There is a lack of information needed for designing a cathodic protection system here.  You need to know are there any compartments inside the storage tanks as well as what is the actual wetted surface area, design life and the kind of anode that you require and etc.  Some examples are magnesium ribbon anodes, bolt-on type anodes as well as ICCP system.

Thanks dlms;

I am really designing the internal and external corrosion protection of the tanks. But I am not clear about the arrangement of the anodes into the tank, for the oil tank and salt water tank. The anodes for each case must be different?

Service temperature, size, access for water tanks.  Please scroll for previous threads on the subject matter.

In General:

Internal Protection:

1.  Zinc/Al (sacraficial anodes)
2.  Impressed Current for water tanks if access is feasiable.
3.  Internal Coating system
  3.1  Fibre-Glass (Floating Roof Tanks up to water phase)
  3.2  Coal Tar/Phenolic Epoxy (In addition to FB for fixed roofs
                             to cover the entire tank internals)

External Protection:

4.  Impressed Current Grid System
    ---> Inclusive of half/cells to measure potential
         either fixed or portable (perforated PVC tubes on bottom
         underside) .


Considerations:

Size, corrosivity, design life, survey intervals, etc...

Cheers