Nace CP 1, Course Manual

Page: 357 / 535

RP0176-2003
NACE International
35
current densities are required to achieve these results, but
current demand decreases (by as much as a factor of 10)
quickly as the protective calcareous deposit is formed. At
the launch of a structure the potential difference between
the sacrificial anode system and the unpolarized steel may
be around 0.45 V, which enables the sacrificial system to
generate a very high current density on the cathode. After
polarization, this potential difference decreases to 0.25 V or
less with an accompanying decrease in current density.
This polarization process normally results in a linear rela-
tionship between steel potential and current density, the
slope of the line being equal to the overall circuit resistance
of the CP system, or the design slope. For any given loca-
tion, galvanic anode designs with similar design slope val-
ues result in similar long-term current densities. The rela-
tionship between design slope and maintenance current
density can be established by on-site experiments or by re-
view of experience at that location.
Current density is strongly dependent on water temperature.
For deep-water structures different design values should be
used for different temperature zones. To optimize the de-
sign the structure should be spilt up into separate zones
over which the temperature does not vary by more than 5
°
C
(9°F). The depth average temperature of each interval
should be used to assess the required current densities.
36
The same approach can apply to resistivity assessment.
Table A1 presents a general guide to the design of CP sys-
tems in eleven major offshore petroleum producing areas.
These data may be used as a starting point for investigation
prior to selection of final design parameters for a specific
application. Table A1 also presents equivalent design val-
ues for the same producing areas.
Figure A1 represents the temperature curves based on cur-
rent density.