Advanced Multiphase CFD Modelling to Understand Impact of Undersea Oil Spill

Abstract

The growing energy demand across the globe has resulted in an increase in the deep water oil exploration activities which in turn has increased the risk of occurrence of associated accidental oil releases. Oil and Gas offshore businesses are important to Qatar's economy. Further, the country is heavily dependent on its marine water resources for fulfilling its potable water needs through the desalination process. So, it is imperative to safeguard the marine environment during an unfavorable accidental oil release, which essentially would put entire ecosystem into peril. Development of reliable models can assist in predicting the extent of damage caused to the environment and can further help in deciding the mitigating strategies during such events. The current research focusses on exploring the of capabilities of Multiphase Computational Fluid Dynamics (CFD) models in simulating various transport processes associated with deep water oil spills. The accidental release of oil in deep oceans results in formation of the plume. The interaction of inertial oil mass with surrounding water results in the formation of droplets which rises in the water column due to buoyancy. Dispersant addition is one of the preferred methods of oil spill remediation which causes the lowering of interfacial tension at the oil/water interface and under the action of local turbulence, it enhances the droplet disintegration process. In deep spill scenarios, droplets spend large amounts of time in the water column, hence, the dissolution process of soluble hydrocarbons which otherwise is detrimental to aquatic life, becomes important. The objective of this work is to develop integrated numerical models which can effectively guide us in predicting the fate of oil mass in such scenarios and help us is estimating the overall impact of such accidents on the environment. Before taking a leap into full scale modelling, it is imperative to grasp a good understanding of above mentioned transport processes at a more fundamental scale. Hence, in the first phase of this project, single droplet dynamics in quiescent systems were studied. We primarily investigated the effect of surfactant (the chief component of a dispersant), on the dynamics of a crude oil droplet rising in a stagnant column. Laboratory scale experiments were performed and a multiphase CFD model based on Volume of fluid method was developed to capture the shape dynamics of the droplet rising in a surfactant laden environment. To capture the subsurface dissolution of hydrocarbons from oil droplet, a unique experiment was devised wherein a binary organic mixture, representing a pseudo oil droplet comprising of volatile and non-volatile hydrocarbons, was employed to study the effect of unsteady mass transport on the overall dynamics of the droplet. Based on the experimental observations, correlations were proposed to estimate the mass transfer rate at various stages of droplet motion. A CFD model capable of evaluating concentration fields of the transported species in both dispersed as well as continuous phases, was developed by coupling VOF approach with species transport model. The above models were also employed to study the jet breakup dynamics in the laminar regime. The next challenge was to extend the applicability of developed models to large scale scenarios. Turbulence is inherent to oceanic environment and hence incorporating it into existing models becomes important. In a real deep spill scenario, a swarm of bubbles and droplets often interact with each other in a turbulent environment and this leads to occurrence of a sequence of coalescence and disintegration processes. The information on size of droplets serves as an important parameter for the evaluation of approximate rise velocities and overall mass transfer rates in the system. Thus, a model capable of predicting droplet size distribution can be employed for determining the fate of droplets in the event of accidental subsea releases. In current work, this objective was achieved by integrating traditional multiphase CFD models and turbulence models, with a population balance (PB) approach. The developed model was validated against the experimental observations reported in Johansen et al (Marine Pollution Bulletin. 2013;73(1), 327–335). Through this work, we were able to demonstrate the capability of an integrated CFD+PB model in analyzing the effect of dispersed (oil) phase flow rates, the presence of dispersants and the presence of air (introduced along with the dispersed phase) on the overall size distribution of oil droplets.