Section 2 Protection of tanks
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 8 Corrosion Control - Chapter 4 Guidance Notes on Design of Cathodic Protection Systems and Coatings - Section 2 Protection of tanks

Section 2 Protection of tanks

Parent topic: Chapter 4 Guidance Notes on Design of Cathodic Protection Systems and Coatings

2.1 Anode resistance

2.1.1 Where large stand-off anodes are used for the protection of tanks, the resistance should be determined using the formula as given in Pt 8, Ch 4, 1.2 Sacrificial anode systems 1.2.5

2.1.2 Where flat plate anodes are used, their resistance is to be determined from the following formula:

however, if the flat plate anodes are close to the structure or painted on the lower face then the resistance is to be determined using:

where ρ is as defined in xPt 8, Ch 4, 1.2 Sacrificial anode systems 1.2.5

= mean length of anode sides, in cm.

2.2 Current density

2.2.1 The design current density to be used for permanent water ballast tanks should be based on a minimum value of 110 mA/m2 but this may have to be increased to at least 130 mA/m2 if hot oil is stored on the opposite side of the bulkhead. For a coating allowance, see Pt 8, Ch 4, 1.1 Current density 1.1.6.

2.2.2 Uncoated tanks used for the storage of crude oil at ambient temperature alternating with water ballast are to have a minimum current density of 90 mA/m2; however, this should be increased for higher temperatures.

2.2.3 Unless otherwise agreed the resistivity of the water in ballast tanks should be assumed to be 25 ohm.cm.

2.3 Anode distribution

2.3.1 Once the number and size of anodes have been determined, they are to be distributed as follows:
  1. Ballast-only tanks: evenly over all the steelwork with some consideration given to the lower sections based on usage pattern and ballasting levels.
  2. Crude oil/ballast tanks: evenly but with some emphasis on horizontal surfaces in proportion to the area of these surfaces.
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