Mechanism-Aligned Simplified Soil–Pile Interaction Models for Offshore Wind Turbine Monopiles in Sand

Monopiles are the predominant foundation type for offshore wind turbines (OWTs). Their diameters have increased substantially to accommodate larger structures, while current design approaches primarily rely on the API “p-y” model to simulate soil–pile interaction (SPI), which significantly underestimates the ultimate lateral pile capacity of large-diameter monopiles. Further, the API model accounts only for lateral soil resistance, neglecting mechanisms that substantially influence the lateral response of piles with low length-to-diameter (L/D) ratios, including pile toe shear, toe moment, and axial interfacial shaft friction. To address these problems, this study proposes a complete set of mechanism-aligned, spring-based SPI models capable of accurately simulating lateral pile response in sand across the full L/D spectrum typical of OWTs. The models include: a one-spring “p-y” model for flexible piles, capturing distributed lateral soil resistance; a two-spring “p-y + MR-θR” model for semi-rigid piles, which additionally accounts for pile toe shear and bending moment resistance against rigid-body rotations; and a three-spring “p-y + MR-θR + Mp-θp” model for rigid piles, which further includes rotational springs to account for distributed moment resistance due to rotation-induced shaft friction effects in sand. The derived spring parameter formulas have been calibrated using readily available engineering parameters, such as soil modulus, friction angle, and pile geometry. The three mechanism-aligned SPI models were validated against full-scale offshore monopile tests, centrifuge tests, and small-scale laboratory experiments, achieving less than 10% error in predicted pile capacities and less than 15% error in soil–pile coupled stiffness evolution.