Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 407

anode resistance, R (ohm), a controlling factor for the current output from
an anode.

6.4.2 Design Criteria
The cathodic protection system for the substructure should be designed in
accordance with DNV RP B401-1993 . The design criteria should also contain
the conditions and assumptions that were taken into consideration.
Retrofit CP design may be limited to the platform structure, pile guides,
piles and, where applicable, any conductors or appurtenances, including risers.
When the focus is on the platform, the design does not include pipelines or
associated subsea structures that might be in close proximity and electrically
connected to the platforms, other than making an allowance for some debris
on the seabed.
The design life of the sacrificial anode cathodic protection (SACP) system
should be defined based on the the owner ' s requirements, which take into

consideration the lifetime of the platform.
Retrofit CP design uses conventional and modified CP criteria to maintain
external structure potentials within the range of 1050 mV for an

Ag/AgCl (seawater) reference electrode, throughout the remaining service life
of the platform.
A seawater resistivity of 20

− 800 to −

Ω /cm and an average seawater temperature of

22 C have been used in the design example discussed here.

In the case of platform jackets that were originally fitted with aluminum
flush-fit, long, slender anodes more than 30 years previously, the anodes will
have been either totally consumed or at end of their life. Because these anodes
have a theoretical utilization factor of 85%, the anode retrofit design will not
consider allowance for any remaining anode alloy.
In the case of retrofit, the designer should check the original coating system
applied to the splash zone. For the purposes of CP retrofit design, an initial coat-
ing breakdown factor of 2% can be used for the first year, and a factor of 1%
can be used for each year thereafter. This is slightly less than what DNV recom-
mends but in some offshore operations it is accepted in practice. Assuming plat-
forms originally installed 30 years earlier, the coating breakdown factors used
in calculations for 25 years are an initial coating breakdown failure (CBF) of
35%, mean CBF of 48% and final CBF of 60%.
Based on DNV RP B401 (2005) , the CP design document should recommend
the following current densities to be used when designing offshore structure CP
systems for wetted areas: 150 mA/m 2 ,70mA/m 2 , 100 mA/m 2 for initial, mean
and final, respectively. In buried areas, 20 mA/m 2 can be used for initial, mean
and final.
The above current densities are recommended for new uncoated steel struc-
tures in tropical waters (

º

C) with a depth of 30 m or less. In practice, the
above criteria are known to be conservative for retrofit designs when structures

> 20 º