ABSTRACT

After the commissioning of monopile foundation systems in many offshore wind farms, internal corrosion problems have been observed inside of the monopiles in the area located beneath the so-called ”airtight deck”.

The cause of these corrosion problems is usually a failure of the subsea cable entrance seal which allows oxygen to penetrate into the monopile.

This is in complete contradiction to the assumption that the submerged internal area is a ”gas sealed area” with a low corrosion risk due to a limited presence of oxygen after initial oxygen depletion.

This paper presents a solution to significantly reduce the internal corrosion of such areas using a highly sophisticated ICCP system.

This paper also covers various other technical, electrochemical and installation challenges in the implementation of such an ICCP system within confined spaces.

This solution was developed in great detail in 2016 and has recently been successfully installed in 39 monopiles in a very short period of time.

INTRODUCTION

Offshore Wind Power has become the main renewable energy source in Europe in the last decade. Many of the installed wind turbines are constructed on so-called monopiles or tripods which are submerged in very corrosive seawater. For the external steel surfaces, various kinds of CP system have been installed to protect the monopiles against corrosion, but the severe corrosion risk for the internal submerged steel surfaces has been underestimated in the past because these internal spaces were previously assumed to contain little or no oxygen.

This paper describes the impressed current cathodic protection (ICCP) retrofitting of monopile internal spaces of 39 wind turbines in the Baltic Sea near to Germany.

The initial design of the corrosion protection system for the monopiles considered only external CP systems without CP for the internal submerged bare steel surfaces.

During the first inspection, it was determined that there was corrosion activity inside of the Monopiles and in 2014 attempts were made to remedy the situation. This consisted of adding a galvanic anode system to counteract the corrosion processes. After the installation of the galvanic anode system, measurements were made to verify the success and the effectiveness of the CP system.

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