Page 301 - Pipelines and Risers

274 Chryiter 15

15 2.0 2.5 30
Safety laclor
Figure 15.7 Safety factory vs. fi and PF.

It must be pointed out that the calibrated safety factor is usually higher than the practical
applied safety factor. For instance, it is generally believed that the target safety level
according to existing code is lo4, while calculation of the implied safety of the existing rules
demonstrated that the implied safety level in the existing codes is of lo3. A necessary
modification based on practical engineering judgement should be applied to the calibrated
safety factor. The history record of safety factor for the considered pipe should be considered
in the judgement.

15.6 Conclusions
A new methodology for fracture assessment of dented pipes with cracks is developed in this
chapter. The calculated fracture strengths are compared with test data and a good agreement is
observed. Uncertainties involved in the evaluation are assessed and measured. A fracture
reliability model is established and applied to evaluate a practical existing pipe further.
Detailed parametric studies is conducted. A new design equation for dented pipes with cracks
in operation with respect to fracture criterion is derived, and corresponding safety factor is
calibrated based on reliability methods. The methodology presented in this chapter has been
used in practical engineering and also accepted by the third party verification.

In order to increase the confidence in the estimated reliability, more refined statistical
presentation of random variables in the analytical model will surely be required, especially
data from pipe field operation. Other failure modes should be investigated in separate studies
and additional information on pipe conditions should be incorporated into the analysis to
produce much more practical, safe and economic results.

15.7 References
1. AFT 5L Specifications, American Petroleum Institutes, (1993).