Above ground empty diesel storage tanks float or not in case tank bund flooded with water

[A_WAHAB]

Hi,
We have above ground diesel storage tanks and they have been decommissioned and empty.
If the manways of tanks are closed and secondary containment (Tank Bund) due to heavy rain or flood with water whether they can float.
Tanks have no anchor bolts they are having diameter of 34 meters and height of 13 meters and having floating roof design.
If tank base plate and foundation seal become damage and water level inside bund raised up whether it can generate enough bouncy force?
I have done some rough calculation, and bouncy force is coming higher as compared to downwards force due to weight
Is there any reference mentioned in API 650
I know by opening manways, water can cross inside tank and does not cause or cancel the bouncy force.
There are some anchoring requirement mentioned in NFPA 30 Aboveground Tank Installation Chapter 4 Tank Storage.
Any thoughts or exprienece

 

[MintJulep]

I have done some rough calculation, and bouncy force is coming higher as compared to downwards force due to weight

It seems that you've already answered the only question in your original post.

 

[johnengkol]

In NFPA 30 indeed there is requirement regarding above mentioned subject which flooding that leads to floating.

such as "Where a tank is located in an area subject to flooding, provisions shall be taken to prevent tanks, either full or empty, from floating during a rise in water level up to the established maximum flood stage"

You can find on another chapter about this provision, which recommend Anchor, Attached to foundation whether it is concrete or structural and the most powerful statement is Secured from floating by other means. But i don't think this is applicable for bis storage tanks (API 650)

I do believe if your tanks located in Gulf, there is a mitigation and action before hurricane you need to fill your tanks with water to prevent floats during flooding.
or if the flood come immediately without notice, I think some action are not to put anchor on your tanks, maybe prepare some pump to take water out of the bund area

 

[LittleInch]

If the tank is empty, why not just drain the bund all the time?

Maybe API 650 has a section but this is just common sense engineering.

 

[HTURKAK]

I have done some rough calculation, and bouncy force is coming higher as compared to downwards force due to weight
Is there any reference mentioned in API 650

- There are some references at API 650 two of them,

B.2.3 Some of the many conditions that require special engineering consideration are as follows:
f) sites where tanks may be exposed to flood waters, possibly resulting in uplift, displacement, or scour.


V.9.3.3 anchorage: For tanks that are mechanically anchored, the anchorage devices shall be adequate to resist
the uplift and shear forces resulting from the pressure due to external flood liquid. If the tank is not mechanically
anchored, provisions should be made to guide the tank back into its original position when the flooding conditions
recede.

I know by opening manways, water can cross inside tank and does not cause or cancel the bouncy force.

This is not true .. if the tank can resist to the uplift developing at a height of bottom of opennings , the water will enter and cancel the bouyancy .

Just for curious , if tanks decommissioned , what is the reason to keep the bund?
If floating is a risk, i would prefer to provide some openings at the bottom level of the shell .

 

[JStephen]

I think most tanks would be floating before liquid level reached the opening of a conventional round shell manway.
You can anchor tanks to prevent floating, and the amount of hassle involved there depends on the depth involved.
Also consider, that while the tank itself may survive the event just fine, the same may not be true of any liners, salt-free sand, cathodic, etc.