Dynamic response of offshore wind turbine considering sand stiffness evolution characteristics and scour effect

Abstract

Monopile offshore wind turbines (MOWTs) are located in complex marine environments and are subject to wind, wave, current loads and seismic action in service. Besides, MOWTs in sand are highly susceptible to scour under environmental loads and the dynamic response of MOWTs is affected by the combination of long-term cyclic loads such as wind, wave and current. In this paper, a numerical model of MOWT is constructed for dynamic response analysis under the combined effect of scour and long-term cyclic loading. The simulation of the cyclic loading characteristics of sand under horizontal cyclic loading is realized by the USDFLD subroutine and the MATLAB program, based on the loading and unloading secant stiffness evolution model of sand. The dynamic response analysis considering scour effects is conducted under different load conditions, which focused on the influence of different cycle times. The research results indicate that the variation patterns of horizontal displacement and bending moment of MOWT under different types of scouring and load combinations are similar, both increasing with the number of cycles. In considering the stiffness evolution characteristics of sand due to long-term cyclic loading, both local scour and global scour have different effects on the response of MOWT under different loading conditions. This suggests that the scour effects needs to be considered in the design of MOWTs under different loading conditions, and that local scour on monopile foundations cannot be directly substituted for global scour.