Pipeline Risk Assessment—Risk Profiling
In earlier installments of this column, we introduced the concept of pipeline risk assessment Essential Elements (EE’s). That guideline is a list of ingredients that must be included in any pipeline risk assessment before that assessment can be considered complete. Following these guidelines helps to ensure a technically sound risk assessment that should satisfy all stakeholders, including regulators, and provide useful decision-making support to owner/operators. The EE risk assessment is a Quantitative Risk Assessment (QRA). There are many reasons why today’s pipeline risk assessment must be quantitative—driven by numbers.
The pipeline EE risk assessment differs in important ways from the QRA typically seen in the nuclear, chemical, and aerospace industries. In this installment, let’s note some key differences and examine the essential element dealing with one of those differences—the need for a risk profile.
The idea of a risk profile—changes in risk over ‘space’—is what sets pipeline risk assessment apart from many other QRA applications. Traditional QRA is normally applied to facilities that do not occupy a constantly changing ‘space’. Even air- and spacecraft QRA’s have limited use of changing environmental conditions—they tend to focus on the extreme conditions that govern design requirements.
There are many similarities in QRA approaches, but some key differences when applied to a pipeline. Pipeline QRA’s are unique from others in several key respects:
- Efficient and independent examination of three distinct PoF ingredients: exposure, mitigation, resistance
- Methodology that accommodates long, linear assets with constantly changing environmental and loading conditions
- Substantially reduced reliance on historical incident rates.
The creation of a risk profile is a key differentiating aspect. A profile acknowledges the unique aspects of long, linear assets, recognizing that there are many ‘individual’ pipe segments among the ‘population’ of segments that make up a pipeline. As with any treatment of populations vs individuals, behavior is much more accurately predicted for the former.
Traditional QRA relies heavily on historical incident rates. When applied to pipelines, this means that a pipeline—a population of pipe segments in varying environments—is modeled to behave as point estimates of populations of other, supposedly comparable pipelines—also collections of individual segments. As the foundation for a risk assessment on a subject pipeline, this is potentially very inaccurate. Let’s examine all of the implied assumptions embedded in this approach:
- Comparison population (collections of pipe segments from similar pipelines) is fairly represented by a single value—ignoring extremes is appropriate; ie, the fact that some segments of pipelines may carry much less or much more risk, is not germain.
- Subject pipeline, considered as a whole, is fairly represented by the comparison population
- All parts of subject pipeline behave similarly—the sum of the parts equals a value represented by the comparison
The full RA solution to any variation in any risk variable is to ‘dynamically segment’ on that variation. This means that a new segment should be created for any feature or length of pipe that has different characteristics from its neighbors. Therefore, each pipe joint, fitting, etc should be an independent segment, reflecting its differing crack potential, corrosion potential, ability to resist external force, etc. This will generate many segments. A potential criticism to this high-resolution approach is that ‘management of such a high count of segments is problematic’. The response is direct and intuitive—these segments are currently already being ‘managed’ in the real world. Each joint really does have failure issues distinct from the adjacent pipe and must be managed accordingly. Every fitting really does introduce new failure issues. The risk assessment should acknowledge this reality.
In RM practice, the high segment count is not burdensome. Proper aggregation allows a ‘summary’ risk value for any stretch of pipe, regardless of the number of changes in risk properties along that stretch. Once a pipe section—a collection of segments—becomes a candidate for RM, the initial drill-down quickly reveals if the risk issue is due to a number of problematic joints, fittings, appurtenances, etc. If so, RM plans are made accordingly.