Jiang, Zhiyu (Department of Marine Technology, Norwegian University of Science and Technology (NTNU), and Centre for Ships and Ocean Structures, NTNU) | Karimirad, Madjid (Centre for Ships and Ocean Structures, NTNU, and Norwegian Research Centre for Offshore Wind Technology, NTNU) | Moan, Torgeir (Centre for Ships and Ocean Structures, NTNU)
Offshore floating wind turbines are subjected to harsh environmental conditions and might experience fault conditions. Load cases considering parked and fault conditions are important for the design of wind turbines and are defined in the IEC61400-3. For a parked wind turbine, the blades are feathered and put parallel to the wind direction. However, if the pitch mechanism fails, the blades cannot be feathered to the maximum pitch set point―the blades are seized. This will likely cause a large drag loading and increase the extreme response values. The study conditions on the 1-year and 50-year environmental contour line for a site in the North Sea. Three parked scenarios are considered: fault with one seized blade, fault with three seized blades and normal condition. Steady state responses of a spar-type wind turbine are investigated. Most of the response extremes and standard deviations are sensitive to the wave direction. For the normal parked conditions, yaw of the platform is sensitive to the blade azimuth while surge and pitch are not. The blade azimuth position also plays a key role in responses such as roll and yaw for the parked conditions with one faulted blade. The fault cases under 1-year environmental condition are also compared with the normal parked ones with an environmental condition corresponding to 50-year recurrence period. Fault with one seized blade often leads to the largest roll resonance and yaw extreme angle and the extremes may exceed the 50-year reference values by more than 16%. The linked structural responses are not as critical, however. Fault with three seized blades causes an average rise of 38% and 23% for surge and pitch extremes over the 50-year references due to the large aerodynamic drag. The tower bottom bending moment and the blade root bending moment may also exceed the 50-year values by more than 10%.