Heading Control of a Turret-Moored FPSO with and Without Forward Thrusters

Abstract A valuable property of a turret moored FPSO is its ability to weathervane or self adjust its heading to the varying metocean environment. Still, heading control with thrusters is necessary when a different heading than the natural weathervaning heading is wanted for operational reasons. In particular, in non-colinear metocean conditions it may be desirable to keep the bow of the FPSO against the waves to reduce rolling. It may happen that the weathervaning becomes unstable. In this case, the vessel will go into fishtailing motion, which is a phenomenon that is governed mainly by interaction between sway and yaw. Fishtailing is weather-dependent. When it happens, heading control is needed to stabilize it. One important objective of the study has been to investigate if satisfactory heading control can be obtained using thrusters aft only, or if thrusters forward are necessary. For a given FPSO design, we study weathervaning stability by formulating a linear model in sway and yaw. Based on this model with damping neglected, we develop three simplified criteria for stability. Using heuristic argumentation, we accept the criteria as conditions for sufficiency. The system's eigenvalues give sufficient and necessary conditions for stability. Using metocean hindcast data covering a time span of 56 years, the simple criteria are tested against the eigenvalues of the sway-yaw model. We found good agreement. The heading controller used is essentially a PID controller. It is found that with aft thrusters only, good heading control can be achieved for all the metocean states in the 56 years span, provided the controller gain is moderate. With high controller gain, the thrusters may excite resonance in the turret mooring. Using additional thrusters forward gives the freedom to reduce turret resonance. Time domain simulation with an accurate 6-degree-of-freedom model shows that thrusters aft and forward gives slightly better control than control using aft thrusters only. Still, using only aft thrusters appears to give satisfactory heading control.