Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Hesham Elnaggar

Abstract

Green energy resources are essential to meet the growing energy demands in the near future while reducing the effects of global warming. Offshore wind energy is one of the main efficient renewable energy sources which drive the ever increasing expansion of offshore wind farms globally. Wind energy technologies are improving making energy production more affordable, which helped Denmark, for example, to produce about 25% of its energy. One of the main challenges for offshore wind projects is the cost of foundation construction, which represents about 40% of the total cost. The investigated hybrid foundation system has the potential to reduce the foundation cost, while meeting the demands for performance and capacity for large wind turbines. The hybrid foundation system comprises a steel pile attached to a concrete plate to increase its lateral and rotational stiffness and capacity.

The main objective of this thesis is to examine the performance of the proposed hybrid system subjected to the environmental loads expected to act on the 5 MW National Renewable Energy Laboratory (NREL) wind turbine. To achieve this objective, both physical and numerical investigations were conducted to address several aspects of the problem. First, wind tunnel tests were performed on a scaled model (with 1:150 ratio) of the 5 MW NREL wind turbine at the Boundary Layer Wind Tunnel Laboratory in Western University. Force balance technique was applied to determine the different base load components under the ultimate wind loading considering different configurations and angles of attack.

A comprehensive parametric study was conducted employing three-dimensional nonlinear finite element models considering different foundations systems installed in sand and subjected to the measured wind loads, along with applicable wave loads for 20m deep water. The foundation systems included: monopile with diameter of 4 and 6 m and a hybrid system with pile diameter of 4 m attached to a concrete plate with and without ribs and plate diameter was 12 m or 16 m. For all considered foundation systems, the pile embedded depth varied from 8 to 36 m long. Different load combinations were examined for ultimate and serviceability static load cases.

The axial and lateral stiffness and capacity of the different foundation systems were evaluated and compared to lineate the advantageous effect of adding the plate to the monopile. The results demonstrated the superior performance and the higher capacity of the hybrid system and the potential cost savings associated with reducing the required pile diameter to support the 5MW NREL wind turbine. In addition, some guidelines are offered to evaluate the capacity of the hybrid system.

Finally, laboratory tests were conducted on scaled down foundation models under 1 g. The tests were conducted to evaluate the long term performance of the hybrid system under monotonic and cyclic wind loading conditions. Both the lateral and rotational responses of the foundation systems were evaluated under monotonic loading and after 10,000 cycles of loading. The test were able to detect the effect of adding the plate in the hybrid system to study its effect and its increasing in the rocking and lateral capacity. The results from the model tests confirmed the superior performance of the hybrid foundation system in terms of increased lateral and rotational stiffnesses, which is important for performance of supported wind turbines, as well as lateral capacity, which increases the factor of safety against excessive lateral displacement. Furthermore, the results obtained from the tests were employed to develop equations to predict the stiffness of the proposed hybrid foundation system.

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