LONG-TERM EXTREME RESPONSE ANALYSIS FOR A FIXED OFFSHORE WIND TURBINE CONSIDERING BLADE-PITCH-ACTUATOR FAULT AND NORMAL TRANSIENT EVENTS

In this paper, the 50-year long-term 1-hour extreme responses of a fixed jacket-type offshore wind turbine with consideration of one-blade-pitch-actuator-stuck fault and the effect of normal transient events such as normal shut-down and start-up process is studied. The long-term extreme results are found based on each short-term extreme response distributions at different environmental conditions. Structure responses such as tower and jacket bottom shear and bending moments as well as blade root bending moments will be focused in this paper. To study the long-term effect of the fault and transient events, the service life of a wind turbine is divided into normal part, faulted part, and transient part. Normal part includes both normal operation and parking of the wind turbine at different wind speed range without any faults. Faulted part includes the parked and emergency shut-down condition of the wind turbine under the fault assuming that the faults are detected soon after they occur but require a longer time before fully repaired. Transient part includes the start-up and shut-down process during the normal operation when wind speed is beyond operation range. The contribution of each part to the long-term extreme response distribution is calculated by weighting factors based on the probability of occurrence of each part. From the results, it is found that in general, the blade-pitch-actuator-stuck fault and the normal transient events generally increase the extreme responses of the wind turbine. The jacket wind turbine is more affected compared to its land based counterpart. In this study since the wind direction is aligned with wind turbine, it is found that the fault primarily increases the tower bottom shear force perpendicular to the wind direction and the bending moments with the axis parallel to the wind as well as the torsional moment, while normal transient events, especially the start-up process at cut-out speed, causes a much greater increase compared to the fault. It contribute mostly to the shear forces parallel and bending moment with axis perpendicular to the wind direction. The azimuth of the blades is found to be very important for blade responses during start-up process especially at higher wind speed.