Experimental simulation study on dynamic response of offshore wind power pile foundation under complex marine loadings

Offshore wind power structures are subjected to complex dynamic loads in marine environment during their service. At present, the real sea conditions can be simulated to the maximum extent by the mixed test technology of boundary layer wind tunnels, wave pools and shaking tables. However, due to the high cost of test and the lack of a complete and mature combined simulation laboratory which can be operated for wind, wave and seismic loads, it is difficult to carry out such joint tests. It is advantageous to use self-developed mechanical devices to simulate marine environmental loads within the controllable errors and which is in simple operation. A series of dynamic model tests were carried out on saturated sand foundation in order to explore the feasibility of studying dynamic response of offshore wind power pile foundations through simulating marine environmental loads by simplified methods. Based on Stokes second-order wave theory and harmonic superposition method, the time history of wind, wave and current load is derived and programmed, then the wind, wave and current load on offshore fan model structure is simulated by using self-developed complex dynamic loading system. Finally, the dynamic response of single pile foundation in saturated sand under complex load is analyzed according to the test results. The test results show that the load displacement curve of single pile foundation in saturated sand is of work hardening type. The maximum pile bending moment at the peak of dynamic load increases with the increase of loading times, but the cumulative effect of bending moment caused by the change of load amplitude should be considered. The dynamic load in marine environment will cause the increase of pore pressure around the pile, but the increase of pore pressure on loading side is not obvious in comparison to that on the opposite side. The displacement change of pile body is basically in accordance with the fluctuation trend of test load and increases with load. The cumulative rate of displacement of single pile decreases with the increase of number of cycles under the long-term cyclic wind wave load. In this study, the method combining theoretical calculation of wind, wave and current load with model test of complex loading system can consider the characteristics of dynamic loads of marine environment more accurately, compared with the empirically determined cyclic load waveforms in conventional tests, the obtained response law is more in accord with actual sea conditions.