Successful Application of ShafTest® Technology in Wind Power

07/12/15

Bureau Veritas Greater China Wind Power Operation Manager, Tian Peng

As the main transmission part to support conversion of kinetic energy from the wind into electrical power, wind turbine main shaft is the most critical component of wind turbine. Main shaft may cause hazardous defects due to factors like large workload and fatigue during wind turbine running. The main shaft is a transmission equipment with hard-to-disassemble components such as bearing on its surface, so it would be a major challenge to check for hazardous defects within the main shaft without disassembling it. ShafTest®, a self-developed technology of Bureau Veritas Australia, can help solve this issue while providing solutions for more reliable and accurate assessment on the structural integrity of in-service wind turbine main shafts. 


About ShafTest® technology


ShafTest® is a portable Ultrasonic (UT) flaw detection system designed specifically to improve current UT techniques for monitoring the condition of shafts and pins. ShafTest® acquires signals through at least one full end surface. A complete plane signal distribution diagram is generated through integrated processing by software. The variation in signal strength is displayed according to a specified hierarchical distance, allowing for assessing the existence of any defects and their sizes. 


ShafTest® is mainly used for detection and prevention of hazardous defects like broken main shafts, tracking and monitoring of the progression of possible defects, and periodic testing and maintenance of main shafts. 


What are the key benefits? 

1. ShafTest® detects cracking early, avoiding unexpected failures and allowing for more efficient maintenance planning. 

2. ShafTest® only requires access to the end (s) of the shafts. 

3. ShafTest® results can be completely analysed away from the test shaft, minimising potential access issues and downtime of equipment. 

4. ShafTest® delivers major cost savings by maximising the service lifetime of shafts and pins and avoiding catastrophic failures.  





General procedure for detecting in-service wind turbine main shaft with ShafTest®


The ShafTest® system comprises an ultrasonic signal module, an electronic computer and a ShafTest® software processing toolset. 


The ShafTest® inspector must be specialized in conventional ultrasonic test techniques. The personnel to acquire signals must hold Level II or above qualification certificates for ultrasonic test. Only authorized personnel who are trained for ShafTest® technology are qualified to evaluate signals. 


The ShafTest® inspectors start by modeling in the ShafTest® device based on the wind turbine main shaft drawing. The wind turbine is shut down and the hub braked after the inspectors arrive at the wind farm. The ShafTest® inspectors plan out the data acquisition points on the end surface of the main shaft within the wind turbine's hub based on the site condition. Signals are acquired at these points with the ShafTest® device. General single-crystal straight probes should be used to acquire data. Depending on the material of the main shaft, a probe with proper frequency and diameter should be selected to suit the need for data acquisition. In the case of fine-grained material, a high-frequency probe (4MHz or 5MHz) is recommended. A low-frequency probe (0.5MHz - 2MHz) would be a good option for a main shaft with ultrasonic signal serious attenuation in material or coarse-grained material. A large-diameter probe is recommended for end surface with a large diameter, and vice versa. Data acquisition for the wind turbine main shaft should be performed twice at a minimum gain difference of 20dB, with the acquired data being stored into the electrical computer.


The ShafTest® test data should be evaluated according to general ultrasonic test standards as well as ShafTest® technical specifications and main shaft inspection standards. The ShafTest® evaluator is responsible for performing software-based analysis on the acquired signals to generate a sound pressure distribution map. The analysis is done according to a specified hierarchical distance to identify the existence of any defects and their sizes. 





ShafTest® Case Study: ShafTest® for In-service Wind Turbine Main Shaft


Background: 


On June 1, 2010, a main shaft with 12-year service in a wind farm cracked down, causing its hub and three blades to fall off the wind turbine and wrecked. 






Analysis of causes:


Being in-service for 12 years, the main shaft had long been subject to the influence of fatiguing workload. As the main shaft and the gearing drifted apart, the gearing surface was under uneven pressure, started to craze and finally cracked. 


Challenge

As restricted by the site condition, there was no contact surface available for inspection by conventional ultrasonic test (UT), magnetic test (MT) and penetrating test (PT) in the area of main shaft behind bearing without disassembling the main shaft. Radiographic test (RT) couldn’t work, either. At that point, the inspection could only be performed on the end surface of the main shaft. Using A-mode pulse signals, conventional UT delivers a relatively low display resolution for the outer edge of the main shaft. On top of that, structural deformation of the main shaft and shear signal and longitude-to-transverse conversion of ultrasonic signals and corner reflected/refracted signals may lead to false positives or false negatives, especially making it hard to evaluate micro-cracks that results in broken main shaft. 


ShafTest® as the Solution

As requested by client, the inspectors tested the main shafts of the other 32 wind turbines using ShafTest® technology. A single-crystal straight probe with a frequency of 4MHz and a diameter of 20mm was used to acquire signals from the grid on shaft end surface. First, signal acquisition was carried out on the wind farm's spare main shafts (new satisfactory ones) using ShafTest®. Signal acquisition was then performed twice for the in-service main shafts, with the first getting done when the amplitude of bottom-wave signals reached 80%, and the second at a gain of increase 20dB to the first time. Subsequently, the quality status of the in-service main shafts was evaluated through comparison with the new satisfactory shaft data. (With ShafTest®, it’s best to evaluate data comparison based on the original data of the part itself. Since no data acquisition had been performed on the in-service main shafts, the new satisfactory shaft data had to be used.) By doing this, the existence of hazardous defects within the main shafts was identified.


Detection results


Only two of the 33 inspected in-service main shafts were detected with abnormal signals, which indicated possible surface cracking. The wind farm was urged to shut down the wind turbine and disassemble these main shafts for maintenance. Conventional UT, MT and PT were done on the disassembled main shafts and both of them were detected with surface cracking in the same position as shown in ShafTest®, which demonstrated the reliability and viability of ShafTest® in detection of in-service wind turbine main shafts.





Conclusion

ShafTest®, an advanced ultrasonic detection technique designed to detect hazardous defects in shafts, has strong sensitivity to fatigue-induced cracks and other similar hazardous defects in in-service wind turbine main shafts. The detection system also proved its reliability and accuracy while being used in the above case. As a novel detection technique for shafts, ShafTest® helps overcome the challenges in in-service detection of wind turbine main shafts and provides effective data support for quality tracking and maintenance of them. 

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