A Design Framework for Monopiles in Sand Based on Large-Scale Pile Load Tests and FE Analysis-Paper

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The paper presents a design framework for large-diameter offshore wind monopiles in sand, whose compact geometry causes them to rotate more like rigid bodies than the slender piles used to develop conventional API p-y curves.<br /><br />Developed through the German TANDEM project, the framework combines large-scale pile load tests with finite-element analysis. The simulations use the Coupled Eulerian-Lagrangian method to handle large soil deformations and a hypoplasticity model for dense sand. They were validated against tests on three impact-driven piles, then used to study piles with diameters up to 8 m.<br /><br />From the simulations, the authors derive four soil-resistance relationships: lateral shaft resistance (p-y), base shear resistance, rotational shaft resistance, and rotational base resistance. A general quadratic/hyperbolic function is calibrated to represent these relationships, with parameters dependent on pile diameter, depth, soil unit weight, and friction angle. The resulting curves fit the finite-element results reasonably well and can be used in one-dimensional beam-on-elastic-foundation design analyses.<br /><br />A redesign of an 8 m reference monopile using the new approach predicts greater soil stiffness but lower capacity than the conventional API-based design. The required penetration increases from 32 m to 35 m, while the first natural frequency rises by 4%. Reduced fatigue loads allow thinner walls, producing an estimated 8% reduction in steel weight—about 100 tonnes—despite the increased pile length.<br /><br />The method currently applies to drained granular soils under monotonic loading. Installation effects were not included, and the framework is based on tests using one sand type. Further testing across different soil conditions and validation for cohesive soils and cyclic loading are needed.

Keywords

offshore wind monopile

large-diameter piles

sand soil resistance

API p-y curves

finite-element analysis

Coupled Eulerian-Lagrangian

hypoplasticity model

rotational shaft resistance

monopile redesign

steel weight reduction