SYSTEMATIC INVESTIGATION OF THE DYNAMICS OF A TURRET FPSO UNIT IN SINGLE AND TANDEM CONFIGURATION

摘要

This work considers nonlinear dynamical aspects of a turret FPSO unit plus shuttle vessel. Firstly, the dynamics of the main FPSO unit alone is investigated under the combined action of wind and current. Several environmental conditions as defined by a systematic variation of relative angles and speeds are considered. The effect of the longitudinal position of the turret is also assessed. The relative importance of wind and current forces also depends on the draft of the vessel, and therefore the analyses are performed for two load conditions. As a first step in characterizing the most relevant aspects of the problem, static equilibrium solutions are calculated. Next, their stability properties are studied. The system displays a variety of different regimes of solutions in which both their number and their stability may change as one or more parameters are varied. The mathematical model of the forces due to the hydrodynamic action of currents in based on a theory of low aspect ratio wings that includes experimentally verified heuristic terms. Wind action is modeled by drag forces using experimental coefficients. The tensions in the mooring lines are approximated by linear spring formulae. Due to the great complexity of the mathematical models involved, solutions are obtained numerically, and their stability is studied via time-domain simulations. Results are summarized in a series of bifurcation diagrams covering the influence of all relevant parameters involved, namely, wind to current speed ratio, wind and current relative angles of incidence, position of the turret, and vessel draft. A potentially more complex situation arises during the offloading operation when a shuttle vessel is attached to the FPSO unit through a hawser. The two vessels form a coupled system for which new equilibrium positions exist with their own stability properties. The shuttle vessel is shown to exhibit at least two equilibrium solutions for the whole range of parameters under investigation. For each equilibrium solution of the shuttle vessel the FPSO unit can have two or more equilibrium positions. The study of the FPSO-shuttle vessel tandem configuration can therefore be quite complex. In this work preliminary results about the dynamics of the two-body floating system are discussed.