Effect of Wind Loads on Heading Stability of FPSOs

Tanker vessels used for offshore oil production and storage are kept at station by turret mooring systems, enabling the vessel to weathervane in the direction of the dominant environmental loads. The disruption of heading equilibrium for a turret moored tanker was predicted by experiments and numerical studies. A vessel was observed to lose heading control in the head sea condition, due to a pitchfork bifurcation that was initiated at a critical wavelength of 0.73L (Thiagarajan et al. 2013). While previous studies focused on heading instability in waves only, the effect of wind can be significant. Wind not only creates wind induced waves, but also directly generates loads on marine structures when the superstructure area (portion above the mean water surface) is significant. This paper reports on a parametric study on the heading stability of a turret-moored tanker in regular waves in the presence of steady wind. The analysis was conducted using the commercial solver AQWA. It is found that the presence of an initially bow wind can minimize the heading instability. Reasons for this behavior are described by analyzing the effect of wind induced moments on the equilibrium condition.