In the paper the relative wave motions along the side of a weathervaning FPSO are investigated. In non-collinear directions of wind, waves and current the vessel does not always encounter head waves. With wave headings of typically 15-30 degrees from head waves, the strongly non-linear relative wave motions along the side can come above the freeboard of the vessel and result in greenwater flow onto the deck. At several existing FPSOs this already resulted in damage, typically at midships and further aft.The paper first discusses the background of the occuring phenomena. Then a method for the prediction of the non-linear relative wave motions along the hull is presented. This is necessary to predict the possible freeboard exceedances. Finally a new estimation method for the prediction of the green water loads on slender structures is presented, based on the exceedance of the freeboard on the side.