Wind-wave combined effect on dynamic response of soil-monopile-OWT system considering cyclic hydro-mechanical clay behavior

Abstract

Based on advanced clay-monopile-OWT numerical model, the wind-wave coupling effect was investigated. Compared to the case of wind load only, both the nacelle top displacement and the maximum monopile’s bending moment are smaller when the wind-wave load coupling is considered. The monopile-head displacement cumulation can cause the drift of the nacelle top displacements. Moreover, the wind and wave loads may cause opposite accumulative direction of the monopile-head displacement due to the difference in load nature. The displacement cumulative magnitude is positively correlated to the load magnitude. Hence, the monopile possesses better energy-dissipating capacity in the coupled load case. The equivalent damping ratio of the soil is about 3.2 ∼ 4.9% under the wind-wave combination, 1.2 ∼ 1.4% under the wind only and 2.3 ∼ 3.5% under the wave only, which agrees with superposition principle. Under the wind-wave loads, noticeable accumulation of excess pore pressure exists in the seabed soil. Compared to the separate load cases, the excess pore pressure in the coupling case may be smaller. Analysis also showed that mean effective stress of soil decreases with loading time due to pore pressure buildup, although the soil mean stress may increase when total stress method is considered.