Generalized lateral, rocking, and coupling stiffness equations for offshore wind turbine monopiles in clay

Harnessing wind energy from deep waters presents unique challenges for foundation design. These challenges arise from the potential for high bending moments and lateral loads, which must be managed while adhering to strict rocking limitations and natural frequency requirements. This research investigates the stiffness properties of commonly used monopile systems. Using finite element modeling, a three-spring soil–structure interaction model is formulated to represent the mechanical response of monopiles at the soil mudline, accounting for both the soil profile and pile rigidity. Closed-form expressions for foundation stiffness are obtained by fitting finite element results, enabling characterization of the stiffness properties with limited information on soil conditions and foundation geometry. Additionally, a normalization procedure is implemented to ensure the applicability of the equations across different pile diameters. Results indicate that the normalized stiffness properties of the monopile deteriorate with increasing Young’s modulus of the soil at 50% shear strain. Furthermore, the stiffness properties increase at different rates with increasing slenderness ratio (length ( L )/diameter ( D ) of the monopile). An examination of the normalization plots revealed three distinct zones: (1) a rigid pile zone where stiffness properties are governed by L / D , (2) a semiflexible pile zone where stiffness is influenced by L / D and pile rigidity, and (3) a flexible pile zone where stiffness properties are determined exclusively by pile rigidity. This research findings provide a theoretical foundation for the design of offshore wind turbine monopiles.