Page 154 - Offshore Electrical Engineering Manual

Cathodic Protection 141

Platinised anodes are generally constructed from solid or copper-cored rod about
12 mm in diameter and 1500 mm long. A layer of platinum approximately 0.005 mm
thick is deposited on this. A similar thickness of platinum may be used on anodes of
other shapes and other materials, such as platinised titanium, platinised niobium or
lead silver alloy. Graphite is not generally suitable for use offshore. For reasons of
economy, platinised titanium anodes should not be used where silt or mud may accu-
mulate, above the lowest tide level, or where fluctuating voltages, single (i.e., low
frequency) ac ripple or anode voltages higher than 7 V are present, since the expected
long life will not materialise in these conditions. The system design should be such
that these anodes are not expected to carry current densities of more than 700 A/
2
m . The 0.005-mm thick platinising dissolves at the rate of about 10 mg/A year, and
although the abrasive action of water-borne sand may reduce this to some degree, a
life expectancy of 15 years may be realistically hoped for.
The anodes are usually mounted on supports cantilevered out, but electrically
insulated from the protected structure. In shallow waters, seabed nonmetallic frames
may be used to support the anodes above the mud or silt level. To protect any offshore
platform, distributed multiple anode arrangements will be required to be mounted
over the members of the complex subsea structure to provide a uniform current den-
sity. The size and design of the cathodic protection scheme will depend on the cur-
rent density required to bring the structure up to the level of protection required. Sea
water velocity and oxygen content will affect chemical activity and hence the current
density.
Statistical data will be required from the platform site, if an accurate figure for
current density is to be determined. However, data may be available for similar envi-
ronments as a guide. Some figures are given below as an indication of the magnitude
required:

Location Environment Typical Current Required (mA/M )
2
Persian Gulf Seawater 20
Below mud level 50
North Sea Seawater 50
Below mud level 90
Mexican Gulf Seawater 30
Below mud level 60

It should be remembered that there will be little or no protection afforded by these
systems above the low tide level and additional forms of protection will be required
above this level.

Impressed Current Systems on Submerged Pipelines
Offshore submerged pipelines generally require a minimum pipe-to-water potential
of −0.9 (ON) volts with reference to a silver/silver chloride half-cell. This corre-
sponds to a current density of 2.5 mA/m .
2