Slow motion dynamics of turret mooring and its approximation as single point mooring

Abstract

The mathematical model for the nonlinear dynamics of slow motions in the horizontal plane of Turret Mooring Systems (TMS) is presented. It is shown that the TMS model differs from the classical Single Point Mooring (SPM) model, which is used generally to study the TMS dynamics. The friction moment exerted between the turret and the vessel, and the mooring line damping moment resulting from the turret rotation are the sources of difference between TMS and SPM. Qualitative differences in the dynamical behavior between these two mooring systems are identified using nonlinear dynamics and bifurcation theory. In two- and three-dimensional parametric design spaces, the dependence of stability boundaries and singularities of bifurcations for given TMS and SPM configurations is revealed. It is shown that the static loss of stability of a TMS can be located approximately by the SPM static bifurcation. The dynamic loss of stability of TMS and the associated morphogenesis may be affected strongly by the friction/damping moment, and to a lesser extent, by the mooring line damping. Nonlinear time simulations are used to assess the effects of these properties on TMS and compared to SPM systems. The TMS mathematical model consists of the nonlinear horizontal plane fifth-order, large drift, low speed maneuvering equations. Mooring line behavior is modeled quasistatically by submerged catenaries, including nonlinear drag and touchdown. External excitation consists of time independent current, steady wind, and second-order mean drift forces.