spill response
and where the resulting ecological impacts accrue emphasises that the vast majority of the oil is retained at depth and, among other response actions, calls into question the efficacy of dispersants.
In the case of Deepwater Horizon, hot oil and natural gas erupted from the seabed and were rapidly mixed and dispersed due to the physics of the pressurised oil jetting from the tip of the wellbore. “Much of that oil never got to the surface, or ever could have gotten to the surface, calling into question the value of dispersant use at depth,” argued co-leader Gary Cherr, director of the UC Davis Bodega Marine Lab. “We have generally hailed the use of dispersants as helpful, but really are basing this on the fact we seemed to have kept oil from getting to the surface.
“The truth is much of this oil probably was staying at a depth independent of the amount of surfactants we dumped into the ocean. And we dumped a lot of dispersants into the ocean, all told approximately one-third the global supply.” The authors argue that had their newly- proposed oil-spill model been in use, responders would have proceeded in a different manner. And, in those critical early weeks and months of the unfolding spill, the response effort could have focused greater attention on the ecological communities most in harm’s way. As near-shore, shallow-water oil reservoirs become depleted, the oil industry has transferred the focus of
its exploration and production
activity to deep and ultradeep reservoirs similar to the one in which the Deepwater Horizon disaster occurred. However, the Outer Continental Shelf Lands Act explicitly excluded the central and western Gulf of Mexico from the otherwise universal requirement to produce a development and production plan, which, the authors argue, effectively allows deepwater drilling to proceed without the need for a full assessment of risks. “Our hope is that this paper brings attention to the fact that deepwater oil-spill response efforts must be extensively revised so that we do not repeat the same mistakes and are better prepared to assess important ecological impacts from day one,” Ms Joye concluded. Researchers at the Virginia Institute of
Marine Science are using a US$350,000 contract from the US Department of the Interior to test whether sound waves can be used to determine the size of oil droplets in the ocean, knowledge that could help guide the use of chemical dispersants in future spills. The effort is also supported by the VIMS-Industry Partnership. Chemical dispersants have conventionally been applied to surface oil to produce smaller droplets that can more easily be mixed downward by ocean turbulence. Dispersal through a larger water volume reduces the immediate threat to the shoreline and to marine organisms. It
58 I Offshore Support Journal I June 2012
Oil spills at great depth, such as Deepwater Horizon, present new challenges for capping and containment, enhancing the value of dispersants
also increases the surface area available for bacterial decay. During the Deepwater Horizon incident,
however, the oil industry for the first time released dispersants directly into a deepsea blowout. Indeed, of the 1.84 million gallons of dispersants used during the spill, 42 per cent, equivalent to 771,000 gallons, was applied at the wellhead, 5,067ft below the surface. The idea was to reduce both the amount of oil reaching the surface and the amount of dispersants that needed to be applied. Today, the effectiveness and safety of this deepsea dispersant
application remains
unknown, at least in part because of the difficulty of monitoring the size of the oil droplets within the subsea plume. That’s where the VIMS research comes in. Project leader Paul Panetta, a scientist with Applied Research Associates and adjunct professor
at VIMS, said: “To maximise biodegradation, dispersants are designed
to produce oil droplets that are less than 100 microns across. But there are currently no tools available to monitor droplet size in deep subsea blowouts. Our goal is to develop acoustic techniques for that purpose, giving spill responders a means to gauge the effectiveness of the dispersants and how much they should use.” Tools exist to measure droplet size within
dispersed oil slicks at and just below the sea surface – including ultraviolet fluorometers and laser in-situ scattering and transmissometers (LISST) – but these optical devices are poorly suited for use within highly opaque plumes of oil. Acoustic instruments and techniques offer a promising alternative. “There’s a reason that many marine mammals use sound rather than sight for long-distance communications,” said team member Carl Friedrichs, chair of physical sciences and head of the Coastal Hydrodynamics
www.osjonline.com
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