Long-term behaviour of offshore wind turbine monopiles in sand

This paper presents a series of physical modelling tests designed to investigate the horizontal displacement, secant stiffness, hysteresis and natural frequency behaviour of a wind turbine with a monopile foundation in sand under cyclic lateral loads. The study addresses the knowledge gap in the long-term assessment of these parameters through centrifuge tests, modelling the complete mass distribution of an offshore wind turbine – including the tower and rotor–nacelle assembly – under macrogravity conditions to provide a realistic simulation of soil–structure interaction and prototype stress–strain fields. The results show a high rate of horizontal displacement increase during the first 1000 cycles, when most of the displacements occur, followed by a marked decrease. An equation is provided describing the increasing behaviour of the secant stiffness with cycles. Hysteresis analysis of horizontal displacement shows a sharp reduction in loop area during the early cycles, followed by stabilisation, indicating reduced damping and energy dissipation as the response becomes increasingly more elastic. For natural frequency variation, measurements show a final increase of approximately 3%, smaller than the values reported in conventional (1g) physical modelling tests. A series of N-increasing free-vibration tests was carried out to show how the system’s natural frequency rises with the soil’s effective stress around the pile. From these results, an equation is derived to estimate natural frequency directly from design parameters such as turbine mass, pile geometry and soil vertical stress.