Page 511 - Pipelines and Risers

478 Chapter 25

q = Usage factor

Equation 25.8,25.9 and 25.10 are combined to give equation 25.11 for short corrosion defects
( L 4.48. n) (from NGdland, Bai and Damsleth, 1997). The increased strength of the pipe
wall in uncorroded sections of the pipe (due to the remaining corrosion allowance) has thus
been taken into account. These calculations allow for no reduction in design pressure during
the lifetime.
(25.1 1)
d, =
1.1-t D-t-
-.c-
t-C D-t
where:
dh= Allowable corrosion depth based on hoop stress.
T= Wall thickness incl. corrosion allowance
&= Wall thickness excl. corrosion allowance

4. Allowable Corrosion Depth Based on Collapse
A corrosion defect may reduce the hoop buckling capacity of a pipe. The allowable corrosion
depth based on collapse may easily be derived based on the formulation in Chapter 3.

5. Limit State Function
The limit state function, g(X), forms the basis for the reliability calculations. This function
expresses ‘Resistance’ - ‘Load’ as a function of X, where X is a vector containing all the
basic uncertainty variables describing the ‘loads’ and ‘resistance’s’. Deterministic values may
also be included in g. The criterion for non-acceptance (or failure) is consequently defined as
g(X) < 0, with the corresponding probability:
(25.12)
P(g(x) < 0) = 5 $(jr)ajC
V
where:
<
V= failure domain = {qg(x) 0)
f,(Sr)= joint density function for X
51 = Realization of TT in the basic variable space

Since two failure modes are investigated (i.e. hoop stress and local collapse for load and
displacement control), two limit state functions are needed to describe the system. The system
probability of failure may therefore be approximated by:
(25.13)
PpyptCm = P(g*(X) c 0) + P(&(X) < 0)

The calculation of the probability of failure is done by the proprietary software SYSREL.
Second Order Reliability Method (SORM) used.
is