This is a debate that will become more important as the focus moves from the earliest demonstration projects to larger, industrial-scale developments further offshore. The indications are that it may cost more than originally thought.
The oldest offshore wind farms are mostly demonstration projects, close to shore and with few turbines, most of which are no longer manufactured. The aim, of course, is to keep them running as long as possible, provided the economics are right. After that, there are three options: partial repowering - installing a new drivetrain and rotor on an existing tower and foundation to increase energy production; full repowering - replacing old turbines with newer, larger units; and total decommissioning - removing turbines without replacing them.
The decision will depend on various factors, including the size and type of project, its distance from shore, wind resource, regulations, power price, operating costs, and the exent to which existing infrastructure can be reused. The expected profitability of a repowered site compared to competing calls on capital will also be critical.
While regulations vary, most jurisdictions require that installations are removed if no longer in use and that the seabed is returned to its original state. Whether monopiles have to be fully removed or cut off a safe distance below the seabed will probably be decided on a case-by-case basis, depending on how much environmental damage might be caused trying to remove them. The same goes for inter-array and export cables, especially where scour protection has been colonised by marine life.
Planning at outset
The project owner also has to submit a detailed decommissioning plan. Nowadays, this will generally be part of the initial permitting process, with regular updates to take account of changing regulations, technological advances and costs.
Because decommissioning is an expensive business, and there is a risk of insolvency, most jurisdictions now require owners to provide financial guarantees, such as paying into a secure decommissioning fund during the life of the facility, bonds or letters of credit.
Since there has been very little decommissioning of offshore turbines so far, costs have often been underestimated, says Gillian Smith, senior engineer in DNV GL's offshore wind group.
A 2006 study by Climate Change Capital for the UK government suggested around £40,000/MW (EUR55,000 at today's exchange rate). Renewable energy and technical advisory DNV GL now estimates it will cost EUR200,000-500,000/MW to decommission offshore turbines, equal to 60-70% of recently quoted installation costs. The lower figure represents earlier projects with smaller turbines in shallower waters needing less sophisticated vessels, and the upper end bigger installations in more challenging conditions.
One way to control costs is to keep decommissioning in focus right from the initial stage of design, Smith says. When welding substation topsides on to jacket foundations, for example, it is sensible to consider how easy it is to access that weld for when it comes to dismantling the project.
Similarly, how easy is it to remove the ballast from gravity-based foundations? An important lesson from the oil and gas industry is to record any work that is done during the life of the project. "This is especially important if the facility has changed hands several times and they have lost track of what has been added or taken away," says Smith. Efficient use of vessels and specialist equipment will also be key.
For repowering, too, the sooner owners start to consider the options the better, says Andrew Ho, offshore wind analyst at the European Wind Energy Association (EWEA). "If you are going to repower, you have to start taking measurements today, to monitor fatigue loads and think if the same foundation can carry the same weight and load," he says.
The industry's experience to date comes almost entirely from decommissioning meteorological masts, which are smaller but similar in many ways. DBB Jack-Up, which has dismantled masts at Horns Rev 1 and 2 and Nysted, uses jack-up vessels to remove the mast and then cuts the monopile 1-2 metres below the seabed. Removing turbines will not be "a huge challenge", says Ole Jacob W Nielsen, DBB's head of sales and marketing. "There is a proven track record that it can be done safely and cost-efficiently," he asserts.
The industry also needs to start thinking about reusing more of the structures in order to cut costs, says Nielsen. It is unlikely the bigger projects being built today will be completely removed, notes Andreas Wickman, CEO of Wickman Wind and one of the original owners of the Bockstigen wind project, Sweden's first offshore wind farm. With all the electrical infrastructure, substations and perhaps accommodation platforms, it is very likely the sites will be retained, he believes.
The 220kW Windworld Svante I turbine erected in Swedish waters in 1991 was the first to be dismantled, in 2007. Following that, Vattenfall's 10MW Yttre Stengrund project, commissioned in 2001, will probably be the first to be fully decommissioned.
Although only 13 years old, it is difficult to find spare parts for the NEG Micon 2MW turbines, and only one of the five is still operating. Vattenfall originally hoped to repower the plant, using the same monopiles, but Sweden's low electricity price means it would not be profitable. Even if the price increases in future, "we have bigger and better projects we would like to focus on," says Per Attemo, asset manager of Vattenfall's Nordic unit. A repowering of the Utgrunden I plant is also "very unlikely" as long as Sweden's support mechanism remains unchanged, he says.
The situation at Bockstigen, commissioned in 1998 four kilometres off the island of Gotland in one of Sweden's windiest regions, is less clear cut. While four of the five Windworld 550kW turbines are still operating, they are "very tired", admits Kim Madsen, director of Momentum-gruppen, which bought the site in January with the aim of repowering it. Ideally, he wants to replace the five turbines with twelve new units totalling around 40MW.
If the permits are not forthcoming, he will install five new turbines of about the same height as or about the same overall size as the current machines, using the same monopiles if possible, with a total capacity of 6-10MW. The biggest uncertainty is the electricity price and whether the government will decide to support offshore wind. An announcement is expected this summer.
In the Netherlands, Vattenfall's 2MW Lely plant has been out of operation since December, when the rotor head and blades fell off one of the four Nedwind 500kW machines. Investigations continue, but the long-term prospects are not attractive, admits Gerben Epema, Vattenfall's onshore and nearshore wind manager. The winds are good, but the site is too small and not designated for wind power in new government plans. However, repowering the 16.8MW Irene Vorrink site might be viable, Epema says, depending on other priorities at the time.
Dong and Vattenfall aim to continue running the 160MW Horns Rev I, "as long as it is profitable to continue operations", says Attemo. The seabed lease expires in 2027. After that, it is a political decision, which could see Denmark auctioning the site. "Whether or not the existing substation and export cable could be used again is up to the Energy Board to decide," Attemo says.
Dong says its Vindeby, Tuno Knob and Middelgrunden projects are all still running well, as is E.on's Blyth facility, though they declined to comment further. The bottom line is that as long as these projects are making money, the turbines will carry on turning.
TEN OLDEST OFFSHORE INSTALLATIONS
Small and near shore, these projects will be cheaper to decommission than today's projects
Denmark 1991 5 11 x Bonus 450kW GBS* Dong
Netherlands 1994 2 4 x Nedwind 500kW MP** Nuon/Vattenfall
Denmark 1995 5 10 x Vestas 500kW GBS Dong
Netherlands 1996 16.8 28 x Nordtank 600kW MP Nuon/Vattenfall
Sweden 1998 2.75 5 x Windworld 550kW MP Momentum Gruppen
Sweden 2000 10.5 7 x Enron Wind 1.4MW MP Vattenfall
UK 2000 4 2 x Vestas 2MW MP E.On
Denmark 2001 40 20 x Bonus 2MW GBS Dong/Middelg's Vind'g
Sweden 2001 10 5 x NEG Micon 2MW MP Vattenfall
Horns Rev I
Denmark 2002 160 80 x Vestas 2MW MP Vattenfall/Dong