Undersea Station: Decompression

NEEMO Diver under Aquarius HabitatTo stay in an underwater habitat longer than 12 hours means to stay under saturated conditions, which requires a decompression sequence of at least several hours. This decompression procedure is very critical: if any of aquanauts gets into an emergency situation, there is no way to take him out of the chamber before the sequence is finished. If the procedure is badly designed there is no way to bring a paramedic into the chamber. For the period of several hours the aquanaut would be alone with his companion.

Before we start with the different designs we should have a look at our depth options. We said the first module should be within the 20m range to allow short visits from the surface. Aquarius habitat uses a 17-hours decompression schedule from 19m depth which proved successful for many years. If experiments are conducted at lower depths like 5m a decompression schedule of 1m/hour seems safe.  Jules Undersea Lodge at 6m depth requires no decompression at all except restrictions in after-dive flights and repetitive dives.

Discussing the following concepts we should always keep in mind: 1. the regular decompression sequence; 2. potential cases of emergency; and 3. serious cases of emergency.

So, what are the alternatives?

1. Wet decompression

The aquanauts would leave the habitat and decompress using their Scuba gear stopping at different depth stages. A depth of 20m would require an ascent dive of more than 17 hours. If you still have skin on your hands afterwards you were lucky that there was no emergency during that ascent, because treatment would have been impossible. Your support team would curse on you for making them bring 20 tanks per diver to you while you loose your brain doing nothing except holding the mooring line. Even the ascent from 5m would be difficult since slightest waves would shake you up and down and from right to left (at least it just takes 5 hours).

In the Tektite programs the Aquanauts just left the habitat, ascended to the support vessel and entered the decompression chamber within a certain time frame in which the effect of decompression sickness would not occur yet.

Therefore wet decompression is out of discussion: too boring, too nervy and far too dangerous.

2. Decompression in the habitat

In Cousteau’s Conshelf III habitat 1965 on 100m (∼300 foot) depth the decompression sequence took 84 hours and was completed inside the habitat.

According to the US Navy Heliox Saturation Decompression Table from 2006  it would still take 65,83 hours ((100÷6)+(100÷5)+(50÷4)+(50÷3)).

US Navy Decompression Table*
Depth range Ascent rate
1600 to 200 fsw 6 fsw per hour
200 to 100 fsw 5 fsw per hour
100 to 50 fsw 4 fsw per hour
50 to 0 fsw 3 fsw per hour

(*US Navy Diving Manual Rev. 6, 2008, p. 661)

A decompression schedule of nearly 3,5 days in a locked environment represents a high risk if something goes wrong like in Sealab III (1969, 186m) where one of the divers Berry Cannon had a severe accident. This created a dilemma, whether to keep him inside the decompression chamber, where he might have died, or risking his life by taking him out before completion of the decompression sequence. Both of the alternatives proved fatal in the end. He lost his life and the Sealab program was terminated.

The decompression from the Aquarius habitat in a depth of 19m (63 ft) takes 17 hours and is completed in the habitat itself. Therefore it ‘consists of three compartments. Access to the water is made via the ‘wet porch’, a chamber equipped with a moon pool, which keeps the air pressure inside the wet porch the same as the water pressure at that depth (‘ambient pressure’), about 2.6 atmospheres, through hydrostatic equilibrium. The main compartment is strong enough, like a submarine, to maintain normal atmospheric pressure, and can also be pressurized to ambient pressure, and is usually held at a pressure in between. The smallest compartment, the Entry Lock, is between the other two and functions as an airlock in which personnel wait while pressure is adjusted to match either the wet porch or the main compartment.’ (Wikipedia)

Beside the Aquarius solution (wet room + entry lock + main compartment) we could use a two-compartment concept (wet room + main compartment). The Aquanauts in the main compartment lock the hatch between the two rooms and start to reduce the pressure inside. After reaching surface pressure they would recompress, open the hatch to the wet room, leave the habitat and ascent diving within the standard no-decompression time. In case of an emergency during the decompression a paramedic enters the wet room, seals the hatch behind him, reduces the pressure until equilibrium in both compartments is obtained, opens then the hatch between them and starts the treatment of the sick diver. He then would complete the final decompression together with the aquanauts.

This concept would prove fatal if the emergency is serious and one of the aquanauts has to be brought to a medical facility urgently. Then we would be in a double trouble: first, a life threatening situation with an aquanaut who might be unconscious, and second, the same diver who is not sufficiently decompressed. There would be no way to bring him to the surface. Instead, all necessary medical equipment (if it is pressure-proof and not bigger than the entrance trunk) has to be brought into the habitat. Conshelf III would have been unable to maintain even the transport of the paramedic to the habitat: it was just to deep.

3. Standard offshore procedure

In the end of the mission the aquanauts leave the habitat and enter the PTC (personnel transfer capsule) nearby. The entrance would be sealed and the PTC lifted to the support vessel. There it would be mated to the DDC (deck decompression chamber). The hatch would be opened and the aquanauts would be able to enter the comfortable DDC, where they complete the decompression while the support vessel starts to return to the harbour.

In a case of emergency a paramedic can enter the DDC over an airlock. If the emergency is serious, the diver would be transferred to the PTC again and could even be brought to the nearest hospital by helicopter, while the other Aquanauts stay in the DDC.

This procedure is the most safe one, although it requires a sufficient support vessel, a corresponding winch, a mounted DDC, the mating interface, the PTC, lots of personnel and lots of money. The expenses would maybe even exceed the feasibility of the project in the first place.

And just not to forget: We also should keep in mind that the PTC and the DDC should be air-conditioned since they would heat up to very serious temperatures under the summer sun.

4. The open PTC

In the end of the mission the aquanauts leave the habitat and move to the PTC that is lifted towards the surface with a speed of e.g. 1 m/hour, while the PTC is fixed to a heavy mooring on the seafloor by steel cables. The hatch remains open the whole time, while the expanding air just bubbles out. In a case of emergency a paramedic would dive to the PTC, enter it and start treatment. If the situation is very serious, the hatch would be locked and transported ashore by helicopter. The PTC would have to be equipped with a standard mating interface to be mated to the decompression chamber in the hospital.

The advantage of this design is the price. There is no need for a large support vessel, winch, DDC and numerous personnel.

The disadvantage is the susceptibility for wave movements since the PTC is ‘hanging’ upwards. It occurred before that aquanauts became heavily seasick because their habitat swayed with the waves. And more than 17 hours of seasickness would be unacceptable especially if one of the divers suffers from a slight emergency.

Sudden weather changes should be taken into account as well. It might be necessary that the decompression sequence will be aborted and the aquanauts return to the habitat. A serious problem appears if the weather change is that strong that it even threatens the habitat. Evacuation has to be done over the PTC, which is sealed and brought back to shore by helicopter.

All these ‘ifs’ show how important a clear planning is. Especially the weather forecast should be followed very attentive and a mission should be aborted long before a potential serious emergency might occur.


If there are no legal regulations available already, that demand a certain way of decompression design, I would suggest to concentrate on decompression procedures inside the habitat (see No.2 above: the two-compartment concept). It has proven very satisfying and safe in many years of Aquarius missions.

Additionally a PTC as backup is not too difficult to build and might be life saving.

As a decompression schedule I would suggest the table of the US Navy Diving Manual Rev. 6 from 2008 until a better proposal is done.

Please join the conversation and contribute your opinion by using the comment option.

Support by Sharing

Leave a Reply

Your email address will not be published. Required fields are marked *