Hybrid Verification of a DICAS Moored FPSO

Vessel motions and mooring line tensions of a DICAS moored FPSO are investigated experimentally and numerically. The model tests are carried out through 1:90 scaled experiments for an equivalent mooring and riser system at 450m water depth. The full-scale depth is 1240m. Extreme metocean conditions for Brazilian waters are simulated. Using the DEEP-C software, time-domain global hydrodynamic analyses of the equivalent system are performed. The numerical model is calibrated to reproduce the model test results for the equivalent model set-up. This is denoted as the model-the-model approach. Good agreement between numerical and experimental results is found for both the statistics and underlying time series. The calibrated model is extrapolated to full depth. Accordingly, numerical simulations of the full-depth system are performed. Comparisons show that an equivalent system can significantly under-predict the wave frequency part of the mooring line dynamics. This may be crucial for the assessment of fatigue life of chain segments and links. The extreme line-tensions which are crucial for the mooring system design, may be both over- and under-predicted by the equivalent system. The hybrid approach, i.e. combination of model tests and numerical simulations, is a viable way to obtain a numerical model of the full-depth system that can describe both the vessel motions and the line tensions well.