The rise in global energy demand, combined with the imperative to reduce CO2 emissions, has driven significant investments in renewable energy technologies. Offshore wind energy is a critical part of this transition, given its potential to provide large-scale clean energy. However, one of the main chal- lenges facing the offshore wind industry is the high levelised cost of energy (LCOE), driven primarily by the costs of operation, maintenance, and the construction of support stuctures and foundations. Reducing the LCOE is essential to make offshore wind energy more competitive. Innovations in tur- bine design and installation techniques can play a key role in achieving this goal. A good example is the introduction of slipjoint connections, which simplify the installation process and reduce material use. Similarly, new turbine concepts, such as the hydraulic pump generator by Delft Offshore Tur- bine (DOT), aim to reduce the weight and complexity of the rotor nacelle assembly (RNA), potentially cutting both maintenance costs and structural demands.

This thesis investigates the effects of the lighter RNA, due to the innovative hydraulic generator pump, on the support structure. The DOT turbine employs a hydraulic pump directly driven by the rotor, eliminating the need for a gearbox and reducing the RNA weight by up to 50%. Moreover, the hydraulic pump uses significantly less space in the nacelle. This reduction provides an opportunity to enhance the support structure and incorporate dampers in the nacelle to mitigate vibrations caused by environmental loads. In the second half of this research, the potential of a gyroscopic damper to improve the dynamic response of wind turbines subjected to wind and wave loads is explored. The focus lays on the reduction of the undamped side-to-side vibrations.

The research objectives include evaluating the impact of the lighter topside on the design of the support structure and evaluating the effectiveness of gyroscopic dampers in reducing fatigue and wave-induced vibrations. To this end a comparison between a conventional wind turbine support structure and the lighter DOT design is made. The dynamic response of the system is analyzed through ultimate limit state (ULS) and fatigue limit state (FLS) evaluations, using simplified, but accu- rate, dynamic models. In addition, the performance of the gyroscopic damper is modeled and tested to determine its ability to reduce the steady-state amplitude of side-to-side vibrations.

The results demonstrate that the lighter topside of the DOT turbine reduces steel use for the support structure by 13%, compared to the conventional support structure. Furthermore, the implementation of a passive gyroscopic damper without any damping elements demonstrates a frequency skipping tool that can easily be tuned by altering the gyricity of the spinning disk. In addition to that, when a damping element is added to the gyrostabiliser, a positive effect in damping the side-to-side vibrations is observed. Compared to the undamped case, a reduction of more than 90% of the maximum bending stress at the mudline is achieved. Additionally, due to the added weight and the ability of a spinning disk to resist changing its orientation, the natural frequency of the total system lowers. A sensitivity analysis of gyricity and mass variations confirms the potential benefits of such a damper to improve the structural integrity and extend the lifetime of the turbine. The findings suggest that further optimization of the damper configuration could lead to more reliable and cost-effective off- shore wind turbines.

The thesis concludes with recommendations for future research, including the exploration of the effects of lighter topsides on the support structure and the implementation of gyrostabilisers in larger turbines, deeper-water and floating turbine applications. ... Read more

On request of Royal Boskalis Westminster N.V. a comprehensive research was performed regarding emission free maintenance dredging in a harbour environment. The project site is the Maasmond at the port of Rotterdam. It covers an area of almost 10 km2 and, on average, a monthly volume of 400.000 cubic meters of sediment needs to be dredged. The operations are currently performed using trailing suction hopper dredgers (TSHD). Several new fully working emission free concept work methods were designed. These were assessed using a multi-criteria analysis, where emphasis was placed on energy reduction, reliability, interference, risk and safety. Given the scope of this research, costs are not decisive. General conclusions for the solutions contain the splitting of the total process. As the energy consumption of a conventional hopper is too high to operate on a battery cell, the work method is split into three different processes being:
(i) gathering, (ii) pumping and (iii) transportation. Two work methods scored best in this research, Sloped Water Injection Dredging (SWID) and the Fully Autonomous Submerged Dredger (FASD). SWID consists of Water Injection Dredging vessels, fixed structures and autonomous barges. FASD contains the design of a submerged dredging vessel. It can be concluded that a harbour environment is suitable to perform emissions free maintenance dredging with only small alterations to the current technology. ... Read more