Structural condition monitoring of wind turbine foundations

Cases of excessive vertical displacement of wind turbines with inserted can foundations have been reported worldwide. Crack occurrence and propagation are among the critical factors that can reduce a turbine's capacity and alter its dynamic response. This paper reports on the use of a combination of two approaches to determine the nature of the defects around the bottom flange of an inserted can. A discrete strain monitoring system that used arrays of sparsely spaced sensors was used to capture the occurrence of cracks and monitor the crack-opening displacement pattern. Ultrasonic testing was adopted to locate defects and identify the approximate boundaries of cracks under the bottom of the inserted can. Two onshore 1·5 MW wind turbine foundations in northern China were selected to evaluate the performance of these monitoring approaches and improve understanding of foundation responses during construction and operation. The monitoring strategies were proven capable of capturing the occurrence and the change of cracks in a full-scale test. Results suggest that cracks in onshore turbine foundations would occur at an early age, grow gradually and fluctuate according to wind speed changes. Long-term monitoring of crack development will be required to ensure foundation integrity.