Coupled effects of long-term cyclic loading and scour on the mechanical responses of monopile-supported offshore wind turbines

Abstract

Monopile-supported offshore wind turbines (OWTs) inevitably undergo the coupled effects of long-term cyclic loads and scour, resulting in cumulative deformation and the migration of the natural frequency of the foundation-superstructure system and the simultaneous development of scour holes around monopiles, which brings notable challenges to the design of OWTs. Hence, a series of model tests on monopile-supported OWTs in sands were conducted in a flume equipped with a custom long-term cyclic loading device. The mechanical responses of the OWT system and the scour process under separate and coupled effects of long-term cyclic loading and scour were studied, with special attention given to the cumulative rotation, natural frequency, ultimate bearing capacity of the OWT system, and scour hole development. A preliminary investigation of the armoring effect of the overlying coarse-grained soil under combined action was also conducted. The experimental results revealed that the coupled effect of long-term cyclic loading and scour not only has a great impact on the mechanical response of the foundation-superstructure system, but it also resulted in an acceleration of the initial scour rate and reduced the equilibrium scour depth. Moreover, the coupled effect reduced the scour resistance of coarse-grained soils under the layered soil conditions.