LI,
I'm surprised you don't know this - I thought your background was in O&G in the USA.
PSM is USA's OSHA mandated program for "Process Safety Management of Highly Hazardous Chemicals" -29 CFR 1910.119.
Any site with above threshold quantities of toxic materials or >20,000 lbs of flammables qualifies as a PSM-covered facility, so it is a very common regulatory requirement. I know there are (or were) some exemptions in the drilling wells, but thought most O&G was covered. Perhaps I am mistaken.
MP08 - I do not know of any PSM standard that specifically address decommissioning or mothballing. You have to satisfy an auditor that you meet the requirement that the limits of the covered process are defined. Without an air gap, I am not sure if an auditor would necessarily say you were not connected to the process. One reason for doubt would be corrosion and eventual pinhole leaks in the blind causing the equipment to become "live" again. As far as I know there is no "hard" standard - it's up to the process owner to define. Hopefully the government agrees with you if that is the route you take.