The safety and reliability margin of offshore floating wind turbines need to be higher than that of onshore wind turbines due to larger environmental loads and higher operational and maintenance costs for offshore wind turbines compared to onshore wind turbines. However rotor cyclic loads coupled with 6 DOFs motions of the substructure, amplifies the fatigue damage in offshore floating wind turbines. In general a lower fatigue design factor is used for offshore wind turbines compared to that of the stationary oil and gas platforms. This is because the consequence of a failure in offshore wind turbines in general is lower than that of the offshore oil and gas platforms. In offshore floating wind turbines a sub-system fault in the electrical system and blade pitch angle controller also induces additional fatigue loading on the wind turbine structure. In this paper effect of selected controller system faults on the fatigue damage of an offshore floating wind turbine is investigated, in a case which fault is not detected by a fault detection system due to a failure in the fault detection system or operator decided to continue operation under fault condition. Two fault cases in the blade pitch angle controller of the NREL 5MW offshore floating wind turbine are modeled and simulated. These faults include: bias error in the blade pitch angle rotary encoder and valve blockage or line disconnection in the blade pitch angle actuator. The short-term fatigue damage due to these faults on the composite blade root, steel low-speed shaft, tower bottom and hub are calculated and compared with the fatigue damage under normal operational conditions considering same environmental conditions for both cases. This comparison shows that how risky is to work under the fault conditions which could be useful for wind turbine operators. The servo-hydro-aeroelastic code HAWC2 is used to simulate the time domain responses of the spar-type offshore floating wind turbine under normal and faulty operational conditions. The rain-flow cycle counting method is used to calculate the load cycles under normal operational and fault conditions. The short term fatigue damage to the composite blade root and steel structures are calculated for 6-hour reference period. The bi-linear Goodman diagram and a linear SN curve are used to estimate the fatigue damage to the composite blade root and the steel structures respectively. Moreover the fatigue damage for different mean wind speeds, sea states and fault amplitudes are calculated to figure out the region of wind speeds operation with the highest risk of damage.
Ultimate structural and fatigue damage loads of a spar-type floating wind turbine