The paper summarizes some aspects of a series of model experiments conducted with a Tension Leg Platform (in fact a Tension Leg Wellhead Platform) in close proximity with a FPSO emphasizing the types of coupled motions taking place. It is observed that as the yaw motion develops increasing amplitudes the sway motion is reduced, pointing out to an interesting exchange of energy between the sway and yaw modes. This should be recognized as a revealing aspect of strongly non-linear coupled parametric resonant motions. A mathematical model is proposed to describe the main aspects of the two-body moored system and hydrodynamic interactions. In principle a 12-DOF model is contemplated. Numerical simulations are compared to the time series obtained from the experiments showing adequate agreement. However, in this paper the essential coupling of sway and yaw is distinguished in order to typify the Mathieu-type instability as being the main mechanism behind the onset of large yaw motions of the TLP unit.

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