Oil droplet size model for ocean surface oil spills: impact of breaking wave height and oil properties

One of the key parameters that has been identified to affect the accuracy of the oil spreading calculations from ocean oil spill models is the oil Droplet Size Distribution (DSD). The oil droplet dynamics in the water column plays a critical role in oil spreading and subsequent oil weathering processes. Thus, accounting for the droplet dynamics under wave conditions is a critical issue in ocean oil spreading models. In this regard, droplet dynamic models have shown promising results. In the present study, a phenomenological droplet dynamic model is employed to analyse the impact of wave parameters and oil properties on the developed oil DSD. The model results confirm the capability of the phenomenological models to calculate the evolution of oil DSD under breaking wave conditions, which is an advantage over equilibrium models. The model was used to analyse how the developed droplet sizes vary with different breaking wave heights, oil types, and differently weathered oils. The model results show that increasing breaking wave height results in a shift of oil DSD towards smaller droplet sizes, however the impact of oil viscosity is comparatively less for the selected range of oils

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