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ABSTRACT
Tanker-shaped vessels, used for offshore oil production and storage, are kept in station by turret-mooring systems, enabling the vessels to weathervane in the direction of the dominant environmental loads. These mooring configurations have been observed in model experiments to be ineffective in swell-dominated long-wave conditions. Over a range of wavelengths from 0.6 <λ/L < 2 (L – ship length), a model tanker was observed to lose heading control in head sea condition, due to a pitchfork bifurcation that was initiated at a critical wavelength of 0.73L. This paper reports on a parametric study on the heading stability of a turret-moored tanker in regular waves and in swells. Viscous damping is systematically varied in different modes of motion and its effect on final heading equilibrium is assessed. It is found that effects of pitch damping are stronger than those in heave or roll, and can eliminate heading instability altogether. Reasons for this behaviour are described by analysing the components of the horizontal mean drift force, which cause yaw moments on the ship.
Disclosure statement
No potential conflict of interest was reported by the authors.