Page 58 - Pipelines and Risers

Wall-thickness and Material Grade Selection 31

= Axial tension in pipe
'0
Teff = Effective tension in pipe

Tv = Yield tension of the pipe
= Axial stress in the pipe wall

Rewriting the Eq. 2.12 yields for external overpressure:
LTefl = LTa + APe . A,, + pi IO.6OSMYS (2.13)
~
A

where:
L T ~effective stress
=
Me = external over pressure

2.4 Equivalent Stress Criterion
The equivalent stress criterion based on von Mises equivalent stress CT,, may be defined as:

LT, =&: +o; -ff,(T, +32f <~,SMYS (2.14)
in which qe, is the usage factor, LT, is the characteristic longitudinal stress,o, is the
characteristic hoop stress, and 2, is the characteristic tangential shear stress. The Tmca
yielding criterion is used in some codes.

ASME B31.8 (1992) specifies a usage factor of 0.90 for both the safety zone and the midline
zone. However, this criterion is not required in situations where the pipeline experiences a
predictable non-cyclic displacement of its support (e.g. fault movement or differential
subsidence) or pipe sag leads to support contact as long as the consequences of yielding are
not detrimental to the structural integrity of the pipeline.

BS 8010 (1993) requires a usage factor of 0.72 for risers and 0.96 for pipelines for functional
and environmental (or accidental) loads respectively, and a usage factor of 1.0 for
construction or hydrotest loads.

The equivalent stress equation is as following:

LT,, =,/LT,? +oh2 -LT,LT, +321 517, .SMYS(T) (2.15)
where:
= the characteristic longitudinal stress
(T!
= the characteristic hoop stress
CT,