Airgap analysis of floating structures: first- and second-order transfer functions from system identification

System identification is used in a detailed comparison of model test data and numerical diffraction results. 'Optimal' quadratic transfer functions based on system identification of measured data are compared with those resulting from WAMIT hydrodynamic analysis and those from Stokes theory in order to understand the implications of a previously proposed 'Stokes substitution', which applies a hybrid of second-order transfer functions resulting from hydrodynamic diffraction and those resulting from Stokes wave theory. The goodness-of-fit of a second-order model, i.e. fraction of power explained through a second-order system identification, is examined to assess the possibility that a higher-order model may be needed. The results of the system identification suggest that (a) the Stokes substitution is reasonable and (b) quadratic transfer functions at high frequencies resulting from WAMIT analysis are not reasonable. (C) 2002 Elsevier Science Ltd. All rights reserved.