Abstract

This work presents a new laboratory study for understanding secondary intrusions in multiphase plumes in quiescent, stratified environments. The study is driven by field observations of secondary intrusions during the Deepwater Horizon (DWH) oil spill. The smaller trap heights observed at DWH for secondary versus primary intrusions could have resulted, in part, from decreasing plume buoyancy (due to gas dissolution) and increasing ambient stratification with elevation above the source. We seek additional mechanisms responsible for the observed smaller secondary trap heights through controlled laboratory experiments where buoyancy and ambient stratification are nominally constant throughout. A novel approach is adopted in the experiments to increase the visibility of secondary intrusions, which are traditionally difficult to visualize, thus investigate. The study reveals that a wider plume source width can also cause the secondary intrusions to trap earlier, providing another plausible explanation for the shallower secondary intrusions observed in the field data.

Highlights

  • Published CTD-CDOM profiles during the Deepwater Horizon (DWH) oil spill showed a prominent intrusion of trapped hydrocarbons near the spill site, typically at elevations about 300– 350 m above the seafloor (Valentine et al, 2010; Socolofsky et al, 2011; Camilli, 2014)

  • Driven by field observations at DWH, we present a laboratory investigation that explores conditions under which secondary intrusions occur in multiphase plumes

  • We conclude that the smaller trap heights observed at DWH for secondary versus primary intrusions could have resulted, in part, from decreasing plume buoyancy and increasing ambient stratification with elevation above the source

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Summary

Introduction

Published CTD-CDOM profiles during the Deepwater Horizon (DWH) oil spill showed a prominent intrusion of trapped hydrocarbons near the spill site, typically at elevations about 300– 350 m above the seafloor (Valentine et al, 2010; Socolofsky et al, 2011; Camilli, 2014). We analyzed 266 CTD-CDOM profiles within a 7 km radius from the wellhead using methods outlined in Socolofsky et al (2011) and the Supplementary Information, and found 19 with a distinct secondary intrusion. In all 19 profiles, the secondary intrusion concentrations were significantly smaller than those in the primary intrusion. This stems from the fact that the more soluble hydrocarbons are dissolved near the source and enter the first intrusion, resulting in fewer hydrocarbons to enter the secondary (or further) intrusions, even if there are apparent hydrodynamic intrusions

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