Page 288 - Pipelines and Risers

Residual Strength of Dented Pipes with Cracks 26 1

Flow stress modification
A more accurate measure of the plastic failure stress would be the collapse stress with a defect
present. Following the B31G, collapse stress for a rectangular defect in a pipe is:
t-a
op = of- (1 5.16)
t-aM;!

in which of is the flow stress for intact pipe and can be estimated from API as:
of =a+o, (15.17)

where oy is the pipe yield strength and parameter a is around 1.25, a decreases when oy
increases.

15.2.3 Burst Strength Criteria
The critical stress at failure is obtained from Equations (15.9 and 15.15) as:

( 1 5.1 8)

Burst strength is given by:
1
P = 2o- (15.19)
D

Based on Failure Assessment Diagram (FAD), the aforementioned burst strength can also be
obtained by use of the procedure presented in PD6493, in which iteratively solving the
equation of assessment will be involved including safety factors, as described for the case for
circumferential cracks, Section 15.3.

15.2.4 Comparisons with Test
Based on the formulae presented in this chapter, comparisons of predicted burst strength and
tests @berg et al. (1982)) including input data used in the calculation are listed in Table 15.3.
This test was conducted as a joint industry research project from which the main achievement
was a compact full-scale test series. Pipes with different diameters were pressure tested to
rupture with varying degree of indentation and gouging combinations. Meanwhile, curves of
strength reduction factors as a function of If(Dt)0.5 were also obtained. Some
recommendations were made based on the results of the pressure tests, fracture mechanic
tests.