Mooring and hydrostatic restoring of offshore floating wind turbine platforms

This paper investigates the restoring stiffness of the main platform concepts proposed for offshore floating wind turbine (FWT) systems; namely, barge, spar, tension leg platform (TLP). The overall system stiffness is partly due to the hydrostatics, and partly due to mooring. The hydrostatic stiffness matrix is formulated using the linear hydrostatic approach that assumes small platform rotation. A new analytical form of the mooring stiffness matrix for a taut-leg platform is presented and subsequently used to formulate the TLP mooring stiffness. While a numerical approach, is used for the other two platform types. The hydrostatic and mooring stiffness coefficients for the surge, sway, heave, roll, pitch and yaw degrees of the freedom (DOF) are computed for the different types of platforms. For each DOF, the magnitude of stiffness from both hydrostatics and moorings are compared.