Page 278 - Pipelines and Risers

Remaining Strength of Corroded Pipes 25 1

-1.0 I I
0 5 10 15
Smice Time (Years)
Figure 14.10 Annual Bursting Failure Probability.

For the probabilistic analysis, the following approaches are applied

The corrosion rate is based on the deWaard & Milliams formula,
The reduced burst capacity is estimated based on the new criteria,
The design pressure for which the capacity model is to resist is developed over the service
life as a function of the operating pressure.

Based on the capacity and loading model, the annual probability for bursting of the corroded
pipelines is illustrated in Figure 14.10. It is shown that the estimated probability of failure
increases slightly with time in spite of the reduced operating pressure due to the increase in
the expected level of corrosion.

Evaluating of Repair Strategies
A minor repair/modification is recommended. The alternatives are summarized as:

A reduction of the operating pressure, de-rating;
Use of corrosion mitigation measures (inhibitors);
Rescheduled inspection;
Combination of the above alternatives.

The life-cycle cost of mitigation measures and lost income are set as the evaluation criteria.
The constraint requirements are:

Acceptable level of safety within the remaining service life, or at least until next
inspection;
The annual failure probability of the pipeline should be less than with the remaining
service life or until next inspection;
Next inspection is scheduled for a service life of 15 years. Meanwhile, an early inspection
can be recommended.