Dynamic Responses of a Wind Turbine Founded in Warm Permafrost

Abstract

Over the last decade, wind turbines have been constructed in several villages throughout western Alaska because of the abundance of wind sources. Because of the extensive existence of warm permafrost and the high cost of concrete foundations, many wind turbine towers are founded on steel-pipe piles connected by a prefabricated RC cap and thermosiphons. This type of foundation is softer than concrete foundations, and its stiffness is subject to variation due to seasonal and long-term temperature changes in permafrost and seasonal freezing and thawing in the active layer. Field performance data of wind turbine towers supported by such foundations are scarce. This paper describes a wind turbine founded on a steel-pipe pile group embedded in warm permafrost; presents climate data, site conditions and foundation performance of a wind turbine in western Alaska; and presents the dynamic response data during the 213–214 winter season. It identifies the seasonal variation of the fundamental frequency of the turbine tower–foundation system and potential operational issues for wind turbines founded in warm permafrost. In particular, this paper discusses in detail the resonance between the rotor and the tower–foundation system observed in the recorded data, and possible solutions to this issue. In addition, it presents the dynamic properties of the wind turbine foundation system, including modal frequencies and damping ratios identified by the Hilbert–Huang transform (HHT) using the resonance data. These findings and the dynamic properties could be useful in design and dynamic response analyses of wind turbines in cold regions.