Page 294 - Pipelines and Risers

Residual Strength ofDented Pipes with Cracks 267

=
a(.) standard normal distribution function.
15.4.4 Target Safety Level
When carrying out structural reliability analysis, an appropriate safety level in a given
reference time period and reference length of pipe is required. This should be selected based
on factors such as; consequence of failure, location and contents of pipes, relevant rules,
access to inspection and repair, etc. Each factor is termed as target safety levels. Target safety
levels have to be met in design in order to ensure that certain safety levels are achieved.

Reliability methods can be applied to verify that the required target safety level is achieved
for the considered structure. The target safety level for dented pipes with cracks is defined in
the same level as intact pipe. The target safety level needs to be evaluated considering the
implied safety level in the existing rules and codes. In the present calibration, however, an
annual target safety level with @=3.71 is adopted. To illustrate the effect of usin different
target safety levels, the annual target failure probability is taken to be IO' or IO4,
corresponding to an annual target reliability index of 3.09 or 3.71 respectively in the present
investigation. However, further considerations on the target level should be made in
connection with actual code implementation.

15.4.5 Calibration

The safety factor is determined so that the calibrated reliability indices @i for various
conditions are as close to the target safety level PT as possible.
An optimization procedure should in principle be applied in determining the actual sets of the
safety factors. In the present case, a trial and error approach is sufficient to find the optimal
sets of safety factors so that:
C, fi (P~.~ PT y = minimum (15.36)
(y)-

where:
fi = relative frequency of the design case number i
P?= target level
P,, = calibrated probability

15.5 Design Examples

Cited from a practical evaluation of an existing dented pipe, an example is given to verify the
presented model and demonstrate its application in assessing structural integrity of damaged
pipes.

15.5.1 Case Description
The analysis is based on the following data given in Table 15.2 from an existing pipe.