A simple method for solving water impact loads underneath decks of offshore structures is developed. In the present paper the emphasis is on the vertical loads, but in principle the suggested method can also be applied to horizontal loading. The suggested method is three-dimensional and valid for general deck geometries and arbitrary incoming wave direction. First and second order wave amplification due to the large-volume structure is included in the analysis. An important feature of the present approach is that added mass of the instantaneous wetted deck area is approximated by the added mass of thin rectangular or elliptical plates. A numerical tool for solving the impact loads is implemented. This tool uses the results from an a priori second order diffraction analysis of the platform hull. In particular the wave-in-deck simulation program applies linear and quadratic transfer functions from the diffraction analysis as input. Since pre-computed hydrodynamic quantities are used in the simulations, very fast computations can be performed. The method is validated against experiments. Results from scaled model tests of the Statfjord A gravity-based structure (GBS) have been compared to numerical results. The comparisons are limited to regular waves. Satisfactory results are obtained from the numerical simulations. The theoretical results compare well with the experiments for the most severe cases. The vertical loads on the deck are well reproduced both during the water entry phase and the water exit phase. Moreover, the duration of the wave-in-deck event is satisfactorily predicted.

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