| dc.description.abstract | Nonlinear effects become more important in predicting the motions of a wave energy
structure (WES), which is subject to large (relative to its dimensions) motion amplitudes.
To model the WES more accurately, a time-domain program (SIMDYN) is further
developed. In particular, SIMDYN’s “blended” option improves the linear option by
accounting for not only the nonlinearity of important external forces but also the
nonlinearity due to large body rotations (i.e., inertia forces). To reveal the significance of
these nonlinear effects forced motion analysis are performed. The simulation results from
SIMDYN under the blended option are examined by model test correlations, which has
seldom been done before for a WES.
Besides that, the other important discrepancy in WES modelling: viscous damping is
studied. By applying an advanced system identification technique, Reverse-Multiple Input
Single Output (R-MISO), to model tests of a WES under random waves, viscous damping
of a realistic (typical catenary moored) system is studied. Based on the comparisons
between the frequency dependent transfer functions from the simulations and those from
the model tests, reasonable linear or quadratic damping have been extracted. In a sense,
this methodology can become a powerful alternative in damping corrections for WES
under random sea states.
Compared to other quantities of interest which have been extensively studied in typical
design practice, the dynamic stability has not been investigated adequately. The Melnikov
function model and the Markov process model are two efficient approaches providing quantitative predictions of capsizing. In the last part, to predict the pitchpoling risks of a moored floating cylinder representing a generic WES under random excitation, the two methods have been explored. Using the Melnikov approach, the rate of phase space flux was evaluated to quantify the dynamic stability. This approach is compared with the Markov approach, which evaluates the mean first escape rate to quantify the vessel’s dynamic stability. The two methods are investigated systematically by varying important parameters, which include the linear and quadratic damping, the mooring systems and the sea states. | en |
