The Chair looks back at 15 years of experience in the dynamics and the design of offshore wind turbines. Recently, the PhD projects focus on the systems dynamics of offshore wind turbines, their behaviour under simultaneous aero- and hydrodynamic loads, and particular design methods that allow for further cost reduction. The research group works on three offshore projects and one related onshore project:

Future offshore wind plants located in water depths of more than 25 meters will have to be designed with a special focus on the occurring hydrodynamic loads. Thus, Tim Fisher develops in his PhD project "Mitigation of Aerodynamic and Hydrodynamic induced Loads of Offshore Wind Turbines" methods for the reduction of loads that make active use of control options of the turbine. In contrast and at present, fixed system properties are assumed for the design of the support structure and control is only used for the reduction of aerodynamic loads. In an exemplary manner, the project uses new approaches and evaluates them for virtual 5 MW and 10 MW wind energy plants:

• Use of collective and individual pitch for the active dampening of tower head vibrations
• Tower mass dampener
• Adaptation of the operation to meteorological and oceanographic conditions (frequencies and energy context of sea states, wind-wave-misalignment, wind-wave-load ratio
• Reduction of the effects of differing local conditions, like water depth and ground properties, through regulation and control

Two additional PhD projects result from research done at alpha ventus. Jan Quappen analyses the "Overall Dynamics and Integrated Design of Offshore Wind Turbines". His research centers around both:

• the verification of overall systems dynamics and loads of a offshore Multibrid M5000 on a tripod foundation at alpha ventus
• the refinement of procedures for the integrated design of OWEA

At first, measurements done by SWE at a prototype Multibrid M5000 located in Bremerhaven are evaluated. Once first offshore measurements are available the characteristics of the plant configuration will be analysed, as the hydrodynamic properties of the tripod, the stiffness of the structure as a whole, the integrated drive train and the controls. Then, the obtained data will be compared with simulation and measurement data from the same type of wind turbine from a different location, possibly with a different support structure.

Daniel Kaufer adresses in his PhD thesis problems related to the „Development of Design Methods and Requirements of Offshore Wind Turbines". He bases his work on research from Patrick Passon (researcher at the SWE until 31.01.2008) and himself, and focuses on the improvement and the validation of computational tools for the integrated analysis of offshore wind turbines with complex support structures. The methods will be validated by measurements of the systems dynamics and the loads of a Repower 5M on a jacket support structure at the alpha ventus wind farm. As in Jan Quappens work the particularities of the plant design will be analysed, as the jacket structure which behaves hydrodynamically fairly transparent, the relatively high stiffness of the structure, the modular drive train, and the specific control mechanisms.
To ensure the result are correct comparisons with other plants should be done. Those could be of the same type at different locations or with a differing support structure, such as gravity foundation and monopile. Simulation methods will be developed that are appropriate for the respective design phase, as parametric models or simplifying fast models for preliminary drafts.

[REpower Systems AG]