After 24 years, my guess is that your roof drain line is leaking / broken or blocked and hence entering the tank through roof vents or the external seals.

Roof tilt could happen, but unless there is a leak in the pontoons seems unlikely.

General inspection of seals, roof, floor, roof drain elbows and piping.

Find an experienced contractor....

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

[quote.... This was also confirmed as little water was found coming out of tank drain line. The roof water drain line also runs inside of tank to opening at bottom of tank.][/quote]

Apparently , there is no leakage of product thru the roof drain piping .
In this case im thinking two probability ;

- The roof inlet sump is leaking or ,

- The drain line is clogged and the rain water entered from emergency roof drains.


..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

We have done some visual inspection from top platform of tank (as ladder steps degree is not suitable) to access on roof is not possible without any crane& man box.

From visual, we have found small holes due to corrosion on the top roof and seems water can seep from top roof. (In this case pontoons should also be leaking).

Regarding Drain line clogging and shear from any swivel joint, we have not witnessed any hydrocarbon coming out from roof drain and there is small quantity of water was found on roof.

Roof to tank wall seals seems to be the least likelihood as there is no HC vapors are coming out from space.

Rain water ingress from shear roof drain line is possible but in that case diesel should also come out which we have not witnessed.

My assumption is the leak from double deck roof but in this case, diesel should be present/found in pontoons.

My though process is following;

1. Inspection of pontoons and if HC/diesel found filled inside of pontoons then leak is from double deck roof.

2. If pontoons will be found empty, then leak will be Definity from drain line and in that case Darin line is sheared off and sitting on the tank floor (as drain line is 250 mm above from tank floor)

Depends where your pontoons are.

You seem to have a lot of water in your tank, Do you not drain it off?

It could be a mixture of things so don't leap to one conclusion.

If the tank hasn't been entered for 24 years there could be a significant amount of damage.

When there is no rain or water on the roof are you still getting water from the drain line?

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

Why don't you suspect the diesel?

Check the upstream units that send diesel of any probable mal operation that has been concealed.

The log books could give you a clue.

DHURJATI SEN
Kolkata, India

The DIP measurement only gives the outcome of increase in water level while the diesel level maintained the same.