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Floating Roof (Double Deck)

Floating Roof (Double Deck)

Floating Roof (Double Deck)

Dear Tanks experts,

Can you see any anomalies in the attached photos for a double-deck-type floating roof, its supports and one of its compartments?

Be aware that, we have inspected the roof leg pads, sleeves and deck compartment weldings for pitting or cutting by MPI and they were acceptable.

Also, in the early morning and when the weather is cold, a clear space can be seen between some peripheral legs near the cleaning door and their supporting pads.

Is the roof balance is accepted in this case?

Thank you for your participation.


RE: Floating Roof (Double Deck)

IFRS might have a better view but there aren't many legs by the look of it. Is this a very lightweight roof?

The roof is supposed to float on liquid so you will get some bending which could lift off a few legs.

I can't see anything that wrong from the limited data we have.

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

RE: Floating Roof (Double Deck)

Your observations are very good and those areas should be looked at a bit nmore. The uneven support of the deck should be reviewed and fixed if possible.

The welds in the stained areas deserve further examination, perhaps VB at 8-10 psig and/or PT. Do the stains contain product residue? Are they near legs? Sometimes cracks close up when the deck is flexed from landing but they open when the deck is flexed from floating. If the product has an ethanol content you might be seeing the start of some ethanol induced stress corrosion cracking. Does the product side of those seams show anything unusual like excessive buildup of rust in the lap or a leg carrying an unusually high load? In the picture of the leg lifted off the floor, it is not clear if the lap behind the leg is within 12 inches of the leg but it does not appear to be welded as it should be if it is that close.

A steel double deck is a heavy structure for a floating roof, while it should be fairly flat there will be some sag in the deck plates and what the pictures show seems not too out of the ordinary. They also are somewhat flexible in long spans but can be very stiff locally. A drawing would show the leg size, quantity and placement; each often supports somewhere around 400 square feet.

It is unusual in my experience for small temperature changes to cause large deflections but I'm not overly surprised at what you are seeing. My guess is that nothing is terribly wrong but the deck is flexing in response to uneven support conditions and the temperature change just happens to be enough to trigger a movement where it can most easily occur. There will be one or more legs in that area that are carying a higher load, and may be subject to premature failure.

It is possible that this deck was either not built flat, or due to floor settlement is not being supported flat (or both). If this is the case then repeated flexing from leg supported to floating may cause more of the type of leak you are seeing. I'd also look to see if any microbial induced corrosion or ethanol stress corrosion is present.

The highly loaded legs may fail sooner than the others especially if they are not perfectly vertical, which changes each time the deck lands. The higher stress in the deck at leg(s) carrying higher loads may lead to leaks in those locations.

A review of the leg corrosion, loading and buckling strength may be instructive. An elevation survey of the deck and floor at the legs would tell you which ones are long or short and where the floor is not in the conical plane of an ideal bottom.

Before you put this deck back in service, I'd want to resolve the mystery of the stains, look at the legs and their connections where the legs are carrying more load than others and make the deck flat when landed.

RE: Floating Roof (Double Deck)

Additional to the post of MR IFR,

- First picture ( Roof Deformation and an inclined leg support ) ; Implies this leg is longer than supposed to be.. A new adjustement for all legs would fix the problem.

-Fifth picture (Strange stains inside one of the roof compartment )
Seems old SALT AIR stains .. Marine environment ?

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