Turret location impact on global performance of a DP turret-moored FPSO

The change of the global dynamic performance of a turret-moored FPSO (Floating Production Storage Offloading) with DP (Dynamic Positioning) control is analyzed for two different internal turret locations: bow and mid-ship. Both collinear and non-collinear 100-yr GOM (Gulf of Mexico) storm environments and two cases (mooring-only, with DP control) are simulated. The horizontal trajectory, 6DOF (degree of freedom) motions, fairlead mooring top tension is compared. The PD (Proportional-Derivative) controller based on LQR (linear quadratic regulator) theory is implemented in the fully-coupled time-domain hull-mooring-riser-DP simulation program. Both in collinear and non-collinear 100-yr WWC (wind-wave-current) environments, the benefit of mid-ship turret is demonstrated by the significant reduction in heave at the turret location due to the minimal coupling with pitch mode, which is advantageous to mooring and riser design. However, in the non-collinear WWC environment, the mid-turret case exhibits unfavorable position keeping characteristics, which can be reduced by employing DP controls as demonstrated in the present case studies. The present study also presents the plausible cause of the failure of mid-turret Gryphon Alpha FPSO in milder environment than its survival condition.