Offshore wind energy is expanding rapidly as governments aim for net-zero emissions, with monopiles and jackets being the primary foundation methods for offshore wind turbines (OWTs). Supply chains, heavier turbines and deeper waters influence the efficiency of jacket foundations. This research considers a jacket-founded OWT. The development of OWTs has sparked its expansion to building sites with a proclivity to earthquakes. Such loading involves soil-structure interaction of which more needs to be known to improve accurate computational modelling.

Current practice of Siemens Gamesa Renewable Energy (SGRE) is to perform aeroelastic simulations in their computational tool BHawC. The Foundation Designer delivers a foundation Superelement to maintain secrecy. The equation of motion for the jacket foundation can then be solved linearly with a reduced amount of unknowns. However, prevalence of nonlinearity in soil raises the question to what degree a linear model adequately captures the response.

This study investigates the impact of soil nonlinearity on OWT dynamic behavior under seismic loads by comparing linear and nonlinear soil models. The analysis involves performing seismic simulations using multiple earthquakes. Another distinction is made through soil models with different characteristics. Nonlinearity is introduced to the soil stiffness and energy dissipation mechanism under cyclic loading. The investigated variations are a linear (elastic), geometrically nonlinear (nonlinear elastic) or both geometrically and physically nonlinear soil model (nonlinear plastic).

The numerical model consists of a Rotor Nacelle Assembly (RNA), tower, transition piece, jacket, piles and soil springs. Beam elements are used for the tower, jacket and piles. The transition piece is simulated by stiffening the top jacket braces. The RNA is modelled using a lumped mass with rotational inertia. It is vertically eccentric to the tower top and connected with a rigid link. The earthquake is applied uniform over depth and only horizontal movements are considered.

The findings underscore a difference in results between the linear and nonlinear models. The evaluated results from simulations consist of forces, displacements and dynamic characteristics of the structure. Also noted should be that the computational time of the linear model is significantly lower. The results found in models can differ greatly due to the loading spectrum with highly varying frequency peaks. Another factor is softening of the stiffness. The frequency domain of the elastic model results consists of narrow peaks at the system's natural frequencies. The peaks for the nonlinear elastic model are wider due to softening of the stiffness. For the structure used in this research, softening introduces coupled modes with greater displacements along the height of the structure. This makes it possible for the evaluated results to have higher values, even with less energy put into the system. The plastic models' peaks are of a width in between the elastic and nonlinear elastic model due to the combined use of isotropic hardening and nonlinear stiffness. When the model falls back on its initial stiffness upon unloading, the eigenfrequencies related to that stiffness become more pronounced. To match the occupancy of wider frequency peaks, loading and unloading should both happen nonlinearly. This can be achieved by using kinematic hardening instead of isotropic hardening. Plasticity generally reduces peak displacement and sectional moment values and nonlinear stiffness broadens the response frequency spectrum. Careful consideration of cyclic material behaviour, eigenfrequencies and loading characteristics are essential for a realistic model.

The increasing global demand of renewable and clean energy has led to the exponential growth, development, and interest in the offshore wind energy industry and expansion towards earthquake-prone areas. Offshore wind turbine structures, typically supported by a tubular monopile foundation, are increasing in size to meet the increasing human demands. In the constant ongoing debate in search of a balance between design accuracy and efficiency, general consensus is yet to be found in search of accurate yet simplified representation of soil-pile interaction. A Winkler foundation principle has shown to be a good compromise regarding this discussion but current knowledge is mainly based on (pseudo-static) small soil-strain inducing wind- and wave load-cases. Adopting this principle, the soil-structure interaction mechanism is represented by local lateral soil reactions (springs with distributed stiffness in mechanical formulation). Strong ground shaking induces shearing and volumetric variation of the soil particles. Through hysteresis the soil material exhibits energy dissipation: hysteretic damping. Accounting for hysteresis is considered computationally more demanding than when the soil continuum is assumed elastic but this is an assumption which only holds when soils undergo very small soil strains.
This thesis explores the amount of energy dissipated as a result of hysteresis during seismic response of an offshore wind turbine and the applicability of such damping in a local linear visco-elastic manner. In order to obtain insight in this nonlinear energy-dissipation mechanism associated with the hysteretic offshore wind turbine model under seismic excitation, a Python code was developed that calculates the energy dissipation of each load-cycle separately. The developed energy dissipation assessment algorithm is effective in application of arbitrary hysteretic response and unloading-reloading rules.
The hysteretic nature of the soil-pile interaction springs in question are calibrated against the widely applied API p-y, force-displacement curves. Unloading-reloading rules are specified to define the load-cycles. Boulanger et al. describes such unloading-reloading rules for pile application under seismic loading. The applicability of these backbone curves and unloading-reloading rules remains questionable in application of rigid monopile foundations. Despite not representing the accuracy of true soil-monopile interaction, obtained results in this research may support the exploration of innovative unloading-reloading rules.
The developed energy dissipation algorithm is proven to be a powerful tool in identifying the amount of energy dissipation over a total timeseries. Reasonable agreement in peak (maximum observed), Ultimate Limit State, deflection and bending moment seismic response at mudline and tower top has been found between a hysteretic supported model and equivalent elastic models using a single (load-dependent and depth-dependent) equivalent damping coefficient in parallel with each soil spring. Representing the hysteretic energy dissipation mechanism using viscous dampers with constant damping coefficients has therefore proven to be an effective modelling strategy to account for the damping mechanism of plastic unloading-reloading rules without accounting for hysteresis. The effectiveness of an equivalent elastic modelling strategy reduces when the response undergoes substantial permanent plastified displacements. A typical property which is unable to be simulated under the application of an elastic modelling strategy.

In the simulation of floating wind turbines, a traditional rigid floater assumption becomes less valid while pursuing large size floating wind turbines with steel-efficient floaters. Up to date, hull flexibility still cannot be efficiently incorporated into aero-servo-elastic-hydro simulation tools, and the possible influence of hull flexibility has not yet been well-understood. Consequently, it is necessary to identify the significance of hull flexibility and the possible effect of it.

Recent researches have been investigating the influence of hull flexibility on substructural internal load, global responses and dynamics of the system. However, little has been done from a tower design perspective. Moreover, tower design for a floating foundation has also been seldom documented. To fill the knowledge gap, two research questions are defined: What is the difference in tower design with a floating foundation? and What is the effect of hull flexibility on tower design?

To answer the first research question, a FEM model with rigid hull is built based on four floating concepts designed for DTU 10MW wind turbine. The tower fore-aft bending natural frequencies are compared between fixed foundation and floating foundation. The second research question is answered by developing a FEM model with flexible hull based on a spar-buoy concept. The rigid hull model and the flexible hull model are compared by implementing structural analysis and fatigue damage estimation under waves load.

The result shows that the 1st tower bending natural frequency increases significantly(except for TLP) from a fixed foundation to a floating foundation, making it difficult to achieve a soft-stiff tower design. Furthermore, it is indicated that hull flexibility can decrease the 1st tower bending natural frequency, and the magnitude varies with different tower designs. A stiff-stiff tower decreases more while a soft-stiff decreases less. Lastly, the fatigue damage estimation implies that a soft-stiff design can be lack of fatigue strength to survive from waves load.

In conclusion, a soft-stiff tower design is difficult for large size floating wind turbine partly due to the increase in 1st tower bending natural frequency from fixed foundation to floating foundation, and partly because of strength requirement for fatigue load. As for a stiff-stiff tower design, without considering hull flexibility, there is a high uncertainty in the 1st tower bending natural frequency. As a result, for large size floating wind turbines, inclusion of hull flexibility is necessary for the tower design.