Seed Variations of Extreme Sea States and Repeatability of Extreme Crest Events in a Model Test Basin

Abstract

New methods for estimation of extreme wave crest height have resulted in an increase of the estimated 10,000 year crest height. In the Central North Sea this increase is typically 2 to 4 m. As a result several fixed platforms designed prior to 2000 may experience negative air gap if being hit by the 10,000 year wave crest height. Numerical methods have been used for assessing wave-in-deck impact loads. The model tests discussed in this paper were conducted to be used as verification of the numerical codes. For the model tests two sea states along the 10,000 year contour line were considered. Several 3-hour (full scale time) realizations were calibrated in order to capture the natural variability of the most extreme crest heights. Realizations were generated both by using random phase (and deterministic amplitude) and by using random coefficients (i.e. random phase and random amplitude), for the purpose of discussing possible difference between the two approaches. For wave deck impact problems, one is merely interested in the few very large wave crests out of a 3-hour simulation. A more efficient test scope would, therefore, be to generate only the largest wave groups of the realizations. For this reason the most extreme crest(s) per sea state were identified and most wave-in-deck tests were conducted by generating only the part of the time series containing the large crest(s). The adequacy of the crest characteristics of the short realization as compared to the crest characteristics of the full series will be discussed. Impact tests will by nature involve rather steep waves that may be in the vicinity of breaking. The repeatability of such waves can be challenging. For the purpose of simplifying the interpretation of wave-in-deck impact loads, the repeatability (the uncertainty in the crest that actually approaches the platform) of these wave groups will be discussed.