By Charlotte Gliddon-Bush
The UK’s first offshore wind farm is expected to contribute almost one-fifth of the government’s target for renewable energy by 2010. The Blyth offshore wind farm will comprise two of the world’s most powerful wind turbines and generate enough energy to power 3000 homes.
The UK is one of the windiest countries in Europe, and offshore resources could theoretically supply enough energy to meet all of the UK’s current electricity needs. The Blyth offshore wind farm has been built 1 km off the coast of Northumberland to extract this abundant source of power.
The wind farm comprises two 2 MW wind turbines that reach 90 m in height and can operate in winds of up to 160 km per hour. It is the first farm of its kind to be built in such a demanding position, receiving the full force of the North Sea. By utilizing this energy force, the Blyth offshore wind project will be able to generate enough energy to meet the electricity needs of some 3000 homes.
Offshore wind farms are becoming more popular throughout the world as a means to produce clean, efficient energy. They have the potential to deliver the large amounts of electricity and the 3028 TWh of potential electricity available from offshore sites in the European Union (EU) is greater than the region’s existing demand.
More than 40 000 modern wind turbines generate electricity throughout the world without producing the harmful emissions or waste products associated with burning fossil fuels. Wind power is the fastest growing energy source world-wide, and as fossil fuel reserves diminish and concerns increase over climate change, the value of renewable power is increasingly being recognized.
Countries around the world have set targets for the generation of electricity from renewable technologies along with targets for reducing greenhouse gas emissions. In the UK, the target for renewable generation is set at ten per cent of the total electricity supply by 2010, and it is believed that 1.8 per cent will come from offshore wind projects. The government is also aiming for reductions in carbon dioxide (CO2) emissions by 20 per cent over the same time period. Wind power therefore has an important role to play.
One of the attractions of offshore projects is that wind speeds are generally higher offshore than on land and the wind turbulence is lower. In addition to this, many people still find wind turbines unsightly and locating them offshore helps to reduce the visual impact.
The £4 million ($5.8 million) Blyth offshore wind project was funded by Blyth Offshore Wind Ltd., a company made up of four shareholders – PowerGen Renewables (a joint venture between Abbot Group and PowerGen), Shell Renewables, Nuon and AMEC Border Winds. A further 30 per cent of the capital cost was provided by the European Commission’s Thermie Programme as part of its strategy to introduce more renewable power. The project is monitored and evaluated as part of the Department of Trade and Industry’s Wind Energy Programme, which aims to enable offshore wind power development and support UK industry.
One of the two turbines at the farm already has a Non Fossil Fuel Obligation-4 (NFFO-4) contract, a special electricity contract for the sale of electricity by renewable generators. The second turbine has two back-to-back contracts for electricity sales: a ‘brown’ electricity contract with PowerGen, and a Renewable Energy Certificate trade with Nuon for the ‘green environmental benefit’ associated with the power from this turbine. This represents one of the first international trades in renewable energy certificates.
Blyth Offshore wind farm is located 1 km off the coast of Northumberland in the UK and will receive the full force of the North Sea
Henk Kouwenhoven, director of Nuon UK Ltd. said, “Nuon has a strong commitment to renewable energy, both onshore and offshore. This project is an important step for the development of wind energy offshore.”
Each partner in Blyth Offshore Wind Ltd. brought to the project a range of complementary skills and experience. Each partner was actively involved in a Construction Steering Committee which sat under the Blyth Offshore company board and took day-to-day decisions on construction issues.
PowerGen Renewables was involved in wind turbine specification and procurement, wind energy analysis, power purchase contracting and lease negotiations with the Crown Estate, which owns the coastal regions around the UK out to 12 nautical miles.
PowerGen Renewables executive director of UK operations, Nick Baldwin, said: “This is a significant benchmark in the development of offshore wind farms in the UK. We are confident that the Blyth project will demonstrate the robustness of the technology and lead to further harnessing of the UK’s substantial offshore wind assets.”
Shell Renewables utilized its extensive marine construction knowledge and experience in its role as project manager, reporting directly to the Construction Steering Committee.
AMEC Border Wind, part of AMEC’s Offshore Services, was the project engineer for the wind farm and worked with Seacore to provide the foundation piles for the turbines, and performed installation activities under the guidance of Vestas.
AMEC is traditionally an oil and gas provider, but it expanded into wind when it bought Border Wind Ltd., one of the UK’s leading wind energy developers. “AMEC is one of the first oil and gas providers to expand into wind energy and this allows us to respond positively to the government’s recently announced commitment to developing wind energy projects,” said Neil Bruce, managing director of AMEC Offshore Services.
The Blyth Offshore wind farm is installed on submerged rock 1 km offshore where the minimum water depth is 6 m, the tidal range is 5 m and waves of up to 8 m are expected. Previous offshore wind farms in the Baltic and elsewhere have been exposed to 0.5 m tidal range and waves of 2 m at most.
Seacore worked closely with AMEC Border Wind on the design and installation of the wind turbine monopiles. Construction of the wind turbines in the North Sea environment was difficult, but Seacore and AMEC combined their knowledge and experience to complete the project. The foundations for the turbines had to be designed to 3.5 m in diameter to support the large wind turbines.
The towers’ natural frequencies are in the same frequency range as the loading from large waves. Large bending movements can be projected in the pile due to the wind loading from a large rotor. To analyse these and ensure that the structure can withstand the static and dynamic loads, various techniques were used in consultation with other engineering design teams.
Seacore completed a sea bed survey two years before construction began to assess the condition of the seabed. It adapted its reverse circulation drill rig and socketing technique to suit AMEC’s Wijslift 6 jack-up platform and the sea bed rock.
The company worked around the clock on the platform where it used special large diameter sacrificial coring bits to assist with drilling 15 m-deep sockets in the seabed rock to receive the turbine’s steel monopile foundations. The 3.5 m-diameter steel monopiles then had to be grouted into these foundations. In September 2000, the two towers were crane lifted by AMEC onto the monopiles and then flange bolted to them.
The Blyth Offshore wind farm supplies the UK mainland with electricity. A 11 KVA subsea power cable was installed by Global Marine’s Shallow Water Services team, linking the wind turbines to the shore. The cables incorporate 16 optical fibres to enable the generators to be remotely monitored and controlled from the shore.
The V-66 wind turbines are connected to medium voltage grid at 6-33 kV where 36 kV is the highest equipment voltage. The transformer has been adjusted to the grid voltage to be within ±5 per cent. Steady variations within +1/-3 Hz (50 Hz) or within +2/-3 (60 Hz) are acceptable. Intermittent or rapid grid frequency fluctuations would cause serious damage to the turbine.
Wind energy will make a significant contribution to renewable energy targets
To ensure the optimum efficiency and life span of the turbines, grid drop-outs must only occur once a week as an average over the lifespan of the wind turbine and a ground connection of a maximum 10 Ω must be present.
Design and operation
The Vestas V66, 2 MW offshore wind turbine is a pitch regulated upwind turbine with active yaw and a rotor with three blades. The rotor is 66 m in diameter and operates using the OptiSpeed concept which enables the rotor to operate at variable speeds to optimize power production and noise levels. An OptiTip feature allows the blades to be angled so that they are pitched at the optimal angle for current wind conditions.
The OptiSpeed, also named Vestas Converter System (VCS), consists of an effective asynchronous generator with round rotor and sliprings, a power converter with IGBT switches, contractors and protection. It controls the current in the rotor-circuit in the generator. This gives precise control of the reactive power, and an exact connection of the generator to the grid.
The blades on the wind turbine are made of glass fibre reinforced epoxy. Each blade consists of two blade shells, bonded to a supporting beam. Special steel root inserts connect the blades to the blade bearing. This is a four-point ball bearing, which is bolted to the blade hub.
The main shaft of the turbine transmits the power to the generator through the combined planetary and helical gearbox. From the gearbox, power is transmitted via a maintenance-free composite coupling to the special asynchronous four-pole generator with wound rotor.
At higher wind speeds, the OptiSpeed and the pitch regulating system keep the power at nominal, regardless of the air temperature and density. At lower wind speeds, the OptiTip system and the OptiSpeed system optimize the power output by selecting the optimal speed and pitch angle. The wind turbine brakes by full feathering the blades. A parking brake is mounted on the gearbox high speed shaft.
The wind turbines’ functions are monitored and controlled by microprocessor-based control units. This control system – including the transformer – is placed in the nacelle. The nacelle has a glass fibre reinforced cover to protect all the components inside against the elements. A central opening provides access to the nacelle from the tower. Inside the nacelle there is also an 800 kg service crane, which can be enlarged to hoist the main components.
Changes in the pitch of the blades are activated by a hydraulic system, which enables the blades to rotate 95°. This system also supplies the pressure for the brake system. Four electrical yaw gears rotate the yaw pinions, which mesh with a large toothed yaw ring mounted on the top of the tower.
Offshore generation potential in the EU is estimated to be 3028 TWh/year
The yaw bearing system is a plain-bearing system with built-in friction and a self-locking function.
Coping with the elements
The V66 wind turbine is designed for ambient temperatures ranging from -20°C to +30°C. Operating outside these temperatures affects the efficiency of the wind turbines. During periods of low wind, heating and de-humidification of the nacelle will increase the consumption of the turbines’ own power. If the turbine is in connection to heavy icing, interruptions in operation may occur.
During certain combinations of high winds, high temperatures, low air density and/or low voltage, power deterioration may occur. This ensures that the temperature of the gearbox, generator, transformer and other major components are kept within acceptable limits. The wind turbines are protected from these elements with corrosion protection.
The future for offshore wind
On the 7 December 2000 the Right Honourable Helen Liddell MP, Minister of State for Energy and European Competitiveness, officially opened the Blyth Offshore wind farm. She said it was a “major signal of the potential for a new energy source and a new industry for the UK”.
Although offshore wind farms are more popular in the rest of Europe, more wind farm like the Blyth project are planned in the near future as the UK tries to reach its target of ten per cent of electricity being produced from renewable sources.
David Still, chairman of Blyth Offshore Wind Ltd. said: “We are delighted with the success of the project. It has clearly shown that the technology is robust and ideally suited to supply sustainable, green energy. It is an important step in the harnessing of offshore wind energy.”