Trends towards higher capacity offshore wind turbines (OWTs) and deeper offshore wind farm sites, make wave-induced vibrations more dominant in its design. During idling phases of these next-generation OWTs, additional damping is required. This can be achieved through dynamic vibration absorbers (VAs), i.e. dampers. More importantly, wave-induced vibrations can decrease the installation workability of these OWTs up to 50%. VAs which are designed for the operational life of an OWT, are not effective at natural frequencies typical for partially installed OWTs. To reduce installation costs to acceptable values, higher damper effectiveness is required.
The main objective of this thesis is, therefore, the reduction of wave-induced vibrations during OWT installation by non-invasive modification strategies of off-tuned VAs.

In the first part of the research, a passive damper modification is proposed. A numerical model was developed of the stand-alone passive modified VA. Using data from small scale-experiments, the steady state response of the model was validated. However, the passive method was shown to be insufficient in supplying additional damping at installation frequencies.

In the second part, semi-active control strategies were deployed. This semi-active controller, which cannot add mechanical energy through the actuator, results in an inherently stable system. A hybrid control law and a clipped-LQR control law were investigated. The damper its performance was tested for a deep-water site case study. The semi-active damper modification was shown to be a robust approach for frequency tuning. Wave-induced vibration reduction of up to 60% could be achieved, independent of the off-tuning of the unmodified damper. Actuator dynamics were included and shown to have insignificant effects on the performance.

It is therefore concluded that, the semi-active damper can be effective over the whole installation and operational range of an OWT support structure. For the representative case-study, the installation workability can be increased from only 50% to approximately 90%.