In Part I of this series, we covered what constitutes an interactive threat. Now that you have a handle on what an interactive threat is, how can you use this information to help you plan for them in your Integrity Management Plan (IMP)?
When identifying threats and their interactions, start by:
- Reviewing the industry failure database
- Reviewing industry papers and past experiences
- Consulting Subject Matter Experts (SME)
- Developing and implementing a risk assessment process to ensure evaluation of threat interaction
In a PHMSA-sponsored study, Eduardo Munoz and Michael J. Rosenfeld proposed a normalizing approach to quantifying the increased likelihood of failure due to threat interactions. "The likelihood of failure from each threat includes a portion that involves that threat alone and a portion that involves interaction with other threats. To evaluate the likelihood of failure from multiple threats, your risk model should appropriately account for both portions."
According to Munoz and Rosenfeld, the likelihood of failure (probability) from the two threats can be expressed as:
𝑃𝑇 = 𝑃1 + 𝑃2 + 𝑃𝑖
P1 = failure probability from threat 1 individual factors
P2 = failure probability from threat 2 individual factors
Pi = increased failure probability from threat 1 and threat 2 interactions
“Pi in this expression is evaluated by considering the increased conditional probability of failure from threat 2, given the interactive factors from threat 1."
For example, the increased likelihood of failure from external corrosion due to ineffective cathodic protection (P1) combined with existing coating damage from previous pipe damage (P2) will be higher than the likelihood of failure from external corrosion alone. Accordingly, that will increase the probability (Pi) in the above expression used to evaluate the probability of failure from both threats combined.