Green water evaluation for FPSOs during cyclones

A numerical tool is developed to predict the heading of a weather-vaning FPSO and the freeboard exceedance distribution around an FPSO for the assessment of greenwater overtopping. The heading prediction is compared with full-scale measurements from an operating FPSO for validation. The validated model is then applied to explore the behavior of FPSOs during the passage of tropical cyclones off the North West Shelf (NWS) of Australia. A synthetic database is used to represent tropical cyclone metocean conditions. The vessel is found to experience oblique wave attack frequently due to the misalignment in waves, current and wind. At some instances the vessel can be exposed to beam seas during the passage of a cyclone. Analysis of freeboard exceedance indicates that three locations around the vessel are susceptible to increased risk of greenwater, these are the bow area, amidships and the stern of the vessel. It is also found that for locations around the bow and stern areas of the vessel, combinations of slightly smaller wave heights and shorter wave periods can lead to larger freeboard exceedance events. The freeboard exceedance statistics are sensitive to the vessel heading for the locations on the vessel where the exceedance event occurs. The incoming waves associated with extreme relative wave motion, i.e. freeboard exceedance, in directionally spread seas typical of tropical cyclones are identified. It is found that the maximum elevations of these incoming waves occur some time (on the order of one wave period) before the maximum freeboard exceedance for the seastates and vessel locations analysed due to the vessel motions.