Slow motion dynamics of dicas mooring systems under steady current, wind, and steady drift excitation

摘要

The slow motion dynamical behavior of a Differentiated Compliance Anchoring System (DICAS), supject to a range of directions of external excitation of constant magnitude, located in the Marlin Field, Campos Basin, Brazil, is assessed based on nonlinear stability analysis and bifurcation theory. Excitation consists of steady current, wind, and second order mean wave drift forces. Catastrophe sets are constructed in a two-dimensional parametric design space, separating regions of qualitatively different dynamics. Stability analysis defines the morphogeneses occurring across bifurcation boundaries to find stable and limit cycle response near the principal equilibrium position. The resulting design graphs allow the designer to select an appropriate orientation and other design variables for DICAS without resorting to trial and error, or extensive nonlinear times simulations. The positioin of the vessel at equilibrium defines the mean horizontal displacement of the system with respect to a prescribed initial orientation. This position depends on the magnitude and direction of the external excitation. Limited simulations or further nonlinear analysis enable the designer to investigate whether or not DICAS slow motions comply with the allowable limits of motions for safe operations. The effect of water depth variation on the stability of DICAS is considered, and it is shown that the dynamics of the system change considerably with relatively small variation in water depth. The DICAS mathematical model consists of the nonlinear, horizontal plane fifth-order, large drift, low-speed maneuvering equations. Mooring lines are modeled by catenaries with touchdown and nonlinear drag. External excitation consists of time independent current, steady wind, and second order mean wave drift forces.