Analysis and optimization of monopile type offshore wind turbines under wind and wave loading

Abstract

The development of offshore wind technology is rapidly improving because of the energy need in the world. One of the main concerns about the design is to make an economically feasible offshore wind turbine system. The design of the support structure should be strong and stiff enough to endure the time varying wind and wave loads because the offshore wind turbines are very large structures and continuously subjected to the time varying loads. In this study, 5MW monopile type offshore wind turbine (OWT) was analyzed according to design guidelines. Cost optimization of monopile regarding to steel weight has been done. The standard analysis procedure for the OWT is such that the first structural natural frequency is far away from the first wave frequency to eliminate the resonance; however, it was found that there is an increase in the response of the structure significantly due to the higher wave harmonics (i.e 2nd and 3rd harmonics). Therefore, the results of the thesis may be valuable to designers in terms of the effects of higher wave harmonics on the structural response. It was found that higher wave harmonics can increase the overturning moment of the monopile (the most important design parameter) about 40% depending on the change in the wave period. The most important outcome of the optimization study was that change in the penetration depth of the monopile into the soil may result in 20% lighter i.e. cheaper structure.