Double-plume Integral Model And Its Application In Deepwater Oil Spill

The exploitation of deepwater petroleum has a broad prospect in the future.In the other hand,it will also increase the risk of deepwater oil-spill.Researches on the numerical model for spreading of spilled oil offer strong application value for pollution assessment and emergency disposal.Based on the double-plume integral model and Lagrangian particle tracking method,a three-dimensional model for underwater oil spill is developed to simulate the whole process of oil and gas rising and spreading from seabed to surface in a blowout accident.The model can be used to analyze the residence of spilled oil underwater and the surface extension of oil slick in preliminary stage.The underwater dynamic process of spilled oil is very complex and to improve its model is the key point of this study.In a blowout,the leaked oil and gas spray out and form a multiphase buoyant jet driven by the initial momentum and the buoyancy of oil droplets and gas bubbles.In the stratified deep-sea,the buoyant jet may have a complicated structure in which the dispersed bubbles and droplet remain in the central part of the plume and the outer part of the plume can leave and spread out horizontally at certain level.Based on the gas-liquid two-phase double-plume model proposed by former researchers,a three-phase(water,gas and oil)double-plume model was presented to simulate the underwater dynamic process of oil and gas,by considering the buoyancy and dissolution of oil into the governing equations.The buoyant jet in the model is divided into two parts,i.e.an inner circular plume and an outer annular plume.By solving the equations of mass conservation and momentum conservation,the representative parameters like mean velocity and plume width can be obtained.The model is appropriate in the condition of vertical stratification and weak cross-flow over the depth.In contrast with the numerical result and experimental data by previous researchers,the present model shows a good agreement in simulating some important parameters such as the trap height and the fraction of peeling flux.Then,we coupled the double-plume integral model with Lagrangian particle tracking method to simulate the whole process of oil and gas transportation from seabed to surface.The movement of particles includes time-averaged movement and random walk,in which the random walk is adopted for representing the turbulent diffusion of particles.The particles follow the movement of buoyant jet after ejecting form leak point and the velocity is given by double-plume integral model.When the particles separate from buoyant jet,they are dominated by ambient flow field which is obtained from large scale ocean model.After the particles reaching water surface,they are driven by the surface current and wind,to simulate the extension and drifting of surface oil slick under the force of astronomical tide and wind field.The Lagrangian particle tracking method coupled with double-plume model can simulate the residence of spilled oil in the water body and its distribution of concentration in the condition of stratified density environment.Finally,the model was applied to simulate ‘Deepwater Horizon' oil spill accident in the Gulf of Mexico in 2010.The unstructured finite volume ocean model(FVCOM)has been used to simulate the current.The surface wind field and the distribution of salinity and temperature were given by combining public observations and reanalysis data.Then the preliminary stage of the oil spill event from April 20 to April 25 has been simulated,and a distribution of spilled oil in the water body and surface is given.The results show several high concentration layers of oil and gas in different depth,which corresponds to the distribution of petroleum pollutants in the in-situ observation.The distribution of simulated oil slick at water surface was also in good agreement with the actual observed results.