The guidelines for direct stability assessment of pure loss of stability are currently under development at IMO for the second generation intact stability criteria. A surge-heave-pitch-roll coupled equation named as 4 DOF is newly established for predicting pure loss of stability in following seas. Firstly, the thrust and the resistance of calm water are varied with the ship forward speed and the excited surge force by waves is varied with the relative position between the ship and waves. Secondly, the heave and pitch motions obtained by a strip method with the speed variation newly considered, are used to determine the simultaneous relative position of the ship to waves in the time domain, and then the nonlinear Froude-Krylov roll restoring variation is calculated by integrating the wave pressure up to the wave surface. Thirdly, the effect of the constant ship speed, the surge motion, the initial heeling angle, the heel-induced hydrodynamic forces and the heave and pitch motions on pure loss of stability are studied for direct stability assessment with the 4 DOF mathematical model. Finally, the new numerical approach on pure loss of stability in following seas is verified by experimental results using the standard ONR tumblehome.
- pure loss of stability
- second generation intact stability criteria
- direct stability assessment