Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 412

The design values recommended are consistently selected using a conserva-
tive approach; therefore, the values are expected to provide a service life that
exceeds the design life of the CP system.
In the case of oil companies that have a fleet of fixed offshore structures,
they may specify less, or (in certain cases) more, conservative design data,
based on their own experience or other special considerations. Furthermore,
the contractor, in addition to the owner, may propose use of alternative design
data; however, any such data should then be accepted by the owner, preferably
before the CP design work has started.
All electrochemical potentials associated with CP will be refered to the Ag/
AgCl/seawater reference electrode. The potential of this reference electrode is vir-
tually equivalent to that of the standard calomel electrode (SCE).
One change to note in the DNV RP 2004 revision is that the number of
depth zones for design current densities has been extended from two to
four, while the number of coating categories has been reduced from four to
three.
Design Lifetime
The owner usually will specify the required lifespan for the CP system. The
design life should take into account any period of time when the CP system
will be active prior to operation of the platform.
It is also very important to take into consideration, when defining the CP
lifespan, that the maintenance and repair of CP systems for fixed offshore struc-
tures are generally very costly and sometimes impractical. It is therefore normal
practice to apply at least the same anode design life as the design life for the
offshore structure platform. However, in certain circumstances, planned retrofit-
ting of sacrificial anodes may be an economically viable alternative to the initial
installation of very large anodes. This alternative should then be planned so that
necessary provisions for retrofitting are made during the initial design and
fabrication.
In general, the design life should be stated clearly on the basis of design and
approved by the owner clearly before the design of the CP system is begun.
Current Densities for Design
The current density, I c , is the cathodic protection current per unit surface area in
A/m 2 . There are initial and final design current densities, I ci (initial) and I cf
(final), respectively, that give a measure of the anticipated cathodic current den-
sity demand to achieve cathodic protection of a bare metal surface within a rea-
sonably short period of time. The main part of design is to calculate the required
initial and final currents, which can define the number and sizing of anodes.
During calculation, the effect of any coating on current demand should be con-
sidered by the application of the coating breakdown factor.