Reviewing fastener standards may lead to fewer wind turbine failure-related accidents
Article originally published in Fastener + Fixing Technology, issue 9, April–June 2013 (Renewable Energy section). Read the original at issuu.com.
The number of wind turbine-related accidents globally reached a record high of 160 in 20111, including 11 fatalities. Turbine failure can also have a serious impact on energy supply security. Studies into wind turbine reliability show that vibration fatigue is the third most common reason for wind turbine failure2. When analysing the root causes of failures, the suitability of design methods and component standards has been called into question3. Introducing standards such as DIN 25201 may prevent vibration-induced fastener self-loosening, thus cutting turbine failure rates and resulting in reduced accident numbers and fatalities, increased energy security and lower operation and maintenance costs.
Wind turbines are heavy consumers of industrial fasteners
According to the American Wind Energy Association (AWEA), an average utility grade wind turbine with an output of 2 MW – 3 MW includes about 8,000 components4. Depending on the design, there may be up to 25,000 fasteners in each wind turbine, including bolts and studs.
As many as 500 large fasteners in the M30 – M45 size range are used in critical bolted joints, such as connecting the rotor to the main shaft and the bearing housings to the turbine’s nacelle5. Between US$15,000 and US$25,000 per turbine is spent on large fasteners5. AWEA estimates that in the United States alone, during 2009, over 5,700 turbines were installed consuming approximately 3.2 million bolts6.
The wind energy sector is a major consumer of industrial fasteners and demand is likely to increase as major windfarm projects continue across North America, Europe and Asia. The United States, for example, must install a further 60,000 new turbines by 2030 to reach its wind energy use targets7.
Impacts of turbine failure
The UK based Caithness Windfarm Information Forum has published a summary of wind turbine accident data, showing that official figures point to an average of 133 accidents per year, during the period 2007 – 20111. However, it acknowledges that the actual number of accidents involving windfarms may be as much as 90% higher.
Blade failure is the principal cause of accidents. It results in either whole blades or fragments being ejected by the turbine and causing damage when striking people and property. Fire is the second most common cause of accidents, followed by structural failure. Although structural failure is often caused by storm damage, the Caithness report notes that “poor quality control, lack of maintenance and component failure” cause turbine accidents.
Turbine failure can have a significant impact on the security of supply and the costs of running a windfarm. The AWEA highlights that “the cost of repairing a nacelle – which must be dismantled on the ground – easily runs upwards of US$250,000 and repairs often cost 10 times that of original manufacturing.”4
Wind turbines are typically designed for a twenty-plus year lifespan. Because windfarms are either offshore or in remote land-based locations, maintenance and repair can be disproportionately expensive. So it is in the interests of turbine original equipment manufacturers (OEMs) to design-out failure modes.
Furthermore, if wind turbine equipment is considered to be less reliable and of increased risk of failing to generate power, then windfarm financing and interest rates become more expensive3, which in turn reduces the commercial viability of wind energy projects.
Why turbines fail
In-depth research into wind turbine failure modes shows that “vibration fatigue is the third most common occurrence of failure in wind turbines, after corrosion and mechanical overload.”2 As one wind power engineering commentator puts it: “Wind turbines, like helicopters, are fatigue machines – always vibrating and working to loosen their fasteners.”
He continues: “More vibration will come as engineers develop longer blades to capture more wind energy and towers reach higher to accommodate them. These increased vibration levels may loosen some tower fasteners.”9
Further research confirms that “bolt fatigue and vibration loosening account for over 75% of all bolted joint failures.”8 This points to fastener failure, or failure of fastener securing and locking mechanisms, as a primary root cause of wind turbine failure.
However, as the AWEA confirms, wind turbine OEMs “are very conservative in their supplier qualification processes and have extremely demanding quality requirements. They require consistent and zero defect quality.”4
Why, then, are fasteners causing wind turbine failures if OEMs insist on such a high quality ‘cost of entry’? Could fundamental design and the types of component standards used be a significant contributory factor? Studies into wind turbine failures agree these factors may be the case.
Are standards and design methods causing wind turbines to fail?
Quantitative research into wind turbine operation and management costs has flagged serious concerns about the suitability of existing design methods and standards. In the conclusions to his 2006 survey of wind turbine reliability in the US Federal wind energy programme, Christopher Walford of Global Energy Concepts says: “The large number of load-carrying component failures has led to concerns about the applicability of the standards that are used for component design. Existing standards and design methods should be reviewed.”3
A European Commission-funded study into the reliability of offshore windfarms reached the same conclusion: “It is necessary to increase our focus on the following aspects: design, procurement, quality, risk assessments and quality assurance. New standards or practices regarding risk assessment and quality assurance of offshore wind-farm installations are required, and existing IEC standards may not be sufficient.”10
These are studies dating back to 2006 and 2007 respectively. The key standard, DIN 25201, which relates to the self-loosening behaviour of fasteners under vibration conditions, was not published until 2010. So, the wind turbine supply chain now has the tools to provide components that reduce failure risk.
Component standards that prevent self-loosening behaviour
The self-loosening behaviour of fasteners under vibration conditions is a well-known engineering phenomenon and was first theoretically explored by German engineer Gerhard Junker. In his paper – ‘New Criteria for Self-Loosening of Fasteners Under Vibration’11, Junker developed what became known as the ‘Junker Test’, which tests the self-loosening behaviour of fasteners under transverse loading conditions by vibration testing.
The Junker Test was incorporated into the German railways standard DIN 6515112 and became a standard fastener industry test to determine the self-loosening behaviour of a nut and bolt. More specifically, DIN 65151 was used to demonstrate the effectiveness of fastener locking mechanisms when under vibration conditions.
In 2010, a much more demanding standard was introduced, DIN 2520113. The underlying principle of the new standard is that vibration testing of fasteners and their locking elements should as closely resemble real life conditions as possible. It sets out exactly how the test should be performed and the equipment that should be used.
Fastener locking elements tested to, and satisfying the requirements of, DIN 25201 are significantly less likely to self-loosen and cause the failure of wind turbines. Adoption of DIN 25201 in the fastener industry is patchy so far, but as it becomes more widespread, specifiers in wind turbine OEMs will be able to genuinely compare the performance of competing fastener products and locking elements.
OEMs will be able to choose better quality fasteners that are far less likely to lead to their turbines failing. This, in turn, will cut the costs of operation and maintenance and the total cost of ownership. It will also increase the security of energy supply, and by adopting DIN 25201, wind turbine manufacturers may be able to prevent turbine failures that lead to accidents and deaths.
References
1. Caithness Windfarm Information Forum. Summary of Wind Turbine Accident Data to 30 September 2012. [Online] Caithness Windfarm Information Forum, 30 September 2012. Available from: http://www.caithnesswindfarms.co.uk/accidents.pdf [Accessed 30 October 2012].
2. Arabian-Hoseynabadi H, Oraeea H, Tavner PJ. Failure Modes and Effects Analysis (FMEA) for Wind Turbines. International Journal of Electrical Power and Energy Systems 2010; 32(7): 817-824.
3. Walford C.A. Wind Turbine Reliability: Understanding and Minimizing Wind Turbine Operation and Maintenance Costs. [Online] Kirkland: Global Energy Concepts LLC; March 2006. Available from: http://prod.sandia.gov/techlib/access-control.cgi/2006/061100.pdf
4. American Wind Energy Association. Wind Energy Industry Manufacturing Supplier Handbook. [Online] Washington: American Wind Energy Association; May 2011. Available from: http://www.awea.org/issues/supply_chain/upload/Supplier-Handbook.pdf
5. The Crown Estate & BVG Associates. A Guide to an Offshore Wind Farm. [Online] London: The Crown Estate; February 2010. Available from: http://www.thecrownestate.co.uk/media/211144/guide_to_offshore_windfarm.pdf
6. American Wind Energy Association. Anatomy of a Wind Turbine. [Online] Available from: http://www.awea.org/issues/supply_chain/Anatomy-of-a-Wind-Turbine.cfm [Accessed 30 October 2012].
7. American Wind Energy Association. Market Growth Potential. [Online] Available from: http://www.awea.org/issues/supply_chain/Market-Growth-Potential.cfm [Accessed 30 October 2012].
8. Srinnivasan S. Advances in bolting technology. Wind Systems Mag. [Online] December 2010. Available from: http://windsystemsmag.com/media/pdfs/Articles/2011_Dec/1211_LCT.pdf [Accessed 30 October 2012].
9. Bushing S. 2012 Trends in Bolting. Windpower Engineering and Development [Online] 1 June 2012. Available from: http://www.windpowerengineering.com/design/mechanical/fastening_joining/2012-trends-in-bolting/ [Accessed 30 October 2012].
10. Holmstrom O. Survey of reliability of large offshore wind farms. Part 1: Reliability of state-of-the-art wind farms. Project Upwind/DONG Energy Generation A/S. Report number 250490, August 2007.
11. Junker G, New Criteria for Self-Loosening of Fasteners Under Vibration, Transactions of the Society of Automotive Engineers, Vol. 78, 1969, pp. 314-335, New York.
12. DIN 65151:2002-08 Dynamic testing of the locking characteristics of fasteners under transverse loading conditions (vibration test), Deutsches Institut für Normung e.V., Berlin 2002.
13. DIN 25201-4:2010-03 Annex B Test specification for demonstrating the resistance to self-loosening of secured bolted joints, Deutsches Institut für Normung e.V., Berlin 2010.
Read the original article: Fastener + Fixing Technology, issue 9, April–June 2013, on issuu
