In Hong Kong, for the construction of an underwater tunnel, the tubists are placed under pressure for a month. A new mode of organization that raises questions.
8h00. Bastien turns off his alarm clock and extracts himself from his bed. Body temperature: 36.2. Everything is fine. A little shower to wash off, it’s time to go. The young man leaves the “leaving habitat”, an 18m2 apartment whose electric door opens and closes, emitting gaseous noises mixed with sound signals, and enters the “capsule”, a shuttle vessel straight out of the door. Men in Black who takes him to his place of work. In eight hours, the same vehicle will bring him back to his home.
Like a Parisian worker, Bastien has his little routine, his life at the metro-work-sleep rhythm. But on closer inspection, it’s not just its futuristic environment that surprises. In fact, Bastien lives at a pressure of five bars, the air he breathes consists of helium and if suddenly the door to his “house” opened to the outside world, he would die instantly.
Bastien, 32, is a tubist. A diver by training, his playground is more deep-sea diving, but this time he has accepted a “dry” mission on a colossal site where 400 people are crowded, led by Bouygues Construction in the bay of Tuen Mun -Chek Lap Kok. The young man is participating in the construction of an underwater tunnel 4.2 kilometers long and 14 meters in diameter. The two tubes of this tunnel will link Hong Kong to Lanteau Island and its international airport. Bastien is employed in the maintenance of the heads of the immense steel drilling machine which digs its furrow in the sediments.
The heads of the tunnel boring machine, on the right, are maintained by the hyperbaric teams – Credit: Bouygues Construction
To prevent the still uncemented structure from collapsing under the pressure of the water, in the drilling area, the air pressure is five times that of the atmosphere at the surface. During his mission, Bastien replaces the worn parts of the head, cleans the mud with Kärsher. A hyperbaric site 50 meters deep, on which a new organization of work is taking place.
Usually, tubists only operate for a few hours under pressure. Before entering the saturated zone, at the level of the drill head, they are placed in a hyperbaric chamber where they breathe a mixture of compressed gases. There, their organism slowly adapts to saturation. Once they have completed their task, which is often very physical, they return to the chamber for a good four hours of decompression. Either a long day playing with its physiological parameters, with a rare accidental risk, but unforgivable (see box).
A profession under pressure
To put the tubers in saturation, the tubers placed in the hyperbaric chamber are made to breathe a tri-mix mixture based on helium, nitrogen and oxygen. The proportion of these gases varies according to the depth of the site. Decompression leads to a degassing phenomenon. The gases dissolved in the blood form small bubbles which become dangerous if they are too big or too many. These “gas emboli” can be the cause of decompression sickness, the damage to which varies according to the organs concerned.
“The most serious are spinal cord injuries in the spinal cord, with a risk of paralysis,” explains Bernard Gardette, specialist in decompression procedures. Some bubbles can also go to the brain and cause brain damage, or into the heart and cause cardiac arrest ”. The decompression stops aim to limit the appearance of these bubbles and allow their elimination through the venous route.
In Hong Kong, site organizers have found a way to reduce the risk of decompression sickness while increasing worker productivity. Decompression does not take place every day but only once, at the end of the four-week mission. Each room occupied by the workers is placed under a pressure equivalent to that of the work area, just like the shuttle that connects them. For 28 days, there are about thirty of them living in permanent saturation. Teams of four tubers maintain the head of the tunnel boring machine and take turns every six hours, before returning to the surface, in a habitat still under pressure. The presence of agents on the intervention site is thus almost permanent.
Credit: The Institute of Materials, Minerals and Mining, Jan 2016
Besides his cartoon voice due to helium which “makes everyone laugh on Skype” (yes, he has Internet), Bastien perceives some physiological effects of this unusual environment. His body temperature is a little lower. That of his “home” is maintained at 26-28 degrees, he explains, but with each micro-variation, he is swimming or shivering. “With pressure, the body is more prone to temperature changes,” he explains.
The friction of the steel which hollows the rock gives off a heavy heat; he feels the efforts deployed to handle this monster of a drilling rig – a world record for the tunnel boring machine used on a construction site, in terms of size. Sometimes she may feel a little “stoned”. In the environment, this is called “nitrogen narcosis”, a kind of intoxication linked to compressed nitrogen, but the gas mixtures present in the pressurized atmosphere are precisely intended to avoid an overly marked effect. Their concentration is regularly readjusted.
Hell is other people
It is neither the most technical, nor the deepest site on which he has worked, but it is certainly one of the most painful, says Bastien. However, according to the young man, the real difficulty lies elsewhere. Living in a caisson means living together for 28 days with a whole team, eight in 18m2, four per dormitory, with no escape. A month in which we are filmed 24 hours a day for security reasons, during which entertainment is particularly scarce once we have returned “home” in the evening.
From the window, they can see the outside world but communication remains limited. A hit on the wall, daily, to indicate to an operator that the toilet should be flushed, an intervention that they cannot carry out themselves at the risk of decompressing the habitat. “Your only private space, c is your bunk (your bed), you put up a little curtain and then you read. A real life as a monk, in the end! No alcohol, no tobacco… no privacy ”.
To get out of there and breathe the air in our atmosphere, he must acquire patience. Wait four days in a decompression chamber, hoping that the small bubbles that form on this occasion in his body keep quiet. Bastien does not apprehend these moments there, nor the temporary effects which occur at the exit of the box. “For four weeks, since we were slightly over-oxygenated, we feel a little tired. It’s hard to climb a staircase, but after a week, it fades ”.
This method of organization, on such a large scale, has been tested on a German site. According to specialists in hyperbaric medicine, it seems safer than frequent decompressions.
>> Read more: Hyperbaric workers: “Long stays are less risky than short interventions”
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