| dc.description.abstract | In recent years wind energy industry has been developing very fast, moving from onshore
to offshore and from shallow water to deep water. Offshore wind has a huge potential for
long-term sustainable energy supply and deeper water sites have better wind conditions
and less noise pollution to the local residence.
Many floating wind turbines have been proposed and some of them have been developed
into prototypes. However, the cost of energy (CoE) for these kind of structures
is too high in terms of design, installation and grid connection as well as maintenance
and operation. One way to reduce the costs is to use a larger wind turbine, absorbing
more wind power. Therefore, many large-scale floating wind turbine concepts have been
proposed.
In this thesis a coupled dynamic analysis of the novel semi-submersible wind turbine
5-MW-CSC designed at CeSOS, NTNU, is performed including both first-and secondorder
effects. The structure is made of a main cylindrical column supporting the wind
turbine and connected to three cylindrical side-columns by three rectangular pontoons.
There are no braces in this concept to avoid the fatigue problems in brace-column joints.
Design of a floating wind turbine requires direct time-domain simulations of the complete
structures considering both wind and wave loads. Unlike the wave-frequency motions of a
floating wind turbine, slowly-varying motions might be excited by both the aerodynamic
loads on wind turbine rotor and the second-order wave loads on floater. It is therefore
interesting to investigate the relative importance of the second-order wave loads on a
semi-submersible wind turbine as compared to the aerodynamic loads.
In this thesis, the effect of second-order wave loads on global motions and structural
responses are examined in the time domain by using the coupled code SIMO-RIFLEX-AeroDyn
to take into account the wind turbine aerodynamic loads and the induced motions.
Based on an initial numerical model of a 5MW wind turbine in SIMO-RIFLEX-AeroDyn,
which was provided by the Post-Doc Constantine Michailides, one first-order model and
two second-order models, which account slowly-varying forces by using full quadratic
transfer functions (QTFs) and Newman s approximation respectively, were developed to
perform time domain simulations in both wave-only and coupled wind-wave conditions.
QTFs were computed by the candidate in a previous work by means of a frequency domain
analysis in which different kind of viscous drag forces linearizations were investigated | en |
