Hi.. I've double wall LPG storage tank (suspended deck at inner tank) sized 44m diameter which had never being internally inspected for 28 years. The inspection group had done the RBI and presented the case, which require the tank to immediately shutdown for internal inspection (of course involved hugh cost & long duration). From design, the tank was designed to API 620 App. R.There are school of practise mentioned that low temp double wall tank is internally free maintenance, not require internal inspection if the temperature maintained at all time subzero (no internal corrosion is expected). As for inspector, there's another risk i.e. crack due to cyclic stress from temperature variation from loading & unloading the product.
The inner tank material is A537 Cl.1 = low temp carbon steel
Design temp (inner) = -30 deg C
Your opinion and sharing is appreciated.