Load response of a two-rotor floating wind turbine undergoing blade-pitch system faults

Abstract

Multi-rotor floating wind turbines are among the innovative technologies proposed in the last decade in the effort to reduce the cost of wind energy. These systems are able to offer advantages in terms of smaller blades deployed offshore, cheaper operations, fewer installations, and sharing of the floating platform. As the blade-pitch actuation system is prone to failures, the assessment of the associated load scenarios is commonly required. Load assessment of blade-pitch fault scenarios has only been performed for single-rotor solutions. In this work, we address the effect of blade-pitch system faults and emergency shutdown on the dynamics and loads of a two-rotor floating wind turbine. The concept considered employs two NREL 5-MW baseline wind turbines and the OO-Star semi-submersible platform. The blade-pitch faults investigated are blade blockage and runaway, that is, the seizure at a given pitch angle and the uncontrolled actuation of one of the blades, respectively. Blade-pitch faults lead to a significant increase in the structural loads of the system, especially for runaway fault conditions. Emergency shutdown significantly excites the platform pitch motion, the tower-bottom bending moment, and tower torsional loads, while suppressing the faulty blade flapwise bending moment after a short peak. Shutdown delay between rotors increases significantly the maxima of the torsional loads acting on the tower. Comparison of blade loads with data from single-rotor spar-type study show great similarity, highlighting that the faulty blade loads are not affected by (1) the type of platform used and (2) the multi-rotor deployment.