Hire a reputable inspection firm that has a 'stable' of API-653 certified Inspectors, with RBI - Risk Based Inspection experience.. If you can have the design drawings ready, or send them out with your Request for Quote, you should get all your answers for 'free', after they look over your tank and give you a quote/estimate.

Plan on having to get an Internal Inspx on at least one of your tanks; if they are actually identical [or nearly so] and have been in the same service, it may be possible to use Risk-Based-Inspx methods to avoid emptying and cleaning both tanks.

Thank you Duwe6.
In fact, these tanks are identical, and of my knowledge, no inspection was done over the 25 years internally. Now it becomes the concern of all technicals and management to prepare inspection considering RBI.

I was involved on the periphery of an inspection/repair of an ethane storage tank that operated at a few psig, sounds similar to what you have here. We used CBI to develop the shutdown, warmup and purging procedures as well as the purging, cooldown and recommissioning procedures and they also did the repairs required by the tank (we had external corrosion of the base CS floor resulting in ethane gas leaks from the annular space between the two tanks into the foundation sand support layer.

I was pretty impressed with the people we worked with at CBI, they knew their stuff.

"...CBI, they knew their stuff."

I know of no firm that exceeds the institutional knowledge, and abilities of Chicago Bridge & Iron. Period.