Offshore wind: Utgrunden leads the way

The development of renewable energy technologies has been identified as one of the key issues for the 21st century, and not just in terms of environmental protection. First steps have been made to combat climate change, at both the national and international levels, and towards the reduction of greenhouse gases like carbon dioxide. More recently, increasing attention has been given to the issue of security of supply and safety of energy systems.

Of all renewable energy technologies, offshore wind energy has possibly the most favourable combination of the key attributes of resource, energy cost and risk, especially in countries around the North Sea and the Baltic Sea, most of which are highly industrialised, with a high demand for energy.

A large resource

The European offshore wind resource is extremely large, energy costs are lower than those for many other renewable technologies (but it is more expensive than onshore wind, at least in the early stages), and the risks are relatively low, as offshore wind energy technology is already moving from demonstration to commercial applications.

Several studies of European resources have confirmed that most states have accessible offshore wind energy resources equal to at least 20 per cent of current consumption, and most have considerably more.

Constraints do, however, need to be taken into account, including shipping lanes, military activity, dredging concessions and environmentally sensitive areas. Most resource studies classify the resource according to water depth and distance from shore, as the cheaper resources (in modest depths, close to the shore) are likely to be exploited first.

Early studies focused on the use of MW size wind turbines, frequently in large arrays, whereas early demonstration wind farms used modest numbers of specially adapted versions of commercial machines around the 500 kW mark. Although these have operated successfully and some have delivered energy in excess of expectations, they are mostly installed in relatively sheltered waters. The conditions in some of the windier regions, for example the North Sea, will be more hostile.

The Utgrunden project

With its commissioning on 21 December 2000, the Utgrunden offshore wind farm became the first operating offshore wind farm with commercial machines in the megawatt class.* It can be regarded as the forerunner of much larger farms to be erected in due course.

The Utgrunden wind farm consists of seven variable-speed, variable-pitch regulated 1.5 MW turbines of the Enron Wind 1.5 offshore model and is situated 12.5 km from the Swedish mainland and 8 km from the island of Øland (Figure 1 and Table 1). The machines are built on monopiles, driven into sand and gravel at water depths ranging from 7.1 to 9.9 m.

The Utgrunden project was a turnkey project planned and implemented by Enron Wind. The first building application was submitted by Vindkompaniet AB in April 1997. Later, Enron Wind purchased the project assets, and successively the land use permit, building permit and environmental permits were gained. Execution was planned for 1999, but in January 1999, the National Juridical Board for Public Lands and Funds Permit (Kamerkollegiat) appealed against the project for commercial and political reasons. After a year, the complaint was rejected by the court but a final decision of the Minister of Environmental Affairs was recommended, which was finally announced on 9 March 2000. The issue of more than one million birds migrating each year was a major concern during the permitting process. Approval was finally granted, with one objective of the project being to gain more knowledge on this aspect through an extensive research programme on bird behaviour, both before and after the installation.

Wind turbine design

Reliability and maintainability are key requirements for offshore wind turbines if high availability is to be achieved.

Therefore, a well-proven turbine with a track record of, at that time, more than 260 machines in the field was chosen and marinised for offshore application (Figure 3).§ Field experience was systematically evaluated, and components and component suppliers were selected, accordingly.

A logistic concept for the exchange of components was implemented in order to reduce the use of heavy and expensive equipment. The permanent trolley crane can access smaller components in the nacelle, while repair work in the hub and exchange of the generator is possible with the aid of two devices fitted temporarily.

The power electronics and switchgear are enclosed in a container, which was assembled and pre-commissioned in the factory. Its location on a platform 8.8 m above sea level enables good access in the event of maintenance of individual components or the unit as a whole.

In addition, the cooling medium for the power electronics was changed from air to water, avoiding direct contact with the corrosive environment.

A number of other measures were taken to improve corrosion protection, reduce exposure to the sea climate, lengthen maintenance intervals, provide better access and simplify installation.

Monopile and platform

The Utgrunden wind farm is located on a subsea ridge originating from an ice-age moraine, partly covered and mixed with gravel. The soil consists of medium dense to dense sand, stones and occasional boulders. Divers indicated up to four boulders per 10 m2 with a maximum diameter of 0.7m. There are also some zones with clay content. The complex geological formation was investigated at every proposed turbine location, with drillings and other testing, where possible.

Early studies showed the advantages of driven monopiles compared with more expensive drilled monopiles or gravity foundations. The foundation and tower were designed together as one structure. Loads and frequencies arising from the combined effects of machine, wind, waves and ice were considered. Application of recent research1 on the dynamics of the entire system and co-operation with AMEC Process and Energy Ltd and IDAS GmbH enabled a lighter, and thus, cheaper pile and tower to be used.

Figure 4 and Table 2 compare the actual built soft–soft‡ design with a considerably heavier soft–stiff concept, showing weight savings of 1/3 of the total weight for the softer pile, and smaller reductions for the transition piece and tower. Rolling and welding of the lower wall thickness material of the monopile is also less expensive.

The relatively small pile diameter of 3 m reduced the installation effort and resulted in lower design ice loads. Because of this no ice cone was required, which itself is sensitive to ice damage and which cannot be applied at sites with large variations in sea level, eg due to tides and storm surges.

The basic internal frequency of the seven monopiles, of approximately 0.28 Hz, lies in the operational range of the mean rotor speed, which is 0.18 to 0.33 Hz, but well below the mean rotor frequency of 0.33 Hz at rated power.

Monopile foundations are very attractive in terms of economics. However, attention is needed to monitor the potential inclination of driven monopiles after installation and possible damage to the pile top arising from hammer impact. Therefore, an innovative grouted joint between the pile and the tower was used (Figure 5), which is similar to the common pile–sleeve connection of offshore oil & gas jackets. The transition piece, with pre-mounted platform for the container and maintenance activities, proved easy and fast to install and provides a means to correct pile inclinations and to compensate for any imperfections in the pile top.

A 20 kV AC power cable with integrated optical fibre connects the turbines. It was buried 0.75 m into the seabed or fixed by other means when jetting was not possible. The extension of the 50 kV station at Degerhamn on Øland island was another task.

Construction and early operation

In May 2000, the major project specific components, monopiles, towers and cables, were finally ordered. Installation activities started onshore on 7 September. After two weeks, the Belgian contractor Hydro Soil Services had driven all the monopiles, with about 2000 blows each, to a penetration of 19 m. Installation of transition pieces and turbines then continued until 9 October, just a few days before the start of the stormy winter season. Laying of the onshore and offshore cables and extension of the substation were executed prior to 24 November. The first turbine delivered power on 27 November.

Commissioning and testing was finalised on 21 December, with the project transferred to the client two days before Christmas Eve. During construction and installation, no major problems were encountered, and the schedule and budget were met.

Working during the autumn and winter season proved to be good training for more exposed sites where similar conditions may arise during the summer period.

For maintenance of the wind farm a special station was set up on the mainland, at Bergkvara harbour. Some 12.5 km away from the wind farm site, this corresponds to 25 or 40 minutes of sailing time, depending on whether the company’s fast Zodiac boat is employed, or a more comfortable vessel with shock-absorbing seats and an enclosed cabin. The existence of a permanent team of Enron technicians and skippers ensures fast reaction during the whole course of the year. In future, onshore turbines on the mainland will also be serviced by this team.

An average energy yield of 38 000 MWh/y is expected for the wind farm, at an estimated annual average wind speed of 8.5 m/s. This corresponds to the consumption of 6000 Swedish households.

So far the turbines have performed well, achieving significantly higher energy yield than the average of the turbines of this type used in onshore installations, mainly in Germany.

From a practical point of view, the availability of offshore turbines must be lower than on land, since access is more difficult and results in loss of production after failures during bad weather.

During the commissioning period, which was in the winter, the availability was somewhat lower than for wind farms erected at the same time in Germany. Since spring the turbines have been operating at high power levels, and the guaranteed availability has been exceeded.

Preliminary results of the bird investigations indicate that migrating birds detect the wind farm at a distance of about 2 km, and change direction or altitude. No evidence for bird collisions or undue disturbance in their behaviour has been found, to date.

Success on three fronts

Overall, Utgrunden can be considered a technological, commercial and political success. Marinisation of a well-proven wind turbine and the innovations of the soft–soft monopile and the grouted joint, have proved successful, as reflected in the good experience during installation and operation to date.

Eigenfrequency measurements have demonstrated the merits of the soft–soft design and of the tailoring of the dynamics of the entire system.

Evaluation of the monitored aerodynamic and hydrodynamic loads is underway at the time of writing.

Preliminary results from investigations of migrating birds and of public acceptance issues are very positive and encourage further exploitation of the offshore wind energy potential in similar projects, in Sweden and elsewhere in Europe.

The next generation

The next generation of Enron offshore wind turbines, with a rating of 3.6 MW (Figure 6), will be prototyped as early as the end of 2001 at an onshore site in Spain. The rotor diameter will be 100 meters, whereas today’s largest wind turbines usually have a rotor diameter of up to 80 meters.

The new turbine is based on proven technology, such as pitch regulated blades and a variable speed concept which reduces loads and stress. Unique design features, including a helicopter platform, allow for annual instead of semi-annual service intervals. An integrated crane system allows for replacement of all major components, eg blades, gearbox, generator etc, without the use of jack-up platforms. With this technology we expect further improvements in terms of efficiency and economics.

Political initiatives needed

As well as the appropriate technology, to allow offshore wind to take off in Europe and to realise its vast potential, there is a clear need also for action by political decision-makers. Their primary objective should be the creation of the right political incentives to give investors sufficient planning and economic security.

In particular, the following measures are necessary2:

• Setting of long-term objectives for offshore wind energy exploitation and specifying appropriate measures to attain this.

• Maintaining and strengthening support for national and EU research activities, in particular relating to optimisation and adaptation of turbine technology, including foundations, grid connection and integration, erection techniques as well as environmental implications.

• Political initiatives to co-ordinate and standardise licensing procedures, not only in Germany but across the EU. Potential conflicts with other users of the sea, eg military and civil aviation as well as navigation, fishing, tourism, nature conservation, need to be resolved at an early stage with all participants involved.

• Establish the requirements and approach for grid integration of large offshore wind farms and the associated grid expansion onshore. And

• Pro-active support for implementation and licensing of the first, smaller offshore wind farms in the near coastal zone.

Tables

Table 1. Utgrunden: main project data
Table 2. Data for the monopile designs