Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 394

TABLE 6.3 Comparison of Impressed Current CP and Sacrificial Anode

CP Systems — cont ' d

Impressed Current CP Systems Sacrificial Anode CP Systems

Installation Requires a high level of detailed

design and installation expertise.

Straightforward. Often bulky and
large numbers involved.

Hazards Diver risk from electrical shock;

system needs to be switched off
when diver is near anodes.

Magnesium anodes can be used in
potable tanks but never in areas
containing hydrocarbons. Alumi-
num and zinc anodes must never
be used in potable water tanks.

Cost Generally, the initial cost is high

but service cost is normally low.

Initial cost depends on design life
but is relatively low; periodic
renewals are necessary and cumu-
lative costs are high.

It is worth mentioning that the impressed current systems may also be cost-
competitive for traditional fixed offshore jacket structures with simple geome-
try, located in relatively shallow water depth. In this case the reference
electrode and anode cables will be installed in substantial conduits and routed
along the outside of the structural tubular members. But impressed current sys-
tems are less likely to be cost-competitive on large jackets of complex geome-
try located in harsh environments. In case of complex node geometries in the
structure jacket, it will be difficult to allow large-capacity anodes to protect all
surfaces adequately, because of shielding effects. Moreover, in a harsh envir-
onment, it will be difficult and expensive to route anode and reference elec-
trode cables inside the structural tubular members in order to ensure their
mechanical safety.
As a general rule, increasing anode operating temperatures causes a decrease
in both anode ampere-hour capacity and driving potential; at temperatures
exceeding 50 C, zinc alloys experience intergranular corrosion and they should

not be used at low anode current densities, and the ampere-hour capacity of alu-
minum alloys tends to decrease significantly. In order to realize the performance
claimed by anode manufacturers and thus to ensure the successful operation of
the cathodic protection system, it is imperative that strict quality assurance and
quality control of the anode manufacturing process should be achieved and main-
tained throughout production. (Quality assurance, quality control and tests are
discussed below.) The requirements contained in DNV RPB, 401 (2005) are con-
sidered the minimum standards for offshore work, with supplementary require-
ments for specific project applications to be determined and specified by the
designer.

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