Page 100 - Pipelines and Risers

Limit-state based Strength Design 73

The PD6493 plastic collapse equation can be applied to calculate allowable defect depth (a)
and length (2c) for a given critical stress.

In addition to PD6493 plastic collapse equation, the following plastic collapse equations are
available from literature, see Bai (1993) and Denys (1992):

CEGB R6 approach (A.G. Miller’s equation)
Willoughby’s equation
the net section yielding collapse solution
the CSA 2184 equation
Denys’s equation

Comparing with other available equations, PD6493 seems to give conservative and reasonable
predictions. The PD6493 suggests that the safety factor for om in Eq. (4.16) is 1.1. The readers
are suggested to define safety factors based on the structural reliability principles described in
Chapters 13 through 15.

Chen et al. (2000) discussed formulae for plastic collapse and fracture of pipe with girth weld
defects. A study of fracture criteria, conducted as part of the DEEPIPE JIP, was summarized
by Igland et al. (2000).

4.6 Fatigue

4.6.1 General
Pipeline components such as risers, unsupported free spans, welds, J-lay collars, buckle
arrestors, riser touchdown points and flex-joints, should be assessed for fatigue. Potential
cyclic loading that can cause fatigue damage includes vortex-induced-vibrations (VIV), wave-
induced hydrodynamic loads, platform movements and cyclic pressure and thermal expansion
loads. The fatigue life of the component is defined as the time it takes to develop a through-
wall-thickness crack of the component.
For high cycle fatigue assessment, fatigue strength is to be calculated based on laboratory
tests (S-N curves) or fracture mechanics.
If no detailed information is available, the F2 curve may be applied as the S-N curves for
pipeline high cycle fatigue. Low cycle fatigue of girth welds may be checked based on A&-N
curves.
The fracture mechanics approach calculates the crack growth using Paris’ equation and final
fracture using a recognized failure assessment diagrams (see Chapter 4.5). It may be applied
to develop cracked S-N curves that are for pipes containing initial defects. If a fracture
mechanics crack growth analysis is employed, the design fatigue life should be at least 10
times the service life for all components. The initial flaw size should be the maximum
acceptable flaw specified for the non-destructive testing during pipe welding in question.