POWER from the Irish Sea

GE Wind Energy and Airtricity are constructing Arklow Bank, Ireland’s first offshore wind power project. The wind park will be the first in the world to use offshore turbines over 3 MW in size.

Offshore wind farms are not new in Europe, which has nearly three-quarters of the world’s installed wind power capacity. The Arklow Bank wind park off the coast of Ireland stands apart from other offshore projects, however, for several reasons: it is Ireland’s first offshore wind farm, it is the first offshore application of some of the world’s largest wind turbines, and potentially it could grow into the world’s largest offshore wind facility.

In the early days of wind power, most wind turbines were fairly small, generating a MW or less of electricity. Today, wind technology has grown not only in size, but also has become increasingly sophisticated and highly reliable. New offshore installations ” such as the 3.6 MW machines being installed by GE Wind Energy on the Arklow Bank ” are becoming increasingly popular, especially in Europe.

Co-developed by GE Wind Energy and Airtricity of Sandyford, Ireland, Arklow Bank marks the world’s first commercial application of offshore wind turbines more than 3 MW in size. The seven GE units installed in the Irish Sea will add 25 MW of electrical capacity to the Irish grid, or enough to serve the annual needs of approximately 16 000 average Irish households.

Arklow Bank will feature some of the world’s biggest offshore wind turbines
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The 25 MW installation is expected to be just the first phase of a much larger wind farm Airtricity proposes to build over the coming years. The company’s offshore wind project proposals have been made possible under a foreshore lease which can provide for more than 520 MW of offshore wind power on the Arklow Bank, or enough power to meet about ten per cent of Ireland’s total energy needs.

“Ireland is 90 per cent dependent on energy imports,” said Eddie O’Connor, chief executive of Airticity, Ireland’s largest renewable energy company. “We are working with GE to demonstrate that the Arklow Bank project, even in this first phase, can already make a viable contribution to future energy self-sufficiency for this country.”

“Developments such as the 3.6 MW wind turbine and the success of the Arklow Bank project will play key roles in establishing offshore wind power as a commercially viable method for producing clean energy worldwide,” said Steve Zwolinski, president of GE Wind Energy, a unit of GE Power Systems.

Initially, GE Wind Energy will operate the Arklow Bank facility as a demonstration site for its 3.6 MW technology. Under the terms of the agreement, Airtricity holds an option to purchase the project after the certification, testing and demonstration stages are complete, approximately two years after first operation.

Winds of Ireland

Ireland has some of the best offshore wind resources in Europe. According to O’Connor, Ireland has enough wind energy potential to provide power for the entire country with enough left over to export to its European neighbours. That view is reinforced by a recent study sponsored by Sustainable Energy Ireland, a government-funded agency that researches and develops new energy efficiency policies. The study determined that Ireland has the potential to develop 2500 MW of wind power over the next decade, using both onshore and offshore resources.

Arklow Bank is a sandbank located approximately 10 km off the coast of Arklow, a town 64 km south of Dublin. In addition to the presence of strong winds, it was selected as the site for Ireland’s first offshore wind farm because the Bank’s shallow waters enable the wind turbine foundations to be installed at reasonable costs.

The foreshore lease to develop the Arklow site was secured by Airtricity in February 2002 following completion of extensive environmental and geotechnical studies. Erection of the project’s seven 3.6 MW machines was completed in only nine weeks. Commissioning of the units is currently in progress and the project is expected to begin producing power before the end of this year.

The wind turbine erection process began with the project’s seven monoplie foundations which were driven into the seabed using the same process used in bridge building. A transition piece which provides access for cables and maintenance workers was fitted over each monopile. Two tower sections were added to the top of each transition piece and bolted into place. Each nacelle, which holds the main working components was lifted to the top of each structure.

Finally, each rotor assembly was lifted from its horizontal shipping position to the vertical fixing position, and attached to the nacelle. All major components were staged and assembled at Rosslare Harbor in Ireland and transported to the project site, approximately 80 km away, as they were needed for erection.

When completed, the Arklow Bank wind farm will be connected to the public grid in Arklow, where GE Wind Energy also is establishing a permanent station to facilitate operation and maintenance of the wind turbines.

Offshore technology

One of the attractions of offshore wind power is that wind speeds are generally higher offshore than onshore, while wind turbulence is lower. However, the costs of construction and operation of wind turbines offshore are considerably higher than onshore applications.

The wind power industry has been striving to develop the technology to produce ever-increasing sizes of wind turbines, to achieve the economies of scale to make offshore projects economically feasible. As the first wind turbine expressly designed and optimized for offshore applications, GE’s 3.6 MW technology is a significant step in that direction.

GE unveiled the first commercial prototype for its 3.6 MW machine in Spain last year. It is installed on farmland leased by GE located 230 km southeast of Madrid in Castilla la Mancha. GE selected this location because the wind conditions there are close to those of offshore sites. The machine began producing power for Spanish energy supplier Iberdrola in September 2002.

Based on the proven technology of GE Wind’s 1.5 MW wind turbine series ” more than 1700 of which are operating worldwide ” the 3.6 MW machine is specifically configured for high-wind sites. Among its key features are an increased generator size, power electronics and a variable speed rotor (8.5-15.3 r/min). GE Wind Energy’s voltage control technology facilitates grid integration by improving grid voltage stability and overall system reliability.

A range of technical innovations was incorporated in the development of the 3.6 MW machine to keep the overall turbine costs down, including new blades, an improved gearbox concept and adjustments to the structure to enhance load absorption and optimize assembly, transport and service logistics.

The height of the 3.6 MW machine at its highest point is 124 m to blade tip, which is taller than a 30-storey building. The blades are 50.5 m in length, and the rotor diameter is 104 m, or longer than the wingspan of two jumbo jets. The swept area of each turbine is 8495 m2, or four times the sail area of a clipper ship.

A complete 3.6 MW unit weighs 290 t. The turbine is supported by a steel monopile foundation which is driven into the seabed by a hydraulic hammer. This equipment and process are widely used for similar applications, including installation of monopiles for bridges. The monopile and transition piece provide cable access to the tower of the turbine from the seabed, as well as boat access to the tower and associated equipment.

GE also has developed and is currently testing a variety of offshore project site access systems, including the use of a helicopter hoisting platform
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Installation and maintenance

To facilitate ease of handling, testing and transport of the 3.6 MW units, GE used a modular design where the lower bedplate, upper bedplate (nacelle), drive train, container and hub are assembled separately.

Offshore installations will require only a few lifting operations, including the tower erection process as well as the lifting of the complete nacelle and rotor. Prior onshore inspection of the nacelle will help to expedite the commissioning process of offshore wind farms such as the Arklow project.

The new wind turbine features an optional 40 t internal crane, allowing for the exchange of all major components including rotor blades, gearbox and generator without external support by heavy cranes. A second optional, two t internal gantry crane facilitates easy repair and maintenance in the nacelle.

The integration of an electrical container that houses sensitive electrical components (such as the control panel, converter, switching systems and transformer) directly beneath the nacelle allows better access during maintenance work, improved protection against corrosion, and excellent logistics for erection and repair work.

GE also has developed and is currently testing a variety of offshore project site access systems, including the use of a helicopter hoisting platform. With this system, a helicopter can hover several meters above a turbine platform as personnel and equipment are delivered and/or retrieved.

Key benefits of this system include safe access that is nearly independent from wind and waves, as well as quicker access to the project site compared to traditional boat transportation. The helicopter system can be employed for emergency evacuations as well as routine maintenance tasks.

Feasibility testing of the helicopter hoisting platform has been performed via simulated helicopter demonstrations at GE Wind Energy’s Salzbergen facility, to validate the operation of the 3.6 MW turbine’s hoisting system. Personnel can be lowered onto the turbine platform or picked up in less than 30 s.

Global showcase

The Arklow Bank wind farm is designed to be a global showcase for large-scale offshore wind energy projects. Ireland’s Marine and Natural Resources Minister, Frank Fahey, has cited Arklow as a significant step toward Ireland’s commitment to the Kyoto Protocol. He called the project, “The dawning of a new age of clean energy, harvested from two plentiful resources, the sea and the wind.” He voiced optimism that the project “will be the first of many and will help establish Ireland as a world leader in this young industry.”

Airtricity’s O’Connor noted, “This is a landmark demonstration project for Airtricity, GE Wind Energy and Ireland in general. We are pleased to be working with GE Wind Energy to facilitate the commercial installation of some of the world’s most advanced and powerful wind turbines.”

GE’s Zwolinski added: “We’re confident the Arklow Bank wind park will provide a showcase demonstration of how abundant offshore winds can play a greater role in helping to meet the world’s ever-growing energy requirements.”

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