MODEL TESTING OF NON-LINEAR MOORING SYSTEM USING AN ACTIVE MOORING SIMULATOR FOR FLOATING OFFSHORE WIND SYSTEMS

This paper presents the development, testing and validity of an active mooring simulator for basin testing of scale model floating offshore wind turbines (FOWTs). The mooring simulator uses closed loop force control and a lookup table to actuate linear and non-linear mooring response for a semi- submersible 11MW FOWT at 1:56.7 scale. Presented is the validation of the system performance on the bench and against traditional springs in the basin. Three mooring systems were actuated by the mooring simulator: 1) Linearized polyester semi-taut mooring, 2) Non-linear polyester semi-taut mooring, 3) Non-linear catenary mooring. Characterization and verification of the simulated mooring system was done by direct comparison to an actual spring system. Validation results include linear actuated free decays and irregular wave tests in comparison to traditional spring setup. Non-linear mooring responses were also simulated under irregular sea states specified by ABS guidelines. The control system uses a proportional-integrator (PI) controller with active force feedback and a lookup table. The force-displacement relationship of the represented mooring systems includes static and dynamic stiffness components developed from OrcaFlex simulations. This relationship is fed into the controller for the active mooring system via a lookup table. The actuation of non-linear force displacement relationships allows for basin test results to be more accurate to the increasingly non-linear mooring systems being developed.