Soil–Structure Interaction Effects in Offshore Wind Support Structures Under Seismic Loading

Abstract

This paper explores the performance of a 10 MW offshore wind turbine (OWT) supported either on a large diameter monopile or a 4-legged jacket emphasizing on the nonlinear response of its belowseabed foundation. The seabed foundation alternatives, a monopile and a multipod foundation, are compared under monotonic, cyclic, and seismic loading. For all nonseismic scenarios considered, the monopile is more flexible than the jacket and transmits higher rotations at the OWT base. The differences between the two alternatives are amplified in the case of nonsymmetric cyclic loading; the monopile not only deforms more than the jacket but tends to accumulate irrecoverable rotation with increasing loading cycles. The seismic performance of the alternative support structures is assessed for a comprehensive set of earthquake motions. It is concluded that both systems are seismically robust especially when subjected to pure earthquake loading, neglecting the simultaneous action of wind and waves. Alarming issues for OWT performance may arise when a nonzero steady wind force is superimposed to the kinematically induced stressing of the seabed foundation due to the seismic wave action. Jacket legs settle unevenly, while monopiles are building up rotations at increasing rates. Assuming a design-level earthquake and a wind thrust of the order 60% of the NC wind loading amplitude, this seismically induced residual rotation for the monopile may often exceed the deformation tolerance criterion. For the same loading combination, the corresponding rotation of the Jacket installation remains safely within the prescribed limits.