Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 422

TABLE 6.15 Recommended Anode Utilization Factors for CP Design

Calculations

Anode Type Anode Utilization Factor

Long, slender stand-off, L ≥ 4 r 0.90

Short, slender stand-off, L < 4 r 0.85

Long, flush-mounted
L

0.85

≥ anode width

L ≥ anode thickness

Short, flush-mounted, bracelet and other types 0.80

structural elements, such as piles and skirts, which do not need corrosion pro-
tection due to their increased wall thickness, which includes a corrosion allow-
ance to account for the expected corrosion rates.
Calculations of current drain use the design current densities and coating
breakdown factors for items requiring CP. Calculations of surface areas and cur-
rent demands are carried out.
The design current densities and coating breakdown factors are important for
calculation of current drains to structural elements that are not considered to need
CP, but will be, or are expected to be, electrically connected to the protection
object by the CP system being designed.
In case of buried steel structural surfaces, such as mud mats, skirts and piles,
a current drain value of 0.020 A/m 2 is accounted for, based on the outer external
surface area that is exposed to sediment. In some cases and based on the own-
er s specification for parts of steel skirts and piles to be buried in sediments, a

current density (initial/final and average) of 0.025 A/m 2 is used. The current
draintoopenpipescoversaninternal surface area equivalent to 10 times

'

π

times the diameter.
For open pile ends, the top internal surface is included for a distance of 5 times
the diameter and is regarded as seawater exposed. Internal surfaces of piles filled
with sediments do not have to be included.
Unless otherwise specified by the owner, a current drain of 5 A/well casing
should be considered in CP design calculations.
Subsea wells that are cemented will have reduced current drain in compar-
ison to platform wells, which are normally not cemented. The CP system design
should consider the worst-case scenario in operations to have a conservative
design. For example, subsea wells may account for significant current drain dur-
ing installation or work-over and other interventions.
Current drain due to anchor chains is accounted for by 30 m of chain for
systems with the mooring point topside only. On the other hand, if the mooring