Nondestructive Evaluation of Wind Turbine Towers

Wind energy is the world’s fastest growing energy resource. With the US goal of 20% wind power by 2030, manufacturers and their suppliers are facing enormous challenges.

One such challenge is the inspection of wind turbine towers. The cylinders forming the wind turbine tower are made from plated steel sheets. The sheets are rolled into rings and welded via submerged arc welding (SAW), forming sections of different lengths and thicknesses. Depending on the application, towers can contain anywhere from four to twelve rings. The current method of inspection on these longitudinal and circumferential welds is conventional manual ultrasound (UT). Welds are inspected with a minimum of two angle beam transducers (60 and 70 degrees). Each weld is scanned manually with each angle, so a total of four scans are performed on each weld (two per side). This method is extremely time-consuming, can vary from inspector to inspector, can miss critical flaws, and can misidentify flaws. Each of these can lead to a lot of unnecessary and expensive rework. A technique the can detect internal flaws quickly and reliably is needed.

Advancements in phased-array UT (PA-UT) technology over the past ten years make it an excellent candidate. Unlike single element conventional UT probes, PA probes are made up of multiple elements that can be fired individually under computer control to electronically scan, steer and focus the sound energy. Figure 1 is a PA-UT beam model showing how electronic focusing can be used to focus and shape the sound beam.

When using the electronic focusing abilities of PA-UT, more sound energy can be brought to bear on the fusion zone resulting in better signal-to-noise ratios and better detection capabilities. In fact, recent PA-UT modeling and simulation work predicted that planar flaws having a diameter in the range of 1 to 2 mm (0.04 to 0.08-in.) could be detected. Follow-up experimental work on samples with known flaws verified the modeling and simulation predictions.

Software used by most PA-UT systems allows large amounts of scan data to be interpreted quickly.

For instance, in a manual free-run PA-UT inspection of a wind tower approximately 14-mm thick, one probe would scan from 35 to 70 degrees. This would reduce the number of scans to two (one per side), but provides information from 36 angles. The use of sector scans also makes vertical (through-wall) sizing of flaws much easier. When PA-UT is used with an automated system (PA-AUT), tower sections can be scanned quickly and efficiently.

Five to 10 years ago PA-UT technology was gaining acceptance at a fairly slow rate. Instrumentation costs, limited portability, and complexity of the systems all contributed to the relatively slow acceptance of PA-UT. However, recent advancements in hand held PA equipment and software has made PA-UT a robust, reliable and cost effective NDT method for many inspection applications, including inspection of wind turbine towers.

EWI is currently conducting internal research to model two different joint configurations commonly used on wind turbine towers. One model will be performed to show the limitations of conventional UT. The next model will show how PA-UT can be used on these same joint configurations to improve the speed and quality of the inspections.

The primary goal of this internal investment is to begin to develop inspection techniques and procedures to meet the needs of wind turbine OEMs and component suppliers. If you are interested in learning more about EWI’s PA-UT capabilities or would like to discuss a specific inspection application, please contact Kevin Clear, kclear@ewi.org, 614.688.5243.

You can see some of the application of this technology used to address a specific issue in the oil industry in a video that EWI has posted on YouTube. http://www.youtube.com/user/edisonwelding