Technology improvements and growing maturity of the offshore wind industry have resulted in significant cost reductions and rise in demand. More can be achieved by focusing on improving the understanding of key design areas of an offshore wind turbine (OWT). Since fatigue is one of the main design criteria for offshore structures and little is known about fatigue cumulative development in time under operating conditions, it formed the basis of this thesis. However, fatigue is a complex phenomenon that is dependent on numerous interlinked parameters, such as damping, loading type, and stiffness of support structure (SUS). For the purpose, data analysis was performed on measurements which were gathered over a period of four years from a monopile-supported OWT site. The measurement data included information about the environmental and operational conditions of the OWT; and strain, acceleration and inclinometer readings from its SUS. Use of different data acquisition systems during the measurement campaign inherently required significant efforts to synchronise and preprocess raw data to appropriate state for data analysis. Time-dependent lag was identified via computation of cross-correlation sequences between data segment pairings, and the lag was successfully corrected. With the use of a standardized rainflow cycle counting algorithm, strain-derived moment time series were converted to constant amplitude events, which could be used to derive damage equivalent bending moments, M-N curves and fatigue damage accumulation frequency spectra. Short-term damage equivalent bending moment (STEL) was revealed to have a linear dependency with turbulence intensity when the OWT was in the run-up operational state. Additionally, the fatigue damage was demonstrated to be higher for downstream turbines due to wake effects. Also, fatigue life consumption of OWT SUS increases above rated wind speed conditions despite decrease in load magnitude due to increase in SUS response frequency. The latter was confirmed with fatigue damage accumulation frequency spectra, which revealed that the importance of high frequency band (1 – 5 Hz) increases in conjunction with wind speed at above rated wind speed conditions. However, most of the overall fatigue damage is accumulated at the low frequency band, which associated with wind and wave loading, first SUS bending modes, 1P and 3P rotor harmonics.

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.