Numerical Experimental Set-up Of Oil Containment By Boom And Investigation On Containment Failure

Marine Oil spills can cause serious damage to natural resources and to those whose livelihoods depend on these resources. Unfortunately, experience shows that even the best safety efforts cannot prevent occasional oil spill accidents on the sea. Hence, it is important to improve techniques and equipment that facilitate spill cleanup under such circumstances. Oil spill containment booms are the most commonly adopted techniques employed to collect and contain oil on sea surface, or to protect specific areas against slick spreading. Oil containment, however, is often attempted under open-sea conditions, where currents, winds, and waves are present, and it is generally accepted that the effectiveness of booms is reduced under these conditions due to oil containment failure. The previous researches about oil containment failure mainly focus on the experimental and numerical study of oil containment by fixed barrier under pure current conditions; even the consideration of the influence of wave parameters on oil containment failure was limited to the case of experimental study of oil containment under a single wavelength of linear wave. So in the dissertation, CFD software FLUENT is re-developed in mathematical model, programming procedure, data access and analysis method to establish a numerical experimental platform based on viscous flow and VOF method, which can be used to simulate oil containment by floating boom under wave and current conditions. When compared with physical experimental platform, the numerical experimental platform has the advantage of low cost, non-contact flow measurement, reducing the scale effect, eliminating sensor size used in physical experimental, model deformation and other factors influencing the flow field, getting more detailed flow field information, and so on.Numerical methods of wave generating and absorbing are key techniques in the establishment of numerical experimental platform of oil containment by floating boom. Under the irrotational potential flow conditions, different numerical modeling methods (the pusher flap wave maker, the method of defining velocity of water particle on the inlet boundary, mass source wave-maker and momentum source wave-maker) have been employed to deal with the wave generating simulation of linear wave, second-order Stokes wave and solitary wave, using the Navier-Stokes or Reynolds Averaged Navier-Stokes (RANS) equation model and featuring closure via the k-ε turbulent model, employing the VOF (volume of fluids) method, which was developed to manage interfaces between multiple phases, i.e., water and air. The above wave generating methods for linear and nonlinear waves are firstly analyzed and compared in detail, the advantage and disadvantage of different methods are summarized and commented, some conclusions on application are then obtained.Wave parameters including boom draft, water depth, flow velocity and oil parameters have great influence on oil containment efficiency. Therefore, the effect analysis of oil containment by boom under wave and current conditions come necessary and urgently. Based on the model established above, the two-dimensional numerical wave tank is re-developed in source function, boundary conditions to establish the numerical wave and current tank of the viscous fluid with constant water depth simulating wave-current interactions, and then the interactions of wave and uniform current were discussed. The numerical results compared well to wave and ray theory and the Doppler Effect derived from wave action balance equation according to variational principle. The variations of wave profile under current action and the influence of the current velocity on the wave parameters were studied. The finds show that wave length is increased (reduced) and wave height is reduced (increased) by coplanar (counter) currents, even wave breaking occurs in counter currents. Currents will also cause wave refraction, wave amplitude converge and diverge by changing the propagating direction of wave. The effects of currents on wave crest with different wave steepness were further studied, for coplanar (counter) current cases, the same current velocity will change wave crest with bigger wave steepness more (less). Then a fully nonlinear numerical wave and current tank was developed based on the linear wave and current tank. The interactions of weakly nonlinear waves, strong nonlinear wave-current coupling were simulated and the effects of currents on wave nonlinearity were analyzed, it shows that opposing (coplanar) current increased (reduced) the wave nonlinearity, the nonlinear wave-current coupling interactions match Doppler Effect well.Finally, the variations of oil shape before boom under waves and currents were simulated and analyzed based on the numerical wave and current tank of the viscous fluid with constant water depth established above combined with using the three-phase flow model. Then the numerical experimental platform was established successfully by adding floating booms to the tank, utilizing the dynamic mesh model in FLUENT and the "half coupling" method to simulate the vertical heave motion of floating boom under waves and currents. Based on the numerical experimental platform, oil containment by floating boom under waves and currents were numerically studied, the effects of waves and currents on variations of oil shape before boom were analyzed. The finds show that oil slick moves toward the boom under the action of currents, the coplanar wave accelerate and the counter wave decelerate the oil slick moving towards boom slightly, oil slick thickness reached maximum in coplanar current conditions and minimum in counter current conditions, oil slick length reached minimum in coplanar current conditions and maximum in counter current conditions, the thickness of oil slick and the free-surface elevation at the boom were approximately in phase. When the wave at the boom is at its trough, the oil slick achieves its minimum thickness, and when the wave at the boom is at its crest, the oil slick achieves its maximum thickness and oil containment failure prone to take place. Then the relationship between oil containment failure velocity and oil boom draft, water depth, current velocity, oil parameters, wave parameters was quantitatively analyzed in detail. The results show that an increase in oil density is detrimental to oil containment under pure current condition, coplanar current condition and counter current condition. The bigger the initial oil volume, the lower the oil containment failure velocity. The greater the oil boom draft, the higher the oil containment failure velocity, and the relationship between oil boom draft and oil containment failure velocity show linear trend. The results also indicate that an increase in wave height will reduce oil containment failure velocity. Another important wave parameter likely to influence oil containment is the wave period. The variation of failure velocity with wave period was investigated. As the finds show, the longer the wave period, the higher the failure velocity and that a short and high wave is most detrimental to oil containment. According to the experimental research and analysis of the numerical results, empirical equations were proposed for the prediction of the initial failure velocity of oil containment by the floating boom under pure current condition, coplanar current condition and counter current condition respectively.