Dynamic Coupling of Near-Field and Far-Field Models

Deepwater spills pose a unique challenge for reliable predictions of oil transport and fate, since live oil spewing under very high hydrostatic pressure has characteristics remarkably distinct from oil spilling in shallow water. It is thus important to describe in detail the complex thermodynamic processes occurring in the near-field, meters above the wellhead, and the hydrodynamic processes in the far-field, up to kilometers away. However, these processes are typically modeled separately since they occur at different scales. Here we directly couple two oil prediction applications developed during the Deepwater Horizon blowout operating at different scales: the near-field Texas A&M Oilspill Calculator (TAMOC) and the far-field oil application of the Connectivity Modeling System (oil-CMS). To achieve this coupling, new oil-CMS modules were developed to read TAMOC output, which consists of the description of distinct oil droplet “types,” each of specific size and pseudo-component mixture that enters at a given mass flow rate, time, and position into the far field. These variables are transformed for use in the individual-based framework of CMS, where each droplet type fits into a droplet size distribution (DSD). Here we used 19 pseudo-components representing a large range of hydrocarbon compounds and their respective thermodynamic properties. Simulation results show that the dispersion pathway of the different droplet types varies significantly. Indeed, some droplet types remain suspended in the subsea over months, while others accumulate in the surface layers. In addition, the decay rate of oil pseudo-components significantly alters the dispersion, denoting the importance of more biodegradation and dissolution studies of chemically and naturally dispersed live oil at high pressure. This new modeling tool shows the potential for improved accuracy in predictions of oil partition in the water column and of advancing impact assessment and response during a deepwater spill

Summary of Contemporary Research on the Use of Chemical Dispersants for Deep-Sea Oil Spills

Mitigation options for deep-sea oil spills are indeed few. In the open ocean, far from land, booming, burning, and mechanical pickup of oil at the sea surface may be of limited value due to wave and wind conditions. The use of chemicals to disperse oil into smaller droplets is predicated on the assumptions that smaller droplets are more easily dissolved into surrounding waters and that smaller droplets are degraded by bacterial action more rapidly than are larger droplets. During the Deepwater Horizon accident, a novel use of dispersants injected directly into the subsurface source of the blowout was undertaken to treat the oil prior to surfacing. The presence of subsurface “plumes” of small droplets and dissolved oil observed during DWH raised the issue of active measures to sequester oil in the subsurface vs. allowing it to surface. Reducing the concentration of volatile organic compounds surfacing near workers was also a stated objective of subsurface dispersant injection (SSDI) application. Aquatic toxicity testing has evolved significantly from a sole focus on short-term mortality to evaluate a variety of sublethal physiological, genotoxic, and immunogenic impacts affecting animal health and fitness of exposed populations. In this chapter we consider a number of pressing – and heretofore unresolved – issues surrounding the use of dispersants as an oil spill mitigation tool. Further, we advocate continued, targeted research to help resolve ongoing controversies regarding dispersant use

Far-Field Modeling of a Deep-Sea Blowout: Sensitivity Studies of Initial Conditions, Biodegradation, Sedimentation, and Subsurface Dispersant Injection on Surface Slicks and Oil Plume Concentrations

Modeling of large-scale oil transport and fate resulting from deep-sea oil spills is highly complex due to a number of bio-chemo-geophysical interactions, which are often empirically based. Predicting mass-conserved total petroleum hydrocarbon concentrations is thus still a challenge for most oil spill models. In addition, dynamic quantification and visualization of spilled oil concentrations are necessary both for first response and basin-wide impact studies. This chapter presents a new implementation of the Connectivity Modeling System (CMS) oil application that tracks individual multi-fraction oil droplets and estimates oil concentrations and oil mass in a 3D space grid. We used the Deepwater Horizon (DWH) blowout as a case study and performed a sensitivity analysis of several modeling key factors, such as biodegradation, sedimentation, and alternative initial conditions, including droplet size distribution (DSD) corresponding to an untreated and treated live oil from subsurface dispersant injection (SSDI) predicted experimentally under high pressure and by the VDROP-J jet-droplet formation model. This quantitative analysis enabled the reconstruction of a time evolving three-dimensional (3D) oil plume in the ocean interior, the rising and spreading of oil on the ocean surface, and the effect of SSDI in shifting the oil to deeper waters while conserving the mass balance. Our modeling framework and analyses are thus important technical advances for understanding and mitigating deep-sea blowouts