Offshore Wind Power Reliability, availability and maintenance (2nd Edition)
The development of offshore wind power has become a pressing energy issue, driven by the need to find new electrical power sources and to reduce the use of fossil fuels. Offshore wind farms can harness tremendous wind resources without annoying citizens and with a comparatively low environmental impact. They are thus becoming a central pillar of a carbon free energy system. However offshore turbines and wind farms are costly to install and maintain, making reliability and cost-effectiveness key issues. This work covers reliability of offshore wind farms as a whole, starting from weather and wind conditions, dealing with wind turbine technology, farm layout, monitoring, safety and maintenance. The thoroughly revised second edition additionally covers turbines of up to 10 MW, turbine design changes, turbine converters, HVDC converter stations and DC links, offshore sub-sea collector and export cables, and the structures supporting large offshore wind farms. Offshore Wind Power is essential reading for scientists, engineers, technicians and advanced students interested or engaged in the design of wind turbines, drivetrain technology and power mechatronics, in academia and industry.
Inspec keywords: wind power plants; wind turbines; offshore installations; safety
Other keywords: wind power plants; safety; offshore installations; wind turbines
Subjects: Reliability; Wind power plants; Plant engineering, maintenance and safety; General electrical engineering topics
- Book DOI: 10.1049/PBPO194E
- Chapter DOI: 10.1049/PBPO194E
- ISBN: 9781839533334
- e-ISBN: 9781839533341
- Page count: 401
- Format: PDF
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Front Matter
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1 Introduction to off-shore wind
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A detailed description of wind turbine development with photographs and key large wind turbine projects is given. Their evolution has been profoundly influenced by the la Cour wind turbine and reliability and availability issues. This design evolution, with competing two or three blade, upwind or down-wind HAWT, geared or direct drive configurations or VAWTs, has affected subsequent developments, which is interesting as the reliability of many of these early on-shore wind turbine prototypes was extremely poor. The large wind turbines at Grandpa's Knob, the USA, Orkney, the United Kingdom (UK) and Growian, Germany only operated for some hundreds of hours, suffering catastrophic failures in the turbine hub or blades.
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2 Reliability theory relevant to off-shore wind
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Off-shore wind turbines are connected by a network of HV collection cables to one or more off-shore AC sub-stations or HVDC converter stations, which in turn are connected via EHV AC or DC export cables to an on-shore AC sub-station or HVDC converter station, where power is transformed or converted to EHV AC and injected into the National Grid. The reliability analysis of such off-shore installations is made more complex because the turbines and off-shore substations are all subject to aleatory uncertainty, due to the stochastic effects of the weather. This relates to the wind itself from which an offshore wind farm extracts energy, the combined effects of wind, waves and weather on their structures and the corrosion of their submerged or exposed parts. Structural reliability of the offshore wind farm components for which predicted failure rates are <10-4 failures/year, but the probabilistic time distribution of those low failure rate events needs to be considered. The reliability analysis of such off-shore installations is made more complex because the turbines and off-shore sub-stations are all subject to aleatory uncertainty, due to the stochastic effects of the weather. This relates to the wind itself from which an offshore wind farm extracts energy, the combined effects of wind, waves and weather on their structures and the corrosion of their submerged or exposed parts. To track reliability changes with time of complex offshore power stations, during different operational phases, reliability growth models should be developed. The purpose of reliability data and analysis is not only to define the way in which wind power plants should operate but also to provide the numerical data to identify how their operational performance could be improved.
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3 Weather, its influence on off-shore reliability
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Having considered in previous chapter, the mathematics needed to understand the reliability of wind turbines, we now need to consider the random effects of the weather. Weather conditions are difficult to describe succinctly for engineering purposes and it is still not clear which aspects are important for reliable wind turbine operation. But the weather has been measured at sea since 1805, using the Beaufort scale, and this is a helpful starting point for understanding the impact of weather on off-shore wind farms.
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4 Practical off-shore wind farm reliability
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This chapter describes the reliability of current wind turbines and farms, using research from Tavneret al.(2006), Ribrantet al.(2007) and Spinato (2008), based upon on-shore and some off-shore data but completes with the most up-to-date data from Cervascoet al.(2021). Wind turbines are effectively unmanned robotic devices and it's relatively rare that that a stoppage can be readily classified as a fault It is important to define what a failure can be in order to define reliability.
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5 Wind turbine configuration and reliability
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This chapter has shown that wind turbine configuration does have an effect on reliability but there are some industry myths that are not supported by evidence. For example, it is simply not proven that a direct-drive wind turbine is more reliable than a geared-drive machine. Electrical sub-assemblies, such as the converter, appear to have improving reliability, MTBF, with time and lower MTTRs, whereas heavy mechanical sub-assemblies, such as the gearbox, have high MTTRs and are a mature technology whose reliability is unlikely to improve significantly. This suggests that in the long-term, all-electric wind turbines should have a more reliable future. On the other hand, there are some emerging drive train technologies, such as semi-direct drives with single-stage gearboxes, or hydraulic drive transmissions which allow the use of fixed speed MV generators, showing great promise for reducing weight, reducing balance of plant costs, and improving reliability, but their full production capital costs are as yet unknown.
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6 Design and testing for wind farm availability
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This chapter follows the previous one focusing on wind turbine design, consisting of several mechanical, electrical and auxiliary assemblies, as part of a larger wind farm, showing that these methods can be applied to assemble the objective of a complete wind farm's reliability.
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7 Early off-shore unreliability and availability
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The first large offshore wind farm in the world, consisting of 80 Vestas V80-2MW wind turbines each of 5,027-m2 swept area, was completed in 2002 in 6-14 m of water in the North Sea at Horns Rev, 14-20 km off the West Jutland coast of Denmark. The project was managed by the West Danish utility Elsam, then DONG Energy, now Oersted and the wind farm is connected via an offshore substation using 30-kV AC collector cables and to shore from the substation via a 150-kV AC export cable. Maintenance of the wind farm was conceived on the basis of using helicopters' access to individual wind turbine nacelles via specially designed access platforms built onto the nacelles to accommodate drops and lifts from the Eurocopter EC135.
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8 Off-shore wind farm layouts and grid connection
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This chapter has shown that there is a balance to be made, for availability, reliability and maintenance, in the device layout and grid connection of off-shore wind turbines and associated substation or converter assets, whether their substations are fixed to the seabed or are detachable, floating and moored. As the size of off-shore wind farms increase and we move from fixed to floating structures, the reliability of individual off-shore wind turbines becomes less important than the overall reliability and availability of the off-shore power stations that they form. In turn, this depends on their layouts and grid connections, the subject of this chapter.
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9 Monitoring for off-shore wind farms
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The monitoring of modern wind turbines and wind farms may include a variety of systems that can be generally classified as follows: Supervisory control, or signal conditioning, and data acquisition (SCADA) system, to provide low-resolution monitoring to supervise wind turbine and farms operations and provide a channel for data and alarms. Condition monitoring system (CMS), to provide high-resolution monitoring of high-risk subassemblies in wind turbines for the diagnosis and prognosis of faults, included in this area are blade monitoring systems, aimed at the early detection of blade defects. Structural health monitoring (SHM), to provide low-resolution signals for the monitoring of key structural items of fixed or floating wind turbines, substations and converter stations structures. These systems each have different data rates as summarised in Figure 9.1, but as the wind industry develops they are slowly being integrated together.
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10 Maintenance for off-shore wind farms
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The maintenance of wind farms has evolved organically from early days when many self-trained technicians maintained large numbers of small turbines on-shore in rural areas in Denmark, Germany and the USA, see Gipe (2016).This has grown to a much larger industry, where wind turbine original equipment manufacturers (OEMs), like manufacturers Enercon and Siemens-Gamesa, trained their own staff, but also included engineers trained by large operators, like Øersted (former DONG Energy). The costs involved in off-shore oil and gas maintenance regimes were initially unsupportable, for the lower value product throughput of inshore wind but gradually, experience and understanding between the oil and gas and the off-shore wind industry has led to improved maintenance methodologies with procedures based upon data and emphasis on accessibility and training becoming more important.
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11 Production safety, training and qualification
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Trained technicians may come into the wind industry from a variety of other relevant maintenance environments, such as power generation, automotive, oil and gas, aerospace and the armed forces. Their knowledge is making an important contribution to improving offshore wind turbine maintenance quality as well as increasing the quantity of trained staff. The off-shore environment is arduous and hazardous, and maintenance staff working off-shore must be well trained, not only to undertake the necessary maintenance tasks on our wind turbines, substations, converter stations and infra-structure but also to protect themselves and their colleagues from harm. Those staff also need to have specific training to undertake this important and very interesting work and the qualifications to reflect their education, training and knowledge.
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12 Overall summary conclusions
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The author suggests that the task of improved reliability and availability for offshore wind farms would be aided by the ready availability of more reliability data from original equipment manufacturers and operators to allow designs to be benchmarked against best practice. In the early days, the wind industry manufacturers were secretive about performance to protect their intellectual property and champion individual improvement. Operators have also been protective of wind farm performance data as those have contractual significance for their investors. But the industry is now of a size and professionalism where it could find a way to share within the wind industry operational data, in a non-competitive way, to improve collective wind power performance, as the industry comes into direct competition with other renewable and non-renewable power sources. The wind industry could share data to deal with the off-shore CAPEX and OPEX challenges and meet the competition head-on, Barberis Negraet al. (2007). An important reliability and availability issue will be to determine maintenance cost-effectiveness. Some operators are setting availability targets for offshore wind farms. There may be dangers in this approach since higher availability can always be achieved with higher O&M investment. The better path would be to determine the optimal O&M costs to achieve an acceptable availability and that will vary from wind farm to wind farm, depending strongly on the location of the site, being affected by distance offshore, local infrastructure and assets and their costs.
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References
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WMEP operators' report form
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Reliability data collection
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Commercial SCADA systems
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Commercial condition monitoring systems
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Reliabilities of key off-shore sub-assemblies
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Wind power historical timeline
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Back Matter
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