Structural energy transfer to the elevated pile-cap foundation of an offshore wind turbine based on extracted transfer

In the long-term operation of an offshore wind turbines (OWT), the tower bears various harmonic excitations generated by the rotation of the impeller and other environmental excitations. The energy generated by excitations will be transferred to the bottom foundation through the tower. For an elevated pile-cap foundation, different piles receive different amounts of energy, and piles that receive the largest amount of energy are more likely to move than others, making it difficult to ensure the stability of the foundation structure. In this study, the structural energy transfer to the elevated pile-cap foundation of the OWT based on extracted transfer path analysis (ETPA) is studied by considering the impeller-tower-pile structure as a source-path-receiver system. The energy transmissibility functions between the impeller and different piles of the foundation are calculated to identify the pile that bears the maximum energy from impeller rotation or other excitations. Compared with the widely applied classical transfer path analysis, the ETPA method can overcome the noise generated by OWT operation by feature extraction and yield more effective results in the case of noise. In general, the pile that bears the most energy is more likely to move compared to other piles. The calculated results can be used to assess the stability of the foundation for taking corresponding measures according to project requirements and be used for vibration-based damage detection and structural health monitoring. To study the effectiveness of the ETPA method, a four-degree-of-freedom energy transfer path theoretical model, finite element model and physical model of OWT are used to numerical study, simulation and physical experiment, respectively. Computed results of transmissibility are compared with the maximum extreme displacement of piles to verify the validity.