Page 263 - Pipelines and Risers

236 Chapter 14

Folias Factor M
The Folias factor M is a geometric factor developed by Folias (1964) to account for the stress
concentration effect of a notch in the pipes. Recent studies (Kiefner and Vieth (1989))
recommended the following expression to improve the accuracy of the Folias factor:
/= 2.51(L/2)2 0.054(L/2)4 for clso

L2 L2 Dt (14.17)
>
0.032- +3.3 for - 50
Dt Dt
14.2.6 Problems excluded in the B31G Criteria
The ASME B31G criterion can not be applied in some practical corrosion problems including
corroded welds, ductile and low toughness pipe, and corroded pipes under combined pressure,
axial and bending loads. Recent studies concluded that the corrosion in submerged-arc seams
(longitudinal welds) should be handled in the same manner as corrosion in the body of the
pipe. Corrosion in Electric Resistance Welds (ERW) or flash-welded seams should not be
evaluated on the basis of the existing B31G criteria. It is recommended that Kastner's local
collapse criteria (Kastner et al. (1981) is to be used to evaluate corrosion in (circumferential)
girth welds.

A fracture mechanics approach (PD 6493) should be applied for assessing corroded welds,
considering possible defects in the welds. The effect of material's fracture toughness (in
ductile and low toughness pipe) is reflected by the critical fracture toughness of the material
used in the fracture assessment criteria.

In the B31G criteria, the effect of axial load is not discussed. In general, tensile longitudinal
stress may delay yielding and pipe bursting. On the other hand, compressive longitudinal
stress may accelerate yielding and result in reductions in bursting pressure. Figure 14.5 shows
the effect of axial load on collapse pressure (Galambos, 1988).

Figure 14.5 Effect of Axial had on the Collapse Pressure.
Figure 14.5 shows that:

0 The internal burst pressure is largely reduced by axial compression