HORIZONTAL MOTIONS OF AN SPM TANKER UNDER ALTERNATIVE MOORING CONFIGURATIONS

Horizontal motions of a tanker single point moored in current, wind, a nd waves were numerically investigated under four alternative configur ations namely, buoy mooring (SBM), articulated tower mooring (ATM), bo w turret mooring (BTM), and internal turret mooring (ITM). The analysi s was based on a previously validated, comprehensive mathematical Lloy d, sive mathematical model of the equations of motion in three degrees of freedom (surge, sway, and yawl that considers Jive sets of forces: 1) nonlinear quasi-steady hydrodynamic response and control forces; 2 ) linear memory effects due to radiated waves; 3) nonlinear mooring re storing force characteristics; 4) empirical wind actions; and 5) first -order wave forces and wave drift forces. Locally linearized stability analyses showed that restoring characteristics have no influence on s tatic bifurcations (typical for BTM and ITM) and only minor influence on dynamic bifurcations (typical for SBM and ATM). However, nonlinear time domain simulations revealed that they do affect the form of asymp totic motion trajectories as well as line tension amplitudes. It was f ound that turret position is a significant parameter and motion behavi or is sensitive to the direction of wind and waves relative to current . Comparison of simulated time histories between autonomous and nonaut onomous modes indicated that the traditional spectral analysis treatin g high-frequency and low-frequency responses separately may not be val id for locally unstable cases.