The structural response and thus the service life of offshore wind turbines is significantly affected by their first natural frequency. However, field measurements from offshore wind farms show that the field response of monopile-founded wind turbines is stiffer than expected, suggesting that improved modelling could lead to optimized cost-efficient structures.

This discrepancy may be associated to conservative foundation modelling. The current industry-standard finite element (FE) approach assumes monopiles as wished-in-place (WIP), thus neglecting installation effects. The goal of this thesis is to calibrate a 3D FE model to better capture the lateral response of impact and vibratory-driven monopiles in predominantly sandy soils under monotonic loading, particularly in the small-strain range that governs the dynamic response of offshore wind turbines, by accounting for installation effects.

PLAXIS 3D is used to model the soil-structure interaction with Hardening Soil elastic-plastic constitutive model that can capture small-strain stiffness (HSsmall). The study has a twofold scope: namely, establishing an interpretation scheme for initial soil properties to model the WIP response and incorporating installation effects through a practical approach that captures the effects of the installation on the soil state, and consequently the lateral capacity, without explicitly simulating the pile installation.

The WIP models, validated against a number of field tests, show that the current modelling approach can accurately predict the lateral response of vibratory driven monopiles, while it underestimates the stiffness of impact driven ones. Therefore, this thesis proposes to artificially incorporate installation effects into the established WIP FE models for impact driven monopiles, by either imposing volumetric strains to the soil plug or by modifying the coefficient of lateral earth pressure at rest. Calibration of these installation parameters is performed against the global monopile response and the post-installation horizontal stress profiles. Both methods lead to increased horizontal stresses around the monopile and result in stiffer global response and improved agreement with the field data.

This thesis offers a comprehensive framework for modelling the lateral response of monotonically loaded monopiles, including installation effects, that could potentially be adopted by industry thanks to its simplicity, computational efficiency and reliance on commonly available data in offshore wind projects.