Building the next-generation Alvin submersible
Posted on 25.11.2008 - 14:00 EST in GENERAL NEWS by Rons_ROV_Links
Three times geologist Adam Soule has climbed inside the deep-diving submersible Alvin and headed to the seafloor. Geochemist Susan Humphris stopped counting after 30 dives. Dan Fornari, who studies deep-sea volcanoes, has descended more than 100 times. Yet for all of them, the deepest seafloor depths have remained out of reach. Alvin is not designed to withstand pressures beyond a depth of 4,500 meters, or 2.8 miles.
"Right now, Alvin allows us to see 63 percent of the ocean," Fornari said. "We want to see 99 percent."
That would require descending to 6,500 meters, or more than 4 miles.
In the late 1990s, ocean scientists steered toward that goal as they began planning to replace the stalwart Alvin, which made its first dive in 1964. They proposed a next-generation vehicle that could go deeper, spend more time on the bottom, have more interior room, and have more and bigger windows, or viewports.
In 2004, the National Science Foundation (NSF) agreed to the plan and awarded $21.6 million to Woods Hole Oceanographic Institution (WHOI), which manages the National Deep Submergence Facility (NDSF), a federally funded center that operates, maintains, and coordinates the use of deep-sea exploration vehicles, including Alvin, for the U.S. oceanographic community. But initial estimates for design, construction, and testing of the new vehicle nearly doubled, and material costs soared. The cost of titanium to forge a new personnel sphere for the sub rose fivefold.
"As detailed planning progressed, it became clear that we couldn't afford the full costs to implement the 6,500-meter, top-of-the-range vehicle in a single hit," said Chris German, chief scientist for deep submergence at WHOI. "Consequently, we developed a new plan, still aiming to achieve the same endgame, but via a longer-term, two-phase approach." Continuing the capacity to bring people deep
Instead of replacing the entire vehicle, engineers will build several new, key components that will be integrated into the existing Alvin in mid-2010, during the submersible's next regularly scheduled major overhaul and modernization, which it undergoes every three to five years. (Over more than four decades of service, nearly every component in the sub has been replaced at least once.)
In this Phase 1, scientists will accomplish most of their desired upgrades, including:
• increased battery capacity, giving researchers more on-bottom time
• improved speed and maneuverability
• better lighting and video systems
• improved performance of Alvin's manipulator arms and an increased payload for its "basket" for carrying back samples.
Most important, Phase 1 will include a new larger personnel sphere, already under construction. It will have improved ergonomics and electronics, more and bigger viewports, and the capability to dive to 6,500 meters. "The catch is that a human-occupied vehicle can only dive safely to the maximum depth of the most shallow-rated component of the submersible," German said.
Additional components needed to reach the larger goal of building the U.S. oceanographic research community's deepest-diving, human-occupied submersible could be integrated in Phase 2, during Alvin's next overhaul around 2015, with additional funding.
"While we may not get everything according to this plan-at least not in the next five years-what this plan does ensure is that the U.S. research community will have access to a much improved, bigger and better Alvin from 2011 onward," German said.
In the meantime, scientists and engineers continue to work on overcoming other technological challenges to build a 6,500-meter sub, including the new personnel sphere inside the sub, new batteries to supply more power without adding more weight, and new syntactic foam to provide buoyancy while withstanding more pressure. Building a new personnel sphere
Many goals of the upgraded submersible will be met by crafting a new $8-million titanium personnel sphere, which carries a pilot and two scientists into the ocean depths. The sphere maintains sea level atmospheric pressure inside for its occupants, while it resists increasing pressure from the increasing volume of overlying water as the sub dives deeper.
The metallic sphere also provides some insulation against cold deep-sea water temperatures, though it isn't warm inside (occupants often wear sweatshirts and hats).
Deep-sea explorers often make crew compartments spherical. Because a sphere has no edges, its geometry distributes external forces evenly over its structure, making it the strongest possible shape. At depths of 6,500 meters, pressure will reach nearly 5 tons per square inch, said Pat Hickey, operations manager for the Alvin Group. Hickey has piloted 631 dives in the sphere-more than any Alvin pilot.
"Other shapes, such as a cylinder, can also be made to withstand such pressure, but at a severe weight penalty because they need to be much thicker than a sphere to go to the same depth," Hickey said. The new sphere will weigh more than 11,000 pounds and its walls will be nearly 3 inches thick, up from Alvin's current 2 inches.
"This sphere is definitely the biggest technical challenge of the project," said Anthony Tarantino, a former Alvin pilot who is part of a team of engineers at WHOI overseeing the development of the new submersible. The sphere, he said, must be nearly flawless-free of any deformities that could weaken its structure and potentially cause it to crumple under pressure. Engineers will run dozens of tests at all stages of its fabrication to insure its reliability.
To make the sphere, engineers needed titanium, more than 34,000 pounds to be exact, about the equivalent weight of a large school bus. Two huge, barrel-shaped ingots were fabricated by a mill in Morgantown, Pa., and reshaped into two giant hemispheres that will eventually be welded together, forming the personnel sphere.
To turn the barrels into hemispheres, the ingots were trucked to Ladish Forging in Cudahy Wisc., a plant more accustomed to turning metal into rocket casings. In July, the ingots were pressed like huge pieces of dough into huge 11-foot discs and then successfully formed into their cup-like shapes.
The spheres will spend the next year or so traveling between Ladish and two companies in Los Angeles, Calif. Bodycote Heat Treatment will heat-treat and anneal the hull at various stages. Annealing relieves stresses that form during the various machining and welding processes during construction, Hickey said.
Stadco Inc. will join the hemispheres using high-energy electronic beam welding. The technique eliminates the need for additional welding material that would add weight, minimizes the amount of heat that needs to be applied to the parts, and reduces the chances that the sphere will be distorted. More room, better views
Stadco will also cut inserts for the hatch, electrical connections between components inside and outside the sphere, and five viewports through which pilots and scientists can view the depths.
Alvin now has three viewports, one in front and one on each side, each about the size of a dinner plate. But because the viewports are funnel-shaped through the sub's thick walls, the actual view is more like looking through a window the size of a teacup saucer.
"Looking out of an airplane window actually offers a much larger field of vision," said Humphris, who is acting vice president for marine facilities and operations at WHOI.
Now, each viewport in Alvin can be used by only one person at a time. Humphris said that when she sees something on the seafloor that she wants to the pilot to see, or to sample with Alvin's mechanical arm, they have to switch places inside the tight confines of the sphere. Or they use up precious time and battery power while the pilot turns the sub to see Humphris' view.
To remedy that, the new sphere will have five larger viewports-including three forward-looking viewports with overlapping fields of view.
When the sphere finally arrives back at WHOI by mid-2010, its inside diameter will be 7 feet-a foot wider than Alvin's current sphere-and its volume will be 18 percent larger. That allows more space for science equipment and more room for some of Alvin's taller pilots and passengers.
Veteran divers eagerly anticipate even slightly roomier accommodations. Before Dan Fornari's first Alvin dive in 1976, his advisor taped a flashlight to his head, turned out the lights, and invited his student to huddle with him under an office desk.
" 'This is what it is going to be like, so come under here with me,' " Fornari recalled him saying. "His description wasn't far off."
"I've always been able to stand up in the sub," said Tarantino, who is 5 feet, 7 inches. "But I know some of my lankier colleagues are really looking forward to those extra inches." The quest for more battery power
Alvin works from the 274-foot research vessel Atlantis. On the ship, before each Alvin dive, engineers recharge 3,500 pounds of lead-acid batteries that power every aspect of the vehicle, from its exterior lights to its six reversible thrusters: three that move the submersible backward and forward, two for up and down movements, and one for turning. The battery power provides scientists with about 10 to 11 hours of time in the water.
About three to four of those hours are taken up by descending and then returning to the sea surface.
"That time is so valuable, so precious," said Soule. "When we're down there, we're squeezing out every moment of energy. When we see the power get low, we start turning off lights and working with a real sense of urgency. So to me the biggest benefit of the new submersible will be the longer time on the bottom."
To get more time, researchers need more power. Fortunately, in the past 10 years, with the development of cell phones and computers, small, powerful lithium-ion batteries have become common. In fact, those types of batteries could power the new submersible. But instead of one or two thumb-size batteries typically used in a cell phone, it would take about 10,000 in total, said engineer Daniel Gómez-Ibáñez, who is designing custom-made batteries for Alvin's replacement.
Gómez-Ibáñez, with degrees in physics and mechanical engineering, came to WHOI three years ago with the goal of "building things that are different than the stuff that already exists."
"I didn't want to build things for the military that kill people, and I didn't want to build cars-we already have too many on the road," he said. Since arriving at WHOI, he's helped to design batteries for two other WHOI-operated, deep-sea vehicles: Nereus and Sentry.
Not only do the batteries need to produce more power than the existing batteries, they must be smaller and lighter-under 3,000 pounds-to offset the weight of the new larger and heavier personnel sphere. To accomplish this feat, engineers plan to mate each lithium-ion cell with a metal bellows, so that compression of the cell at depth squeezes the bellows, like an accordion, but does not permanently warp the thin, sheet-metal can that houses the batteries.
Alvin's battery requirements are unique because it carries people. Safety and reliability are crucial to their design, Gómez-Ibáñez said. To ensure their reliability, he said, engineers must investigate dozens of possible ways for the batteries to fail and plan for them, so that any failure cannot possibly endanger human lives. Syntactic foam: the stuff that keeps Alvin afloat
Another critical and expensive component of the sub is the material that keeps it floating: syntactic buoyancy foam, a matrix of billions of microscopic hollow glass spheres embedded in a hard epoxy resin. The resulting material is hard enough to resist crushing under extreme pressure, yet it is lighter than water and thus provides buoyancy to lift a 36,000-pound vehicle, said Rod Catanach, a WHOI engineer overseeing the foam's development.
The Phase 1 submersible will require additional syntactic foam to accommodate the new shapes and weights of the new personnel sphere and batteries, Hickey said. Anticipating that the submersible will eventually be used to go to 6,500 meters, researchers will use a combination of 4,500-meter syntactic foam from the current vehicle, along with newly developed syntactic foam capable of being used at depths of 6,500 meters. Catanach is working with Teledyne Technologies Inc., which created the 4,500-meter-depth foam now used in Alvin.
A tricky part to manufacturing the new syntactic foam is perfecting the ratio of the light material, the microspheres, and the heavy "glue" material, the resin, Catanach said, so that a minimum amount of foam can provide maximum buoyancy. Otherwise, too much is needed, and that would make the sub too bulky.
Syntactic foam doesn't come cheap. Catanach estimates that they will eventually need about 140 cubic feet of new foam, at a cost of $2,500 to $3,500 per cubic-foot block. That cost easily doubles when you factor in costs to machine and glue new foam into required shapes and to test the new foam.
"We want to make sure it doesn't come back to the surface looking like a crushed Styrofoam cup," Catanach said. "It must be reliable; we have people going down in Alvin. The testing procedure for this high-pressure syntactic foam to be rated for manned submersible use is quite extensive."
In June 2009, WHOI and NSF researchers will meet to discuss the Phase 1 vehicle's preliminary design and costs, as well as estimates for the second phase of the project. A decision is expected in August 2009 on whether to pursue the two-phased approach that could lead to a human-occupied submersible that can reach 6,500 meters.
"At the end I'll probably have no hair and a couple of ulcers," Tarantino said. "But it'll be a fulfilling legacy." © 2008 - Woods Hole Oceanographic Institution