An Experimental Deployment of a Portable ﬂatable Habitat ... · of realizing permanent undersea resi-dence and colonies suffered setbacks that can be traced to the fall of the Sealab

P A P E R

An Experimental Deployment of a PortableInflatable Habitat in Open Water to AugmentLengthy In-Water Decompressionby Scientific DiversA U T H O R SMichael LombardiLombardi Undersea Resource Center,Middletown, Rhode Island

Winslow BurlesonArizona State University

Jeff GodfreyUniversity of Connecticut

Richard FryburgSubsalve USA, North Kingstown,Rhode Island

A B S T R A C TUndersea living in the science community has effectively risen and fallen within

the last half century. The paradigm of residing on the seafloor within a fixed, per-manent structure, while body tissues are saturated with inert breathing gasses, pro-vides for extended-duration excursions from such a structure, although limitsgeographical productivity to within reasonable proximity of the habitat structureitself. Saturation diving exploration with science motives provided an exciting op-portunity during the 1960s and 1970s, with timing lending itself well to providing asea-to-space analog for human residence in a remote and confined space, as thespace race was underway.With limited saturation diving for science occurring pres-ently, today’s marine science paradigm is trending toward advanced autonomousdiving technologies and techniques, including mixed-gas use, rebreathers, andstaged decompression. These emerging technologies afford an enhanced“commodity-style” approach to exploration, in which diving scientists can travelto any remote locale and spend longer durations underwater than they can withthe previous and more common paradigm of lightweight, travel-friendly, conven-tional open-circuit scuba (using air as the breathing medium). Amiss in the newparadigm is the practical extension of depth. This is well within reach with theuse of emerging technologies; however, end-users are often dissuaded from theincurrence of lengthy decompression (exposure to the marine environment duringwhat is effectively an extended idle time) that is required when scientists return fromrelatively short working periods at extended depths. In an effort to address these is-sues, we describe here the development and experimental deployment of a new classof portable inflatable underwater habitats that provide for rapid deployments, freefrom surface support augmentation requirements typical of the existing alternativesfor lengthy decompression dives. In the context of vastly expanding the commodity-style diving requirements of today’s marine scientist and engineers, particularly interms of increased depth and duration, we also discuss the further research anddevelopment applications that these habitats make possible.Keywords: underwater habitat, rebreather, mixed-gas, scientific diving, decompression

IntroductionHistorical Overview

The promise of undersea living orimproving the experience of scientistsand explorers along continental shelfmargins has largely eluded modernocean exploration efforts for morethan half a century. The grander visionof realizing permanent undersea resi-dence and colonies suffered setbacksthat can be traced to the fall of theSealab program in 1969 (Hellwarth,2012). Although efforts in undersealiving and habitation for science con-tinue with government support in theUnited States, meaningful advance-ments and ambitions in terms ofdepth and duration have been consid-erably scaled back and remain precari-ous, with all missions taking place inrelatively shallow waters. Most signifi-cantly, despite a record of successfulmissions, NOAA’s Aquarius Reef

Base Habitat in Key Largo, Florida,recently faced imminent shutdowndue to loss of federal support in thewake of the United States’ financialcrises. Fortunately, the Aquarius Pro-

gram was rekindled with supportfrom Florida International Universityin 2013 (Mesophotic.org, 2013) andcurrently remains the last perma-nent undersea habitat dedicated to

52 Marine Technology Society Journal

supporting scientific missions in theUnited States.

Living underwater has found somejustifiable merit in the commercialsector, where industry-driven profitsfrom resource exploitation providesufficient capital to underwrite thesubstantial costs of the required tech-nology and infrastructure to operatesafely. Although it is commonly un-derstood that risk aversion supportsa trend toward replacing humansworking underwater with new appli-cations of advanced robotics and au-tomated systems, such technologiesare often viewed as permanent replace-ments. This misperception dates backto the early 1960s; the same time frameof the undersea habitat ion andresource exploitation boom—a timewhen the Shell Corporation claimedto have a robot that would “replace”divers (The Times News, 1962).Today, the fact remains that, morethan a half century later, this re-placement has not occurred. Placinghumans on the seafloor to conductmeaningful work, across a multitudeof industry and scientific communi-ties, is still occurring with great fre-quency and is in high demand. In thescientific diving community, a smallsubset of occupationally classed divingoperations, more than 100,000 divesare conducted annually within theUnited States (Dardeau et al., 2012).However, many of the same strugglesexposed in the Sealab program (andsimilar habitat programs) remain: pre-dominantly the aversion to acceptingrisk associated with the fundamentaldriver of placing humans on the sea-floor, at unexplored or unstudieddepths, and for periods of time toallow for productive work. At thecore of these struggles is the challengeof effectively coupling human physio-logical limits with emerging tech-

nological approaches in ways thatfundamentally improve the capacityfor human intervention and innova-tion within the marine environmentand on the seafloor, across the spec-trum of both depth and time.

The Need for NovelTechnologies That SupportImproved HumanIntervention Techniques

Marine scientists represent a popu-lation whose work requires scientificdiving as a tool to afford observationand data gathering of diverse environ-mental parameters and personal inter-action with seafloor flora and fauna.Given massive costs for operations,permanent undersea habitats offervery limited and largely ineffective op-portunities for a growing scientific div-ing population that generally operateson lean field program budgets and thusincreasingly can only afford to conductfieldwork opportunistically.

According to the American Acad-emy of Underwater Sciences (AAUS)statistics reviewed from 1998 to2012, there is a declining trend inboth number of diver end-users andcumulative dives conducted from satu-ration (Figure 1). This is likely the re-sult of realizing the limitations withshallow saturation diving as a tool forscientific diving, where geographicalrange is restricted to the habitat’shome base and depth is restricted tothe immediate bathymetry. As the sci-ence community considers extendingtime underwater, increasing mobility,investigating a variety of sites, and ex-tending operations to remote locales,the adoption of more advanced divingtechnologies and techniques are beingexplored. and their use is on the rise(Figure 1) with the desired intent ofusing these as “commodity tools.”This commodity-style approach to

diving has already occurred in the con-vention afforded by open-circuitscuba, where this mode of diving hasbecome a standard tool in every divingscientists’ toolbox (in contrast to theinaccessibility of saturation or habita-tionmodels of fixed, controlled, costly,and permanent assets). This trend to-ward the broader adoption (or thecommoditization) of increasingly ad-vanced diving modalities is evidentin Figure 1. These advanced modesoffer opportunities for deeper depthsand longer durations—effectively im-provements on the prior commodityconvention—and are evidence thatscientific divers and exploration com-munities are both interested in andbeginning to exert significant effortsto spend more time underwater in acost-effective manner and with the op-portunity to employ such capabilitiesacross broader geographical localesthan afforded with saturation diving.

Commodity technologies (open-circuit scuba) enabling end-user diversto effectively “pick up and go diving”have become effective for surface-to-surface scientific excursions in numer-ous types of environments, especiallyin remote environments, and to abroad spectrum of depths for decades.While trends in wet diving techniquesindicate increasing use of frequencyfor decompression, rebreather, andmixed-gas diving, scientific diving ac-tivities below 60 msw (190 fsw thresh-old for AAUS depth reporting) haveremained relatively constant (Figure 1).This may be due to several factors,including difficulty in attaining andmaintaining deep diving proficiencywithin short field seasons, regulatorycomplexities such as insurance and lia-bility concerns, and somewhat complexoperational standards enforced at thehome or host institution. Furthermore,limitations of working time at these

November/December 2013 Volume 47 Number 6 53

depths, due to both the equipmentrequired to be carried and the prac-ticalities of carrying out lengthy de-compression while exposed to themarine environment, can be psycho-logical deterrents from carrying outdecompression dives.

With the increasing availability ofnew commodity tools that improvehuman intervention within the marineenvironment and the resulting evi-dent trend (Figure 1) in an increaseof mixed-gas, decompression, and re-breather dives over time (but notably,not with increased depth), scientificdivers are seeking to spendmuch greatertime durations in the water. Lookingahead, as the existing advanced modesreach their limits in terms of theseparameters, it is logical to considerthe next steps in improving technolo-gies or developing new technologiesthat support human intervention andhow to make these increasingly acces-sible to the community that desires touse and benefit from them.

Our Realized Justificationto Address the Need forNovel Technologies

Scientific field programs conductedin the Tongue of the Ocean (TOTO),

Andros, Bahamas (2010) and ExumaSound, the Exumas, Bahamas (2011)provided novel access to mesophoticcoral ecosystems (general ly 60−130 msw; Puglise et al., 2009) usingmixed-gas closed-circuit rebreathers(CCRs) (Lombardi & Godfrey, 2011).Dives were conducted to greater than120 msw at each respective locationby an autonomous team of two indi-vidual self-contained scientific diverswithout incident. While the diveswere productive, with >35 min spentworking at depths deeper than80 msw on each dive, shallow decom-pression requirements accounted forthe vast majority of the in-water divetime, that is, over 85 min or about66% of total in-water time (Figure 2).Generally, these lengthy decompres-sion exposures were cause for keepingplanned total run times to less than3 h for subsequent dives. This affordedcontingency dive profiles of up to 4 h,which remains within the functionallimitations of the divers’ autonomouslife support package. This packageincluded a rebreather device and open-circuit bailout supplies that can be rea-sonably carried by each independentdiver (not more than three AL80 cylin-ders each) while working from a small

vessel. Predive plans indicated that lon-ger dives would push the limits of thefull primary life support capacity rea-sonably carried by a single diver whileworking independently, free of surfaceor in-water support. Working bottomtimes were consequently restricted.

The scientific success and incident-free nature of the dives are encouragingwith respect to extending both depthand duration of future dive profiles inways that aim to improve interven-tion capacity and discovery potential.Lombardi and Godfrey (2011) de-scribe approaching these workingdives within the vertical MesophoticCoral Ecosystem (vMCE) using aphased approach. While each identi-fied dive phase (Figure 2) warrants fur-ther evaluation, the foremost resultinginterest is to address the decompres-sion phase of these deep dives. Decom-pression was carried out according toonboard rebreather computers usingthe Buhlman algorithm with gradientfactors. Varied gradient factors, GF5/95, 10/95, and 30/85, were anecdot-ally experimented with in an effort tomaximize time spent on the verticalwall habitat. While ascending acrossthe vertical plain, the ascent and earlydecompression can be coupled with

FIGURE 1

Trends in scientific divers’ use of mixed-gas and rebreathers as modes of diving and in required decompression and saturation environments since1998. Left graph indicates number of divers over time. Right graph indicates number of dives conducted over time. Data extracted fromAAUS statisticsdatabase, accessed May 9, 2013.


more productive work and thus reduceidle time spent in midwater while solelydecompressing. Generally, the diversnoted minimal work productivity ben-efits while forcing deeper decompres-sion stops (GF 5/95) and thereforeused GF 30/85 for all subsequentlyplanned dives.

While mission objectives are typi-cally completed before reaching thefinal stages of decompression, the lon-gest portion of the in-water exposureremains. In this phase of the dive,ascending to the surface, without em-

ploying cumbersome surface technicalsupport such as a pressurized bell, isnot a viable option in the event ofany in-water incidents, given the im-minent threat of decompression sick-ness. Utilizing GF 30/85 encourageslonger and shallower decompressionstops. These stops account for the lon-gest phase of in-water time, greaterthan 66% of the full surface to surfacein-water immersion (Figure 2). Thedive team described the final decom-pression phases for lengthy Mesopho-tic zone scientific exploration dives as

“wet, cold, and cumbersome.” Duringworking dives conducted to 120 mswand greater, it was not uncommon toincur greater than 90 min of decom-pression at the 6 msw stop alone.These decompression phases are gen-erally regarded as unproductive blocksof dive time.

Prior to engaging in a 2012 fieldseason, the dive team sought to addressthe activity hazards associated withthis lengthy decompression in thisopen-water environment (Figure 3).While hazards associatedwith equipment

FIGURE 2

Sample scientific diving profile while working within the vMCE (in Lombardi & Godfrey, 2011). Profile breaks dive into phases based on specificoperating procedures within that portion of the dive. Phase 4 indicates the lengthy decompression phase of the dive.


FIGURE 3

Activity hazard analysis for the Phase 4 portion of decompression dives. Categorical hazards include those associated with environmental exposure,equipment used, and diver operations. Benefits and controls afforded by habitat use are identified for each categorical hazard.


and operational procedures are fac-tors, it became evident that manyof these lengthy decompressionactivity hazards could be remediedby mitigating the risks associatedwith environmental exposure/immersionby the diver—removing the diverfrom the wet environment.

With the intent of extending work-ing bottom times for future work andconsequently incurring longer decom-pression obligations, the dive teammoved forward with the design, con-struction, experimental deployment,and subsequent mission integrationof a portable inflatable habitat as a pos-sible solution to mitigate the activityhazards of lengthy decompression con-ducted by an autonomous team in anopen-water environment.

Materials and MethodsMission logistics for operating in a

remote location with an autonomoustwo-person team, as well as study ofprior art including Edwin’s Link Sub-mersible Portable Inflatable Dwelling(MacInnis, 1966), led to the choiceof a flexible form-factor over a rigidstructure for the purpose of ease oftransport. Concept sketches (Figure 4)were generated to describe the basicenvelope sought for the developmentproject. The envelope pursued for de-velopment was a collapsible under-water tent, supported by a bridleattached to an anchorage, to be de-ployed to a fixed depth correspondingto the lengthy decompression that ex-tended depth and duration dive excur-sions require. The tent structure wouldbe equipped with bench seats and pro-vide valve assemblies for inflation anddeflation as necessary for both deploy-ment and atmospheric management.This structure allows for variable lifesupport systems to be integrated de-

pending on mission requirements. Thebasic design envelope was developedover a period of 1 year and reflectedthe product of industry, academic,and private sector collaboration.

Design and ConstructionSystem components include the

following:

Tent/ShellThe primary structure is a 60-inch

diameter by 60-inch tall fabric rein-forced open-bottom vinyl shell sup-ported by six 2-inch nylon safetystraps and six stainless steel triangularfixation points. A tubular aluminumframe is affixed to 16 perimeter grom-mets RFwelded to the inside perimeterof the shell, which support two benchseats. The shell includes three windowsat eye level to permit outside obser-vation. The tent is plumbed with two¾-inch NPT ports allowing fixation ofquarter turn ball valves, one on the in-terior and one on the exterior, to allowfor venting gas from the system. At100% inflation, the tent/shell exerts5,500 lb of buoyant force across thebridle.

BridleSix 20-foot-long straps (rated at

2,500 lb working strength each inchoke configuration) are fixed one toeach of the six stainless steel triangularfixation points. These join to two20-foot long straps (rated at 8,800 lbworking strength each in vertical con-figuration) connected by a 1-inch gal-vanized shackle (17,000 lb ratedworking strength). Each of the twodownward straps is fixed to the an-chorage, one via a three-quarter-inchshackle (9,500 lb rated workingstrength) and the other choked directlyto the anchor pin.

AnchorageThe anchorage consists of a prior

installed stainless eye pin drilled andepoxied into coral reef under an envi-ronmentally friendly mooring pro-gram installation coordinated byEnvironmental Moorings Interna-tional while under contract withNOAA’s Caribbean Marine ResearchCenter. Previous tests in similar sub-strate indicate that the stainless pinwill deform under a 20,000-lb pulltest, with no pullout (Halas, personalcommunication, 2010).

FIGURE 4

Concept sketches of portable inflatable habitat structure in use. Habitat deployed at the reef crestserving as hub for dive staging (left). Two divers rest in habitat while each breathing independentprimary life support used during the dive excursion (middle). Two divers rest in habitat whileeach breathing independent habitat-staged primary life support (right).


Life SystemsFor the initial experimental deploy-

ment, divers shed their open-circuitbailout cylinders and donned onlytheir CCRs inside the habitat duringlengthy decompression. No life sup-port is provided by the surface.

Experimental DeploymentAn experimental deployment

was carried out at the Bock Wall divesite, Exumas, Bahamas (N 23.832,W −76.1529). This is one of theprior sites included in a regional envi-ronmental mooring program im-plemented by NOAA’s CaribbeanMarine Research Center. This sitewas chosen given the opportunisticstainless eye pin anchorage (Figure 5)that sits at the reef crest at approxi-

mately 20 msw and is immediately ad-jacent to the vertical MCE. Suchpermanent mooring hardware is nowcommonly used in tropical and sub-tropical environments with moderateto heavy vessel and dive traffic to avoiddamage to coral reefs by dropping ananchor from the vessel.

The tent/shell and bridle wereassembled at the surface prior to thedive, with careful inspections made ofeach fixation point. The system wasfolded and tied into a reasonably man-aged package to be carried by a singlediver (Figure 5). The system was de-ployed in less than 30 min by thetwo-person dive team. Diver 1 enteredthe water, and the habitat system washanded to them from the vessel.Diver 2 entered the water and washanded the bench seat array. The

divers descended together, with thesystem, to the anchorage point and se-cured the two bottom straps makingup the lower portion of the bridle.Lines used to pack the habitat fortransport were removed and stowed.A small quantity of air was expelledfrom an independent aluminum 80cubic foot scuba cylinder (AL80) intothe bottom of the habitat, causing itto rise and extend the bridle.

The divers ascended, following theoutstretched bridle, again inspectingits condition. The habitat was thenreached, which rested at a depth of ap-proximately 7 msw. The balance of thescuba cylinder was expelled into thehabitat, causing it to displace all butthe lower 1 foot of water in the habitatand to exert its maximum force on thebridle. Diver 2 placed the bench seatarray into the habitat and held inplace while both divers fixed the seatsto the perimeter grommets using cableties. The divers made a final visual in-spection of all critical components andthen entered the habitat one at a time.Each sat on a bench seat. Mouthpieceswere removed briefly to communicateabout the deployment process and toexchange initial thoughts on the over-all comfort of the system.

The divers exited the habitat one ata time, then surfaced, with a total divetime of less than 45 min.

ResultsMission Integration

As proposed, the 2012 field pro-gram was to include the initial habitatdeployment, followed by a series of fiveworking dives within the vMCE, eachprogressing in depth and durationwhile making use of the habitat forthe Phase 4 portion of the dive. In-clement weather delayed habitat de-ployment for 3 days, which resulted

FIGURE 5

Image plate revealing system components. Bench seat array from below (top left). Shackle matingbottom and top portions of bridle assembly (middle left). Stainless eye pin anchorage (bottomleft). System packed for transport (bottom second from left). Habitat tent outstretched for inspec-tion (bottom third from left). Authors Lombardi and Fryburg discuss design (bottom right).System successfully deployed and in use during lengthy decompression (main frame).


in a shorter mission; the habitat wasused for only two working dives.Once deployed, the habitat remainedin-water for the duration of the proj-ect, totaling an inflated and deployedduration of approximately 96 h on site.

The two working dives conductedwere to 91 and 125 msw. respectively.Dives were planned to limit surface tosurface time within the functional lim-itations of their primary life support,while providing for some contingency(approximately 4 h total immersiontime). For this experimental deploy-ment, the habitat was not relied on asdecompression life support, but ratheras augmentation of the planned divefor experimental purposes. It servedto address the fit of portable habitatsin mitigating the activity hazards (Fig-ure 3) associated with lengthy decom-pression or Phase 4 of these deepscientific dives within the vMCE.

The two working dives were carriedout successfully and without incident.Upon reaching the habitat, nearing thefinal decompression stop at 6 msw, thedivers removed open-circuit bailoutsupplies and affixed these cylinders tothe exterior of the habitat. Divers en-tered the habitat one at a time, whilecontinuing to utilize their closed-circuit rebreather as primary life sup-port. Once seated on the benches, anAL80 scuba cylinder of air was usedto add gas to the habitat, displacingwater to a satisfactory level near thebench seats. The divers found that,when sitting with the CCRs on theirbacks, it was more comfortable toallow water levels to increase to justabove the diver’s waist to displace aportion of the weight carried on thedivers’ backs. This arrangement satis-fied the objective of removing thedivers from the ambient environment,though it illustrates the need toimprove system configuration for

comfort. Remaining decompressionwas carried out while at rest in thehabitat.

Divers would occasionally removetheir mouthpieces for a brief timeperiod to communicate, though itshould be noted that best efforts weremade to reduce breathing the habitat’satmosphere via mouth or nose as thisexperimental deployment did not in-clude habitat atmospheric manage-ment systems. The divers could alsoobserve the outside environment viathe three peripheral windows. Obser-vations included small fishes peeringthrough the windows and, generally,an attraction of larger reef fishes tothe habitat structure.

Upon completing decompression,one diver exited the habitat at a time,picked up stowed open-circuit bailoutcylinders, and then made a controlledascent to the surface, completing thedive.

DiscussionBenefits

Habitat benefits include providinga “dry” environment for diver rest,affording more direct diver-to-diverinteraction and communication andproviding an environment potentiallysuitable for conducting ancillary taskssuch as preliminary sample processing.Diver function within this type ofshort to mid-duration space warrantscontinued evaluation to assess psycho-logical benefits of dry versus wet im-mersion for these stays.

The most significant benefit stemsfrommitigating the activity hazards as-sociated with Phase 4 lengthy decom-pression. These have been identifiedfor previous vMCE scientific missionsand categorized into environmental,equipment, and operational hazards(Figure 3). The convention for surface-

to-surface vMCE excursions has been toprovide for in-water controls in responseto the activity hazards. Throughout theexperimental deployment, the diversmade note of the benefits affordedfrom habitat integration (Figure 3).Benefits were apparent in each cate-gory of environmental, equipment,and operational hazards. Benefitsstemmed from simplifying the divers’task load and removing them fromthe wet or ambient environment.

A future benefit that is immediatelyevident is being able to shed the pri-mary life support system, which isoften a closed-circuit rebreather. Thecomplications of atmospheric manage-ment using a CCR can be limited, al-lowing for diver-operators to focus onatmospheric management of the habi-tat’s atmosphere, independent of com-plex personal life support. While thiswill likely include a similar fundamen-tal operating principle of carbon dioxideabsorption and oxygen addition, man-agement tasks can be shared by thedive team in a more relaxed manner.

Probable ComplicationsComplications were discussed prior

to pursuing development. Obviouscomplications include added expenseand increased logistical complexitywhen compared to an in-water onlysurface to surface mixed-gas CCRdive operation. The habitat assemblyitself adds costs to the equipment re-quired for these types of dives, thoughit is too early to assess the full cost-benefit and true financial benefit to aproject as this portable habitat hasnot yet been applied in circumstanceswhere its benefits are fully exploited.To do this, future work of increaseddepth and time at depth will forcethe habitat to be used as an essentialand dependent piece of equipment,as dives will exceed the capacity of


personally carried life support (approx-imately 4 h immersion time). This de-pendence is also cause for consideringfurther developments of habitat-specific life support systems, furtherredundant life support systems (Fig-ure 6), and additional features to pro-vide for comfortable lengthy stays.Using these types of habitats forextended stays will require dedicatedatmospheric management and opera-tor discipline for atmospheric con-trol risks, including but not limitedto elevated carbon dioxide levels andhypoxia.

While the system described is aportable unit for transport, there re-mains geographical restrictions in de-ployment, for example, an anchorageor other suitable means for securingthe system to the seafloor is required.An advance mission, deploying anarray of anchor points or mooringsacross a geographical region of interest,would provide for semipermanentstudy sites where one or more portablehabitats can be deployed for mission-specific purposes. Using the bridle sys-tem also poses geographic restrictions

as the bridle is prefabricated at a spe-cific length to place the habitat withinthe appropriate depth zone for the lon-gest decompression obligations. Whilea variable depth system using a hoistmechanism with infinite adjustment,such as incorporated by Stone inWakulla Springs, would allow forgreater applicability across varyingdepth study sites, such a system wouldthen pose the complication of ballast-ing the system such that the net buoy-ant force remains within the operatinglimits of a hoist mechanism. With re-quired space within the habitat beingfixed, ballast would come in the formof add-on weight, which may provecomplex when working in a remotearea. Alternatives to variable depthvia a hoist mechanism may include aseries of habitats at variable depths ortransfer of the single habitat across aseries of fixed loops or rigging. Theseevolutions are being considered care-fully for future work, as variabledepth capabilities will also allow diversto enter the habitat earlier in their de-compression schedules. They will thuseffectively be removed from the wet

environment for a greater portion ofthe idle time spent decompressing,again, mitigating risks associated withwet exposure (Figure 3).

Design ConsiderationsWhile numerous in-water hazards

(Figure 3) are controlled by using theportable habitat, the habitat structureitself must also be assessed for variousfailure modes (Figure 6). While eachcomponent was constructed to meeta minimum 2:1 safety ratio, the un-likely event of a component failureneeded to be addressed. Generally, amajor component failure would leavethe occupants immediately exposedto the ambient environment. As such,should the habitat be further developedfor diver dependence as a primary lifesupport system, efforts must be placedon redundant systems, and fly away sys-tems where wet escape and further de-compression could be safely carried out.

The consideration for mobility andcost-effectiveness of a wet dive teamusing CCRs needed to be maintainedthroughout the development process.Key design considerations included

FIGURE 6

Portable inflatable habitat failure mode analysis and recommended controls.


ability to meet rapid deploymentrequirements of a mission, a design toreduce activity hazards specific to themission (Figure 3), a design to meetan accommodating form factor forthe dive team, and should be inclusiveof life systems integration and environ-mental control systems which matchteam equipment configuration andcompatibility.

Generally, the design used for thisexperimental deployment matched allidentified requirements. Four areaswill continue to be the focus of on-going development of this portablesystem. These include (1) basic lifesupport systems for atmospheric man-agement and redundant atmosphericmanagement such that the habitat it-self may be used for primary life sup-port; (2) a variable depth capabilitycomparable to the aforementionedStone habitat used at Wakulla Springs;(3) improvements to permit makingeffective use of idle decompressiontime such as sleeping quarters, digitalentertainment, work stations to beginsample processing, or improved obser-vation capabilities; and (4) redundancyin the anchorage and bridle assembly.

With respect to the improvementof observation capabilities, a previousdesign by Clark (1972) sought toaddress this using a transparent filmmaterial for shell construction. Thesystem offered vastly enhanced periph-eral viewing of the environment.While transparent, the film requireda net or mesh to provide shape andstrength to the inflatable tent struc-ture. Advances in available materialsmay provide for a more integratedand truly transparent system in thenear future. This prospective addedbenefit of marine life observationwith a transparent shell adds anotherdimension in making use of idle de-compression time. The working divers

are still effectively immersed, with im-mediate and ready access to the shallowenvironments where decompression isbeing carried out. Limited wet excur-sions to carry out a task or the deploy-ment of a remote observation tool orcollection apparatus, in response toan opportunistic observation in theenvironment, will likely increase workproductivity, thus reducing or elimi-nating truly idle time.

Newly Exposed Frontiers andAssociated Risk

Previous efforts in habitation havelargely focused on the idea of underseapermanence or living and working inthe water column and on the seafloor.Efforts to pursue this have come atvarying scales, though, until now, allhave come with the commonality ofrequiring massive “top-down” infra-structure and support, which haverestricted operations to fixed studysites. The expense associated withthis is cost-prohibitive and lends littlevalue to today’s human undersea sci-ence paradigms, in which the marinescience community is eager to exploreand study vast new geographic areasand multiple study sites.

The current diving paradigm isconducted through surface-to-surfaceapproaches using commodity divingstyles—predominantly scuba. Modesof scuba including mixed-gas, re-breathers, and decompression divingcertainly fill an important niche, thoughthey still come with limitations. Porta-ble inflatable habitat technologies as apossible commodity tool, as opposedto permanent fixed habitats, offer a ve-hicle to (1) increase safety, by limitingdiver’s wet exposure during decom-pression; (2) buffer idle time spentduring decompressing; and (3) whenused on shallow no-decompressiondives, improve and vastly extend obser-

vation time. When coupling deepvMCE dives with this type of shallowoutpost, day-to-day commodity-stylediving can be meaningfully enhanced,opening up an increasingly significantnew region of ocean space. This is anew “bottom-up” approach, providingfull system autonomy that is at thesame time highly portable.

This new found accessibility comeswith newly exposed risks, though anincremental approach to continued in-novation in this area will provide formitigating these risks in a controlledmanner.

Potential Role of UnderwaterTents in the Not-Too-Distant Future

The integration of portable inflat-able underwater habitats as a new com-modity diving technology has thepotential to open up a broad range ofnew diving experiences and profiles.Here we present a few of the possibili-ties that we anticipate emerging andmaturing in the coming decade.

Extended Visitation CapacityUsing Diving Outposts

As witnessed by the benefits of ourinitial deployments and the dramaticincrease in demand for utilizing ad-vanced diving modes by the scientificdiving community, there is great po-tential for the type of flexible under-water habitats described in this paperto bewidely employed by early adopters.We expect that this course of eventswill subsequently lead to a range of in-teresting features, configurations, andnovel deployments. For example, theimmediate next steps for our own de-velopment are to design and integratemultiple rebreather systems for use inthe habitat itself. This will make thehabitat capable of providing integratedlife support that will, in turn, extend


the duration of the deepest portion ofdeep scientific dives.

In subsequent developments, ex-tending the duration of integrated lifesupport will lead to the mid-term real-ization of flexible habitats that willsupport divers’ overnight stays. Thiscould enable vastly extended decom-pression or prepare the crew for multi-day activities. As discussed, a variabledepth capability would allow for de-compression to occur while the habitatslowly rises through the water column,for example, during an overnight stay.This would enable the divers to realizesignificantly greater extensions of div-ing profiles—longer and deeper dives,with safe and relaxed decompression,while at rest overnight which is accept-ed as normal downtime during everyhuman’s diurnal cycle. While criticsargue that similar technologies existfor modern era saturation and bell div-ing, the development effort to beplaced is on increased portability, dra-matically reduced costs, and eliminat-ing required surface infrastructure anddependence. For consideration, the ex-perimental system described and suc-cessfully deployed was transported ina suitcase and deployed rapidly froma small vessel with only minimal ad-vance investment of the epoxied stain-less pin used as an anchorage. Suchinvestment includes just 1 day ofadvance labor per pin installation bya small team. This is a stark contrastto previous evolutions emphasizinghuman permanence with massivetop-down requirements.

An ability to reside in a habitatovernight, coupled with the deploy-ment of multiple tents within reason-able proximity, would enable a newclass of multiday dives that could dra-matically enrich the human experienceunderwater, ranging from more effi-cient scientific and engineering endeav-

ors within a greater range of depthsand environments, to the opening upof a new class of saturation sport divingor tourism that has largely eluded thecommunity for the past half-century.These could include a combinationof deep and shallow outposts equippedwith hookah type excursion capability,conventional scuba tanks, and/or re-breathers. Divers could traverse diversesaturation profiles, allowing them to,for instance, saturate at 20 msw andwork laterally for as long as they like,followed by deeper downward excur-sions to carry out ancillary exploratoryobjectives. The dive might then culmi-nate with variable depth decompres-sion on a final overnight ascent to thesurface.

Semipermanent Living QuartersIn the course of developing this

flexible habitat, we have asked our-selves, “What would it take to make ahabitat reasonably livable?” Flexibleunderwater tents might open the wayfor commoditizing semipermanentunderwater living quarters in a widerange of environments across the planet.For instance, marine reserves or pro-tected areas may include a series ofstaging areas to support deploymentof these structures and facilitate a vastlyenhanced human presence to carry outamplified science operations duringprecious field time.

A reference model, in terms of theuse of space, is the Apollo Command/Service Module (CSM), which is ca-pable of supporting a 14-day spacemission. The CSM used for lunar mis-sions had a cabin volume of 218 cubicfeet (6.2 m3) living space. This spacewould yield 14,824 lb of buoyancyif submerged; adding a safety factorof 4 indicates a ballast requirement of59,296 lb. While this is considerable,there are multiple ballast/anchorage

approaches that could be consideredgiven today’s mooring technology.First, having multiple smaller habitatswith separate anchor points wouldgreatly reduce the ballast requirementfor any single anchor point. Anchoring/mooring technologies such as epoxiedpins and drilled helical installationsprovide for relatively easy and inexpen-sive one-time setup across a variety ofgeographical locales and within vary-ing substrates. The authors attest tothis simplicity with firsthand experi-ence installing such hardware acrossthe spectrum of depths (to 100 fsw)and in varying substrates. Other ap-proachesmight includemaking oppor-tunistic use of the local environment ortarget of interest—placing an inflat-able habitat within a wreck, within incave, under a ledge or mouth of a cave,within an iceberg (with suitable expand-able insulation), or within manmadeindustrial spaces requiring explorationsuch as pipelines, tunnels, sumps, oraqueduct shafts.

While atmospheric management isthe foremost consideration for nearterm evolutions of this innovation,other features required to support ex-tended missions would certainly needto be addressed such as sustenanceand waste products. While these haveonce been considered “rocket science”they are no longer; with multiple solu-tions having proven reliable by previ-ous missions in space, on earth, at sea,and, most directly relevant, at depth.Many of these subsystems are off theshelf technologies at this point, requir-ing systems integration level engineer-ing rather than complete novel systemdesign.

Human–Robot TeamsBeyond the explicit features of the

habitat and its capabilities, anotherexciting opportunity that a habitat


technology affords is a human plat-form for in situ mission planning andoperation. While there are many excit-ing possibilities, we expect to see flexi-ble habitats serving as command andcontrol centers for hybrid humanrobot teams in the near term. Forth-coming collaborations will enable theintegration of robotics in the formof both autonomous underwater vehi-cles and remotely operated vehicles(ROVs) to form hybrid human-robotexploration teams. A collocated under-water habitat in such a deploymentwould enable a “local” command cen-ter for multiple robotic activities, forexample, scouting, routine monitoringor repair, or related standard activities,coupled with the ability to have closeto real-time human intervention.This would enable, for example, arobot to prescout a vertical deep reefhabitat and send imagery back to thedivers in the habitat, until multipletargets of interest (and their corre-sponding precise depth and locationcoordinates) along the wall had beenacquired and mapped. With thesedata, the divers could plan their divefor multiple contingencies that couldserve to optimize scientific return andsafety, planning which targets to visitfirst, for how long, and how to proceedin a considered manner. Conversely, ahuman exploration excursion mayidentify scientific targets of interest,which may be followed by immediatedeployment of an ROV, from a com-mand outpost or even controlled bythe diver, allowing for the ROVs’ spe-cialized instrumentation to be moreaccurately deployed via human inter-vention and guidance. This interactionwill substantially reduce idle decom-pression time in between dive eventsor subsequent robotics deployments.Such in situ deployments involvingrobot and human teams operating at

close range will allow for a new divingparadigm enabling mutual supportand ready follow-up with humanand/or robotic intervention, as appro-priate, when opportunities arise.

AcknowledgmentsThe authors would like to thank

the National Geographic Society/WaittGrants Program (Award W196-11 toML) and Ocean Opportunity, Inc.,for funding this field program. Wealso extend our thanks to the BahamasMarine EcoCentre on LittleDarbyCay,Exumas, Bahamas, and Mr. WendellMcKenzie for providing field sup-port. Habitat illustrations providedby Anthony Appleyard. The authorsalso acknowledge their home institu-tions of Lombardi Undersea ResourceCenter, Arizona State University, theUniversity of Connecticut at AveryPoint, and Subsalve, Inc., for provid-ing various in-kind assistance.

Corresponding Author:Michael LombardiLombardi Undersea Resource Center307 Oliphant Lane #25, Middletown,RI 02916Email: [email protected]

ReferencesClark, J.F. 1972. Lightweight Readily Porta-

ble Underwater Habitation and Method for

Assembly and Emplacement. US Patent

3,706,206. Issued December 19, 1972.

Dardeau, M.R., Pollock, N.W., McDonald,

C.M., & Lang, M.A. 2012. The incidence of

decompression illness in 10 years of scientific

diving. Diving Hyperb Med. 42(4):195-200.

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Halas, J. personal communication. 2010.

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Hellwarth, B. 2012. Sealab: America’s For-

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take-over-operations-of-aquarius-underwater-

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P A P E R

Manufacturing Imperfection SensitivityAnalysis of Spherical Pressure Hullfor Manned SubmersibleA U T H O R SBhaskaran PraneshDharmaraj SathianarayananSethuraman RameshGidugu Ananda RamadassNational Institute of OceanTechnology, Chennai, India

A B S T R A C TAny pressure hull invariably has imperfections as a result of the manufacturing

procedure. Imperfections in a spherical pressure hull are the basis for localizedbuckling and deformation behavior. Numerical analysis and analytical calculationsare carried out to predict the buckling behavior and strength of a pressure hull madeof titanium alloy (Ti-6Al-4V) for both perfect and imperfect pressure hulls. Finiteelement analysis is carried out for different imperfection angles to see the effecton strength and buckling. Results of numerical analysis show that there is consid-erable reduction in both buckling pressure and strength as a result of imperfections.Hence, allowable deviation due to imperfection for a spherical pressure hull has tobe considered for thickness calculations.

Abbreviations:P external pressure (Design pressure)Dm mean diameter of the pressure hullRm mean radius of the pressure hullRi imperfect radius of the pressure hullt thickness of the pressure hullΔR imperfect deviationδ imperfection angleσ hoop stressPy pressure at yield strength of the materialPb buckling pressureE Young’s modulus of the materialμ Poisson’s ratioMSW meters of sea waterAPDL ANSYS Parametric Design Language

Keywords: manned submersible, pressure hull, buckling, imperfection, finiteelement analysis

Introduction

Manned submersibles are designedand developed for applications likedeep sea exploration, inspection, engi-neering intervention, etc. These opera-tions are carried out by one, two, orthree scientists sitting inside a pressurehull along with a pilot and at timeshave work capabilities by using roboticarms or other custom tooling. A pres-sure hull may be considered as a thinsphere, and it is a curved shell wherethickness is small as compared to di-ameter of the body. A pressure hull isthe main component in a manned sub-mersible because it is a safe living spacefor pilots and scientists in deep water.The pressure hull must have enoughstrength to withstand high hydrostaticpressure, yet should be as light as pos-sible for easy handling.

Analytical calculations are availablein the literature for perfect geometrybut rarely for imperfect geometry.The comparison of collapse pressureof a cylindrical shell may be obtainedfrom numerical and experimentalmethods (Boote, 1997). The numeri-cal results are in agreement with exper-imental results for actual imperfections

measured. The authors carried outnumerical analyses for different imper-fect deviations at a central angle of20° and derived the ultimate pressureformulae from finite element analysis.The results are verified through the ex-perimental method (Pan&Cui, 2010;Pan, Cui, Shen, & Liu, 2010; Pan,Cui, & Shen, 2012).

Once the material crosses the yieldlimit and undergoes plastic deforma-tion, it will not regain its original di-mensions, so it adversely affects thefunctionality of the system. It is betterto have yield strength as the limitingfactor in pressure hull design. It is nec-essary to understand the reduction ofstrength and decrease in the buckling


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