Comparative Assessment of Geotechnical Design Concepts for Offshore Monopile Foundations in Sand and Clay

Modern offshore wind turbines require increasingly larger monopile foundations, often with low embedded length-to-diameter ratios that fall outside the calibration range of the conventional standard p–y approach. Consequently, existing code-based p–y methods may provide an incomplete representation of the three-dimensional soil–structure interaction mechanisms mobilized under lateral loading. This study presents a comprehensive assessment of current geotechnical design concepts for laterally loaded monopiles embedded in clay and sand. To this end, three-dimensional finite element (FE) analyses were performed and used as the benchmark for comparison. The analyses employed the NGI-ADP constitutive model to simulate clay and the HSsmall constitutive model to represent sand. The FE models were validated against the PISA field test results from the Cowden and Dunkirk sites prior to their use in the subsequent evaluation. The validated FE models were then used to extract PISA-based p–y curves, which were compared with the code-based formulations for sand and clay at both the local p–y level and the global load–displacement level using 1D Winkler analyses. In addition, a moment contribution ratio (MCR) analysis was carried out to quantify the relative contributions of the individual resistance mechanisms governing monopile behaviour in both sand and clay. FE-extracted PISA p–y curves consistently produced the closest predictions of monopile behaviour captured in the FE analysis, while conventional code-based p–y formulations underestimated resistance in clay and overestimated it in sand. MCR analysis confirmed that lateral p–y reaction is the dominant resistance component, but shaft friction and base shear are important for short monopiles, especially at small displacements; incorporating these components into FE-calibrated 1D Winkler models substantially improves agreement with 3D FE predictions while retaining computational efficiency.