decompression of rescue personnel during australian submarine rescue

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Decompressing rescue personnel during Australian submarine rescue operations Michael, P. REID 1 Andrew, FOCK 2 and David J. DOOLETTE 3

1 Submarine Underwater Medicine Unit, Royal Australian Navy, Sydney, Australia; 2

Hyperbaric Unit, The Alfred Hospital, Melbourne, Australia; 3 Navy Experimental Diving Unit, United States Navy, Panama City, United States of America. Correspondence Michael Reid Submarine Underwater Medicine Unit HMAS Penguin Middle Head Road MOSMAN, NSW 2008 Ph: +61 2 9647 5572 Fax: +61 2 9960 4435 Email: [email protected] This version is a candidate for publication in the Journal of Diving and Hyperbaric Medicine. Key Words: Decompression, Decompression Tables, Submarine, Rescue, Risk Management.

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ABSTRACT Introduction: James Fisher Submersible Rescue Vehicle and Navy Medical

Attendants rescuing survivors from a pressurized, distressed submarine (DISSUB)

may themselves accumulate a decompression obligation, which may exceed the limits

of the Defense and Civil Institute of Environmental Medicine (DCIEM) tables

presently used by the Royal Australian Navy (RAN). This study compared DCIEM

tables with the following alternative decompression strategies for rescue personnel:

United States Navy XVALSS_DISSUB 7, VVAL-18M and Royal Navy 14

(Modified) tables.

Methods: Estimated risk probability of decompression sickness (PDCS), the

cumulative pulmonary oxygen toxicity dose, the volume of oxygen required and the

total decompression time were calculated for hypothetical single and repetitive

exposures to 253 kPa air pressure for various bottom times and the resulting

decompression schedules.

Results: Single DCIEM profiles had PDCS estimates ranging from 1.4 % to 6.5 % with

CUPTD between 0 to 101 units. XVALSS_DISSUB 7 tables produced the lowest

PDCS, less than 3.1 % for single dives, and with cumulative pulmonary toxicity dose

between 36 to 350 units.

Conclusions: XVALSS_DISSUB 7 tables were specifically designed for submarine

rescue3 and, unlike DCIEM tables, covered the maximum required bottom time of

345-minutes to effect a DISSUB rescue. Lower PDCS comes at the cost of longer TDT

and greater required oxygen supply.

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Introduction:

The ambient pressure inside a DISSUB may be elevated above 101 kilopascals (kPa)

due to compression of the remaining gas space by partial flooding or leakage from on-

board high-pressure gas supplies.1 Surviving DISSUB crew may be rescued by a

Submersible Rescue Vehicle (SRV) which can mate to the escape hatch of the

DISSUB. 1 To accomplish this evacuation, the ambient pressure inside the SRV must

be equalized with the DISSUB.1 Survivors may have a lengthy decompression

obligation so the SRV must remain pressurized above 101 kPa during the transit to

the surface where the survivors are again transferred under pressure (TUP) to a

recompression chamber (RCC) located on-board a rescue ship in order to complete

their decompression. 2

A RAN Collins class submarine can accommodate sixty personnel. The current RAN

DISSUB rescue system uses the James Fisher Defence LR5 SRV (Figure 1), which

can rescue 14 survivors per sortie, and two Type B RCCs that can accommodate

seven survivors and one medical attendant each. Survivors are transferred one at a

time between the SRV and the Type B RCC via the TUP compartment and one-man,

portable chambers (Figure 2).

Pilots housed in the forward LR5 SRV compartment remain at 101 kPa and do not

require decompression but rescue personnel attending survivors within either the aft

section of the LR5 SRV or the TUP compartment accrue their own decompression

obligation. Each LR5 sortie requires approximately four hours and LR5 hyperbaric

personnel will be subjected to the DISSUB pressure along with the rescued survivors

for approximately 180-minutes by the time they return to the rescue vessel. The LR5

aft compartment remains pressurized during the 150-minutes required to transfer

fourteen survivors from one LR5 sortie to the deck RCC. Closing the Type B RCC

inner lock and commencing survivor decompression will take an additional 15-

minutes, after which the Type B RCC outer lock is available for oxygen

decompression of rescue personnel. LR5/TUP compartments do not have the

necessary equipment for oxygen decompression. Therefore LR5 hyperbaric personnel

could have bottom times up to 345-minutes and TUP personnel could have bottom

times up to 150-minutes.

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Transfer under pressure personnel can be decompressed within the TUP compartment

using DCIEM Table 1 (air decompression), which takes 29-minutes 3,4 whereas LR5

personnel are prioritised for accelerated oxygen decompression in the Type B RCC

outer lock because of their longer bottom times. Rescue sorties are separated by a

surface interval time (SIT) of 12.5 hours as the LR5 can only redeploy back to the

DISSUB two hours before the first cohort of survivors are due to complete

decompression. This ensures chamber availability for the second cohort of survivors.

A future system is being considered that will combine the James Fisher SRV with two

larger 36-man RCCs similar to the RAN’s previous Remora rescue system.2 This may

incorporate a larger TUP compartment that will connect directly to the RCCs via a

flexible hyperbaric tunnel or double convolution bellow unit. This system could

reduce the transfer time of fourteen survivors to 60-minutes by eliminating the

requirement for portable one-man chambers and the TUP system remaining

pressurised between patient transfers (existing system is depressurised between

transfers). The total exposure time for LR5 hyperbaric personnel could therefore be

reduced to 240-minutes. The spare RCC capacity within the future system also

enables the LR5 SRV to immediately redeploy back to the DISSUB upon transferring

her first cohort of survivors potentially reducing the SIT between each rescue sortie to

6 hours.

Figure 1* (NEAR HERE)

Figure 2† (NEAR HERE)

* James Fisher Submersible Rescue Vehicle (LR5). † Submersible rescue vehicle (LR5) connected to the TUP compartment and portable, one-man RCC. James Fisher Defence refers to its TUP compartment as the Universal Deck Reception Chamber.

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Due to the relatively small size of the submarines compartments; rescue from a

disabled Collins class submarine with an internal pressure greater than 253 kPa is

unlikely (Pennefather J, personal communication, 2012). At present, the only

decompression tables authorized for decompression of RAN medical attendants are

the DCIEM tables.4, 5 These tables are designed for underwater diving operations and

have a maximum bottom time at 253 kPa (15 metres sea water) of 280-minutes,4

insufficient to accommodate the 345-minute exposures likely for LR5 hyperbaric

personnel.

Additional rescue personnel could lock-in before the limits of DCIEM tables are

exceeded; however there may not always be sufficient personnel for the longest

exposures. Current RAN policy requires rescue personnel who have exceeded the

limits of DCIEM tables to be decompressed on the same ‘saturation’ decompression

schedule as survivors (Hissink J, personal communication, 2013). Survivors are

considered to be saturated (all body tissues equilibrated with inspired gas pressure)

after 24 to 48 hours exposure to hyperbaric pressure.6 The National Oceanic and

Aerospace Administration (NOAA) ‘Standard’ Table is currently favoured for

saturation decompression of survivors owing to its relatively short TDT.7

Nevertheless, TDT from 253 kPa can range from 11 to 33 hours depending on

whether oxygen is used to accelerate decompression.8 Saturation decompression of

rescue personnel from exposures of 345-minutes or less is unnecessary and costly in

terms of oxygen supply, RCC space, and human resources on-board the rescue ship.

These constraints prompted an investigation into alternative decompression tables that

would permit decompression of personnel with bottom times up to 345-minutes at

253 kPa. VVAL-18M is the algorithm underlying the air decompression tables in the

United States Navy (USN) Diving Manual, Revision 6 and is intended for diving

operations but has schedules for bottom times up to 420-minutes at 253 kPa.9,10 The

Royal Navy 14 (Modified) table 14 was originally a diving table retrospectively

modified for submarine rescue and has schedules for bottom times up to 350-minute

at 253 kPa.11,12 The USN XVALSS_DISSUB 7 tables were specifically designed for

submarine rescue and has schedules for bottom times up to 460-minutes at 253 kPa.13

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To evaluate the utility of these alternative tables, this study compared the estimated

PDCS, CUPTD, the volume of oxygen required and the TDT of single and repetitive

exposures to 253 kPa followed by decompression prescribed by DCIEM, VVAL-

18M, XVALSS DISSUB 7, and Royal Navy 14 (Modified) tables.4, 9-13

Methods:

The approach was to analyse hypothetical dive profiles (pressure/time/breathing gas

histories) representing single or repetitive exposures to a DISSUB ambient pressure of

253 kPa for various bottom times followed by decompression prescribed by each of

the four candidate decompression tables that had a schedule for that exposure. These

profiles were based on a worse case scenario in which sixty survivors need to be

evacuated from a Collins submarine with a DISSUB internal pressure of 253 kPa,

thus placing maximum strain of human resource management.

Single dive bottom times ranged from 60 to 460-minutes followed by decompression

prescribed by the DCIEM In-Water Oxygen Decompression tables (for dives up to

280-minutes bottom time),4 Royal Navy 14 (Modified) tables (for dives up 350-

minutes bottom time),11,12 VVAL-18M In-Water Oxygen Decompression tables (for

dives up 420-minutes bottom time),9,10 and XVAL_DISSUB 7 tables (for all dives).13

Repetitive exposure comprised two identical bottom times ranging from 60 to 345-

minutes separated by SITs of 6 and 12.5 hours with decompression from each dive as

prescribed by the candidate tables. Most repetitive combinations could not be planned

using printed tables. Royal Navy 14 (Modified) tables do not accommodate repetitive

diving as residual nitrogen time (RNT) cannot be calculated.11-12 Defense and Civil

Institute of Environmental Medicine tables allow a repetitive dive after a first dive to

253 kPa with bottom times up to 140-minutes.4 Bottom times exceeding this limit

require a minimum 18-hour SIT credit, after which a subsequent dive is no longer

considered repetitive.4 VVAL-18M and XVALSS_DISSUB 7 tables allow a repetitive

dive after a first dive to 253 kPa with bottom times up to 140 and 200-minutes

respectively.9,13 Although both technical reports do not specify the minimum SIT for

dives exceeding this limit, an 18-hour SIT was assumed.9,13 In all, 160 single and 40

repetitive dive profiles were examined.

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The PDCS, CUPTD and oxygen consumption was calculated for each of these dive

profiles. Oxygen consumption was based on a conservative respiratory minute

ventilation of 15 litres per minute adjusted for Boyle’s law. Fifteen litres per minute

was based on 10 mL per Kg tidal volume and a resting adult respiratory rate of

15 breaths per minute,14 and a body weight of 100 kg. The later based on the 95th

percentile for weight in Australian submariners being 96 kg (Ponton, K. personal

communication, 2013). Cumulative pulmonary oxygen toxicity dose was calculated

using the Harabin et al method.15 Total decompression time was calculated including

the recommended air-breaks. The VVAL-18M and Royal Navy 14 (Modified) tables

recommend a 5-minute air-break after every 30-minutes of oxygen breathing,9-12

whereas XVALSS_DISSUB 7 tables require a 15 air-break after every 60-minutes of

oxygen breathing.13 The DCIEM 2 tables do not require air-breaks.4

The PDCS for each complete dive profile was calculated using the Navy Medical

Research Institute 98 (NMRI-98) and Bubble Volume Model 3 [BVM(3)]

probabilistic models for DCS incidence and time of occurrence.16,17 For repetitive

dives, the cumulative PDCS is the sum of the risks of all dives (within 18-hours). For

example if 1% risk accumulates during the first dive and then a further 1.2% risk

accumulates during the second dive then the cumulative risk is 2.2 %. In the NMRI-

98 model, instantaneous risk of DCS is a function of the gas super-saturation in three

modelled tissue compartments.16 In the BVM(3) model, instantaneous risk is a

function of the bubble volume in three modelled tissue compartments.17 Parameters of

these models are calibrated by fit to large, diverse database of dive profiles

(approximately 5% incidence of DCS) from carefully controlled and monitored air

and nitrox man-dives. These models are therefore generalized expressions of the

experience embodied in these calibration data. Insights into the reliabilities of the

PDCS estimates were obtained by comparing estimates from the two models. Because

the models have different structures but provide statistically similar correlations of the

calibration data, divergences of model estimates indicate extrapolation beyond the

calibration data and should be interpreted cautiously.

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Results: Single Dives:

Figures 3 to 6 compare NMRI-98 and BVM(3) estimated PDCS for DCIEM, VVAL-

18M, XVALSS_DISSUB 7 and Royal Navy 14 (Modified) decompression following

single dives at a depth of 253 kPa. Owing to differences in table increments and table

limits; a direct comparison could not be made between all dive profiles, but several

things are evident in the figures. First of all, the PDCS increases with increasing bottom

time. This increase in PDCS is only modest for XVALSS_DISSUB 7, which was

designed to produce risk-constrained schedules.13 For single dive profiles

XVALSS_DISSUB 7 tables produced the lowest PDCS risk estimates, whereas

DCIEM 2 produced the highest. For example decompressing with DCIEM 2 tables

following a 280-minute dive at 253 kPa gave PDCS of 6.4 % and 5.1 % (NMRI-98 and

BVM(3), respectively), whereas decompressing with XVALSS_DISSUB 7 tables for

the same profile gave PDCS of 1.4 % and 1.5%.

Figure 3‡ (NEAR HERE)

Figure 4§ (NEAR HERE)

Figure 5** (NEAR HERE)

Figure 6†† (NEAR HERE)

‡ PDCS (%, y-axis) in rescue personnel for single dives with bottom times from 60–280 minutes (x-axis) at 253 kPa estimated using the NMRI-98 model. At each bottom time the cluster of bars gives the PDCS for decompression according to each table that has a schedule for that bottom time. The order in the bar cluster is always the same — DCIEM 2 (blue); XVALSS_DISSUB 7 (green); VVAL-18M (orange); Royal Navy 14 (Modified) (red) — but not all bars may appear. For instance, the VVAL-18M table does not have an entry for 75-minute bottom time and consequently there is no orange bar in the cluster at this value of the x-axis. § PDCS (%, y-axis) in rescue personnel for single dives with bottom times from 290–460 minutes (x-axis) at 253 kPa estimated using the NMRI-98 model. These profiles exceed the 280 minute limit at 253 kPa for DCIEM tables. ** PDCS (%, y-axis) in rescue personnel for single dives with bottom times from 60–280 minutes (x-axis) at 253 kPa estimated using the BVM(3) model. †† PDCS (%, y-axis) in rescue personnel for single dives with bottom times from 290–460 minutes (x-axis) at 253 kPa estimated using the BVM(3) model. These profiles exceed the 280 minute limit at 253 kPa for DCIEM tables.

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The second observation is that lower PDCS comes at the expense of longer

decompression times and greater oxygen exposure. Figure 7 compares TDT for

DCIEM 2, VVAL-18M, XVALSS_DISSUB 7 and Royal Navy 14 (Modified) tables.

XVALSS_DISSUB 7 tables had the longest TDT’s whereas DCIEM 2 had the

shortest.

Figure 7‡‡ (NEAR HERE)

Figure 8 compares the total oxygen consumption for personnel decompressing with

DCIEM 2, XVALSS_DISSUB 7, VVAL-18M and Royal Navy 14 (Modified) tables.

To facilitate a direct comparison between tables; standard table round up conventions

were used. For example a person decompressing after a 115-minute dive at 253 kPa

would require 144 litres of oxygen if decompressed with DCIEM 2. This profile

would be rounded up to the next highest bottom time of 120-minutes when

decompressing with XVALSS_DISSUB 7 and VVAL-18M tables9,13, requiring 1037

and 169 Litres of oxygen respectively. The same profile would be rounded up to 125-

minutes for Royal Navy 14 (Modified),11,12 requiring 1051 Litres. As illustrated in

Figure 8, XVALSS_DISSUB 7 and Royal Navy 14 (Modified) tables had higher

oxygen requirements in comparison to VVAL-18M and DCIEM tables.

Figure 8§§(NEAR HERE)

‡‡ Total Decompression Time. Minutes (y-axis) in rescue personnel for single dives with bottom times from 75–460 minutes (x-axis) at 253 kPa. At each bottom time the cluster of bars gives the TDT for decompression according to each table that has a schedule for that bottom time. The order in the bar cluster is always the same — DCIEM 2 (blue); XVAL SS_DISSUB 7 (green); VVAL-18M (orange); Royal Navy 14 (Modified) (red) — but not all bars may appear. For instance, the VVAL-18M table does not have an entry for 75-minutes bottom time and consequently there is no orange bar in the cluster at this value of the x-axis. §§ Total oxygen consumption. Litres (y-axis) in rescue personnel for single dives with bottom times from 50–460 minutes (x-axis) at 253 kPa. Respiratory-minute ventilation rate based on 15 litres per minute. At each bottom time the cluster of bars gives the total oxygen usage for decompression according to each table that has a schedule for that bottom time. The order in the bar cluster is always the same — DCIEM 2 (blue); XVAL SS_DISSUB 7 (green); VVAL-18M (orange); RN Table 14 (MOD) (red) — but not all bars may appear. For instance, the VVAL-18M table does not have an entry for 75-minutes bottom time and consequently there is no orange bar in the cluster at this value of the axis.

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Figure 9 summarizes the CUPTD units for single dive profiles at 253 kPa following

decompression with either: XVALSS_DISSUB 7, DCIEM, Royal Navy 14

(Modified) and VVAL-18M tables.

Figure 9***(NEAR HERE)

LR5 personnel performing dives of 240 and 345-minutes (future and current systems)

respectively will have bottom times too long to be accommodated by table based

repetitive procedures and will require an 18 hour SIT before their second dive

whereas TUP personnel will have bottom times sufficiently short to conduct repetitive

diving. Figures 10 and 11 compare the PDCS after decompression with DCIEM 2,

VVAL-18M and XVALSS_DISSUB 7 tables for a repetitive 60-minute dive at

253 kPa with SITs of 6, 12.5 and 18 hours. Table 12 compares the cumulative PDCS for

a repetitive 150-minute dive at 253 kPa with a SIT of 12.5 and 18 hours using

DCIEM 2, VVAL-18M and XVALSS_DISSUB 7 tables. Defense and Civil Institute

of Environmental Medicine and XVALSS_DISSUB 7 tables do not permit a

repetitive 150-minute dive at 253 kPa with a SIT of 12.5 hours. No printed tables

permit a repetitive 150-minute dive with a SIT of 6-hours. This gives a sense of the

overall risk to an individual TUP operator if they are required to conduct repetitive

diving.

Figure 10††† (NEAR HERE) Figure 11‡‡‡ (NEAR HERE)

Table 12§§§ (NEAR HERE)

*** Cumulative Unit Pulmonary Toxicity Dose. Units (y-axis) in rescue personnel for single dives with bottom times from 75-460 minutes (x-axis) at 253 kPa. At each bottom time the cluster of bars gives the CUPTD for decompression according to each table that has a schedule for that bottom time. The order in the bar cluster is always the same — DCIEM 2 (blue); XVAL SS_DISSUB 7 (green); VVAL-18M (orange) and Royal Navy 14 (Modified) — but not all bars may appear. For instance, the VVAL-18M table does not have an entry for 75-minutes bottom time and consequently there is no orange bar in the cluster at this value of the x-axis. ††† PDCS (%, y-axis) in rescue personnel performing 60-minute repetitive dives at 253 kPa with surface interval times of 6, 12.5 and 18-hours (x-axis). Estimated using NMRI-98 modelling. ‡‡‡ PDCS (%, y-axis) in rescue personnel performing 60-minute repetitive dives at 253 kPa with surface interval times of 6, 12.5 and 18-hours (x-axis). Estimated using BVM(3) modelling. §§§ PDCS (%) in rescue personnel performing 150-minute repetitive dives at 253 kPa with surface interval times of 12.5-hours. Estimated using NMRI-98 and BVM(3) modelling. * Decompression not possible using printed table protocols.

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Discussion:

The RAN does not have a policy on acceptable PDCS for diving or DISSUB rescue

operations. The DCIEM tables, which are approved for RAN use, had DCS incidence

of 3.2 – 3.5 % during development and validation.18-20 Most U. S. Navy air and nitrox

decompression procedures have an upper limit of about 5% PDCS for normal exposure

diving.21 Severe central nervous system DCS is uncommon in air dives with less than

about 7% estimated PDCS.22 Together, these provide objective criteria to evaluate

suitable replacement tables. Tables that provide mean DCS rates less than 5%,

although ideally less than 3.5% will be in the range of acceptable RAN diving

operations. However any DCS in rescue personnel will result in serious strain on

resources and therefore the lowest practicable PDCS is desirable. Nevertheless, low

PDCS must be balanced against TDT, CUPTD and oxygen use.

During each rescue sortie a James Fisher Submarine Rescue Operator (SRO) and

RAN medical attendant work within the TUP compartment and aft section of LR5. A

single RAN medical attendant is deployed within each RCC and as part of the

DISSUB medical entry team. This team also comprises a RAN medical and

engineering officer who along with the RAN medical attendant, enter the DISSUB to

render to assistance to remaining survivors (Figure 13). The remaining rescue

personnel co-ordinate the rescue operation and augment existing teams if required.

Figure 13**** (NEAR HERE)

All the tables evaluated provided a reasonably low risk of decompression for single

dives with very few profiles exceeding 5% PDCS (Figures 3 to 6). For single dives, the

major advantage of using XVALSS_DISSUB 7, VVAL-18M and Royal Navy 14

(Modified) over DCIEM tables are their longer table limits, 4,9-13 which covers the

worst case 345-minute dive profile for LR5 personnel operating within the current

system.

**** Current rescue system schematic and personnel requirements. LR5 schematic - http://www.navy.gov.au/fleet/ships-boats-craft/submarines/submarine-rescue-vehicles

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Procedural solutions to remain within DCIEM table limits have been considered.

Upon recovering LR5 back on-board the rescue ship and before commencing patient

transfer; the first LR5 team may be locked-out and replaced so as to reduce their

bottom time to 200-minutes whilst a second team completes the remaining 145-

minute component of the dive. This ‘split team’ approach is within the scope of

DCIEM tables but requires eighteen personnel, divided into six LR5/TUP teams.

There are insufficient James Fisher SROs currently on-board to create six teams.

Furthermore, the current rescue ship; MV Seahorse Standard can only accommodate

sixty personnel, 24 limiting the number of hyperbaric rescue personnel on-board to

seven RAN medical officers, twelve RAN medical attendants, fourteen Diver Medical

Technicians (DMTs) and four James Fisher SRO’s (total of 37 personnel). 23 The

remaining 23 non-hyperbaric rescue personnel include RCC operators, James Fisher

mechanics and Defence Maritime Service (DMT) personnel. Post decompression

survivors will need to be accommodated on-board other RAN vessels.

An alternative solution is to keep LR5/TUP personnel in saturation and decompress at

the end of the rescue operation (5-6 days later) using the NOAA Standard table. The

entire rescue could be performed by two LR5/TUP teams (four James Fisher SROs)

but would require an additional RCC, large enough to facilitate rest periods between

rescue sorties. At present there is insufficient deck space on-board MV Seahorse

Standard to accommodate a third RCC. Crew size and deck space limitations can be

resolved by using larger rescue vessels such as Australian Defence Vessel (ADV)

Ocean Shield 25 or the future replacement vessel (Damen Class Escape Gear Ship

8316). 26 ††††

†††† ADV Ocean Shield, length 105.9 metres, beam 21 metres. Crew 100; MV Seahorse Standard, length 72 metres, beam 16 metres. Crew 60 and Damen Class ERS 8316, length 83 metres, beam 16 metres. Crew 88.

13

The availability of only four James Fisher SROs means that only two LR5/TUP teams

are currently available with each team needing to perform three of the six dives

required to rescue sixty survivors, assuming all personnel can evacuated and

decompressed whilst seated (casualties on stretchers will occupy more RCC space).

The risk of a DCS incidence in the course of a series of dives is greater than the PDCS

of a single dive. If identical dives are separated by a sufficiently long SIT (at least 18-

hours); the risk associated with a preceding dive does not influence a subsequent dive

with no persistent excess dissolved inert gas or bubbles. The risk of at least one case

of DCS in the course of the series of dives can be approximated using binomial

theorem (making the non-rigorous assumption that all exposures are independent).

For instance LR5 hyperbaric personnel undertaking a single 253 kPa dive with 240-

minute bottom time (future system) with decompression according to DCIEM tables

has only a 5.4% PDCS (NMRI-98 modelling). However, the risk of at least one incident

of DCS in the course of six sorties is one minus the probability of no DCS in six dives

or 1-(1-0.054)6=0.283. So it can be seen that the risk of a DCS event in a LR5

operator and the associated strain on resources is 28.3 %.

XVALSS_DISSUB 7 tables were specifically designed to mitigate this risk of DCS

over the course of multiple dives. The same, single dive conducted using

XVALSS_DISSUB 7 tables carries a 1.1% PDCS. The risk of at least one incident of

DCS in the course of six dives is 1-(1-0.011)6=0.064 (6.4%), so the risk of DCS in the

course of six sorties is lower for XVALSS_DISSUB 7 compared to DCIEM 2.

Human validation trials of these tables tested eight, two-dive, repetitive profiles at

depths between 193 to 284 kPa.13 There were a total of three cases of DCS during 125

dives yielding a DCS risk estimate of 2.4 % for XVALSS_DISSUB 7 tables.13

Importantly all three DCS cases occurred after completion of repetitive dives to

284kPa, deeper than the RAN requirement.13 This validation process accurately

reflects the type of diving likely to be performed by Australian submarine rescue

personnel.

14

Reducing the DCS risk will inevitably require additional personnel to either reduce

the number and/or duration of their dives unless as previously noted LR5/TUP teams

remain saturated and decompressed using the NOAA Standard table. Decompressing

with either XVALSS_DISSUB 7 or VVAL-18M tables (within the current system)

will require at least three LR5/TUP teams (total of twelve personnel) with each team

performing two dives. At least six James Fisher SROs will be required.

Improved engineering within the future rescue system decreases the total time rescue

personnel must remain pressurized but also reduces the SIT between rescue cycles

from 12.5 to 6 hours. This shorted SIT decreases the time frame for ‘off gassing’

residual nitrogen. Consequently, the faster operational tempo requires more rescue

personnel but facilitates the evacuation of survivors over a shorter time period. Using

XVALSS_DISSUB 7 and VVAL-18M tables in this situation confers no advantages

in decreasing minimum personnel requirements, irrespective of which decompression

tables are used (including DCIEM) as a minimum of six LR5 and three TUP teams

will be required (total of eighteen personnel). The future system will require at least

nine James Fisher SROs, however LR5 personnel will only need to perform a single

dive, whereas TUP personnel will perform two, 60-minute dives. Regardless of which

decompression tables are used, decreasing the bottom time and number of dives will

decrease the risk of DCS in rescue personnel. The introduction of the larger Damen

Class rescue vessel will accommodate these increased personnel numbers.

“REPEX” recommendations for daily CUPTD dose limits are based on the number of

dives being performed and the duration at which personnel are exposed to an elevated

partial pressure of oxygen (PPO2) greater than 51 kPa.27 For example, rescue

personnel performing a single dive are permitted a daily and total cumulative dose of

850 units,27 whereas LR5 personnel performing three dives during the rescue

operation are restricted to a daily and total cumulative CUPTD of 620 and 1860 units

respectively.27 No estimated CUPTD (Figure 9) exceeded REPEX recommendations.

15

The risk of central nervous system (CNS) oxygen toxicity is also considered low as

the deepest decompression stops for Royal Navy 14 (Modified),

XVALSS_DISSUB 7, DCIEM and VVAL-18M tables are 243, 223, 193 and 162 kPa

respectively4, 9-13 and within a dry environment, the incidence of oxygen toxicity

seizures ranges from 1:3000 to 1:12000 at depths between 243 to 303 kPa.28

The shorter TDT and lower oxygen requirements for DCIEM and VVAL-18M tables

justifies their retention and/or introduction for Australian submarine rescue (Figure 7

and Figure 8). Oxygen consumption is an important consideration as the current

rescue vessel is limited in the number of cylinders that can be embarked. Larger deck

space on-board either the Damen Class or ADV Ocean Shield will permit greater

oxygen carrying capacity. Although decompressing personnel with these tables

carries a higher risk of DCS, there may be exceptional circumstances such as severe

limitation of oxygen supply or time constraints where the Fleet Medical Officer or

Senior Hyperbaric Physician may have to tolerate higher DCS risks in order to

preserve oxygen for patients or expedite the decompression of rescue personnel.

In conclusion, reducing PDCS in rescue personnel inevitably comes at the cost of

increased oxygen requirements, longer TDTs and the availability of more personnel,

however ultimately, the choice of decompression table will depend on balancing DCS

risk with competing human, time and oxygen supply constraints. The current oxygen

supply does however accommodate the introduction of candidate tables. Improved

engineering of rescue platforms will reduce operator bottom times and the number of

dives required reducing the risk of DCS. Larger rescue vessels, such as ADV Ocean

Shield or the replacement Damen Class vessel will accommodate the greater numbers

of rescue personnel or a third RCC providing a saturation decompression option.

16

Acknowledgements:

Technical advice was provided by Mark Carey (James Fisher Defence), Kate Ponton

(Defence Science and Technology Organisation), Commander Brett Westcott

(Submarine Escape and Rescue Manager), Lieutenant Commander Joel Hissink

(Submarine Underwater Medicine Unit), Mr John Pennefather (Submarine

Underwater Medicine Unit) and Mr Mark Carey (James Fisher Defence). Special

thankyou to the late, Lieutenant Commander Giselle Mouret (RANR) who kindly

provided French translation services.

17

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5. Royal Australian Navy Subsunk Medical Guide. Canberra: Department of

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