To illustrate the inclusion of the Essential Elements in a risk assessment, the following example is offered. The Elements are inherently flexible allowing for alternative, yet effective, approaches. The Elements' methodology establishes a desirable level of rigor for assessing risk.
Scenario: A 120 mile pipeline is to have a risk assessment performed. For the assessment, failure is defined as loss of integrity leading to loss of pipeline product. Consequences are measured as potential harm to public health, property and the environment.
|'Failure' and 'Consequences' are clearly defined.|
|Probability of Failure (PoF) ||failures/mile-year|
|Consequence of Failure (CoF) ||$/failure|
|Time to Failure (TTF) ||years|
Measuring in verifiable units keeps the process honest by using
understandable terms that can be calibrated to reality.
Minimum data (as defined in ASME B31.8S) are collected and includes Subject Matter Expert (SME) estimates where actual data is unavailable. The collected data show changes in risk along the pipeline route—6,530 segments are created by the changing data with an average length of 87 ft. This ensures that a risk profile with adequate discrimination is generated.
|Full data integration with sufficient resolution|
A level of conservatism to be used is defined as P90 for all inputs that are not based on actual measurements. This means that an ‘undesirable result’ will arise once for every ten inputs (i.e. the input will only overestimate the true value 10% of the time). The risk assessors have chosen this level of conservatism to account for plausible (albeit extreme) conditions and ensure that risks are not underestimated.
|Bias Control: Clearly defined level of conservatism|
For assessing PoF from time-independent failure mechanisms, the top level equation selected by risk assessors is as follows:
PoF_time-independent = exposure x (1 - mitigation) x (1 - resistance)
|Grounding aspects of risk such as PoF in sound engineering|
principles provides a firm basis in science and helps to provide a rational, defensible position.
As an example for applying this to PoF due to time-independent third-party damage, the following inputs are identified (by SME’s) for certain portions of the subject pipeline. Here, by independently measuring the attacks, defenses and survivability, the analysis promotes a full understanding of the PoF:
- Exposure (unmitigated ‘attack’) is estimated to be third-party damage events per mile-year.
- Using a mitigation (defense) effectiveness analysis, SME’s estimate that 1 in 50 of these exposures will not be successfully prevented by existing mitigation measures. This results in an overall mitigation effectiveness estimate of 98% mitigated.
- Of the exposures that result in contact with the pipe, SME’s perform an analysis to estimate that 1 in 4 will result in failure, not just damage. This estimate includes the possible presence of weaknesses due to threat interaction and/or manufacturing and construction issues. So, the pipeline in this area is judged to have a 75% resistance to failure (survivability) from this mechanism, once contact occurs.
These inputs result in the following assessment:
PoF_third-party damage =
(3 damage events per mile-year) x (1 - 98% mitigated) x (1 - 75% resistive) =
1.5% (0.015) per mile-year (a failure about every 67 years along this mile of pipeline)
Produce useful estimates even when data is scarce.
Note that a useful intermediate calculation, probability of damage—but short of failure—emerges from this assessment and can be verified by future inspections.
(3 damage events per mile-year) x (1 - 98% mitigated) =
0.06 damage events/mile-year (damage occurring about once every 17 years)
Verifiable units of measure, even for estimates
This same approach is used for other time-independent failure mechanisms and for all portions of the pipeline.
In assessing PoF due to time-dependent failure mechanisms, the previous algorithms are slightly modified:
PoF_time-dependent = ƒ(TTF_time-dependent)
TTF_time-dependent = resistance / [exposure x (1 - mitigation)]
To continue the example, SME’s have determined that, at certain locations along the 120 mile pipeline, soil corrosivity leads to 5 mils / year (mpy) external corrosion exposure (unmitigated). Examination of coating and CP effectiveness leads SME’s to assign a mitigation effectiveness of 90%. Recent inspections, adjusted for uncertainty, result in a pipe wall thickness estimate of 0.220” (remaining resistance). Use of these inputs in the PoF assessment is shown below:
TTF = 220 mils / [5 mpy x (1 - 90%)] = 440 years
PoF = 1 / TTF = [5 mpy x (1 - 90%)] / 220 mils = 0.23% PoF
Risk assessment uses data as would an SME
In this case, SME’s have analyzed potential consequences and determined the range of possible consequence scenarios generated by a failure. The range of possibilities is characterized by a set of scenarios. This company has decided to monetize all potential consequences. After assignment of probabilities to each scenario, a point estimate representing the distribution of all future scenarios yields the value of $11,500 per failure.
Risk assessors calculate all risk elements for each of the 6,530 segments. To estimate PoF for any portion of the 120 mile pipeline, a probabilistic summation (using an 'OR' gate) is used to ensure that length effects and the probabilistic nature of estimates are appropriately considered. To estimate total risk, an expected loss calculation for the full 120 miles yields an average value of $210/mile-year.
The company uses this value to compare to, among other benchmarks, a US national average for similar pipelines of $350/mile-year.
|Conducting the risk assessment in this manner helps the operator define maintenance and operational strategies as well as in carrying out other risk-informed decision making.|
Date: 24 May 2012