This year we have to perform our 5 year water storage tank inspection according to NFPA 25, so we have to perform an internal inspection, and we thought that is possible that we will have to make some maintenance in the coating and the bottom plates.
The thing is that we just have one Storage tank (340 m3) which supplies water to the fire network with one fire water pump (1000 GPM), so if we take the tank out of service, we won't have water, we estimate that just inspection will take 5-10 days, and maintenance work probably 10 days more, that is a total of 20 days with tank out of service.
We were researching alternatives such diving inspector, or ROV inspection, but it's limited just to visual inspection that not covers the full scope of required by NFPA 25, also we look for Acoustion Emission Testing, but it is just a qualitative method inspection. Finally we look for use a portable tank (like a bladder tank), the thing is that we are not sure if we make an hose arrangement to connect to our main fire pump suction, it would works fine. The suction connections are in the top of portable tank. I found that NFPA 25 Ed. 2014 in 15.5.2 (4) (c) considers the use of portable tanks, but it looks like that just between 10 to 24 hours, and I'm not sure that if the proposed arrange is ok.
I would like that you could tell me your experience how you manage those inspections, or how you would manage this if you were in this situation.