Page 481 - Pipelines and Risers

448 Chapter 24

The equivalent stress criterion limits von Mises stress to a fraction of SMYS. For D/t ratio
larger than 20, the bi-axial form of equivalent stress should be calculated and the criterion
reads as:

up =&;+a,? -uhul +%’ Sq-SMYS (24.6)

where:
oh -hOOpStreSS

01 - longitudinal stress
z - shear stress
fl - usage factor

For high pressure pipes with D/t ratio less than 20, the tri-axial fom of equivalent stress
should be calculated and the criterion reads as:

(24.7)

where:
OR - radial stress

Strain Based Design Criteria
For high temperature (e.g. above 12OoC) pipe, the stress based design criteria might severely
limit the hoop stress capacity due to internal pressure. In this case, the strain based design
criteria can applied instead of stress based criteria. The maximum equivalent plastic strain
shall be calculated by
E P =d+E:h+E:) (24.8)

where:
q,~ - longitudinal plastic strain
%h - hoop plastic strain
qr - radial plastic strain

The accumulated plastic strain shall satisfy q, < 0.5%. Otherwise, fracture assessment shall be
performed.

Local Buckling Design Criterion
Local buckling capacity check for both internal and external pipes shall in line with the
criterion given in Chapter 3.