Time-domain Global Response Analysis of a Novel 5MW Semi-submersible Wind Turbine Considering the Effect of Second-order Wave Loads
Master thesis
Date
2015Metadata
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- Institutt for marin teknikk [3472]
Abstract
In recent years wind energy industry has been developing very fast, moving from onshoreto offshore and from shallow water to deep water. Offshore wind has a huge potential forlong-term sustainable energy supply and deeper water sites have better wind conditionsand less noise pollution to the local residence.
Many floating wind turbines have been proposed and some of them have been developedinto prototypes. However, the cost of energy (CoE) for these kind of structuresis too high in terms of design, installation and grid connection as well as maintenanceand operation. One way to reduce the costs is to use a larger wind turbine, absorbingmore wind power. Therefore, many large-scale floating wind turbine concepts have beenproposed.
In this thesis a coupled dynamic analysis of the novel semi-submersible wind turbine5-MW-CSC designed at CeSOS, NTNU, is performed including both first-and secondordereffects. The structure is made of a main cylindrical column supporting the windturbine 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 completestructures considering both wind and wave loads. Unlike the wave-frequency motions of afloating wind turbine, slowly-varying motions might be excited by both the aerodynamicloads on wind turbine rotor and the second-order wave loads on floater. It is thereforeinteresting to investigate the relative importance of the second-order wave loads on asemi-submersible wind turbine as compared to the aerodynamic loads.
In this thesis, the effect of second-order wave loads on global motions and structuralresponses are examined in the time domain by using the coupled code SIMO-RIFLEX-AeroDynto 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 andtwo second-order models, which account slowly-varying forces by using full quadratictransfer functions (QTFs) and Newman s approximation respectively, were developed toperform 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 domainanalysis in which different kind of viscous drag forces linearizations were investigated