A computer program system for coupled numerical analysis of moored vessels is described, where the vessel is integrated as a part of a finite element model of the complete mooring/riser system. This gives a proper time-domain interaction relationship between the vessel motions and the line dynamics. Experimental verification of the program is carried out by detailed numerical reconstructions of time series from model tests with a semisubmersible. By use of empirical drift forces estimated from the tests, the reconstructions compare well with the measurements, while standard potential theory significantly underestimates slow-drift in large waves. The program is then applied in an investigation on a hybrid model testing technique, where simplified tests in reduced depth are reconstructed and extrapolated numerically. By reproducing almost realistic catenary lines in the reduced depth model tests, one can calibrate the numerical model by the reconstruction at this depth, and calibrated parameters are kept in the extrapolated simulations. Comparisons to full-depth model tests show that the numerical extrapolations of slow-drift and line tension work reasonably well, although the line dynamics are slightly underestimated.


Model test verification of floating production systems moored in very deep water meets a limitation in the size of test facitilies. With water depth exceeding 1000m, conventional methods require wide basins with an overall depth of more than 10- 20m for complete mooring and riser modelling. Such facilities are hardly available today. Alternative methods for reliable verification are needed. To some extent, one can bring further conventional complete system testing techniques into deeper waters by developing ultra-small scale techniques, as described by Moxnes & Larsen (1998). Alternatively, or in combination with ultra-small testing, hybrid testing techniques may be applied, where full mooring model tests are replaced by tests at a reduced water depth, combined with numerical simulations to predict full-depth results.

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