Numerical and experimental investigation of modal-energy-based damage localization for offshore wind turbine structures

Offshore wind turbine structures are prone to deterioration and damage during their service life in harsh marine environment. To explore highly efficient and robust damage detection methods for offshore wind turbine structures, three well-known modal strain energy indices are reviewed first and then a new index named total modal energy method is proposed. The innovation of the new index is the simultaneous use of modal strain energy and modal kinetic energy. To investigate the feasibility and robustness of the four modal-energy-based methods, numerical and experimental studies are conducted on a tripod-type offshore wind turbine structure with simulated and measured data. It is indicated that all the four modal-energy-based methods work well with limited incomplete modal data, especially for the single-damage cases. While for the cases of multiple damage locations, the new total modal energy index significantly outperforms the traditional modal strain energy indices. Moreover, high robustness is shown for the indices, when the measured mode shapes of undamaged and damaged structures are polluted with the same noise level. However, when their noise levels have some difference, two of the modal strain energy indices turn invalid, but the new total modal energy index still shows stronger robustness. As frequencies are also used in the total modal energy index, its robustness to the noise in modal frequencies is also studied. It is shown that the results are slightly affected by the measurement noise in modal frequencies. Besides, the influence of finite element modeling errors is also investigated with both simulated and experimental data. Results show that all the four modal-energy-based methods are all very stable and insensitive to certain modeling errors. So, finite element model updating is not necessary in the test structure herein.