Hybrid operational modal analysis of an operative two-bladed offshore wind turbine

Abstract

Two-bladed offshore wind turbines regained interest in finding the most profitable way of generating wind energy. Wind industry companies demand the safe operation of two-bladed offshore wind turbines. To guarantee safe operation, the companies perform operational modal analyses to investigate modal properties variation which might be allocated to damage. However, the operational modal analysis of an operative two-bladed offshore wind turbine faces multiple challenges. (1) Fundamental operational modal analysis assumptions about the applied loads are violated by environmental and operational loads. (2) The closely spaced modes of an offshore wind turbine are hard to identify. (3) An operative two-bladed offshore wind turbine is a linear time-variant system. This paper introduces an enhanced operational modal analysis procedure to overcome some of the mentioned challenges. The enhanced procedure incorporates a post-processing technique in a transmissibility-based approach. A developed representative model of an operative two-bladed offshore wind turbine is used to compare the enhanced procedure with the frequency domain decomposition method. Based on the comparison, this paper proposes a new operational modal analysis method that combines a transmissibility-based approach, the post-processing technique, and the frequency domain decomposition method. This paper proves that this proposed combined method is a promising new operational modal analysis technique that outperforms the enhanced procedure and the frequency domain decomposition method in identifying the modal properties of a two-bladed offshore wind turbine.