Experience to date on E.ON’s Arkona offshore windfarm project, the first use of a new corrosion protection concept, suggests that it is significantly less expensive than conventional techniques
As first highlighted in the 3rd Quarter 2017 issue of OWJ, the concept of using thermally sprayed aluminium to prevent corrosion is widely used in the offshore oil and gas industry but has only recently begun to be used in the offshore wind industry.
Corrosion protection and management of foundations for offshore wind turbines is a potentially costly undertaking in terms of initial capital outlay and in long-term maintenance. The current solution of paint and sacrificial anodes not only slows foundation construction time but has a limited lifetime that requires frequent inspection and maintenance activities. Anodes require additional secondary steelwork to be added to a foundation, and paint requires many days to dry, slowing production times and adding cost. The first application of thermally sprayed aluminium was on the Arkona offshore windfarm, for which the foundations have now been installed.
Speaking at a recent conference, representatives of E.ON Climate & Renewables, consultant Rambøll Germany (who designed the monopile foundations for the Arkona project) and monopile contractor EEW provided an insight into their experience with the process, which they anticipate can significantly reduce the environmental impact during operations compared to conventional methods.
The new technique replaces conventional coatings and anodes and reduces manufacturing and maintenance costs. As highlighted earlier by OWJ, the technique, thermally sprayed aluminium, has been widely used to protect structures in the offshore oil and gas industry but has not been used until now in the offshore wind industry.
E.ON chose to protect all 60 foundations for the Arkona offshore windfarm in the Baltic with thermally sprayed aluminium. It believes this will be more cost-effective than conventional coatings and will be more environmentally friendly because it will eliminate the gradual deposit of tonnes of paint-based products on the seabed as corrosion takes place. EEW developed innovative solutions on behalf of E.ON to implement the process on an industrial scale at its facility in Rostock and built what is believed to be the world’s first fully automated coating line for thermally sprayed aluminium. Because the process is automated, it can lead to significant cost savings compared with conventional corrosion protection.
The foundations for the project are being installed in water depths ranging from 23 m to 37 m, have a diameter ranging from 7 m to 7.75 m, are 55–80 m in length, weigh between 667 tonnes and 1,247 tonnes and are being driven between 27 m and 43 m into the seabed. A number of different ways of protecting them were investigated, including anode rings combined with conventional coatings to DNV GL standards.
However, the team working on the corrosion protection aspects of the project also investigated thermally sprayed aluminium and its use in projects such as bridges and the Gjøa offshore platform. They conducted a comparison of the advantages/benefits and costs of a conventional approach to corrosion control and a cost-benefit analysis comparing existing techniques with the new one. They found that the thermally sprayed aluminium technique was less expensive overall by approximately 30%, mainly as a result of the elimination of anodes and a reduction in fabrication costs.
Having conducted a comparison of the various codes used for corrosion protection, they also estimate that thermally sprayed aluminium will have a service life of in excess of 27 years, which compares well with conventional corrosion protection techniques.
As with any corrosion protection technique, there are issues to address when using thermally sprayed aluminium, E.ON and Rambøll acknowledge. These include the fact that, when monopiles are driven into the seabed, some damage to the coating must be expected. Experience in other areas suggests that, although the technique works well in water, in the splash zone, where conditions differ, the level of protection provided may be less effective. Not all of a monopile needs to be coated with thermally sprayed aluminium, they note, but partial coating can reduce the lifetime of the aluminium coating, so a conservative approach in which the entire structure is coated might be best.