pipes with Cracks 259

By applying a safety factory, the allowable pressure can be calculated from:
P = P,/y (15.6)

Safety factory can be calibrated by reliability methods as discussed in the following section.
If no calibration is conducted, it is suggested that ~2.0.

15.2.2 Burst Pressure of Pipes Containing Combined Dent and Longitudinal Notch
The fracture condition for the Bilby-Cottrell-Swinden dislocation model (Bilby, Cottrell and
Swinden (1963)) is given as, (Heald et al. (1971))

(15.7)

where:
o = stress at failure (bursting)
o,, collapse stress for a pipe with an infinitely long defect notch of depth a.
=
This model has been used successfully to describe the failure of part-wall defects in pipes, but
modifications are needed before it can be used for dented pipes with defects, as discussed
below.

Toughness modification
Pipe toughness is measured in terms of the Charpy energy, C,. This measure has been shown
to be a good qualitative measure for pipe toughness but has no theoretical relation with the
fracture toughness parameter, Kmt. It is, therefore, necessary to use an empirical relationship
between K,, and C,.

The Battelle Kma,-Cv relationship has been derived based on non-linear regression on full-
scale tests of mechanical damaged pipes. But the deterioration of the fracture toughness
caused by the material deformation as a result of denting has not been taken into account. The
K,,-C, relationship has been modified in Nederlanse Gasunie as:
E
K:at = 1000-(C, - 17.6) (15.8)
A

where:
K,, = material toughness (N/mm3n)
C, = Charpy energy (J)
E= Young’s modulus (N/mm2)
A= section area for Charpy test (mm’), normally A=80 mm’.

Compliance modification: geometry function