Page 247 - Pipelines and Risers
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220 Chapter I3
The purpose of this chapter is to present the experience gained from reliability-based limit-
state design applied in practical design and re-qualification of pipelines to achieve cost-
effective solutions (e.g. Bai et al. (1997), Bai and Song (1998)).
This chapter summarizes methods for reliability analysis, uncertainty measures, target
reliability levels and calibration of safety factors. A limit-state design case study is then
presented and discussed.
13.2 Reliability-based Design
13.2.1 General
In principle, reliability-based design of offshore pipelines involves the following aspects:
Identification of failure modes for specified design cases;
Definition of design formats and Limit State Functions (LSF);
Uncertainty measurements of all random variables;
Calculation of failure probability;
Determination of target reliability levels;
Calibration of safety factors for design;
Evaluation of design results.
13.2.2 Deterministic vs. Probabilistic Design
Structural design codes commonly specify loads and strength and appropriate safety factors
for design use. Generally, two design approaches are being adopted namely deterministic and
probabilistic designs. In traditional (deterministic) design, the relevant loads, load effects and
material propcrties are defined as deterministic quantities. Two basic design equations are
explicitly specified for the yielding check the hoop stress criterion and the equivalent stress
criterion.
In reality, most of the design variables, such as wall-thickness and material properties, contain
uncertainty. In addition, the idealized analytical model is also a source of uncertainties.
Hence, a probabilistic approach is required to provide an appropriate method to deal with
these uncertainties and to achieve a consistently safe level of design. Besides, different
failures may occur in different design scenarios and, hence, lead to different failure
consequences. Reliability methods may be applied to achieve a cost-effective design that
balances both safety and costs.
13.2.3 Load Effects and Combinations
In general, the following loads and load combinations in pipeline structural design should be
considered:
