Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 414

for harsh weather conditions, including waves and sea currents, but not ero-
sive effects on calcareous layers by silt or ice. They further assume that the
seawater at the surface is saturated with air (i.e., at 0.2 bar oxygen partial
pressure).
Based on DNV B401, the data in Table 6.11 reflect the expected influence
of seawater temperature and depth on the properties of a calcareous scale
formed by cathodic protection and of the dissolved oxygen content. The prop-
erties of such layers are dependent on the seawater ambient temperature and,
moreover, on certain depth-dependent parameters other than temperature.
Note that oxygen is dissolved in the surface layer by dissolution from air
and photosynthesis, so that the oxygen content at a great depth in a tropical
region is likely to be substantially lower than in temperate or arctic surface
waters of the same seawater temperature. Higher design current densities
usually apply in the uppermost zone due to the wave forces and marine
growth on degradation of calcareous scales and convective mass transfer of
oxygen. Therefore, the design must account for the fact that, in certain
areas, decomposition of organic material may reduce and ultimately consume
all oxygen in the seawater. No such reduction in oxygen content is accounted
for in Table 6.11 .
In the case of bare steel surfaces buried in sediments, a design current den-
sity (initial/final and average) of 0.020 A/m 2 is recommended irrespective of

geographical location and depth.
Bacterial activitymay be the primary factor determining the CP current demand
in the uppermost layer of seabed sediments. Further down into sediments, the
current will be related to hydrogen evolution.
The design also has to take into consideration the effect of heat transfer due
to convection in increasing the oxygen; this is usually done by an additional
margin of CP current density.
The design current densities in Table 6.11 also apply for surfaces of any
stainless steel or non-ferrous components of a CP system, including compo-
nents in C-steel or low-alloy steel. For calculation of anode current output, a
protective potential of 0.80 V then also applies to these materials.

Based on DNV, for aluminum components, or those coated with either alu-
minum or zinc, a design current density of 0.010 A/m 2 is recommended for
initial and final currents, as well as mean values. For internally heated compo-
nents, the design current density should be increased by 0.0002 A/m 2 for each
°

−

C.
The main goal of the CP design system is that the current density available
to the structural steel surfaces will be sufficient, at any time during the design
life of the protected offshore structure, to achieve the required potential range
and maintain a calcareous deposit.

C that the metal/seawater is assumed to exceed 25 °