Section
1 External steel protection
1.1 Current density
1.1.1 The current density required for the external protection of the
submerged zone of units will depend on many factors such as water temperature,
oxygen content, resistivity of the water, suspended solids, water currents and
biological activity.
1.1.2 Design current density values are given in Table 4.1.1 Current density values for
design purposes for guidance purposes, but
the values to be used should be based on the environmental conditions prevailing at
the site. It should be noted that these values may be appreciably different from
values actually measured on steelwork in the vicinity of the site.
Table 4.1.1 Current density values for
design purposes
| Location
|
Current density
mA/m2
|
| Initial
|
Mean
|
Final
|
| Cook inlet
|
400
|
400
|
400
|
|
North Sea (Northern) Above 62°N
55°N to 62°N
|
220
|
100
|
130
|
| 180
|
90
|
120
|
| North Sea (Southern) Below
55°N
|
150
|
80
|
100
|
| Arabian Gulf, Africa, Brazil,
China, India
|
130
|
70
|
90
|
| Mediterranean, Australia
(Western), Gulf of Mexico, Adriatic Sea, US West Coast
|
110
|
60
|
80
|
| Mud – Most locations
|
25
|
20
|
20
|
| Drainage per
well
|
5A
|
|
NOTES
1. The current density values are intended for
guidance purposes in the design of sacrificial anode systems
using the methods as outlined in this Chapter. However, other
values may be accepted provided that there is adequate
justification.
2. For impressed current cathodic protection
systems, current densities higher than the values given in the
Table may be necessary but this will depend on the type,
location and quantity of the anodes.
|
1.1.3 In order to minimise pitting, the cathodic protection system must be
capable of rapidly polarising the steelwork and the cathodic protection design must
demonstrate that the system is capable of initially polarising the structure rapidly
In order to minimise pitting.
1.1.4 The cathodic protection system must be capable of re-polarising the
steelwork rapidly after storms, even when the anodes are well wasted, this should be
demonstrated in design calculations.
1.1.6 Coatings will deteriorate with time and there is likely to be mechanical
damage. In order to take this into account at the design stage, appropriate coating
breakdown factors should be applied and these are to be based on the percentages
given in Pt 8, Ch 4, 1.1 Current density 1.1.7.
1.1.7 For an epoxy or coal tar epoxy coating applied to give a dry film
thickness of 250 to 500 microns, an initial coating breakdown factor of one to two
per cent for the submerged zone and an annual degradation rate of one to one and a
half per cent per year should be used, in line with ISO 13173, unless agreed
otherwise with Class. Coating breakdown factors for high build coatings, applied to
give a dry film thickness of 1000 to 3000 microns should be lower and agreed with
Class.
1.1.8 For other coating systems an initial breakdown factor of minimum 5%
should be used and added to any area that is visibly damaged. Due allowance should
be made for further breakdown during the service life given in Pt 8, Ch 4, 1.1 Current density 1.1.7.
1.1.9 Current drain due to risers, mooring lines and other electrically
connected structures shall be considered in the current requirement calculations and
be fully documented.
1.1.10 Current density of propeller and rudder components and surrounding areas
are likely to require a significantly higher current density to polarise and
maintain protection through life. Design of cathodic protection systems for
propeller and rudder components should be based on a minimum current density of
400mA/m2 and 200mA/m2 respectively.
1.2 Sacrificial anode systems
1.2.1 The following indicates an acceptable method for determining the number
and weight of anodes to achieve the required level of polarisation on most
structures. Other methods may be accepted provided they give reasonable
equivalence.
1.2.2 The type of anode selected must be of sufficient mass with appropriate
dimensions to ensure an adequate current output throughout its design life.
1.2.3 The current output of the anode should be calculated using the following
formula:
where
 |
= |
current output of anode, in amps |
 |
= |
driving potential, i.e. the difference between the potential of
the mode and the protected steel potential, in volts |
 |
= |
anodic resistance, in ohms. |
1.2.4 The potential of the polarised steel should be taken as –0,8 volt ( /sea-water reference electrode), although a more negative value
may be used for those locations where sulphate-reducing bacteria may be active,
see
Pt 8, Ch 2, 1.3 Criteria for cathodic protection.
1.2.5 The resistance of an anode, R, with small cross-section in
relation to its length (4r≤L) and with a stand-off distance from the bottom of the
anode surface to the structure of not less than 300 mm, is given by:
-
where
|
ρ |
= |
resistivity of sea-water, in ohm.cm |
 |
= |
length of anode, in cm |
|
r |
= |
equivalent radius of anode, in cm |
|
ln |
= |
 |
 |
= |
 |
|
a |
= |
cross-sectional area of the anode, in cm2
|
- When bracelet anodes are used, the resistance may be determined
using:
where
 |
= |
the exposed surface area of the anode, in
cm2. |
1.2.6 In order to achieve a suitable anode distribution on tubular structures,
each appropriate section of steelwork should be considered separately.
1.2.7 The current required for each section may be determined from the
following:
where
 |
= |
current, in amps |
|
A |
= |
area of steelwork, in m2
|
|
I |
= |
current density, in mA/m2. |
1.2.8 The number of anodes, N, required should satisfy both of the
following:
where
 |
= |
current, in amps |
 |
= |
current output of anode, in amps |
 |
= |
net weight of anode material, in kg |
 |
= |
net weight of individual anode, in kg |
 |
= |
 |
|
C |
= |
practical electrochemical capacity of the alloy, in Ah/kg |
|
U |
= |
utilisation factor, i.e. proportion of net weight consumed at
end of anode life. For fully supported tubular inserts
- U = 0,9
- U = 0,8 for bracelet (half shell)
- U = 0,75 for bracelet (segmental type).
|
In order to optimise the performance and efficiency of the anodes the
values for both equations should be similar.
1.2.9 It is to be shown by appropriate calculations that the system is capable
of polarising the structure initially and also when the anodes are consumed to their
design utilisation factor.
1.2.10 It should be assumed that, at the end of its life, the anode length has
been reduced by 10 per cent and that the remaining material is evenly distributed
over the steel insert.
1.3 Location of anodes
1.3.1 Having determined the number and size of the anodes to comply with the
recommended nominal current density and the required life, the anodes should be
distributed over the steel surfaces according to the required level of protection on
the steelwork but with some emphasis on the area adjacent to joints, etc. CP
potential modelling can be considered to aid distribution to ensure full coverage.
The anodes associated with the structure likely to become buried, such as footings,
etc. should be positioned on the steelwork immediately above the mudline.
|