Page 248 - Pipelines and Risers

Reliability-Based Strength Design of Pipelines 22 1

0 Functional loads, e.g. internal and external pressure load effects, thermal forces, pipe
weight and residual lay forces.
0 Environmental loads, e.g. wave (in shallow water) and current loads.
0 Accidental loud efsects, e.g. fishing gear impact, dropped objects impact, anchor impact,
etc.
Combinations of the above.

The functional load and environmental load effects are related to the pipeline system. While
accidental load effects and load combinations may be critical to the local components. Two
design phases are defined: temporary and operational.

13.2.4 LRFD Design Format
To achieve a uniform safety level for a range of parameter variation, an appropriate design
format, which should be simple to use in design, will be selected. The design format is usually
based on LRFD (Load Resistance Factored Design). The selected design format should be a
simplified representation of the actual limit state condition under consideration. The most
significant variables should be included in the design format.

A representative LFWD design format is expressed as:
YES, +rB, 5 yR (13.1)

where SC and RC are characteristic load effect and resistance of the considered failure mode, y
is the partial safety factors to be calibrated, Subscripts E and F denote environmental loads
and functional loads respectively.

The design values of load effects and capacity are estimated as the product of characteristic
values and partial safety factors. Four kinds of limit states and related failure modes for
pipelines are generally identified namely serviceability limit state (SLS), ultimate limit state
(ULS), fatigue limit state (FLS) and accidental limit state (ALS).

13.2.5 Calculation of Failure Probability
Generally, limit-state function (LSF) is introduced and denoted by g(Z) where Z is the vector
of all uncertainty variables. Failure occurs when g(Z)IO. For a given LSF g(Z), the probability
of failure is defined as:
P,(t)=P[g(Z)<O] (13.2)

The results can also be expressed in terms of a reliability index p, which is uniquely related to
the failure probability by:
p(t) = -W(P, (t)) = @-I(- P, (t)) (13.3)