Using a simple von Karman approach, Baarholm (2005) obtained good agreement between measured and predicted impact force underneath the deck of a GBS. This method rested, however, on a very careful second order modelling of the diffracted free surface elevation underneath the deck. In particular, the method was found to be very sensitive to the maximum wave elevation (i.e. the magnitude of the negative airgap). These requirements limit the applicability of the method since an accurate representation of the surface elevation is not usually available. In the design of an offshore structure, however, much effort is spent on determining the airgap underneath the deck. Zones with negative airgap are found by approximate calculations or by extrapolating on model test results. The method described in the present paper simplifies the problem by using Baarholm’s method subject to a known negative airgap height above the deck rather than attempting to solve the whole problem. With a known negative airgap height and using the well known expected linear shape of the wave around this maximum, a robust and conservative estimate of the deck loading can be obtained. The nonconservative property of linear theory that it underestimates the crest height is thus removed and replaced with the conservative property that too much water is present above the deck level. The paper describes this method, provides a practical design example and compares the results with model test results. The agreement with model test results is good for the single event which has been compared, but there is a clear scope for further comparison.

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