Civil Engineering Reference
In-Depth Information
The number of anodes is calculated once the total weight of anode material
has been determined for each area to be protected, by dividing the total anode
weight by the specified anode unit net weight, as in the following formula
Number of anodes
=
Total weight/anode net weight
The designer should ensure that the number of anodes for each area is cap-
able of providing sufficient current throughout the remaining design life of the
structure to achieve the target by maintaining polarization during the structure
s
lifetime. This can be achieved by calculating the initial, mean and final current
output of each anode type used and comparing the results with the platform cur-
rent demands based on surface areas and specified current densities.
The above equation is valid for anodes with a minimum stand-off distance
of 300 mm from the structure. A correction factor of 1.3 is applied for anodes
between 150 mm and 300 mm from the structure.
'
Anode Design Precautions
The contractor specifies in the CP design report the tentative dimensions and
net mass for anodes to be used.
The design report should include the design of anode-fastening devices for
anodes that will be affected by significant forces during installation and opera-
tion. There are special considerations for large anodes that will be installed on
structural members subject to fatigue loads during pile-driving operations.
Therefore, doubler or gusset plates may be used for large anodes.
To use the anode resistance formula in
Table 6.16
for stand-off type anodes,
the minimum distance from anode to protection object is 300 mm. However, for
distances down to 150 mm, the formula can still be used by multiplying the
anode resistance by a factor of 1.3.
Detailed anode design ensures that the utilization factor assumed during
calculations of required anode net mass is met. Hence, it ensures that the anode
inserts are still likely to support the remaining anode material when the anode
has been consumed to its design utilization factor. In most cases, anode cores of
stand-off anodes protrude through the end faces.
With the exception of stand-off anodes, a marine-grade paint coating (min
100
μ
m DFT) is specified for anode surfaces facing the protection object.
Anode Distribution on the Offshore Jacket
The calculated number of anodes, N, for a CP unit is distributed to provide a
uniform current distribution, taking into account the current demand of indivi-
dual members based on the different surface areas and coatings used. On fixed
offshore platforms, special areas to be considered when distributing anodes are
nodes, pile guides and conductors. The location of all individual anodes is
shown on drawings.
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