Dynamic Properties of Offshore Wind Turbine Foundations

Increasing oil prices and energy demands combined with a general acceptance that fossil fuels drive the climate changes justify the development of new sustainable energy solutions. Although offshore wind energy has proven potential to produce reliable quantities of renewable energy, there is a general consensus that offshore wind-generated electricity is still too expensive to be competitive with conventional energy sources. As a consequence, the overall weight of the turbine and foundation is kept to a minimum resulting in a flexible and dynamically active structural system—even at low frequencies. The highly variable and cyclic loads on the rotor, tower and foundation, caused by wind and wave loads as well as low-frequent excitations from the rotor blades, all demand special fatigue design considerations and create an even greater demand for a fuller appreciation of how the wind turbine ages structurally over its service life.

Well-covered in the field of earthquake engineering, the dynamic response of civil engineering structures is highly dependent on the impedance of the soil–foundation system. For offshore wind turbine applications, however, the hysteretical and geometrical dissipation effects in the soil are difficult to incorporate for time-domain simulations. Accurate assessment of the fatigue limit state requires simulations of several thousands of load cases, and the consequential high computational burden necessitates a structural model with few degrees of freedom that capture the most important effects of the dynamic wind turbine response. To overcome this, sequential or fully coupled aero-hydro-elastic simulations are often conducted where the soil–structure interaction is incorporated via the principle of an equivalent fixity depth or by a so-called Winkler approach with static springs along the foundation and soil damping applied as modal damping. The methods, however, do not account for the dynamic stiffness due to inertia forces, and a welldefined representation of the dissipation effects in the soil is neglected. This in turn forms the basis of the current thesis that examines the soil–foundation interaction and its influence on the natural and dynamic vibration characteristics of offshore wind turbines, and presents a novel, time-efficient coupled aero-hydro-elastic model of the wind turbine system accounting for the dissipation effects through wave radiation and material damping in the soil.

Modal properties in terms of natural frequencies and corresponding damping ratios of offshore wind turbines are investigated by full-scale modal testing and simple numerical quasi-static simulations. The analyses show distinctly time-varying inherent modal properties that, supported numerically, may be caused by moveable seabed conditions. In addition, “rotor-stop” tests and ambient vibration tests indicate the same level of damping related to the lowest damped crosswind eigenmode. The tendency is caused by the fact that the hysteretic soil damping, caused by the slippage of grains with respect to each other, is high during “rotor-stop” tests with low contribution of aerodynamic damping. The opposite holds for normal wind turbine operations.

Although the dynamic soil–foundation response can be calculated rigorously based on threedimensional elastodynamics with the coupled boundary element and finite element methods, these approaches are not applicable for coupled wind turbine simulations from a computational point of view. As a consequence, lumped-parameter models with frequency-independent real coefficients are applied in the thesis and successfully implemented into aeroelastic wind turbine codes. Time-efficient, semi-analytical solutions for the dynamic impedance functions of gravity base foundations and monopiles underlie the model calibration. Application of the fully coupled aero-hydro-elastic substructuring approach with deterministic and random linearised models of the soil indicates that the modal properties and cross-wind fatigue loads of offshore wind turbines are strongly affected by the interrelation effects between the foundation and subsoil.