Any reason for the above ground storage? Why not blend on demand to the user? Online measurement instrumentation exists for degAPI or viscosity. Density transducers such as manufactured by SolartronMobrey or Peek (two of the industry standard instruments) provide degrees API online and in real time. Other technologies such as some "coriolis" density meters (mass meters used as densitometers) may also provide the necessary calculations. Jiskoot (www.jiskoot.com) provide jet mixers which will provide effective homogenisation, and blenders as a package to provide the complete blending solution. Storage may provide problems; if the crudes are incompatible then they will tend to separate so you will need to mix in the tank. On the other hand, if you inline blend then the product delivered is homogenised by the blender and the product is no different as the user receives it.
The other main reason for storage tanks is for test and correction. In the belnding of heavy fuel oils, even using process capillary viscometers, the fuels are blended into storage where top, middle and bottom samples are taken for lab analysis and for corrective action (adding further quantities of one of the components). This is usually because of the problem of obtaining accurae on-line measurement but new viscosity technology now overcomes this and allows inline blending without the need for the intermediate storage tank... fuel is blended straight into the loading facility. Measurement is not a problem for degrees API as the technology has existed for 40 years or more (magic figure, 40!)
In the oil fields it is necessary to have strict controls on the allowable density and viscosity of the crudes at the pipe line temperature. Where heavry crudes are involved it is not unusal to blend with distillate using on-line density and viscosity measurements.
Sorry, i can't help with the other questions.
If you would like more help with online measurements, post again.

jmw:

Many thanks for your aid, i wasn't think in line blend because the client dont want that.
He need a AST! why? i'm not completelly sure.

Any ideas about where i can found the answer to the other questions?

Many thxs again!

The best i can suggest is to try the ASTM and API web pages for design standards.