UT data taken during on-stream vs shutdown - the difference?

Good day all.. i'm from oil & gas company.
I've an issue to resolve with regard to ut data.
Is there any difference between ut data taken during on-stream vs shutdown?
For both equipment operated at ambient & above 65 deg celcius.. is there any paper / study done for this?

I am a retired engineer with 45 years of background in the petroleum industry, production, refining & petrochemical but I still have no idea what "UT" specifies. So you may have a better chance of getting responses if you define that term.