Hello Ksenia,
Good discussion.
Many engineers that work in this field differentiate regulatory barrier compliance and objective risk - recognising they are not the same thing and that both need to be properly considered.
You could think about the use of DHSVs in that context.
Regulatory barrier compliance tends to focus on barrier counting and the condition of those barriers, in a layers of protection approach. The basis of the dual barrier approach is very simple – that if a barrier fails in a well that is capable of flowing somewhere you don't want it to flow to, there is another barrier in the system to prevent a leak.
However regulatory barrier compliance isn't very good with the severity / consequences side of the risk picture. It addresses the basic question of whether a barrier leaks, though with little consideration for what that really means. Approximation factors are added to this picture, such as degradation, verification status, acceptable leak rates, and MAASP and operating limits, though those variations are often not definitive and can be confusing.
On the other hand, risk can be more definitive if you can build confidence in how likely your well is to leak, and how severe that leak would be. Clearly there are challenges in gathering data to calculate failure probability and to do a meaningful severity assessment. However, when you have built that confidence you can be in a much better place for objective decision making.
It can also be helpful to think about well types for risk assessment. Key considerations for defining a well type can be architecture (which drives leak probability) and leak severity (determined by location, fluid properties and flow potential).
Additionally, if you can build into your analysis what you are doing with the well at a particular time, such as valves being open or closed, plugs set, containment broken and operational activity, that makes your risk model flexible to any scenario.
However, well integrity engineers cannot rely on quantitative assessment of risk without also considering barrier compliance. For one thing, as you pointed out in your original post, there is a regulatory imperative in barrier management. Secondly it would be foolhardy to have so much faith in the input data and assumptions built into a quantitative risk assessment to believe its conclusions absolutely.
This is why objective risk assessment and barrier compliance management are both needed. If you get your data or assumptions wrong and a barrier component fails unexpectedly it is vital to have an additional layer of protection to prevent a leak.
Whether a DHSV can be used as a barrier or not depends on many things, including the well type, the condition of the DHSV and other well components, and what you are doing with the well at the time. It can be a barrier in some circumstances and not in others. But if you can build understanding and confidence based on objective risk AND barrier compliance assessment you are in a much better place to make that decision.
Thanks,
Stuart
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Stuart Girling
GMVi
Aberdeen
stuart.girling@gmvi.co.uk------------------------------