Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 391

TABLE 6.2 Comparison of Different CP Systems

Sacrificial Anode Impressed Current Hybrid

Advantages Simple, reliable and

free from in-service
operator
surveillance.
System installation
is simple.
Permanent potential
monitoring system
not essential.

Flexible under widely
varying operating
conditions.
Weight advantage for
large-capacity, long-life
systems.

Flexible under
widely varying
operating
conditions.
Weight advantage
for large-capacity,
long-life systems.

Disadvantages Large weight penalty

for large-capacity,
long-life system.
Response to varying
operating conditions
is limited.
Hydrodynamic
loadings can be
high.

Relative complexity of
system demands high
level of detailed design
expertise.
System installation is
complex, and it requires a
power source.
Perceived diver risk from
electric shock.
In-service operator
surveillance required.
Permanent potential
monitoring system
essential.
Vulnerable to loss of
power.
It is not recommended for
North Sea without full
sacrificial back-up (i.e.,
part of a hybrid system).

Relative complexity
of system demands
high level of detailed
design expertise.
System installation is
complex, and it
requires a power
source.
Perceived diver risk
from electric shock.
In-service operator
surveillance
required.
Permanent potential
monitoring system
essential.

The principal technical advantages and disadvantages of sacrificial,
impressed current and hybrid systems are summarized in Table 6.2 .
The use of the term impressed current system can be misleading, because for
most offshore applications, an impressed current system is used in combination
with a small number of sacrificial anodes, forming a hybrid system. Sacrificial
anodes are provided in hybrid systems to ensure that adequate polarization of
the critical nodes is maintained at all times, even if the power supply to the
impressed current anodes fails or is switched off temporarily (e.g., to permit
manual underwater inspection or cleaningofthestructurebydivers).Some
early impressed current systems were provided with inadequate sacrificial
anode back-up and significant corrosion damage was reported in times of
unplanned (and planned) impressed current shut-downs.