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 Pt 8, Ch 4, 1.1 Current density 1.1.2 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
Location	Initial	Mean	Final
Cook inlet	400	400	400
North Sea (Northern) Above 62°N 55°N to 62°N	220	100	130
North Sea (Northern) Above 62°N 55°N to 62°N	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.

1.1.5 Where suitable high resistance coatings are used, consideration will be given to use of current densities lower than those given in Pt 8, Ch 4, 1.1 Current density 1.1.2.

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

resistivity of sea-water, in ohm.cm

length of anode, in cm

equivalent radius of anode, in cm

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

area of steelwork, in m²

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

life of structure or appropriate dry-docking interval in years, see Pt 8, Ch 2, 1.1 Objective 1.1.1

practical electrochemical capacity of the alloy, in Ah/kg

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.