Airgap analysis of floating structures subject to random seas: Prediction of extremes using diffraction analysis versus model test results

Statistical and dynamic non-linearities in ocean waves and wave-structure interaction are considered. Focus is on prediction of extreme airgap events for semisubmersibles. It is recognized that ocean waves are inherently nonlinear, and that this non-linearity affects the statistics of extreme crests. Two sources are assumed to account for all nonlinearity: incident waves, and wave-structure interaction. Various methods of predicting extremes based on post-processing hydrodynamic analysis and model test results are proposed. Methods are tested against model test results for the Veslefrikk semisubmersible.; First, methods using regression and fractile trend-lines to predict extreme airgap events from model test results are developed and confirmed.; Second, methods to predict extreme airgap events based on linear diffraction results are developed and confirmed. All of these new models include Stokes second-order incident waves.; Third, full second-order (WAMIT) hydrodynamic panel diffraction is applied. Two new methods of modifying quadratic transfer functions (QTF's) are developed: In one, QTF's predicted by WAMIT are replaced with those predicted by Stokes theory for short periods only. In the other, known on-diagonal QTF's predicted by multi-column analysis (WACYL) are extrapolated to estimate off-diagonal terms. WACYL QTF magnitudes are found to be reasonable for all period ranges, so no Stokes substitution is necessary.; Fourth, black-box system identification is used to extract first- and second-order transfer functions from measured data. The reasonableness of the Stokes substitution is confirmed, as is the capability of second-order modeling of airgap demand.; Finally, a system identification based on Stokes second-order theory is applied to incident waves. First- and second-order components of a specified wave history are identified, and results are used predict consistent first- and second-order wave time-histories at other spatial locations.; The most significant conclusions are: second-order effects are important to prediction of airgap demand; WAMIT over predicts these effects for high frequencies; incident waves are a meaningful source of second-order effects, and application of first-order diffraction with second-order incident waves is reasonable when estimating airgap extreme statistics.