As offshore reservoirs are depleted, the seabed may subside. Bottom fixed installations which have previously had sufficient clearance between the deck and the sea surface may be in a situation where wave impact with the deck must be considered at relevant probability levels. Some statistical aspects associated with the calculation of a deck impact load with a prescribed probability of occurrence are the subject of the present paper.

The Short Crest JIP addressed the distribution of the crest height in extreme sea states, the properties of the largest crests and the deck impact loading on a closed deck. It was concluded that the largest waves in the sea may be in the process of breaking and thus have properties which deviate significantly from estimates found from weakly nonlinear irregular or regular wave theory.

The present paper addresses findings from the Short Crest JIP regarding

• long-term analysis of wave heights and crest, including the effect of wave breaking

• air gap analysis for jacket, TLP and semisubmersible using 2nd order time domain simulations over the platform area

• statistics for horizontal wave-in-deck impacts for short crested sea versus for long crested sea

• structural reliability analysis of jackets for some simple limit states that are governed by loads caused by impact of extreme crests

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