Research On Mechanism Of Yaw Instability Of Turret Moored FPSO

Floating Production Storage Offloading vessels are one kind of offshore oil exploitation engineering equipment which integrating various functions such as oil and gas exploitation,production and processing,storage and transportation,etc.Most FPSOs are kept in station by single point moored system,which allows the vessel to weathervane to adapt its orientation,so the damage of wind and waves can be avoided.Anecdotal evidence from offshore practice indicates that an FPSO may lose yaw control under regular waves of a particular wave length,under which condition the weathervane effect fails and the hull will balance near the condition of transverse wave state.This paper studied the phenomenon of unpredictably changing in the yaw angle which is called yaw instability.Model test of yaw instability of internal turret moored FPSO in the regular waves was carried out.A generic FPSO hull was modelled in the scale ratio 1:80,and truncated mooring system was designed.The relationship of equilibrium yaw angle versus wave length-ship length ratio was studied.The uncoupled nonlinear equation of yaw motion was built and simulation in SESAM was established to compute hydrodynamics parameters.Lyapunov theorem of stability and Hamiltonian phase flow were adopted to obtain pitchfork bifurcation of equilibrium yaw angles versus wave length-ship length ratio,the results of which can fit well with those of model test and the accuracy of theoretical analysis was verified.Parameter sensitive analysis was conducted to study the influence of roll and pitch damping,position of internal turret and load condition on equilibrium yaw angle.Based on the 6-DOF full coupled equation,a numerical solution code was established to study the influence of yaw instability on the 6-DOF response and mooring load,and the influence of wave steepness and initial yaw angle on yaw instability were also studied.The mechanism of yaw instability of FPSO in irregular waves was studied.The equation of yaw motion under random parametric excitation was established with the slow drift excitation loads considered by Newman’s approximation.The stationary probability density function of yaw angle was solved by stochastic averaging method of energy envelope.By comparing with the results of Monte Carlo simulation,the reliability of the analytical method was verified and the application range of yaw damping while applying stochastic averaging method of energy envelope was determined.Parametric sensitive study was carried out to research the influence of wave parameters,roll and pitch damping and position of internal turret on yaw response.