Analysis of green water events on a typical FPSO design

Abstract

This study dealt with the problems caused by green water waves, waves that reach so high that significant overflow onto the deck occurs. The most important hazard caused by green water is the direct impact load on deck, threatening equipment, hull structure and personnel. The goal of current green water research is to be able to (better) predict the risks of green water occurrences and the possible damage such an event might cause. Specifically our research has focused on the adequacy of the prevailing method of predicting green water flows, a method that draws on the analogy of the water flows released after a dam breaks (the so called Dam Break theory). Although this model is validated and widely used, we are not able to accurately predict the amount of water on deck, given a certain exceedance of the freeboard. To be able to do predictions for engineering purposes, physical model tests are the only accurate tool to predict green water susceptibility of an FPSO, as the software is limited. Since model tests are expensive, a better understanding of the green water problem is clearly needed. The hypothesis is that Ritter's Dam Break theory does not predict the water on deck accurately because different physical phenomena lead to different types of green water waves, not only the Dam Break type of flow. Instead of modifying the initial Dam Break theory to get to an accurate prediction method, an alternative approach to a better description of water height on deck is chosen, in which some, but very limited research is done. The assumption is that one theory does not cover the physical characterisations of all green water events. By comparing results from earlier tests conducted by SBM in the MARIN basin, we found significant differences between the Dam Break theory predictions and the empirical test results. By comparing information extracted from video footage with sensor measurements made simultaneously, we explored various explanations for these different trends.