Design of Laterally Loaded Monopiles for Offshore Wind Using Small-Strain Properties of Sands-Paper

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The study evaluates how to improve predictions of offshore wind monopile behavior in sand under small lateral displacements, where the conventional API RP 2GEO (2014) p-y method can underestimate initial stiffness and miss its nonlinear variation.<br /><br />Researchers developed a three-dimensional finite element model in Abaqus using a soil constitutive model calibrated to field and laboratory measurements of sand’s small-strain properties. The model represents the reduction in shear stiffness as strain increases and the increase in stiffness with mean effective stress. It was validated against the Mustang Island lateral pile test, reproducing both the measured load–displacement response and the pile’s capacity. The team then extracted p-y curves and examined the effects of pile diameter, soil-layer depth, and embedded-length-to-diameter ratio (L/D).<br /><br />The analyses indicate that API p-y curves underestimate initial stiffness and do not capture the nonlinear response at very small displacements. The difference between API and finite-element predictions diminishes with depth. When p-y curves are normalized by pile diameter, their shapes are broadly similar across the pile diameters and depths studied; diameter and depth affect ultimate resistance more than normalized curve shape.<br /><br />For service-level displacements, L/D has little effect on predicted response when L/D exceeds approximately 5. At larger displacements, embedment length, pile-tip resistance, and pile–soil interface friction become more influential, particularly at lower L/D ratios.<br /><br />The authors recommend revising the initial portion of sand p-y curves using site-specific small-strain properties, obtainable from field and laboratory dynamic tests. This could improve predictions of monopile stiffness under service loading, which is important for estimating offshore wind turbine natural frequencies.

Keywords

offshore wind

monopile

sand

p-y curves

finite element model

small-strain stiffness

Abaqus

API RP 2GEO

lateral pile response

service-level displacements