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Range of corrosion protection

Power transmission lines are submerged for shorter and longer sea crossings. What length requires cathodic protection at both landfalls?

Protection range

As a rule of thumb, until 2000 m sea cable can be protected from the landfall, by one ICCP system.

Will the anode location impact the maximum length of sea cable that can be protected with only one ICCP system? The SINTEF report STF34 F91195 discuss and show calculations and modelling of the potential profile for a 3000 m long sea cable protected by a single cathodic protection system located at one of the landfalls.

Anodes close to landfall

The potential reduction due to cathodic protection (polarisation of the sea cable steel armour) is dominated by the potential drop in the steel armour. The potential drop in seawater along the cable amounts to less than 25 % of the armouring potential drop. Hence, there is no ideal (optimum) anode locations near the landfall area.

With anodes located 50-150 m aside for the sea cable and 20-200 m from the shore line, only 40-50 % of the sea cable will be fully cathodically protected. This applies both with anodes at 20 m depth or buried in the tidal zone at 1-1.5 m depth.

Only 50 % of the cable length is protected with the anodes located close to the landfall area due to most of the potential drop will be caused by the current flow in the steel armour. The modelling indicates a potential drop in the steel armour around 350 mV, while the potential drop in the seawater is around 100 mV. As the cable potential drop is very dominant, anode location variations have minor impact.

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Anodes far from shore line

If the anodes are moved a very far from the shore line, the potential drop in seawater can be eliminated and up to 75 % of the sea cable can be protected. However, the technical aspects with such locations are obvious and hence such locations are not recommended.

Cathodic protection verification for long sea cables

As a rule of thumb, sea cables longer than 2000 m require cathodic protection system at both landfall areas. A common approach to protect sea cables up to 2000 m is to install one cathodic protection system at one of the landfalls and test the effect at the opposite side. If cathodic protection is not obtained for the entire sea cable, cathodic protection should be established at both landfall areas.

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