Page 32 - Reliability and Maintainability of In service Pipelines

Introduction 21

Figure 1.3 A compound yield of a pipe.

stresses are compound stresses that act on the pipe and alter the yield stress
(Whidden, 2009).
The compression component in longitudinal stresses is related to external
pressure being applied to the pipe, and the stress found in the pipe wall is defined
as ring compression stress, measured by the formula below (Whidden, 2009):
PðD O Þ
S 5 ð1:10Þ
2w
External pressures can be caused by soil embedment (including water
table height above ground surface) with additional pressures being taken into con-
sideration as these pressures need to be supported by the embedment. In the case
of external pressures including water table heights above ground surface, it is
essential that the water table height is added to the internal vacuum pressure as
the internal vacuum pressure is measured at the height of the water table. It is in
this instance where soil slip occurs and it is important to ensure all external pres-
sures are taken into account when designing the pipe ring.
Although the pipe does not exactly fail at yield stress, this measure is set as a
performance limit where the performance limit of pipes is generally wall buckling
or crushing of the pipe wall at yield stress, σ y .
The ring compression design for pipes is determined via the formula below
with the safety factor, sf (Whidden, 2009):

σ y PðD O Þ
5 ð1:11Þ
sf 2w