| dc.description.abstract | In this study, start-up load and failure assessment of NREL 5MW OWT offshore wind turbine tower were analyzed. Start-up conditions of DLC 3.2 and 3.3 in IEC 61400-3 standard were selected. Coupling conditions of wind and operating conditions set in DNVGL-ST-0437 standard and the ocean parameters of Taiwan west coast were adopted for simulation. Firstly, the load analysis of wind turbine tower is carried out by using GH-Bladed software, and then the load conversion is input to ANSYS Workbench for stress analysis. Finally, semi-elliptical cracks were set at the circumferential and longitudinal welds of the tower, and the failure assessment analysis of the tower was carried out by BS 7910.
The results show that the top of the tower produces a great axial force in the direction of the rotating axis of the wind turbine during the start-up and the extreme operating gust coupling period. When start-up coupled with the extreme wind direction change, the axial force at the top of the tower will decrease, but the lateral force will increase due to the wind direction change. At the rated wind speed (11.4 m/s), the tower withstands the start-up and gust coupling condition without cracks, and the safety factor is 1.62, which is in line with the recommended value of IEC 61400-3 standard. When the circumferential welding crack depth of the tower is 20 mm and the crack length is 50 mm, the tower will be in danger of collapse, and maintenance procedures should be carried out immediately. The maximum stress intensity factor of the longitudinal welding cracks of the tower is very small and will not cause serious damage and collapse. For the circumferential welding cracks of the tower, the maximum increase of stress intensity factor is 1.20 MPa√m with the increase of crack depth 1 mm, and 0.34 MPa√m with the increase of crack length 1 mm. It can be concluded that the increase of crack depth is more likely to cause the tower to fracture than the increase of crack length. | en_US |