Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 426

in Equations (6.23) and (6.24) are the initial and final current output for the
individual anodes.
If anodes of different sizes and hence different anode current capacities, C a
(Ah), and current output, I a (A), are utilized for a CP unit, N

⋅ ⋅

C a and N I ai /

⋅

N{ts} {ts}I af will have to be calculated for each individual size and then

added for calculation of the total anode current capacity (C at ) and total anode
current output (I ati /I atf ).
If the above criteria cannot be fulfilled for the anode dimensions and net
mass initially selected, another anode size is selected and the calculations are
repeated until the criteria are fulfilled.
Optimizing the requirements in Equations (6.22), (6.23) and (6.24) is an itera-
tive process where a simple computer spreadsheet may be helpful. In general, if
Equation (6.22) is fulfilled, but not Equation (6.23) and/or (6.24) , a higher num-
ber of smaller anodes, or the same number of more elongated anodes, are to be
used. On the other hand, if N

⋅

I a in Equations (6.23) and (6.24) is much larger
than I ci and I cf , fewer and/or more compact anodes may be applied.
Unless a high initial current capacity is deliberately aimed for, as with pro-
tection objects consisting primarily of uncoated metal surfaces, the anodes to be
installed should have a similar anode current output (I a ) to net anode mass (m a )
ratio. It is worth mentioning that small anodes with high anode current output to
net mass ratio will be more rapidly consumed than large anodes with a higher
ratio, which could result in an insufficient total anode current capacity toward
the end of the design life.
For anodes with the same anode resistance and hence the same anode cur-
rent output, but a major difference in net anode mass due to different anode geo-
metry, the anode with the lowest net anode mass will be consumed first. The
anodes with the lowest anode resistance will be consumed first if anodes
with the same net anode mass but with major differences in anode resistance
and, therefore, anode current output are used.
From a practical point of view, the following factors are used:

Design protective potential (E oc )= 0.800 V Ag/AgCl/seawater

Design closed circuit potential (E oa )=

−

− 1.050 V Ag/AgCl/seawater

The anode current output is calculated for the initial and final life of the CP
system. In the latter case, anodes of 10% remaining mass are used.
The number of anodes computed for the structure

' s initial and final currents,

and the structure ' s current capacity, by zone, is as follows:

Anodes for initial current needs (N)=I c (initial)/I a (initial)
Anodes for final current needs (N)=I c (final)/I a (final)
Anodes for final current needs (N)=M/m[m= mass of anode material per anode]

The number of anodes provided within each zone is greater than the required
number as calculated by zone, and also greater than the minimum number of anodes
required to ensure adequate current capacity and proper current distribution.