Design of Monopiles in the North Sea with the New PISA Springs, a Sensitivity Study-Paper

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The study compares PISA soil–pile interaction springs with conventional empirical p–y curves for offshore wind turbine monopiles. Using representative North Sea conditions, it models a 7.8 m diameter pile in homogeneous sand and clay under lateral loading, and evaluates deflection, permanent rotation, soil resistance mobilisation, bending moments and natural frequency.<br /><br />Overall, PISA springs generally predict stiffer behaviour than standard p–y models, particularly in clay. In sand, results are more comparable, with some conventional models producing responses similar to PISA. PISA predictions are highly sensitive to the soil’s small-strain stiffness, making reliable field and laboratory measurements of this parameter important.<br /><br />PISA springs are more nonlinear than traditional curves. This can lead to greater predicted permanent deformation in sand, although PISA clay models produce smaller permanent rotations than several conventional clay models in the cases examined. Maximum bending moments change only modestly, while PISA models generally predict slightly lower maxima and shift their depth closer to the seabed. Natural-frequency predictions also vary with the stiffness selected for calculation.<br /><br />The authors conclude that the choice of soil–structure interaction model can materially affect monopile design assessments. The rule-based PISA curves examined represent static loading and are intended for preliminary design; detailed design requires further calibration, including numerical modelling.

Keywords

PISA springs

p-y curves

offshore wind turbine

monopile

soil-pile interaction

lateral loading

North Sea

small-strain stiffness

bending moments

natural frequency