Theory and simulations have shown that the maximum crest height in an area can be substantially greater than the single point maximum. Theoretical results have been tested and confirmed using data from a 10 by 10 array of wave probes in the Marin Offshore Basin. Estimation of the maximum crest height under a TLP’s deck is more complicated than it is in unobstructed waters. A TLP’s large columns diffract and radiate incident waves. Fortunately, the short term statistics of the diffracted waves have the same form as the short term statistics of the incident waves. The maximum crests under the TLP are estimated by using diffraction theory to find the wave spectra at each point. Then, linear simulations are used to find time series at each point. Finally, the linear maxima are multiplied by the ratio between the theoretical linear and second order crest heights. Comparisons with measurements from wave probes placed under a TLP model in the Marin Offshore Basin support the accuracy of this process. These results mean that the maximum crest heights in the area under a deck must be considered in air gap calculations.

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