Many researches about green water on FPSO tend to ignore or simplify details like bilge keel, mooring and risers, leaving some gap between academic study and industrial application. This paper presents an efficient numerical approach in frequency domain for FPSO green water assessment, considering nonlinear effects from bilge keels, spread mooring and asymmetrically arranged risers. Comparisons with experimental measurements show that the adequacy of relative motion response prediction is significantly improved by incorporating the equivalent nonlinear effects. Numerical analysis indicates that the green water from sides of the FPSO in oblique and beam sea waves are affected by the presented effects. Application and discussion are further made on the freeboard exceeding frequency at various locations on the FPSO.
Green water impact often occurs on ships and offshore platforms. When large relative motion between wave and platform occurs, sea water may exceed the freeboard and run up over the deck, threatening the safety of topside structure, equipment and working crew. Ersdal and Kvitrud (2002) summarized a number of Norwegian production ships being attacked by green water in 1998 to 2000, getting various damages on topside structures and equipment, as well as economic loss due to repair and operation downtime. In some extreme situations, green water had even broken windows and ceilings, and ingress into rooms and corridors. The trend of operating offshore platforms in more severe environmental conditions has increased the importance and challenge of keeping the severity and frequency of green water impact under control. It is therefore important to assess the risk of green water numerically during the initial design phase of an offshore project, before sophisticated experiment being conducted at a much later stage.
The entire green water phenomenon is roughly consisted by two stages: the global stage and local stage. In the global stage, wave exceeds freeboard due to relative motion of water and vessel. In the local stage, local interaction occurred between water and structure above water level, like bow flare, deck and topside structures, etc., which can be further divided into several steps for more detailed description. Among various methods of analyzing and predicting green water on ships and FPSOs, the most efficient approach is to calculate the relative wave motion and freeboard exceeding from the simulation of global stage using linear potential theory, and assess the green water risk by comparing the results directly with some simplified criteria. Safety criteria applicable to this approach include considering deck wetness probability to be below 30hits/hr as safe (Karppinen, 1987; Smith and Thomas, 1989; NATO STANAG 1454–2000), or below 5% (NORDFORSK, 1987; Faltinsen, 1990), or to consider the freeboard exceeding of 3m and 6m as threshold for classifying low/medium and medium/high risk of green water (Morris et al., 2000). By using such type of occurring frequency-based or height-based criteria, preliminary green water risk assessment can be conducted directly, using freeboard exceeding frequency or height. Application of this type of approach can be found in Bales (1977), Morris et al. (2000), HSE UK report (2001), Fyfe and Ballard (2003), etc.