Habitat Entrance

The moonpool of habitat Hydrolab
Moonpool of Hydrolab

(Updated 24.05.2018; added wet room configuration, scuba tank refilling installations and summarizing requirements list) Surely one of the most complicated area of an underwater structure for human occupation is the habitat entrance. It is the local water-air interface, vulnerable to changes in pressure by tidal movements on the surface of the sea; its hatches have to bare potential pressure differences between the habitat interior and the surrounding water; no object that is only a little bigger than the greatest diameter of the entrance can be brought into the habitat. It is constantly wet and humid and the only gateway for medical assistance. But to see in detail let’s have a look at the different sections of the entrance complex.

The Trunk

The trunk is the skirt under the entrance hatch and it is generally holding a certain amount of air. Even if the hatch above is closed it can serve as an emergency air pocket allowing divers to emerge partially or completely from the water. There they are able to talk face-to-face, solving hatch problems in a calm way or just save themselves from any emergency. Above the trunk is the hatch, that can be opened up to the inside of the habitat, down to the trunk itself or (with two hatches) to each side. The first trunks were small and simple, but several projects showed that they should at least have the size for two divers and seats just below the water allowing returning divers to sit and take off their diving gear. Therefore they should have well lighting, handles and a system for fresh air supply.

Some habitats had scuba tank refilling installations inside the trunk. This avoids lifting of heavy equipment; it simplifies taking equipment on and off; and it cools the tanks while being refilled.

The underwater access space

The underwater access space is located below the trunk between the seafloor and the trunk entrance. It requires enough space to easily enter the trunk. Therefore it determines the height of the entire habitat over the sea bottom. Several projects like Tektite or Conshelf had a “shark cage” at the beginning of this area preventing uninvited animals from approaching the entrance. The access of this area should be very well marked (e.g. by position lights) to guide divers in bad visibility.

The Hatch

The hatch seals the interior of the habitat from pressure differentials during (de)compression or habitat lowering. There are three different options:

  • opening to the inside of habitat
  • opening to the outside of the habitat (into the trunk)
  • two hatches; one to each side

The hatch is only openable if the pressure on both sides of the hatch is equal. If the pressure is different it might be impossible to be opened or (in unsecured hatches) the hatch would spring open once unlocked, and therefore would be a risk for the person trying to open. A pressure differential of the hatch was one of the reasons of a fatal accident in Sealab III leading to the termination of the multi-million dollar project.

Two mutual hatches require a valve in the space between them to equalize the pressure. Otherwise the hatches would be unopenable (for any reason) even if the pressure inside the habitat and the ambient pressure are the same.

The crew of NEEMO 6 in the moonpool of Aquarius
NEEMO 6 in the moonpool of Aquarius

In Aquarius there is no direct hatch over the trunk. The air pocket of the trunk consists of a complete room, the so called “wet room”, that is attached to the actual habitat. For decompression the hatch between the actual habitat and the wet room is locked, while the wet room always remains at ambient pressure. The following room can also be used as an airlock, e.g. if medical assistance is required during decompression. In this case the paramedic closes the hatch behind him after entering the air lock and opens the next hatch after the pressure between the airlock and the actual habitat is equalized. In normal conditions, however, the airlock is used as a standard working place. This is a very coherent system and very desirable solution for future ventures.

If there is a probability that the pressure inside the habitat changes during a certain period of non-occupation (between missions), then there has to be a possibility to equalize the interior pressure from the outside; e.g. remotely or by a mechanism in the entrance trunk.

The Moonpool

The moonpool is the gateway between the habitat interior and the surrounding water. Its level is maintained by the internal pressure preventing water from rising and flooding the entire habitat. Pressure lower than ambient pressure would cause the level to rise, higher pressure would cause the air to bubble out from the trunk. Only by using a hatch between the moonpool and the habitat interior, different pressures can be applied, e.g. for lowering the habitat to the seafloor or for decompression in the end of a mission.

A moonpool on a ship

Moonpools are used in diving support vessels to lower/lift objects from the center of the vessel. (image from Wikimedia Commons: model of Ceona Amazon by Dr. Karl-Heinz Hochhaus)

There were several different types of moonpools. Smaller habitats used a huge pipe as a trunk ending in a horizontal hatch. Larger habitats like La Chalupa and Aquarius used (and still use) rectangle moonpools, while the “wet room” is separated from the actual habitat by a vertical door-hatch.

The moonpool of Hydrolab (see image at the beginning of this article) directly led to a very small airlock. Divers had to enter this airlock, close the hatch behind them and wait in a squatted position till the pressure equalization is completed to open the upper hatch and to enter the habitat.

The garage of Conshelf II
Conshelf II: Garage (2017)

The biggest moonpool to our knowledge was inside the “garage” of Conshelf II. The garage itself was separated by all means from the actual habitat. It served as a hangar for a submarine for two persons. It allowed the submarine to emerge and be lifted by a winch over the water, while the moonpool itself could be covered to allow maintenance of the sub. To reach the habitat the aquanauts had to dive, since the habitat and the garage were two different buildings. The moonpool was several meters in diameter. (Image from Wikimedia: Tauchsafari Sudan mit UMEX Tauchsport 2017 by wikiumex)

One specific difficulty with moonpools is the direct reaction of the water level to tidal changes on the sea surface. Water depth and pressure is changing if a wave passes above. This pressure has a direct effect on the volume of air inside the habitat leading to a change water level in the trunk/moonpool. Even a few centimeters would require the aquanauts to equalize their ears constantly. Closing the hatch would surely solve the problem. To open the hatch again there has to be a direct air connection between the air pocket in the trunk and the habitat interior to equalize the pressure in both compartments.

A curious thing happened in Sealab II: a great number of poisonous scorpion fish gathered in the trunk and hampered the divers from passing. What sounds like a funny incident could become a serious threat. Most likely they were attracted by the light coming from the wet room. To avoid similar situations in the future we should consider to install a mechanism that automatically turns the light inside the wet room into a red light (with the option to open normal light on demand) whenever the hatch is opened. This would accustom the divers eyes to the outside twilight and prevents incoming divers from glare. Additionally a distraction light on a less critical location could be activated prior to hatch activities to create an attraction to any marine animal.

Even if there are no information on pollution of the moonpool surface it is clear, that after a while there will be several particles swimming on the water, like hair, dust etc. Our proposal is to install an overflow mechanism that regularly pumps off the upper layer of the pool and disposes it to the outside the habitat.

The Wet Room

The wet room is located between the hatch and the main living and working area. Due to its natural high humidity it is well separated from the other areas by sealable doors. Ventilation is an important feature for the wet room. To avoid humidity built-ups there should be a procedure to regulate opening of the hatch if possible, especially in tropical waters, when the water temperature rises over the habitat interior temperature at night.

Showers are generally located in the wet room since the sewage is easily disposed over the entrance . One of the most desired features of former habitats was the availability of hot water to permit the aquanauts to warm up after excursions. Comfortable body temperatures are the top priority for successful missions. We should consider a separate room to maintain a certain degree of privacy. The WC will also be located in a separate cabin.

A rinsing basin for scuba gear should be available inside the wet room. After excursions this gear has to be cleaned from salt water to be stored for drying.

The scuba gear drying and storage facilities should be very well ventilated to avoid mold. In order to use the thermal discharge of compressors and machineries it may be a good idea to position the storage close to the separating wall of the machinery section. Maybe there is a possibility to lead used (warm) air from the habitat interior through the storage area before it arrives at the scrubber devices. It is important that the storage area is large enough to avoid lack of ventilation around the single items.And there should be enough space to change clothes without hindering any other procedure.

Besides there should be locker space for individual diving equipments and to store dry clothes while the aquanaut is outside the habitat.

A winch over the moon pool is necessary to lift heavy items out of the water. This could be daily cargo, heavy machinery spare parts or simply diving gear.

General Consideration

Like in all major habitats there should be alternative exits in different parts of the habitat for the case, that the main entrance is blocked for any reason. These can be hidden hatches, but they have to be well marked, always accessible and regularly controlled. Of course, there should be corresponding diving equipments nearby for emergency exits.

Aquanauts of Sealab requested outside cameras to oversee activities around the trunk. They are vital for the coordination of emergencies.

The most desirable moonpool would look like the one  seen in the movie Abyss. A large pool for all kinds of submarines, equipments and divers. Well, beside the difficulty of balancing the structure, compensating the tidal changes,… and the money… why not?

Summary

Here is summarizing table of all required features of the entrance complex. I divided it into two different habitat types: the experimental habitat, which is small and mobile; and Module 1 of the large hangar shaped structure. Primary features are necessary from the beginning; secondary features are subject to addition afterwards.

StructureFeatures
Experimental
Habitat

(small mobile version)
primary features
- distraction light on habitat exterior
- hooks and shelves on exterior
- exterior camera for supervision from inside
- guidance light next to trunk entrance
- sufficient underwater access space (>=1m)
- sufficient trunk diameter (≈120cm)
- sufficient air pocket (>=70cm)
- handles, foot leans, sitting installations
- light inside the air pocket
- communication installation
- single hatch (opens to the outside)
- additional emergency hatch
- hatch decompression valve (?)
- safety measures for hatch handling
- ladder to the wetroom

- shower (=rinsing facility)
- standard boat WC (separate cabin)
- sink, mirror, electric socket
- hooks, shelf

- personal locker
- equipment drying/storage installation
- dehumidification facilities
- automated red light upon hatch opening
- optional white light switches
- (mountings for secondary features)

secondary features
- emergency breathing installations
- moonpool overflow for surface cleaning
- hot shower (=rinsing facility)
- winch
Module 1
(of large structure, hangar type)
primary feaatures
- distraction light on habitat exterior
- hooks and shelves on exterior
- exterior camera(s) for supervision from inside
- guidance light next to trunk entrance
- sufficient underwater access space (>=1,50m)
- sufficient trunk diameter (≈200cm)
- large air pocket (≈200x300cm, ≈150cm height)
- sitting facilities, grating (≈200x80cm)
- scuba tank securing installation
- lighting inside the air pocket
- communication installation
- two mutual hatches
- additional emergency hatch(es)
- hatch decompression valves (?)
- safety measures for hatch handling
- emergency breathing installations
- stairway to the wetroom

- hot shower (=rinsing facility)
- shower cabin
- WC in separate cabin
- sink, mirror, electric socket
- hooks for towels and clothes
- shelves for personal belongings
- storage locker

- personal locker
- equipment drying/storage installation
- sealable door to other sections
- dehumidification facilities
- automated red light upon hatch opening
- optional white light switches
- (blue light when wetroom is not in use)
- (mountings for secondary features)

secondary features
- "shark cage" (if necessary)
- scuba tank refilling installation
- airlock
- moonpool overflow for surface cleaning
- rinsing basin
- advanced drying installation

This article needs to be improved. Help us by using the comment box below.

Still to be added:

  • sketch of the different entrance sections
  • Link the regular maintenance procedures of the emergency exits to the maintenance manual
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