4.2 Ocean Space Habitat
Decompression.
It’s the hard reality faced that comes with human exposure to pressure, and the challenge we divers have endeavored to address for centuries. There are only two options to mitigate limitations imposed through decompression. First, we [humans] can evolve towards a more aquatic state – think Cousteau’s manfish prophecy. This is fun to think about, but in facing the realities of our terrestrial stronghold in the nearer term, we’re left with only the second option - and that is to adapt.
My first exposure to staged decompression diving came at 18 or 19 years old. This was the late 90’s, when mainstream technical diving was just starting to emerge. I recall one summer in particular when one specialty company released their product catalog full of technical diving tools that many of us hadn’t seen before. It was easy to get the bug to buy new toys and then put them to work. In hindsight, it was a bit of a backwards approach to more advanced diving. We had the tools but didn’t know what to do with them; as opposed to finding an unmet need and then identifying or developing the tool for the job. It seemed like everyone wanted to be a deep technical diver, and, as a divemaster working New England wreck charters, it was scary. This new emergence meant lots of divers over-laden with heavy equipment that they may or may not have actually needed jumping over the side of the boat and starting to run dive profiles that put them into required decompression.
At the time, a last decompression stop of 20-30 minutes was typical after diving wrecks in the 100-190fsw range. As a young divemaster, I studied these types of profiles and prepared myself for how I might need to intervene, particularly if the diver omitted this required decompression. Training for these types of dives back then was fairly rudimentary, and I honestly don’t believe much thought was placed on the many ‘what if’s’ that needed to be accounted for, particularly regarding the realities of gas management contingencies.
In 2002, I was involved in one of the first modern era mixed-gas projects for science that took us to 300 feet. By today’s standards, that’s a walk in the park, but the dives did shed some light on what’s been a recurring theme in advanced diving - limited bottom times (only minutes) come with the expense of very lengthy decompression requirements. This was exciting work but cost ineffective and prohibitive of really moving forward with any more meaningful scientific work at depth.
| Dive profiles have evolved to become more complex, largely made possible with dive computers that can leverage an algorithm in real time. Habitats add another element to this, since the ability to safely surface becomes a longer foray. |
Over the decade that followed, I was on a professional trajectory in commercial diving, though also embraced rebreather diving and quickly saw the benefits if the technology were applied in both communities (science and industry); though it was slow going with all sorts of challenges at programmatic levels. In 2010, I organized a scientific expedition with the intent of establishing some techniques for working vertically along deep coral reefs to depths approaching 500 feet. The mission was successful, but it became ever more obvious that the d-word, 'decompression', was the limiting factor, with literally hours spent doing nothing but hanging around on a line. That afforded plenty of time to appreciate that something significant had to change if this type of diving would ever become more commonplace as a scientific vehicle.
Sometime after this 2010 mission, it dawned on me that the challenge with decompression is that it is unproductive time. It represents a time investment that is counterproductive to the very nature of scientific fieldwork where being efficient with precious field time is critical. So, quite simply, I set out to determine how to make productive use of otherwise unproductive time - that meant taking the diver out of the unproductive environment.
With decompression conventionally carried out while the diver is immersed, hanging on a line, and still burdened with staying alive underwater, it became obvious that this convention is what needed to change. By contrast, in commercial diving this decompression obligation is often managed in a surface-based chamber. There was obvious middle ground to be explored.
With that, Ocean Space Habitat was born.
System Description
Portable habitats are quite literally small voids of space, created either with soft inflatables, plastic boxes, or other simple structures that provide a controlled area that water can be displaced from for divers to enter. The habitat may or may not contain seats, windows, other fixtures, various plumbing for inflation/deflation, and atmospheric management tools. Small decompression habitats are not new. They have been used by the cave diving community over the years, but as of 2012, we hadn’t seen them used in open water. The issue that surfaced immediately was securely anchoring such a buoyant system. A space large enough for two people to sit with only minimal space is about 2000 pounds buoyant. In a cave, the ceiling becomes the most opportune location to secure this significantly buoyant space – just displace the shell and it will wedge itself in the ceiling. Secondary anchor bolts can be used for stability but managing the buoyant load itself is rather straightforward. In open water, anchoring is everything. I don’t want to stray too far into the specifics of portable underwater habitat deployment and operations, particularly anchoring, but will say that the decade and a half of commercial diving prior to beginning the habitat experimentation paid off. I had learned how to drill into rock, use hydraulic torque tools, use underwater epoxy, drive duckbill style anchors, set heavy anchors, work with heavy hardware, and so on – all indispensable on-the-job training that just isn't the routine within the scientific community.
In 2012, we forayed back to the deep reef environment and demonstrated use of a portable inflatable habitat in open water to provide respite from lengthy decompression following several deep mixed gas dives. For the very first time, rather than hang on a line, we were able to sit, relax, talk, stay warm, and decompress in relative comfort. We were making productive use of previously unproductive time and within the conventions of mainstream technical diving techniques. This came with one caveat. Now that we could stay down there comfortably and presumably for lengthy periods of time, we had to think through the new diver responsibilities that came from making use of newly created confined space.
| Our Gen 1 Ocean Space Habitat was deployed at the reef crest, allowing excursions throughout the mesophotic zone to over 400' depth. |
Atmospheric management within any confined space becomes the name of the game. Thanks to my time commercial diving, I was familiar with the industrial protocols for atmospheric monitoring and management, and then the value of rebreathers come into play very quickly. The occupants of the habitat space must ensure that the atmosphere is stable and maintained by way of removing carbon dioxide and replenishing oxygen at a rate matching their metabolism. Sound familiar? This is rebreather diving, though the main difference being that the occupant is essentially inside of the breathing loop. The habitat itself is displaced with some known displacement (‘diluent’) gas, and then simply scrubbed of carbon dioxide with oxygen metered in.
To take advantage of not needing a mouthpiece shoved in your face, the habitat makes use of an open loop oxygen rebreather with a fan that circulates the ambient atmosphere through the scrubber, and then mechanically adds oxygen at a rate equal to the occupants’ metabolism. Very simple, and, given the community’s generally safe and thoughtful reception to rebreather diving, it is not too far reaching to put habitat atmospheric management responsibility in the hands of a well-trained diver within the technical diving training regimen.
Technical Overview & Principles of Design
My work on habitat development continues even at time of this publication; however, the fundamental principles are well developed enough to present here. Recent work on the full system has resulted in a 2018 patent on the technology of a portable inflatable habitat that includes a replenishable life support payload, inferring that once life support is consumed, it can be replenished such that the habitat stay can theoretically become indefinite. Removing the habitat shell itself from discussion, here I will focus on the life support payload given its consistencies with the rebreather technology described throughout this content.
Basic Envelope and Fundamental Configuration
We’ve worked through a couple of habitat life support configurations – one being built into a frame which doubles as the occupant’s seats, and, more recently, have reverted back to a configuration more consistent with a standalone rebreather. This provides what we refer to as a ‘tower’. The tower can be towed in a streamlined fashion and hung vertically within the habitat space. When hanging vertically, the system’s controls are about waist level when the occupant is sitting, and monitoring/alarm devices are placed near the occupant’s head for easy and immediate reference.
| One iterations of the life system assemblage utilized within the rebreather envelope. Current variants are highly modular, allowing plug and play interoperability for redundancy and consumable replenishment through extended stays. |
Breathing Loop
The breathing loop is virtually identical to that described previously, with a few key features specific to habitat stays. Functioning as an open-loop, there is no counterlung(s), however both an ADV (oxygen only, perhaps ‘AOV’) and pressure relief valve are present to compensate for changes in depth during deployment and recovery. The loop is kept closed during these wet periods and then opened once within the displaced habitat space.
Carbon Dioxide Removal
The scrubber is a simple axial design and is keyed to mission requirements. Considering ergonomics during diver transport and what we envision as typical use, we have selected a scrubber size that provides for 8 hours of use by a single diver at 1ATA, thus 4 hours for two divers. This 8-hour period has been considered since it represents a ‘shift’ of work, after which support personnel may be tasked with rotating out this consumed payload for a fresh payload. Similarly, if used for an overnight, 8 hours of occupancy would cover a typical sleep period.
A powered fan circulates the atmosphere through the scrubber with gas drawn in from the low portion of the scrubber (effectively the exhale side) and distributed through the upper portion of the scrubber (effectively the inhale side) where the oxygen sensors are located. The inlet and outlet for the scrubber are separated with the inlet lower positioned given the assumption that CO2 is heavier than the desired atmospheric gas composition, so it is logical to scrub [heavier] gas lower in the habitat, sending fresh gas to the upper portions of the habitat. Should the fan fail, BIBS style oro-nasal masks can be fit to the inhalation side of the scrubber such that the occupants can make use of a lung-powered scrubber. This feature is used routinely in mini-submersibles and atmospheric diving suits, so while new to divers, this open-loop rebreather concept is not new within the submersible community.
Gas Distribution
When the loop is closed during underwater transport, pressure is regulated via oxygen supplying an ADV (AOV) device. Once the loop is opened, this same device can be used to manually inject oxygen; however, the primary oxygen is supplied passively using a needle valve, which covers the spectrum of one or two persons’ oxygen metabolism demands.
In habitat diving, rather than reference ‘diluent’ gas, we reference a ‘displacement’ gas, since this is the supply making up the primary atmosphere that has displaced the water inside the habitat. Displacement gas is selected according to depth and the desired decompression strategy. Displacement gas is carried in cylinders external to the system, such as stage cylinders, which are sized to account for displacement the system at the desired depth plus provide some contingency, plus provide for minimal occupant use during the ingress and egress protocol.
The life support tower includes a displacement gas distribution manifold consisting of a quick disconnect inlet on a long hose that extends to the habitat exterior, one or two second stage regulators on long hoses also positioned exterior to the habitat, and a ball valve which allows for aggressive manual displacement inside the habitat. The displacement gas cylinder is attached to the quick disconnect, similar to the process described for a rebreather gas switch, then allowing the occupant divers to proceed with ingress, displacement, and egress from this exterior gas supply. We've become partial to long hoses fit to the displacement cylinder manifold (for easy pee breaks).
Oxygen Monitoring
Habitat atmospheric/oxygen monitoring uses the same basic principles as RD1 where two sensors supply a simple digital pO2 meter. Critical to the habitat metering system is that it must function when removed from the water. Many off the shelf oxygen monitoring devices are water or pressure activated and will time out once dry, believing that the device is back at the surface. In the habitat, the display must function independently of pressure or depth. A simple loop flush chart can be used to establish benchmarks for atmospheric management through the stay, particularly if the displacement gas composition might be changed at some point during the decompression profile.
Opportunities
Once space is established within the habitat and that atmosphere can be managed, very real range extensions can be considered. When coupled with rebreather technology and supply rations, these spaces can provide for full work shifts, if not multiple days of a controlled and comfortable underwater stay. It becomes entirely feasible to maximize the life support of a typical rebreather exterior to the habitat (say 4 to 6 hours) and then become dependent on the habitat for the decompression phase of the dive. This represents a massive range extension within the technical diving regimen as no longer are the full surface-to-surface excursion required to be conducted with the domain limitations of the primary life support system. This capability effectively puts cost-effective ‘near saturation diving’ capabilities in the hands of the masses.
For immediate practice, the utility of the system can be applied in several contexts where:
- the space can provide simple respite or a pressurized workspace for lengthy no-decompression dives.
- the space can provide rest during decompression.
- the space can serve as a space for emergent retreat where First Aid can be rendered.
- the habitat can function as a controlled area for conducting in-water recompression (IWR) treatments.
| Renderings of a variable depth habitat concept, allowing traverse of the shallower decompression stops. In practice, there are multiple decades of benefit to first exploiting the 20' stop alone before this variable depth capability becomes an advantage. |
For IWR considerations, integration of voice and video telemetry could provide a space where medical intervention can be guided for field emergencies. When a hyperbaric treatment center is greater than 24 hours away, IWR has been considered previously though while recognizing limitations of the victim/patient being immersed in water, potentially cold, and in a compromising environment if conducting aggressive oxygen therapy under pressure. A habitat mitigates these risks substantially.
Future applications of portable habitats will benefit from multilevel excursions, relying on the structure as a push downward of the decompression ceiling. By staging dives from 20', very full workdays can be spent underwater, within the range of depths we just temporarily visit now for short durations. This opens vast expanses of Ocean Space for the ambitious and capable technical diver.
| Conceptual profiles to illustrate use of a 20' staging area. Planned well, multilevel excursions can be made across several days, all while never incurring more decompression than what can be comfortably dived out (roughly 4-5 hours) to surface. |