"Until the Titan disaster on June 18, no one had ever died
while piloting or riding a submersible into the deep’s unending darkness. This
remarkable safety record stood for nearly a century, despite explorers making
many thousands of dives.
Federal investigators say it could
take them up to 18 months to determine why Titan imploded, killing its five
passengers. But engineers interviewed by The New York Times point out possible
weak points in the sub’s design.
Titan had several cost-saving
departures from proven submersible designs.
Unlike most other submersibles, Titan’s hull was shaped like
a pill, which fit more passengers. A sphere has been the industry standard,
known to be better suited for deep-sea pressures.
A 3-D diagram of the Titan
submersible. The outer layer of the submersible is shown half-transparent,
revealing the oblong pill-shaped interior hull.
The hull’s central cylinder used carbon fiber, not the more
expensive titanium used in other submersibles that safely returned passengers
from the abyss.
A 3-D diagram of the Titan
submersible highlighting a hollow cylinder located in the center of the
pill-shaped hull.
And Titan’s carbon fiber cylinder was attached to titanium
hemispheres, creating several joints of dissimilar materials that are
challenging to bond properly.
A 3-D diagram of the Titan
submersible highlighting ring-shaped connections between a cylinder and two domes.
Titan’s owner, OceanGate, took a unique approach with the
vessel, working to minimize the costly overhead of other submersibles. The
lightweight craft was relatively easy to transport. It required no dedicated
mothership, but instead could be towed on a flotation device behind a rented
ship. Compared with rivals, the company said, the savings made Titan “more
financially viable.”
What follows is a comparison of
Titan’s design with a standard vehicle that, in typical fashion, relies on
conservative engineering rules that have proven themselves over many decades.
Teams of investigators are now examining such differences in trying to
understand why Titan imploded during its dive to the Titanic wreckage.
Titan’s
Unorthodox Shape Fit More Passengers
Submersibles must withstand the deep
ocean’s crushing pressures, which squeeze with equal force from all sides.
At Titanic’s depth — two and a half miles down — every
square inch of a submersible experiences three tons of pressure.
A spherical hull distributes the stress evenly, making it
the best shape for resisting the compressive forces of the abyss. Any other
shape, experts said, will tend to deform unevenly.
Take a look at the difference
between Titan and Alvin, a research submersible with an all-titanium hull.
Since 1973, it has completed more than 4,500 dives.
Titan
Hull
The pressure applied to a pill shape
is distributed disproportionately and may cause collapse similar to a soda can
being crushed. Source: Oceangate Expeditions
Alvin
Hull
A spherical shape allows for a
uniform distribution of pressure, making it less susceptible to distortion. Source:
Woods Hole Oceanographic Institution
Titan’s hull was larger and held two
more passengers than Alvin, which fits three. In three years of dives,
OceanGate charged up to $250,000 per person to visit the
Titanic.
Titan
5 ft.
Five people sat inside Titan during
a previous dive. Oisin Fanning
Alvin
5 ft.
Alvin seats up to three people,
nestled among sensors and instruments. Woods Hole Oceanographic Institution
Tim Foecke, a retired forensic metallurgist who has done
mechanical testing and failure analysis on metals and carbon fiber, said the
change in hull geometry from a tight sphere to a lengthy tube may have
contributed to Titan’s catastrophic failure. A larger hull needs to be stronger
and thicker to withstand the same pressure as a smaller one. In two hulls of
the same thickness, he said, the larger one would “collapse or buckle” first.
Unproven
but Economical Material
OceanGate created most of Titan’s
hull out of carbon fiber, rather than the conventional titanium used for Alvin.
Experts said the risky design saved money.
A machine wraps a large metallic
cylinder with layers of carbon fiber.
Photographs published by OceanGate
show that Titan was constructed by winding layers of carbon fiber around a
cylinder form. OceanGate Expeditions
Titanium is strong against both compression and tension.
That means it can withstand forces that are crushing it or pulling it apart.
Carbon fiber, though, is much more effective in resisting pulling forces than
crushing forces, such as compression. It resists pulling for a while before
breaking, but collapses or buckles if pushed on or compressed.
“I was very surprised” by the fiber
construction of Titan, Mr. Foecke said, because compression was the main force
that the submersible encountered during its long descent.
Carbon fiber is often used in the aerospace industry because it’s
strong and lightweight. It reduced Titan’s weight to 21,000 pounds, compared
with Alvin’s 45,000 pounds.
“This weight reduction allows us to
carry a significantly greater payload, which we use to carry five crew
members,” said Stockton Rush, OceanGate’s chief executive, in a company news release last year. Mr. Rush was
serving as Titan’s pilot when it imploded.
To further cut costs for the 2023
season, Mr. Rush rented a mothership that was smaller, older and less expensive
than those on previous expeditions. Called the Polar Prince, it was too small
and cramped to carry Titan on its deck. The ship thus towed the lightweight
craft on the three-day voyage from St. John’s, Newfoundland, to the Titanic
site.
“I thought the sub and platform were being tossed around
pretty roughly,” recalled Arnie Weissmann, the editor in chief of
Travel Weekly, about his expedition in May aboard Titan, with the same
mothership. In contrast, Alvin travels to its dive sites aboard a dedicated
mothership outfitted with custom winches, hangars and a machine shop. A large
crane places it into the ocean.
Titan
The Polar Prince towed the Titan
submersible through a harbor in St. John’s, Newfoundland, in May. Dolores
Harvey/Alamy
Alvin
Alvin is brought out to sea aboard a
scientific ship like Atlantis, shown here lifting Alvin over the water. Woods
Hole Oceanographic Institution
Asked if towing Titan risked damage,
in addition to other questions, a company spokesperson, Andrew Von Kerens said:
“OceanGate is unable to provide any additional information at this time.”
As a class, submarines go down for
days, weeks and months. They operate autonomously. In contrast, submersibles go
down for hours and rely on a mothership for such things as crew sustenance,
communications, sleeping bunks and proper toilets. The combination of a support
ship and the undersea craft can be costly. Recently, a pair went up for sale at $50 million.
Joining
Dissimilar Materials Was Challenging
The engineers who were interviewed also expressed design
concerns over areas of Titan where dissimilar materials were joined. Because
different materials change shape at different rates when under pressure,
achieving and maintaining a seal in these areas can be challenging.itanium
At Titanic depths, Titan would have
been subject to water pressures of three tons per square inch.
A 3-D diagram of the Titan
submersible.
Titan’s hull was designed so that a carbon fiber shell was
glued to titanium rings on either end.
A 3-D diagram of Titan’s hull,
showing where the carbon fiber was attached to the titanium.
This cross section of the hull shows
where the two different materials were joined.
A 3-D cross section diagram of the
Titan submersible. The outer layer of the submersible is shown cut in half. The
joint between the domed titanium and the carbon fiber cylinder of the hull is
highlighted.
Under deep-sea pressures, carbon fiber would compress in
diameter more quickly than the titanium, placing stress on the glue joint.
(Animation is an illustration of how
the deformation could work.)
A zoomed in view of the joint
between the titanium and the carbon fiber sections of the hull. An animation
shows the carbon fiber and the titanium compressing at different speeds as the
water pressure outside the Titan increases.
The dissimilar materials used in the craft’s hull
construction, said Alfred S. McLaren, a retired Navy submariner and president
emeritus of the Explorers Club of New York City, “have different coefficients
of expansion and compression, and that works against keeping a watertight
bond.”
Moisture or sea salt could have degraded the hull’s carbon
fiber and the glue joining it to the titanium, creating another potential weak
point, said Kedar Kirane, a mechanical engineer with expertise in damage,
fracture and fatigue in fiber-reinforced composites. Mr. Foecke also said that
the acrylic of the viewport might have failed where it met the titanium or that
uneven tightening of the hatch bolts might have caused uneven stresses along
the porthole, causing a fracture.
The best way of outwitting the many dangers, the experts
agreed, would have been to subject Titan to rigorous testing under the
anticipated conditions and stresses. Fatigue of various materials also would
have had to be considered and continually monitored. Manufacturing defects or
any damage could build up over time as the Titan endured the cycles of stress
associated with repeatedly going down miles to the bottom of the Atlantic and
back.
A detailed timeline stretching back to 2013 shows
OceanGate did extensive developmental work and testing for Titan. Still, left
out of the public record are any proprietary tricks and improvements that
OceanGate may have made as it gained field experience with its experimental
sub.
Skirting
Certification Saved Time and Costs
Most deep-sea craft undergo costly
rounds of inspection and testing by reputable marine organizations that
specialize in certifying the deep-diving craft as safe. But Mr. Rush obtained
no certification for Titan, saying it stifled innovation. In a documentary, he said: “You are remembered for the
rules you break, and I’ve broken some rules to make this. The carbon fiber and
titanium — there’s a rule you don’t do that. Well, I did.”
Mr. Kirane said that if he were
designing the Titan, his top priorities would be testing and certification.
“I would probably emphasize the
actual testing itself because that’s very critical,” he said. “Safety is at
stake, so before actually using it in a real-world application, I would make
sure it passes all the certification required and a lot of experiments.”
Sources: Tim Foecke, former director
of the Center for Automotive Lightweighting at the National Institute of
Standards and Technology; Kedar Kirane, assistant professor, Department of
Mechanical Engineering, Stony Brook University; Arun Shukla, co-director,
National Institute of Undersea Vehicle Technology, Department of Mechanical,
Industrial and Systems Engineering, University of Rhode Island; Woods Hole
Oceanographic Institution (U.S. Navy vessel Alvin information) | Note: Alvin is
a U.S. Navy vessel that is operated by Woods Hole with funding from the
National Science Foundation."
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