Numerical Analysis of Offshore Wind Turbine Foundations in Dense Sand Considering Partial Drainage-Paper

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The paper examines whether offshore monopile and suction-bucket foundations in dense North Sea sand can be modelled as fully drained. Because these sands often contain fines and have relatively low permeability, large foundations under realistic loading rates may instead experience partial drainage or near-undrained behaviour.<br /><br />Coupled flow–deformation finite-element analyses were conducted for two case studies. For monopiles, a calibrated sand model was first checked against lateral pile tests at Dunkirk, then used to assess realistic storm loading. Partial drainage increased predicted capacity relative to the fully drained case. However, neglecting limits on negative pore pressure from water cavitation substantially overestimated resistance. The authors demonstrate a method for incorporating a cavitation limit in coupled analyses.<br /><br />For suction buckets, simulations using a calibrated bounding-surface plasticity model were compared with centrifuge pull-out tests. The predictions agreed reasonably well with the tests. Including inherent soil fabric anisotropy was important: omitting it produced unconservative estimates of tension capacity, particularly at faster pull-out rates, when drainage was more restricted.<br /><br />The study concludes that assuming fully drained behaviour can be conservative for dense sand, but partial-drainage analyses are only reliable when the constitutive model is carefully selected and calibrated. Models should capture state-dependent dilatancy and, where relevant, anisotropy; analyses should also account for permeability and cavitation pressure limits.

Keywords

offshore foundations

monopile

suction bucket

dense North Sea sand

partial drainage

fully drained behaviour

finite element analysis

pore pressure cavitation

soil anisotropy

coupled flow-deformation