Sweden: The first decommission in the world of an offshore wind farm is now complete

Work on dismantling the five wind turbines with a total capacity of 10 megawatt (MW) which made up the Yttre Stengrund wind farm in Kalmar Sound began at the end of November. The wind farm had been in operation since 2001 and had been owned by Vattenfall since 2006. The reason for the decision to dismantle the entire wind farm rather than replacing the turbines with new, more modern ones was both financial and technical.

"The turbines that were installed at Yttre Stengrund were an early model and only about 50 of them in total were actually produced. The difficulty of getting hold of spare parts and the huge cost involved in upgrading the turbines and gearboxes meant that it wasn't financially viable to replace the turbines," says Maria Hassel, who project managed the dismantling operation for Vattenfall.

The job of removing and lifting down the rotor blades and the turbine housings fell to regional company Svensk Sjöentreprenad (SSE). They also dismantled the five masts and cut the concrete foundations off level with the sea bed. There were no major issues with the work itself but the operation was delayed by a month due to bad weather, which was a major risk throughout the dismantling process.

Taking the decision to decommission a wind farm when all Vattenfall's investments in new generation are in renewable energy sources may seem odd.

"Yes, it may seem a bit of a contradiction in terms but Yttre Stengrund was one of our first wind farms and we've learnt a lot over the almost 10 years that we've been operating it. That knowledge, combined with the prohibitive cost and technical difficulty of replacing the turbines, made this a relatively straightforward decision. We have locations with better wind conditions in other parts of the country where we can build new wind power and take with us the knowledge we've gained from Yttre Stengrund," continues Maria Hassel.

The work has been carried out with the necessary consideration for the environment, in accordance with the decisions made by the environmental authorities involved. Vattenfall plans to restore the site in such a way that any trace of the former wind farm will be negligible. Underwater cables will be removed in the summer of 2016.