The General Nonlinear Stiffness Matrix of Taut Mooring Systems and Suspension Systems of Cable-Connected Floater-Keel Platforms

Abstract The static and dynamic behavior of taut-leg and tension leg platforms (TLPs) are highly characterized by the stiffness properties of their mooring systems. That also applies to the innovative offshore floating wind turbine platforms composed of a floater connected to a keel only via suspension cables. Several works have tried to derive the nonlinear stiffness matrix of such systems. Yet, the results are only partially similar. In this study, a general approach is developed for deriving the nonlinear tangent stiffness matrix of a floating platform moored with a single taut line. The stiffness matrix is derived using the virtual work principle as a function of the platform's general displacement. The derivation method is analytically partially verified. Furthermore, the stiffness matrix of a rectangular taut-leg platform and a rectangular TLP mooring systems is constructed and its initial forms are computed and compared to previous work. In addition, the innovative floating platform concept cable-based truss spar (CBTS) is introduced and the previously presented methodology is adopted to derive the general nonlinear stiffness matrix of its suspension system. The floater's initial pitch stiffness of the CBTS platform is further investigated at different initial positions of the platform.