Nonlinear dynamics and stability of spread mooring with riser

Abstract

The dynamics of a floating vessel with its spread mooring and riser subjected to environmental excitation from current, wind, and waves is studied to investigate the effect of riser dynamics on the stability and bifurcation sequences of stationkeeping. A design methodology is developed to reveal the dependence of slow motion dynamics of spread mooring systems (SMS) on several design parameters, such as riser location, water depth, length and fairlead of mooring lines, orientation and pretension in the mooring lines. Nonlinear stability and bifurcation theory are used to produce catastrophe sets in the design space defining regions of qualitatively different dynamics. Limited time simulations are performed to verify the richness of the non-linear system dynamics identified in the catastrophe sets. The mathematical model is based on the third-order maneuvering equations in the horizontal plane including hydrodynamic memory. The riser dynamics is modeled quasi-statically by a nonlinear three-dimensional large deformation finite element model. Mooring lines are modeled by deep-water catenaries with drag and touchdown effect. External excitation consists of time-independent current, wind, and mean wave drift forces.