New Insights in Turret Mooring Systems Yaw Dynamics

Owing to friction in the weathervaning system and in the swivel stack of a turret-moored FPSO, the turret can be locked to the FPSO in day-to-day environments meaning that no relative yaw motions occur in such conditions. When the FPSO yaws sufficiently, the mooring yaw restoring moment builds up to a point where the friction torque is overcome freeing within seconds the turret from the FPSO which then yaws back towards its neutral yaw position. This phenomenon is referred to as a turret release event. Evidence of the existence of such a phenomenon has been confirmed for one North Sea FPSO. In this paper, the basic physics of turret release are investigated from first principles. Incorporation of a friction module in a readily available time domain simulation software is described. Application of this enhanced tool towards a better understanding of the FPSO/Turret relative yaw motions & velocities during the FPSO service life is then discussed in the context of existing SBM Offshore turret systems.