Soil-Monopile Interaction: from Elastic to Elastoplastic soil reaction modelling under Quasi-Static Monotonic Loading

Abstract

Offshore wind energy's rapid expansion underscores the need for accurate and efficient methods to analyze the behavior of monopile foundations supporting wind turbines. While three-dimensional (3D) analyses provide comprehensive insights, their computational demands are significant. As an alternative, one-dimensional (1D) models with spring elements to simulate the interaction between the structure and the surrounding soil, offer efficiency and simplicity. Realistic soil behavior, characterized by elastoplasticity, necessitates proper calibration of the spring models in 1D analysis.
This thesis addresses the challenge of soil-monopile interaction analysis, specifically focusing on the monopile response under lateral static monotonic loading. The research commences by highlighting the development imperatives in monopile-founded offshore wind turbines. The first phase involves calibrating elastic springs through a comprehensive review of existing literature. This calibration accounts for variations in spring stiffness along the monopile's length. Subsequently, the study progresses to elastoplastic soil modelling, adopting a linear elastic perfectly plastic approach and employing only lateral shaft springs. Acknowledging the limitations of linear elastic perfectly plastic p-y response, new material models, namely a bilinear and an exponential model, are examined. A parametric analysis encompassing various monopile geometries and lateral load eccentricities is conducted. An optimization routine refines the bilinear and exponential model parameters to closely match 3D responses. The results demonstrate satisfactory agreement for the analyzed high L/D monopiles, yielding valuable insights and conclusions. However, the low L/D monopiles exhibit a less successful match, primarily attributed to the absence of rotational shaft springs in the analysis.
Furthermore, empirical design processes for applying the bilinear and exponential models are outlined. These processes are founded on the relationships between the model parameters and the length-to-diameter (L/D) ratio as well as the eccentricity-to-diameter (e/D) ratio. The study highlights the applicability of the bilinear model across various soil conditions, monopile geometries and lateral load eccentricities. In contrast, the exponential model's efficacy is constrained by the examined L/D ratios, warranting further analyses for expanded application.
In conclusion, this thesis presents a systematic transition from elastic to elastoplastic modelling for soil-monopile interaction analysis under static monotonic loading. The proposed bilinear and exponential models enhance the accuracy of 1D simulations, facilitating efficient design and analysis of monopile-founded offshore wind turbines. These methodologies contribute to the advancement of sustainable offshore wind energy, catering to diverse soil conditions and design scenarios.