A numerical study into the behaviour of monopiled footings in sand for offshore wind turbines

Behrang Pedram

    Research output: ThesisDoctoral Thesis

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    Abstract

    [Truncated] As demand for renewable energy sources increases, foundation systems that are capable of withstanding higher load capacities are required to generate more energy per structure. One such approach for wind powered generators is to construct a hybrid system that increases the lateral load capacity of the entire foundation. The proposed hybrid structure consists of a hollow pile/tower firmly attached to a circular footing.
    It has been discussed that for this proposed hybrid structure to be beneficial, its stiffness and ultimate lateral capacity must be higher than that of a single pile/tower. Additionally, the bending moments acting on the pile section under the ground for the piled footing structure must be smaller than that of a single pile/tower.
    A few scholars have discussed that the hollow pile/tower controls the initial behaviour of the proposed hybrid structure and benefits of the bearing plate were clear after the structure had rotated during their testing programme in a sand deposit, concluding that the stiffness of the hybrid structure was similar to the stiffness of the monopile at small displacements.
    In the first section of this thesis, equations that have been developed for short elastic piles embedded in an elastic soil medium are upgraded using 20 noded elements and an equation has been developed for calculating the horizontal displacement of a restrained elastic pile.
    In this thesis, the lateral capacity of monopiles, un-piled footings and piled footing structures installed in sand are explored. The behaviour of the structures were investigated through 3-dimensional finite elements analysis and a series of small-scale centrifuge model tests conducted by Harloe (2010). In the numerical models constructed for comparison purposes with the test results, the pile/towers were treated as an elasto-plastic material (with strain hardening) and the soil was idealised using the Mohr-Coulomb constitutive model with a linear and parabolic increase in the soil modulus.
    As there were close agreements between the test results and the FE predictions, it was clear that the ultimate lateral capacity and the stiffness of the hybrid structures were higher than that of a monopile. It was evident that the stiffness of the hybrid structures were higher than of a monopile from the initial point; this was also confirmed by the centrifugal tests. It was also demonstrated that the stiffness of the un-piled footing structure, used as a wind turbine, was higher than that of a monopile at low displacements; this point was also confirmed by the small-scale centrifuge tests. Moreover, from the results of the FE analysis, it was obvious that the initial behaviour of the hybrid structure was totally controlled by the bearing plate and not by the pile/tower.
    From the results of the numerical models and the centrifugal tests, it was apparent that the ultimate lateral capacity of the un-piled footing structures used as wind turbines, which are installed at shallow depths in a sand deposit, could be accurately calculated by a combination of the limit equilibrium equations and the equations used for VHM loadings. In addition, it was illustrated that for the un-piled footing structures resting close to the ground level in a sand deposit, the overturning capacity could also provide a reasonable estimate to the ultimate lateral capacity of the structure.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2015

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