The characteristics and modelling of oil dispersion in the ocean mixed layer from underwater blowout spill accidents

Open Access
Author:
Chen, Bicheng
Graduate Program:
Meteorology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 23, 2017
Committee Members:
  • Marcelo Chamecki, Dissertation Advisor
  • Chris E. Forest, Committee Chair
  • Marcelo Chamecki, Committee Member
  • Sukyoung Lee, Committee Member
  • Francesco Costanzo, Outside Member
Keywords:
  • oil transport
  • ocean mixed layer
  • turbulence
  • LES
  • ENDLESS
Abstract:
Underwater oil spill accidents are the most challenging oil spills in the ocean and the ensuing oil transport is impacted by a wide range of physical, chemical and biological processes through the entire ocean column. In particular, the final fate of oil plume is strongly affected by the various scales of motions present in the ocean mixed layer (OML) (i.e. Ekman transport, submesoscale/mesoscale eddies, small-scale turbulence). Due to the large extension of oil plumes, the numerical prediction of oil trasnport is most often investigated by regional models, which requires turbulence models for the OML. In these models of OML, turbulence and mixing have been developed based on knowledge of atmospheric boundary layer (ABL) turbulence, because of the similarity between these two boundary layers and the lack of observation and understanding about the OML. Recent studies have found that the unique wave-induced phenomena (i.e. Stokes drift, Langmuir circulatons) in OML are critical for both horizontal advection and vertical mixing in the OML, but important issues related to oil transport remain unsolved. In this work, we focus on i) the modulation of turbulence field by surface gravity waves in the OML and responses of oil plumes to this modulation using the high fidelity large eddy simulation (LES); ii) a new multi-scale numerical approach to inlcude both accurate representations of small-scale turbulence and large-scale eddies; and iii) impacts of dispersant application on oil plume from an underwater blowout and the corresponding implication for remediation. The surface gravity wave induces two phenomena--the Stokes drift and the Langmuir circulation, which are important for transport processes in the OML. Both phenomena are modulated by the misalignment angle between wind and wave. In the first work, we focus on the effects of swell waves on the modulation of turbulent flow in the OML and its impact on oil transport. Results show that when the wind-swell misalignment is small, Langmuir cells develop and significantly enhance the vertical mixing of the oil plume. Conversely, when the misalignment is large, vertical dilution is suppressed when compared to the no-swell case. Due to the strong directional shear of the mean flow within the OML, plume depth significantly impacts the mean transport direction. The size of oil droplets in the plume also plays an important role in vertical mixing and mean transport direction. Oil plumes being transported in the OML experience the action of shear-generated turbulence, Langmuir circulations, Ekman transport and submesoscale/mesoscale eddies. To resolve such turbulent processes, grid sizes of a few meters are desirable while horizontal domain sizes of LES are typically restricted from hundreds of meters to a few kilometers, for LES to remain practically affordable. Yet transported oil plumes evolve to large scales extending to tens or even hundreds of kilometers. In this work, the Extended Nonperiodic Domain LES for Scalar transport (ENDLESS) is proposed as a multi-scale approach to tackle this challenge while being computationally affordable. The basic idea is to simulate the shear turbulence and Langmuir circulations on a small horizontal domain with periodic boundary conditions while the resulting transport velocity field is replicated periodically following adaptively the large-scale plume as it evolves spatially towards much larger scales. This approach also permits the superposition of larger-scale quasi two-dimensional flow motions on the oil advection, allowing for coupling with regional circulation models. A validation case and two sample applications to oil plume evolution in the OML are presented in order to demonstrate key features and computational speedup associated with the ENDLESS method. The results shows that the ENDLESS method is a promising multi-scale approach. The application of oil dispersants in oil spill accidents is controversial due to the unclear effects on oil transport in a real environment and their toxicity to oceanic organisms. Because of the velocity shear in Ekman layer, the oil experiences different advection velocity in different depth, therefore, the vertical transport also impact the horizontal transport of oil. In this study, the oil transport of an idealized underwater blow-up accident is simulated by LES, which is able to resolve the high fidelity vertical mixing. The oil spill accident is simulated by two parts: i) the near-field simulation which includes the water column where the oil plume rises from the source; ii) the far-field simulation which the oil plume disperses mainly horizontally in the ocean mixed layer (OML) using the ENDLESS approach. The result from LES shows that after applying dispersants, the advection velocity of oil plume reduces to about 1/8 of its original value and the lateral diffusion is significantly enhanced.