Yaw motion analysis of a FPSO turret mooring system under wave drift forces

In order to evaluate the dynamic behavior of a FPSO-turret system, experimental tests in engineering wave basin have been performed. During the realization of experimental test large slow yaw rotations were detected in the dynamic behavior of the vessel during regular and irregular wave conditions tests. In order to study this phenomenon, an analytical method was developed in order to predict the maximum yaw angle reached under the slow yaw varying oscillations of a single point moored FPSO-turret system dependent of the regular mean wave drift forces and the turret position. The results for the proposed analytical method were verified by comparing it with experimental and simulated results. Then, the proposed method allows to identify the critical regular wave periods than can provoke large yaw rotations as an alternative for the stability tests by bifurcation analysis. On the other hand, the performed yaw motion simulations take into account the following aspects: the turret position, and the force and momentum associated with the mooring system variations (in function of the FPSO center of gravity positions and regular mean wave drift forces). Analyzing the time domain simulations it is possible to identify that large yaw motion is reached faster when the turret is near the CG and so, as the turret moves away, the yaw motions become slower.