Investigation into heading stability of turret-moored vessels in regular waves

Abstract

In practical marine engineering, single-point mooring (SPM) vessels under heading regular wave excitation manifest sustained and large-amplitude slow yaw motions. To clarify the essential mechanisms, this work employs the nonlinear dynamics method to investigate the bifurcation characteristics and stability of the system. Firstly, the phenomenon of yaw instability was observed in experiments. Then, the surge-sway-yaw horizontal motion equations were established, linear stability analysis was conducted, and the stability criterion was obtained. Comparison with experimental and numerical results demonstrates great agreement. Subsequently, critical parameters, including turret position and pitch damping, are selected to analyze the bifurcation characteristics and evaluate the yaw stability region. The results show that wave frequency, turret position, pitch damping, and load condition affect equilibrium heading angle and yaw stability pronouncedly. In contrast, wave height and mooring stiffness demonstrate negligible impact on equilibria and yaw static stability. It is concluded that yaw instability under regular waves manifests as a transition process where the SPM system shifts from its zero equilibrium point to a non-zero equilibrium state, indicating a loss of static stability. The qualitative analysis of system parameters presented in this paper could offer practical guidance for the preliminary design stage of SPM vessels, particularly in optimizing turret positioning, load condition, and metocean conditions.