Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 425

Anode Number Calculation
From the anode type selected, the number of anodes, N, anode dimensions and
anode net mass are defined to meet the requirements for 1) initial/final current
output, I ci /I cf (A) and 2) anode current capacity C a (Ah), which relate to the CP
current demand, I c (A), of the protection object.
The preliminary sizing of anodes should be based on commercially available
products, which requires liaison with potential anode vendors.
The individual anode current output, I a (A), required to meet the current
demand, I c (A), is calculated from Ohm ' s law:

N ð E co − E ao Þ

N Δ E o

R

I c = NI a = = (6.20)

R

where I a (A) is the individual anode current output, E ao (V) is the design closed-
circuit potential of the anode material and R (ohm) is the anode resistance. The
initial and final current output, I ai and I af , are calculated using the initial and
final anode resistance, R ai and R af , respectively.
For calculation of anode resistance, E co (V) is the design protective poten-
tial, which is

− Δ

E o (V) is termed the design driving voltage.
Because the design driving voltage in Equation (6.20) is defined using the
design protective potential for C-steel, the initial/final design current densities
that define the anode current output capacity, and hence the driving voltage,
refer to the required anode current output at this potential. Hence, the initial/
final design current densities given in Table 6.11 are based on a protection
potential of

0.80 V.

0.80 V. This means that they are always used for calculations
according to Equation (6.20) in combination with this potential, as well as if
a more negative protection potential (e.g.,

−

0.90 V) is aimed for.
The individual anode current capacity, C a (A

≤−

⋅

h), is given by:

C a = m a ε u (6.21)

where m a (kg) is the net mass per anode. The total current capacity for a CP unit
with N anodes thus becomes N

⋅ ⋅

h).
Calculations are carried out to demonstrate that the following requirements
are met:

C a (A

⋅ ⋅

C at = NC a ≥ 8760 I cm t f (6.22)

I ati = NI ai ≥ I ci (6.23)

I atf = NI af ≥ I cf (6.24)

C at in Equation (6.22) is the total anode current capacity. I cm ,I ci and I cf
in Equations (6.22), (6.23) and (6.24) are the current demands of a CP unit,
including any current drain. 8760 is the number of hours per year. I ai and I af