Soon, the wind turbines in Sweden's second oldest offshore wind farm will stop turning, and then, the Yttre Stengrund wind farm site will be restored to its original state. But first, the nacelle and rotors must be dismantled.
"Vattenfall is number two in the world for offshore wind power and this will be a good lesson for the future. Dismantling the towers and nacelle and pulling up the undersea cable won't be a problem. What will be difficult will be cutting the foundations in such a way as to have as little impact as possible on the environment," says Fredrik Forslund, site manager for Yttre Stengrund.
No impact on the environment
The five wind turbines are erected on a base consisting of boulders, stone and sand. Hard beds like this are important for many species and have a high level of biodiversity. Studies have shown that the wind turbine foundations act as artificial reefs and habitats for mussels and barnacles.
Dan Wilhelmsson, scientific advisor on the environmental impact of offshore wind power at the Swedish Secretariat for Environmental Earth System Sciences., has investigated the impact of wind power on the underwater environment at Yttre Stengrund.
"That effect disappears when the foundations are removed. So, if the restoration is done properly, the long-term impact on the environment is more or less zero," he says.
CUT FROM THE INSIDE
The foundations must be removed because otherwise they will rust and fall apart. They will therefore be cut using a diamond wire or water jet cutter. The advantage with a water jet cutter is that it cuts the steel pipes off from the inside and can therefore cut them underneath the seabed without the need for excavation. Both methods are controlled remotely from an unmanned submarine known as an ROV, or Remote Operated Vehicle.
During the actual restoration process, there will be a certain environmental impact in the form of noise and muddying of the water but once this is done the only evidence of the installation will be below the seabed. These components will gradually rust away with no impact on fish or seabed vegetation.
"Leaving these in the seabed will have no significant impact on the environment," says Rutger Rosenberg, research scientist at Gothenburg University's Department of Biological and Environmental Sciences.
Wherever possible, anything that remains of the old turbines will be sold, recycled or used as spare parts in other power plants. The project is expected to take one month to complete and is estimated to cost SEK 10 million.
IN OPERATION SINCE 2001
The wind farm, which is located in the southern part of the Kalmarsund Bay in southeast Sweden, came into operation in 2001 and was taken over by Vattenfall in 2006 when it acquired Elsam and Energi E2. Currently, only one of the five turbines is in operation and it is not financially viable to repair or replace the other four turbines.
"There are only a limited number of spare parts available for these old models and the undersea cable needs replacing. With the current forecasts for electricity prices, it's not financially viable to build new wind power, even if we re-use the foundations," says Fredrik Forslund.