Photo: MIT Sea Grant College Program |
As the search for oil and natural-gas resources moves into
deeper and deeper water, companies are facing increasing costs. Building and
installing a single offshore drilling platform now costs more than a billion
dollars, so companies are using their platforms as efficiently as possible.
Advances in technology have enabled them to service several oil fields from a
single platform, and much of the infrastructure for well operations has moved
to the seafloor, which may be as much as 4,000 m—almost 2.5 miles—below the
surface. As a result, inspecting, servicing and repairing underwater equipment
has become an ever-greater challenge.
Many
companies accomplish those tasks using remotely operated vehicles (ROVs),
robots that are operated by a person aboard a surface vessel. Because radio
signals do not propagate through seawater, the ROVs are connected to the vessel
by cables that carry data as well as power. But as the distance between a
platform and its wells has increased, the cables, or “tethers,” have become
longer and heavier. To support that weight, the vessels needed to launch and
recover them have become larger and more expensive. Running an ROV and its ship
now costs around $250,000 per day.
For
the past two decades, Massachusetts Institute of Technology (MIT) researchers
have been working on a different approach, motivated by the notion that “small
is good”—the operating premise of Chryssostomos Chryssostomidis, director of
the MIT Sea Grant College Program, the Doherty Professor of Ocean Science and
Engineering and professor of mechanical and ocean engineering.
“In
the late 1980s, I suggested a revolutionary concept: an underwater vehicle that
has no tether and travels in the deep ocean without input from an operator,”
Chryssostomidis says.
Chryssostomidis’
proposed AUV would be fully functional, but small enough so that deploying it
wouldn’t require a huge ship. Getting rid of the tether would make it far more
maneuverable and flexible. It could get into small spaces without worrying
about the tether dragging along and getting tangled.
The
goal, therefore, was to make an AUV that the offshore industry could use to service
its deepwater operations—and that researchers could use to explore and monitor
the deep ocean. To that end, in 1989 Chryssostomidis founded the Autonomous
Underwater Vehicles Laboratory within the MIT Sea Grant College Program, and he
and his colleagues began developing a series of AUVs.
A
first challenge was how to navigate without knowing details of the deep-sea
landscape. Early efforts were helped by insights from Rodney Brooks, now the
Panasonic Professor of Robotics emeritus. Brooks’ idea was that the AUV—as with
other robots—didn’t need to know anything about its environment. It only needed
to know when it was approaching an obstacle and should go right, left, up, or
down to avoid a collision. “So he enabled me to start developing AUVs without
having to address that problem,” Chryssostomidis says.
Since
then, the laboratory has developed and demonstrated a series of AUVs, all of
them small, relatively inexpensive, and artificially intelligent. Of particular
note was the early “Odyssey” series of vehicles, which had a torpedo shape with
a streamlined horizontal axis designed for efficient cruising. For a decade,
Odyssey II vehicles have run successful surveying missions, demonstrating rapid
long-distance travel and good battery life due to their hydrodynamic
efficiency.
But
while surveys are important, they are not enough. “The next frontier is going
to be intervention,” Chryssostomidis says. An AUV will examine, say, the
footing of an oil platform or another piece of subsea equipment and then
perform a task. An Odyssey II vehicle isn’t suited to such close study. Like a
shark, it must keep swimming forward in order to maintain its maneuvering
capability. As a result, it can prepare a detailed image of an object only by
repeatedly circling over it, taking a photo at each pass.
Performing
close-up inspections, service, and repairs would be better accomplished by an
AUV that could stop and hover in one place. Members of the AUV laboratory and
their collaborators therefore designed a hovering AUV, which has a full six
degrees of freedom while standing still and is extremely maneuverable. However,
its lack of a streamlined axis doesn’t allow for efficient cruising, and its
small thrusters and battery don’t provide enough force to withstand any but the
smallest of currents.
Enter
the Odyssey IV, a hybrid cruising/hovering vehicle that gains advantages from
both vehicle designs. This two-meter-long craft has a smooth, teardrop profile
derived from the streamlined body of the Odyssey II, and four commercial
off-the-shelf thrusters: one in the bow, one in the stern, and two mounted on
arms that protrude from the sides of the vehicle and can be rotated about its
lateral axis. A custom-designed battery consisting of 648 lithium-ion cells
provides the vehicle with the power necessary to fight currents and the
longevity to dive to full ocean depth. The vehicle’s conservative size and
weight make it deployable from small, less-expensive boats, but it still has
room inside for a substantial payload.
In
sea trials, the Odyssey IV has demonstrated that it can both move quickly and
hover in place. It can travel through the deep ocean—6,000 m below the
surface—at a rate of 1.4 m/sec when going straight ahead. Having located its
objective, the Odyssey IV can hold its position to within centimeters of the
desired location. If a current pushes it in one direction, its controller activates
the appropriate thruster and brings it back into position. It can thus hover as
a helicopter does, making detailed inspections of particular subsea structures
or the natural landscape. It can also pick up samples and other cargo from the
deep sea and bring them back to the surface for inspection and analysis.
Remaining
challenges include developing better power storage and communications
capabilities so that the vehicles can stay underwater longer and send back more
information to operators on shore. Chryssostomidis notes that his team has made
great strides in both areas. The Odyssey IV has enough power that it can
operate for a day or more without refueling, and an onshore operator can
receive still frames of what the AUV is “seeing” with only a few seconds’ delay
(due to the time it takes for sound to travel through water). The researchers
continue to improve the efficiency with which video is transmitted underwater,
largely due to new techniques of data compression developed by Milica Stojanovic,
a visiting scientist at MIT Sea Grant and associate professor at Northeastern
University.
What’s
the future? To illustrate, Chryssostomidis refers to the blowout preventer: the
device that’s at the heart of underwater operations and that dominated news reports
about the April 2010 Deepwater Horizon accident. For now, he is sure he will be
allowed to send an AUV within 5 m of a blowout preventer and observe. If all
goes well, he predicts that in a few years he’ll be allowed to have an AUV
touch that critical piece of equipment. In a few decades, the AUV may perform a
real repair. And one day, Chryssostomidis says, an AUV will be able to do tasks
without a human in the loop. It will head underwater, locate a problem, make a
judgment, formulate a plan, and perform a repair—all on its own.
