Feature Articles—December 2009 Issue
A Personal Rebreather System For Rescue Submarine OperationsTeam Meets Design Challenges in Developing A Wearable Rebreather System for Australian
Navy’s Remora
By Mark Johnson
President
O2 Dive Technologies Inc.
Houston, Texas
and
Gene Melton
President
HydroSpace Engineering Inc.
St. Augustine, Florida
There are many operational scenarios in which an operator is required to enter or transit an area that may be contaminated with toxic chemicals, fire byproducts or chemical or biological agents. In some of these situations, it is not acceptable for personnel to use negative-pressure filtered respirators due to the nature of the contaminant or even its physical state (e.g., super-heated air). In these situations, a self-contained breathing apparatus (SCBA) is required. Conventional SCBA units consist of a cylinder of compressed gas (usually ambient, filtered air) with flow controlled by a demand valve, or regulator.
One of the major inefficiencies of these units is that the exhausted, or exhaled, air still contains a significant amount of usable oxygen (O2) that is vented to the environment and lost to the user. Much greater efficiencies (translating into smaller, lighter units) can be attained by recycling the exhaust air and either recovering the O2 or removing the undesirable products of respiration, mainly carbon dioxide (CO2). A device utilizing this approach is commonly referred to as a rebreather.
In response to contract requirements from OceanWorks International (Vancouver, Canada), a rescue rebreather, called SubPack, was designed and constructed by O2 Dive Technologies Inc. for the Remora rescue submarine to be used by the Australian navy.
The goal of the SubPack project was to develop a small, lightweight, low-cost emergency rebreather system with breathing gases delivered via umbilical hoses from the Remora’s life support system. This rebreather would be issued to the Remora crew. The project team was charged not only with designing and testing the technology, but also with training rescue submariners on protocols on a very short schedule.
Vessel Setup
The Remora is a 16.5-ton remotely operated rescue vehicle built around a diving bell. It has room for seven people: the operator/attendant and six survivors. It is capable of operating at more than 500 meters’ depth in a current of 1.5 meters per second and of mating to a distressed submarine lying at angles of up to 60° from the vertical.
Rescue and transfer under pressures of up to five bars is achieved by mating to a transfer-under-pressure chamber that is connected by spool pieces. When docked to a distressed submarine, the interface area between the distressed and rescue submarines may be contaminated with toxic chemicals or fire byproducts to the extent that rescue personnel must use closed-circuit breathing systems rather than negative-pressure filter respirators.
System Requirements
A major functional requirement of the new design was a form and fit that would allow rescue submariners to function in a small sphere and assist distressed submarine crew members from their submarine into the Remora.
The Remora’s chamber has interior space restrictions because the inside of the bell is a spherical surface in which it is difficult for submariners to stand up straight. Furthermore, the submarine crew would have to employ SubPack when the two hatches between the distressed and rescue submarines are opened to allow “normal” operations in a contaminated environment. Because the operator would have to be able to reach down through the hatch areas, the unit had to be small, back-mounted and easily donnable. It was also required that the unit possess automatic controls and fail-safe operation.
Other functional requirements of the new design were that no gases could be exhaled into the chamber and that the electronic control system had to operate on its own power source to monitor and control O2 in the breathing loop. O2 and diluent gas also had to be supplied via umbilical hoses so that no gas cylinders were required. Additionally, it was required that the unit weigh as little as possible (less than nine kilograms) and have minimal breathing resistance, a functional duration of six hours at less than 40° Celsius and a design service life of 10 years. The system also had to meet Høvik, Norway-based Det Norske Veritas (DNV) requirements.
Besides these initial, fundamental requirements, OceanWorks International provided valuable meetings during the design process to ensure various other considerations were incorporated into the final design.
Design Concept
The team proposed a compact, simple rebreather as a solution. To recover the usable O2 from the exhaled air, a rebreather cycles expired air through a “scrubber” that removes the CO2 and adds O2 as necessary. This closed-loop supply is the basis of a highly efficient and completely self-contained breathing system.
SubPack is a new design for this project, based on unconventional CO2 scrubber technology combined with innovative flow-control electronics and hardware. The low-cost SubPack unit is ergonomically small, lightweight (at less than nine kilograms), rapidly donnable and capable of 360 minutes of life support.
The unit is designed to be as low maintenance as possible and to have the highest feasible heat dissipation. Materials were selected for their machining characteristics and heat dissipation, in anticipation of meeting DNV testing requirements. The SubPack interfaces with standard oral/nasal masks, is easy to maintain and has a short training period for operators.
System Description
The system consists of seven major subassemblies: outer can, counterlung, manifold, dual CO2 scrubbers, O2 and diluent add valves, electronics pod and oral/nasal mask.
The outer can is made from high-impact plastic. This ensures a very robust and lightweight outer protective cover for the counterlung and other internal components.
The counterlung is an airtight bag that acts as a breathing volume gas accumulator and mixing zone while providing housing for the scrubber manifold subassemblies. The bag is fabricated from a material tailored to the mission that can withstand chemical toxins. The counterlung is incorporated onto a special proprietary manifold that provides heat dissipation.
The main manifold is the backbone of the system. It acts as the main weight-bearing component and provides hardware mounting, heat dissipation and high-pressure to low-pressure gas assembly components. The manifold is fabricated from 6061 T6 aluminum and is hard-coat-anodized to resist oxidation.
At the heart of the rebreather is the dual scrubber, which removes the CO2 from the expired air. For optimal operation of a scrubber, it is important that the gas not only comes in contact with the absorbent material but also that the gas remains in contact with the absorbent material long enough for the desired chemical reaction to occur.
Traditional scrubber designs utilize a single external cylindrical housing filled with absorbent material in which the gas travels from one end to the other. In a dual scrubber configuration, two scrubbers are arranged in a series to increase efficiency.
The SubPack system incorporates a dual scrubber design and then maximizes its functionality by increasing the contact area and the variables, like temperature and contact time, that affect CO2 absorption.
CO2 reduction chemicals work best at a stable temperature between 27° and 38° Celsius. Placing the scrubber canisters inside the counterlung surrounds it with warm, moist breathing gas, which stabilizes the reaction temperature and protects it from cooling. The chemical reaction is thus optimized and provides the user with warm, moist breathing gas. The scrubber temperature varies only 3° Celsius over ambient temperatures ranging from 27° to 2° Celsius, and the SubPack utilizes refillable scrubber canisters rather than disposable cartridges to minimize operational costs.
The set point controller is designed by HydroSpace Engineering to operate with three O2 sensor cells and is powered by a common six-volt battery pack. Sensor values are analyzed to determine loop O2 partial pressures so the controller can make the proper adjustments to the breathing atmosphere. The controller wrist electronics are attached to an electronics pod mounted into the manifold.
Testing Performance
DNV testing of the SubPack consisted of a six-hour duration test at a working temperature of less than 33° Celsius. The SubPack demonstrated that a rescue submariner could move rapidly between resting, medium and heavy metabolic work rates.
Breathing Resistance. During light workloads, the SubPack maintains consistent inhalation and exhalation resistance. Peak resistance between inlet and outlet breaths is 0.25 joules per liter, or 0.5 joules per liter peak-to-peak total breathing resistance. This is well below the acceptable rate of three to four joules per liter.
During moderate workloads, peak resistance was found to be 0.25 joules per liter on the exhale side and 0.40 joules per liter on the inhale side, or 0.65 joules per liter peak-to-peak total breathing resistance.
During heavy workloads, the peak resistance was found to be 0.37 joules per liter on exhalation and 0.76 joules per liter on the inhalation, or 1.10 joules per liter peak-to-peak total breathing resistance.
This means the system performed well at moderate and heavy workloads. After each 30-minute run at increased work-rate levels, the breathing machine was automatically cycled down to the light workload level. After the SubPack was cycled from one workload to another, it returned to temperature and percent of oxygen ranges in less than a minute.
Gas Delivery. A total of 244 liters of CO2 was absorbed during the three test series, 175 percent of the design requirement. Maximum gas temperature during the six-hour light workload test was found to be approximately 33° Celsius. This temperature was below the required 40° Celsius. In subsequent tests of medium and heavy workloads the temperature remained below design requirements.
Electronics. Batteries were installed at the beginning of the test and powered the display and solenoid for the 26-hour total test duration. Throughout all tests, the electronics maintained the percent of oxygen between 24 percent and 28 percent.
The displayed percent of O2 content was the same as the analyzed percent of O2 reading. The solenoid was capable of actuating through multiple breathing rates and maintaining the system O2 levels. The percent of O2 liquid crystal display screen could be easily read in normal light from four to five feet away.
Training
Training in the use of the SubPack is paramount to operational safety. The training of key personnel was conducted by HydroSpace at the OceanWorks facility in Vancouver.
Topics covered included discussion of the physiological aspects of using a rebreather, the theory of operation, potential hazards and postoperations cleaning. Hands-on training included system setup, assembly, calibration and operational procedures.
Conclusions
The SubPack rebreather design is a considerable success. In a situation in which life support is required, the SubPack rebreather can provide rescue missions with a three-hour window for the performance of operations.
The SubPack unit was subsequently redesigned and built to meet the differing requirements of long-duration, high-exertion operations.
Designed for mine search-and- rescue personnel, the Multi Mission ReBreather System is a portable breathing system that has a six- hour capacity which incorporates gas cylinders into a wearable unit. It has a low breathing resistance and no requirement for subsequent gas cooling.
This device is of commercial interest to port security agencies; federal, state and local law enforcement agencies; urban and rural rescue teams; and industrial first responders.
The SubPack unit and all components herein are proprietary to O2 Dive Technologies Inc.
Mark Johnson is president of O2 Dive Technologies Inc. and an ocean engineer who designs, constructs and tests custom life support equipment. He is also president of Deepwater Research Inc. and has more than 20 years of experience designing autonomous underwater vehicles, remotely operated vehicles (ROVs) and ROV tooling for the oil industry.
Gene Melton is president of HydroSpace Engineering Inc. and the designer and manufacturer of the HydroSpace Explorer dive computer and partial pressure oxygen monitors and Neptune rebreathers. He has worked as an engineer on the superconducting super collider and the space shuttle program and as an electronics engineer, pilot and diver with the Johnson-Sea Link submersibles.
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