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RE: Foundation for large tanks

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Rajendran,

For your 150 foot tank example, typically we check with the tank vendor
for the allowable maximum differential settlement criteria that they
recommend for the tank to remain in service (the tank comes out of
service if the floating roof hangs up on an out of plumb wall).

For the 150 foot tank example, we use center to edge differential
settlement numbers around 3.5 inches, and perimeter differential
settlement numbers of about 1/1000 along the ring arc length.

Robert Randig, P.E.
  
 

-----Original Message-----
From: Padmanabhan Rajendran [mailto:rakamaka(--nospam--at)yahoo.com] 
Sent: Monday, October 22, 2007 2:40 PM
To: seaint(--nospam--at)seaint.org
Subject: Foundation for large tanks

A few of the list members serve the oil/gas industry. Large storage
tanks, say 150' diameter, are commonly used to store products. Among
other things, the design of the tank foundations, depends on the level
of tank settlement.

Typically, three types of tank settlement are addressed. Total
settlement, differential settlement between the tank center and the
circumference and the differential settlement along the circumference.
Maximum settlement occurs at the center of the tank.

Problems can arise in the wall or bottom plate of the tank or at the
nozzle connections if the settlement is "large". In floating roof tanks,
the operation of the roof itself may be affected if the settlement issue
is not adequately addressed in the design stage. So, what is a "large"
settlement?

I have not come across any prescription for limiting any of the above
three in the API codes, which commonly govern tank design for the
gas/oil industry. Nevertheless, common sense suggests there must be
threshold values to each of the above parameter for settlements. 

What has been the experience of the list members? What performance
criteria should be included in a typical geotechnical specification
associated with developing a foundation design? 

Thanks.

Rajendran


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