All blades are large, white, aerodynamic, and made of composite materials. But that is where the commonalities end. So when a large fleet includes a number of different turbine and therefore blade models, the right blade-maintenance programme requires it to be tuned to the idiosyncrasies of the blade, the environment and the ownership philosophy.
While a higher speed of blade means greater energy, it also means increased collisions between air-borne particles and the blade, so more damage. The longer the blade, the faster the tip speed. That means more restoration work and at a higher frequency.
Blade construction and materials vary by manufacturer, but the basic materials used are fibreglass and carbon fabrics, polyester and epoxy resins, foam and wood sandwich cores, and resin gelcoats. The precise materials used, and how they are applied and cured, will affect the resistance to UV rays, their hardness and resistance to erosion.
Additional features that are added to blades, such as spoilers, fences and vortex generators, may enhance performance by improving airflow on the blade, but because they are often applied with an adhesive, they can crack, split and even detach from the blade.
The leading edge of the blade, another common area of damage, can bubble, blister or peel from impact damage, which leads to excess noise, loss of erosion protection and increased blade drag.
Some manufacturers and owners apply a polyurethane material to the blade near the tip to toughen the leading edge. It commonly comes as a tape or a liquid that is brushed or rolled on to the surface. This, too, is subject to wear and will need to be replaced during the blade's life: every two years in an extremely erosive environment, every seven or less often in benign environments.
The wind farm owner's philosophy has a significant influence on the type of maintenance programme. If they are planning to sell the project in a few years, then investment in the long-term reliability of the blades will not be a consideration. But if they have a long-term outlook for the site, they will be inclined to keep the blades in top shape. Quite often it is the budget that determines the quality and amount of inspections and repairs that are completed each year.
Blade inspections vary greatly in quality and cost. The most basic inspection can be performed with binoculars from the ground or atop the nacelle. Unless augmented by a camera, the binocular inspection gives no photographic record, and the location and description of any anomalies found is by annotation only.
Spotting scopes - small telescopes that are commonly used by wildlife enthusiasts - can provide an inspection similar to binoculars, but with the option of variable magnification. Again, it does not provide a photographic record of the blade anomalies.
Telephoto camera inspections — usually using a 35mm lens with 300-600mm focal length — performed on the ground near the wind turbine provide a higher magnification inspection and a photographic record of the blade anomalies. The longer the focal length of the lens, the better to detect small fine defects.
Ground-based photographic inspection can go one step further by using a computerised and motorised scanning system that produces a complete photo-scan of the blade — from root to tip. This semi-automatic system allows for a complete record of the blade surface, giving a permanent visual history for future reference. It also provides a single mosaic image of each blade surface, putting them altogether as one image that has been optimised by image-enhancement algorithms.
By putting all the images together, the mosaic can be a significant time-saver in that it allows the inspector to review just one image of a blade surface rather than 20 to 50 singular images of segments of the blade surface.
Drones add another dimension to optical inspections, allowing the camera to scan the blade from a very close distance. This can be beneficial in certain weather conditions such as fog or rain, where the image from ground-based cameras can be degraded. The drawbacks to the drone approach include the added complexity and cost in the piloting of the drone, and legal restrictions — certainly within the US.
If your fleet inspections involve thousands of blades, it is important to keep all the inspection reports, images, blade condition and repair status well organised. Frequently, there is an interest in viewing the history of the blade by referring back to images taken during previous inspections to understand how the defect, such as a crack, has changed over time and its rate of development.
Categorisation of the defect should be based upon the repair criticality — the impact upon energy production, blade reliability, additional costs if repairs are deferred, etc. A simple A-B-C-D categorisation can be useful to assist in identifying and planning the most critical repairs that need the most immediate attention.
If all blades have been inspected and their conditions categorised, this is the starting point for blade repair planning. The most critical defects are identified first, and then there are other factors that will need to be considered to create the most cost-effective repair plan. These include other blade defects on the same turbine or at the same site, proximity and availability of the blade repair crews, the weather in region of the sites.
So, what is the right inspection and the pace of repair? That is the million-dollar question for every owner, but unfortunately, the answer is rather disappointing.
Vast differences in defect rates are observed and can be due to factors such as the blade materials, the quality of the blade manufacture, the site conditions and the air-borne particle density and hardness, the wind velocity and variability, and lightning intensity and frequency. These are some of the factors that present a complex relationship that determines the rate that the blades deteriorate at each site. The most effective inspection frequency for a given site is therefore best established empirically.
Ideally, the blades should be fully inspected each year until the suitable inspection regularity is revealed. Cost is a major consideration in the inspection schedule, but the inspection and repair costs should always be contrasted with the cost of not repairing the blades, which, when taken to the extreme, becomes a replacement cost rather than a repair cost, with additional outlays for lost energy production and removal and replacement costs.
Jon Salmon is technical services manager for EDF Renewable Services. He is speaking at the Windpower Monthly Blade O&M Forum, 10-12 November in Houston, US
Windpower Monthly Blade Operations And Maintenance Forum, Tue 10 Nov 2015 - Thu 12 Nov 2015, Houston, US