A Study on Unstable Motions of a Tension Leg Platform in Close Proximity to a Large FPSO

The present paper elaborates on model experimental results obtained from tests conducted with a TLP connected to a nearby positioned FPSO. The tests revealed, at a given range of wave periods, the onset of unexpected large oscillatory yaw motions of the platform whereas the FPSO remained rather stable when the TLP was directly excited in sway. The paper summarizes the model experiments 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. A mathematical model is proposed to describe the main aspects of the two-body moored system. In principle a 12 DOF model is contemplated. Numerical simulations are compared to the time series obtained from the experiments showing adequate agreement.