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224 Chapter 13
xfi(Pr,i(y)-P:y =minimum (13.5)
where fi is the relative frequency of the design case number i.
13.4.2 Target Reliability Levels
When conducting structural reliability analysis, target reliability levels in a given reference
time period and reference length of pipeline should be selected. The selection is based on
consequence of failure, location and contents of pipelines, relevant rules, access to inspection
and repair, etc. The target reliability levels have to be met in design to ensure that certain
safety levels are maintained.
The following safety classes are proposed:
Low safety class: where failure implies no risk of human injury, minor environmental damage
and economic consequences.
Normal safety class: classification for temporary conditions where failure implies risk of
human injury, significant environmental and economical consequences.
High safety class: classification for operation conditions where failure implies risk of human
injury, significant environmental and economical consequences.
Target reliability levels may be specified by the operator guided by authority requirements,
design philosophy and risk attitude in terms of economics. The target reliability level for
damaged pipelines should be defined in the same level as intact pipeline. The target reliability
level needs to be evaluated considering the implied safety level in the existing rules and
codes. Sotberg et al. (1997) proposed target reliability levels as below:
Table 13.1 Target reliability levels (Sotberg, et al. (1997)).
13.5 BucklinglCollapse of Corroded Pipes
13.5.1 BucklinglCollapse
In the DNV’96 rules for submarine pipelines two alternative criteria are defined for local
bucklinghollapse for internal over pressure cases (and load-controlled situations):
