Researchers Paul Panetta and Carl Friedrichs of the Virginia Institute of Marine Science (VIMS) compared the performance of acoustic and optical instruments using this oil slick in the Ohmsett wave tank. Credit: Photo by Paul Panetta. |
Two
years ago this week, oil began streaming from the seafloor into the
Gulf of Mexico following the explosion of the Deepwater Horizon
platform. All told, the disaster cost 11 lives, released 4.9 million
barrels of crude oil, and caused still unspecified impacts to marine
life and the Gulf economy.
Now,
a pair of researchers at the Virginia Institute of Marine Science is
using a 1-year, $350,000 contract from the U.S. Department of the
Interior to test whether sound waves can be used to determine the size
of oil droplets in the subsea—knowledge that could help guide the use of
chemical dispersants during the cleanup of future spills. The effort is
also supported by the VIMS-Industry Partnership.
Chemical
dispersants have conventionally been applied to surface oil slicks to
produce smaller droplets that can more easily be mixed downward by ocean
turbulence. Dispersal through a larger water volume lessens the
immediate threat to the shoreline and to organisms such as seabirds,
marine mammals, and turtles. Dispersion also increases the surface area
available for bacterial decay.
During
the Deepwater event, however, the oil industry for the first time
released dispersants directly into a deep-sea blowout. Indeed, of the
1.84 million gallons of dispersants used during the spill, 42%—771,000
gallons—was applied at the wellhead, 5,067 feet 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 deep-sea 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,
Inc., and an adjunct professor at VIMS, says “To maximize
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
do exist to measure droplet size within dispersed oil slicks at and
just below the sea surface—including ultraviolet fluorometers and LISSTs
(for Laser In-Situ Scattering and Transmissometers). 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,” says team member Carl Friedrichs, Chair
of Physcial Sciences and head of the Coastal Hydrodynamics and Sediment
Dynamics lab at VIMS. “Light can’t go nearly as far in water—let alone
turbid water—as compared to sound waves.” Friedrichs notes that acoustic
instruments also tend to be less delicate than their optical
counterparts, and are better able to withstand “biofouling” and the high
pressures of the deep sea.
Experiments
Panetta
and Friedrichs conducted the first experiments for the project in
December 2011 at the Ohmsett Wave Tank in Leonardo, New Jersey, which
serves as the National Oil Spill Response Research & Renewable
Energy Test Facility for the U.S. Department of the Interior. This
2.6-million gallon concrete basin—one of the largest wave tanks in the
world—measures 666 feet long by 65 feet wide by 11 feet deep. It
features an immense piston for generating waves up to 3 feet high, an
oil distribution and recovery system, and a motorized bridge for
deploying instruments.
During
their Ohmsett tests, Panetta and Friedrichs compared the performance of
optical and acoustic instruments borrowed from their labs at VIMS,
transmitting, receiving, and interpreting sound waves and light as they
reflected against an aqueous slurry of 20 parts of oil to 1 part
dispersant.
In
a second experiment at VIMS, the pair performed a similar experiment
but on a much smaller—and simpler—scale. This time they compared the
performance of their optical and acoustical instruments in a small
bucket, adding dispersants to the same crude oil used at Ohmsett and
creating turbulence with a drill-powered paint mixer.
They
recently conducted a third test in Norway, in a tank operated by
SINTEF, the largest independent research organization in Scandinavia.
This “tower tank”—specifically created for studying subsurface releases
of oil—measures 21 feet tall by 9 feet wide and allows room for various
instruments including video cameras, a LISST, and, in this case, the
acoustic equipment supplied by the VIMS team.
Preliminary results are promising
In
all three cases, the team’s preliminary results qualitatively confirm
the potential superiority of an acoustic approach to monitoring oil
dispersion. “Our tests showed that acoustic techniques were effective at
penetrating the plume,” says Panetta, “whereas the LISST would have
been ineffective. Our initial measurements indicate the acoustic
measurements can track the droplet size for a subsurface release of
oil.”
The
next step, says Panetta, is to “take these data and turn them into a
measurement method that would tell us exactly what the droplet size is.
That would be valuable to the people spraying the dispersants, and
valuable to the people modeling the fate of the oil, because during the
cleanup of an oil spill, the size of the oil droplets affects
everything.”
Panetta
and Friedrichs say their ultimate goal is to partner with the private
sector so that commercial sonar manufacturers can adapt the new
technology to their existing instruments for use by the oil and gas
industry. “That’s the longer term technology plan,” says Panetta, “but
we obviously have to first figure out the science behind it to make it
work. We have to solve the physics problem—to figure out which signals
to analyze and how to interpret them so we can get a quantitative
measure of the oil-droplet size.”
