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.
A number of monopile constructed windfarm turbines were retrofitted with a galvanic anode system comprising of aluminium alloy anodes, however a trial monopile was first tested, primarily to monitor the performance of the CP system and extent of hydrogen evolution within the monopile.
However, within two weeks of the offshore trial commencing the remotely monitored data provided unexpected results. A paper has been published to discuss the trial results and offer an explanation of the reasons for the sudden drop in water pH2.
This paper concentrates on the trial results, the reasons for the evolution of the different gases detected during the trial, namely Hydrogen, Hydrogen Sulphide, Carbon Monoxide and Carbon Dioxide. It also discusses why Zinc anodes would technically be the preferred anode system to select, given that Aluminium alloy anodes were the preferred choice due to the total cost installation.
A number of anodes were originally connected to the structure, although other anodes were installed as a backstop. It should be noted the monopiles in question were designed to be sealed with stagnant seawater, which was the case until the project decided otherwise based on the trial results and subsequent further tests.
The monitoring system was remotely monitored, measuring anode current output, potentials within the monopile and hydrogen evolution amongst other data. Unfortunately, the trial was restricted with what could be measured due to the hazardous area classification inside the trialled monopile, primarily the pH levels.