You're not supplying a lot of information here....

If there is access safely onto the roof without falling through, then weld from the outside.

Inside you need to drain the tank, make it gas free and erect scaffolding inside. Even then hot work inside a tank is notoriously dangerous and you have the issue of fumes etc. What does your contractor think?

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

This tank is under maintenance. Tank internal scaffolding erected. Thanks for your guidance.

Tank needs to be safe for hot work whether roof patch plates are put inside or out.
But I would assume patch plates would normally be welded outside, due to welding position.

Check the condition of the trusses/ rafters. If those are also corroded, then patching could be dangerous.
Patching of roof plates is not a good engineering practice as you can replace those quite easily.

DHURJATI SEN
Kolkata, India

You don't say why the corrosion happened - can you share the results of your root-cause-analysis? Do you have a reliable corrosion rate?

It's not all that uncommon for high sulfur products to corrode the underside of carbon steel cone roof plate. From what I've seen, the structure is often OK, but certainly bears inspection from underneath with a high-reach. Then, if it is OK, remove and replace the cone roof plate.

This is not all that hard or particularly dangerous if the structure underneath is OK and the contractor has a good work plan. I'm not sure patching is profitable since it may just postpone the inevitable (failure that will occur just when you don't want it to) and leaves you with a bad roof with patches.

Always patch from the topside.

Depending on the root cause, you might consider looking at a high-quality floating roof to reduce emissions and extra shell and roof circulation vents to help remove the vapors.

Also, you might consider replacing with thicker roof plate, or plate that is factory coated with a weldable or zinc-rich primer.

Let us know what happens!!!

I think it is safe and effective to weld the tank from outside rather than inside.