Float-over method has been developed and successfully applied to offshore installations due to its relatively low operational costs and higher installation capacity. Such installations generally involve multibody dynamics with wave-induced motions and nonlinear constraints. The dynamics of float-over installations may exhibit subharmonic motion and chaotic motion, under environmental loads. This paper establishes a time domain model for the float-over installation based on a linear potential flow approach. A combination of hydrodynamic analysis of the wave-induced dynamics, and bifurcation analysis of dynamic motion, is applied to investigate the characteristic of float-over system. The three degrees of freedom motion equation (including sway, heave and roll) is established and numerically solved using 4th Runge-Kutta method. The techniques of Poincare maps, bifurcation diagram and power density spectrum are used in this paper.
Load transfer is one of the critical tasks during the float-over installation and involves considerable risks and technical challenges. As the topside is lowered by the transportation barge using the ballasting tanks, the docking pins on the topside will make great and nonlinear impact on the mating cones under the environmental loads. And the guiding systems such as leg mating unit (LMU), deck supporting unit (DSU) and sway fender are designed to provide the positive alignment and adequate control of the system motions. In order to ensure an efficient and safe float-over installation, it is vitally important to predict the dynamic motions of such system during the load transfer stage. In the past, considerable numerical efforts have been made to understand the nonlinear dynamics of marine structures using the methods of nonlinear stability analysis. Virgin, et al. (1988) studied the nonlinear motion of a typical semi-submersible in regular waves using a nonlinear oscillator model. Kim, et al. (2001) presented the complex dynamics of a floating vessel with mooring and riser under the environmental loads.
In their paper, the catastrophe sets differentiating the stable regions from the dynamic responses were obtained using nonlinear stability and bifurcation theory. Lu, et al. (2013) established the one dimensional motion equation of a submerged moored floating structure. A series of period doubling bifurcations and chaotic motions were found in their models.
