This work presents an enhanced hybrid methodology for the analysis and design of floating production systems (FPS). The semi-coupled (S-C) procedure exploits the advantages of coupled and uncoupled models, incorporated into a three-stage sequence of analyses that can be fully automated within a single analysis program, presenting striking reductions of computational costs. The procedure begins by determining, through a full nonlinear static coupled analysis, the mean equilibrium position of the FPS with its mooring lines and risers. Then, it automatically evaluates equivalent six degrees-of-freedom (6DOF) stiffness matrices and force vectors representing the whole array of lines. Finally, these matrices/vectors are transferred to the dynamic analysis, solving the global 6DOF equations of motion restarted from the static equilibrium position. This way, the S-C methodology represents all nonlinear effects associated with the lines and considers their influence on the dynamic behavior of the hull. However, in some situations, it could still overestimate dynamic amplitudes of low-frequency (LF) motions and/or underestimate amplitudes of line tensions. Thus, to improve the overall accuracy, enhanced procedures are incorporated to better represent damping and inertial contribution of the lines. Results of case studies confirm that this methodology provides results adequate for preliminary or intermediary design stages.

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This work presents and evaluates a hybrid methodology for the analysis of FOWT, taking advantage of the SITUA-Prosim code developed by LAMCSO/COPPE/UFRJ, and the NREL FAST code. SITUA-Prosim has been originally developed for the coupled analysis of floating platforms for oil production. It fully couples a validated hydrodynamic model, which represents the hull motions, with a full nonlinear dynamic Finite-Element based structural model, which represents mooring lines and risers. The first step of the methodology is to employ the FAST code to generate time-series of forces on the bottom of the tower. These forces are then applied as input for an analysis with the SITUA-Prosim code where the platform and mooring lines are fully coupled, the latter being represented by a full nonlinear dynamic FE model. This may be considered as a “hybrid” methodology since it uncouples the platform motion response from the forces from the wind turbine, but couples the platform motions with the nonlinear dynamic structural response of the mooring lines. The methodology is applied to analyze the OC4 DeepCwind semisubmersible, taking different wind velocities, and evaluating the influence of the elastic behavior of the blades on the global motion and line tension responses. The results are compared with a standard procedure where the whole system is analyzed entirely by the FAST program. Due to its simplification, this hybrid methodology can be employed for preliminary or intermediate design stages. The main goal of the studies presented here is to comprise an initial step toward a full integration between the Prosim and FAST programs. In future works a full coupling between the wind turbine forces and the platform motions will be implemented.