A new method for the prediction of non-linear relative wave motions

The problem of green water on the bow decks of floating offshore production units in large survival waves is an important design problem. An accurate description of the relative wave motions around the bow is important for the prediction of green water on the deck. In the paper a new method is presented to predict these relative wave motions, which are highly non-linear. Input for the method is the calculation of the relative wave motions in front of the bow with linear (3D) diffraction theory. Based on this input the proposed method is able to account for the non-linearities which occur as a result of the non-linearity in the waves, the influence of the above water hull shape (bow flare) and the effect of green water on deck on the ship motions. The method is based on a modification of the Raleigh distribution and allows a fast evaluation of the effect of the above water hull shape (bow flare angle and freeboard height) on the relative wave motions. The prediction of the extreme relative wave motions can be used for the prediction of the required bow height for dry fore decks or to determine the maximum freeboard exceedence as input to the study of green water on the deck.