Preliminary Design And Fatigue Strength Analysis Of Support Structure On Floating Offshore Wind Turbine

Wind power is a kind of clean and renewable energy, and offshore wind power farm isbecoming new direction of developing renewable energy. At present, because deep sea windpower has more advantages than offshore, it is inevitable that offshore wind turbinedevelops from shallow fixed style to deep floating style. Support structure is an importantcomponent of floating offshore wind turbine, which is subjected to cyclic loads caused byboth hydrodynamic force and aerodynamic force. During the design life of20years, thecycle counts of the fatigue loads are so large that fatigue strength of the support structureneeds particular attention.Assume that floating wind turbine is located in the Stevenson of North Atlantic Ocean.Foundation structure of three pontoons offshore wind turbine is designed preliminarilybased on design standard of offshore steel structures of DNV. Load cases of motionresponse analysis and overall strength analysis are confirmed according to design standardof offshore wind turbine IEC61400-3. The result of overall structural motion response isobtained and overall strength result subjected to both environmental loads and turbine loadsis also obtained. Besides that the location of fatigue hot spot can be knew. In the end, thefatigue load spectrums of support structure fatigue hot spot under fatigue load cases are gotthrough analysis. Then fatigue life of wind turbine support structure is calculated. Keyresearch contents and relevant conclusions in this paper are as follows:(1) Plate thickness of column and modulus of girder section of floating wind turbinefoundation structure are calculated with load and resistance factor design method (LRFD) ofdesign standard of offshore steel structures of DNV. The best draught of floating foundationunder operation and survival is got through analysis. In the end, preliminary design offloating offshore wind turbine support structure is completed.(2) The integrated model of three pontoons offshore wind turbine is built with theprofessional design software Samcef for Wind Turbine. The normal operating sea states (sixgroups) and sea state with a return period of100years are confirmed based on IEC61400-3.Then dynamic response analysis of floating wind turbine under the combination of wind,wave and current is conducted. The results show that motions of all kinds of load cases arereasonable, and the effect of wave load on heave motion is large, the effect of wind load onsurge motion is large. Surge motion of structure increases first and then decreases with theincrease of wind speed, which reaches summit when wind speed gets to the rated windspeed of wind turbine. (3) Overall strength of wind turbine support structure is analyzed under the combinedaction of aerodynamic load, wind load, wave load and current load. Then overall strength ischecked based on DNV standard. The result shows that strength of support structure meetsthe demands, and stress concentration exists at the joint of tower and foundation structure.The higher stress area is confirmed as fatigue hot spot in this paper.(4) Local refinement model of the joint between tower and foundation structure is built,and its fatigue load spectrums under different fatigue load cases are obtained with SWT.Fatigue life of wind turbine support structure is calculated according to S-N curve, linearfatigue cumulative damage theory along with the fatigue load spectrums. The result showsthat the life of support structure is more than20years, which meets the minimumrequirement of standard.