Nonlinear wave loads on monopile foundations and structural response in severe wave conditions

This study investigates nonlinear wave loading on monopile-like offshore structures subjected to steep, intermediate-depth focused wave groups—conditions representative of offshore wind turbine environments. Physical model tests in a wave flume were conducted to isolate and analyse first- and higher-order force components via decomposition techniques, made possible by repeating tests at different initial phase angles. A Stokes-type fitting of nonlinear force harmonics based on the linear component is improved by generalising the analytical formulation to arbitrary order. The new formulation is now limited only by the separability of individual harmonics used in the fitting. The Stokes approach effectively captures key harmonic features, preserving phase information more robustly than the nonlinear Transformed-FNV (T-FNV) force model, particularly at higher wave steepnesses. Nonetheless, the T-FNV model captures the fundamental force behaviour. To assess structural response under complex nonlinear loading, a simplified damped oscillator model is used. Dynamic response analysis reveals that the secondary load cycle contributes negligibly to the overall response in adequately damped structures. In contrast, slam loads impart sharp impulses to the system. Although damping mechanisms effectively dissipate this energy at the system's first eigenfrequency and lower modes, reduced damping efficiency at very high frequencies allows a small residual response to emerge. These findings support the use of computationally efficient nonlinear modelling approaches in wind turbine foundations and structural design, affirming that secondary nonlinearities and impulsive effects can be safely neglected for inertia-dominated loading and compact monopile geometries, with adequate damping, without compromising response accuracy.