Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 416

Maintenance of anode performance includes underwater inspection, as
presented in Chapter 8 , and should be documented for follow-up and monitor-
ing of the CP system.
Coating Breakdown Factors
The use of nonmetallic coatings is very important, as they reduce the CP current
demand for the protection of the structure, which will reduce anode weight cor-
respondingly. For weight-sensitive structures with a long design life, the com-
bination of a coating and CP is likely to give the most cost-effective corrosion
control. If a very long design life is required by the owner, CP may be imprac-
tical unless combined with coatings.
If the CP of bare metal surfaces is known or expected to be high from the
calculation, coating applications will be essential. In deep water, the formation
of calcareous deposits may be slow. Coating should also be considered for sur-
faces that are partly shielded from CP by geometric effects.
In the case of large and complex offshore structure platforms, such as a
multi-well production structure platform, the extensive use of coating is
essential to reduce the overall current demand and to ensure adequate current
distribution. To compensate for this, the design coating breakdown factors to
be used for CP design (see Table 6.12 ) are deliberately selected conserva-
tively to ensure that a sufficient total final current output capacity is installed.
Consequently, calculations of the electrolytic voltage-drop away from the
anodes using these coating breakdown factors and result in excessively
high cathodic protection in terms of the estimated protection potential. This
applies primarily to relatively long design lives when the calculated coating
breakdown, and hence current demands and electrolytic voltage-drop,
increase exponentially.
Use coatings is not a practical solution in the case of submerged parts of the
offshore structures that need frequent inspections for fatigue cracks based on the
underwater inspection plan, because this location is usually the critical welded
joints of the jacket structure.
Metallic coatings on zinc or aluminum are compatible with galvanic
anode CP. The organic coatings have no advantage in decreasing the current
demand for CP. In addition, it is advisable to avoid zinc-rich primers
because they are unsuitable for application with a CP system, due to their
susceptibility to low electrical resistivity, which will require a high CP cur-
rent demand.
The coating breakdown factor, f c , represents the anticipated reduction in ca-
thodic current density due to the application of an electrically insulating coating.
When f c is equal to 0, it means that the coating is 100% electrically insulating,
thus decreasing the cathodic current density to zero. If f c is equal to 1, the coating
has no current-reducing properties.