Proof of the benefit In that case, blades are like all other components. Minor damages or regular wear and tear are generally easy to manage, and there exists a range of inspection methods to identify these. So even though blades lack the monitoring systems that enable predictable maintenance on other components, blades can be maintained much more cost efficiently than we see in many cases today. A good case in action is a study carried out on a large blade population (2200 at study start, 4200 at study end) in Vestas’ fleet (see figure 1). In Vestas, blade damages are classified in 5 categories (see figure 2), depending on the severity of the damage and intervention urgency. Reducing the number of serious damages As the study reveals, the more damages detected and managed in the lower damage categories (non-serious), the less are registered in the higher damage categories (serious). In practice, this means that the amount of blade damages in category 4 and 5 (serious and very serious damage) dropped from 20.7% of all damages to 3.4%. The average cost of repairing category 4 damage is almost 6 times higher than the cost of repairing category 2 damage, while category 5 damage repairs are more than 12 times costlier than category 3 repair. This is a good example of the substantial impact an inspection and early intervention program can have on the condition of the blades, and subsequently on the costs required to maintain them. More support for increased focus on blade maintenance is found in the direct impact wear and tear damage can have on a turbines performance. Leading edge erosion, one of the most common wear and tear damage types, has a direct and accelerating effect on a turbines annual energy production (AEP). In cases where leading edge erosion is not detected and handled in time, it can reduce AEP by up to 3.5%. Thus, lack of proper blade maintenance not only results in higher costs, it will also lead to lower production and thereby impact asset revenue as well. Figure 1 Blade repairs by category, showing a decrease in category 4 and 5 damage from 20.7% of all damages in 2009 to 3.3% in 2014. Category 1 Category 2 Category 3 Category 4 Category 5 Scrap Total blades 2009 29.3% 24.8% 20.7% 14.2% 6.5% 1.7% 2203 2010 33.7% 27.4% 20.7% 11.7% 4.8% 1.7% 1806 2011 38.8% 31.7% 13.6% 11.0% 3.8% 1.0% 1437 2012 35.0% 26.7% 22.0% 12.8% 3.0% 0.4% 2797 2013 43.8% 24.0% 16.2% 12.3% 3.3% 0.3% 2847 2014 39.2% 39.8% 17.6% 2.6% 0.7% 0.1% 4176 Mapping an inspection and intervention program In practice, it takes careful planning and strong analytical resources to carry out an effective inspection and intervention program. It all starts with a site-specific inspection program that takes climatic conditions, turbine conditions, and resource availability into account. A standard blade inspection process flow includes the following steps: • Perform blade inspection by drone or a high-resolution camera and telescope., including photography of the entire blade surface — shells, root, leading edge and trailing edge. • The blade is reviewed for damage, such as leading-edge erosion, lightning damage, impact damage, cracks, broken or missing vortex generators and severe contamination. • Damages are evaluated and compared with historical data base points. • Possible repairs are generated and evaluated. • A detailed report, with recommended action, is provided based on the most cost-effective repairs and industryleading upgrades. • Repair is completed. The most challenging aspect in this process is often to avoid bottlenecks in the analysis and report creation work, where blade experts are required not only to identify and classify damage correctly, but also to recommend the most optimal solution. Here, higher quality of the inspection material, engineering know -how and increasing automation of the damage identification process are key drivers to improve the effectiveness of blade inspections. Currently, these areas are in rapid development as asset owners increasingly look for blade maintenance solutions. When damage has been identified and reported and a solution has been recommended, detailed planning is required to make sure that the chosen means of rectification is performed at the most sensible time. For most minor damages, the turbine can easily continue to operate for some period of time without potentially increasing the damage or in other ways affecting the production. Therefore, it should be an objective in a blade maintenance strategy to not intervene unnecessarily, as this would only lead to downtime that could be avoided with the proper planning. As a result, most blade maintenance work could be conducted in combination with scheduled maintenance work, when there are resources at the site and the turbines are down anyway.
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