Zigzag Mooring Line Design for Shallow Water Semi-Submersible FOWT

Abstract A novel mooring design with zigzag-shaped mooring line profile (patent pending) is proposed as a small footprint soft mooring system for floating offshore wind turbine (FOWT) in shallow water. The mooring system utilizes a combination of clump weights and buoys to make the mooring line into a ‘zigzag’ shape for absorbing high dynamic load without over stretching rope material. The mooring system’s restoring force mainly comes from geometrical change of the mooring lines. Any required mooring stiffness can be achieved by adjusting the clump weights, buoys, rope lengths between them, and pretension, within a relatively small anchor radius. A zigzag mooring is not sensitive to the rope length increase due to rope fabrication stretch and creep and therefore potentially eliminates the need for future rope re-tensioning. A design example of small footprint zigzag moorings for shallow water FOWT is given and its performance is analyzed. It is found that the proposed zigzag mooring design meets the intended design objectives.