Semi-Submersible FOWT with Small Mooring Footprint

A novel Vertically Moored Semisubmersible (VMS) for deep water is proposed as a small footprint and cost effective floating offshore wind turbine (FOWT). In contrast to the stiff tendons in a conventional Tension Leg Platform (TLP), VMS is a ‘soft’ system with long natural periods in its heave, pitch and roll modes and alleviates the technical difficulties and cost challenges associated with the tendons of a TLP in deep water while maintaining a small footprint to avoid interference with neighboring mooring systems and power lines. In this paper the motion response of a VMS is investigated in both frequency domain and time domain for extreme and operating sea states. It is demonstrated that a VMS meets all the relevant design requirements and has a superb redundancy compared to a spread moored system. The heave stiffness of a vertical mooring system makes it possible to tune the heave natural frequency closer to the heave cancellation frequency thus minimizing heave resonance response. A VMS also have better pitch and roll motions than a conventional spread moored semisubmersible because of the additional rotational stiffness from the vertical moorings.