Effect of Wind Loads and Damping on Heading Stability of FPSOs

Abstract

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 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 control in head sea condition, wherein for wavelength from 0.73 < λ/L < 2 (L-ship length) the model drifted to a large angle of 45–60 degrees (Thiagarajan et al. 2013). Previous numerical analyses conducted by the authors identified that this heading drift reduced remarkably in the presence of wind. This finding is confirmed by an experimental study and reported in this paper. A geometrically scaled down version of a turret-moored FPSO at 1:120 scale of a prototype VLCC was tested at the Alfond W2 Wind & Wave Ocean Engineering facility of the University of Maine. This lab is a unique facility equipped with a high-performance wind machine over a multidirectional wave generator, and can create regular or random sea-states with wind speeds up to 7 m/s. The tests reported here were conducted with regular waves under two wind speeds (12 and 25 m/s full scale). It was observed that the presence of an initially bow wind can minimize the heading instability. The reason for this observation is described by analyzing the effect of the wind induced moments on the equilibrium condition. Free-decay tests were also conducted to investigate the contribution of the wind damping to the total damping. Measured results show that in the presence of wind, the damping values are higher than those estimated due to hydrodynamics only. It also has been discussed that this wind induced damping on FPSOs, can result in smaller heading angles. From this study, it is concluded that the wind can play a large role in the station-keeping dynamics of the moored-tankers.

Theoretical and Experimental Study of Heading Stability and Heading Control of a Turret-Moored FPSO

“…Stability of a turret-moored ship has been investigated by a number of authors, [5], [7], [2], [18], [19], [20].…”