Non-Gaussian Air Gap Response Models for Floating Structures

The air gap response of a specific semisubmersible platform subjected to irregular waves is considered. Statistical analyses are performed on model test data for the absolute near-structure wave elevation, and these measured data are compared with predictions resulting from probabilistic models. Models applied are first-order and second-order diffraction models typical of standard practice, and two new hybrid models that include second-order effects in the incident wave, but not in the diffracted wave. The first of these hybrid models is moment based, while the second relies on narrow-band random process theory. Either of these new models can be implemented in place of the standard linear-only model with little additional computational effort, as only linear diffraction analysis is required. Both new models are found to better predict the air gap demand than standard linear diffraction analysis.