Incorporating the nonlocality of a nonlinear continuum in an effective 1D model for monopile foundations
T. Ntouras (TU Delft - Mechanical Engineering)
A.V. Metrikin – Mentor
J.S. Hoving – Graduation committee member
Federico Pisano – Graduation committee member
Karel van Dalen – Graduation committee member
Willem Geert Versteijlen – Graduation committee member
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Abstract
World energy consumption rises every year. Therefore, producing renewable and
sustainable energy is necessary to protect the viability of this planet. The most well-known and promising form of renewable energy is that of wind power. The limited space in terrestrial areas yielded motivation to the growth of offshore wind industry. Offshore wind turbines (OWT) are most often founded on large-diameter monopiles (MP). Despite of the significant amount of units installed, their design engages Soil Structure Interaction (SSI) models based on slender pile response. The current research focuses on developing a nonlinear nonlocal 1D effective model able to predict the nonlinear response of such a foundation under static loading, incorporating their rigid behavior. This is considered a first step so as advanced nonlinear SSI is incorporated in the design of monopile foundations.
The 1D model is based on Timoshenko beam theory, assuming the beam (representing the MP) is founded on a distributed, Winkler-type foundation, consisting of two types of fully coupled springs, lateral and rotational. These springs allow the model to capture the 3D continuum reaction of the soil towards rigid behaving monopiles.
This type of behavior mobilizes a large volume of the continuum. The term
nonlocality describes this global soil response towards the pile. In a nutshell, perturbations in a 3D Finite Element (FE) continuum are performed to capture the global reaction.
The soil profile considered in this study is homogeneous sand. In the Finite Element model it is modeled with a nonlinear constitutive model. This means that a nonlinear relation between the loading on the monopile and the respective displacement exists. The resulting nonlinear response should be captured in the 1D effective model. This is achieved by calculating the stiffness terms of the 1D model for more than one loading combinations on the MP. The obtained nonlocal stiffness matrices contain a 3rd dimension along which the nonlinear soil response is captured. The nonlinear nonlocal stiffness matrices are directly applied in the 1D effective model.
To visualize the performance of the developed model, a comparison between
horizontal force - head displacement and overturning moment - head rotation curves calculated by 3D Finite Element Analysis and computed by the 1D effective model is made. The comparison is promising for the examined cases. However, there are still steps to be taken in order to further validate the method and expand its applicability.