Comparative Assessment of Hydrodynamic Models in Slow-Motion Mooring Dynamics

Abstract

The slow-motion dynamics of a turret mooring system is analyzed and compared for four of the most commonly used ship hydrodynamic maneuvering models. Each of those utilizes a different approach to model and then to calculate or measure the hydrodynamic forces and moment acting on the vessel. The four hydrodynamic maneuvering models are studied first by a physics-based analysis of each model and then by numerically comparing their prediction of equilibria, nonlinear stability analysis, bifurcation sequences, and morphogeneses of turret mooring systems. Catastrophe sets are constructed in two-dimensional parametric design spaces to determine the qualitative behavior of the system, and nonlinear time simulations are used to assess its quantitative properties. Static bifurcations of the principal equilibrium are compared to determine the nature of alternate equilibria. A turret-moored tanker is modeled with anchored catenaries, including nonlinear drag. External excitation is time independent, and for the numerical applications it is limited to steady current. Of the four models used, the Abkowitz and Takashina models show similar qualitative dynamics. The Obokata and Short-Wing models are also qualitatively similar, but very different from the first group. Limited sensitivity analysis pinpoints the source of discrepancy between the two schools of thought. [S0892-7219(00)01401-1]