Experimental investigation of mooring line damping for floating offshore wind turbines

This thesis investigates how different mooring line configurations affect the damping of floating offshore wind turbines. To study this, forced oscillation experiments were carried out at a 1:60 scale in the Memorial University of Newfoundland towing tank. A studless chain was tested in three setups: a semi-taut line, a bare catenary, and a buoy-assisted catenary. The tests were performed using a programmable linear actuator to impose low-frequency surge motions, while line tension and motions were carefully recorded. The semi-taut line showed an increase in energy dissipation with larger oscillation amplitudes, especially when the line assumed a taut state. At shorter oscillation periods, drag forces dominated, resulting in higher energy dissipation but lower equivalent damping. For the catenary cases, static tests showed that the bare chain had higher initial stiffness than the buoy-assisted line, where the buoy reduced restoring stiffness by lifting the chain mid-span. Dynamic tests confirmed that energy dissipation increased with amplitude in both cases, but the buoy-assisted system showed stronger nonlinear effects, including slack–snap events. These led to much higher dynamic tensions, up to 8.2 times the minimum tension, compared with 2.9 times for the bare chain. The bare chain followed a near-linear trend between energy dissipation and the excitation parameter (ω²q³); the buoy case showed clear deviations.