Offshore wind power is undergoing intense growth in order to meet the European energy targets laid out for 2020. Research and development is being carried out into all aspects of the industry as it attempts to emerge as the main source of renewable energy throughout Europe.
An integral component of a wind turbine, the drivetrain is continually evolving as new concepts and designs are explored. As larger turbines are developed, and more wind farms are installed in deeper water, the service and maintenance of each component must also be considered.
The design of drivetrains is moving towards direct drive technology, and new initiatives are also being developed in hydraulic and hydrodynamic technology. All of the new concepts and designs have reliability as a central aim in development, as drivetrain failure is a big contributor to the time a wind turbine spends out of commission.
Operation and Development
Planned maintenance at sea is a much more complicated routine than on land, and unexpected service visits are highly unwelcome. A wind turbine drivetrain may have a yearly maintenance routine consisting of generator brush inspections, gearbox oil and filter changes, LSS bearing grease refill and container removal. The extra overheads of a boat, a crew, and sea trained technicians can easily amount to several thousand pounds; and the weather can delay schedule and increase the costs.
According to data gathered from existing offshore wind farms, drivetrain, generator, and gearbox failures account for around 14% of all down time for offshore turbines. Due to the complexity of repairs, however, they account for 39% of the time a turbine is out of commission per year. Typical failures include HSS generator and bearing damage, coupling degradation and internal gearbox component failures.
There are several causes associated with drivetrain failure, some specific to the offshore environment. Wind speed can increase considerably even when only a short distance from the shore; while this improves the potential for energy production, it significantly increases the stress and pressure placed on components and can cause fatigue damage far quicker than in onshore applications. Turbulence is also greatly increased at sea, and although designers plan the layout of wind farms to avoid it, turbines can still be subject to downstream turbulence from other turbines in the grid.
Direct drive technology is being developed to eliminate the gearbox from the drivetrain assembly altogether. While Hybrid technology is being developed to combine very simple and reliable gearboxes with direct drive systems. These are being designed with maintenance fully in mind, and monitoring systems are being developed which can highlight weaknesses before they become failures to proactively prevent down time.
ETI Drivetrain Test Rig
To support the Crown Estates’ round three offshore programs in the UK, the Energy Technologies Institute is building a wind turbine drivetrain test rig in Blyth, Northumberland. The rig will be open access and will be the world’s largest test center, able to test turbines up to 15MW. The indoor test rig is being designed so that the whole nacelle can be tested prior to installation, giving much more commercial security to large scale deployment.
Built on the Narec (New and Renewable Energy Center) site in Blyth, and known as Project Fujin, the test rig will enable drivetrain testing in different dynamic scenarios, and will test the whole drivetrain system. New technologies and prototypes can be rigorously tested before trial installation, helping to speed up the design process.
Opening in late 2011, the center will test gearbox design, validation and development, converter and control validation, grid disconnection simulation, component testing and research, environmental impact simulation, and lightening strike protection systems.
ETI’s Chief Executive, David Clarke, said of the test center, “This world leading facility will allow turbine manufacturers and engineering teams to test the reliability of their equipment under realistic load conditions without the expense and risk of deploying them offshore.”
Ricardo MultiLife Bearing
Ricardo was selected in collaboration with HORIBA to design plans for the construction of the ETI drivetrain test rig, and the company was also given a grant of three million pounds by the Northern Wind Innovation Program (NWIP) to develop durable gearbox bearings for wind turbine gearboxes.
A global company, Ricardo is a key developer within the wind turbine drivetrain sector, and is involved in the design of modular solutions for drivetrains. The MultiLife bearing project was initiated to increase the durability and reliability of gearbox bearings for use offshore. Bearings are prone to several different types of failure and in-service maintenance is very difficult due to conditions.
Research by Ricardo found that Wind turbine bearings with fixed inner races can wear in a 40 degree arc, resulting in premature failure. Ricardo has been tasked with developing a prototype bearing which will have a five times increase in lifespan. The final design of the MultiLife bearing provides several benefits; it can be retro-fitted to existing turbines with very little change needed to the gearbox. Ricardo is confident that this year it will complete the project and reveal a bearing with a five times increased life expectancy.
Serviceability of Drivetrains
The design and innovation of components and of the drivetrain assembly as a whole will lead to greater reliability, less breakdowns and less down time; but the turbines still have to be regularly serviced and maintained. Current data shows that offshore wind turbines run at an availability level of 97% on average. In practical terms this equates to four visits per year either for maintenance or repairs.
It is estimated that operation and maintenance costs could account for up to 30% of the energy cost, and manufacturers recently indicated that the cost of operation and maintenance is around £30,000 per turbine per year in the UK. The cost of offshore servicing is exaggerated by the cost of equipment, particularly vessels, as the price is driven up by competition from the oil and gas industry. Weather constraints can also affect budget and increase costs.
The current generation of wind turbines require servicing once every six months, and take between 40 and 80 man hours per year. Typically a major overhaul will be carried out every five years, and would need 100 man hours. Cancellations due to weather are estimates at 15% of service visits, so can have a significant effect on schedule and cost.
Substantial investment is being put into the development of more reliable drivetrain systems, to reduce the amount of maintenance. One area of development is computer controlled monitoring systems to react to faults, or highlight potential failures that can be rectified before they cause complete breakdown. SCADA (System Control and Data Acquisition) systems transmit signals and alarms between the turbine and the onshore control center. Minor faults can be resolved remotely from the control center, while more serious problems can be diagnosed early to reduce down time.
Summary
Wind turbine drivetrains as a concept, and at component level are undergoing rapid development. As one of the most significant contributing factors to the down time for wind turbines each year, the drivetrain is being revolutionized; along with the wind energy industry as a whole. It is unclear at this stage which technology will prove most suitable for mass production in the long term, so planning an operation and maintenance routine is a fluid process that must evolve with the design of each new system. As wind farms venture further out to sea in search of higher winds, so the turbines will increase in size, and may even exceed 10MW. The drivetrain solutions to suit that next generation of wind turbines will require their own custom maintenance plans.
Thousands of wind turbines will need to be installed over the next ten years if European energy targets for 2020 are to be met. This increase in volume of maintenance will change the landscape of the operation and maintenance sector of the wind energy industry. The cost of servicing is already at a very high level, because it requires specialist equipment, highly trained technicians and is reliant upon weather conditions.
Better reliability of the components within the drivetrain and longer service intervals built into design, can reduce the frequency of visits and cost of maintenance. Computer monitoring systems should also be utilized to relay detailed information back to the control center, but also used interactively to diagnose and repair faults. Automated maintenance, such as filter changes, can be implemented into design to make the turbine as ‘self sufficient’ as possible.
The next round of development across Europe will see several wind farm installations, and much of the new technology being developed will be utilized to make this generation of wind turbines more economic and efficient; some of the innovations will be on ‘trail’ under very real conditions. To minimize the cost surrounding breakdowns and repairs due to unreliable components, strategies to improve the reliability and serviceability of drivetrain systems should be implemented, and should be seen as a vital part of the design process.
Article by IQPC, a leading organizer of about 2,000 worldwide conferences, seminars, and related learning programs every year. The company is organizing the 2nd Drivetrain Concepts for Wind Turbines from 17 – 19 October, 2011 at the Swissôtel Bremen, Germany. Free whitepapers, articles and podcasts on drivetrains are available on the website.
