Experimental study of improved modal strain energy method for damage localisation in jacket-type offshore wind turbines

An improved modal strain energy method is proposed for damage localisation in jacket-type offshore wind turbines by defining a series of stiffness-correction factors that can be employed to calculate the modal strain energy (MSE) of the measured model without utilising the stiffness matrix of the finite element model (FEM) as an approximation. The theoretical contribution of this article is that the MSE of the measured model could be estimated with better accuracy, and the advantage of the proposed indicator is that it is more sensitive to damage locations than the traditional MSE method. Numerical studies on a tripod offshore jacket wind turbine reveal that the proposed method could locate the damage positions for jacket-type offshore wind turbines when limited number of lower-order modes is available, even when these modes are spatially incomplete. The performance of the proposed method is also investigated using real measurements from a steel jacket-type offshore wind turbine experiment conducted in a water tank of Ocean University of China. The experimental results demonstrated that the proposed method outperforms the traditional MSE method, and damages in jacket-type offshore wind turbines could be properly located utilising the first two measured modes excited by environmental loadings, such as waves, currents, or the vibration of the wind turbine.