Insight into the Cyclic Response of OWT Pile Foundations in Sand: Numerical Simulation of PISA Field Tests-Paper

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The study evaluates a three-dimensional numerical method for predicting the monotonic and cyclic response of offshore wind turbine monopiles in sand. The authors used the Ta-Ger sand constitutive model in FLAC3D to simulate medium-diameter piles tested at the PISA site in Dunkirk, France. The modeled piles were 0.76 m in diameter and 4 m long; analyses covered a monotonic test (DM4) and part of a cyclic test (DM2).<br /><br />The simulations reproduced key observed behaviors, including pile capacity, near-rigid rotation, soil gapping, and the cyclic increase in system stiffness. Under repeated loading, the model also captured the decreasing rate of accumulated pile displacement and rotation. Numerical response curves generally agreed with field measurements in both magnitude and shape.<br /><br />The analyses provided depth-dependent soil reactions, including lateral resistance (p-y curves) and distributed moments caused by vertical shear stresses around the pile. These results showed that calculated lateral resistance exceeded API estimates near the surface but was lower at greater depth, underscoring the limitations of applying conventional p-y relationships for smaller piles directly to monopiles.<br /><br />The findings indicate that the method can help explain pile–soil interaction and support the development of site-specific foundation design parameters. However, the sand model used a default calibration because site-specific cyclic laboratory data were unavailable, and the cyclic analysis required adjustment to account for loading that preceded the simulated test sequence. The field tests were conducted above the water table, unlike typical offshore conditions. Further testing and data—especially for saturated soils and depth-dependent pile response—are needed to assess drainage effects and refine the methodology.

Keywords

offshore wind turbine

monopile

sand

FLAC3D

Ta-Ger model

cyclic loading

pile-soil interaction

p-y curves

PISA site

numerical simulation