A closer look into offshore wind decommissioning

The view at Kalmar Sound in Sweden has changed considerably since November of last year. The five NEG Micon 2MW turbines have been dismantled. It had been operational since 2001 and Vattenfall has had ownership since 2006 when DONG Energy handed over to them. Why was the Yttre Stengrund wind farm dismantled? Offshore WIND magazine delves deeper into this world first with Maria Hassel, project manager of the decommissioning for Vattenfall.

Now, decommissioning may seem strange in a relatively young industry as offshore wind, but as Hassel points out, it was a pretty straightforward decision. “Yttre Stengrund wind farm was one of our first wind farms and we’ve learnt a lot over the almost ten years that we’ve been operating it. It is a relatively small site with only five turbines, which were quite old and too costly to repair and maintain. Also, the technical difficulty of replacing the turbines played a part in our decision making. The NEG Micon 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. Furthermore we want to focus on bigger projects and our locations with better wind conditions in other parts of the country where we can build new wind power and take the knowledge we’ve gained from Yttre Stengrund with us”, says Hassel.

So the reason was both financial as well as technical. “Planning a decommissioning is a lengthy process and I believe it took us around two years, including the dismantling”, Hassel continues. “The procurement took around half a year and the actual decommissioning took two months. Really, the decommissioning is not complicated as you can reverse the order in which you installed it, yet it is still a risky assignment. You are working offshore. I remember cutting the first turbine and that being a great moment of interest and excitement. From that first turbine we could learn and improve with each that followed.

“We also took experience from other industries, such as oil and gas. Regional company Svensk Sjöentreprenad (SSE) were tasked with removing and lifting down the rotor blades and also the nacelles. Furthermore, SSE dismantled the five masts and cut the concrete foundations off level with the sea bed.”

“We had planned the dismantling in the winter period and we knew we would have bad weather. I am really pleased that we completed the decommissioning in two months time and the whole process developed better than I had hoped. Although we had some bad weather, which caused some delay, more often than not it worked out that when we had dismantled one turbine the bad weather would hit just as we arrived in the harbour to offload”, explains 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. Hassel: “We worked closely with the authorities. Initially, to get the permits necessary for the decommissioning and after that we updated them continuously during the dismantling.”

Vattenfall plans to restore the site in such a way that any trace of the former wind farm will be negligible as they will no longer use the site in the future. All the cables will be removed from the seabed in the summer of 2016.

Offshore WIND magazine also spoke to Catapult about decommissioning. Peter MacDonald, Head of Innovation Engineering for ORE Catapult, said: “Looking to the future, it is important that decommissioning be researched today to ensure that workable solutions are considered and designed into the equipment. Much of the offshore wind industry plant and equipment is still relatively new, but many owner/operators are starting to turn their attention to decommissioning, and the engineering challenges it will bring.

“Decommissioning of offshore wind shares many aspects with installation, but unlike installation, decommissioning plans remain unproven. The main driver will be to achieve decommissioning in the most cost effective way, whilst minimising impact on the environment. Wind farm owner/operators will need to learn lessons from other sectors, such as oil and gas, and consider factors such as port infrastructure and vessels and the decommissioning of foundations, substations, towers and turbines.”

Rebecca van den Berge-McFedries

This article first appeared in the February 2016 edition of the Offshore WIND Magazine.