In approximately one year from September 2004 to September 2005, three Category 5 hurricanes (Ivan, Katrina, Rita) hit the Gulf of Mexico. Well over 80% of the 4,000 oil and gas production platforms in the Gulf were directly impacted by the hurricanes. The hurricanes destroyed or caused extensive damage to 190 platforms. In most cases the platform damage was caused by green water wave loading on the deck.
Green water damage to floating structures results from high pressures and dynamic loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. In situations where one must consider the possibility of wave overtopping and green water rushing onto the deck, the mechanics of wave loading become very complex and are poorly understood. One of the major sources of uncertainty is the velocity field of the green water flow itself.
The objective of this research is to develop a robust procedure to estimate local and global green water loads on structures due to extreme wave crests. Through the combined efforts of laboratory measurements and numerical simulation, the results will allow designers to avoid or minimize the impact of green water on new floating structures through design, and help the industry and regulators to develop associated design guidance. This research is a continued effort after the successful formulation of green water over a 2D platform through laboratory measurement, and a continuation on the development of a 3D computational fluid dynamics (CFD) code for the simulation of green water. The prior study has shown that the traditional prediction method often used in design, i.e., the dam breaking model, results in significant discrepancy between the model and the laboratory measurements. Since the more realistic 3D prediction model is not yet established, the continuation of the research is critical for the prediction of green water and subsequently its mitigation. The study considers 3D structure geometries such as TLP’s, spars, and ship-shaped FPSO’s.
In the experimental approach, we first use laboratory measurements to investigate the void fraction as well as the flow volume, flow rate, water velocity, water elevation and momentum flux of an overtopping flow on a 2D structure. The flow structure on a 3D model was then measured with preliminary results presented. Green water was generated by the impingement of a plunging breaking wave on the structure following the Froude similarity of an extreme hurricane wave and a simplified offshore structure. The flow is multi-phased and turbulent with significant aeration. A fiber optic reflectometer (FOR) and bubble image velocimetry (BIV) were employed to measure the void fraction and velocity in the flow, respectively, and to determine the water level on the deck. Mean properties of void fraction and velocity were obtained by ensemble averaging
and time-averaging the repeated instantaneous measurements. The temporal and spatial distributions of void fraction reveal that the flow is very highly aerated near the front of green water and has relatively low aeration near the deck surface. The mean void fraction and velocity distributions were also depth-averaged for simplicity and potential use in engineering applications. Using the measured data, similarity profiles for depth-averaged void fraction, depth-averaged velocity, and water level were found. The study suggests that using only the velocity data is insufficient if the flow momentum or the flow rate is to be determined. The accuracy of the void fraction measurements was validated by comparing the directly measured water volume of the overtopping flow with the calculated water volume based on the measured velocity and void fraction.
In the numerical approach, an interface-preserving level set method was incorporated into the Reynolds-Averaged Navier-Stokes (RANS) method for the simulation of the green water effect. In the method, free surface flows are modeled as immiscible air-water two-phase flows and the free surface itself is represented by the zero level set function. Calculations were performed for several benchmark free surface flows including dam break flows, free jets, solitary wave propagation and the impingement of dam break flow on a fixed structure. After some validation, the method is applied to simulate sloshing flows in an LNG tank and green water over an offshore platform. The good agreement between numerical and experimental results prove the level set RANS method is a powerful and accurate CFD methodology in free surface
flow simulations.
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