Page 410 - Pipelines and Risers

Use of High Strength Steel 377

It is likely that fracture occurs in the weldment. Then the CTOD requirements made to pipe
base material are not relevant. However, the CTOD value for HAZ may be relevant for
fracture in HAZ. Weldability of the pipe is a more important parameter than CTOD value.

19.6.3 Material Property Requirement in Longitudinal Direction
The CTOD value for line pipes in longitudinal direction is influential for fracture limit-state
when ECA such as PD 6493 is applied to calculate the limiting loading condition to avoid
fracture.

The CTOD value needed to avoid fracture depends on the extent of girth weld defects likely
to exist and the applied load. For a defect depth of 3 mm, a wall thickness of 25.4 mm and
loading up to 0.5% total strain a defect length of 177 mm (7 x wall thickness) was shown to
be safe when CTOD is minimum 0.10 mm, see Knauf and Hopkins (1996).

The discussions on unstable fracture and CTOD for hoop direction are also valid for
longitudinal direction.

The fact is that the yield stress in longitudinal direction does not significantly affect pipe
strength as long as strain-based design is applicable to the design situation. The reasoning for
this statement is that strain acting on pipelines in operating condition is typically as low as
0.2% unless the pipeline is under a high pull-over load.

With exception of some special material problems, the Y/T (SMYS/SMTS) ratio requirements
can be replaced by introducing strain-hardening parameters such as OR and n used in a
Ramberg-Osgood equation. In Bai et al. (1994), a set of equations are given to relate SMYS
and SMTS with strain-hardening parameters OR and n.

The material strain-hardening effect may be accounted for in fracture mechanics assessment
and local bucklinglcollapse checks through use of the stress-strain curves. In fact, a set of
design equations was given by Bai et al. (1997) and Bai et al. (1999) for local
bucklingkollapse. In the papers by Bai et aI. (1997, 1999), the effect of material strain
hardening parameter on buckling/colIapse have been discussed in detail.

The level-2 and level-3 failure assessment diagrams in PD 6493 do also account for strain-
hardening effects.

19.6.4 Comparisons of Material Property Requirements
Which material properties are dominant in local bucklinglcollapse? The answcr is dependent
on loads as the following:

0 For internal pressure containment, hoop SMTS;
For external-pressure induced buckling, hoop SMYS;
For bending collapse, longitudinal SMYS;