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Impacts of Mooring Line Lengthening on Position Controller Design for a Floating Offshore Wind Turbine

This paper presents how longer mooring lines impact the ability to command a Floating Offshore Wind Turbine (FOWT) to a desired position. This starts with an analysis of the asymmetrical platform motions that are magnified by the longer mooring lines. These asymmetrical platform motions are most notable when comparing motion in the positive and negative sway directions. These asymmetries are caused by the rotor gyroscopic torque and generator reaction torque, and the combined effects of wind shear, rotor yaw direction, blade pitch and rotor rotation direction. These asymmetries cause differences in motion dependent on the direction of the commanded position. The asymmetries appear as an ineffectiveness of position control when positive sway movement is commanded. Then a position controller is designed to address some of these asymmetries, and to highlight the impacts of the asymmetries which cannot be mitigated with the proposed controller structure. The position controller is designed as a Proportional-Derivative (PD) controller to control the FOWT position perpendicular to the wind direction by manipulating only nacelle yaw. The results demonstrate that, except at especially high wind speeds with FOWT motion in the positive direction, the controller is able to control the FOWT to a commanded position. At high wind speeds, the asymmetries of the system with respect to nacelle yaw direction become too large to be overcome. The recommendations for position control design are to include platform rotational degrees of freedom in all control design models, to explicitly account for platform yaw in the controller design and to have the wind farm supercontroller account for wind-shear based asymmetries.