Crude Oil Floating Roof Storage Tank

OK, a bit more data required.

What happens whne it rains? Same thing?
Diameter of the tank?
How many roof drains?
Size? Type?
How long since last inspection?
Any drawings or diagrams?
Any photos?
Can you access these drain ends easily?
Sounds like there is debris / blockage, but I wouldn't recommend blowing stuff into them, more like jetting from the bottom end using a lance and a flexible hose. You need to think drain cleaning equipment here.

And / or along boroscope to find the blockage.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Do not ignore this. You have one or more blocked drains. Fix them before you really need them!
My thoughts:
Remove the bottom valve first, maybe it's just a bad valve. Up on the deck, are there sumps with check valves or shutoff valves? If so, make sure the sumps are clean and those valves are OK. Having eliminated both ends, contact the manufacturer of the articulating joints or the hose depending on your type. They should be able to help you with cleaning suggestions and pressure limits. Don't randomly inject high pressure anything until you know you won't damage the materials or parts. Creating leaks and then getting product in the drains is terrible.

Plesae let us know how you proceed and what the results are.

The internet country domain of OP is (LY) and i worked there about 40 yrs ago.. If the thread is for this country, IMO, the problem probably sand accumulation ( could be died seagulls clogging the roof inlets also ) at roof drain and tank bottom drain pipework rather than articulated joints.

This thread reminds to me, what is the job of maintenance team when their simple task is do observation and make maintenance periodically..

Dear Gents,

You are right concerning the probabilities.

First, one drain was having some accumulated sludge just beneath the roof sump and the other has a problem with its roof shut-off valve; and we succeeded to clean them and fix the valve.

The third one was clogged by a previous crude oil leak, so when the product heated we managed to clear it.

Thanks for sharing your experiences.

BR