The security conditions around the expedition of the Titan submarine, which was to join the wreck of the Titanic, are now at the heart of the investigation. Was the private submersible really cut out for such a descent into the ocean depths?
The five passengers of the submersible Titan disappeared for four days near the wreck of the Titanic are considered dead, announced Thursday, June 22 the company OceanGate, organizer of the expedition. At the same time, the US Coast Guard said an underwater vessel had located a “debris field”, located at 3,800 meters deep.
If the circumstances of the accident are not yet known, this drama raises questions about the safety of such expeditions.
The context in which the Titan has disappeared is worrying. Reports detailing court documents from a 2018 case show OceanGate fired an employee, David Lochridge, after he raised concerns about the safety of the submersible.
Lochridge disagreed with OceanGate on how best to demonstrate the craft’s seaworthiness, and objected to OceanGate’s decision to conduct dives without prior “non-destructive testing” of the craft’s hull. vessel to prove its integrity. The latter aim to characterize the state of integrity of the structures or materials of the machine.
Also in 2018, a letter sent to OceanGate by the Committee on Manned Underwater Vehicles of the Marine Technology Society, signed by 38 experts, expressed reservations about the safety of the submersible. In particular, they state that “the experimental approach adopted by OceanGate could lead to incidents (from minor to catastrophic) that would have serious consequences for all players in the industry”.
As can be seen from reading these exchanges, the engineering and regulation of deep-sea submersibles remains somewhat uncharted territory. And like the Titan operates in international waters, it is technically free from governance by single country regulations.
In this case, most submersible designers choose to have the vessel design certified by a classification society. It appears that OceanGate would not have done this for the Titan.
The seaworthiness of submarines in the ocean
When we talk about the “seaworthiness” of a ship, we ask ourselves above all whether it is suitable for the use for which it is intended, whether it can be operated in complete safety and whether it complies with the standards of Environmental Protection.
For the Titanfitness for purpose could be boiled down to the ability to safely launch from a mothership to the water surface, operate autonomously down to 4,000 m (the approximate depth of the wreckage of titanic) and resurface to be picked up by the mothership after a dive of a few hours.
Operational safety means that no equipment is damaged and no passenger is likely to be injured (or worse) on board. Environmental protection means that the submersible will not have a significant impact on its environment, for example by polluting or disturbing the ecosystem.
However, this is an optimistic scenario. Deep-sea submersibles operate in a hostile environment and things can go wrong.
The challenge of resisting enormous pressures
The shape of submersibles and submarines is due to the fact that spheres and cylinders are geometrically more resistant to crushing pressures.
Instead of operating in a 1 bar breathable atmosphere, the Titan must withstand a pressure of 370 bars in seawater at the depth of the titanic. Any flaw in the hull could result in an instant implosion. So what is the threshold below which an “out of circle” geometry becomes a defect?
Industries that use underwater vessels at depths of a few hundred meters often use steel hulls, which typically have an out-of-circularity threshold of less than 0.5% of the vessel’s diameter. Would this criterion be safe enough for the pressurized hull of the Titan at 4000 meters?
THE Titan consists of a carbon fiber and titanium composite shell. It is extremely complicated to design and structurally evaluate these materials, compared to a metal-only material. Presumably this is why OceanGate has equipped the Titan a “real-time hull condition monitoring system”.
It is unclear whether this system actually measures stress using gauges in the hull, or whether it is (as Lochridge warned) an acoustic analysis that would only alert people to impending problems “often milliseconds before an implosion”.
Pressure hull integrity safety requires the analysis of different failure modes, before determining a factor of safety for each mode, based on the target dive depth.
Once the design has been verified (by calculations), the validation in real conditions must be done in two stages.
Non-destructive testing should be performed on the fabricated pressure hull to verify geometry accuracy and any out-of-circle appearance.
Then, actual (ideally unmanned) dives should be made to progressively increasing depths, with strain gauges used to measure actual against forecast. We don’t know if the Titan has undergone such tests.
Redundancy and backup procedures
When designing the functional architecture and selecting equipment, the designer should consider a number of fallback scenarios:
- What happens if the main energy sources fail?
- What if my computer crashes and the pilot loses control?
- What happens if my main communication system fails?
- How can the submersible signal to the mother ship that there is a problem?
These scenarios require naval architects to ensure what is known as SFAIRP security (so far as is reasonably practicable, or as far as possible). This is not only about mitigating the consequences of an accident, but also about preventing it from happening.
In concrete terms, this means having:
- A supply of oxygen (for example, while waiting for a rescue team);
- Reliable primary energy sources And back-up systems;
- Another source of power (hydraulic, for example) in the event of a loss of power – which would release the safety cables to achieve positive buoyancy and return to the surface.
Each of these systems should be subject to specific verification (theoretical) and validation (testing) for the environment in question.
Commercially available equipment can be installed on board, provided it can be demonstrated that it is suitable for different scenarios. However, most external components (due to crushing pressure) and safety systems would have to be custom designed.
According to reports, the Titan used some “off-the-shelf” equipment, but it is unclear if they were certified for the intended use at these depths.
Security systems and means of being detected
In the case of Titan, a link cable to the mothership would have provided instant two-way communication and a higher data exchange rate. But these cables can get tangled up with potential hazards at the site of a wreck.
This is why cables are mainly used for unmanned vehicles; manned submersibles prefer to trust the pilot. GPS, satellite cell phones and automatic identification systems cannot be used underwater either. These tools use electromagnetic waves which do not propagate in depth (although they can be used on the surface).
Some submarines are equipped with an emergency beacon, the equivalent of an EPIRB. This beacon can be triggered by order of the captain or via a “dead man” switch; if the pilot responds to a test at regular intervals, a sudden lack of response leads the system to assume that the crew is unable to do so. This is a technique taught to the crews of military submarines when they run aground on the seabed.
A high-frequency acoustic transmitter would be even more effective, as it would provide directional accuracy to locate a submersible in distress.
A number of situations can also occur on the surface, in case the Titan would have floated to the surface. Even if he did (or will), the crew and passengers cannot open the ship’s bolted hatch. They will likely have to continue to deal with the potentially polluted atmosphere inside.
The white color of Titan further complicates matters, which would make it harder to spot in the sea due to the foam of the waves. This is why floating objects detected from the sky are usually orange or yellow in color, giving them greater visibility.
What future for submersibles on the high seas?
The crew and passengers of the Titan may be saved. But if the worst happens, the forensic expert will inevitably wonder if the Titan has met the basic thresholds to demonstrate its airworthiness.
Although various classification societies offer a set of rules for commercial submarines and submersibles, choosing to follow these rules is still a voluntary process (which the craft insurer usually encourages).
It’s time to recognize that going deep is as complex, if not more so, than going to space – and keeping submersibles safe should be more than a matter of choice.
Eric Fusil, Associate Professor, School of Electrical and Mechanical Engineering, University of Adelaide
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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