Civil Engineering Reference
In-Depth Information
The coating breakdown factor is a function of coating properties, operational
parameters and time. As a simple engineering approach, f
c
can be expressed as:
f
c
=
⋅
a
+
b
t
(6.10)
where t is the coating lifetime in years, and a and b are constants that are
dependent on coating properties and the environment (see
Table 6.12
).
It is worth mentioning that the effect of marine growth is highest in the
upper 30 m, where wave forces may further contribute to coating degradation.
Another factor is periodic cleaning of marine growth in this zone.
Preferably, the owner will specify constants a and b for calculation of coat-
ing breakdown factors based on his own practical experience of specific coating
systems in a particular environment. When the owner has not specified any such
data, the default values in
Table 6.12
should be used.
Once a and b are defined, mean and final coating breakdown factors, f
cm
and f
cf
,
respectively, to be used for CP design purposes are to be calculated by introducing
the CP design life, t
f
(years):
f
cm
=
a
+
0
:
5bt
f
(6.11)
⋅
f
cf
=
a
+
b
t
f
(6.12)
For certain protection objects, with large uncoated surfaces, the initial coat-
ing breakdown factor, f
ci
= a, may be applied to calculate the initial current
demand to include coated surfaces.
If the calculated value exceeds 1, f
c
= 1 is applied in the design. When the
design life of the CP system exceeds the actual calculated life of the coating
system, f
cm
may be calculated as:
2
ð
1
−
a
Þ
f
cm
=
1
−
(6.13)
2bt
f
To account for the effect of a coating system on coating breakdown factors,
three coating categories have been defined for inclusion in
Table 6.12
.
The criteria for the three categories are:
Category 1: One layer of epoxy paint coating not less than 20
μ
mofdry
film thickness
Category 2: One or more layers of marine paint coating by epoxy, polyurethane
or vinyl base, with total nominal of the dry film thickness not less than 250
m
Category 3: Two or more layers of marine paint coating by epoxy, polyur-
ethane or vinyl base, with total nominal of the dry film thickness not less
than 350
μ
μ
m.
Note that category 1 includes the shop primer type of coating. It is assumed
for categories 2 and 3 that the supplier will provide specific coating materials to
be applied and will ensure its performance by documented qualification or by
performing the required tests. It is very important to know that the assumption
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