Section 1 General requirements

1.1.1 The cathodic protection system for the external submerged zone is to be designed for a period commensurate with the service life of the structure or the dry-docking interval and it should be capable of polarising the steelwork to a sufficient level in order to minimise corrosion at any point in the service life.

1.1.2 This may be achieved using either sacrificial anodes or an impressed current system or a combination of both, see Pt 8, Ch 2, 3.2 Protection after launching and during outfitting 3.2.1.

1.2.1 All parts of the structure should be electrically continuous and, where considered necessary, appropriate bonding straps should be fitted across such items as propellers, thrusters, rudders and legs, etc. and the joints of articulated structures are to be efficiently completed to the Surveyor’s satisfaction.

1.2.2 Where bonding straps are not fitted, a supplementary cathodic protection system should be considered.

1.2.3 Particular attention to earthing and bonding is required in hazardous areas where flammable gases or vapours may be present, see Pt 7 Safety Systems, Hazardous Areas and Fire.

To avoid dangerous sparking between metallic parts of structures, potential equalisation is always required for installations in Zone 1 and may be necessary for installations in Zone 2 areas; this is achieved by connecting all exposed and extraneous conductive parts to the equipotential bonding system. Notwithstanding this, cathodic protection installations are not to be connected to the equipotential bonding system unless the cathodic protection system is specifically designed for this purpose. See IEC 61892-7 Section 5.6.3.

Cathodically protected metallic parts are live extraneous conductive parts. If located in hazardous areas, they are to be considered potentially dangerous (especially if equipped with the impressed current method) despite their low negative potential.

No cathodic protection is to be provided for metallic parts in Zone 0 unless it is specially designed for this application. See IEC 61892-7 Section 6.7.

1.2.4 Consideration should be given to the influence of any connecting structures, such as risers and pipelines, on the efficiency of the cathodic protection system. A floating structure may be permanently or temporarily connected to another neighbouring structure. In this situation, the requirements of BS EN 13173 are to be met, including the taking of measurements to ensure that there are no deleterious effects of electrical stray current on the protected structure.

1.3.1 Cathodic protection systems are to comply with BS EN 13173 – Cathodic protection for steel offshore floating structures or BS EN 12495 – Cathodic protection for fixed steel offshore structures unless local legislation requirements dictate otherwise; replacement standards shall be listed and submitted to LR for approval..

1.3.3 Potentials more negative than –1,10 volts Ag/AgCl must be avoided in order to minimise any damage due to hydrogen absorption and reduction in the fatigue life. For steel with a tensile strength in excess of 700 N/mm² the maximum negative potential should be limited to –0,95 volt. But where the steel is prone to hydrogen assisted cracking the potential should not be more negative than –0,83 volt (Ag/AgCl reference cell).

1.3.4 High strength fastening materials should be avoided because of the possible effects of hydrogen, and the hardness of such bolting materials should be limited to a maximum of 300 Vickers Diamond Pyramid Number.

1.3.5 The potential for steels with surfaces operating above 25°C should be 1 mV more negative for each degree above 25°C.

1.3.6 For guidance on the design of sacrificial anode systems, see Pt 8, Ch 4, 2 Protection of tanks.