That dependds on what your soil conditions are and what capacity pile you are driving.  If your total load is 100 kips, and your driving 20 kip piles, then you'd need 5 piles.  How you space them around the perimeter is dependant on how many piles you need.  If your tank can span to the perimeter, design a ring grade beam and support it on piles.  If not, design a mat slab with evenly spaced piles.

Lets deal with this without numbers

40000 barrels? That seem to be a lot and heavy. API650 is not going to tell you how to design your foundation. It should be just like any other foundation. Just define your design criteria properly-pressure, displacement, base stiffness, mechanical and drainage requirements.

Most of your weight will attack the base slab. And perhaps that is where most of your piles will have to go.

However,if there is a concern for seismic induced peripheral pressures then I guess yes, you may elect to place some piles close to the periphery.

Another thing I know is that the slabs of these tanks are usually elevated some 2-3ft above ground,because you usually have mechanical requirements including pumping and piping, hence their seems to be no choice other than utilizing a ring foundation. My guess is you will have to provide a few piles to support the ring, and a couple to support your slab.

Your slab could be reinforced with a grid of beams for stiffness and pile capping,DEPENDING ON YOUR DESIGN CRITERIA.

Good luck
IJR

My company designes a lot of tanks for the petroleum
industry. I have not been directly involved, but have a number
of colleagues who are.

40,000 barrel is about 6000 m^3, which, incidentally, is at
the smaller end of tank sizes! (IJR). The largest tanks are in
the >70000 m^3 range. (80 m dia, 20 m high).

They normally do not have a slab foundation, and are not
necessarily elevated. (It would be cheaper to put the pumps in
a pit!). My colleagues have designed 'stone columns' to
consolidate and improve the soil strength. Ring beams are
normally provided.

You should consult someone who has actually designed
foundations for tanks (Geotech. person working in a company
designing refineries. The answers you have got, including mine,
are not conclusive, and authoritative! There is a lot at stake, and
experience is vital. Forum replies are not adequate.

Hariharan

I agree with Hariharan in the general aspects, including consulting tank foundation design experts

But Hariharan, why no base slab? I have a tank farm drawing with me, with a base slab, for a fuel tank built in Sri-Lanka and it is one from as-built drawings. The tank was detailed as per API650.

Do you want to let us know why there would be no base slab?

regs
IJR

I also agree with Hariharan.  Our company has been involved in several bulk fuel tank designs in Alaska and some in Japan.  Most all have ringwalls.  The Japan tanks were 100,000 barrels.  They were placed over poor soils, where Sand Compaction Piles and preloading are being used to improve the soils.  Standard driven piles to a bearing layer were considered, but would have been cost prohibitive, and they would lose lateral support due to soil liquefaction in an earthquake.

I've not seen a base slab used. Maybe it's that our designs have been in seismically active zones. Also,a membrane is installed below the tank floor for secondary containment.  And a slab probably interferes with impressed current cathodic protection of the floor. Sand/gravel fill is used to support the tank, with a sand layer just below the floor.