A combined rotor transient responses and analysis tools/Computational Fluid Dynamics (CFD) effort was completed to assess rotor response in a complex flow field. A generic helicopter on an oil rig platform was chosen because the bluff nature of the rig presents unique challenges to rotorcraft operations. The rotor start-up and shutdown simulation model was validated with the test data. Comprehensive studies of rotor transient responses under various wind directions and wind speeds were conducted in order to establish the safe operation envelope. The study found that horizontal wind velocities may be accelerated by the oil-rig building corner, and cause significant rotor flap motions. The upwash and downwash flow due to the blockage of the helicopter fuselage to the cross wind had great impact on rotor transient responses; and the peak blade flap responses can be 2.5 time higher than the simulation results when the effect of airframe on airwake was ignored. The influence of unsteady airwake on rotor transient responses was also investigated, and the results showed that it could increase the blade tip deflections by 70% if the unsteady flow components were included. The sensitivity of rotor blade initial azimuth positions for rotor start-up operations was also examined.