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PIPELINE RISK ASSESSMENT

THE ESSENTIAL ELEMENTS

This document sets forth essential elements for a pipeline risk assessment. Their utilization ensures that meaningful risk estimates are produced. Furthermore, adoption of these minimum elements facilitates efficient and consistent regulatory oversight and helps manage expectations of all stakeholders.
The essential elements are intentionally a very short list, providing a foundation from which to build a comprehensive (or modify an existing) risk assessment program. They supplement existing industry guidance documents on risk assessment and therefore do not repeat the important issues addressed there.
Application of these elements is easy and intuitive, as is shown in a sample case starting on Page 5. These essential elements and other aspects of pipeline risk assessment will be fully detailed in subsequent guidance documents.
 

1. Measure in Verifiable Units

The risk assessment must include a definition of ‘failure’ and produce verifiable estimates of failure potential. Therefore, the risk assessment must produce a measure of probability of failure (PoF) and a measure of potential consequence.
Both must be expressed in verifiable and commonly used measurement units, free from intermediate schemes (such as scoring or point assignments). Failure probability, which can also be expressed as a frequency, must capture effects of length and time, leading to risk estimates such as ‘leaks per mile per year’ or ‘costs/km-year’, etc.
 

2. Ground Probability of Failure in Sound Engineering Principles

All plausible failure mechanisms must be included in the assessment of PoF. Every failure mechanism must have the following three elements independently measured:

• Exposure (attack) - The type and unmitigated aggressiveness of the force or process that may precipitate failure. Example measurement units are ‘events per mile-year’ or ‘mils per year metal loss’.

• Mitigation (defense) - The type and effectiveness of every mitigation measure designed to block or reduce an exposure. The benefit from each independent mitigation measure, coupled with the combined effect of all mitigations, is to be estimated.

• Resistance (survivability) - The inherent ability of a pipeline to sustain forces and deformations in the event of mitigation failure. Resistance characteristics are to be evaluated separately to determine the probability of ‘damage without failure’ vs. ‘damage resulting in failure’.

For each time-dependent failure mechanism, a theoretical remaining life estimate must be produced and expressed as a function of time.
 

3. Fully Characterize Consequence of Failure

The risk assessment must identify and acknowledge the full range of possible consequence scenarios associated with failure, including ‘most probable’ and ‘worst case’ scenarios.

4. Profile the Risk Reality

The risk assessment must produce a continuous profile of changing risks along the entire pipeline recognizing the changing characteristics of the pipe and its surroundings. The risk assessment must be performed at all points along the pipeline.
 

5. Integrate Pipeline Knowledge

The assessment must include complete, appropriate, and transparent use of all available information. Appropriateness is evident when the risk assessment uses all information in substantially the same way that a subject matter expert (SME) uses information to improve his understanding of risk.
 

6. Incorporate Sufficient Granularity

For analysis purposes, the risk assessment must divide the pipeline into segments where risks are unchanging (i.e. all risk variables are essentially unchanging within each segment). Due to factors such as hydraulic profile and varying natural environments, most pipelines will necessitate the identification of at least five to ten segments per mile with some pipelines requiring thousands per mile. Compromises involving the use of averages or extremes (i.e. maximums, minimums) to characterize a segment can significantly weaken the analyses and are to be avoided.
 

7. Control the Bias

The risk assessment must state the level of conservatism employed in all of its components—inputs, defaults (applied in the absence of information), algorithms, and results. The assessment must be free of inappropriate bias that tends to force incorrect conclusions for some segments. For example, the use of weightings based on historical failure frequencies may misrepresent lower frequency albeit important threats.
 

8. Unmask Aggregation

A proper process for aggregation of the risks from multiple pipeline segments must be included. For a variety of purposes, summarization of the risks presented by multiple segments is desirable (e.g. ‘from trap to trap’). Such summaries must avoid simple statistics (i.e. averages, maximums, etc.) or weighted statistics (length-weighted averages, etc.) that may mask the real risks presented by the collection of segments. Use of such summarization strategies often leads to incorrect conclusions and is to be avoided.

Source:

An initiative through collaboration of DNV and W. Kent Muhlbauer.May 2012.PIPELINE RISK ASSESSMENT-The Essential Elements (First published in Pipeline & Gas Journal).www.dnvusa.com/Binaries/Pipeline%2520Risk%2520Assessment%2520Essential%2520Elements%2520with%2520Sample%2520Case_tcm153-573274.pdf+&cd=1&hl=en&ct=clnk