if the tank cannot be taken off line, there is no way to inspect it. The -33°C temperature means that ice will interfere with any ultrasonic measurement. Also, you would have to effectively compromise the insulation to get to the vessel wall.
fertilizerseurope has some info on line. you should look up some of the online resources such as
i have been involved getting a 30,000 Ton tank ready for inspection (6-10" w.c. pressure). our tank was ~170 feet in diameter and 65 feet sidewall.
you need to figure something like 4-6 weeks. it is not an easy task to pump out and then vaporize the remaining liquid depending on your design. they you will need a LOT of nitrogen to purge out to a flare.
going inside the tank also has risks as oxygen has been identified as causing SCC. we found some inside our tank in the roof support structure.
commissioning is not an easy task either. you need to purge with nitrogen to get back down to a low enough O2 content to not have SCC and also to not have an explosive mixture of air/ammonia going to the refrigeration compressors. also, the fill rate is limited at first due to amount of ammonia that will flash to cool the thermal mass of the tank back down to operating temperature.
forcetechnolgy has some info for going inside the tank when the ammonia fumes are still present. the robotic style inspection methods were not really ready when i was involved back in the 80's and 90's. this is a huge task. good luck!
