Just some of the issues you might have spotted with the process include:
- Risk-Based Assessment/Inspection on mature assets.
Though it is ‘standards driven’ and has been in use since the mid 90’s, the RBA / RBI process is really at its most useful on components that are all in the same or very similar condition. That is if they are new (<10 years old) or have been recently maintained to the “exact” same standard.
- Inspection Frequencies.
The time invested in integrity inspections is expensive, and often at a premium. Current methods of deciding where these hours might be best spent either fall-out of the statistical techniques discussed earlier, or are driven by an ‘Anomaly Reporting’ procedure. These anomalies are typically raised as ‘exceptional’ reports where the unacceptable condition of a component has been noted during routine inspection or on an ad-hoc basis (somebody noticed something untoward etc.).
- The RBI process relies upon statistical calculations; the number of likely failures over a given time, and the probability of spotting those potential failures when inspecting a given (small) percentage of the component. For example looking at 5% of the pipe may, according to some RBA/RBI tools, offer confidence that the remainder of the pipe (the other 95%) is in the same or similar condition.
Inspecting such a small percentage of substrates reduces the time and expense invested in the integrity inspection process and the overall integrity management of the facility, however it is obviously seen as a cost driven process. In fact, with very little further development and additional considerations, it would be seen as a true integrity driven methodology.
The ‘assumption’ of condition based on a ‘supposedly’ representative-sample is possibly the strongest reason that the RBA/RBI process cannot, on its own, demonstrate ‘Objective Evidence’ of corrosion management and control on mature process plant. The probability of failure from external corrosion of mature components is entirely dependent upon the substrate makeup and its lifetime micro-climate.
Beyond year 10 or so, there may be little or no representative-sample of corrosion condition available. Our (SCM) own quest to determine this is only just now becoming available through our degRATE® process. We believe this or a similar tool is essential to properly enable the ‘probability‘ aspect of the RBA / RBI process to achieve the expected and desired capability to predict corrosion condition into the future.
That is assuming a different 5% of pipe is considered, at each round of inspection, which is unlikely as where defects are discovered (wall thinning for example) then that same location will be a preferred inspection point in future, further reducing the actual unique substrate area inspected over time.
The nature of this statistical approach, cost effective in the early years though it may be, means that, even if a different 5% of a pipe undergoes inspection at each round, over the course of 5 years only 50% of that pipe would have been inspected (based on our example), between 25-50% would never have been seen.
Can you imagine working on a plant where as much as 50% of the highly critical pipework may not have been seen in 10 years or more? It seems most of us do this daily!
For young plant that may well be acceptable, the unseen components ‘may’ be in a similar condition to those inspected, but for mature assets that assumption can, and often does, spell ‘disaster’!
I was a member of an integrity failure Near-Miss examination committee for a major Oil Operator in Aberdeen several years ago. The near failure was on a gas pipe and the mode of the failure was a severe thinning of the pipe wall thickness to within 0.5mm of its minimum allowable wall thickness. On investigation, it was discovered that this pipe had indeed been inspected annually, over 5% of its length, however for 3 of the previous 4 annual inspections the same section of pipe had been examined. The remaining 90% had not been seen since it was originally installed. The failure was, of course, in the unobserved 90% of the pipe.
Thankfully this this technique is not very often seen these days, but that trait of human nature to select the easiest to reach, closest or most obvious inspection points can seem really tempting.
Cost plays an ever-increasing part in these decisions too. If an access needs to be built to reach part A, then why not consider part B instead? That idea is, of course, supported by the assumptions inculcated in the ‘representative sample’ statistical process. If one section of the pipe is representative today, then why not tomorrow (or next year) too? Engineers are human too.