Two computer programs, WINPOST-FPSO and WINPOST-POLY, are developed for hull/mooring/riser coupled dynamic analysis of a deepwater floating platform in waves, winds, and currents. WINPOST-FPSO is developed for the coupled dynamic analysis for a turret moored FPSO (Floating Production Storage and Offloading). The program is extended from the current coupled dynamic analysis program WINPOST by adding the capability of a large-angle rotation about the vertical axis (yaw rotation). WINPOST-POLY is developed to perform coupled dynamic analyses with polyester mooring lines that allow large strain and nonlinear stress-strain relationships. In these two programs, the floating platform is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass, and radiation damping are calculated from the hydrodynamics program WAMIT. The wind and current forces in WINPOST-FPSO are modeled following the method suggested by OCIMF (Oil Company International Marine Forum).
The mooring line dynamics are modeled using a rod theory and finite element method, with the governing equations described in a generalized coordinate system. A complete derivation of rod theory for polyester lines is presented. The dynamic modulus of polyester is modeled following a regression formula derived by Bosman and Hooker (1999), which is based on experimental data and depends on loading conditions. The effect of large elongation and nonlinear stress-strain relations are separately assessed.
The nonlinear coupled responses of a turret-moored FPSO in a 100-year hurricane condition (non-collinear winds, waves, and currents) are investigated, and the results are compared with those from MARIN’s computation and experiment. The statistical nature of the responses is also studied. A comprehensive sensitivity/parametric study is conducted to figure out the role of each hydrodynamics parameter.
The numerical predictions in time domain of a spar and a tensioning buoy with polyester lines are carried out and the results with WINPOST-POLY are systematically compared with those from the original program WINPOST. Both the large elongation assumption and the time dependent modulus have a significant effect on the tension and motion. The statistics of the two programs are also studied.