Nace CP 1, Course Manual

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RP0176-2003
36
NACE International
Table A1: Design Criteria For Cathodic Protection Systems
Environmental
Factors
(A)
Typical
Design
Slope
Typical Design Current
Density
(C)
mA/m
2
(mA/ft
2
)
Production Area Water
Resistivity
(B)
(ohm-cm)
Water
Temp. (°C)
Turbulence
Factor
(Wave
Action)
Lateral
Water
Flow
Initial
(E)
Mean
(F)
Final
(G)
ohm-m
2
(ohm-ft
2
)
Gulf of Mexico 20 22 Moderate Moderate 110 (10) 55 (5) 75 (7) 4.1 (44)
U.S. West Coast 24 15 Moderate Moderate 150 (14) 90 (8) 100 (9) 3.0 (32)
Cook Inlet 50 2 Low High 430 (40) 380 (35) 380 (35) 1.0 (11)
Northern North Sea
(D)
26 to 33 0 to 12 High Moderate 180 (17) 90 (8) 120 (11) 2.5 (27)
Southern North Sea
(D)
26 to 33 0 to 12 High Moderate 150 (14) 90 (8) 100 (9) 3.0 (32)
Arabian Gulf 15 30 Moderate Low 130 (12) 65 (6) 90 (8) 3.5 (37)
Australia 23 to 30 12 to 18 High Moderate 130 (12) 90 (8) 90 (8) 3.5 (37)
Brazil 20 15 to 20 Moderate High 180 (17) 65 (6) 90 (8) 2.5 (27)
West Africa 20 to 30 5 to 21 Low Low 130 (12) 65 (6) 90 (8) 3.5 (37)
Indonesia 19 24 Moderate Moderate 110 (10) 55 (5) 75 (7) 4.1 (44)
South China Sea 18 30 Low Low 100 (9) 35 (3) 35 (3)
Resistivities (ohm-cm)
Temperature (°C [°F])
Chlorinity (ppt) 0 (32) 5 (41) 10 (50) 15 (59) 20 (68) 25 (77)
19 35.1 30.4 26.7 23.7 21.3 19.2
20 33.5 29.0 25.5 22.7 20.3 18.3
___________________________
(A)
Typical values and ratings based on average conditions, remote from river discharge.
(B)
Water resistivities are a function of both chlorinity and temperature. In the Corrosion Handbook
37
by H.H. Uhlig the following resistivities are
given for chlorinities of 19 and 20 parts per thousand (ppt).
(C)
In ordinary seawater, a current density less than the design value suffices to hold the structure at protective potential once polarization has been
accomplished and calcareous coatings are built up by the design current density. CAUTION: Depolarization can result from storm action.
(D)
Conditions in the North Sea can vary greatly from the northern to the southern area, from winter to summer, and during storm periods.
(E)
Initial current densities are calculated using Ohm’s Law and a resistance equation such as Dwight’s or Crennell’s (McCoy’s) equation with the
original dimensions of the anode. An example of this calculation is given in Appendix D, which uses an assumed cathode potential of -0.80 V
(Ag/AgCl [sw]).
(F)
Mean current densities are used to calculate the total weight of anodes required to maintain the protective current to the structure over the
design life. Examples of these calculations are given in Appendixes D and E.
(G)
Final current densities are calculated in a manner similar to the initial current density, except that the depleted anode dimensions are used. An
example of this calculation is given in Appendix D.