Abstract
With the gradual increase in offshore wind turbine capacity, large monopiles having over 10 m diameter for offshore wind turbines (OWTs) are progressively employed in deeper water to exploit more steady and reliable wind power. Numerical simulations are preferred for the prediction of the behaviour of the large-diameter pile. Given the crucial importance of the cyclic long-term constitutive behaviour of the soil to the numerical simulation, appropriate laboratory testing, to determine and calibrate constitutive soil parameters, is required. The current laboratory practice typically relies on triaxial and simple shear tests. However, it is generally recognised that soil element surrounding a laterally loaded pile is subject to a complex stress path.In this research, finite element analyses of large diameter monotonically and
cyclically laterally loaded pile are presented to evaluate the actual stress paths
experienced by soil element around the pile foundation. It is found that soil
elements typically follow multiaxial stress paths involving rotation of principal stress axes which are more complex than assumed by current practice. Assessments of the limitation of laboratory element testing in reproducing these stress paths are provided alongside a comparison of the soil behaviour observed between these complex and the standard testing practice.
Informed by the FE analysis, granular soil element samples have been tested in
the HCTA under cyclic stress paths which simulate loading conditions of soil
elements around an offshore monopile foundation. The influence of the simultaneous application of high number (up to about 3∙104) of axial and torsional stress cycles on the soil strain accumulation and stiffness evolution is presented.
Results show that the accumulated stain and degradation of small strainstiffness
are dependent on the direction of the principal stress axes. The results under the triaxial stress condition well fit the high cycles accumulation model (HCA) by
Niemunis et al. (2005). However, a discrepancy between the experimental results
under the combination of axial and torsional cycles and the HCA fitting curve is
observed. A new parameter is therefore incorporated for better capturing the
influence of multiaxial stress on the accumulated strain. These data will be valuable for the development and calibration of soil constitutive models to be used in numerical analyses and contribute to the pile design.
| Date of Award | 20 Jun 2023 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Erdin Ibraim (Supervisor) & Andrea Diambra (Supervisor) |