Research Of Offshore Wind Turbine Supporting System With Jacket Structure

Offshore wind energy is a new type of promising energy. Nowadays, the supportstructure of the offshore wind turbine becomes an important research topic owing to the harshenvironment, difficult construction, and high construction costs. The support structure ofoffshore wind turbine can be categorized to gravity type, single-column type with suctionbarrel, single-column type with single-or multiple-pile, jacket type with three or four legs,floating type and so on. Generally, the support structures of the offshore wind turbine are thegravity concrete foundation and the single steel pile foundation.The support structure of the offshore wind turbine, having a jacket in the lower part anda cylindrical tower in the upper part, will be investigated in this thesis. Due to the lack ofdesign and fabrication standards, the support structures with jackets are not a popular style.However, compared with the traditional gravity concrete and single steel pile supports, thesupport structure with jacket can be used in deeper water areas, up to20–50m, andmoreover, lots of well developed technology can be borrowed for its construction.In this study, the analysis of environmental loads, modal analysis, and non-linear timehistory analysis will be conducted. The environmental loads, including wind, wave, current,and earthquake loads, are evaluated, and the wind-induced vibration coefficient is alsocalculated using both frequency and time domain methods. The results show that although thewave and current loads have a large effect to the structure, due to the loading location, theireffect to the structure is less than that of the wind loads.In order to prevent from resonances, the natural frequencies of the structure should besteered away from the rotational frequency of the wind turbine. Generally, the structuralfrequencies are designed to1P±10%and3P±10%(where P is the rotational frequency of theturbine). The conventional simplified method for modeling pile foundation of marinestructures is to assume a fixed end at the location of6times of the pile diameter below thesoil surface. With this method, the modal analysis results will have big errors. It can only beused in a preliminary estimation of the structural modal parameters. In order to accuratelyanalyze the dynamic properties of the support structure of the offshore wind turbine, thepile-soil interaction must be taken into account.A nonlinear time history analysis of the wind load is carried out in this study. The results show that the structure may be collapsed if the averaged wind speed at hub height is morethan65m/s.A variety of factors considered in practical structure design are also analyzed in detail inthis thesis. All the work will be helpful to construct a support structure of the offshore windturbine from an old discarded jacket platform.