| dc.description.abstract | In the last 25 years, offshore wind turbines (OWTs) have moved from small machines of less that 500 kW in a few metres of water to large multi-megawatt machines in water depths exceeding 30 meters.
On monopile foundations, which dominate the industry, this will increase the importance of hydrodynamic loads, as both the water depth and monopile diameter increase.
Since operating OWTs have a significantly different damping levels in-line with the wind and in the cross-wind direction, an increased sensitivity to hydrodynamic loads can result in an increased importance of the misalignment between wind and waves.
Similarly, the importance of shortcrested waves can increase.
Recently, there has also been an increased concern as to whether the model for soil-structure interaction most frequently used is overly conservative.
This thesis investigates several aspects with regards to the modelling and simulation of OWTs using the DTU 10 MW reference wind turbine placed on a large-diameter monopile foundation.
In a comparison study between three different computer programs, the differences caused by modelling uncertainties and program differences were found to result in a difference of 14 \% in the damage equivalent moment at mudline.
The difference is mainly found to be caused by differences in the distribution of aerodynamic properties, and emphasizes the importance of correct modelling.
Further, the combined effect of wind-wave misalignment and soil stiffness is investigated by using three different estimates for the soil stiffness.
On a model with large soil stiffness little dynamic excitation from the waves is observed, while the importance of loads in the wave-frequency range increases with decreasing soil stiffness.
This means that a structure in stiff soil will have a larger fatigue contribution from wind speeds close to rated, while sea states associated with large waves will be more important for a structure in soft soil.
The influence of wave loads is even more clearly demonstrated when wind-wave misaligned and shortcrested waves are included in the analysis.
While there is an increase in the sideways damage irrespectively of the soil stiffness, the effect is far more significant for the models with low soil stiffness.
These results can then be used to provide some guidelines in the design of monopile-supported wind turbines.
While the soil properties at a location are difficult to influence, much of the same effects can be achieved by changing the stiffness of the monopile.
At locations with a harsh wave climate, a design with a stiff monopile would therefore be beneficial, while a soft monopile and stiff tower can be more feasible at locations with dominating wind loads. | en |
