Why measuring static forces is not enough
6. May 2021
The bathtub curve and the lifetime of an asset
4. June 2021The blades of wind turbines keep growing larger. But so do the forces on the structure. We offer a new way to offset this problem by monitoring the loads on screws and bolts to help ensure safer and longer operations.
Wind turbines keep getting bigger and bigger, especially the rotor blades. The longest blade has been recently produced, measuring in at 108 meters. The reason for building bigger blades lies in the relationship between wind power output and the rotor swept area. This is analogous to the relationship between the radius of a circle and its area. Thus, if the blade length increases by a factor of 2, the power production increases by a factor of 4. All things being equal, it increases electricity production. But living in a three-dimensional world means that the weight of the blade increases by a factor of 8. Ergo, the forces increase by a factor of 8 as well. So, whereas power output scales with the power of 2 of blade length, the forces increase with the power of 3. This is significant when we look at how forces affect the blades of a wind turbine.
The rotor hub and the rotor blades are exposed to external loads that are caused by wind and gravity. Apart from the dead weight of the turbine parts, gravity also gives rise to cyclic loads. They cause the blades to bend in the edgewise direction. The larger the blade, the more significant is the gravitational load. Loads derived from the force of the wind cause a flapwise bending because of natural variations in wind speed. Thus, the rotor and the blades are subject to high-level fluctuating forces that cause fatigue.
There are several ways to mitigate some of the disadvantages of having super long blades, but it is crucial to minimize the weight of the blades to reduce the forces that try to rip the blades off the hub and tip over the wind turbine. Additionally, continuous monitoring of critical components can help offset this problem.
The bolt as a force measuring device
The blades are attached to the hub via the blade root, which is the end of the blade nearest the hub. The root experiences the highest loads and is also the location that must provide the connection to the hub. Nowadays the blade-hub connection is held together by bolts, that when tightened creates a tension known as preload. These bolted connections sit right in the flux of forces at a privileged position because they are part of the rotating system of the turbine. That is why measuring the forces the bolts experience gives valuable insights into the operation of the whole structure. On the other hand, that is also one of the reasons why blade bolts are a load-carrying, critical component vital to structural safety.
The importance of maintaining the correct bolt preload cannot be overstated. The loosening of a bolt can lead to connectors slipping and over-tightening to bolt strength failure. Both are damaging to the structure. The Sensorise SmartScrew System offers continuous monitoring of bolt preload in real-time. Alerts are given based on pre-defined values that are indicative of failure. Although proper preload is imperative to the reliability of bolted connections, it is certainly not the only problem operators face. We know that a wind turbine is in constant movement and interacting with its environment. Therefore, capturing variations in loads due to the cyclic motion and wind conditions further improves our ability to predict failures.
Sensorise’s patented sensor technology also measures dynamic forces on bolted connections. SmartScrews are standard, normed bolts that also function as a force measuring device. So, retrofitting is like replacing old bolts. By installing SmartScrews at selected points in a blade flange, it detects load cycles, load spectra, eigenfrequencies, and bending moments. This opens up new avenues to explore predictive analytics techniques. In this way, wind turbines can grow bigger while we keep an eye on the forces affecting the structure. With the Sensorise SmartScrew System, we help you push the limits of wind technology.
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