1. Evaluating Non-Leak Threats
Focusing on the Likelihood of Failure
Andy Benedict - Opvantek, Inc.

2. Agenda The Classic Risk Equation Focus on the 8 DIMP Threats
Analyzing High Probability Threats
Analyzing Low Probability Threats
Some Specific Examples
Using Cathodic Protection Information
Selective Seam Corrosion Seam Failure

3. The Classic Risk Equation
Risk = Likelihood of Failure X Consequence of Failure
Focusing on Likelihood, the Goal of Most Risk Assessment Models is to Develop an Accurate Probabilistic Failure Model
Some threat types, e.g. Corrosion, have sufficient likelihood of failure (i.e. Leaks) that enable development of robust and accurate probabilistic functions.
Other threat types, e.g. Incorrect Operations have extremely low likelihood (although have high consequence), that other methods need to be used. These methods naturally introduce variability and uncertainty, but remember that uncertainty is analogous to higher risk.

4. The 8 DIMP Threats – Hi vs. Low Failure Rates
High Failure Rate Threats
Corrosion (Corrosion Leaks)
Natural Forces (CI Breaks)
Excavation Damage
Equipment Failure (Mechanical Couplings, CI Joint Seals)
Low Failure Rate Threats
Other Outside Force Damage (Vandalism/Vehicular/Latent Damage)
Material or Welds (Manuf. Defects)
Incorrect Operations (Procedures)
Other

5. High Failure Rate Threat Analysis
Select homogeneous groups of assets that are affected by a threat. E.g. 2” dia welded Bare Steel installed between 1955 and 1971
Total segments in this group : 1000
Use Leaks where Corrosion is the cause
An ‘a priori’ probability algorithm provides a good means of estimating the number of leaks likely to occur during the next year, given the number of leaks that currently exist on that segment.
Count Corrosion Leaks that occur each year for each segment, given the number of prior leaks on the segment.
For each number of prior leaks, track the number of opportunities to fail against the occurrences. This provides a simple failure rate ratio : Occurrences/Opportunities.

6. High Failure Rate Analysis (example)

7. Low Failure Rate Threat Analysis
Consider Bayesian Causal Networks
Example, Likelihood of Traffic Jam given given bad weather
Each Node is a random variable
Arrow denotes a Dependence relationship between variables
Each node can assume a True/False state
Probabilities can be assigned each to state
Considers interactive threats

8. Low Failure Rate Threat Analysis
Consider Fault Tree and Event Tree analysis with estimated probabilities at each branch

9. Cathodic Protection Impact on Failure
Problem: On CP Systems, there may not be enough Corrosion Leaks to develop an a priori probability function. How can we use CP information to develop a failure likelihood?
Solution:
Link the DIMP/TIMP Risk Assessment Model to the CP System.
Develop Fault Tree Parameters based on CP measurements, primarily Pipe-to-soil readings.

10. Cathodic Protection Impact on Failure
Recommended Fault Tree Parameters
CP_Section Number of Times Down. These are the number of excursions greater than -850mv, and back under protection. If the CP_Section is currently down, it counts. Note that 100mv shift criteria may also be used.
CP_Section Total Days Down. Total number of days where the last Pipe-to-soil reading was greater than -850mv (or less than 100mv shift)
CP_Section Down Severity. This is weighted average pipe-to-soil read times the number of days down. Acts like an ‘integral’ to approximate pipe wall loss.

11. Selective Seam Weld Corrosion Seam Failure
Problem: Gas Operators who have Electric Resistance Welded (ERW) pipe that was installed prior to 1980 have the potential threat of SSWC, that can result in catastrophic seam failure. This is due to manufacturing defects such as low frequency current; improper heat treating, sulfur inclusion, etc. (Kiefner Studies)
Solution:
Develop an asset installation database of installation years to identify the likelihood that specific pipe installations were ERW prior to 1980
Develop Fault Tree Parameters based on the identified failure factors for ERW pipe

12. Selective Seam Weld Corrosion Seam Failure
Recommended Fault Tree Parameters
Probability ERW. This is the probability that any pipe segment is ERW pipe
Probability Inclusion. This is based on three primary year bands of manufacture.
ERW-CP Impact. A true/false factor based on effectiveness of Cathodic Protection (see prior section). This takes into account the interactive threats of Manufacturing Defects combined with Corrosion.

13. Selective Seam Weld Corrosion Seam Failure
Recommended Fault Tree Parameters
Weighted Stress. This is based on the pipe stress formula: Stress = OperatingPressure * PipeRadius/WallThickness and is based on the MOP for the particular pipe segment. Once we have taken into account the diminished mechanical properties of the defective ERW pipe; the presence of corrosion potentially at the seam; and the increased stress at the seam due to the stress and diminished wall thickness at the seam, this provides the final needed parameter to estimate failure.

14. Questions? Thank you! Andy Benedict, President Opvantek, Inc.