Page 99 - Pipelines and Risers

72 Chapter 4

It was stated by Canadian Standards Associations that the pipeline industry has used a
longitudinal tensile strain limit of 0.5%. This limit prevents fracture initiation and plastic
collapse from Circumferential weld flaws small enough to be accepted by the specification or
that may have been missed by inspection. Zimmerman et al. (1992) and Price (1990) reports
that the 0.5% tensile strain limit is a subjective limitation, chosen to coincide with the API
yield strength specifications and does not reflect an objective failure limit.

The Troll Phase I project applied an allowable strain level of 0.4 % for a 36" gas export line,
which was approved by NPD, (Koets and Guijt (1996)).

4.5.2 Plastic Collapse Assessment
It has been observed that all fracture mechanics calculations based on PD6493 lead to S,=l.
Where S, is defined as:
(4.14)

The flow stress of is according to PD6493 defined as the average of yield stress oy and
ultimate tensile stress ou of the weld material. For a flat plate with surface flaw under tension,
equations for the net section stress on from PD6493 lead to:
(4.15)

where:
o,, : critical stress
a :defectdepth
c : half-width of the defect
t : wall-thickness of the plate

The applied PD6493 assessment criteria can then be re-expressed as:

(4.16)

The above PD6493 plastic collapse equation may be valid provided that brittle fracture is not
a relevant failure mode, e.g.:

The defect depth (a) is less that 3 mm and the length (2c) is less than t (or 25 mm)
The material CTOD is more than e.g. 0.2 mm at operating temperature