by Mark Stephens, C-FER Technologies
As published in Pipelines International, June 2012
Maintaining pipeline integrity in the face of progressive deterioration mechanisms, like corrosion and cracking, is an ongoing challenge for pipeline operators. In general terms, the process for defect specific integrity management involves four key steps:
Identify significant features;
size the features;
project feature growth with time; and,
assess the features' remaining life.
The listed steps would suggest a relatively straight forward, sequential approach to the management of time-dependent damage; however, a complicating factor is the fact that each step in the process is associated with significant uncertainty. Consider the following:
The detection of significant damage features is not certain and random measurement error will always be associated with the reported sizes of detected features. In addition, the likelihood of non detection and the magnitude of possible measurement error are dependent on the type of damage feature and the inspection technology employed to find it. Conventional deterministic assessment methods assume certainty with respect to detection and do not generally consider feature sizing uncertainty. When sizing error is considered in a deterministic assessment, a simplistic and overly conservative approach is often employed wherein a prescribed sizing tolerance is added to the reported feature depth and/or length.
Feature growth uncertainty
The potential for feature growth is a key consideration since without growth, existing features can be considered stable and thereby unlikely to cause failure unless a pipeline experiences an overload. Due to the complexities associated with feature growth mechanisms, both the likelihood of growth and the rate of growth are highly uncertain. Deterministic approaches generally lack consistency in the way that growth rates are selected, and the assumed rates are often made unduly conservative in an effort to account for the inherently high level of uncertainty.
Consistent levels of protection
The time to defect remediation or time to re-inspection, as dictated by the time required for significant defects to grow to a failure-critical size, is influenced by the sources of uncertainty described above, the added uncertainty associated with the accuracy of the adopted failure prediction model, and the uncertainties associated with the joint-specific pipe properties that are also inputs to the adopted capacity model.
Deterministic methods implicitly account for capacity model and input parameter uncertainties through a safety factor. However, given the potential for line-specific variability in the level of uncertainty associated with the key inputs to the remaining life assessment, it follows that a single safety factor in the case of liquid pipelines, and a single safety factor for each location class in the case of gas pipelines, cannot achieve consistent levels of environmental protection and/or public safety in all cases.
A quantitative approach
Given the above, an integrity management approach that formally and systematically accounts for the uncertainties inherent in the process is highly desirable. It is all the more attractive, if not essential, given recent regulatory pronouncements on the importance of acknowledging parameter uncertainties, such as feature sizing error, in the assessment process.
In this context, a quantitative analysis approach based on structural reliability methods is considered ideally suited to managing time-dependent damage as identified and characterized through inspection. This alternative to conventional deterministic assessment methods combines appropriate failure prediction models, defect attributes obtained from inspection data, data on the accuracy of the inspection method, and the physical and operational characteristics of the pipeline, all within a probabilistic analysis framework. The output of this type of analysis is a quantitative measure of pipeline integrity (or reliability) in terms of the likelihood of line failure as a function of time. It explicitly accounts for the key sources of analysis uncertainty in an objective and consistent manner, and it provides the information required to determine what to repair and when to re-inspect.