This paper provides a comprehensive theoretical analysis to determine the nonlinear hydrostatic restoring loads and stiffnesses of a floating offshore platform. A new approach is developed to calculate the buoyancy forces and the corresponding moments for general 3D displacements of offshore platforms that utilize cylindrical floaters, such as barge (rectangular cylinder), spar, tension leg platform (TLP), and semisubmersible (circular cylinders) offshore platforms. The exact buoyancy force magnitude and point of action (center of buoyancy) and hydrostatic restoring moments for general fully coupled 3D displacements are derived. Exact expressions for the coupled water plane area restoring moments in pitch, roll, and yaw are derived in the body and inertial frames. The analysis is subsequently used to evaluate the hydrostatic loads and stiffness of floating cylinders that undergo large displacement, such as floating wind turbine (FWT) platforms. Moreover, it can be used to determine the equilibrium positions and orientation of free floating cylindrical bodies.

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