Study On Structural Reliability Of Tower And Mooring System Of Offshore Floating Wind Turbines

The development of offshore wind energy is growing at an unprecedented pace. The style of offshore wind turbines has changed from fixed foundation to floating foundation. As sea conditions will be more complicated with water depth increased, great environmental loads of offshore wind turbines will be caused, which may make a negative impact on normal performance of offshore wind turbines. It's very important to make the reliability evaluation of supporting structures of offshore wind turbines. This paper makes the structual reliability calculation of tower and mooring system of NREL 5MW semi-submersible offshore wind turbine.Firstly, the calculation of wind-resistant dynamic reliability of the tower has been made.With harmonic superposition method used,The average wind speed,fluctuating wind speed and wind loading are simulated. The probability density function of top displacement and positive stress can be get by solving the probability density evolution equation. With thresholds given, the structural wind-resistant reliability index can be calculated at maximum operating state and limit stop state. The result shows that the reliability index of top displacement is less than the design value at the limit stop state, which should be concerned in the design of the tower.Secondly, time domain curve of axial tension of the mooring system will be reached with AQWA used at the rated state, maximum operating state and limit stop state. By using extremum principle,the probability distribution models of the load and the broken strength of mooring chain can be constructed. Parameters of the probability distribution models can be solved by data fitting.Thirdly, the structural reliability index of any mooring chain can be calculated by using FOSM method. The failure model of the mooring system can be described as series model and the reliability index of the mooring system can be get also. The result shows that the index is similar to the design value at the limit stop state, which should be concerned in the process of design.