Optimizing use of inspection results.
A limitation that continues to plague older risk assessment methodologies is the inability to fully include the results of inspections. None dispute the value of inspection and its critical role in failure avoidance. Yet, neither the classical statistics-centric QRA nor once common relative risk ranking methodologies[1], can capture inspection activity in a meaningful way.
A modern risk assessment methodology should recognize exactly what has been learned from an inspection and how that knowledge should be captured. The age of each inspection and its detection/sizing capabilities are crucial, as are the inspection findings themselves.
Inspection of Integrity vs Inspection of Defenses
Recall a previous article (“ILI vs DA—The Risk View”, Apr 2016) comparing In Line Inspection (ILI) vs Direct Assessment (DA, and, in particular, External Corrosion DA or ECDA). This article highlighted a fundamental difference between the two types of inspection. The main conclusion was that one is forward-looking while the other is backward-looking:
ECDA is a forward-looking technique since it mostly yields information on mitigation: how well protected is the system, suggesting future failure potential. ECDA and especially its key aspect of overline surveys, tell us of deficiencies in protection that may lead to future damages.
ILI and pressure testing are primarily backwards-looking techniques, showing damages that have already occurred (resistance issues, although ILI also provides useful knowledge of exposure and mitigation).
Both are important. Neither tells us everything. Both contribute to risk reduction. But they do so in different ways. Good risk assessment allows us to clearly see the role of each and make better decisions.
When ‘Nothing is Found’
Another aspect of fully capturing the learnings from an inspection involves components that have been ‘inspected with no findings of deficiencies or weaknesses’. As with findings of deficiencies, something has been learned from an inspection reporting no deficiencies. Obviously, inspecting and finding no deficiencies is not the same as not inspecting, and these must be treated differently in a risk assessment.
In the case of ILI with no deficiencies detected, inspection has shown that there are no[2] weaknesses existing on the inspection date. Previous to the ILI, the risk assessment should have conservatively assumed that some damages had been occurring from corrosion, cracking, external forces, etc. Since none was found, the ‘clock has been reset’. The assumed damages did not actually occur, and so the original strength (wall thickness) can be used in risk calculations[3].
Similarly, an ECDA that finds no deficiencies has provided valuable information. It has confirmed, to some confidence level, that the mitigation—cathodic protection (CP) and coating—are performing as intended at all points.
Understanding the differing roles of inspection in risk assessment, we can explore how to best utilize the information from an inspection.
Age and Accuracy
We can build datasets of inspection results that provide both inputs into mitigation effectiveness and strength or resistance estimates. See Evidence . The risk assessment algorithms use these datasets, but only after adjusting their values for two key considerations:
- What might the inspection have missed?
- What might have happened since the inspection?
Consistent with the chosen level of conservatism accompanying the risk assessment, answers to both of these must accompany the use of the inspection data. But how can that be done efficiently over many kilometers of pipeline operating over multiple decades?
A Broadcast Solution
It is not unusual for single locations along a pipeline to undergo multiple inspections of various types spanning many years. In addition to ILI, pressure testing, and DA, many other types of surveys/inspections are common. Inspections like soil resistivity inform our estimates of failure mechanism aggressiveness (exposure). Inspections like depth of cover, pipe-to-soil voltage inform estimates of mitigation. Many integrity-centric inspections like in-ditch UT measurements and magnetic particle inform estimates of resistance. Inspections also inform estimates of consequences—eg, population density surveys, spill dispersion analyses, etc.
Each inspection probably provided valuable information to risk knowledge. Each is subject to aging and accuracy limitations. There will sometimes be overlapping, conflicting, and confounding findings from inspections. There will also be gaps—portions of the system that have not been inspected.
In order to efficiently capture and utilize findings from a wide variety of inspections performed over many years, with disparate findings, a broadcast solution is needed. That is, a way to efficiently compare inspection results, resolve differences in findings, override older and less accurate information in favor of newer and more accurate information, and include in the risk assessment only the best information. This solution should be able to analyze information over many kilometers of pipeline[4] very rapidly and without much ‘manual’ examination of specific locations.
The key to obtaining this level of efficiency is to first identify how the inspection impacts risk estimates. Then, the findings of every inspection should be adjusted for age and accuracy. This is simply[5] a matter of understanding the rate of emergence of various weaknesses and abilities of the various inspections to detect them. When this adjustment is done consistently and conservatively, the more optimistic findings will almost always appropriately override the more pessimistic findings. This makes it easy for the solution that is being broadcast across the many kilometer-years of a system, to select the most useful information to include in the risk assessment.
Better Risk Analyses
Learnings from inspections are valuable and usually intuitive. Unfortunately, their use in formal risk assessments has been questionable in the past. The full and appropriate use of knowledge gained from inspection is yet another distinguishing feature of modern risk assessment methods compared to the older techniques.
[1] Not to mention “SME” or “scenario” risk assessment techniques—whatever those are! See “Updating Guidance Documents for Pipeline Threat Identification and Risk Assessment”, Oct 2016.
[2] Within detection limits
[3] The ILI also provides some indirect evidence regarding exposure and mitigation. While valuable to the big picture, let’s disregard that for now.
[4] Or among many components in a facility such as tank farm, compressor station, underground storage facility, etc.
[5] “Simple”, conceptually, sometimes challenging in practice.