An offshore wind turbine requires a stable support structure to support the tower and nacelle. The support structure in the form of a monopile is lowered to the seabed and hammered in the soil. During the lowering process the monopile may show excessive motions, that occur on an irregular basis. The cause for this is unknown. This thesis is devoted to find the cause, to prevent this from happening in future operations. First off a literature study and analysis of the environmental conditions is carried out. A study in the mechanical behaviour of the system will provide a good perspective on the situation. Finally, the cause is verified with model tests and a numerical model is validated.
From the environmental conditions wind and current are expelled as (main) contributor. Data analysis and system analysis conclude that waves are the main cause for the problem. In the model tests this cause is validated together with the mechanical behaviour (natural frequency) of the system. The natural frequency of the monopile is depending on the waterlevel.
Wave frequencies (in the spectrum) close to the natural frequency cause a resonance phenomenon, with large monopile motions as consequence.
A numerical model, based on a mechanical system and the Morison's equation is validated. The numerical model is a good and fast solution for modeling monopile motions, from wave conditions. The model can be used to predict future scenarios to prevent excessive monopile motions to occur, a (potential) tool for engineering purposes (determining workability).