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51318-10494- The Water pH and Gas Evolution Risks Associated with the Use of Aluminium and Zinc Anodes for the Internal Cathodic Protection of Offshore Monopiles

Within a few weeks in a 2012 cathodic protection (CP) trial for monopiled windturbine structures in the North Sea, the seawater pH inside the monopile dropped from 8 to 5 and toxic gas (H2S and CO) alarms were energised. This paper explains why.

Product Number: 51318-10494-SG
Author: Alex Delwiche / Patrick Lydon
Publication Date: 2018
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In 2012, a cathodic protection (CP) trial was undertaken to establish the current requirements to protect the submerged and buried bare steel internal surfaces of monopiled windturbine structures in the North

Sea. Aluminium alloy anodes were utilized for the CP trial system and a remote monitoring system was also installed to measure the time dependent variations in anode current, structure to seawater potential, cathode current density and monopile hydrogen gas concentration produced from operation of the CP system.

Within a few weeks of the trial commencing, the seawater pH inside the monopile, had changed from near neutral pH 8 to less than pH 5 and when access to the confined spaces within the monopiles was attempted toxic gas alarms were energised. Several papers have been written discussing the strategy and the reasons why the pH reduced1,2. This paper discusses the reasons why different gases were produced and explains why the water pH changes were greater using Aluminium anodes than with Zinc anodes in situations where there is limited water replenishment and essentially stagnant water conditions exist.

Key words: Monopiles, Aluminium, Zinc, Hydrolysis, Anodes, Hydrogen Sulphide, pH

In 2012, a cathodic protection (CP) trial was undertaken to establish the current requirements to protect the submerged and buried bare steel internal surfaces of monopiled windturbine structures in the North

Sea. Aluminium alloy anodes were utilized for the CP trial system and a remote monitoring system was also installed to measure the time dependent variations in anode current, structure to seawater potential, cathode current density and monopile hydrogen gas concentration produced from operation of the CP system.

Within a few weeks of the trial commencing, the seawater pH inside the monopile, had changed from near neutral pH 8 to less than pH 5 and when access to the confined spaces within the monopiles was attempted toxic gas alarms were energised. Several papers have been written discussing the strategy and the reasons why the pH reduced1,2. This paper discusses the reasons why different gases were produced and explains why the water pH changes were greater using Aluminium anodes than with Zinc anodes in situations where there is limited water replenishment and essentially stagnant water conditions exist.

Key words: Monopiles, Aluminium, Zinc, Hydrolysis, Anodes, Hydrogen Sulphide, pH

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