Page 155 - Offshore Electrical Engineering Manual

142 CHAPTER 13 Subsea Supplies and Cathodic Protection

GALVANIC ANODE SYSTEMS

In the galvanic or sacrificial anode system, the driving voltage between the structure
to be protected and the anodes is developed directly by the electrolytic potential
between the two (different) metals involved. The anodes are usually of aluminium
alloy or magnesium, types which are less affected by insulation from oil wetting such
as ‘Galvanum III’ being often preferred. Galvanic anodes may be placed very close
to, or in contact with, the structure to be protected. Galvanic anode systems should
be designed for a life of 25 years. Galvanum III or similar anodes are suitable for use
on both subsea pipelines and offshore structures and is particularly suited for use in
saline muds or silted-over conditions. On subsea pipelines it is used in the form of
‘bracelets’. On subsea structures, bar type anodes are mounted at suitable intervals.
Although used extensively on ships, zinc and magnesium anodes are not suit-
able for offshore use mainly due to their faster rate of consumption and hence costs
involved in replacement. Supplementary magnesium anodes may be used in order
to provide a temporary boost in polarising current, however. As cathodic protection
starts to operate, a layer of alkaline material is formed on the protected structure
by cathodic electrode reaction. Provided this material is not dislodged, its presence
reduces the current density required to maintain protection. On offshore structures
the polarisation-boost anode may take the form of a 10 mm × 20 mm ribbon of mag-
nesium attached to the structure supplementing the permanent system.

CATHODIC PROTECTION CALCULATIONS
This section is for guidance only and results should be heavily weighted by previous
experience and measured data from previous structures and tests carried out for the
new structure to be protected.
Impressed current circuits basically consist of a dc power source driving a current
through the anode, the electrolyte (i.e., seawater) and back through the structure. The
empirical rule used for calculating the current is:
(
Surface Area m 2 ) × mA/m 2
Total Current (Amps) =
1000
The surface area used is that of all metal surfaces submerged in the electrolyte at
mean high water.
Having determined the total current, the component resistances

Cable resistance (both poles) R C
Anode to electrolyte resistance R A
Cathode (structure) to electrolyte resistance R E
Linear resistance of the structure R S

are adjusted in order to match the total dc output of the available power source. The
linear resistance of an offshore platform may be neglected in most cases, but where