Geotechnical Analysis of Monopile Foundations for Offshore Wind Turbines under Cyclic Loading
Pitkänen, Joonas (2025)
2025
Rakennustekniikan DI-ohjelma - Master's Programme in Civil Engineering
Rakennetun ympäristön tiedekunta - Faculty of Built Environment
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Hyväksymispäivämäärä
2025-11-12
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-2025111210547
https://urn.fi/URN:NBN:fi:tuni-2025111210547
Tiivistelmä
This thesis investigates the behavior of offshore monopile foundations under cyclic loading, with a particular focus on soil response in partially drained conditions. The primary objective is to evaluate an industry-friendly method for modeling cyclic soil behavior and to compare it with more advanced approaches from the literature, while also assessing the influence of drainage assumptions (1D vs. 3D) on monopile design.
A simplified iterative approach to cyclic loading is implemented in static finite element (FEM) simulations (PLAXIS 3D) and benchmarked against the advanced PDCAM model by Jostad et al. (2022) for North Sea conditions. The results show that the simplified approach, when combined with FEM analysis, reproduces the lateral pile deformations at seabed level obtained with the 3D PDCAM model, while remaining practical for engineering applications. The analyses demonstrate that cyclic degradation has a significant influence on soil strength and pile–soil interaction. The approach was further applied to a monopile structure considering an ideal soil profile in the Gulf of Bothnia, comprising glacial till and sand, showing how the cyclic degradation develops in layered seabed.
Three-dimensional FEM simulations of pore pressure accumulation under cyclic loading reveal a critical depth of maximum excess pore pressure at approximately 0.25D (where D is the monopile diameter) in homogeneous sand layer. Above this depth, pore pressures are dissipated by three-dimensional groundwater flow, while below it, accumulation increases until reaching a peak at around 0.5D. Nevertheless, the analysis demonstrated that modelling of three-dimensional flow towards seabed especially in shallow layers does not have meaningful effect to the monopile performance because of the low effective stresses and high strength utilization near the seabed level.
A simplified iterative approach to cyclic loading is implemented in static finite element (FEM) simulations (PLAXIS 3D) and benchmarked against the advanced PDCAM model by Jostad et al. (2022) for North Sea conditions. The results show that the simplified approach, when combined with FEM analysis, reproduces the lateral pile deformations at seabed level obtained with the 3D PDCAM model, while remaining practical for engineering applications. The analyses demonstrate that cyclic degradation has a significant influence on soil strength and pile–soil interaction. The approach was further applied to a monopile structure considering an ideal soil profile in the Gulf of Bothnia, comprising glacial till and sand, showing how the cyclic degradation develops in layered seabed.
Three-dimensional FEM simulations of pore pressure accumulation under cyclic loading reveal a critical depth of maximum excess pore pressure at approximately 0.25D (where D is the monopile diameter) in homogeneous sand layer. Above this depth, pore pressures are dissipated by three-dimensional groundwater flow, while below it, accumulation increases until reaching a peak at around 0.5D. Nevertheless, the analysis demonstrated that modelling of three-dimensional flow towards seabed especially in shallow layers does not have meaningful effect to the monopile performance because of the low effective stresses and high strength utilization near the seabed level.