Page 153 - Offshore Electrical Engineering Manual

140 CHAPTER 13 Subsea Supplies and Cathodic Protection

systems are normally only used where power supplies are unavailable, or for tem-
porary protection during construction, tow-out and commissioning. A galvanic
anode system may also be used to complement an impressed current system in order
to achieve rapid polarization and as part of an overall design to protect the struc-
ture. Although there are guides and codes of practice (such as the one listed in the
Bibliography) to assist the engineer, the design of a good system depends to a large
extent on past experience with similar structures and subsea environments.

TYPES OF SYSTEM
Impressed Current Systems
In this type of system, an external source of power, usually a transformer-rectifier
unit provides the driving voltage between the anode and the structure to be protected.
The negative terminal of the dc source is electrically connected to the structure and
the positive terminal, similarly, connected to the anode. The output voltage of the
dc source is adjustable so that the protection current may be varied either by a trim-
ming potentiometer or automatically through a control signal loop to provide the
required voltage at the reference half-cell. Transformer-rectifiers for this use are usu-
ally rated in the range of 20–500 A and from 30 to 120 V. Impressed current anodes,
unlike galvanic ones, must be well insulated from the protected structure if they are
to be mounted on it. Where it is found necessary to place impressed current anodes
very close to the protected structure, dielectric shielding between anode and structure
must be provided.
As it has a uniformly low electrical resistivity, sea water is an excellent medium
for the application of cathodic protection and facilitates an even current distribution
over the surface to be protected. Bare steel submerged in seawater is easily polarised
if an adequate current density is maintained.
Impressed current anodes may be of silicon cast iron, lead alloy or graphite. Lead
silver alloy or platinised anodes, although more expensive at the installation stage,
may be used to provide a longer life and hence maintenance cost savings. Impressed
current anode systems should be designed for a life of at least 10 years. High silicon
cast iron corrodes relatively slowly and in seawater, where chlorine is produced at the
anode surface, chromium may be added to further improve longevity. The resistance
of a single anode in free flowing seawater is less than that of the same anode in mud
or silt, and it may be necessary to install groups of anodes, sometimes mounted on a
wood or concrete framework in order to reduce silting up and maintain effective con-
tact with the seawater. Care must be taken to avoid incidental rubbing or scraping of
the active anode surface by suspension or stabilising rods or cables, since the effect
of this is to accelerate dissolution at that point. For obvious reasons, suspending the
anode by the conducting cable is not advised. The number of anodes is determined
by the required anode life, the allowable current density and the circuit resistance of
the system. Anode life will be affected by the operating current density and the total
2
current magnitude. In free flowing seawater, a maximum output of about 10 A/m
is normal practice, giving an anode consumption rate in the region of 0.4 kg/A year.